Heterogeneity of competition at decameter scale: patches of high canopy leaf area in a shade-intolerant larch stand transpire less yet are more sensitive to drought

Abstract

Small differences in the sensitivity of stomatal conductance to light intensity on leaf surfaces may lead to large differences in total canopy transpiration (EC) with increasing canopy leaf area (L). Typically, the increase of L would more than compensate for the decrease of transpiration per unit of leaf area (EL), resulting in concurrent increase of EC. However, highly shade-intolerant species, such as Larix principis-rupprechtii Mayr., may be so sensitive to increased shading that such compensation is not complete. We hypothesized that in such a stand, windfall-induced spatial variation at a decameter scale would result in greatly reduced EL in patches of high L leading to lower EC than low competition patches of sparse canopy. We further hypothesized that quicker extraction of soil moisture in patches of lower competition will result in earlier onset of drought symptoms in these patches. Thus, patches of low L will transition from light to soil moisture as the factor dominating EL. This process should progressively homogenize EC in the stand even as the variation of soil moisture is increasing. We tested the hypotheses utilizing sap flux of nine trees, and associated environmental and stand variables. The results were consistent with only some of the expectations. Under non-limiting soil moisture, EL was very sensitive to the spatial variation of L, decreasing sharply with increasing L and associated decrease of mean light intensity on leaf surfaces. Thus, under the conditions of ample soil moisture maximum EC decreased with increasing patch-scale L. Annual EC and biomass production also decreased with L, albeit more weakly. Furthermore, variation of EC among patches decreased as average stand soil moisture declined between rain events. However, contrary to expectation, high L plots which transpired less showed a greater EL sensitivity to decreasing stand-scale soil moisture, suggesting a different mechanism than simple control by decreasing soil moisture. We offer potential explanations to the observed phenomenon. Our results demonstrate that spatial variation of L at decameter scale, even within relatively homogeneous, single-species, even-aged stands, can produce large variation of transpiration, soil moisture and biomass production and should be considered in 1-D soil-plant-atmosphere models.

Ecophysiological variation of transpiration of pine forests: synthesis of new and published results

“…However, L is also greatly affected by the large spatial variation in tree density and sapwood area per unit of ground area (Oren et al 1998a), reflecting local variation of resource availability, such as water and nutrients as affected by rooting volume (e.g., in relation to rock content and soil depth; McCarthy et al 2006), weatherrelated disturbances (McCarthy et al 2006, Xiong et al 2015, or management (Zhang et al 2012). Modification of site resources, such as water and nutrients, may differentially affect A S and A L , decoupling stand transpiration from L (Ewers et al 2001).…”

Modeling the Response of Daily Evapotranspiration and its Components of a Larch Plantation to the Variation of Weather, Soil Moisture, and Canopy Leaf Area Index

“…To some extent, the LAI can reflect the total leaf area and the number of stomata (Wang et al, ). Within the low range of LAI, the evaporation surface area and stomata number increase with rising LAI, the ability of trees to transfer water from soil to leaves accordingly enhances; thus, the stand T increases (Gong et al, ); the stand T will no longer increase with rising LAI when the LAI is within the high range, this is because stomatal closure and the decrease of received light for lower leaves with increased mutual shading directly translates to a decrease of transpiration per unit of leaf area with increased LAI (Xiong et al, ). In this study, stand T with rising LAI exhibited a saturated exponential increase relation, being in line with the study of Wang et al ().…”

“…Thus, coupling the different responses of ET components to variation of the main driving factors is required. At a given site with certain soil and landform conditions, the main factors driving the ET components can be simplified and divided into several aspects: the depth of precipitation (P) and the potential evapotranspiration (ET ref ), which reflect the effect of changing weather (Granier et al, ; Ungar et al, ); the relative extractable water (REW) or volumetric soil moisture (VSM), which reflects the water supply ability from the root zone soil layer (Lagergren & Lindroth, ; Sinclair et al, ; Ungar et al, ); and the forest canopy leaf area index (LAI), which reflects the water conveying capacity of vegetation or canopy structure (Bucci et al, ; Xiong et al, ).…”

The Variation in Water Consumption by Transpiration of Qinghai Spruce among Canopy Layers in the Qilian Mountains, Northwestern China

“…In related studies, trees were distinguished by DBH, and larger trees were found to generally transpire more than smaller trees [24,26], but it was also found that the mean sap flow of trees was not strongly correlated with DBH [18,22,44]. In addition to DBH, tree dominance in forest was also the main influencing factor on transpiration [22], because it was related to the ability to acquire living resources, including water and energy resources.…”