Washington State Department of Natural Resources, Box 47014, Olympia, Washington, 98506, USA.
School of Environmental & Forest Sciences, University of Washington, Seattle, Washington, 98195, USA.
Ecol Appl. 2020 Jan;30(1):e02013. doi: 10.1002/eap.2013. Epub 2019 Oct 30.
The natural range of variation (NRV) is an important reference for ecosystem management, but has been scarcely quantified for forest landscapes driven by infrequent, severe disturbances. Extreme events such as large, stand-replacing wildfires at multi-century intervals are typical for these regimes; however, data on their characteristics are inherently scarce, and, for land management, these events are commonly considered too large and unpredictable to integrate into planning efforts (the proverbial "Black Swan"). Here, we estimate the NRV of late-seral (mature/old-growth) and early-seral (post-disturbance, pre-canopy-closure) conditions in a forest landscape driven by episodic, large, stand-replacing wildfires: the Western Cascade Range of Washington, USA (2.7 million ha). These two seral stages are focal points for conservation and restoration objectives in many regions. Using a state-and-transition simulation approach incorporating uncertainty, we assess the degree to which NRV estimates differ under a broad range of literature-derived inputs regarding (1) overall fire rotations and (2) how fire area is distributed through time, as relatively frequent smaller events (less episodic), or fewer but larger events (more episodic). All combinations of literature-derived fire rotations and temporal distributions (i.e., "scenarios") indicate that the largest wildfire events (or episodes) burned up to 10 -10 ha. Under most scenarios, wildfire dynamics produced 5th-95th percentile ranges for late-seral forests of ~47-90% of the region (median 70%), with structurally complex early-seral conditions composing ~1-30% (median 6%). Fire rotation was the main determinant of NRV, but temporal distribution was also important, with more episodic (temporally clustered) fire yielding wider NRV. In smaller landscapes (20,000 ha; typical of conservation reserves and management districts), ranges were 0-100% because fires commonly exceeded the landscape size. Current conditions are outside the estimated NRV, with the majority of the region instead covered by dense mid-seral forests (i.e., a regional landscape with no historical analog). Broad consistency in NRV estimates among widely varied fire regime parameters suggests these ranges are likely relevant even under changing climatic conditions, both historical and future. These results indicate management-relevant NRV estimates can be derived for seral stages of interest in extreme-event landscapes, even when incorporating inherent uncertainties in disturbance regimes.
自然变异性范围（NRV）是生态系统管理的重要参考，但对于由不频繁、剧烈干扰驱动的森林景观，其范围却很少被量化。这些情况下的极端事件（如每隔几个世纪发生一次的大规模、连片的野火）很典型；然而，关于这些事件特征的数据本来就很稀少，而且对于土地管理来说，这些事件通常被认为太大且无法预测，无法纳入规划工作（俗语中的“黑天鹅”）。在这里，我们估计了由间歇性、大规模、连片野火驱动的森林景观中晚成（成熟/老龄）和早成（干扰后、林冠关闭前）条件的 NRV：美国华盛顿西部喀斯喀特山脉（270 万公顷）。这两个演替阶段是许多地区保护和恢复目标的重点。我们使用一种包含不确定性的状态和转换模拟方法，评估了在广泛的文献衍生输入下，NRV 估计值在以下方面的差异：（1）总体火灾周期，以及（2）火灾面积随时间的分布方式，是相对频繁的较小事件（非间歇性更强），还是较少但较大的事件（更具间歇性）。文献衍生的火灾周期和时间分布的所有组合（即“情景”）表明，最大的野火事件（或期）燃烧了多达 10 到 10 公顷。在大多数情景下，野火动态产生了晚成林的 5%至 95%分位数范围，约占该地区的 47%至 90%（中位数为 70%），结构复杂的早成林条件约占 1%至 30%（中位数为 6%）。火灾周期是 NRV 的主要决定因素，但时间分布也很重要，更具间歇性（时间上更集中）的火灾产生了更宽的 NRV。在较小的景观（20000 公顷；典型的保护区和管理区）中，范围为 0%至 100%，因为火灾通常超过景观规模。目前的情况超出了估计的 NRV，该地区大部分被茂密的中龄林覆盖（即，一个没有历史类似物的区域景观）。广泛的火灾制度参数的 NRV 估计值之间存在广泛的一致性，这表明即使在气候变化条件下，这些范围也可能是相关的，包括历史和未来的情况。这些结果表明，即使在纳入干扰制度固有不确定性的情况下，也可以为极端事件景观中的感兴趣的演替阶段得出具有管理意义的 NRV 估计值。
