Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon, United States of America.
U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon, United States of America.
PLoS One. 2021 Feb 16;16(2):e0246822. doi: 10.1371/journal.pone.0246822. eCollection 2021.
Resource managers in the Pacific Northwest (USA) actively thin second-growth forests to accelerate the development of late-successional conditions and seek to expand these restoration thinning treatments into riparian zones. Riparian forest thinning, however, may impact stream temperatures-a key water quality parameter often regulated to protect stream habitat and aquatic organisms. To better understand the effects of riparian thinning on shade, light, and stream temperature, we employed a manipulative field experiment following a replicated Before-After-Control-Impact (BACI) design in three watersheds in the redwood forests of northern California, USA. Thinning treatments were intended to reduce canopy closure or basal area within the riparian zone by up to 50% on both sides of the stream channel along a 100-200 m stream reach. We found that responses to thinning ranged widely depending on the intensity of thinning treatments. In the watersheds with more intensive treatments, thinning reduced shade, increased light, and altered stream thermal regimes in thinned and downstream reaches. Thinning shifted thermal regimes by increasing maximum temperatures, thermal variability, and the frequency and duration of elevated temperatures. These thermal responses occurred primarily during summer but also extended into spring and fall. Longitudinal profiles indicated that increases in temperature associated with thinning frequently persisted downstream, but downstream effects depended on the magnitude of upstream temperature increases. Model selection analyses indicated that local changes in shade as well as upstream thermal conditions and proximity to upstream treatments explained variation in stream temperature responses to thinning. In contrast, in the study watershed with less intensive thinning, smaller changes in shade and light resulted in minimal stream temperature responses. Collectively, our data shed new light on the stream thermal responses to riparian thinning. These results provide relevant information for managers considering thinning as a viable restoration strategy for second-growth riparian forests.
太平洋西北地区（美国）的资源管理者积极地对次生林进行疏伐，以加速后成林条件的发展，并寻求将这些恢复性疏伐措施扩展到河岸带。然而，河岸森林疏伐可能会影响溪流温度——这是一个关键的水质参数，通常受到监管以保护溪流生境和水生生物。为了更好地了解河岸疏伐对遮荫、光照和溪流温度的影响，我们在美国加利福尼亚北部红杉林的三个流域采用了一种复制的前后对照影响（BACI）设计进行了操纵性野外实验。疏伐处理旨在减少河岸带内的树冠覆盖或基面积，在 100-200 米的溪流段沿溪流两侧减少高达 50%。我们发现，对疏伐的反应范围很广，这取决于疏伐处理的强度。在处理强度更高的流域，疏伐减少了遮荫，增加了光照，并改变了疏伐和下游流域的溪流热状况。疏伐通过增加最高温度、热变异性以及高温的频率和持续时间来改变热状况。这些热响应主要发生在夏季，但也延伸到春季和秋季。纵向剖面表明，与疏伐相关的温度升高经常在下游持续，但下游效应取决于上游温度升高的幅度。模型选择分析表明，遮荫的局部变化以及上游热条件和与上游处理的接近程度解释了溪流温度对疏伐响应的变化。相比之下，在疏伐强度较低的研究流域，遮荫和光照的变化较小，导致溪流温度的响应最小。总的来说，我们的数据为河岸疏伐对溪流温度的响应提供了新的认识。这些结果为考虑疏伐作为次生林河岸带恢复策略的管理者提供了相关信息。
