Recruitment Patterns and Radial Growth Response of
High‐Elevation Pines to Climatic Variability, 1883‐2013, Western Great Basin, USA
# 681
Constance I. Millar, Robert D. Westfall, and Diane L. Delany
USDA Forest Service, Pacific Southwest Research Station
800 Buchanan St., Albany, CA 94710
INTRODUCTION
Subalpine forest communities are assumed to respond to rising temperatures by migrating upslope, with the concomitant expectation that habitat will be lost as area diminishes approaching summits (“elevational squeeze”). Our prior studies of Great Basin forests and historic climate lead us to hypothesize that responses are more complex, with species having individualistic and often unexpected responses. We report studies of recruitment response and radial growth over the past 130 years in subalpine limber pine (Pinus flexilis; PiFl) and bristlecone pine (P. longaeva; PiLo, White Mtns
only) forests of the western Great Basin.
STUDY QUESTION
What is the nature of recruitment and radial growth in PiFl and PiLo at upper‐, middle‐ (internal), and lower treelines
from 1883—2013, and what climatic relationships influence these responses?
STUDY SITES
14 study sites were selected from 4 mountain ranges of E. California and W. Nevada, representing 3 elevation zones (Figs. 1, 2). At each site, pines were advancing across forest borders > 50 m into alpine (high elevation), shrubland (middle), or woodland (low) habitat.
METHODS
Fig. 1: Map of the study region showing distribution of pine recruitment and tree‐ring sites. Black dots are recruitment sites; Open diamonds are tree‐ring sites.
Recruitment. We established 30 m‐wide belt transects that crossed the forest border into non‐forest community at upper (high), middle (interior), and lower (low) treeline sites. Belts were divided into 30 m intervals, establishing 30x30 m plots along the transects. We tallied and aged all live trees < 10 cm diameter from either whorl count or increment cores, and assessed densities for all live trees/plot aged ≤ 130 yrs in 2013, which we designate as recruits. Radial Growth. To assess radial growth, we developed four tree‐ring chronologies from mature PiFl trees in the vicinity of the recruitments sites (Fig. 1). More than 15 trees were included in each final chronology, and only trees with correlations to the master > 0.4 were included. Climatic Assessment. We developed composite climate indices from four instrumental NOAA/NWS HCN stations: Tahoe City, CA; Yosemite Valley, CA; Mina, NV; and Independence, CA. The composite record had higher correlations to recruitment data Fig. 2: Recruitment of pines across high (A&B), mid (C&D), and low (E&F) ecotones in western Great Basin ranges.
than other sources (other stations, PRISM model), and we used these data for analysis as well as cumulative water deficit (CWD) and snowpack, both extracted from the Basin Characterization Model (Flint et al. 2013). For both recruitment and radial growth, we analyzed simple linear correlation as well as non‐linear relationships, including a second‐order least squares response‐surface model (JMP, SAS), including an initial large set of climate variables. We filtered the model to select only significant variables a best‐fit model for the data, and evaluated the behavior of these variables in second‐order response surfaces. Lead and lag year cross‐correlations were assessed in the time series analysis platform in JMP (SAS). Models were fit by minimizing AICc and SBC scores.
RESULTS (Figs. 3,4)
• In general, recruitment across forest borders occurred rarely in the mountain ranges we studied. Sites were mostly on north aspects but otherwise shared no obvious environmental correlates. Many sites with similar physical features did not have recruitment.
• Recruitment was most abundant at upper treeline (high elevations), but also occurred at moderate levels at middle and lower treelines. PiFl greatly exceeded PiLo
recruitment at all elevations.
• Recruitment was highly episodic, with a dominant pulse for PiFl 1965—2000 and for PiLo 1955—1973 with a smaller pulse from 1978—1988. Recruitment was low in the 21st
century for both species.
• In ranges where both species occurred, PiFl was leap‐frogging at upper treeline above PiLo by as much as 300 m. In these situations, recruitment occurred in areas only in locations with remnant dead PiLo.
• Radial growth at 3 of 4 sites showed a common pattern wherein a dominant pulse of high growth coincided with the recruitment episode, 1965—2000. The anomalous site, Owens Gorge, is highly disjunct and very low elevation, located > 800 m below usual lower range limit.
Fig. 3: Density of pine recruitment by birth year and elevation zone, and climate records. A‐C, limber pine, all sites: A. High. B. Middle. C. Low. D‐F, 120‐yr climate records from composite weather stations. D. Minimum annual temperature. E. Maximum annual temperature. F. Water year precipitation. G. Cumulative Water Deficit (CWD) modeled from the Basin Characterization Model (Flint et al. 2013). Shaded panel highlights the recruitment episode.
• Synchronous trends implicate regional climate control over recruitment and radial growth. Significant correlations were similar for recruitment and radial growth and included positive correlations with minimum annual temperature, water‐year precipitation, summer and autumn precipitation, and snowpack. Negative correlations were with maximum annual temperature, summer maximum temperature and CWD. Significant lead and lag correlations existed for six years before and after recruitment year, and six years before radial growth year.
CONCLUSIONS
• Uphill migration of subalpine pines in the Great Basin is one but not the only response to warming of the last 130 yrs; recruitment is also occurring across middle‐elevation ecotones and below lower treeline.
• Recruitment across these ecotones have been episodic, not directional, over 130 yrs; recruitment in the 21st century has been low.
Fig. 4: Standardized ring widths for 4 PiFl tree‐ring chronologies, 1750‐2013. The period assessed for recruitment extended to 1883 (marked by solid vertical line). Shaded panel highlights the recruitment episode.
REFERENCES
• Change in species dominances above upper treeline might become a common response to 21st century climate change, as is occurring in the White Mtns, CA.
Flint, L.E., Flint, A.L., Thorne, J.H., and Boynton, R. 2013. Fine‐
scale hydrologic modeling for regional landscape applications: the California Basin Characterization Model development and performance. Ecological Processes 2: 25.
• Climate appears to be a primary influence on recruitment in PiFl and PiLo, and in radial growth in PiFl. Recruitment and growth, however, are complexly related to climate, requiring appropriate conditions in many climate factors combined, and low interannual variance for stable establishment of young trees. Millar, C.I., L.J. Graumlich, D.L. Delany, R.D. Westfall, and J.K. King. 2004. Response of subalpine conifers in the Sierra Nevada, California, U.S.A., to 20th‐century warming and decadal climate variability. Arctic, Antarctic, and Alpine Research, 36(2): 181‐200.