Reforestation practices and microsite effects on the performance of contrasting sympatric tree species: a case-study for adaptive silviculture
Date
2019
Authors
Hill, Edward, author
Ex, Seth, advisor
Nagel, Linda, committee member
Fonte, Steven, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
In the central-southern Rocky Mountain region, warming climate in low-elevation Engelmann spruce forests may limit future viability of spruce but favor sympatric species like ponderosa pine, prompting consideration of both species in adaptive reforestation efforts. We used a planting experiment to systematically evaluate survival and root growth of these contrasting species to microsite conditions resulting from silvicultural regeneration treatments in a spruce forest on the Uncompahgre Plateau, Colorado. Our assessment targeted the effects of varying levels of canopy cover generated by different regeneration treatments, paired with and without microsite shelter from coarse woody debris. For explaining survival, we also considered the potential for covarying microsite influences of vegetation, soil, or litter cover, soil moisture and depth, surrounding natural tree regeneration, and seedling size. Survival of pine was twice that of spruce, but the relative effects of microsite variables were similar, possibly due to the severe drought during our study and the stress of first-season establishment. Coarse woody debris shelter benefitted survival of both species, likely from shading of succulent stem tissue and improved soil moisture retention. Influences of canopy cover were comparatively indistinct overall, which may reflect reduced capacity for temperature and moisture buffering on extremely dry sites. Survival was also strongly negatively affected by seedling height, suggesting a potential benefit of lower above-to-belowground biomass ratios for establishment in stressful environments. Root growth was seemingly limited by light for both species in the most dense, unharvested canopy environments, and for pine in coarse woody debris shelter. Our results imply that adaptive reforestation efforts in similar environments should consider more drought-adapted, sympatric species as viable alternatives or supplements to moisture-dependent species at their current lower range limits. Additionally, our results show that first-season seedling survivorship is strongly dependent on facilitating influences of adjacent, non-living shelter, especially compared to canopy cover effects, and seedling development prior to outplanting, favoring lower shoot-to-root ratios.