USDA Forest Service, Eastern Forests Environmental Assessment Threats Center, Southern Global Change Program, 920 Main Campus Dr. Suite 300, Raleigh, NC 27606, USA.
Environ Pollut. 2010 Jun;158(6):2053-8. doi: 10.1016/j.envpol.2009.11.028. Epub 2009 Dec 31.
Federal agencies of several nations have or are currently developing guidelines for critical forest soil acid loads. These guidelines are used to establish regulations designed to maintain atmospheric acid inputs below levels shown to damage forests and streams. Traditionally, when the critical soil acid load exceeds the amount of acid that the ecosystem can absorb, it is believed to potentially impair forest health. The excess over the critical soil acid load is termed the exceedance, and the larger the exceedance, the greater the risk of ecosystem damage. This definition of critical soil acid load applies to exposure of the soil to a single, long-term pollutant (i.e., acidic deposition). However, ecosystems can be simultaneously under multiple ecosystem stresses and a single critical soil acid load level may not accurately reflect ecosystem health risk when subjected to multiple, episodic environmental stress. For example, the Appalachian Mountains of western North Carolina receive some of the highest rates of acidic deposition in the eastern United States, but these levels are considered to be below the critical acid load (CAL) that would cause forest damage. However, the area experienced a moderate three-year drought from 1999 to 2002, and in 2001 red spruce (Picea rubens Sarg.) trees in the area began to die in large numbers. The initial survey indicated that the affected trees were killed by the southern pine beetle (Dendroctonus frontalis Zimm.). This insect is not normally successful at colonizing these tree species because the trees produce large amounts of oleoresin that exclude the boring beetles. Subsequent investigations revealed that long-term acid deposition may have altered red spruce forest structure and function. There is some evidence that elevated acid deposition (particularly nitrogen) reduced tree water uptake potential, oleoresin production, and caused the trees to become more susceptible to insect colonization during the drought period. While the ecosystem was not in exceedance of the CAL, long-term nitrogen deposition pre-disposed the forest to other ecological stress. In combination, insects, drought, and nitrogen ultimately combined to cause the observed forest mortality. If any one of these factors were not present, the trees would likely not have died. This paper presents a conceptual framework of the ecosystem consequences of these interactions as well as limited plot level data to support this concept. Future assessments of the use of CAL studies need to account for multiple stress impacts to better understand ecosystem response.
一些国家的联邦机构已经或正在制定森林土壤酸负荷临界值指南。这些指南被用来制定旨在将大气酸输入量维持在低于已知会损害森林和溪流的水平的法规。传统上，当临界土壤酸负荷超过生态系统可以吸收的酸量时，就有可能损害森林健康。超过临界土壤酸负荷的部分被称为超标，超标越大，生态系统受损的风险就越大。这个临界土壤酸负荷的定义适用于土壤暴露于单一的、长期的污染物（即酸性沉降物）。然而，生态系统可能同时受到多种生态系统压力的影响，并且当受到多种、间歇性的环境压力时，单一的临界土壤酸负荷水平可能无法准确反映生态系统健康风险。例如，美国东部的北卡罗来纳州西部的阿巴拉契亚山脉受到了美国东部最高的酸性沉降物的影响，但这些水平被认为低于会导致森林破坏的临界酸负荷 (CAL)。然而，该地区在 1999 年至 2002 年经历了一场为期三年的中度干旱，2001 年，该地区的红云杉（Picea rubens Sarg.）树木大量死亡。初步调查表明，受影响的树木是被南方松甲虫（Dendroctonus frontalis Zimm.）杀死的。这种昆虫通常无法成功地在这些树种上定殖，因为树木会产生大量的松脂，排斥钻孔甲虫。随后的调查显示，长期的酸性沉降可能改变了红云杉森林的结构和功能。有一些证据表明，升高的酸性沉降物（特别是氮）降低了树木的吸水潜力、松脂产量，并使树木在干旱期间更容易受到昆虫的定殖。虽然生态系统没有超过 CAL，但长期的氮沉降使森林更容易受到其他生态压力的影响。这些因素结合在一起，最终导致了观察到的森林死亡。如果这些因素中的任何一个不存在，树木可能就不会死亡。本文提出了一个概念框架，说明这些相互作用对生态系统的影响，以及有限的斑块水平数据来支持这一概念。未来对 CAL 研究的使用评估需要考虑到多种压力的影响，以便更好地理解生态系统的反应。
