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Facilitation and competition
Underlying the development of aspen forest communities are interactions between aspen and a broad suite of understory plant and tree species. These plant-plant interactions can be positive (facilitative) or antagonistic (competitive) in nature. Across much of its range, aspen is commonly found in association with conifers. Recent research in our lab demonstrates that fir seedlings rarely establish in open meadows but readily establish under adjacent aspen stands suggesting that aspen facilitates fir establishment. Further analysis showed that fir trees are not randomly distributed within stands but instead were strongly aggregated at the base of mature aspen trees Our principal goal on this project is to understand how facilitation and competition between aspen and conifers shape the development of these forest systems.
Spatial analysis showed that fir trees are not randomly distributed within stands but instead were strongly aggregated at the base of mature aspen trees (see photo), particularly on the north aspect. We found that 42% of subalpine fir seedlings establishing in aspen stands did so within 10 cm of the base of a mature aspen tree. These patterns were experimentally tested by placing fir seed at increasing distances from the base of mature aspen and subalpine fir trees in aspen, mixed and conifer stands. Subalpine fir seedling establishment was significantly greater in aspen stands and next to aspen trees (Buck and St.Clair, unpublished results).
This strong facilitative relationship increases the proximity of aspen and fir trees several fold. While the outcome of this relationship in early stages is clearly beneficial to the fir seedling and likely neutral for the mature aspen tree, the relationship changes as the firs rapidly increase in size. We found that close proximity of overstory aspen and maturing fir trees drove mortality patterns of the two species in opposite directions. Proximity drastically increased aspen mortality while increasing fir survival (Calder and St.Clair, 2012). [s1]
In western North America, aspen commonly grow along with conifers. While we don’t yet know the complete process that determines whether aspen maintains dominance in the forest or gives way to conifers, we do know that relatively frequent fire (40-80 years) removes conifers and triggers aspen re-growth. Fire creates young aspen stands that remain vigorous and healthy, but aspen stands by themselves generally won’t carry fire. They are dependent on the presence of more flammable conifers to burn. Longer periods between fire (150 or more years) favor conifers. These longer periods are what we are now experiencing due to climate conditions and fire suppression by humans
Conifer dominance in the West has important implications for hydrology. Up to 65% of water used in the western US comes from mountain snowpack. When snow falls in aspen stands, it reaches the ground. Conifers, on the other hand, keep their needles through winter and catch significant amounts of snow. This moisture is diverted back to the atmosphere resulting in lost water from the watershed. Utah is the second driest state in the US. With demands on water resources growing with the population, and drier conditions expected, water scarcity is a major environmental challenge, and losses of aspen cover adds another level of complexity.
Aspen is a primary host to well over 100 species of herbivores, including mammal, bird, and arthropod species. Although aspen has well-developed defense mechanisms, it is susceptible to herbivory. Herbivory by insects and ungulates can significantly reduce aspen vigor and can result in aspen mortality. Both experimental and long-term correlative studies indicate that browsing by livestock and wildlife can be a major barrier to successful aspen regeneration. The impact of browsing on aspen regeneration varies greatly depending on forest management practices, habitat conditions, and climate characteristics. While evidence suggests that both wildlife and livestock can have negative effects on aspen regeneration, relatively little research has been done that examines how browsing pressure influences the development and resilience of aspen forests.
Aspen communities are particularly vulnerable to ungulate browsing damage as young aspen stems emerge soon after fire. Current studies in the lab are examining how fire size and severity influence aspen susceptibility to browsing. Preliminary results indicate that aspen are less vulnerable to animal browsing when fires are larger and more severe.