Multi-scale subalpine forest dynamics, Rocky Mountain National Park, Colorado.

摘要

In the present study I examine fire history and fire regime variations at multiple spatial and temporal scales, fire-climate relationships, for subalpine forests of Engelmann spruce-subalpine fir (Picea engelmannii, Abies lasiocarpa) and lodgepole pine (Pinus contorta), and the stand level effects of surface fires, Mountain Pine Beetle (MPB) outbreaks, and wind blowdowns in lodgepole pine forests of the southern Rocky Mountains. The study area for this investigation is the subalpine zone of spruce-fir and lodgepole pine forests in the southern sector of Rocky Mountain National Park (ROMO), which straddles the continental divide of the northern Colorado Front Range. I used a combination of dendroecological and Geographic Information System methods to reconstruct fire history, and dendroecological methods to reconstruct stand level dynamics.; The fire histories covered c. 30,000 ha of forest and were based on a total of 676 partial cross-sections of fire-scarred trees and 6,152 tree-core age samples. The subalpine forest fire regime of ROMO is dominated by infrequent, extensive, stand-replacing fire events, whereas surface fires affected only 1 to 3% of the forested area. The fire regime varies at the local scale within drainages, and across the continental divide. However, the primary driver of fire occurrence is regional scale climate variability. Fire occurrence is related to severe regional drought conditions. Fire occurrence at interannual and centennial timescales is also strongly related to broadscale climate drivers in the Pacific and Atlantic Oceans, and phase combinations of broad-scale climate drivers.; The investigation into the effects of surface fire, MPB, and blowdown included c. 3,000 tree-cores age samples from 33 sites. The results demonstrate that surface fires do not have any detectable influence on lodgepole pine stand characteristics. In contrast, MPB, blowdown greatly alter lodgepole pine stand characteristics. The effect of both MPB and blowdown on stands is contingent on the severity and timing of disturbance in relation to the time since the last stand-replacing fire event. In general, high severity events in relatively young stands resulted in new lodgepole pine cohorts establishing, and less severe events in older stands resulted in a shift in species composition to subalpine fir.