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Complex mountain terrain and disturbance history drive variation in forest aboveground live carbon density in the western Oregon Cascades USA

机译：美国俄勒冈西部小瀑布的复杂山区地形和干扰历史驱动着森林地上活碳密度的变化

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Forest carbon (C) density varies tremendously across space due to the inherent heterogeneity of forest ecosystems. Variation of forest C density is especially pronounced in mountainous terrain, where environmental gradients are compressed and vary at multiple spatial scales. Additionally, the influence of environmental gradients may vary with forest age and developmental stage, an important consideration as forest landscapes often have a diversity of stand ages from past management and other disturbance agents. Quantifying forest C density and its underlying environmental determinants in mountain terrain has remained challenging because many available data sources lack the spatial grain and ecological resolution needed at both stand and landscape scales. The objective of this study was to determine if environmental factors influencing aboveground live carbon (ALC) density differed between young versus old forests. We integrated aerial light detection and ranging (lidar) data with 702 field plots to map forest ALC density at a grain of 25 m across the H.J. Andrews Experimental Forest, a 6369 ha watershed in the Cascade Mountains of Oregon, USA. We used linear regressions, random forest ensemble learning (RF) and sequential autoregressive modeling (SAR) to reveal how mapped forest ALC density was related to climate, topography, soils, and past disturbance history (timber harvesting and wildfires). ALC increased with stand age in young managed forests, with much greater variation of ALC in relation to years since wildfire in old unmanaged forests. Timber harvesting was the most important driver of ALC across the entire watershed, despite occurring on only 23% of the landscape. More variation in forest ALC density was explained in models of young managed forests than in models of old unmanaged forests. Besides stand age, ALC density in young managed forests was driven by factors influencing site productivity, whereas variation in ALC density in old unmanaged forests was also affected by finer scale topographic conditions associated with sheltered sites. Past wildfires only had a small influence on current ALC density, which may be a result of long times since fire and/or prevalence of non-stand replacing fire. Our results indicate that forest ALC density depends on a suite of multi-scale environmental drivers mediated by complex mountain topography, and that these relationships are dependent on stand age. The high and context-dependent spatial variability of forest ALC density has implications for quantifying forest carbon stores, establishing upper bounds of potential carbon sequestration, and scaling field data to landscape and regional scales.
机译：由于森林生态系统固有的异质性，森林碳（C）密度在整个空间中变化很大。森林C密度的变化在山区地形中尤为明显，在该山区，环境梯度被压缩并在多个空间尺度上变化。此外，环境梯度的影响可能随森林年龄和发展阶段而变化，这是一个重要的考虑因素，因为森林景观通常具有过去管理和其他干扰因素带来的不同林分年龄。在山区地形中量化森林碳密度及其潜在的环境决定因素仍然充满挑战，因为许多可用的数据源都缺乏林分和景观尺度所需的空间粒度和生态分辨率。这项研究的目的是确定影响年轻地和老森林的地上活碳（ALC）密度的环境因素是否不同。我们将航空光检测和测距（激光）数据与702个场区图进行了集成，以绘制横跨美国俄勒冈州卡斯卡德山脉6369公顷流域的H.J.安德鲁斯实验森林的25 m处森林ALC密度的地图。我们使用线性回归，随机森林集成学习（RF）和顺序自回归建模（SAR）来揭示映射的森林ALC密度与气候，地形，土壤和过去的干扰历史（木材采伐和野火）之间的关系。在年轻的人工林中，ALC随林分年龄的增加而增加，而与旧的非人工林中的野火以来相比，ALC的变化更大。木材采伐是整个流域中ALC的最重要驱动力，尽管仅占整个景观的23％。在年轻的可管理森林模型中，解释了森林ALC密度的变化要大于旧的未管理森林模型。除林分年龄外，年轻人工林中ALC密度还受影响站点生产力的因素驱动，而老龄非林中ALC密度的变化也受到与庇护所相关的更精细地形条件的影响。过去的野火对当前ALC密度的影响很小，这可能是起火时间长和/或非台架替代火的流行。我们的结果表明，森林ALC密度取决于复杂的山地地形介导的一系列多尺度环境驱动因素，并且这些关系取决于林分年龄。森林ALC密度的高且取决于上下文的空间变异性对量化森林碳储量，建立潜在碳固存的上限以及将田间数据缩放到景观和区域尺度具有影响。

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