A biophysical gradient analysis of climate for understanding
treelines in mountain ecosystems of the western U.S.
Jeremy S. Littell1 and Lisa J. Graumlich2
1JISAO
CSES Climate Impacts Group, University of Washington jlittell@u.washington.edu
2School of Natural Resources, University of Arizona
ABSTRACT Conifer establishment at upper treeline is controlled by both physical and ecological phenomena.
The physical limitations on tree establishment and growth as well as the ecological and edaphic factors
moderating climate vary significantly across western mountain ranges, from the more maritime Cascades
through the basin-and-range to the heavily continental central Rockies. In order to understand the factors
limiting tree establishment and estimate rates of ecosystem change under future climate change, it is critical to
understand the climatic factors limiting tree establishment. We use a multiscale approach to identify climatic
patterns associated with upper treeline in nine mountain ranges: the north Cascades, central Cascades, Eagle
Cap, Bitterroot, Teton, Beartooth, Wind River, Snowy, and Zirkel mountain ranges. We examined NCDC
divisional and SNOTEL and snowcourse, seasonal averages/totals for temperature and precipitation variables
to compare the climates at treeline sites identified for conifer establishment research. Divisional data from
1948-2004 indicate a strong geographical gradient in winter precipitation/PDO correlations, but according to
SNOTEL and NRCS snowcourse data from all mountain ranges, these differences are much weaker in the
more recent past. Snow water equivalent at all the sites near the PDO dipole evident in the 1948-2004
correlations appears negatively correlated with PDO. We present DAYMET, SNOTEL, and snowcourse
normals for the mountain environments near the treeline sites and relate them to species composition and the
nature of recent establishment.
Tab.1 Site, state, elevation, and dominant species across the transect. Numbers correspond to
Fig1 and axes for other figures.
Number
Site
State
Elev
1
E. Cascades
WA
2000
2
Goat Rocks
WA
1950
3
Eagle Cap
OR
2630
4
Bitterroot
MT
2960
5
Beartooths
MT/WY
2970
6
Tetons
ID
3000
7
Wind River
WY
3130
8
Snowy Range
WY
3300
9
Zirkels
CO
3350
*LALY = Larix lyallii; PIAL = Pinus albicaulis; PCEN = Picea engelmannii;
ABLA = Abies lasiocarpa; TSME = Tsuga mertensiana
1
2
WA
MT
3
4
5
WY
OR
6
ID
7
8
9
CO
RESULTS The SWE anomalies for site averages of all available snow data (Fig.4a) indicate that there is little
evidence of a consistent dipole. During several periods of the record, there is notably increased variance
between the regional time series, and these periods could be ENSO related. The distributions of mean April 1
SWE values for the constituent series show an increase in SWE in cool (-) PDO years and a decrease in (+)
warm PDO years across the sites (Fig.4b). There is substantial variability, however, within these groups, and
the relationship is strongest in the northwest portion of the transect. Correlations between PDO and April 1
SWE are strongest and least variable where mean April 1 SWE is highest (Fig.5a) , but this may have more to
do with autocorrelations associated with proximity to the coast. However, correlations between April 1 SWE
and the Pacific North America pattern (PNA) index are nearly as strong as with PDO (Fig.5b) and point toward
an interannual relationship for these sites between circulation and snowpack variability.
Certainly at both upper treeline and lower treeline, the role of temperature and solar radiation in
growing season length and potential evapotranspiration cannot be excluded, and further analyses will focus on
characterizing these variables across the sites.
A
Fig.1 Nine sites form a transect from maritime to continental
climate and serve as a basis for comparing the factors limiting and
facilitating conifer establishment at treeline
Species*
LALY, PIAL, PCEN, ABLA
TSME, ABLA
PIAL, PCEN
PIAL, PCEN
PIAL, PCEN, ABLA
PIAL, PCEN, ABLA
PIAL, PCEN
PCEN, ABLA
PCEN, ABLA
NARRATIVE In this analysis, we sought to understand climate variability, particularly winter
precipitation, as a potential control on conifer establishment at upper treeline. It has been speculated that
conifer recruitment at upper treeline is episodic in nature and that decadal climate variability may play an
important role in structuring treeline environments. We established a series of nine sites along a NW - SE
transect between the Washington Cascades and the Colorado Rockies (Fig.1). This transect has three
gradients that make it useful for testing assumptions about treeline establishment. These are (1) maritime
to continental climate, (2) variable ecological communities (Tab.1), and (3) regional dipole in sensitivity
to PDO (Fig.2). Sites were chosen on the basis of relatively gentle topography and soil development to
mitigate the confounding effects of microtopography.
We obtained the SNOTEL/snowcourse records for every possible site within 50km of the chosen
study sites and developed seasonal and monthly indices for available snow water equivalent (SWE) data
(usually Jan - Jun). In this exercise, we focus on April 1 SWE, although some sites have peak SWE in
May or March. Elevation of the snow data vary, and are in Fig.3 We analyzed a total of 98
SNOTEL/snowcourse records.
This analysis is a preliminary component of a larger NICCR-funded study that will examine the
coupled climatic and ecological controls on conifer establishment . We have deployed temperature
microsensors at 12 upper and 8 lower treeline sites along the transect.
ACKNOWLEDGEMENTS: Matt Germino (ISU) and Philip Mote (CIG) contributed advice
and data, respectively, to this effort.
Fig.4a (left) SWE anomalies averaged within sites. Note periods of coherence and periods of
increased variance. 4b (right) shows distributions of means for all analyzed sites for all years (top),
cool PDO years (center), and warm PDO years (bottom).
Fig.2 Nov-Mar winter precipitation
correlation with Nov-Mar PDO,
1948-2006. Note transition in
central WY and CO.
Fig.3 Elevation range of
SNOTEL and snowcourse sites
associated with each treeline
Fig.5 PDO (left) and PNA (right). Correlations with SWE Anomalies.
The correlations are very similar in sign and magnitude.
IMPLICATIONS The decadal variability present in the SWE time series suggests a possible mechanism
for decadal pulses of recruitment in mountain ecosystems, and the correlations between SWE and PDO
suggest an external decadal driver. However, it is noteworthy that interannual variability seems to be
associated with variations in SWE as much as, if not more than, decadal climate. Differences in conifer
species’ tolerances to heavy snow, cold temperatures, and lack of snow across the transect point to the
potential for very different climatic factors limiting establishment. It is also possible that the interaction
between these differences in climate and the local conifer communities structure treeline and determine
what thresholds exist for pulses of establishment to occur.
B