Pinkard Elizabeth A, Battaglia Michael, Mohammed Caroline L
School of Agriculture, University of Tasmania, Hobart, TAS 7001, Australia.
Tree Physiol. 2007 Jul;27(7):1053-63. doi: 10.1093/treephys/27.7.1053.
Plant responses to defoliation are complex. We established a field experiment in a nine-month-old Eucalyptus globulus Labill. plantation to examine the effects of pattern (upper crown versus lower crown removal), frequency (single, double or triple defoliation within a 12-month period) and severity (25 versus 38% of leaf area removed) of defoliation and the effect of soil nitrogen (N) on photosynthetic processes and stem growth. The photosynthetic responses observed following defoliation could be attributed to changes in source:sink ratios. Light-saturated CO(2) uptake (A(max)) increased with increasing severity and frequency of defoliation irrespective of defoliation pattern. Seedlings defoliated in autumn did not exhibit increases in A(max) until the following spring, whereas there was no such delay in photosynthetic responses associated with spring defoliation. Application of N before defoliation allowed trees to compensate for the effect of defoliation on stem diameter growth, which could not be explained simply in terms of increases in A(max). The observed increases in stem diameter increment following N fertilization of defoliated trees suggested increases in leaf area development, and there were changes in the leaf area:leaf dry mass ratio that may have increased light absorption by the crown. Nitrogen fertilization also increased partitioning of dry mass to branches at the expense of main stems, suggesting that N supply was important in rebuilding crowns following a defoliation event.
植物对落叶的反应很复杂。我们在一片9个月大的蓝桉人工林中开展了一项田间试验，以研究落叶的模式（去除树冠上部与下部）、频率（12个月内单次、两次或三次落叶）和严重程度（去除25%与38%的叶面积）以及土壤氮（N）对光合过程和茎干生长的影响。落叶后观察到的光合反应可能归因于源库比的变化。无论落叶模式如何，光饱和二氧化碳吸收量（A(max)）都随着落叶严重程度和频率的增加而增加。秋季落叶的幼苗直到次年春季A(max)才增加，而与春季落叶相关的光合反应则没有这种延迟。落叶前施氮使树木能够弥补落叶对茎干直径生长的影响，这不能简单地用A(max)的增加来解释。对落叶树木施氮后观察到的茎干直径增量增加表明叶面积发育增加，并且叶面积与叶干质量比发生了变化，这可能增加了树冠对光的吸收。施氮还增加了干物质向树枝的分配，而以主茎为代价，这表明氮供应在落叶事件后重建树冠方面很重要。
