GEOMOR-04238; No of Pages 14
Geomorphology xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
Climate and local geomorphic interactions drive patterns of riparian forest decline
along a Mediterranean Basin river
John C. Stella a,⁎, Jess Riddle a, Hervé Piégay b, Matthieu Gagnage b, Marie-Laure Trémélo b
a
b
Forest and Natural Resources Management, State University of New York College of Environmental Science and Forestry, One Forestry Drive, Syracuse, NY, 13210, USA
University of Lyon, CNRS, ENS Lyon, ISIG Platform, EVS, F-69007, France
a r t i c l e
i n f o
Article history:
Accepted 11 January 2013
Available online xxxx
Keywords:
Riparian forest decline
Mediterranean
Dendrochronology
Populus nigra (Salicaceae)
In-channel gravel mining
Climate change
a b s t r a c t
Dynamic fluvial processes strongly influence ecological communities and ecosystem health in riverine and riparian
ecosystems, particularly in drought-prone regions. In these systems, there is a need to develop tools to measure impacts from local and regional hydrogeomorphic changes on the key biological and physical processes that sustain
riparian ecosystem health and potential recovery. We used dendrochronology of Populus nigra, a riparian tree that
is vulnerable to changes in local hydrology, to analyze ecosystem response following channel incision due to gravel
mining along the Drôme River, a Mediterranean Basin stream in southern France. We cored 55 trees at seven floodplain sites, measured ring widths, and calculated basal area growth to compare the severity and timing of local
growth decline along the river. Current basal area increment (BAI) growth per tree ranged almost 10-fold among
sites (7.7±1.3 to 63.9±15.2 cm2 year−1, mean±SE) and these differences were significant. Mean BAI was correlated positively with the proportion of healthy trees at a site, and negatively with proportion of dead canopy area.
Regime shift analysis of the tree-ring series indicates that tree growth declined significantly at four sites since
1978, coincident with documented channel incision. In addition, patterns of low growth and crown dieback are consistent with stress due to reduced water supply. The most impaired sites were not directly adjacent to local mining
pits visible on aerial photographs, nor did the sequence of growth regime shifts suggest a pattern of channel incision
progressing from these areas. The initiation of site growth declines was most typically associated with drought years,
and the most impaired sites were spatially distributed to suggest the influence of local bedrock controls on soil
depth. Climate in the Drôme basin and in the Mediterranean region is trending significantly toward hotter growing
seasons with a decrease in summer river discharge, and this will increase both chronic and acute water shortage for
riparian trees. This study shows that drought-prone riparian forests are vulnerable to hydrogeomorphological
changes, but the severity of impacts is conditioned by interactions between drivers at different scales, including regional climate variability, reach-based geomorphic alteration, and local lithological controls.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Dynamic fluvial processes influence riparian forest and woodland
ecosystems in numerous ways that affect the initial habitat conditions, resource fluxes, and ecological succession pathways. Important hydrogeomorphic processes include: (1) creating new growing
space through bank erosion, horizontal channel migration, channel
cutoff, and accretion of new floodplain surfaces (Piégay et al., 2005;
Florsheim et al., 2008; Stella et al., 2011); (2) influencing the recruitment and survival of riparian seedlings through scour, burial and
desiccation, all of which depend on the flooding regime and floodplain morphodynamics (Mahoney and Rood, 1998; Dufour et al.,
⁎ Corresponding author at: Department of Forest and Natural Resources Management,
State University of New York College of Environmental Science and Forestry, One Forestry
Drive, Syracuse, NY 13210, USA. Tel.: +1 315 380 9759; fax: +1 315 470 6535.
E-mail addresses: stella@esf.edu (J.C. Stella), jdriddle@syr.edu (J. Riddle),
herve.piegay@ens-lyon.fr (H. Piégay), tanatocle@hotmail.com (M. Gagnage),
marie-laure.tremelo@ens-lyon.fr (M.-L. Trémélo).
2007; Moggridge and Gurnell, 2009); and (3) influencing groundwater table dynamics via channel bed incision and aggradation, which
in general reduces or increases, respectively, water availability to
the root zone (Scott et al., 2000; Pont et al., 2009).
In drought-prone regions, these hydrogeomorphic processes create
riparian ecosystems that are particularly resource-rich, productive,
structurally complex, and biodiverse compared with the surrounding
landscape (Patten, 1998; Gasith and Resh, 1999; Underwood et al.,
2009; Stella et al., 2012). Phreatophytic poplars and cottonwoods
(Populus spp.), which dominate sub-humid riparian communities in
much of the northern hemisphere (Karrenberg et al., 2002), are important foundational species (sensu Ellison et al., 2005) in providing or
regulating water quality, microclimate, structural habitat for wildlife
and fish, an energy base for the food web, and bank stability (Braatne
et al., 1996; Lewis et al., 2009; Young-Mathews et al., 2010).
The distribution, composition and health status of Populus-dominated
riparian forests are directly influenced by current and historical hydrology
(Braatne et al., 1996; Rood et al., 2003) and geomorphic conditions
(e.g., Cooper et al., 2003a; Stella et al., 2011). Over the last century,
0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.geomorph.2013.01.013
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
2
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
humans have modified the dynamic processes that sustain riparian
ecosystems in multiple ways, including construction of dams and
flow diversions, groundwater pumping, and in-stream gravel mining
(Steiger et al., 2005; Palmer et al., 2009; Stromberg et al., 2010). As a
result, pioneer riparian forest stands have proven vulnerable to these
disturbances and are in decline in many ecosystems (Scott et al., 1999;
Cooper et al., 2003b; Dufour et al., 2007). Temporary reductions in
water availability can result in reversible changes such as slower growth
and increased water use efficiency (Horton et al., 2001; Amlin and Rood,
2003; Rood et al., 2003; Lambs et al., 2006; Stella and Battles, 2010), but
more severe and abrupt water table declines can cause permanent damage and high mortality (Tyree et al., 1994; Scott et al., 1999; Rood et al.,
2000; Stella et al., 2010). Plant water availability may be limited not
only by modifying water supply, canopy demand and vapor pressure deficits (e.g., Cooper et al., 2003b), but also by altering geomorphological
processes that affect groundwater depth and capillarity (e.g., Scott et al.,
1999, 2000).
Negative impacts to riparian tree survival, growth and function are
likely to continue as increasing human populations compete for water.
Climate change will further alter the water balance in many areas
(Steiger et al., 2005; Palmer et al., 2008; Perry et al., 2011). Because of
its phreatophytic physiology and low tolerance to drought, sudden
and severe damage to Populus stands often occurs when the water
table declines beyond a threshold (Scott et al., 1999; Cooper et al.,
2003b). Furthermore, local geomorphic changes in conjunction with
water competition and climate change could act synergistically to injure
Populus forests in areas where their separate impacts would be minor
(Albertson and Weaver, 1945; Stromberg et al., 1996). The spatially
and temporally discontinuous pattern of riparian forest decline after
some channel incision events suggests that local geomorphic manipulation can moderate the impacts on phreatophytic floodplain trees (Graf,
1982; Scott et al., 1999; Amlin and Rood, 2003). However, few investigations of riparian forest decline have examined the simultaneous effects of
hydrogeomorphic alteration, climate change, and local geomorphic factors (Palmer et al., 2009).
