Assessing conifer seedling regeneration on two substrates at timberline-alpine meadow borders

Background/Question/Methods

Seedling regeneration in alpine meadows may be enhanced by available microsites.  Microsites enhance growing season by facilitating snow melt and by providing additional warmth.  Wood microsites, known as nurse logs, form when large trees at the edge of timberline fall into alpine meadows, and subsequently decay.  To discern possible advantages of wood substrates over adjacent soil substrates, we examined substrate temperature pattern. Specifically, for wood and soil substrates, each having three levels of overstory canopy at timberline-alpine meadow sites, four comparisons were made: (1) conifer seedling density, (2) survival of seedlings, (3) timing of spring thaw, and (4) mean summer temperature.

For six study sites at timberline-alpine meadow borders in the North Cascades of Washington State, seedling density was compared on wood and soil.  Air, soil, and wood temperature measurement were made every 4.2 hours for two years.  Temperature monitors were installed in the midst of seedlings at depths of 6 cm under areas categorized as having low, medium, and high overstory canopy (means of 68, 79, and 88%, respectively). 

Results/Conclusions

Seedling density was significantly greater on wood substrates than on soil substrates (p = 0.026, with mean density of 6.85 vs. 3.64 seedlings per m², respectively).  For sites with low levels of overstory canopy, seedling survival was slightly greater on wood microsites than on soil substrates (p = 0.04, 37% vs. 35% survival, respectively).

In general, soil substrates thawed several days earlier than wood microsites (thaw occurring mid-June in 2010, mid-July in 2011).  Mean summer substrate temperature was significantly higher for wood microsites than adjacent soil substrates (p < 0.001, 6.79 vs. 6.66^◦C).  Post-hoc tests (all with p < 0.001) indicate wood temperature was significantly greater than soil substrates for sites with low overstory (7.39 vs. 7.00^◦C) and medium overstory canopy (6.86 vs. 6.48^◦C), whereas in areas of high overstory canopy, mean wood microsite temperature was cooler than soil substrates (6.23 vs. 6.60^◦C). 

Wood microsites appear to facilitate seedling growth and thereby enhance meadow invasion at timberline-alpine meadow borders.  This conclusion is supported by our finding of warmer temperatures and corresponding greater seedling survival for wood microsites with least overstory canopy.