Geomorphic principles of terrain organization and vegetation gradients

Moisture and nutrient gradients consistently explain much of the variation in plant species composition and abundance, but these gradients are not spatially explicit and only reveal species responses to resource levels. This study links these abstract gradients to quantitative, spatial models of hill-slope assembly. A gradient analysis in the mixed-wood boreal forest demonstrates that patterns of upland vegetation distribution are correlated to soil moisture and nutrient gradients. Variation in species abundance with time since the last fire is removed from the gradient analysis in order to avoid confounding the physical environment gradients. The physical-environment gradients are related to qualitative positions on the hill slope i.e. crest, mid-slope, bottom-slope. However, hill-slope shape can be quantitatively described and compared by fitting allometric equations to the slope profiles. Using these equations, we show that hill-slope profiles on similar surficial materials have similar parameters, despite coming from widely separated locations. We then quantitatively link the moisture and nutrient gradients to the equations. Moisture and nutrients significantly increase as distance down-slope from the ridgeline increases. Corresponding vegetation composition changes too. These relationships characterize the general pattern of vegetation change down most hill slopes in the area. Since hill slopes are a universal feature of all landscapes, these principles may characterize landscape scale spatial patterns of vegetation in many environments.