C33C - 1289
Elevational Gradients and Differential Recruitment of
Limber Pine (Pinus flexilis) and Bristlecone Pine (Pinus longaeva);
White Mountains, California, USA
C.I. Millar, R.D. Westfall, and D.L. Delany
USDA Forest Service, Sierra Nevada Research Center, Albany, CA
Introduction
Subalpine tree species and forest communities are assumed to respond to rising temperatures by migrating
upslope. This generates a conservation hypothesis that habitat will be lost as area diminishes approaching
mountain summits (“elevational squeeze”). Our prior studies of Great Basin forests and historic climate lead us to
hypothesize that responses are more complex, with species following individualistic and often surprising paths.
We began a study to investigate the effect of ongoing climate change on subalpine limber pine (Pinus flexilis; PiFl)
populations in the Great Basin. Here we present results from the White Mtns portion of that study. In this range,
bristlecone pine (P. longaeva; PiLo) currently and historically dominates the highest elevations and on dolomite
soils (Fig. 1), PiFl occupies the middle subalpine range on non-dolomitic soils (Fig. 2), while singleleaf pinyon pine
(P. monophylla; PiMn) historically occurred disjunctly below the other conifers (Fig. 3).
Fig 1. PiLo at upper treeline
Fig 2. PiFl at mid elevation
Current Treelines (m)
Bristlecone Pine
Limber Pine
Pinyon Pine
Upper
3400
3300
2800
Lower
3000
3000
1000
Fig 3. PiMn zone at low elevation below PiLo
and PiFl
Study Question
What is the nature of limber pine recruitment (past 120 years) in the
White Mtns relative to:
-- elevation (lower treeline, middle elevations, and upper treeline)
-- vegetation structure (live forest, meadow, relict forest zone)
-- slope, aspect, and soils (esp dolomite vs non-dolomite substrates)
-- species (limber pine, bristlecone pine, pinyon pine)
Study Sites
Seven study sites were chosen in the White Mtns
between Sierra View and Mt. Barcroft including
three at upper PiLo treeline (3341-3570m), two at
middle elevations (3184-3223m), and two at lower
PiFl/PiLo treeline (2946-3114m) (Fig. 4). These
represent a range of slopes, aspects, elevations,
vegetation communities, and substrates (Table 1).
Fig 4. Study area with high (red), mid (blue), and
low (yellow) elevation sample sites
Table 1. Physical attributes of the study sites
Methods
At each site, we established 30m-wide belt transects that traversed 17-100 m elevation. The belts were divided at 30m intervals, establishing 30x30m plots
along the transects.
We tallied all live trees and dead wood within the plots for tree species and diameter. In addition we estimated age of live limber pines in the plots using
whorl counts (trees < 2 m ht) and increment coring (trees > 2 m ht).
We designated PiFl trees that were ≤120 yrs old and PiLo trees <20cm dia at 0.5 m height as recruits.
We assessed PiFl recruitment age versus elevation with least squares analyses (SAS 2006).
To test relationships of climate and year of recruitment, we first developed composite climate indices (see Millar et al. 2004) from four instrumental records
(Independence, CA, Mina, NV for temperature; Independence, Mina, Tahoe, CA and Yosemite National Pk, CA for precipitation; periods of record were
1927-2005, 1897-2005, 1909-2005, and 1906-2005, respectively, WRCC 2006; HCN 2006). We analyzed simple linear correlations (SAS 2006) as well as nonlinear relationships. For the latter, we used a second-order least squares response-surface model of recruitment versus minimum and maximum annual
temperature, respectively, annual precipitation, as well as standard indices of the Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), and
Atlantic Multidecadal Oscillation (AMO). We evaluated the behavior of these variables in second-order response surfaces.
Results
PiFl exceeded PiLo in abundance of recruits at all sites except the Relay
Ridge middle elevation location (Table 2). The excess ranged from 2803700% and was most prominent at upper treeline locations (Fig. 5). PiFl
recruitment was greatest on dolomite substrates at upper elevations,
occurred on all aspects, and to elevations above current live PiLo or PiFl,
up to 300m above current PiFl treeline.
