Department of Forest and Ecosystem Science, Melbourne School of Land and Environment, The University of Melbourne, Water Street, Creswick, Victoria 3363, Australia.
Tree Physiol. 2012 Mar;32(3):280-93. doi: 10.1093/treephys/tps001. Epub 2012 Feb 23.
While edge effects on tree water relations are well described for closed forests, they remain under-examined in more open forest types. Similarly, there has been minimal evaluation of the effects of contrasting land uses on the water relations of open forest types in highly fragmented landscapes. We examined edge effects on the water relations and gas exchange of a dominant tree (Eucalyptus arenacea Marginson & Ladiges) in an open forest type (temperate woodland) of south-eastern Australia. Edge effects in replicate woodlands adjoined by cleared agricultural land (pasture edges) were compared with those adjoined by 7- to 9-year-old eucalypt plantation with a 25m fire break (plantation edges). Consistent with studies in closed forest types, edge effects were pronounced at pasture edges where photosynthesis, transpiration and stomatal conductance were greater for edge trees than interior trees (75m into woodlands), and were related to greater light availability and significantly higher branch water potentials at woodland edges than interiors. Nonetheless, gas exchange values were only ∼50% greater for edge than interior trees, compared with ∼200% previously found in closed forest types. In contrast to woodlands adjoined by pasture, gas exchange in winter was significantly lower for edge than interior trees in woodlands adjoined by plantations, consistent with shading and buffering effects of plantations on edge microclimate. Plantation edge effects were less pronounced in summer, although higher water use efficiency of edge than interior woodland trees indicated possible competition for water between plantation trees and woodland edge trees in the drier months (an effect that might have been more pronounced were there no firebreak between the two land uses). Scaling up of leaf-level water relations to stand transpiration using a Jarvis-type phenomenological model indicated similar differences between edge types. That is, transpiration was greater at pasture than plantation edges in summer months (most likely due to greater water availability at pasture edges), resulting in significantly greater estimates of annual transpiration at pasture than plantation edges (430 vs. 343lm(-2)year(-1), respectively). Our study highlights the need for landscape-level water flux models to account for edge effects on stand transpiration, particularly in highly fragmented landscapes.
虽然边缘效应对封闭森林的树木水分关系已有很好的描述，但在更开阔的森林类型中仍未得到充分研究。同样，对于在高度破碎化的景观中，不同土地利用方式对开阔森林类型水分关系的影响，也很少有评估。我们研究了澳大利亚东南部开阔森林类型（温带林地）中一种优势树种（桉树）水分关系和气体交换的边缘效应。在被清理过的农业用地（牧场边缘）毗邻的重复林地和被 7 到 9 年生桉树人工林毗邻的林地（人工林边缘）的边缘效应进行了比较，桉树人工林之间有 25 米的防火隔离带。与在封闭森林类型中的研究一致，在牧场边缘，光合作用、蒸腾作用和气孔导度都比林地内部的树木更大，这表明边缘树木的光可用性更大，树枝水分势显著高于林地内部。然而，与之前在封闭森林类型中发现的相比，边缘树木的气体交换值仅比内部树木高约 50%，而不是 200%。与被牧场毗邻的林地相反，在被人工林毗邻的林地中，冬季边缘树木的气体交换量明显低于内部树木，这与人工林对边缘小气候的遮荫和缓冲效应一致。尽管边缘树木的水分利用效率高于内部林地树木，但在夏季，人工林边缘效应不那么明显，这表明在较干燥的月份，人工林树木和林地边缘树木之间可能存在水分竞争（如果两种土地利用之间没有防火带，这种效应可能更为明显）。使用 Jarvis 型现象学模型将叶片水分关系扩展到林分蒸腾表明，边缘类型之间也存在类似的差异。也就是说，在夏季，牧场边缘的蒸腾量大于人工林边缘（很可能是由于牧场边缘的水分可用性更大），导致牧场边缘的年蒸腾量明显大于人工林边缘（分别为 430 和 343lm(-2)year(-1)）。我们的研究强调了需要在景观水平上的水分通量模型中考虑到边缘效应对林分蒸腾的影响，特别是在高度破碎化的景观中。