In this study, we investigated patterns of riparian tree growth over
time, both for individuals and for whole stands, in the context of local setting, reach-wide geomorphic changes and climate change. The Drôme
River in southeastern France provides an excellent opportunity to study
these interactions at varying temporal and spatial scales. The Mediterranean region, including the Drôme basin, is undergoing climatic drying
that is projected to increase in the 21st Century (Giorgi and Lionello,
2008; García-Ruiz et al., 2011). Along a 5-km reach downstream of the
town of Luc-en-Diois, the river is situated in a narrow valley confined
by marl and limestone deposits where the active channel was mined for
gravel beginning in the 1970s and subsequently has incised along its
length (Landon et al., 1998; Kondolf et al., 2002; Toone et al., 2012). Adjacent to the channel, riparian forest stands dominated by Populus nigra currently display patchy, local evidence of severe crown dieback (Gagnage,
2008; Dunford et al., 2009). This species is the dominant European poplar
species and, because of its high ecological and economic value, has been
the focus of international efforts to conserve its habitat and genetic diversity (Lefèvre et al., 1998; Hughes and Rood, 2003).
In this context we asked the general question, ‘How are patterns
in tree growth and decline consistent with overall climate drivers versus
local environmental conditions such as valley morphology and humaninduced geomorphic changes?’ Specifically, we investigated: (1) relationships between tree growth and independent measures of riparian stand
health, and how these vary among sites within the reach; (2) trends
in the spatial extent and timing of P. nigra growth decline within the
reach; and (3) regional climate-based versus local site-based environmental correlates with growth patterns evident across the tree-ring
chronologies. We expected that if gravel mining was the immediate
cause of tree mortality, crown dieback and reduced growth, two predicted patterns in the tree-ring record should be evident. First, any
growth declines should have occurred relatively suddenly and only
after the initiation of gravel mining (approximately 1975–1980 from
aerial photograph evidence; Landon et al., 1998). Secondly, floodplain
sites closest to the pits should show the earliest and strongest declines,
with more distal sites demonstrating later and/or more subtle declines.
These spatio-temporal patterns are consistent with other studies of
water table decline associated with in-channel mining (Scott et al.,
1999; Amlin and Rood, 2003; Rollet et al., 2006) and channel bed incision from sediment starvation after dam construction (Kondolf, 1997).
In addition, the presence of both confining slopes on one side of the
river channel and tributary confluences upstream of the reach may contribute to edaphic heterogeneity in the floodplain, with potentially local
effects on riparian forest stands. Conversely, if tree growth along the
Drôme River responds primarily to climate rather than local drivers,
growth patterns should show a more regional signal that is coordinated
with either short-term droughts (Singer et al., 2012), or long-term climate change in the Mediterranean basin (Giorgi and Lionello, 2008;
García-Ruiz et al., 2011).
2. Materials and methods
2.1. Regional setting and study sites
The Drôme, which is a tributary of the Rhône River in southeastern France (Fig. 1), is a fourth-order stream with a mountainous preAlpine catchment of 1640 km2 of Cretaceous age (elevation range
800–1400 m), that is underlain primarily by marl deposits in its valley
with steep slopes rising to limestone ridges. Throughout the watershed, the climate follows a weakly Mediterranean pattern of warm,
relatively dry summers and cool, moister winters. The study area receives 889 mm average annual precipitation (gauged at Luc-en-Diois;
Fig. 1); at lower elevations in the watershed, winter precipitation falls
as rain, but snow accumulates at higher elevations.
The catchment area for the study reach is approximately 225 km2
(194 km2 at the Luc-en-Diois gaging station) and annual discharge averages 2.4 m3 s−1 (1961–2007 for Luc-en-Diois). Mean discharge in spring
(March–May) is 3.8 (±1.7 SD) m3 s−1 and exceeds the summer
baseflow by an order of magnitude (0.6±0.4 SD m3 s−1 for July–Sept).
Between 1948 and 2009, major floods occurred in 1951 (Q16), 1978
(Q17), 1994 (Q100) and 2003 (Q45). Major growing-season droughts,
with multiple years of lower-than-average streamflow, occurred in
1964–65, 1989–91, and 2003–2006.
The study was undertaken within a 5-km study reach downstream of
the town of Luc-en_Diois (Fig. 1). The channel in this reach is characterized by alternating straight and braided sections, an average longitudinal
slope of 0.1%, an active width ranging from 10 to >200 m, and no significant bank protection (Toone et al., 2012). As a result, unconstrained
channel migration occurs over the floodplain, which supports an extensive riparian forest that developed since the 1950s and is currently dominated by poplar (P. nigra), willow (Salix alba), ash (Fraxinus excelsior),
and alder (Alnus glutinosa) (Piégay and Landon, 1997; Gagnage, 2008).
The channel still experiences significant flood events (Landon et al.,
1998; Kondolf et al., 2002), including one in December 2003 that
resulted in failure of the bridge at Die.
The Drôme channel and floodplain were subjected to intensive gravel
mining beginning in the 1970s. Gravel extraction through channel dredging and strip mining occurred at a rate of 250,000 m3 yr−1 within and
downstream of the study reach during this period, dwarfing the Drôme's
natural annual bedload flux, estimated at 35,000 m3 yr−1 (Landon et al.,
1998). Though the most concentrated activity was downstream of the
5-km study reach, several areas of intensive extraction within the study
reach are evident on 1980 aerial photographs, but not on the previous series flown in 1971 nor the subsequent one in 1991 (Fig. 1; Toone et al.,
2012). This activity was centered downstream of the Béoux confluence
and downstream of the Recoubeau bridge. Geomorphic evidence of
in-stream mining includes pits in the gravel bars, sections of straightened
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
3
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
d)
Recoubeau
bridge and
protective weir
a)
b)
France
lake
Geneva
b)
Lyon
Valence
c)
7
c)
Mediterranean Sea
44.6° north
VERCORS
5.4° east
6
Livron
5
Loriol
Drô
me
Grane
Die
Crest
3
DIOIS
e
nn
Roa
4
Châtillon-en-Diois
Bez
Saillans
STUDY
REACH
d)
Luc-en-Diois
Dr
ôm
e
flo
w
Valdrôme
BARONNIES
2
2
0
5
10 km
Gaging station at Luc-en-Diois
Sampling plots
Mining sites visible on aerial photos
of 1980 but not in 1971
Active channel in 2006
Floodplain age classes
< 36 years old
Confluence
of the Béoux
36 to 58 years old
> 58 years old
Sources : IGN - Aerial photographs from1948 to 2006
N
250 m
1
Luc-en-Diois
Fig. 1. The Drôme River study reach location in: (a) France; (b) the Rhône Basin; and (c) the greater Drôme basin. Panel (d) shows the local geomorphic and riparian forest context digitized
from 2006 aerial photographs. Outlined and numbered polygons denote Populus nigra stands sampled in this study, and circles denote local areas of gravel extraction evident on 1980 aerial
photographs.
low-flow channel, and local gravel levees built to protect the mining
zones from flooding.
