The establishment of Festuca pallescensin arid grasslands in Patagonia (Argentina): the effect of soil water stress

The role of soil water stress on the survival and development of vegetative structures (shoot and root biomass) of new seedlings of Festuca pallescenswas investigated in laboratory experiments under controlled conditions and with field observations. Experiments in the laboratory consisted of evaluating the influence of three levels of water stress, in the range from −1·7 to −8·0 MPa, on the survival and development of seedlings. Field observations were conducted in a representative stand of the semi-arid grasslands of F. pallescens. During two growing seasons, at approximately bimonthly intervals, the soil water potential as well as the density and survival of seedlings of F. pallescenswere estimated. In the laboratory, the development of F. pallescensseedlings depended on water availability in the soil. Survival of seedlings, approximately 2-months old, varied between 65 and 100% among treatments. Significant differences among treatments in above- and below-ground biomass, leaf number and leaf length were also observed in the surviving plants. In range conditions soil water potential showed values ranging from −1 to −9 MPa. Survival reached only 30% in seedlings 2-months old. No seedling survived after summer. These trends depart from those observed in the laboratory, where survival rates were higher even under soil water conditions similar to those observed in the field. The observed differences may be connected with the emergence of seedlings in the vicinity of established plants of F. pallescensin the field where competition for soil water probably occurred. Also, desiccation caused by strong dry winds during spring and summer could have affected seedlings in field conditions. Based on these results the establishments of new plants of F. pallescensin the semi-arid grasslands of western Patagonia might be possible only in years with exceptionally humid summers. The alternative of managing the grassland by seed introduction from ecotypes showing improved drought responses is discussed.