Co-Authors:
Ungar, E.D., Department of Natural Resources, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Rotenberg, E., Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel
Raz-Yaseef, N., Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel
Cohen, S., Department of Environmental Physics and Irrigation, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Yakir, D., Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel
Schiller, G., Department of Natural Resources, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Abstract:
Transpiration is a fundamental datum in understanding the ecophysiology of planted forests in dry regions and is central to the construction of an ecosystem-level water balance. The present aims were: (i) to measure daily transpiration in a dryland Pinus halepensis Mill. (Aleppo pine) forest and to examine its relationship to environmental conditions such as soil water content and evaporative demand; (ii) to determine the seasonal and annual water balances of the ecosystem; and (iii) to explore management implications in the context of a climate-change scenario of increasing aridity. The study was conducted in the Yatir forest (300treesha-1) in Israel's semiarid northern Negev, during three consecutive years (starting 2003/4) in which rainfall (R) was 231, 334 and 224mm, the last designated a drought because of relatively long dry spells between major rain events. Tree transpiration was measured by the heat-pulse method and values were upscaled to the forest canopy level. Independent estimates were obtained for daily ecosystem-level evapotranspiration (ET), and soil and understory vegetation evapotranspiration (E). Daily canopy-level transpiration rate (T) ranged from 0.1 to 1.6mmd-1 and showed a highly dynamic and irregular pattern during the rainy season. For the two non-drought years a large part of the variation in T could be attributed to the following simple relationships. When soil water content (SWC) ≤0.15m3m-3, the primary driver of T was SWC; regression of T on SWC yielded highly significant quadratic relationships indicating little response below SWC of approximately 0.12m3m-3, and a steep linear response above it. For SWC >0.15m3m-3, potential evapotranspiration (PET) was of paramount importance; quadratic regression of T on PET yielded highly significant relationships and explained a high proportion of the variance in T. During the wet season (215d), on average, cumulative ET (201mm) accounted for 0.76 R (R=263mm) and cumulative T (116mm; range 103-126mm) - an independently estimated component of cumulative ET - accounted for 0.45 R. Cumulative E was 70mm. On an annual basis, total evapotranspiration losses were approximately equal to R, with 58% exiting the system via the trees and 39% via soil and undergrowth vegetation. Water balance data combined with assumptions regarding tree minimum transpiration led to a first approximation of sustainable forest density. This approach indicated that the Yatir forest should be thinned to stands of 250 or 190treesha-1 in order to remain sustainable under annual rainfall regimes of 200 or 150mm, respectively. © 2013 Elsevier B.V.
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