Abstract
Whitebark pine (Pinus albicaulis Engelm.), an ecologically important tree species in high-elevation ecosystems of western North America, is threatened by white pine blister rust (Cronartium ribicola Fischer) and increased pressure from mountain pine beetle (Dendroctonus ponderosae Hopkins) due to climate warming. In addition, there is concern that fire suppression may be leading to successional replacement of whitebark by late-seral trees. Despite widespread knowledge that the tree is in decline, there is limited understanding of its successional dynamics, particularly in forests disturbed by white pine blister rust and mountain pine beetle. Our objective was to examine how disturbances have affected forest composition, structure, and seedling regeneration over a 22-year period (1990–2012) at 19 sites in the Cascade Mountains of Washington State (USA). Over that time, 13 sites (68%) were infected by white pine blister rust, 11 (58%) were disturbed by mountain pine beetle, and 5 (26%) experienced wildfire. Tree community composition changed significantly during the study period, primarily due to significant mortality of mature (≥20-cm diameter at breast height) whitebark pine. Despite loss of mature whitebark trees, we found little evidence of successional replacement by other tree species. Whitebark seedling density was unrelated to basal area of mature whitebark pine, but positively correlated with the presence of herb and shrub cover. Our results demonstrate the value of long-term repeated measurements for elucidating successional dynamics.
Highlights
IntroductionThere is concern that changes in disturbance regimes are contributing to successional replacement of whitebark pine by killing whitebark pine trees and facilitating recruitment of shade-tolerant species such as subalpine fir (Abies lasiocarpa (Hooker) Nuttall) and Engelmann spruce (Picea engelmannii Parry ex Engelm.)
Whitebark pine (Pinus albicaulis Engelm.), an ecologically important tree species [1] in high-elevation ecosystems of the western United States and Canada [2,3], is declining due to widespread outbreaks of a native insect, mountain pine beetle (Dendroctonus ponderosae Hopkins) [4], and an invasive fungal pathogen that causes white pine blister rust (Cronartium ribicola Fischer) [2].There is concern that changes in disturbance regimes are contributing to successional replacement of whitebark pine by killing whitebark pine trees and facilitating recruitment of shade-tolerant species such as subalpine fir (Abies lasiocarpa (Hooker) Nuttall) and Engelmann spruce (Picea engelmannii Parry ex Engelm.)
Range-wide, whitebark pine is facing novel conditions due to white pine blister rust, increased bark beetle presence, altered fire regimes, and climate change. These disturbance agents and their interacting effects will reduce the predictability of forest succession, especially given that, with climate change, patterns of disturbance from insects and disease on all tree species are likely to change [69] and wildfires are likely to grow larger and more intense [65]
Summary
There is concern that changes in disturbance regimes are contributing to successional replacement of whitebark pine by killing whitebark pine trees and facilitating recruitment of shade-tolerant species such as subalpine fir (Abies lasiocarpa (Hooker) Nuttall) and Engelmann spruce (Picea engelmannii Parry ex Engelm.). Increases in these shade-tolerant species may reduce the availability of canopy openings, which are thought to increase the likelihood of whitebark seedling recruitment [5,6,7,8,9].
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