Balance of competitive and facilitative effects of exotic trees on a native Patagonian grass

MARIA ELENA FERNANDEZ; GYENGE, J.; SCHLICHTER, T.

PLANT ECOLOGY

Año: 2007 vol. 188 p. 67 - 76

1385-0237

The balance between facilitation and competition in plants changes with species characteristicsand environmental conditions. Facilitative effects are common in natural ecosystems, particularly in stressful environments or years. Contrarily, in artificial associations of plants, such as agroforestry systems, some authors have suggested that even when facilitative effects mayoccur, net balance of tree effects on grasses is usually negative, particularly in dry environments.The aim of this study was to determine the net effect of the exotic ponderosa pine on thenative grass Festuca pallescens (St. Ives) Parodi in agroforestry systems in Patagonia. Soil watercontent, plant water status, and relative growth were measured in the grass growing in differenttreatments (determined by tree cover level) during two growing seasons with contrasting climaticconditions. Facilitative effects of trees over grass water status were recorded only when wateravailability was high. A net negative effect was detected on dates when soil water content was very low and evaporative demand was high. The strength of these negative effects dependedon tree density and climatic conditions, being higher in treatments with lower tree canopycover. These results indicate that the positive effect of trees could only be expected underrelatively low stress conditions. However, relative growth of grasses was always similar inplants growing in forested plots than in open grassland. Differences in biomass allocation forgrasses growing in shade and open habitats may reconcile these contrary results. Our resultshighlight the importance of the physiology of a species (relative drought and shade tolerance) indetermining the response of a plant to a particular interacting species.