Increase in water-use efficiency and underlying processes in pine forests across a precipitation gradient in the dry Mediterranean region over the past 30 years
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- Maseyk, K., Hemming, D., Angert, A. et al. Oecologia (2011) 167: 573. doi:10.1007/s00442-011-2010-4
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Motivated by persistent predictions of warming and drying in the entire Mediterranean and other regions, we have examined the interactions of intrinsic water-use efficiency (Wi) with environmental conditions in Pinus halepensis. We used 30-year (1974–2003) tree-ring records of basal area increment (BAI) and cellulose 13C and 18O composition, complemented by short-term physiological measurements, from three sites across a precipitation (P) gradient (280–700 mm) in Israel. The results show a clear trend of increasing Wi in both the earlywood (EW) and latewood (LW) that varied in magnitude depending on site and season, with the increase ranging from ca. 5 to 20% over the study period. These Wi trends were better correlated with the increase in atmospheric CO2 concentration, Ca, than with the local increase in temperature (~0.04°C year−1), whereas age, height and density variations had minor effects on the long-term isotope record. There were no trends in P over time, but Wi from EW and BAI were dependent on the interannual variations in P. From reconstructed Ci values, we demonstrate that contrasting gas-exchange responses at opposing ends of the hydrologic gradient underlie the variation in Wi sensitivity to Ca between sites and seasons. Under the mild water limitations typical of the main seasonal growth period, regulation was directed at increasing Ci/Ca towards a homeostatic set-point observed at the most mesic site, with a decrease in the Wi response to Ci with increasing aridity. With more extreme drought stress, as seen in the late season at the drier sites, the response was Wi driven, and there was an increase in the Wi sensitivity to Ca with aridity and a decreasing sensitivity of Ci to Ca. The apparent Ca-driven increases in Wi can help to identify the adjustments to drying conditions that forest ecosystems can make in the face of predicted atmospheric change.