Department of Biology, Virginia Commonwealth University, Box 842012, 1000 West Cary Street, Richmond, Virginia, 23284, USA.
Department of Civil, Environmental and Geodetic Engineering, Ohio State University, 2070 Neil Avenue, Columbus, Ohio, 43210, USA.
Ecol Appl. 2021 Oct;31(7):e02417. doi: 10.1002/eap.2417. Epub 2021 Aug 11.
Many secondary deciduous forests of eastern North America are approaching a transition in which mature early-successional trees are declining, resulting in an uncertain future for this century-long carbon (C) sink. We initiated the Forest Accelerated Succession Experiment (FASET) at the University of Michigan Biological Station to examine the patterns and mechanisms underlying forest C cycling following the stem girdling-induced mortality of >6,700 early-successional Populus spp. (aspen) and Betula papyrifera (paper birch). Meteorological flux tower-based C cycling observations from the 33-ha treatment forest have been paired with those from a nearby unmanipulated forest since 2008. Following over a decade of observations, we revisit our core hypothesis: that net ecosystem production (NEP) would increase following the transition to mid-late-successional species dominance due to increased canopy structural complexity. Supporting our hypothesis, NEP was stable, briefly declined, and then increased relative to the control in the decade following disturbance; however, increasing NEP was not associated with rising structural complexity but rather with a rapid 1-yr recovery of total leaf area index as mid-late-successional Acer, Quercus, and Pinus assumed canopy dominance. The transition to mid-late-successional species dominance improved carbon-use efficiency (CUE = NEP/gross primary production) as ecosystem respiration declined. Similar soil respiration rates in control and treatment forests, along with species differences in leaf physiology and the rising relative growth rates of mid-late-successional species in the treatment forest, suggest changes in aboveground plant respiration and growth were primarily responsible for increases in NEP. We conclude that deciduous forests transitioning from early to middle succession are capable of sustained or increased NEP, even when experiencing extensive tree mortality. This adds to mounting evidence that aging deciduous forests in the region will function as C sinks for decades to come.
许多北美东部的次生落叶林正接近一个过渡阶段，在此阶段中，成熟的早期演替树种正在减少，这使得这个拥有一个世纪之久的碳（C）汇的未来充满不确定性。我们在密歇根大学生物站启动了森林加速演替实验（FASET），以研究在 >6700 株早期演替的杨树（如白杨）和桦木（纸皮桦）因树干环剥导致死亡后，森林 C 循环的模式和机制。自 2008 年以来，我们一直在基于气象通量塔的 33 公顷处理林进行 C 循环观测，并将其与附近未受干扰的森林进行配对。经过十多年的观测，我们重新审视了我们的核心假设：由于树冠结构复杂性的增加，在向中晚期演替物种主导的过渡之后，净生态系统生产力（NEP）将增加。支持我们的假设，在干扰后的十年中，NEP 保持稳定，短暂下降，然后相对于对照增加；然而，不断增加的 NEP与上升的结构复杂性无关，而是与中晚期演替的 Acer、栎属和松属快速恢复到树冠主导地位的总叶面积指数相关。向中晚期演替物种主导的过渡改善了碳利用效率（CUE=NEP/总初级生产力），因为生态系统呼吸下降。对照和处理林中相似的土壤呼吸速率，以及叶片生理学上的物种差异和处理林中中晚期演替物种相对生长率的上升，表明地上植物呼吸和生长的变化是 NEP 增加的主要原因。我们得出结论，从早期演替向中期演替过渡的落叶林即使经历了广泛的树木死亡，也能够保持或增加 NEP。这增加了越来越多的证据表明，该地区老化的落叶林在未来几十年内将作为碳汇发挥作用。
