A Decade of Streamwater Nitrogen and Forest Dynamics after a Mountain Pine Beetle Outbreak at the Fraser Experimental Forest, Colorado
A Decade of Streamwater Nitrogen and Forest Dynamics after a Mountain Pine Beetle Outbreak at the Fraser Experimental Forest, Colorado
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科罗拉多州弗雷泽实验森林山松甲虫爆发后十年的溪流氮和森林动态
DOI：
10.1007/s10021-016-0027-6
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发表时间：
2017
期刊：
影响因子：
3.7
通讯作者：
K. Elder
中科院分区：
文献类型：
--
作者：
C. Rhoades;R. Hubbard;K. Elder
北美西部的森林目前正经历着各种树皮甲虫物种造成的大规模树木死亡，预计在温暖干燥的气候下，昆虫的爆发将增加。与野火和木材采伐后典型的突然生物地球化学变化不同，昆虫爆发的结果知之甚少。2002年，山松树皮甲虫（Dendroctonus ponderosae）开始在弗雷泽实验森林（Fraser Experimental Forest）攻击黑松（Pinus contorta），并于2007年在整个研究区域蔓延。我们比较了2003年至2012年的溪流氮（N）与过去二十年的数据在四个流域不同的森林管理历史，林分结构，并响应甲虫爆发。以老树林为主的流域有较大的树木，失去了85%的上层松和44%的总断面积的树皮甲虫。相比之下，管理集水区包含的第二次增长（30-60岁）和老增长（250- 350岁）的混合物有较高的密度，树冠下的树木，较小的平均直径，和较低的小蠹引起的死亡率（约26%的总断面积）。溪流硝酸盐浓度显着较高，在老增长流域在爆发期间比爆发前的水平在融雪和基流季节。在混合年龄的立场，溪流硝酸盐浓度不受疫情的影响。甲虫的爆发使两个古老流域的无机氮输出量增加了43%和74%，尽管相对于大气输入量（<2%的年氮沉积量），河水中释放的氮量较低（0.04和0.15 kg N ha−1）。增加的高度，直径，和叶片N的测量在剩余的活树增加了对N的需求，远远超过了N出口的变化在爆发期间。松死亡后释放的土壤资源重新分配到上层和下层植被有助于解释高养分保留在小流域受小蠹爆发。
Forests of western North America are currently experiencing extensive tree mortality from a variety of bark beetle species, and insect outbreaks are projected to increase under warmer, drier climates. Unlike the abrupt biogeochemical changes typical after wildfire and timber harvesting, the outcomes of insect outbreaks are poorly understood. The mountain pine bark beetle (Dendroctonus ponderosae) began to attack lodgepole pine (Pinus contorta) at the Fraser Experimental Forest in 2002 and spread throughout the research area by 2007. We compared streamwater nitrogen (N) from 2003 through 2012 with data from the previous two decades in four watersheds with distinct forest management histories, stand structures, and responses to the beetle outbreak. Watersheds dominated by old-growth had larger trees and lost 85% of overstory pine and 44% of total basal area to bark beetles. In contrast, managed watersheds containing a mixture of second-growth (30–60 year old) and old-growth (250- to 350-year old) had higher density of subcanopy trees, smaller mean tree diameter, and lower bark beetle-induced mortality (~26% of total basal area). Streamwater nitrate concentrations were significantly higher in old-growth watersheds during the outbreak than pre-outbreak levels during snowmelt and base flow seasons. In mixed-age stands, streamwater nitrate concentrations were unaffected by the outbreak. Beetle outbreak elevated inorganic N export 43 and 74% in two old-growth watersheds though the amounts of N released in streamwater were low (0.04 and 0.15 kg N ha−1) relative to atmospheric inputs (<2% of annual N deposition). Increased height, diameter, and foliar N of measured in residual live trees augmented demand for N, far in excess of the change in N export during the outbreak. Reallocation of soil resources released after pine mortality to overstory and understory vegetation helps explain high nutrient retention in watersheds affected by bark beetle outbreaks.