Is distribution of hydraulic constraints within tree crowns reflected in photosynthetic water‐use efficiency? An example of <i>Betula pendula</i>

Abstract

Abstract Spatial and daily variation in photosynthetic water‐use efficiency was examined in leaves of Betula pendula Roth with respect to distribution of hydraulic conductance within the crown, morphological properties of stomata, and water availability. Intrinsic water‐use efficiency ( A n / g s ) was determined from gas‐exchange measurements performed both in situ in a natural forest stand and on detached shoots under laboratory conditions. In intact foliage, sun leaves demonstrated significantly higher ( P &lt; 0.001) A n / g s than shade leaves, as photosynthesis in the lower canopy was chronically limited by low light availability. However, this difference reversed in the mid‐day period under sufficient irradiance ( I &gt; 800 µmol m −2 s −1 ): A n / g s averaged 28.8 and 24.0 µmol mol −1 ( P &lt; 0.01) for shade and sun leaves, respectively. This last finding coincided with the data obtained in laboratory conditions: under equivalent leaf water supply and light, A n / g s in shade foliage was greater ( P &lt; 0.001) than in sun foliage across a wide range of irradiance. Thus, shade foliage of B. pendula is characterized by inherently higher A n / g s than sun foliage, associated with more conservative stomatal behavior, and lower soil‐to‐leaf ( K T ) and leaf hydraulic conductances. Under unlimited light conditions, a within‐crown trade‐off between A n / g s and K T becomes apparent. Differences in stomatal conductance between the detached shoots from sunlit and shaded canopy layers were largely attributable to the variation in stomatal morphology; significant relationships were established with characteristics combining stomatal size and density (relative stomatal surface, stomatal pore area index). Stomatal morphology is very likely involved in long‐term adjustment of photosynthetic WUE.