Control of plant frequency on an environmental gradient: effects of abiotic variables, neighbours, and predators on Poa pratensis and Poa compressa (Gramineae)

Abstract

Effects of abiotic variables, neighbours, and predators on seedling establishment and tiller production of two perennial grasses (Poa pratensis 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 (hollows) of hills. Seedling establishment and tiller number per plant were each compared where seeds (or plants) were exposed to abiotic variables with and without the presence of neighbours and (or) predators. For P. pratensis, which occurred naturally more often in hollows than on ridges, abiotic variation appeared to be most important in controlling its frequency on the gradient. Tiller production was significantly lower on ridges than in hollows where plants were exposed only to abiotic variables (i.e., neighbours removed and predators excluded). For P. compressa, which occurred naturally less often in hollows than on ridges, greater suppression of seedling emergence by neighbours in hollows than on ridges appeared to be the factor controlling its frequency on the gradient. These results provide support for the view that variation in plant frequency across an environmental gradient reflects abiotic variation and differential effects of neighbours. Key words: competition, predation, abiotic stress, Poa, environmental gradient.