Journal
OECOLOGIA
Volume 167, Issue 2, Pages 573-585Publisher
SPRINGER
DOI: 10.1007/s00442-011-2010-4
Keywords
CO2 fertilization; Dryland productivity; Stomatal conductance; Aleppo pine; Isotope dendrology
Categories
Funding
- International Arid Land Consortium (IALC)
- GLOWA-Jordan River, The Henry Gutwirth Fund for Research
- Philip M. Klutznick Fund for Research
- European Community [HPRN-CT-1999-00059, NETCARB]
- Weizmann Institute
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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 (W-i) with environmental conditions in Pinus halepensis. We used 30-year (1974-2003) tree-ring records of basal area increment (BAI) and cellulose C-13 and O-18 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 W-i 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 W-i trends were better correlated with the increase in atmospheric CO2 concentration, C-a, than with the local increase in temperature (similar to 0.04 degrees 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 W-i from EW and BAI were dependent on the interannual variations in P. From reconstructed C-i values, we demonstrate that contrasting gas-exchange responses at opposing ends of the hydrologic gradient underlie the variation in W-i sensitivity to C-a between sites and seasons. Under the mild water limitations typical of the main seasonal growth period, regulation was directed at increasing C-i/C-a towards a homeostatic set-point observed at the most mesic site, with a decrease in the W-i response to C-i with increasing aridity. With more extreme drought stress, as seen in the late season at the drier sites, the response was W-i driven, and there was an increase in the W-i sensitivity to C-a with aridity and a decreasing sensitivity of C-i to C-a. The apparent C-a-driven increases in W-i can help to identify the adjustments to drying conditions that forest ecosystems can make in the face of predicted atmospheric change.
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