The study reach also experienced bed incision, as observed from comparing long profiles from 2003 to 2005 to one surveyed in 1928 by the
Service des Grandes Forces Hydrauliques (Toone et al., 2012). Between
1928 and 2003, incision averaged 0.8 m but locally exceeded 2 m, and
varied partly because of limestone outcrops that are discontinuous longitudinally and which anchor most of the single-channel reaches (Fig. 1;
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
4
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
Toone et al., 2012). Though the period 1928–2003 is too long to assess
whether the recent gravel mining initiated channel bed incision, some
of the most incised areas are associated with local mining sites, including
the Recoubeau bridge, which was undermined during the mining period
and subsequently protected by a concrete weir. Since the mid-1970s, this
area experienced both channel incision that propagated upstream, and
local aggradation (+0.33 cm between 1928 and 2003) near the weir
after its installation in the early 1990s (Toone et al., 2012).
2.2. Field sampling
We sampled trees in seven floodplain stands located throughout the
study reach (Fig. 1). These stands were selected by first examining a
chronosequence of aerial photograph series to locate floodplain surfaces that had been created prior to 1971 but were still young enough
to be dominated by P. nigra and other pioneer riparian trees. Coniferdominated patches, which indicated xeric, high-elevation areas outside
the riparian zone, were filtered out using an NDVI index on recent aerial
photographs (Quickbird image flown 7 Dec. 2001). This process identified 11 mid-aged sites that ranged from 36 to 58 years old. From this
group we selected seven sites for field sampling that were relatively
well-distributed within the reach and that supported deciduous riparian
forest stands dominated by P. nigra (Fig. 1; Gagnage, 2008; Lejot et al.,
2011).
In each stand we surveyed the relative elevation of its floodplain surface above the current baseflow elevation using DGPS (Trimble 5800 Differential Global Positioning System) with a vertical resolution of b 10 cm,
at 1–7 points (mean 3.4) per stand. Surveys were conducted from May to
July 2008 on days when the river discharge was b2 m3 s−1. Using a soil
corer, we measured the fine sediment thickness at 6 to 14 locations distributed across the stand.
At the center of each stand, we surveyed a circular plot of 500 m2 to
sample tree composition, density and crown health. Canopy cover was estimated visually for the entire stand using a range of values to represent
the high heterogeneity in cover present. All canopy trees within the plot
were measured and the species composition was recorded. From the center of each plot, we selected 20 dominant trees (i.e., those structuring the
canopy) for which we estimated the proportion of live crown; these measurements were aggregated across the sample to assess the health of the
stand (Scott et al., 1999).
The estimates of crown integrity were complemented using lowelevation aerial photograph analysis of the sites to identify the area
of dead branches within the canopy (Gagnage, 2008; Dunford et al.,
2009). In this method, digital photographs taken from an unmanned
aerial vehicle were orthorectified to the project basemap and dead
branches were identified using the object-oriented classification approach detailed in Dunford et al. (2009). Dead branch area was
summed and divided by the sampling area for each of the stands to
estimate the proportion dead canopy.
2.3. Dendrochronological analysis
Of the live trees in each stand, we selected the largest dominant
P. nigra individuals for coring, regardless of health status. A total of
55 trees were cored across the seven sites; samples ranged 6–9 trees
per site. Cores were extracted at breast height (1.4 m) using a 5-mm
increment borer, with two cores collected per tree wherever possible
(N=114 cores).
Following standard dendrochronological procedures, we mounted
cores and clarified ring anatomy by sanding each core with a series of progressively finer sand papers (Stokes and Smiley, 1968). Ring widths were
measured to 0.001 mm resolution using a sliding stage micrometer
(Velmex Inc., Bloomfield, NY) and Acu-Rite encoder (Heidenhain Corp.,
Schaumberg, IL). Potentially misdated cores were identified with the software program COFECHA (Holmes, 1983) and checked for false rings,
missing rings, and other sources of dating errors. All ages reported are
minimum ages, as they do not account for time for the tree to reach coring
height or the time between the innermost ring on the core and the pith.
To examine long term trends in tree growth, we converted the raw
ring widths to basal area increments (BAI) using the formula
!
"
2
2
BAI ¼ π r n −r n−1
ð1Þ
where r is the radius at the end of year n. BAI typically increases as trees'
crowns expand or understory trees are exposed to higher light levels,
and stabilizes or continues to increase in healthy, undamaged, mature
trees (Biondi and Qeadan, 2008). The BAI series for multiple cores per
tree were averaged, and then site chronologies were constructed by
taking the arithmetic mean of all BAI series from the site for each year.
We analyzed the BAI chronologies using Rodionov's regime shift detection (RSD; Rodionov, 2004) to explore the possibility of rapid and
sustained shifts in the site mean growth rates, and to determine the
timing of any shifts. The algorithm uses sequential t-tests to identify shifts
in the mean without an a priori hypothesis of timing, and uses an index
based on normalized anomalies to determine if the shifts are maintained.
We applied the algorithm with the RSD add-in for Microsoft Excel 2002
(Rodionov, 2006a) with significance level=0.05, cut-off=10 years,
Huber's weight parameter=2, and autocorrelation estimated by the inverse proportionality method on 7 year subsamples (Rodionov, 2006a,b).
To examine correlations with growing season climatic and hydrological variables, we used a data-adaptive detrending procedure within the
program ARSTAN (Cook, 1985) to produce a chronology that emphasizes
high-frequency variation (Cook and Peters, 1981). This series was more
appropriate to use than the BAI series, because lower-frequency signals
such as decadal growth trends due to site conditions would be preserved
in the BAI chronologies and would obscure the high-frequency climate
correlations (Cook et al., 1990). Nine of the 114 total cores were removed from this analysis due to poor cross-dating; in some cases,
short series may have compromised the ability of statistical correlations to identify misdated cores (Grissino-Mayer, 2001). A Friedman
variable span smoother (alpha = 5) was fit to each remaining series
(Friedman, 1984), which was then divided by its fit curve to produce
a time series of dimensionless index values; the autocorrelation was
then estimated and removed from each series. All residual series were
averaged by calendar year using a robust bi-weight mean to generate
the final residual chronology. The chronology was truncated where the
expressed population signal, a measure of chronology strength with maximum of 1, fell below 0.85 (Wigley et al., 1984).
We tested differences in recent growth among sites using analysis of
covariance (ANCOVA). Recent growth, the response variable, was calculated by summing the BAI for the most recent four years (2004–07); this
metric was log-transformed in the ANCOVA model to satisfy residual assumptions. Predictor variables in addition to site included tree age and
early tree size, which was represented as basal area in 1980. This latter
variable was included in order to control for differences in tree size at
the start of the growth decline period, when larger trees would be
expected to have a more extensive root system and therefore potentially greater access to water. To estimate individual tree basal areas at the
time when growth declines might have begun, we summed for each
core all ring widths prior to 1980, then averaged the radius estimates
for all cores within trees, and converted these to basal areas.
