LIGHT-FIELD HETEROGENEITY AMONG TUSSOCK GRASSES - THEORETICAL CONSIDERATIONS OF LIGHT-HARVESTING AND SEEDLING ESTABLISHMENT IN TUSSOCKS ANDUNIFORM TILLER DISTRIBUTIONS

Although the tussock growth caespitose graminoids is widespread, the e ffect of this growth form on light interception and carbon gain of til lers has received little attention. Daily incident photosynthetic phot on flux density (PFDinc) and carbon gain in monospecific stands of tus sock grasses were compared with those of a hypothetical distribution w ith the equivalent tiller density per total ground area, but evenly di stributed rather than clumped in tussocks. This was computed for two t ussock grasses Pseudoroegneria spicata (Pursh) A. Love (bluebunch whea tgrass) and Agropyron desertorum (Fisch, ex Link) Schult. (crested whe atgrass) at different plant densities. Daily PFDinc and net photosynth esis (A) were greater if tillers were distributed uniformly rather tha n clumped in tussocks, except when the density of tussocks was so grea t as to approach a uniform canopy. When tussock density per ground are a was low much of the difference between tussock and uniform tiller de nsities in PFDinc and A was due to shading within the tussocks; up to 50-60% of the potential carbon gain was lost in A. desertorum due to s hading within tussocks. In a matrix of tussocks, the light field for e stablishing seedlings was very heterogeneous; potential A ranged from 7 to 96% relative to an isolated seedling. The mean of daily PFDinc an d A for seedlings in a tussock stand were nearly identical to the valu es in corresponding stands of uniform tiller distributions. It is hypo thesized that the loss of A resulting from clumping tillers into tusso cks is offset by benefits of protecting sequestered belowground resour ces from invasion by seedlings of competitors.