Timberline Forest Upward Advance Facilitated by Moisture and Disturbance
Abstract
Although climate warming is generally expected to facilitate timberline upward advance, tree regeneration will be hindered by low substrate moisture, high radiation, and both low and high snow accumulation. To better predict factors promoting regeneration in the alpine treeline ecotone (ATE), this study examined microsites at timberline-alpine meadow borders both regionally (Pacific Northwest, PNW) and globally. In the PNW, 14 sites each having three levels of incoming radiation were randomly selected along a west-east decreasing precipitation gradient. Associations among forest disturbance, tree regeneration, radiation, precipitation, site moisture, and plant stomatal conductance were assessed. Globally, relationships among annual precipitation, annual temperature, and dominant species were evaluated for five microsites types. In the PNW, highest seedling densities at timberline-alpine meadow borders were associated with sites having greatest volumetric water content (VWC), locations typically associated with downed highly decayed wood. Wood microsites had greater seedling survival, greater temperature, and greater number of growing degree hours, as compared to adjacent soils. Greater seedling densities were positively associated with VWC > 12%, conditions most commonly associated with wood substrate presence. For sites having > 25% percent transmitted radiation, positive relationships existed between stomatal conductance and VWC. Global temperature increases, associated with both increases in rainfall amount and drought occurrence, are likely to increase importance of wood microsites at timberline-alpine meadow and timberline-grassland locations with mean annual temperatures < 5°C. We found that ATE wood microsites are common globally and are typically associated with blowdown, but are currently under-recognized as an important factor for tree regeneration above timberline. World-wide, wood microsites had annual precipitation from 86 cm to 320 cm and annual temperatures from 1.5°C to 5.0°C. Temperatures > 5°C are likely to increase the role of shade-related microsites for successful conifer regeneration. Wood microsites role with warming climate will depend on precipitation pattern, timing, magnitude, and frequency.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H23C1274J
- Keywords:
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- 1813 HYDROLOGY Eco-hydrology;
- 1630 GLOBAL CHANGE Impacts of global change;
- 1851 HYDROLOGY Plant ecology;
- 1852 HYDROLOGY Plant uptake