2.4. Climate data analysis
Climate data for the period 1961–2007 were obtained from several
sources in order to evaluate the climate signal within the tree-ring chronologies. Monthly maximum and minimum temperature data (i.e., daily
highs and lows averaged by month) were extracted from the E-OBS
dataset (Haylock et al., 2008), which are gridded (0.25° cell) interpolations from regional station data with missing data estimated and stations
with >20% days with missing data excluded. Daily precipitation totals
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
5
47.4 (±10.4)
63.9 (±15.2)
7.7 (±1.3)
27.3 (±6.8)
51.3 (±10.3)
12.5 (±3.7)
48.5 (±16.4)
58
35.5
47
37
36.5
48
43.5
47.1
49.7
37.6
54.5
50.2
44.0
31.5
(±5.4)
(±5.1)
(±4.1)
(±5.6)
(±4.7)
(±4.3)
(±2.9)
(38,
(31,
(44,
(24,
(24,
(26,
(32,
71)
37)
64)
62)
43)
61)
50)
Age of cored trees
(years, median,
min and max)
150
8
6
9
9
8
7
8
1.4%
2.0%
2.5%
2.5%
0.5%
2.4%
1.6%
6%
0%
65%
11%
50%
61%
26%
c
a
bc
c
ab
bc
1
2
3
4
5
6
7
100
50
Recent BAI (cm2 year-1)
25%
25%–50%
0%–25%
80%
90%
0%–25%
25%
2.2
1.8
1.7
1.0
0.3
1.2
1.1
c
10
5
1.55
2.09
1.45
2.79
0.73
1.87
1.09
1
2
3
4
5
6
7
(±14.5)
(±10.5)
(±6.9)
(±11.2)
(±10.9)
(±5.8)
(±3.7)
70.3
31.9
46.5
54.2
54.2
25.0
15.5
ð2Þ
where μ and σ are the overall mean and standard deviation, respectively, for a particular month throughout the period of record and x is the
monthly value in a given year. Following transformation, monthly scores
were then averaged for the growing season, June through September.
We used a modified Mann–Kendall test proposed by Yue et al. (2002)
to estimate the significance of trends in monthly and seasonal climatic
and discharge variables over the period 1961–2007. In that procedure,
trend is first estimated as the median slope of all possible point pairs
(Theil, 1950a–c; Sen, 1968) then removed from the time series. Next, autocorrelation is modeled as a lag-one autoregressive process and also removed from the time series. Finally, the trend is mixed back into the time
series, and significance of the trend is assessed by the Mann–Kendall test
(Mann, 1945; Kendall, 1975). We completed all trend analysis with the
zyp package in R (version 2.12.1, R Development Core Team).
740
1040
240
1340
740
320
540
Dead canopy
portion of total
stand area
Density of canopy
trees (trees ha−1)
Canopy cover
Maximum incision
between 1928 & 2003
long profiles (m)
Fine sediment
depth (cm,
mean ±1SE)
Floodplain
elevation above
channel (m)
Site
Table 1
Characteristics of the Populus nigra stands studied along the Drôme River.
were obtained for the Luc-en-Diois station (Météo France station
#26167001) and daily river discharge values from the Luc-en-Diois
gauging station (#V4214010, French National Office for Water and
the Aquatic Environment). Monthly metrics were calculated by averaging the daily discharge values and summing the daily precipitation totals.
We used Pearson correlations to assess the relationship between annual radial growth, as indicated by the high-frequency chronology, and
both monthly and growing season climate and discharge variables. Because conditions during the year prior to ring formation often influence
ring width (Fritts, 1976), we used a 19-month window beginning in the
previous April and extending to the current October to assess annual
growth correlations with monthly values for discharge, precipitation,
and minimum and maximum temperatures. The best-correlated months
were further aggregated as a single period (i.e., growing season) to compare the sensitivity of tree-ring growth among climate metrics. The
monthly discharge values had substantially unequal variances, therefore
we transformed all data metrics to standard scores (i.e., z-scores) prior
to averaging months together for the growing season. Standardization
was necessary because the unequal variances could lead to biased growing season discharge estimates whereby only one or two months dominated the seasonal signal. The discharge standard scores were calculated
z ¼ ðx–μ Þ=σ
Proportion of
trees with >50%
branches dead
No. trees cored
Diameter of cored trees
(cm, mean ±1SE)
Basal area increment
of cored trees
(cm2, mean ±1SE)
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
1
Site Number
Fig. 2. Boxplots of recent average growth (basal area increment means for 2004–07)
for Populus nigra trees at the seven Drôme River floodplain sites. Thick solid lines indicate site means, boxes indicate 25th and 75th quartiles, and whiskers span the range of
the data. Different letters at the top denote significant differences between sites based
on the Tukey Honestly Significant Difference test and a threshold p b 0.05.
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
6
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
3.1. Site conditions, tree health, and recent growth
The seven sites are located on floodplain surfaces 0.7 to 2.8 m above
the river channel (Table 1), with fine sediment depth above the coarse
gravel layer ranging from 15.5 ± 3.7 to 70.3 ±14.5 cm (mean± SE).
Tree density among the seven sites ranged from 240 to 1340 trees per
hectare, corresponding to canopy cover ranging from sparse (0–25%)
to closed (90%). Tree diameters ranged from 31.5 (±2.9 SE) to 54.5
(±5.6 SE) cm dbh, and median minimum tree age ranges from 36.5 to
58 years among the sites. Healthy P. nigra trees (i.e., those with b 50% of
branches dead) were uncommon at some sites, but represented over
75% of individuals at other sites. The number of trees with severe crown
damage (>50% branches dead) ranged from 6% to 65% among sites.
a)
100
50
r = 0.56
2
5
1
7
4
10
6
3
Cores from P. nigra trees showed substantial site-based variability
in current growth. Mean basal area increment for the last four years
varied by an order of magnitude among sites, from 7.7 ± 1.3 to
63.9 ± 15.2 cm 2 year − 1 (mean ± 1SE). The great range in recent radial increment includes three sites sustaining high growth (Sites 1,
2, and 5), two maintaining moderate growth (Sites 4 and 7), and
two others (Sites 3 and 6) yielding substantially lower average
growth among trees (Fig. 2). The ANCOVA model indicated that
growth differences were significant among sites (F6,46 = 8.56,
p b 0.0001), and post-hoc Tukey tests of 95% family-wise confidence
levels indicate that Sites 3 and 6 were significantly different from Sites
1, 2 and 5 (Fig. 2). The continuous covariates in the ANCOVA were
a)
60
Diameter at breast height (cm2)
3. Results
r = 0.7
55
4
5
50
2
1
45
6
40
3
35
7
30
5
0.2
b)
0.6
0.8
100
50
1
5
1
7
4
10
200
400
600
800
1200
100
r = -0.72
2
b)
6
3
5
1800
r = 0.77
50
7
2
5
1
4
10
6
3
5
0
0.2
0.4
0.6
0.8
1
c)
Proportion of trees in poor health
( >50% dead crown)
c)
100
r = -0.69
50
2
5
1
7
4
10
6
3
5
0
0.5
1
1.5
2
2.5
3
Dead canopy area (% of stand polygon)
Fig. 3. Relationship of stand health metrics with recent (4-year average) basal area increment: (a) proportion of healthy trees (b50% branch dieback); (b) proportion of unhealthy
trees (>50% branch dieback); and (c) proportion of the stands' canopies comprised of dead
branches evident from air photograph analysis. Numbered symbols indicate sites identified
in Table 1.
