Climate responses of Pseudotsuga menziesii and Pinus flexilis in the greater Yellowstone (USA)

Transcription

1 Climate responses of Pseudotsuga menziesii and Pinus flexilis in the greater Yellowstone (USA) Joey Pettit, Lauren Stachowiak, Anna Sala, Sean Pinnell, Jasmin Sykora, Todd Ellis Introduction Climate change is projected to increase the frequency and intensity of droughts. Because sensitivity to drought and warming varies widely among tree species, some species are expected to be more vulnerable to climate change than others. Species-specific vulnerability to climate change could have important consequences on mortality rates and consequently, on forest structure and function. However, predicting species-specific vulnerability to climate change remains challenging. A useful approach is to make inferences from responses to past climate. The integrated response of trees to their environment is registered in their annual tree rings (Cook 1985; 1992). Dendrochronology therefore, provides a powerful tool to evaluate tree responses to past climate. Plants experience drought when the supply of water from the roots is not sufficient to replace water lost through transpiration in leaves. As a consequence, the tension of the water column in the xylem increases. Excessive xylem tensions are dangerous because the water column may break (cavitation), potentially leading to the irreversible interruption of water transport (hydraulic failure) and subsequent death. Broadly speaking, plants cope with drought with three distinct strategies: they may escape it all together (e.g. dormancy), tolerate it (ability to endure high xylem tensions), or avoid its consequences (conserve water and minimize xylem tensions). Among conifers, species of the genus Pinus tend to be more drought avoiders (employ water conservation strategies) than other species in the same family (firs, Douglas-fir) or other families (e.g. junipers). Avoiders conserve water by closing stomata. However, this also prevents carbon assimilation. Under intense and/or long lasting drought, therefore, carbon supply via

2 photosynthesis may not be sufficient to meet demand for respiration and metabolism, in which case stored usable carbon (e.g. starch) must be used to meet the deficit. Therefore, while the ability of drought tolerators to endure high xylem tensions allows them to maintain stomata open and photosynthesize, water conservation strategies in drought avoiders renders them more dependent on storage. These strategies may have consequences on growth responses to climate in the short and long term. Here, we evaluated climate responses of Douglas-fir (Pseudotsuga menziesii Franco) and limber pine (Pinus flexilis James) in the Greater Yellowstone of the northern Rocky Mountains. Based on the literature, we assume that Douglas-fir is relatively more drought-tolerant relative to limber pine, which is more drought avoider. We expect both species to be sensitive to drought in the short term (month-years). The relatively greater reliance of storage in limber pine may render this species less sensitive to climate fluctuations at longer time scales (decades). That is storage buffers climate fluctuations in the mid-term. If so, mean climate sensitivity should be lower in limber pine than in Douglas-fir. Methods The study area (44.754, ) is located on White Mountain, part of the Shoshone National Forest in the GYE (Figure 1). White Mountain is ideal as a study site, meeting the environmental criteria of south-facing, steep slopes comprised of talus deposits and an openstand structure. Twenty-six trees were selectively sampled based on a visual age assessment, looking for flat-topped, spiral-grained trees. Trees were found on aspects ranging from southeast to southwest, with soil accumulation ranging from nonexistent to minimal. At least two cores

3 were taken per tree, except in samples WMI07 and WMI020 with three cores extracted due to rot or mistletoe infestation. Samples were crossdated using standard dendrochronologic techniques (Stokes & Smiley 1996; Speer 2010) and verified using COFECHA (Holmes 1983). Samples WMI13B, WMI19A, and WMI19B had significantly suppressed chronologies and were removed from the study. Chronologies were standardized with a 100-year cubic smoothing spline using the program ARSTAN, and the ARS chronology was chosen for both species to preserve the maximum amount of climate signal (Cook et al. 2014). Resultant ring-width indices were then correlated with climate data for exploratory climate response analysis. Precipitation and temperature measures during growing and water years, an annual measure of the Palmer drought severity index (PDSI), as well as monthly measures of precipitation, temperature, and the previous year s precipitation and temperature were extracted from climate data dated to 1895 and supplied by the PRISM Climate Group at 4- km spatial resolution. Pearson s correlation analysis was then performed with a P-value of 0.01 using R statistical software, and the most influential limiting factors were isolated. Results ARS chronologies were created for 14 Douglas-fir and 12 limber pine specimens after problematic series were removed. Core series dated from for 26 Douglas-fir cores, and for 12 limber pine cores. Douglas-fir cores returned 1 flagged segments and a series intercorrelation of Limber pine cores returned 2 flagged segments and a series intercorrelation of (Figure 2). Average mean sensitivity for Douglas-fir was 0.374, and

