RESOURCE HETEROGENEITY GENERATED BY SHRUBS AND TOPOGRAPHY ON COASTAL SAND DUNES

In early stages of primary succession, colonizing plants can create re source patches that influence the abundance and distribution of other species. To test whether different colonizing shrubs generate contrast ing patches on coastal sand dunes, we compared soil characteristics an d light availability under the nitrogen-fixing shrub Lupinus arboreus, under the non-nitrogen-fixing shrub Artemisia pycnocephala, and betwe en shrubs on dunes at a site in northern California. Concentrations of inorganic nitrogen and net nitrogen mineralization rates were general ly 1-10 times greater in soil under Lupinus than under Artemisia or be tween shrubs. Soil water content was mostly lower under shrubs. Mean p hoton flux density near ground level was reduced by at least 80% at gr eater than or equal to 35 cm inside shrub canopies. Topography appeare d to have more effect on soil moisture but less direct effect on nitro gen availability than did Lupinus. However, Lupinus probably increases nitrogen levels more on higher, drier dunes. Microhabitats under and between nitrogen-fixing shrubs constitute a mosaic of individually poo r but complementary patches in which high levels of light and moderate levels of soil nitrogen are present but tend not to occur together.