The effects of multiple resources on forest regeneration: Microsite variation and seedling response
Forest gaps play a major role in forest dynamics because increased resource availability favors plant species that differ from those in undisturbed forest. This dissertation investigates the spatial heterogeneity of resource availability in gaps, how it varies by site, and how it affects regeneration. Gap effects on resource availability and seedling growth were studied at three sites, which differed in drainage, using large gaps (108 m × 30 m) where light ranged from understory shade to full sun. Light levels in plots were measured using hemispherical photos; soil moisture, with a capacitance probe; soil temperature, with a portable thermocouple; and nitrogen, with buried-bag incubations.
The strong asymmetric, light-induced temperature gradient drove nitrogen mineralization rates. Patterns of mineralization were modified by variation in soil moisture within sites and increased with the average moisture levels among sites. Nitrogen availability was highest in the center of gaps, while root uptake reduced nitrogen availability at the north edge. Contrary to predictions, relative height growth of Betula lenta growing in the north center of gaps was greater than that of B. papyrifera. For both species, the difference between understory and gap growth increased from least to most fertile sites.
To separate the effects of resources on plant- and leaf-level responses in the two species, a greenhouse experiment was conducted with high and low levels of light, nitrogen, and water. High nitrogen increased height growth even in shade. Nitrogen also increased leaf area through greater leaf initiation and retention. High rates of growth in gaps is caused by nitrogen effects on leaf display and light effects on realized carbon gain.
This research shows that increased nitrogen availability only occurs if gaps are large enough to increase soil temperatures that drive mineralization and to overcome the effects of root uptake near gap edges. It also highlights how the effects of canopy openings varies by site and how this can influence the regeneration of gap-dependent species.