Data from: Resilience and regime change in a southern Rocky Mountain ecosystem during the past 17000 years

Paleoecological records indicate that subalpine forests in western North America have been resilient in response to multiple influences, including severe droughts, insect outbreaks, and widely varying fire regimes, over many millennia. One hypothesis for explaining this ecosystem resilience centers on the disruption of forest dynamics by frequent disturbance and climatic variability, and the resulting development of non- steady-state regimes dominated by early-succession conifers with broad climatic tolerances, such as lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.). To evaluate this hypothesis, we independently reconstructed the vegetation, fire, and effective-moisture histories of a small, forested watershed at 2890-m elevation in southeastern Wyoming, using sedimentary pollen and charcoal counts in conjunction with sedimentary lake-level indicators. The data indicate that prominent vegetation shifts (from sagebrush steppe to spruce-fir parkland at ca. 10.7 ka and spruce-fir parkland to pine-dominated forest at ca. 8.5 ka) coincided with changes in effective moisture. However, after lodgepole pine forests established at ca. 8.5 ka, similar hydroclimatic changes did not produce detectable vegetation responses. Fire history data show that other aspects of the ecosystem were responsive to changes in effective moisture at centennial timescales with prolonged fire-free episodes coinciding with periods of low effective moisture ca. 7.2-5.6 and 3.7-1.6 ka. Throughout our record, the ratio of ecosystem perturbation time (i.e., fire frequency and changes in effective moisture) to recovery time (assuming 200-600 year successional processes) falls within estimates of the ratio for non-steady state ecosystems. Frequent perturbations, therefore, may have prevented this ecosystem from reaching compositional equilibrium with the varied climatic conditions over the past 8.5 ka. Equilibrium states could have included more abundant spruce (Picea spp.) and fir (Abies spp.) than presently observed based on brief increases in pollen abundances of these taxa during prolonged dry, fire-free intervals. Our results show that although current climate changes favor widespread disturbance in Rocky Mountain forests, the composition of these ecosystems could be highly resilient and recover through successional dynamics over the next few decades to centuries.

古生态学记录表明，北美西部的亚高山森林在数千年来，已对多重干扰因素展现出卓越的恢复韧性，这些因素涵盖极端干旱、虫害暴发以及高度多变的火制度（fire regime）。针对该生态系统恢复韧性的一种假说认为，频繁的干扰与气候变异会扰乱森林动态过程，进而形成以气候耐受范围较广的早演替针叶林为主导的非稳态格局（non-steady-state regimes），例如扭叶松（Pinus contorta var. latifolia Engelm. ex Wats.）。为验证该假说，我们通过沉积花粉、炭屑计数结合湖泊水位沉积指标，独立重建了怀俄明州东南部一处海拔2890米的小型森林集水区的植被、火历史与有效水分演化记录。数据显示，显著的植被转变——约10.7 ka时从蒿灌丛草原转向云杉-冷杉疏林，约8.5 ka时从云杉-冷杉疏林转为以松为主的森林——与有效水分的变化同步发生。然而，在约8.5 ka扭叶松森林建立后，类似的水文气候变化未引发可检测到的植被响应。火历史数据表明，该生态系统的其他组分在百年尺度上对有效水分变化存在响应：长期无火时段与约7.2~5.6 ka、3.7~1.6 ka的低有效水分时期相吻合。在本次重建的全部记录中，生态系统扰动时长（即火频率与有效水分变化）与恢复时长（假设演替过程耗时200~600年）的比值，处于非稳态生态系统的比值估算区间内。因此，频繁的干扰可能阻碍了该生态系统在过去8.5 ka内，达到与多变气候条件相匹配的群落组成平衡状态。基于长期干旱无火时段内这些类群的花粉丰度短暂上升，平衡状态下的云杉（Picea spp.）与冷杉（Abies spp.）丰度可能高于当前观测水平。本研究结果表明，尽管当前气候变化倾向于引发落基山脉森林的大范围干扰，但这些生态系统的群落组成仍具备极强的恢复韧性，可在未来数十年至数百年间通过演替动态完成恢复。