Recruitment of PiFl and PiLo occurred primarily in areas of current or
relictual forest, except at middle elevations where PiFl was recruiting
into sagebrush (Artemesia tridentata) meadows (Fig. 6). In the lower
treeline zone, PiFl recruitment occurred in narrow west- and north-facing
ravines on sandstone substrates and along transient watercourses (Fig. 7).
Fig 5. PiFl recruits in relict PiLo above
upper treeline
Table 2. Density and age statistcs
Average age of PiFl trees at high elevation sites (22.3 yrs) was lower than at
mid- (35.7 yrs), and low elevation sites (33.8 yrs) (Table 2, Fig 8). Age was
significantly and negatively correlated with elevation at high elevation sites
(Fig. 9A) indicating that younger trees were higher in elevation above upper
treeline. At middle elevations, significant correlations indicated younger trees
in meadow compared to forest locations (Fig. 9B). At lower treeline, tree age
was not significantly correlated with elevation in three of the four transects
(Fig. 9C).
Recruitment in PiFl during the past 120 years at all sites was highly episodic
and clustered between 1975-1991 (Figs. 8, 10A). Recruitment was very low
before 1945, increased slightly from 1945-1975, pulsed very high between 19751991, and decreased to a low level between 1991-2006.
Fig 6. PiFl recruiting into sage meadows,
mid elevation
Fig 7. PiFl recruiting in ravines,
lower treeline
Significant partial correlations of PIFL recruitment levels with climate were in linear and
in higher-order interaction trends:
Min T
AMO
PDO x AMO
AMO x Min T
p=0.01
p=0.04
p=0.04
p=0.06
Fig. 10B illustrates the correlation of recruitment with multi-decadal periods of AMO over
the past 120 years: recruitment was higher during “cold ocean” AMO periods and lower
during “warm ocen” AMO periods, although asymmetry is strong during the 20th
century.
Fig 8. Mean plot density PiFl by age class at 7 sites
Fig 9. Correlation of PiFl recruit age with elevation at A) high, B) mid, and C) low elevation sites
Discussion
Discussion and Summary
Fig 10. A) Number of PiFl recruits established/ per year (overall dataset)
B) Standardized values of mean annual temp, precip, and AMO
We find that PiFl is recruiting abundantly and unexpectedly at low to high elevations in the White Mtns in diverse habitats, including cool ravines at low
elevations, into sagebrush meadows at mid-elevations, and extending above upper treeline in areas of relict bristlecone pine forests. Contrary to assumptions,
PiFl recruitment is greatly exceeding PiLo above current upper PiLo treeline and 300m above current PiFl treeline, and on high-elevation dolomitic soils.
Recruitment in PiFl has been episodic during the past 120 years, concentrated during 1975-1991, with much lower recruitment in the past 15 years and earlier in
the 20th century. Significant interactions with climate variables suggest that recruitment passed a threshold during the 20th century, whereby recruitment was
triggered by warmer and wetter conditions, but that the favorable combination of conditions did not persist during the past 15 years. Recruitment correlates
with multi-decadal periods of the AMO, with weak but positive recruitment in the cool early 20th century AMO phase, and strong recruitment in the cool late
20th century phase. Warm phases of AMO have been associated with drought in SW US, and in the White Mtns may relate to timing of precipitation as well as
overall amount. Warming temperatures during the second half of the 20th century may have increased the likelihood of precipitation thresholds being crossed
at these high elevations such that more abundant recruitment occurred between 1975-1991.
PiFl appears to be advancing in dominance over PiLo throughout their elevation ranges in the White Mountains, finding favorable conditions above upper
treeline (warm, wet), mid elevation meadows (release from inversion?), and in cool damp ravines at low elevations.
References
Millar, Westfall, King, Graumlich, Delany. 2004. Arctic, Antarctic, and Alpine Research. 36(2):181-200
WRCC 2006. Western Regional Climate Center. Historical Climate Archives. http://www.wrcc.dri.edu/
HCN 2006. U.S. Historical Climate Network, NOAA. http://www.ncdc.noaa.gov/oa/climate/research/ushcn/ushcn.html
SAS 2006. SAS Institute Inc. JMP Statistics and Graphics Guide, version 6.03.