Estimates of White Spruce density at two elevations from 1600-present

Paleoecological investigations of ecosystem responses to past climate change can provide insight into plausible scenarios of response to future change and can elucidate factors that may influence the overall predictability of such responses. There is particular interest in the rate at which subarctic Picea populations respond to climate change, as the rate of conversion from tundra to forest may affect the rate of future climate warming. We investigated recent changes in the structure and distribution of treeline white spruce forests in the White Mountains and the Alaska Range. Treeline advance was ubiquitous, but asynchronous in time, occurring significantly earlier in the White Mountains in interior Alaska than in the Alaska Range. The mean lag between initiation of recruitment and forest development was estimated at approximately 200 years, similar to what modeling studies have found. Although continued advance of Picea is the most likely scenario of future change, the recent history of treeline advance suggests that nonlinear responses to warming may be likely due to nonlinear growth responses of individual trees to temperature, limitation of Picea establishment in highly permafrost-affected sites, and decreasing seed availability as the advancing front moves farther from dense seed sources. All of these factors may cause Picea populations to respond nonlinearly to future warming, and these potential nonlinearities caution against uncritical extrapolation from recent trends.古生态学研究针对过去气候变化下的生态系统响应展开，可为未来气候变化下生态系统的合理响应情景提供科学洞见，并阐明可能影响此类响应整体可预测性的关键因素。其中，亚北极云杉（Picea）种群对气候变化的响应速率尤为受学界关注，因为苔原向森林的转换速率可能会影响未来气候变暖的进程。本研究针对怀特山脉与阿拉斯加山脉范围内的林线白云杉林的结构与分布的近期变化开展了调查。结果显示，林线扩张现象普遍存在，但时间上存在异步性：阿拉斯加内陆的怀特山脉的林线扩张启动时间显著早于阿拉斯加山脉。研究估算得出，从种群更新启动到森林发育完成的平均滞后期约为200年，这与相关模型研究的结果一致。尽管云杉持续扩张是未来变化最可能的情景，但林线扩张的近期历史表明，树木对升温的非线性响应或大概率出现——这一现象源于单株树木生长对温度的非线性响应、云杉在受永久冻土强烈影响的生境中定植受限，以及随着林线扩张前沿远离密集种源区而导致的种子可获得性下降。上述所有因素均可能导致云杉种群对未来变暖呈现非线性响应，而这些潜在的非线性特征警示我们，不可盲目依据近期趋势进行外推预测。