CONTROL OF PLANT FREQUENCY ON AN ENVIRONMENTAL GRADIENT - EFFECTS OF ABIOTIC VARIABLES, NEIGHBORS, AND PREDATORS ON POA-PRATENSIS AND POA-COMPRESSA (GRAMINEAE)

Effects of abiotic variables, neighbours, and predators on seedling es tablishment and tiller production of two perennial grasses (Poa praten sis L. and Poa compressa L.) were determined on a topographic gradient in an abandoned pasture. Seeds and plants of each species were added to experimental plots set up both at the top (ridges) and bottoms (hol lows) of hills. Seedling establishment and tiller number per plant wer e each compared where seeds (or plants) were exposed to abiotic variab les with and without the presence of neighbours and (or) predators. Fo r P. pratensis, which occurred naturally more often in hollows than on ridges, abiotic variation appeared to be most important in controllin g its frequency on the gradient. Tiller production was significantly l ower on ridges than in hollows where plants were exposed only to abiot ic variables (i.e., neighbours removed and predators excluded). For P. compressa, which occurred naturally less often in hollows than on rid ges, greater suppression of seedling emergence by neighbours in hollow s than on ridges appeared to be the factor controlling its frequency o n the gradient. These results provide support for the view that variat ion in plant frequency across an environmental gradient reflects abiot ic variation and differential effects of neighbours.