Ecological footprints in the geomorphic record: reciprocal feedbacks versus diametric processes

Background/Question/Methods

Evidence of reciprocal biotic feedbacks to in geomorphology can be seen in many locations in the form of patterned landscapes. In some of these locations, such as Sahelian Tiger Bush, it is the occurrence of biota (i.e., trees) that creates the pattern. In others, such as peatlands, the historical occurrence of biota creates the patterned geomorphology, in the form of accumulated organic matter. In some cases, however, the interaction of hydrology and ecological processes might leave their imprint on bedrock itself. Using measurements of landform elevation, vegetation communities, and patterning, I investigated a subtropical karst-platform landscape (Big Cypress National Preserve, Florida, USA) to ask two questions: first, can reciprocal biotic feedbacks produce an observed arrangement of depressional landscape features? Second, do contemporary geomorphic processes act to obscure this signal?

Results/Conclusions

The spatial arrangement of depressional landscape features observed from classified imagery and ground-truthed observations indicates overdispersion, or even distribution, of depressional wetlands. Occurrence of these vegetation communities corresponds to the occurrence of depressions in soil surface elevations as well as limestone bedrock lying a few centimeters to a few meters underneath the shallow mantle of calcareous and organic soils of the region. Alternating positive/negative autocorrelation values occur with pseudoperiods that roughly correspond to depression size. Additionally, elevation measurements indicate bimodal distribution of bedrock elevation measurements. These findings support predictions of landscape patterning and reciprocal feedbacks between biota and geomorphology. Measurements of soil-surface elevation are less strongly bimodal, or in some cases unimodal, in their distributions. Also, thickness of the soil layer is negatively correlated with bedrock elevation, indicating a “smoothing” of soil by transport and productivity. These findings appear to indicate the occurrence of reciprocal biota-landform interactions, as well as the obfuscating influence of contemporary processes on this geomorphic record. This leads to the possibility that patterned landforms may occur more commonly than first appearances might lead us to believe.