Proportion of trees in poor health
( >50% dead crown)
Recent mean tree BAI (cm2 yr-1)
0.4
Proportion of trees in good health
(minimal branch dieback)
Recent mean tree BAI (cm2 yr 1)
0
200
400
600
800
1200
1800
r = -0.83
1
0.8
3
0.6
6
5
0.4
7
0.2
4
1
0
200
400
600
2
800
1200
1800
-1)
Density (trees ha
Fig. 4. Relationships of stand density with indicators of tree health: (a) diameter at breast
height (cm); (b) recent mean tree BAI (cm2 yr−1); and (c) proportion of trees in poor
health (>50% dead crown). Overall, the densest stands contained the healthiest trees.
Numbered symbols indicate sites identified in Table 1.
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
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J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
not significant, including tree age (F1,46 = 0.35, p = 0.56) and tree
size normalized to estimated 1980 basal area (F1,46 = 2.36, p =
0.13); nor were any variable interactions significant. Therefore
site-based growth differences were not influenced by variation in the
intrinsic biotic factors of tree age and size early in stand development.
At the site level, recent radial growth was positively correlated with
the proportion of healthy (i.e., minimal branch mortality) P. nigra trees
(r=0.56; Fig. 3a), and negatively correlated with the proportion of
trees with > 50% dead crown (r = − 0.72; Fig. 3b), and the proportion
of the stand's aerial image comprising dead branches in the upper canopy (r= −0.69; Fig. 3c). Metrics of stand vigor were positively correlated
with tree density, with the sparsest stands supporting the least productive and healthy trees. For example, stand density was positively correlated with tree diameter (r = 0.70; Fig. 4a) and recent growth
(r = 0.77; Fig. 4b), and negatively correlated with the proportion of
trees with severe crown damage (r = − 0.83; Fig. 4c).
250
200
a) Site 1
mean+1SE (grey)
trees
3.2. Dendrochronological and growth results
Variability among the BAI site chronologies increased over the time
period examined (1961–2007), and different sites show contrasting
trends (Fig. 5). Prior to the mid-1970s, the chronologies show either little
trend in growth or an increase, which is typical of younger trees still undergoing crown expansion. All sites sustained comparable growth during
this period of 30–100 cm2 average BAI per tree. After that time, variance
250
200
150
150
100
100
50
50
1960
250
200
1970
1980
1990
2000
1960
2010
250
c) Site 3
200
150
150
100
100
50
50
0
0
1960
250
200
1970
1980
1990
2000
250
e) Site 5
200
150
100
100
50
50
1980
1990
2000
2010
1970
1980
1990
2000
2010
1980
1990
2000
2010
f) Site 6
0
0
1960
200
1970
d) Site 4
1960
2010
150
250
b) Site 2
0
0
Basal area increment (cm2 yr-1)
Of the geomorphic variables measured, elevation above the channel
was correlated positively with areal proportion dead canopy in the
stand (r= 0.74), indicating greater damage in higher stands. However,
there was no relationship evident between recent growth and elevation
above the channel (r= −0.16), nor between recent growth and maximum bed incision in the adjacent channel calculated from the 1928
and 2003 long profiles (r= −0.09, t = −0.21, df = 5, p = 0.84).
1970
1980
1990
2000
1960
2010
125
g) Site 7
100
150
75
100
50
50
25
1970
S1
S2
S3
S4
S5
S6
S7
h) All site means
0
0
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
Fig. 5. Annual time series of Populus nigra basal area increment (BAI) at Drôme River sites. For individual sites (panels a–g), thin lines show BAI for individual trees, thick lines show the
annual site means, and gray regions show ±1SE around the mean. Panel (h) shows all site means plotted together for comparison; note the different vertical scale from the other panels.
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
8
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
among chronologies increased and sites diverged steeply in average
growth, with Sites 1, 2, and 5 maintaining 50–100 cm2 BAI per tree to
the present, and the other sites showing growth declines at various times.
Regime Shift Detection (RSD) analysis identified significant shifts in
annual growth rate throughout the study period, both in individual
trees and in site chronologies (Fig. 6). Chronologies for sites 1, 2 and 5
exhibited no regime shifts, and growth was relatively steady through
time. The RSD analysis detected significant declines at Site 3 (in 1978),
Site 7 (in 1990), Site 4 (in 1995), and Site 6 (in 1997), as well as an increase at Site 7 in 1999 following its decline nine years earlier. At all
sites, individual trees displayed either upward or downward shifts, but
not both (Fig. 6).
season response varied more among months (r=0.09–0.28; Table 2).
The correlation analysis detected no significant relationships between
growth and either monthly mean minimum or maximum temperature.
Of the aggregated growing season climate variables, river discharge and
precipitation correlated significantly with growth (r=0.40; Table 2).
Over the 1961–2007 period, the Drôme River basin became significantly hotter and river discharge has decreased. Average discharge
between June and September at the Luc-en-Diois gauge has declined
by an average of 0.09 m 3 per second per decade (tau = − 0.21,
p = 0.04; Fig. 7a), mean June–September precipitation has been stable
(tau=0.0087, p = 0.94; Fig. 7b), daily minimum temperature has increased by 0.16 °C per decade (tau= 0.27, p =0.008; Fig. 7c), and
daily maximum temperature has increased by 0.40 °C per decade
(tau=0.36, p b 0.001; Fig. 7d).
3.3. Climatological correlations with annual growth and climate trends
4. Discussion
P. nigra radial growth responded positively to monthly mean stream
discharge during the growing season, especially June through September
(r=0.26–0.32; Table 2). Responses to discharge during the dormant season are generally negative (r=−0.18–0), but are much weaker than the
response to growing season discharge. Correlations with monthly precipitation totals produce a similar pattern to discharge, but the growing
Riparian forest stands within the Drôme River have experienced sharp
growth declines of their P. nigra trees in some areas, in conjunction with
3
Site 1
2
100
150
3
Site 2
2
100
1
1
0
50
0
50
-1
-1
1960
150
1970
1980
1990
2000
1960
2010
3
Site 3
2
100
-2
0
-2
0
150
1970
1980
1990
2000
2010
3
Site 4
2
100
1
1
0
50
0
50
-1
-1
BAI (cm2)
1960
150
1970
1980
1990
2000
1960
2010
3
Site 5
2
100
-2
0
-2
0
150
1970
1980
1990
2000
2010
3
Site 6
2
100
1
1
0
50
0
50
-1
-1
-2
0
1960
150
1970
1980
1990
2000
Regime Shift Index
150
4.1. Patterns of poplar decline along the Drôme River
-2
0
1960
2010
1970
1980
1990
2000
2010
3
Site 7
2
100
1
0
50
-1
-2
0
1960
1970
1980
1990
2000
2010
Year
Fig. 6. Regime shift detection (RSD) analysis for each site. Each panel shows the site-averaged basal area increment chronology (thin line, left vertical axis), the RSD curve for the
site mean (thick line, right vertical axis), and the RSD break points for individual trees (circles, right vertical axis).
