Contrasting effects of plant neighbours on invading Ulmus rubra seedlings in a successional grassland1

Abstract

Intense competition for soil water can be an important factor regulating rates of woody plant invasion into grasslands that experience periods of drought stress. We conducted a plant neighbour-removal and irrigation experiment in successional grassland at a prairie-forest ecotone in eastern Kansas, U.S.A.: i) to test whether the effects of resident herbaceous vegetation on invading red elm (Ulmus rubra) seedlings varied predictably with water supply, and ii) to assess whether the magnitude and sign of these effects depended on the measure of seedling performance evaluated: seedling survival, growth, or biomass. We found that the impact of neighbours on U. rubra seedlings included strong facilitation, neutral effects, and strong competitive suppression, depending on the level of water supply and on the aspect of plant performance examined. Survival was facilitated by plant neighbours under ambient soil moisture conditions. However, neighbours had no impact on survival in the presence of irrigation, suggesting that facilitation of survival in the absence of irrigation was mediated by neighbour amelioration of water stress. Seedling growth rate was increased by water supply and inhibited by plant neighbours, with the magnitude of inhibition varying independently of water supply. Our results suggest that i) during years of moderate-to-severe drought stress, neighbouring herbaceous plants can have very different impacts on woody seedling recruitment in this grassland and ii) neighbour effects on woody seedling survival should vary more strongly among wet and dry years than neighbour effects on seedling growth do.