GC23E-0674
Evidence that drought-­‐induced stomatal closure is not an important constraint on white spruce performance near the Arc8c treeline in Alaska Patrick Sullivan, Annalis Brownlee, Sarah Ellison, Bjartmar Sveinbjörnsson Environment and Natural Resources Ins?tute, University of Alaska Anchorage, Anchorage, AK 99508 INTRODUCTION Increment cores collected from trees growing at high la?tudes have long been used to reconstruct past climates, because of close posi?ve correla?ons between temperature and tree growth (e.g., Jacoby and Cook 1981). However, in recent decades and at many sites, these rela?onships have deteriorated and become nega?ve in some instances (e.g., D’Arrigo et al. 2008). The observa?on of declining tree growth in response to rising temperature has prompted many inves?gators to suggest that high la?tude trees may be increasingly exhibi?ng drought stress (e.g., Wilmking et al. 2004). In the Brooks Range of northern Alaska, the observa?on of low and declining growth of white spruce is more prevalent in the central and eastern parts of the range, where precipita?on is lower, providing superficial support for the drought stress hypothesis. In this study, we inves?gated the importance of drought-­‐induced stomatal closure to white spruce growth in four watersheds along a west to east gradient near the Arc?c treeline in the Brooks Range. We obtained a historical perspec?ve on tree growth and water rela?ons by collec?ng increment cores for analysis of ring widths and carbon isotopes in tree-­‐ring alpha-­‐cellulose. Meanwhile, we made detailed assessments of contemporary water rela?ons at the level of the needle and the whole canopy. SITE LOCATIONS TEST OF HYPOTHESIS 2 CLIMATE TRENDS TEST OF HYPOTHESIS 4 TEST OF HYPOTHESIS 1 HYPOTHESES If drought-­‐induced stomatal closure can explain deteriora?ng temperature-­‐
growth correla?ons and declining growth in the eastern Brooks Range, then we would expect: 1. Varia?on in Δ13C over the tree-­‐ring record should indicate decreasing needle intercellular CO2 concentra?on in the eastern Brooks Range. 2. Canopy transpira?on should decline under warm and dry condi?ons, par?cularly in the eastern Brooks Range. Figure 1. Trends in June-­‐July air temperature for Fairbanks and Bemles and June-­‐July precipita?on for Bemles. The rela?vely warm and dry 2013 growing season is indicated by the red circles. 3. Stomatal conductance should be low rela?ve to photosynthesis, such that needle intercellular CO2 concentra?on is lower in the eastern Brooks Range. 4. Across sixteen sites in the Brooks Range, there should be a posi?ve correla?on between growth and needle Δ13C. METHODS 1. Increment core sampling, ring width measurement, alpha-­‐cellulose extrac?on and carbon isotope analysis VARIATION IN SITE CLIMATES Table 1. Varia?on in stand climates between June 9 and August 24, 2013. Site Air Temperature (°C) Rain (mm) Agashashok 13.1 150.6 Kugururok 12.9 156.7 Dietrich 13.0 86.6 Wind River 13.2 53.6 2. Hourly measurements of xylem sap flow during the snow-­‐free season 3. Monthly measurements of light-­‐saturated needle gas exchange 4. Measurements of branch extension growth and needle Δ13C at three addi?onal treeline sites (xeric, mesic, hydric) in each of the four watersheds REFERENCES D’Arrigo R, Wilson R, Liepert B, Cherubini P (2008) On the ‘divergence problem’ in northern forests: a review of the tree-­‐ring evidence and possible causes. Global and Planetary Change 60: 289-­‐305. Jacoby GC, Cook ER (1981) Past temperature varia?ons inferred from a 400-­‐year tree-­‐ring chronology from Yukon Territory, Canada. Arc4c and Alpine Research 13: 409-­‐418. Wilmking M, Juday GP, Barber VA, Zald HSJ (2004) Recent climate warming forces contras?ng growth responses of white spruce at treeline in Alaska through temperature thresholds. Global Change Biology 10: 1724-­‐1736. Figure 3. Daily maximum sap flow as a func?on of the daily maximum vapor pressure deficit at terrace sites during the warm and dry summer of 2013. Sap flow was not measured at the Agashashok site in 2013. Data from 2010 are shown instead. Each point is the mean of 10 trees. TREE GROWTH TRENDS Table 2. Classifica?on of tree growth responses to temperature (n=25-­‐29 trees/site). Posi?ve, neutral and nega?ve responders were iden?fied by linear regression between ring width devia?ons and Bemles air temperature between 1951 and 2012. Site Posi8ve Neutral Nega8ve Agashashok 22 7 0 Kugururok 7 14 4 Dietrich 12 13 0 Wind River 3 15 7 CONCLUSIONS
Figure 5. Branch extension growth during 2014 plomed versus needle Δ13C at four sites with contras?ng soil moisture in each watershed. Points represent individual trees (10 trees/site). At a given atmospheric CO2 concentra?on, lower Δ13C indicates greater water-­‐use-­‐efficiency. Figure 2. Varia?on in ring widths and Δ13C, along with es?mates of water-­‐use-­‐efficiency and intercellular CO2 concentra?on, for the six trees analyzed for tree-­‐ring Δ13C at the Wind River terrace site. All sites showed increasing Ci over the course of the tree-­‐ring record. TEST OF HYPOTHESIS 3 1. Needle intercellular CO2 concentra?on increased over the course of the tree-­‐ring record at all sites, including our driest site on the the Wind River in the eastern Brooks Range, indica?ng that photosynthesis is not becoming limited by stomatal closure. 2. Canopy transpira?on remained at near peak levels at all sites under the driest condi?ons observed during the warm and dry summer of 2013, poin?ng to limited stomatal closure. 3. Es?mates of needle intercellular CO2 concentra?on made during measurements of needle gas exchange showed inconsistent differences across sites and no evidence of a decline in mid-­‐summer, again sugges?ng that photosynthesis was not limited by stomatal closure. Figure 4. Seasonal varia?on in es?mates of needle intercellular CO2 concentra?on made during measurements of light-­‐saturated needle gas exchange at terrace sites (n=10 trees, Bars are S.E). paddy@uaa.alaska.edu 4. Across sixteen sites in the Brooks Range, there was no evidence that greater water-­‐use-­‐efficiency (a common response to drought) was associated with limited growth.