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
9
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
Mean
discharge
(m3 s−1)
Cum.
precipitation
(mm)
Mean minimum
daily temperature
(°C)
Mean maximum
daily temperature
(°C)
prevApril
prevMay
prevJune
prevJuly
prevAugust
prevSeptember
prevOctober
prevNovember
prevDecember
January
February
March
April
May
June
July
August
September
October
June–September
0.06
−0.21
−0.10
−0.02
−0.09
0.12
0.13
−0.15
−0.12
0.00
−0.14
−0.18
0.09
0.10
0.32*
0.26
0.26
0.32*
0.20
0.40*
−0.03
−0.18
−0.13
−0.22
−0.01
0.34*
−0.09
−0.31*
0.11
−0.24
−0.03
−0.13
0.10
0.11
0.28
0.28
0.09
0.24
−0.01
0.40*
−0.05
−0.07
−0.17
−0.07
0.04
0.11
−0.10
−0.07
−0.15
0.08
0.11
−0.13
0.28
0.11
0.10
0.11
−0.12
0.24
0.06
0.14
−0.08
0.04
−0.07
−0.11
0.15
−0.02
−0.18
−0.04
−0.13
0.24
0.10
−0.15
0.21
−0.04
−0.08
−0.08
−0.27
−0.05
−0.05
−0.19
a)
2.5
Discharge (m3 s-1)
Month
3.0
2.0
1.5
1.0
0.5
0.0
1960
120
Precipitation (mm month-1)
Table 2
Monthly climate and discharge correlations with the high frequency growth tree-ring
chronology from all sites. Values are Pearson correlation coefficients; asterisks denote significant correlations at pb 0.05. Correlations for the aggregate June–September growing
season metrics (last row) controlled for unequal variance among months by calculating
standardized scores for all months prior to averaging. The designation ‘prev’ in month
names indicates the month of the previous year.
1970
1980
1990
2000
2010
1970
1980
1990
2000
2010
1970
1980
1990
2000
2010
b)
100
80
60
40
20
0
4.2. Evidence for local and reach-scale drivers of tree decline
Broad-scale deterioration in stand health within this reach of the
Drôme River is consistent with rapid changes in water availability, including hypothesized groundwater decline associated with the channel incision that occurred over the last several decades (Landon et al., 1998;
Kondolf et al., 2002). Other reaches of the Drôme also experience tree
stress responses to hydrogeomorphic changes, namely severely reduced
1960
25
Temperature (C)
poor stand health. Long-term growth trends showed strong site-based
differences, varying among steady growth (Sites 1, 2 and 5), sustained
low growth and/or sudden decline following dry years (Sites 3, 4 and
6), and initial decline and recovery (Site 7). Canopy and stand health
were strongly correlated with basal area increment (Fig. 3); therefore
these patterns do not merely reflect random variation in radial growth.
These symptoms of forest stand decline are coherent with impairment from water limitation, and this finding is consistent with research
on other declining riparian Populus stands throughout the Northern
Hemisphere (Stromberg and Patten, 1990; Scott et al., 1999; Amlin
and Rood, 2003; Lambs et al., 2006; Hultine et al., 2010). No available evidence supports an alternative cause of the decline in tree health on the
Drôme floodplain apart from water scarcity. Patterns of decline are not
consistent with biotic factors such as intrinsic life history, stand dynamics, or herbivory, disease or pathogens. Trees at all sites were too young
for natural declines in vigor to have occurred across so much of the study
reach (Table 1; Braatne et al., 1996), and recent growth was unrelated to
tree age or their size near the time when declines began. Therefore,
variation in recent growth was not a function of inherent development or tree architecture. Density-dependent competition for resources
(e.g., light or soil nutrients) was not likely a limiting factor either, because
all available signs of stand health were positively, not negatively correlated with tree density (Fig. 4). The densest stands sustained the largest
trees, highest annual productivity and best canopy health. These relationships suggest that higher tree density is a result, rather than a cause of
different stand conditions, and that these differences are driven by
density-independent controls such as abiotic stress rather than densitydependent resource competition among individuals (Braatne et al.,
1996; Stromberg and Patten, 1996).
c)
20
15
10
1960
Fig. 7. Long-term trends in (a) discharge, (b) precipitation, and (c) minimum and maximum
temperatures during the growing season (June–September) for the Drôme River. Linear fits
for discharge and temperature indicate significant trends (1961 through 2007, pb 0.05) as
determined by modified Mann–Kendall tests (Yue et al., 2002).
diameter growth in ash (F. excelsior) trees several kilometers upstream
of the study reach in areas with local aggradation and presumably higher
levels of flooding and anoxic stress in the root zone (Pont et al., 2009;
Rodríguez-González et al., 2010).
Within the present study, significant growth reductions (i.e., negative
regime shifts) occurred only since the initiation of intensive gravel extraction activity in the early 1970s (Fig. 7; Landon et al., 1998). Though the
RSD method has a limited ability to detect declines in the early years of
a chronology, visual inspection of the site growth series indicates no
major reductions in growth prior to 1975 (Figs. 5–6). In-stream gravel
mining introduces several changes in channel morphology and
hydrogeomorphic processes (Kondolf, 1997). These include a local
deepening of the channel with direct effects on the low flow levels
and associated groundwater, an increased slope at the upper edge
of the mining zone, and a loss of bedload supply within the reach.
These changes initiate a disequilibrium between sediment transport
capacity, which is increased, and sediment supply, which is reduced,
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
3
Site
3
Site
7
Site
4
Site
6
2
1
0.0
0
Regime shift index
and typically results in local channel bed incision that propagates
downstream of the mining site, and a regressive erosion upstream
that eventually stabilizes the local channel slope. Cascading effects
of channel incision on floodplain habitats due to mining has been
also showed by Rollet et al. (2006) on the Doubs River, in which incision propagated both upstream and downstream of mining sites.
Channel bed incision also tends to propagate downstream locally
from dams for similar reasons (Kondolf, 1997).
