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.

干扰可通过引发广泛的种群死亡、启动演替进程来催化生态系统的快速变化；在气候变化时期，干扰还可通过启动全新的演替路径，推动生态系统状态发生转变。北美太平洋西北地区（Pacific Northwest of North America, PNW）内，野火干扰强烈塑造了低地森林的群落组成与结构，但厘清气候变化背景下野火的作用颇具挑战——这是因为火灾轮回间隔极长（可达数千年），且占优势的针叶树种寿命可达数百年之久。本研究构建了覆盖近14000年的林分尺度植被与火灾古记录，以探究林分尺度（1~3公顷）下火灾与森林植被状态转变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的西方铁杉（Tsuga heterophylla）带内两处相距约1公里的小型洼地沉积岩芯为材料，重建了马克沃思州立森林（Marckworth State Forest）的森林历史。两处样点的花粉/孢子组合变化中值速率相近（分别为0.12%/年与0.14%/年），但在火灾发生后，两处样点的植被变化速率均显著提升（最高可达1%/年，中值分别为0.28%/年与0.38%/年，P < 0.003）。当气候移动速率较低时，森林群落组成对野火具有恢复力，火灾启动的演替进程最终会回归至原优势植被类型。与之相反，在气候变异性与移动速率较高的时期（如早全新世），森林群落对野火不再具备恢复力，火灾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：以单次野火事件为代表的干扰，可作为快速生态系统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。