Coweeta Hydrologic Laboratory, Center for Forest Watershed Research, Southern Research Station, USDA Forest Service, 3160 Coweeta Lab Rd, Otto, NC 28763, United States of America.
Sci Total Environ. 2021 Mar 20;761:143270. doi: 10.1016/j.scitotenv.2020.143270. Epub 2020 Oct 28.
In the past century, the evergreen woody shrub, Rhododendron maximum, has experienced habitat expansion following foundational tree species die-off in eastern US deciduous forests. Rhododendron can potentially alter stream chemistry, temperature, trophic dynamics, and in-stream decomposition rates, given its dominance in riparian areas. Here we conducted two operational-scale (3 ha) riparian treatments that removed rhododendron through cutting alone (CR, canopy removal), or removing both the rhododendron canopy and forest floor using cutting and prescribed fire (CFFR, canopy and forest floor removal). We expected that rhododendron shrub removal, with or without soil organic horizon removal, would increase soil nutrient availability and subsequently alter stream pH, acid neutralizing capacity (ANC), inorganic nitrogen (NO-N, NH-N), total dissolved inorganic nitrogen, dissolved organic carbon (DOC), calcium (Ca), potassium (K), and magnesium (Mg). We hypothesized that responses would occur more quickly in the CFFR treatment. Treatments reduced shrub-, but not tree basal area. Treatments lowered soil N, but not C. Stream chemistry responses to treatments varied between CR and CFFR and were transient, generally with pH, N, and some cations declining, and aluminum (Al) and DOC showing a pulse increase. By removing rhododendron, the remaining deciduous trees likely accelerated N uptake as soil moisture availability increased. This could partially explain why we observed lower than expected stream nutrients (NO-N, Ca, and Mg) after treatments. Initial rhododendron slash on the forest floor coupled with incomplete consumption of the O-horizon on the CFFR treatment likely elevated DOC in the upper soil horizons and mobilized Al. From a management perspective, using these treatments to restore structure and function to riparian forests in the wake of eastern hemlock mortality, with or without fire, would most likely not result in short-term diminished water quality that is common when overstory trees are harvested and may even lower stream NO-N concentrations long term.
在过去的一个世纪里，常绿木本灌木杜鹃花属最大值在美国东部落叶林的主要树种死亡后，其栖息地不断扩大。由于杜鹃在河岸地区占主导地位，它有可能改变溪流的化学性质、温度、营养动态和溪流中的分解速率。在这里，我们进行了两项操作规模（3 公顷）的河岸处理，分别通过单独砍伐（CR，树冠移除）或使用砍伐和计划火烧（CFFR，树冠和林地移除）去除杜鹃。我们预计，无论是否去除土壤有机层，杜鹃灌木的移除都会增加土壤养分的有效性，随后改变溪流的 pH 值、酸中和能力（ANC）、无机氮（NO-N、NH-N）、总溶解无机氮、溶解有机碳（DOC）、钙（Ca）、钾（K）和镁（Mg）。我们假设 CFFR 处理中的反应会更快发生。处理降低了灌木的，但不是树木的基部面积。处理降低了土壤中的 N，但不是 C。溪流化学对处理的反应在 CR 和 CFFR 之间有所不同，且是短暂的，通常 pH 值、N 和一些阳离子下降，而铝（Al）和 DOC 显示出脉冲增加。通过去除杜鹃，剩余的落叶树可能会随着土壤水分可用性的增加而加速吸收 N。这可以部分解释为什么我们在处理后观察到低于预期的溪流养分（NO-N、Ca 和 Mg）。初始杜鹃林地上的采伐加上 CFFR 处理中 O 层不完全消耗，可能会导致上层土壤中 DOC 升高，并使 Al 移动。从管理的角度来看，在东部铁杉死亡后，使用这些处理方法来恢复河岸森林的结构和功能，无论是使用还是不使用火，都不太可能导致短期水质恶化，这种情况在采伐上层树木时很常见，甚至可能长期降低溪流中的 NO-N 浓度。
