Belowground Effects of Canopy Gaps in a Tropical Wet Forest

Abstract

Canopy gaps are a fairly well-studied phenomenon in tropical forests, but less well known are their belowground effects on patterns of root length and biomass and the consequences of such changes on regeneration and community structure. I examined canopy gaps and adjacent understory sites on two soil types of contrasting fertility in a lowland rain forest in Costa Rica to determine (1) whether canopy gaps create root gaps beneath the canopy opening, and (2) whether the effects of such root gaps on root ingrowth rates, root competition, and root responses to nutrient heterogeneity are related to soil type. Based on soil coring, canopy gaps had less fine (<2 mm) root length and biomass than comparable closed canopy sites. Reductions in root length and biomass were greater on the infertile (residual) soil type than the fertile (alluvial) type but were unrelated to gap size, age, or percentage canopy openness. Additionally, I measured root competition in a trenching experiment using the pioneer tree Hampea appendiculata as a bioassay species. Light appeared to be the only factor limiting relative growth rate, as seedlings grew faster in the canopy gaps than in the understory, regardless of soil type or trenching treatment (trench lined with root restriction cloth, trench open to colonization by neighboring roots, or no trench). The accumulation rate of live fine roots was unaffected by soil or light conditions. Finally, I altered nutrient heterogeneity of the gap and understory sites through the creation of fertilized microsites. Root proliferation into these microsites was enhanced only in the canopy gaps on the infertile soil. The differences between the two soil types in the amount of root length and biomass and in root proliferation responses suggest that the consequences of canopy gap formation may be dependent on background levels of soil fertility.