Timberline expansion associated with climate change facilitated by microsites

Background/Question/Methods Upward advance of timberlines, associated with climate warming, is occurring in the Pacific Northwest as well as many other mountainous regions of the world.  Forest advance may be enhanced by available microsites assuming minimal affects of orographic disturbance.  Growing on favorable microsites helps seedlings cope with stresses that exist at high elevation sites including wind, cold temperatures, high radiation, drought, animal predation, and infestation by fungal pathogens in snow and soil. Wood substrates called nurse logs, known as key sites of forest regeneration in lower elevation temperate forests, may be important microsites for seedling establishment along advancing timberlines. If large trees at the edge of timberline fall into alpine meadows, they may decay providing sites for seedling establishment. To determine factors associated with seedling establishment and survival on nurse logs at timberline, 14 sites, randomly located across a precipitation gradient in the Washington North Cascades Mountains, were examined.  At each site, one - 60 m transect was made along the forest – meadow edge of timberline noting number of wood pieces, wood decay class, disturbance mechanism introducing woody debris, wood diameter, wood length, seedling height, slope gradient, one of three shade categories, and slope aspect. Near seedling locations on logs and adjacent soils, temperature was measured and substrate samples were taken to determine total organic carbon, total nitrogen, and percent water content.  In addition, seedling density on and off nurse logs was determined for a subset of six locations representing seedling regeneration at the highest timberlines locations having nurse logs, least snow avalanche activity, and no obvious disturbance by recent fires. 
Results/Conclusions Nurse logs were found at 13 out of 14 sites; shady locations associated with highly decayed downed wood originating from blowdown, snow avalanches, and forest fires.  Although nitrogen content of nurse logs and adjacent soil was not significantly different (P = 0.98), higher temperatures (p = 0.015) and higher moisture contents (p = 0.019) were found on nurse logs as compared to the adjacent soil. Seedling densities on nurse logs ranged from 1.6 to 6.7 (mean = 2.6), as compared to the adjacent soil with seedling densities of 0.5 to 8.8 (mean = 3.2), and indicated no significant difference per substrate type (p = 0.93).  Given predictions of climate warming and associated summer drought conditions in Pacific Northwest forests, moisture provided by nurse logs may be critical for conifer survival and subsequent timberline expansion in some landscapes.