Bréda N, Granier A, Aussenac G
INRA, Unité de Bioclimatologie et Ecophysiologie, Champenoux, F-54280 Seichamps, France.
Tree Physiol. 1995 May;15(5):295-306. doi: 10.1093/treephys/15.5.295.
To quantify the effects of crown thinning on the water balance and growth of the stand and to analyze the ecophysiological modifications induced by canopy opening on individual tree water relations, we conducted a thinning experiment in a 43-year-old Quercus petraea stand by removing trees from the upper canopy level. Soil water content, rainfall interception, sap flow, leaf water potential and stomatal conductance were monitored for two seasons following thinning. Seasonal time courses of leaf area index (LAI) and girth increment were also measured. Predawn leaf water potential was significantly higher in trees in the thinned stand than in the closed stand, as a consequence of higher relative extractable water in the soil. The improvement in water availability in the thinned stand resulted from decreases in both interception and transpiration. From Year 1 to Year 2, an increase in transpiration was observed in the thinned stand without any modification in LAI, whereas changes in transpiration in the closed stand were accompanied by variations in LAI. The different behaviors of the closed and open canopies were interpreted in terms of coupling to the atmosphere. Thinning increased inter-tree variability in sap flow density, which was closely related to a leaf area competition index. Stomatal conductance varied little inside the crown and differences in stomatal conductance between the treatments appeared only during a water shortage and affected mainly the closed stand. Thinning enhanced tree growth as a result of a longer growing period due to the absence of summer drought and higher rates of growth. Suppressed and dominant trees benefited more from thinning than trees in the codominant classes.
为了量化树冠疏伐对林分水分平衡和生长的影响，并分析树冠开放对单株树木水分关系所引起的生态生理变化，我们在一片43年生的栓皮栎林分中进行了疏伐实验，通过移除上层树冠层的树木来实现。在疏伐后的两个季节里，对土壤含水量、降雨截留量、液流、叶水势和气孔导度进行了监测。还测量了叶面积指数（LAI）和胸径生长的季节性时间进程。由于土壤中相对可提取水量较高，疏伐林分中树木的黎明前叶水势显著高于郁闭林分。疏伐林分中水分有效性的提高是截留量和蒸腾量减少共同作用的结果。从第1年到第2年，疏伐林分中观察到蒸腾量增加，而LAI没有任何变化，而郁闭林分中蒸腾量的变化伴随着LAI的变化。根据与大气的耦合关系解释了郁闭和开放树冠的不同行为。疏伐增加了树木间液流密度的变异性，这与叶面积竞争指数密切相关。树冠内部气孔导度变化不大，处理间气孔导度的差异仅在缺水期间出现，且主要影响郁闭林分。由于没有夏季干旱且生长速率较高，疏伐延长了生长季节，从而促进了树木生长。被抑制和优势树木比共优势等级的树木从疏伐中受益更多。
