Forest restoration treatments may enhance resilience to disturbance in part by creating spatially heterogeneous tree structure. Structural elements like tree density, crown architecture, and species composition can drive resource heterogeneity by creating gradients of light, moisture, and temperature availability. Heterogeneity in forest structure results in a range of microsite suitability for different tree species and life-stages that have varying tolerances for temperature extremes, moisture deficits, and competition for resources. In dry, mixed-conifer forests of the Rocky Mountains which frequently experience hot and dry growing conditions, canopy buffering of microclimates can be vital to survival of young tree seedlings. To effectively understand the ecological impacts of heterogeneous forest structure on tree regeneration, we conducted a seed-sowing and planting experiment to evaluate survival responses of ponderosa pine and Douglas-fir to microsite conditions following a spatially heterogeneous forest restoration treatment. We planted one-year-old seedlings and sowed seeds of each species in 54, 1m2 plots stratified according to overstory density, topographic position, and inter-plot distances to capture spatial variability in microsite conditions following this restoration treatment. We monitored temperature, humidity, and soil moisture at each plot throughout the growing season and assessed how microclimate and light availability affected survival of germinant and planted seedlings.
Results/Conclusions
The restoration treatment resulted in considerable structural heterogeneity as evidenced by canopy cover ranging from 22.0-73.5% (mean 47.5% ±14.9 SD) across plots. Mean daily vapor pressure deficit (VPD) ranged from 1.01-1.32 kPa (1.13 kPa ±0.08), while mean daily maximums were highly variable (3.15 kPa ±0.34). Mean soil temperatures were 13.1-25.1 °C (17.8 °C ±3.0), and soil available water capacity varied from 7.0-14.0% (10.0% ±0.2). Microclimate buffering was evident in strong negative correlations between canopy cover and VPD (r = -0.71) and soil temperatures (r = -0.84). Survival of planted seedlings was high for both species (95.7% ±20.4), providing little statistical power to assess relationships with microsite conditions. Ponderosa pine germinant survival (13.0% ±19.2) was highest in microsites with low canopy cover and high soil available water. Douglas-fir germinant survival was 5.0% (±19.0), but no factors we measured explained variation in survival. Overall, regeneration responses seemed largely detached from microsite conditions, likely because this study occurred during average climate conditions, rather than during a hot and dry year. While spatially heterogeneous restoration treatments create a range of microclimate conditions that can buffer seedlings from high temperatures and moisture deficits, this may only be critical during abnormally hot and dry years.