Though channel incision and associated groundwater decline would
be expected to have system-wide effects (i.e., affect all sites), they are
also expected to generate coherent spatial and temporal patterns in the
relative response of individual sites. For example, we expected floodplain
stands adjacent to gravel extraction areas on the Drôme to decline most
strongly and earlier as they experience the effective deepening. Scott et
al. (1999) documented that widespread cottonwood mortality and
crown damage occurred within two years of intensive sand mining in
the adjacent channel, and that effects of depressed groundwater and
tree decline were most severe closest to the excavation site. Amlin and
Rood (2003) observed extensive leaf senescence, abscission, and reduced stomatal conductance in a stand of cottonwoods adjacent to an
excavated gravel pit that depressed the water table by 2.5 m, but saw
no effect in a reference stand farther away. On the Drôme, we expected
not only this proximity effect, but also that the sequence of impacts
would progress from the near-pit sites to farther ones upstream and
downstream over time with propagation of channel bed incision and associate water table lowering.
Contrary to expectations, only one stand's regime shift coincided
with incision following mining (Site 3, with a negative regime shift in
1978), whereas the others occurred more recently in the 1990s (Fig. 6).
Site 3 is not the closest to a mining site, which would have explained
its early and severe growth decline, and it is adjacent to Site 4, which
did not experience a regime shift until 1995 (Fig. 6). The two most upstream areas, Sites 1 and 2, are adjacent to a mining site (Fig. 1), yet
they experienced no decline and currently support healthy, dense forest
stands with vigorous tree growth (Figs. 5–6). The downstream stands
closer to other gravel pits had more variable growth, with one sustaining
stable growth (Site 5), one in severe decline (Sites 6), and one that experienced decline then recovery (Site 7). Overall, the sites with the most severe declines were distributed throughout the reach, and their sequence
of negative growth regime shifts started with Site 3 (1978), followed by
Site 7 (1990), Site 4 (1995) and Site 6 (1997) (Fig. 8). These examples
cumulatively indicate that, in contrast to the expected sequence of
impacts, growth declines did not occur exclusively, or earliest, at sites
close to the gravel extraction activities, nor were impacts sequenced
as would be expected with incision propagating upstream and downstream of local mining (Kondolf, 1997; Scott et al., 1999; Florsheim
et al., 2001; Marston et al., 2003). Therefore, gravel mining alone cannot
explain the finer-scale (within-reach) stand deterioration patterns, and
suggests that other factors contributed significantly to the pattern of
local impacts.
However, other local geomorphic influences in addition to gravel mining are operating within the study reach, and these do show coherence
with the spatial pattern of growth declines. For example, it is notable
that three of the four most heavily-impaired sites are on the right
bank of the river (Sites 3, 6, and 7) in an area with shallow alluvium
and marl bedrock outcrops near the surface (Fig. 9). All of these sites
maintained relatively low baseline growth (b50 cm2 year−1 average
BAI; Fig. 5) during their most productive years, and experienced negative regime shifts between 1978 and 1997 (Fig. 6). Site 4, on the left
bank in this reach, had maintained substantially higher baseline growth
than the other three and also experienced a sharp decline (Fig. 5). Thus,
the sites with the shallowest depth to bedrock were the most vulnerable, which is consistent with incision and water table recession occurring generally throughout the reach and preventing shallowly-rooted
stands (due to bedrock constraints) from accessing perennial groundwater. Additionally, the positive regime shift and complete recovery in
June-Sept. discharge (mean monthly z-score)
10
-0.5
-1.0
-1.5
-1
1960
1970
1980
Year
1990
2000
Fig. 8. Annual growing season discharge and initiation year of negative regime shifts for
Populus nigra growth at Drôme River sites. The discharge series was calculated using monthly
average discharge transformed to z-scores and averaged for the growing season months of
June through September (left vertical axis). Gray vertical bars show the year of negative regime shifts for site chronologies, with sites indicated at top. Inverted triangles show the dates
and magnitude of negative regime shifts for individual trees (right vertical axis).
growth rate that occurred at Site 7 after 2000 is compelling evidence
of local geomorphic drivers, as it occurred adjacent to the recent aggradation of the channel bed following construction in the 1990s of the
protective weir at the Recoubeau bridge (Figs. 1 and 9). This suggests
that, as river stage recovered, nearby trees were able to access a more
regular groundwater source and thus grow vigorously again.
Populus and other phreatophytic pioneer species have some ability to
compensate for water scarcity by growing roots deep into alluvial soil horizons where the groundwater supply is dependable (Rood et al., 2011;
Singer et al., 2012). However, when water scarcity is sudden and
prolonged, or else when bedrock or other confining soil layers (e.g., hardpan) prevent roots from accessing the perennial water table, the trees'
high canopy demand and poor drought tolerance mechanisms often result in increased xylem cavitation, crown dieback and tree mortality
(Scott et al., 1999; Cooper et al., 2003b; Rood et al., 2003). Conversely,
when water resource conditions improve, for example by raising the
groundwater level or increasing the rooting zone through sediment deposition, previously stressed riparian trees are capable of full recovery
(e.g., Hultine et al., 2010). This process is apparent at the downstream
site, which recovered following base-level rise at the new weir. If groundwater recovery is so great as to inundate the rooting zone for prolonged
periods, trees can also experience water excess and slow growth
(Kozlowski, 2002; Pont et al., 2009; Rodríguez-González et al., 2010).
4.3. Climate and discharge trends, and relationship to tree growth
Climate and river discharge also appear to have exerted an influence
on riparian tree growth on the Drôme River over the last 50 years. Correlations between the detrended ring-width chronology and physical variables were strongest for growing season months (June–September), and
significant for both discharge and precipitation (Table 2). Thus these
trees are sensitive to hydroclimatic variables even though they grow in
a habitat where water is abundant relative to the surrounding landscape
(Stromberg and Patten, 1996; Stella et al., 2012). In Mediterranean-type
regions, streamflow and water table levels are at their annual minima
during the growing season, and as a result, abiotic stress on aquatic organisms is highest during this period (Gasith and Resh, 1999). Therefore,
riparian trees experience stress that varies annually with hydroclimatic
water supply, similar to sympatric upland species (e.g., pines and oaks)
that are adapted to drier habitats and that are more typically used
for dendrochronological studies (e.g., Cherubini et al., 2003; Meko
and Woodhouse, 2005). In contrast to discharge and precipitation,
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
11
J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
a)
7
Recoubeau
6
5
4
3
2
1
c)
b)
LucenDiois
Bedrock
outcrop
Weir
flow
Bedrock contact
!
Fig. 9. Panel (a), topographic map of the Drôme River study reach showing the river channel alignment (dashed line), steep valley walls on the right bank, and location of Populus nigra stands
(numbered sites). Sites 3, 4, 6, and 7 suffered negative growth regime shifts. Panel (b), ground-level photograph of marl outcrop and shallow alluvium depth on the Drôme's right bank at Site 3.
Panel (c), aerial photograph of the Recoubeau bridge, protective weir, and bedrock outcrop on the right bank; these are just downstream of Site 7, where tree growth declined in 1990 and
recovered in 2000. Map data from SCAN 25®; copyright© IGN, 2008.
growth correlations with temperature were poorer and not significant
(Table 2). Though riparian tree phenology in Mediterranean-type regions
is sensitive to seasonal temperature differences (Stella et al., 2006), temperature is not typically limiting to growth for most of the leaf-on period,
except for high daytime vapor pressure deficits in mid-summer that
induce stomatal closure (Lambs et al., 2006; Hultine et al., 2010). Therefore, temperature is likely limiting primarily through its effects on water
demand.
