Resource availability and seedling establishment

Question:
To what degree does the availability of resources affect seedling establishment?

Why do we care?
Resource availability may influence seedling recruitment by affecting productivity. As resource availability increases, productivity increases. Seedling establishment may respond in one of two ways. (1) If negative interactions among residents intensify concomitantly, then seedling establishment will decrease. (2) If, on the other hand, increased resource availability boosts productivity without intensifying competition, seedling establishment will increase. The evidence available, primarily from temperate grasslands, supports hypothesis (1). To my knowledge, parallel studies have not yet been performed in closed-canopy rainforests. Because of recruitment limitation and suppression by adults, the understory of these forests may be not be saturated with seedlings, and competitive interactions among seedlings may be weak. I predict that in closed-canopy tropical rainforest, hypothesis (2) will be supported; seedling establishment will increase with increases in resource availability.

Approach:
I water four 400-meter-squared plots, each paired with an un-watered control, to maintain soil moisture near field capacity. I run a sprinkler system to simulate wet-season rainfall throughout two dry seasons (June � October). In each plot, I monitor the growth and survival of seedlings. As well, I performed seed additions to control for differences in seedling community composition among subplots.

Results:
After just six months, seed additions had a striking effect on subplot density and richness. Resource addition (watering) had no effect over that time. This is not particularly surprising, however, since seedling germination, relative to survival, is expected to be insensitive to soil moisture.