Spatial patterning of Artemisia tridentata neighborhoods and relative crowding

Plants reflect resource use in their spatial patterning. Competition for
limited resources—such as available soil water in a dryland
ecosystem—drives establishment, growth, and mortality, resulting in shifts
of spatial arrangement over time. We characterized the spatial patterning
of two big sagebrush (Artemisia tridentata subspecies wyomingensis)
communities in the upper Green River Basin of Wyoming, USA. We mapped big
sagebrush canopies in two, 100-square meter sites and calculated plant
neighborhoods as the area closer to a target plant than to any other
plant. We assumed that neighborhoods were areas in which the target plant
dominates resource use. We found that plant neighborhoods had strong,
positive correlations with plant size, indicating that larger
neighborhoods may access more belowground resources. We also found that
the relationships between experienced crowding, i.e. Crowding Index (CI)
by an average neighbor, and neighborhood size, were consistently negative
regardless of calculation method. We also found that the residuals of a
regression of target plant biomass and neighborhood area were strongly
related to the CI calculated via all methods. This means that plants with
smaller neighborhoods than expected also experience the greatest crowding
by an average neighbor. These results are consistent with negative density
dependence and show that greater static crowding predicts smaller
neighborhoods in two, undisturbed, intermediate-successional big sagebrush
communities. In the future, similar studies of spatial patterning that
include interspecific plant-plant interactions will be useful for
understanding the relationship between spatial patterning and negative
density dependence.