Understanding connections between ecosystem nitrogen (N) cycling and invasive insect defoliation could facilitate the prediction of disturbance impacts across a range of spatial scales. In this study we investigated relationships between ecosystem N cycling and tree defoliation during a recent 2015–18 irruption of invasive gypsy moth caterpillars (Lymantria dispar), which can cause tree stress and sometimes mortality following multiple years of defoliation. Nitrogen is a critical nutrient that limits the growth of caterpillars and plants in temperate forests. In this study, we assessed the associations among N concentrations, soil solution N availability and defoliation intensity by L. dispar at the scale of individual trees and forest plots. We measured leaf and soil N concentrations and soil solution inorganic N availability among individual red oak trees (Quercus rubra) in Amherst, MA and across a network of forest plots in Central Massachusetts. We combined these field data with estimated defoliation severity derived from Landsat imagery to assess relationships between plot-scale defoliation and ecosystem N cycling. We found that trees in soil with lower N concentrations experienced more herbivory than trees in soil with higher N concentrations. Additionally, forest plots with lower N soil were correlated with more severe L. dispar defoliation, which matched the tree-level relationship. The amount of inorganic N in soil solution was strongly positively correlated with defoliation intensity and the number of sequential years of defoliation. These results suggested that higher ecosystem N pools might promote the resistance of oak trees to L. dispar defoliation and that defoliation severity across multiple years is associated with a linear increase in soil solution inorganic N.

中文翻译：

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多年入侵昆虫入侵后，落叶严重程度与土壤溶液氮的有效性呈正相关，与土壤氮浓度呈负相关

了解生态系统氮 (N) 循环与入侵昆虫落叶之间的联系可以促进对一系列空间尺度的干扰影响的预测。在这项研究中，我们调查了最近 2015-18 年入侵性吉普赛蛾毛虫 ( Lymantria dispar )爆发期间生态系统 N 循环与树木落叶之间的关系，这会导致树木压力，有时在多年落叶后死亡。氮是限制温带森林中毛虫和植物生长的关键营养素。在这项研究中，我们通过L. dispar评估了 N 浓度、土壤溶液 N 可用性和落叶强度之间的关联在个体树木和森林地块的规模上。我们测量了马萨诸塞州阿默斯特和马萨诸塞州中部森林地块网络中个体红橡树 ( Quercus rubra ) 的叶子和土壤 N 浓度以及土壤溶液无机 N 的有效性。我们将这些实地数据与来自 Landsat 图像的估计落叶严重程度相结合，以评估地块尺度落叶与生态系统 N 循环之间的关系。我们发现，在氮浓度较低的土壤中的树木比在氮浓度较高的土壤中的树木经历了更多的食草性。此外，具有较低 N 土壤的林地与更严重的L. dispar 相关落叶，与树级关系匹配。土壤溶液中无机氮含量与落叶强度和连续落叶年数呈强正相关。这些结果表明，较高的生态系统 N 库可能会促进橡树对L. dispar落叶的抵抗力，并且多年的落叶严重程度与土壤溶液无机 N 的线性增加有关。