Vegetation pattern on landscapes is the manifestation of physical gradients
, biotic response to these gradients, and disturbances. Here we focus on th
e physical template as it governs the distribution of mixed-conifer forests
in California's Sierra Nevada. We extended a forest simulation model to ex
amine montane environmental gradients, emphasizing factors affecting the wa
ter balance in these summer-dry landscapes. The model simulates the soil mo
isture regime in terms of the interaction of water supply and demand: suppl
y depends on precipitation and water storage, while evapotranspirational de
mand varies with solar radiation and temperature. The forest cover itself c
an affect the water balance via canopy interception and evapotranspiration.
We simulated Sierran forests as slope facets, defined as gridded stands of
homogeneous topographic exposure, and verified simulated gradient response
against sample quadrats distributed across Sequoia National Park. We then
performed a modified sensitivity analysis of abiotic factors governing the
physical gradient. Importantly, the model's sensitivity to temperature, pre
cipitation, and soil depth varies considerably over the physical template,
particularly relative to elevation. The physical drivers of the water balan
ce have characteristic spatial scales that differ by orders of magnitude. A
cross large spatial extents, temperature and precipitation as defined by el
evation primarily govern the location of the mixed conifer zone. If the ana
lysis is constrained to elevations within the mixed-conifer zone, local top
ography comes into play as it influences drainage. Soil depth varies consid
erably at all measured scales, and is especially dominant at fine (within-s
tand) scales. Physical site variables can influence soil moisture deficit e
ither by affecting water supply or water demand; these effects have qualita
tively different implications for forest response. These results have clear
implications about purely inferential approaches to gradient analysis, and
bear strongly on our ability to use correlative approaches in assessing th
e potential responses of montane forests to anthropogenic climatic change.