Insect Defoliation and Nitrogen Cycling in Forests
Insect Defoliation and Nitrogen Cycling in Forests
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森林中的昆虫落叶和氮循环
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发表时间：
2002
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通讯作者：
M. Mitchell
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作者：
G. Lovett;L. Christenson;P. Groffman;Clive G. Jones;Julie Hart;M. Mitchell
食叶昆虫对森林生态系统有着巨大的影响。研究表明，落叶可降低蒸腾作用和树木生长，增加树木死亡率、光线穿透森林地面和排水（Stephens et al. 1972，坎贝尔和Sloan 1977，Houston 1981）。碳在树的各个部分的分配可能会改变，叶子中防御性化合物的产生可能会增加（Schultz和Baldwin 1982），而种子产量可能会在落叶后多年下降（McConnell 1988，Gottschalk 1990）。树种组成的变化（Doane和McManus，1981年; Glitzenstein等人，1990年）以及食虫鸟类和其他野生动物种群规模的变化也可能发生（Holmes等人，1986年; USDA Forest Service，1994年）。几项关于昆虫爆发的研究也表明，森林生态系统中的氮在落叶后的排水中损失增加，这表明土壤中可利用的氮增加，易被沥滤（Swank等人，1981年; McDonald等人，1992年; Webb等人，1995年; Eshleman等人，1998年; Reynolds等人，2000年）。通过淋溶造成的大量氮素损失将减少N有限生态系统的长期森林生产。此外，硝酸盐（NO3 -）输出到溪流中会使下游沃茨酸化（Webb等人，1995年），并导致沿海沃茨和河口的富营养化（Fisher和Oppenheimer，1991年）。乍一看，许多研究人员认为森林生态系统在落叶后大量泄漏N的观点符合受干扰生态系统中氮行为的一般概念。在对诸如密集收割（Likens等人，1970年）、火灾（Bayley和Schindler，1991年）和严重风暴（Schaefer等人，1996年）等干扰的反应中，已经观察到显著的氮损失。然而，落叶在三个方面与其他扰动有质的不同。首先，大多数树木在被昆虫落叶后通常仍然活着，其木质结构完好无损。（其中包括硬木树反复严重落叶或针叶树严重落叶造成的高死亡率。）第二，对土壤的物理扰动很小，因此不太可能发生严重的侵蚀。第三，如果树木没有被杀死，树冠恢复的时间通常是以周而不是年来计算的。在这篇文章中，我们研究的机制和幅度的N周期扰动落叶，借鉴大量的研究机构对舞毒蛾（舞毒蛾），一种引进的鳞翅目昆虫，在过去的50或60年里，它是美国东北部硬木森林的主要食叶者（Doane和McManus 1981）。我们试图建立一个更连贯的观点，落叶N循环的可能后果，我们的情况下，与普遍认为，森林生态系统落叶的反应主要是一个重新分配，而不是损失，氮。
O of defoliating insects can have dramatic effects on forest ecosystems. Studies have shown that defoliation can decrease transpiration and tree growth and increase tree mortality, light penetration to the forest floor, and water drainage (Stephens et al. 1972, Campbell and Sloan 1977, Houston 1981). The allocation of carbon to various parts of the tree may be altered, production of defensive compounds in foliage may increase (Schultz and Baldwin 1982), and seed production may decline for many years after defoliation (McConnell 1988, Gottschalk 1990). Shifts in tree species composition (Doane and McManus 1981, Glitzenstein et al. 1990) and changes in the population size of insectivorous birds and other wildlife may also occur (Holmes et al. 1986, USDA Forest Service 1994). Several studies of insect outbreaks have also indicated an increased loss of nitrogen (N) from forest ecosystems in drainage water following defoliation, suggesting an increase in soil-available nitrogen that is subject to leaching (Swank et al. 1981, McDonald et al. 1992, Webb et al. 1995, Eshleman et al. 1998, Reynolds et al. 2000). Large losses of nitrogen via leaching would reduce long-term forest production in Nlimited ecosystems. In addition, the export of nitrate (NO3 –) to stream water can acidify downstream waters (Webb et al. 1995) and contribute to eutrophication of coastal waters and estuaries (Fisher and Oppenheimer 1991). At first glance, the view held by many investigators that forest ecosystems leak N in large quantities after defoliation fits the general notion of nitrogen behavior in disturbed ecosystems. Significant nitrogen losses have been observed in response to disturbances such as intensive harvesting (Likens et al. 1970), fire (Bayley and Schindler 1991), and severe windstorms (Schaefer et al. 1996). However, defoliation differs qualitatively from these other disturbances in three ways. First, most of the trees usually remain alive with their woody structure intact after defoliation by insects. (Exceptions are the high mortality rates caused by repeated severe defoliations of hardwood trees or by severe defoliation of conifers.) Second, physical disturbance of the soil is minimal and significant erosion is therefore unlikely to occur. And third, if the trees are not killed, the time for substantial canopy recovery is often measured in weeks rather than years. In this article we examine the mechanisms and magnitudes of N-cycle perturbations by defoliation, drawing heavily on the considerable body of research on the gypsy moth (Lymantria dispar L.), an introduced lepidopteran that has been the major defoliator of hardwood forests in the northeastern United States during the last 5 or 6 decades (Doane and McManus 1981). We attempt to establish a more coherent view of the likely consequences of defoliation for N cycling, and we make the case that, contrary to the commonly held view, the response of forest ecosystems to defoliation is primarily one of redistribution, rather than loss, of nitrogen.