Soil resource-based niches in temperate and tropical forests

Background/Question/Methods

Spatial patterns in soil nutrients arise from multiple sources, including weathering and plant-soil feedbacks.  Regardless of how spatial structure in soil nutrients arises, finer-scale spatial heterogeneity provides more seedling regeneration niches in a given area, enabling a greater number of seedling species with different resource requirements to coexist. In this study, we hypothesized that tropical forest soils have more soil resource-based regeneration niches than temperate forest soils (i.e. greater fine-scale nutrient heterogeneity), which could reinforce the maintenance of high tree species richness in tropical forests. We examined the spatial structure of critical plant nutrients across soil fertility gradients in temperate (Manistee National Forest, Michigan, USA) and wet tropical forests (La Selva Biological Station, Costa Rica).  Calcium (Ca), potassium (K), magnesium (Mg), total extractable phosphorus (P) and inorganic nitrogen (N) were measured from soil samples taken in a lattice plus fill-in design across 200-m transects from temperate outwash, ice-contact, and moraine landform sites and tropical volcanic and alluvial soils.

Results/Conclusions

Without respect to spatial structure and despite tremendous variation across landforms within biomes, temperate sites exhibited higher mean P and Mg levels, lower N and K levels, and similar Ca levels as our tropical sites.  Across biomes, over half of the total variance in mineral resources was spatially structured, indicating a high degree of spatial dependence, while low structural variance in N suggested its heterogeneity was due to unmeasured factors or occurred at finer spatial scale than we measured.  Regardless of landform, coarse-scale heterogeneity of base cations (Ca, K and Mg) characterized temperate sites with large continuous gradients; on average, pairs of sampling points were correlated to r = 0.5 even at~40 m.  For the tropical sites, in contrast, pairs of points were correlated to r=0.5 at just 10 meters.  In both the temperate and tropical sites, P and N were characterized by small, discontinuous patches (~3-5m). Overall, our results support that there are more soil-resource based niches in these tropical than temperate forest sites.