Woody plants were recorded in 159 samples along nine peninsulas in Maine (USA) to determine whether species richness varied with (1) distance from the mainland, (2) environmental gradients along the peninsulas, or (3) both distance and environment. Species richness decreased significantly with distance from the mainland, with an average loss of approximately seven species over a distance of 16 000 m, thus supporting Simpson's (1964) hypothesis of peninsular diversity patterns. Vegetational gradients were constructed using detrended correspondence analysis (DCA), and the DCA sample scores were used as indicators of environmental conditions at each sample location. Plant community structure varied both along the peninsulas from mainland to the tip (e.g., Pinus resinosa and Acer spicatum near the mainland, Potentilla tridentata and ericaceous shrubs at the tip), and across the peninsulas from edge to interior (e.g., shrub—dominated communities containing Empetrum nigrum and Juniperus communis were common at ...