Please cite this article as: Stella, J.C., et al., Climate and local geomorphic interactions drive patterns of riparian forest decline along a Mediterranean
Basin river, Geomorphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.01.013
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J.C. Stella et al. / Geomorphology xxx (2013) xxx–xxx
Despite a somewhat limited sample size, the temporal pattern
of growth declines in individual trees and sites suggests a general
correspondence with severely dry years or longer droughts (Fig. 8).
The trees at Site 3 initiated a growth decline in 1978, a year with average
summer discharge, but this followed severe droughts in 1974 and 1976
and very high annual variability throughout the 1970s (Fig. 8). This period also coincides with the intensive gravel mining activity and, assuming
that channel incision had occurred, trees would have been predisposed
to water stress. Negative regime shifts for individual trees also tended
to start in drought years, though in some cases they began the year
after, and in other cases do not appear related to discharge.
Factors other than climate and geomorphic change may also have
influenced the stand-level pattern of growth regime shifts. One methodological issue is that the regime shift detection method recognizes step
changes in time series, which can mask the timing of a more gradual
and sustained trend. Most trees and sites show fairly sudden rapid shifts
in growth rate (e.g., Sites 3, 4, and Site 7 recovery in 1999), but others
(Sites 6 and 7 declines) show some evidence of a more gradual decline
that initiated in the late 1970s or early 1980s (Fig. 5). Another reason
for the asynchrony in timing of site growth declines with climate and
geomorphic change may have to do with the differential response of individual trees at a site. There is great variability in both early growth rate
and onset of slow growth among trees at a site (Fig. 5), suggesting that
background site conditions were quite varied spatially, and that following the onset of multiple environmental stressors, site conditions for
trees reached negative thresholds for growth at different times.
4.4. Synergistic drivers and global change
Though neither channel and groundwater alteration from gravel mining nor the sequence of short-term droughts is sufficient alone to explain
the fine-scale patterns of forest stand decline along the Drôme River,
there is compelling evidence that these factors acted jointly to influence
the timing and location of the strongest impacts. First, the recentness of
all stand-level declines (i.e., since the early 1970s) suggests that channel
bed incision and associated groundwater table decline in adjustment to
mining contributed to overall forest vulnerability throughout the reach.
Secondly, dry years following the early 1970s appeared to provide environmental triggers that induced major growth declines. For example,
the first decline at Site 3 (1978) occurred after several major dry years
(1974 and 1976), and the declines at Sites 4, 6 and 7 were associated
with individual dry years that occurred subsequent to the mining period
(Fig. 8). Finally, the positive regime shift at Site 7 in 2000 following construction nearby of the Recoubeau bridge weir (Figs. 1 and 9c) suggests
that local geomorphic manipulation can reverse growth declines by improving sediment/groundwater conditions. Together, these examples
demonstrate the high sensitivity of forest stands to resource availability
from a combination of drought and local geomorphic factors that include
reach-wide incision, locally-induced aggradation, and bedrock controls
on soil depth.
It is clear that, in addition to these stressors, the Drôme basin is
undergoing long-term climate change to a hotter, drier growing season and discharge regime (Fig. 7). Though precipitation trends were
not significant at the Luc-en-Diois station (Fig. 7b), precipitation has
declined significantly over the basin as a whole (E-OBS gridded data
not shown). These trends are expected to continue and accelerate in
the future throughout the Mediterranean Basin (Giorgi and Lionello,
2008; García-Ruiz et al., 2011). For mature riparian forest stands, it
is likely that local water availability will decrease as the river stage
and groundwater table recede farther, faster, and more frequently
during the growing season. These processes will stress mature trees
that have developed root distributions, crown size and other adaptive
morphological traits in response to wetter conditions such as higher
water tables and less variability (Rood et al., 2011). Climate change
impacts can be expected to be most severe at sites with local geomorphic and/or geologic controls. Existing pioneer stands may decline or
transition to other vegetation types faster and irreversibly under these
conditions, and new stands may have trouble establishing (Shafroth
et al., 2000). Riparian studies in other water-limited ecosystems show a
common response of forest decline following rapid hydrologic change
(Stromberg and Patten, 1996; Scott et al., 1999; Dufour and Piégay,
2008), and in some cases an irreversible shift to alternative system states
(Stromberg, 2001; Palmer et al., 2008). This process has occurred locally on the Drôme (Sites 3, 4, and 6 in particular), in which stands that
sustained high and variable growth initially switched to permanently suppressed growth with little variation annually or amongst trees.
Though there remains some uncertainty as to the specific ecological
trigger(s) that would precipitate widespread mortality or growth declines in any one location (Groffman et al., 2006), it is clear that the climate sensitivity and accelerating hydroclimatic change, combined with
multiple local stressors due to geologic setting and human-induced
changes, will make drought-prone riparian zones exceedingly vulnerable in the future (Rood et al., 2008; Perry et al., 2011).
5. Conclusions
In the case of Drôme River poplars, major declines in growth were
coincident with a period of reach-based geomorphic alteration from
instream gravel mining, and these impacts were most severe on floodplain surfaces with thin alluvial substrates overlaying shallow bedrock.
Furthermore, these declines were triggered for many trees in drought
years, and the climate in the region is trending significantly toward hotter growing seasons with a decrease in summer river discharge. These
trends, which are projected to continue, will increase both chronic and
acute water shortage for riparian trees in drought-prone ecosystems,
with phreatophytic species such as P. nigra particularly vulnerable.
Chronic stress from climate change may induce growth declines sooner
and more severely in the future.
This study has shown that tree-ring growth is a sensitive indicator
of water stress for riparian trees, and is a useful tool to understand the
effects of hydrogeomorphic controls and human modifications on river
ecosystems. Riparian forests in semi-arid systems are sensitive to many
physical drivers and can demonstrate threshold responses to multiple
stressors. Pioneer trees respond to decreased water availability through
greater mortality and crown loss, and decreased radial growth. However, the severity of these impacts depends on interactions with physical
drivers at multiple scales, and not all floodplain sites are affected equally. Site-based variation in environmental conditions needs to be taken
into account to understand specific vulnerability to hydrogeomorphic
alteration.
Acknowledgments
This work was supported financially by a grant from the International Collaborative Scientific Project (PICS) and a seed grant from
the State University of New York Research Foundation. We gratefully
acknowledge the E-OBS dataset from the EU-FP6 project ENSEMBLES
(http://ensembles-eu.metoffice.com) and the data providers in the
European Climate Assessment and Dataset project (http://eca.knmi.nl).
Finally, we thank Alex Fremier and Maya Hayden for their assistance in
the field, and three anonymous reviewers and Adrian Harvey for improvements to the manuscript.
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