Tree Intrinsic Water-use Efficiency Research Articles

Gas exchange and 13C-based estimates of intrinsic water-use efficiency show different responses to CO2 enrichment: a global meta-analysis of experimental studies

ABSTRACT Tree intrinsic water-use efficiency (iWUE) is a key characteristic of ecosystem functions central to the global cycles of carbon, water, and energy. Gas exchange and stable carbon isotope compositions have been widely used to assess iWUE. However, it remains unclear to what extent the iWUE estimates obtained using different approaches are comparable, particularly in combination with changes in temperature, precipitation, and experimental duration. Herein, the responses of different estimators of iWUE (as A/g s or through isotopic composition of leaves or tree rings) to elevated CO2 concentrations (eCO2) were assessed through a meta-analysis of 361 observations from 98 independent experiments. Our results show that (1) different proxies for iWUE provide inconsistent estimates of the response of iWUE to eCO2. (2) Throughout the growing season, the average response of g s to eCO2 was smaller than the single-point values estimated by gas exchange and the eCO2-induced reduction in g s was stronger for evergreen species than for deciduous species. (3) Precipitation, rather than temperature, significantly affected the responses of 13C-based estimates of iWUE to eCO2. (4) There was no significant response of leaf 13C-based iWUE estimates to eCO2. (5) The importance of photorespiration in elevated CO2 enhancement responses may be overestimated in evergreen species.

Disentangling the impact of photosynthesis and stomatal conductance on rising water-use efficiency at different altitudes on the Tibetan plateau

Tree intrinsic water-use efficiency (iWUE) has dramatically increased in recent decades in global forests. The rising iWUE can be a result of either enhanced photosynthesis rate (A), or decreased stomatal conductance (gs) or both. The underlying physiological mechanisms are still not well understood. Here, we investigated tree-ring isotopes δ13C and δ18O from two tree species in three altitudinal transects on the southeast Tibetan plateau. We found that the relationship between iWUE and ∆18Olw was negative at lower-altitude forests, whereas it was positive at the treeline, indicating that enhanced photosynthesis was the main driver of the increasing iWUE at the treeline, whereas reduction in stomatal conductance was the prime regulator at lower-altitude forests. Furthermore, vapor pressure deficit (VPD) and precipitation during the main growing season showed the highest and significant correlations with leaf water 18O (Δ18Olw) at lower-altitude forests, suggesting that gs was strongly controlled by moisture conditions in the growing season particularly in the lower-altitude forests. These findings shed light to a better understanding of the regulatory mechanism of iWUE changes under the background of rising atmospheric CO2 and climate change and serve to improve the reliability of ecophysiological modeling.

The effect of long-term atmospheric CO2 enrichment on the intrinsic water-use efficiency of sour orange trees.

Every two months of 1992, as well as on three occasions in 1994-1995, we obtained leaf samples together with samples of surrounding air from eight well-watered and fertilized sour orange (Citrus aurantium L.) trees that were growing out-of-doors at Phoenix, Arizona, USA. These trees had been planted in the ground as small seedlings in July of 1987 and enclosed in pairs by four clear-plastic-wall open-top chambers of which two have been continuously maintained since November of that year at a CO2 concentration of 400 micromol mol(-1) and two have been maintained at 700 micromol mol(-1). In September 2000, we also extracted north-south and east-west oriented wood cores that passed through the center of each tree's trunk at a height of 45 cm above the ground. Stable-carbon isotope ratios (13C/12C) derived from these leaf, wood and air samples were used to evaluate each tree's intrinsic water-use efficiency (iWUE). The grand-average result was an 80% increase in this important plant parameter in response to the 300 micromol mol(-1) increase in atmospheric CO2 concentration employed in the study. This increase in sour orange tree iWUE is identical to the long-term CO2-induced increase in the trees' production of wood and fruit biomass, which suggests there could be little to no change in total water-use per unit land area for this species as the air's CO2 content continues to rise. It is also identical to the increase in the mean iWUE reported for 23 groups of naturally occurring trees scattered across western North America that was caused by the historical rise in the air's CO2 content that occurred between 1800 and 1985.