Influence of silvicultural treatments, overstory, and understory vegetation on quaking aspen (Populus tremuloides) regeneration in southeastern Idaho Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/p2677043d

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  • Quaking aspen (Populus tremuloides Michx.) is known to be a widely distributed, shade-intolerant and short-lived hardwood found in both seral, even-aged and stable, uneven aged stands. There have been reports of extensive aspen mortality, crown thinning, and branch dieback across North America that have been linked to the occurrence of severe droughts since 2001-2002. Because of reports of low aspen regeneration across the Intermountain West, as well as predictions of increases in aspen regeneration in the Northeastern US, researchers and land managers have now focused on managing aspen stands under the assumption that there are multiple aspen types. They have focused on improving resilience within quaking aspen stands with changing ecological conditions. For this thesis I focused on a project the Bureau of Land Management (BLM), Pocatello Field Office initiated in part to improve aspen restoration and resilience of stands in Soda Springs, ID. The BLM conducted two mechanical removal treatments: cut and pile, and slash/lop and scatter. In addition several sites were broadcast burned to reduce fuel loads and conifer density, to enhance aspen regeneration and improve aspen stand resilience. According to the Soda Springs Hills Fuels Reduction and Ecosystem Restoration Environmental Assessment (EA), the BLM aimed to meet the objective of 2500 quaking aspen suckers per ha (1000 suckers per ac) within the two years following treatment, an index of treatment adequacy. My primary objective for this thesis was to assess the influence of each silvicultural treatment, including the change in overstory and understory vegetation, on regeneration of aspen. Mean aspen regeneration two growing seasons after treatment was 11,532 suckers/ha on sites that received slash/lop and scatter treatment, followed by broadcast burning. With these high levels of suckering, there were also low densities of residual overstory conifers (≤ 4 trees/ha with a basal area ≤ 2 m²/ha). In comparison, sites that received the cut and pile treatment followed by a broadcast burn had a mean aspen regeneration of 44 suckers/ha, with higher densities of overstory conifers (≥ 32 trees/ha with a basal area ≥ 26 m²/ha). In slash/lop and scatter treatments without burning, sucker densities were as high as 1117 suckers/ha with low densities of conifers (0 trees/ha). In comparison, the site that received the cut and pile treatment without burning had an aspen regeneration of 0 suckers/ha, with a high density of conifers (36 trees/ha with a basal area of 47 m²/ha). Overall, sites with low residual overstory cover of large conifer trees (< 4 trees/ha), regardless of the treatment, had higher sucker densities two growing seasons after treatment (6244 suckers/ha, on average) than those seen in sites with a remnant overstory of >16 trees/ha (29 suckers/ha, on average). Also, sites that were burned, regardless of the mechanical treatment used, had higher sucker densities (11,244 suckers/ha) than those seen in sites that were not burned (576 suckers/ha). When comparing aspen sucker densities to competing understory woody cover following mechanical treatment, aspen sucker density was lowest (411 suckers/ha) on the site where both tree and shrub percent cover were highest (10 and 16%, respectively). Suckering appeared to be positively correlated with grass cover, however, with as high as 1117 suckers/ha growing with a high percentage of grass cover (≥ 26 %), on sites measured for change in understory following mechanical treatment. Results were collected on a small number of sites and thus have limited statistical significance. However, we are confident that observed trends have values for managers. We suggest that transects should continue to be monitored to observe the long-term effects of silvicultural treatments on overstory and understory vegetation, which are likely to be influenced by climate variability and other disturbances into the future.
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