title: 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.
authors.affiliation: Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, P.R. China xwcaf@163.com.
authors.forename: Wei
authors.initials: W
authors.lastname: Xiong
authors.affiliation: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83 Umeå, Sweden.
authors.forename: Ram
authors.initials: R
authors.lastname: Oren
authors.affiliation: Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, P.R. China.
authors.forename: Yanhui
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, P.R. China.
authors.forename: Pengtao
authors.initials: P
authors.lastname: Yu
authors.affiliation: Monitoring Station of Environmental Protection Center in Erdos, Bureau of Environmental Protection of Erdos in Inner Mongolia of China, Erdos 017000, P.R. China.
authors.forename: Hailong
authors.initials: H
authors.lastname: Liu
authors.affiliation: Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, P.R. China.
authors.forename: Gongxiang
authors.initials: G
authors.lastname: Cao
authors.affiliation: Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, P.R. China.
authors.forename: Lihong
authors.initials: L
authors.lastname: Xu
authors.affiliation: Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, P.R. China.
authors.forename: Yunni
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, P.R. China.
authors.forename: Haijun
authors.initials: H
authors.lastname: Zuo
mesh_terms: D002681:China; D055864:Droughts; D028221:Larix / Q000502:physiology* / Q000528:radiation effects; D008027:Light; D018515:Plant Leaves / Q000502:physiology / Q000528:radiation effects; D054046:Plant Stomata / Q000528:radiation effects; D018526:Plant Transpiration; D014197:Trees / Q000502:physiology* / Q000528:radiation effects
keywords: light absorption; net primary production; soil moisture; soil water potential; spatiotemporal analysis; transpiration