This research aimed to determine whether wood microsites ("nurse logs"), which are regeneration sites in Pacific Northwest (PNW) subalpine forests, supported regeneration at timberline-alpine meadow borders. Upward advance of forests and conifer invasion into alpine meadows, which may be occurring in conjunction with climate warming, have gained worldwide attention. Successful alpine meadow seedling regeneration depends on suitable substrate availability, or microsites, for seedling establishment. To better understand factors associated with wood microsite occurrence, mechanisms of wood input were determined and four specific hypotheses were posed to assess: (1) seedling density and seedling survival; (2) growing season length, summer mean growing temperature, and growing degree hours (GDH); (3) active measures of seedling growth; and (4) global wood microsite climate associations.
Of four studies, three were conducted in the Cascade Mountains of Washington state along a west - east precipitation gradient and one study, assessed various microsites globally. For Cascades-related research, wood and adjacent soil substrate temperature, moisture, and associated seedling density, survival, stomatal conductance, water potential, and leaf nitrogen were compared by percent transmitted radiation at 4 to 14 study sites. Analysis of variance (ANOVA), t-tests, regressions, and classification and regression trees (CARTs) were used to assess significance of comparisons.
Wood microsites, common at 13 of 14 random Cascade sites, had greater seedling densities, greater seedling survival, greater volumetric moisture content (VWC), greater temperature, and greater number of GDH, 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. Globally, high-elevation forests with wood microsites had mean annual precipitation from 86 cm to 320 cm and mean annual temperatures from 1.5°C to 4.7°C.
In general, wood microsites facilitated alpine meadow regeneration better than adjacent soils. Management implications included enhanced understanding of factors associated with upward forest advance and wood use for restoration. Globally, wood microsites importance is likely underrepresented. Wood microsites role with warming climate will depend on precipitation pattern, timing, magnitude, and frequency.
| Identifer | oai:union.ndltd.org:pdx.edu/oai:pdxscholar.library.pdx.edu:open_access_etds-2055 |
| Date | 07 August 2013 |
| Creators | Johnson, Adelaide Chapman |
| Publisher | PDXScholar |
| Source Sets | Portland State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations and Theses |
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