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.

干扰可通过造成大范围生物死亡、启动演替进程推动生态系统快速变化；在气候变化时期，干扰还可通过启动新的演替路径，促使生态系统发生状态转变。在北美太平洋西北岸（Pacific Northwest of North America, PNW），野火干扰强烈塑造了低地森林的群落组成与结构，但厘清气候变化时期火的作用颇具挑战——因为火轮回期极长（可达数千年），且占优势的针叶树种寿命可达数百年。本研究构建了覆盖近14000年的林分尺度植被与野火古记录，以探究林分尺度（1–3公顷）下野火与森林植被状态转变及快速动态的关联机制。我们以美国华盛顿州西部普吉特低地生态区的西部铁杉（Tsuga heterophylla）带内两处相距约1公里的小型洼地沉积物岩芯为研究材料，重建了马克沃思州立森林（Marckworth State Forest）的森林历史。两处样点的孢粉组合变化中值速率相近（分别为0.12%/年与0.14%/年），但均在野火事件后发生显著上升，变化速率最高可达1%/年，中值分别为0.28%/年与0.38%/年（P < 0.003）。在气候迁移速率较低的时期，森林群落组成对野火具有恢复力，野火启动的演替进程最终可回归至原优势植被类型；与之相反，在气候变异性与迁移速率较高的时期（如全新世早期），森林群落对野火不再具备恢复力，野火会触发大规模的生态系统状态转变。本研究的古记录清晰表明，单次野火事件这类干扰，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尺度气候变化的更替进程。