Fire-mediated pathways of stand development in Douglas-fir/ western hemlock forests of the Pacific Northwest, USA.
Fire-mediated pathways of stand development in Douglas-fir/ western hemlock forests of the Pacific Northwest, USA.
复制标题
美国太平洋西北地区花旗松/西部铁杉林中火介导的林分发育途径。
作者：
A. Tepley;F. Swanson;T. Spies
美国太平洋西北部以道格拉斯冷杉和西部铁杉为主的森林极大地影响了有关保护老树森林的概念和政策。尽管人们注意到了它们的老生长特征，但仍有一种趋势是用相对简单的术语来看待它们的干扰生态，强调不频繁的、林分替代(SR)火以及相关的线性途径来发展这些老生长特征。利用俄勒冈州中西部梯级带124个林分的林分和年龄结构数据，构建了在该地区广泛使用的混合烈度火灾制度下林分发展的概念模型。描述耐荫和耐荫树种年龄分布的变量的系统聚类法确定了6个类群，代表了火灾频率和严重程度对林分发展的不同影响。在84%的林分中发现了有400年树龄的道格拉斯冷杉树，但自这些老树建立以来，只有18%的林分(15%)没有火灾的证据，而73%的林分显示至少有一次非林分替代(NSR)火灾的证据。火灾频率和严重程度的不同导致了当代林分结构和主要树种演替作用的多条发展路径和相关的变化。耐荫树种在SR火灾后形成一个队列，或每个林分最多四个队列，以应对反复发生的NSR火灾，这些火灾使活树的密度高达45株/公顷。在幸存的树木密度保持在60-65株/公顷的情况下，火灾后的队列只由耐荫树种组成。这项研究表明，该地区的火灾历史和老生林的发展比目前的林分发展模式要复杂得多，这对维护现有的老生林和恢复木材经营的林分具有重要意义。
Forests dominated by Douglas-fir and western hemlock in the Pacific Northwest of the United States have strongly influenced concepts and policy concerning old-growth forest conservation. Despite the attention to their old-growth characteristics, a tendency remains to view their disturbance ecology in relatively simple terms, emphasizing infrequent, stand-replacing (SR) fire and an associated linear pathway toward development of those old-growth characteristics. This study uses forest stand- and age-structure data from 124 stands in the central western Cascades of Oregon to construct a conceptual model of stand development under the mixed-severity fire regime that has operated extensively in this region. Hierarchical clustering of variables describing the age distributions of shade-intolerant and shade-tolerant species identified six groups, representing different influences of fire frequency and severity on stand development. Douglas-fir trees > 400 years old were found in 84% of stands, yet only 18% of these stands (15% overall) lack evidence of fire since the establishment of these old trees, whereas 73% of all stands show evidence of at least one non-stand-replacing (NSR) fire. Differences in fire frequency and severity have contributed to multiple development pathways and associated variation in contemporary stand structure and the successional roles of the major tree species. Shade-intolerant species form a single cohort following SR fire, or up to four cohorts per stand in response to recurring NSR fires that left living trees at densities up to 45 trees/ha. Where the surviving trees persist at densities of 60-65 trees/ha, the postfire cohort is composed only of shade-tolerant species. This study reveals that fire history and the development of old-growth forests in this region are more complex than characterized in current stand-development models, with important implications for maintaining existing old-growth forests and restoring stands subject to timber management.