Forest restoration treatments in a ponderosa pine forest enhance physiological activity and growth under climatic stress.
Forest restoration treatments in a ponderosa pine forest enhance physiological activity and growth under climatic stress.
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DOI：
10.1002/eap.2188
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发表时间：
2020-06
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影响因子：
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通讯作者：
A. Tepley;S. Hood;Christopher R. Keyes;A. Sala
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文献类型：
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作者：
A. Tepley;S. Hood;Christopher R. Keyes;A. Sala
随着气候变暖，气温升高时干旱将日益发生，使森林生态系统面临大范围枯死和死亡的风险越来越大。在某些情况下，20 世纪初灭火后树木密度的增加可能会加剧这种风险。旨在恢复历史林分结构和增强对高强度火灾抵抗力的治疗方法也可能通过减少竞争来缓解干旱压力，但这些影响的持续时间和潜在机制仍然知之甚少。为了阐明这些机制，我们评估了蒙大拿州西部黄松林在有或没有规定火的情况下，树木的生长、死亡率和树轮稳定碳同位素对林分密度降低处理的反应。中度和较重的切割实验（断面积分别减少 35% 和 56%）于 1992 年开始，随后对一部分被减薄的单位进行了规定的燃烧。所有处理都会导致生长释放，并持续到重新采样时。这些处理对气候与增长的关系影响不大，但显着改变了季节性碳同位素信号及其与气候的关系。在燃烧和未燃烧处理中，相对于对照，早材（EW）的碳同位素辨别力（Δ13 C）增加，晚材（LW）减少。在所有处理中，LW Δ13 C 对夏末气候的敏感性也有所增加，但在对照中没有增加。这种敏感性的增加表明，竞争的减少使树木能够继续固定碳以促进新茎的生长，即使气候变得足够紧张，足以阻止未经处理的单位中生长较慢的树木进行新的同化。如果我们没有将 EW 和 LW 分开，这些发现就会被掩盖。在夏末气候胁迫下加快生长和增强碳同化的重要性在处理后的第二个十年中变得明显，当时山松甲虫的活动局部增加，两个实验对照中的树木死亡率增加到各自处理的两倍以上。这些发现强调，当通过间伐来恢复历史森林结构或增强对高强度火灾的抵抗力时，可能会在气候压力下增强生长和生理活动带来额外的好处，而且这种影响可能会持续二十多年。
As the climate warms, drought will increasingly occur under elevated temperatures, placing forest ecosystems at growing risk of extensive dieback and mortality. In some cases, increases in tree density following early 20th -century fire suppression may exacerbate this risk. Treatments designed to restore historical stand structure and enhance resistance to high-severity fire might also alleviate drought stress by reducing competition, but the duration of these effects and the underlying mechanisms remain poorly understood. To elucidate these mechanisms, we evaluate tree growth, mortality, and tree-ring stable-carbon isotope responses to stand-density reduction treatments with and without prescribed fire in a ponderosa pine forest of western Montana. Moderate and heavier cutting experiments (basal area reductions of 35 and 56%, respectively) were initiated in 1992, followed by prescribed burning in a subset of the thinned units. All treatments led to a growth release that persisted to the time of re-sampling. The treatments had little effect on climate-growth relationships, but they markedly altered seasonal carbon-isotope signals and their relationship to climate. In burned and unburned treatments, carbon-isotope discrimination (Δ13 C) increased in the earlywood (EW) and decreased in the latewood (LW) relative to the control. The sensitivity of LW Δ13 C to late-summer climate also increased in all treatments, but not in the control. Such increased sensitivity indicates that the reduction in competition enabled trees to continue to fix carbon for new stem growth, even when the climate became sufficiently stressful to stop new assimilation in slower-growing trees in untreated units. These findings would have been masked had we not separated EW and LW. The importance of faster growth and enhanced carbon assimilation under late-summer climatic stress became evident in the second decade post-treatment, when mountain pine beetle activity increased locally, and tree mortality rates in the controls of both experiments increased to more than twice those in their respective treatments. These findings highlight that when thinning is used to restore historical forest structure or increase resistance to high-severity fire, there will likely be additional benefits of enhanced growth and physiological activity under climatic stress, and the effects may persist for more than two decades.