Data from: Fire catalyzed rapid ecological change in lowland coniferous forests of the Pacific Northwest over the past 14,000 years

Disturbance can catalyze rapid ecological change by causing widespread mortality and initiating successional pathways, and during times of climate change, disturbance may contribute to ecosystem state changes by initiating a new successional pathway. In the Pacific Northwest of North America (PNW), disturbance by wildfires strongly shapes the composition and structure of lowland forests, but understanding the role of fire over periods of climate change is challenging, because fire-return intervals are long (e.g., millennia) and the coniferous trees dominating these forests can live for many centuries. We developed stand-scale paleorecords
of vegetation and fire that span nearly the past 14,000 yr to study how fire was associated with state changes and rapid dynamics in forest vegetation at the stand scale (1–3 ha). We studied forest history with sediment cores from small hollow sites in the Marckworth State Forest, located ~1 km apart in the Tsuga heterophylla Zone in the Puget Lowland ecoregion of western Washington, USA. The median rate of change in pollen/spore assemblages was similar between sites (0.12 and 0.14% per year), but at both sites, rates of change increased significantly following fire events (ranging up to 1% per year, with a median of 0.28 and 0.38%, P < 0.003). During times of low climate velocity, forest composition was resilient to fires, which initiated successional pathways leading back to the dominant vegetation type. In contrast, during times of high climate variability and velocity (e.g., the early Holocene) forests were not resilient to fires, which triggered large-scale state changes. These records provide clear evidence that disturbance, in the form of an individual fire event, can be an important catalyst for rapid state changes, accelerating vegetation shifts in response to large-scale climate change.

干扰可通过引发大范围植被死亡、启动生态演替路径，催化生态系统快速变化；在气候变化背景下，干扰还可通过开启新的演替路径，推动生态系统状态发生转变。北美太平洋西北地区（Pacific Northwest, PNW）内，野火干扰强烈塑造着低地森林的群落组成与结构特征，但理解气候变化时期野火的生态作用仍颇具挑战——这是因为火轮回期极长（可达数千年），且占优该类森林的针叶树寿命可达数百年之久。我们构建了覆盖近14000年的林分尺度（stand-scale）植被与野火古记录（paleorecord），以探究林分尺度（1~3公顷）下野火与森林植被状态变化及快速动态的关联机制。本研究以美国华盛顿州西部普吉特低地生态区西部铁杉（Tsuga heterophylla）林带内、相距约1公里的两处小型洼地沉积物岩芯（sediment core）为研究材料，对马克沃思州立森林（Marckworth State Forest）的森林历史展开分析。两地的孢粉组合（pollen/spore assemblage）变化中位速率相近（分别为每年0.12%与0.14%），但野火事件发生后，两地的植被变化速率均出现显著提升（最高可达每年1%，中位速率分别为0.28%与0.38%，P < 0.003）。在气候变异性与气候移动速率（climate velocity）较低的时期，森林群落对野火具备恢复力，野火启动的演替路径最终可回归至优势植被类型；而在气候变异性与气候移动速率较高的时期（如早全新世（Holocene）），森林对野火缺乏恢复力，野火会触发大规模的生态系统状态转变。本研究记录清晰表明，以单次野火事件形式存在的干扰，可作为快速生态状态转变的重要催化剂，加速植被群落响应大范围气候变化的更替过程。