Abstract
Aspen ecosystems (upland Populus-dominated forests) support diverse species assemblages in many parts of the northern hemisphere, yet are imperiled by common stressors. Extended drought, fire suppression, human development, and chronic herbivory serve to limit the sustainability of this keystone species. Here we assess conditions at a renowned quaking aspen (Populus tremuloides) grove—purportedly the largest living organism on earth—with ramifications for aspen biogeography globally. The “Pando” clone is 43 ha and estimated to contain 47,000 genetically identical aspen ramets. This iconic forest is threatened in particular by herbivory, and current management activities aim to reverse the potential for type conversion, likely to a non-forest state. We set out to gauge agents affecting recent deterioration through a network of monitoring plots and by examining a chronosequence of historic aerial photos to better understand the timing of putative departure from a sustainable course. Sixty-five permanent forest monitoring plots were located in three management regimes existing within Pando: no fencing, fencing with active and passive treatments, fencing with passive-only treatment. At each sample plot we measured live and dead mature trees, stem recruitment and regeneration, forest and shrub cover, browse level, and feces counts as a surrogate for ungulate presence. Ordination results indicate that aspen regeneration was the strongest indicator of overall forest conditions at Pando, and that mule deer (Odocoileus hemionus) presence strongly impacts successful regeneration. Additionally, fencing with active/passive treatments yielded the most robust regeneration levels; however, a fence penetrable by ungulates in the passive-only treatment most likely played a role in this outcome. The aerial photo sequence depicts various human intrusions over the past seven decades, but perhaps most telling, a decline in self-replacement beginning 30–40 years ago. Aspen communities in many locations in North American and Europe are impacted by unchecked herbivory. The Pando clone presents a unique opportunity for understanding browse mechanisms in a forest where tree genotype, closely aligned with growth and chemical defense, is uniform.
Highlights
Aspen forests ( Populus tremuloides, P. tremula) are among the most widespread tree systems in the world, yet their sustainability is threatened by human-induced impacts such as warming climates, development, fire suppression, and unchecked herbivory
In a North American management context, these elements—forestry, wildlife, disturbance ecology—are often addressed within distinct disciplinary “silos.” This situation constitutes a challenge for sustainable stewardship in the face of expected near-term climate warming and altered precipitation patterns leading to extended seasonal droughts [10]
Other than the road alignment through the study area, which remains constant throughout, few of the forest openings found at the end of the sequence (2011) are evident in the 1939 photo
Summary
Aspen forests ( Populus tremuloides, P. tremula) are among the most widespread tree systems in the world, yet their sustainability is threatened by human-induced impacts such as warming climates, development, fire suppression, and unchecked herbivory. Much of the research focus across the western U.S addressed Rocky Mountain Elk (Cervus elaphus) browsing, with less energy examining long-term impacts of mule deer (Odocoileus hemionus) on aspen systems [11]. Where these species co-exist, often alongside domestic cattle (Bos spp.) or sheep (Ovis spp.), elk seem to have greater impacts [15,16], perhaps because their dietary needs are seasonally more flexible than other regional ungulates [17]. When focusing on only deer and domestic cattle, both species favor browsing aspen suckers late in the season when other forage senesces, deer will generally avoid cattle when the two comingle [18]
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