4 limber pine Raw running r-bars were reported as / for Douglas-fir and / for limber pine (Figure 3). Moving response analyses were made for both species using data from ARS which compared the growth responses to precipitation and temperature with a 25 window lagged by one. Positive tree ring growth relied heavily on precipitation throughout the year where most of it was significant. High temperature during the growing season resulted in decreased tree ring growth. Douglas fir and Limber pine had very similar temperature and precipitation responses. The correlation coefficient for both species using an average of the residual, raw, and ARS data were plotted with the most significant responses to precipitation, temperature, and PDSI at 0.01 significance with a lag of one and two. The highest correlations between ring growth response of both Douglas-fir and limber pine with climatic variables were associated with total water year precipitation, previous total water year precipitation, and annual PDSI (Figures 4, 5, and 6). Precipitation from the previous year influences the next growing season in both Douglas fir and Limber pine. The unique tree ring growth responses between Douglas fir and Limber pine is affected by temperature individually more often than precipitation. An interesting find was that the tree ring growth in Limber pine was being influenced by the temperature two years ago in January. Discussion & Conclusion We did not see dramatic differences in the inter series correlations between Douglas fir and limber pine, however Douglas fir was slightly more sensitive to climatic variations in temperature and precipitation compared to limber pine. Both species responded similarly to precipitation and both shared a common decreased growth response to growing season temperatures. We did observe some interesting correlations in lag growth responses unique to

5 each species. Douglas fir responded positively to the previous year s October precipitation and two year s prior September precipitation, possibly indicating an autocorrelated lagged growth response to increased carbon assimilates from more productive growing years. What is of particular interest is the effect of January s temperature from two years prior on the current growth rate of limber pine. Limber pine is particularly responsive to past winter temperature, possibly resulting in a greater effect of warming on limber pine. Although common responses to both species indicate similar growth trends to temperature and precipitation, limber pine expresses a unique response to a lag growth response to past temperature. This may indicate a more complex interaction between limber pine s ability to partition (i.e. store) carbon assimilates to be utilized in future growth. This strategy coincides with limber pines conservative strategy to more tightly regulate stomatal conductance under higher temperatures and low precipitation conditions, possibly resulting in more complacent ring growth patterns. This may allow limber pine to rely more on stored carbon reserves accumulated during ideal growing conditions. Douglas fir on the other hand is less prone to xylem cavitation and can continue to photosynthesize carbon asslimates under climatic conditions that would be less favorable to limber pine. However more study involving isotope analysis and non-structural carbohydrate concentrations would need to be employed to tease out any of these possibilities. However, this strategy would potentially explain why we did not observe higher significant correlations from past climate responses to current growth in Douglas fir compared to that of limber pine. From this study it s apparent that both species are sensitive to precipitation, and both species respond strongly to water (positive effect) and temperature (negative effect). Water is most often the driving abiotic influence and limiting factor in tree growth. Precipitation is more

6 of a driving factor in photosynthesis then temperature, giving Douglas fir the ability to continue to photosynthesize and continue growth under dryer conditions compared to limber pine, resulting in a higher climatic response to inter-annual growth in Douglas fir. Figure 1: Field site and the introductory group.

7 Figure 2. Chronology development and dating of the Douglas fir (PSME) and limber pine (PIFL). Figure 3: Running r-bar showing the chronology strength through time.

8 Figure 4: Dendroclime2002 response function results for Douglas fir. Figure 5: Dendroclime2002 response function results for limber pine.

9 Figure 6: Significant responses to temperature and precipitation.

10 References Cook, E.R., P.J. Krusic, R.H. Holmes, K.Peters. Program ARSTAN Ver.44, 2014 (/ Speer, J.H. (2010). Fundamentals of Tree-Ring Research. The University of Arizona Press.