Plant productivity responses of upland and lowland grasslands to contrasting water availability

ROXANA ARAGÓN; MARTÍN OESTERHELD

Montreal, Canadá

Congreso; 90Th Ecological Society of America Annual Meeting; 2005

Ecological Society of America

Increasing climate variability is one of the most important components of climate change. For this reason, understanding how plant communities respond to extreme events represents a growing concern. Drought and flooding are common disturbances in numerous grassland ecosystems and many plant communities are subjected to both disturbances during a single year. Previously,  we found that, in grasslands of the Flooding Pampa, Argentina, the temporal variation of the Normalized Difference Vegetation Index  (NDVI) along 19 years, was lower in landscapes dominated by lowland communities than in those dominated by upland communities. Since NDVI is related to absorbed photosynthetic active radiation, this can have important implications for the study of ecosystem function responses to environmental fluctuations. The results of the study mentioned above could be explained by either a difference in water availability along different portions of a topographic gradient, or by  differential resistance of lowland grasslands to changes in water availability. To answer this question we experimentally exposed mesocosms from upland and lowland grasslands of this area to three contrasting water regimes (drought, flooding, and field capacity) during 60 days. Water treatments applied to upland and lowland grasslands were equivalent in terms of soil water potential. We recorded NDVI of each mesocosm every week, performed plant surveys at the beginning and at the end of the experiment, and collected aerial and belowground biomass. Biomass data were analyzed by a two-way ANOVA, and NDVI records were analyzed by repeated measurement ANOVA. Drought reduced NDVI and green biomass in both plant communities, but lowland grasslands were far more sensitive. On the contrary, this community was highly resistant to flooding. Flooding increased NDVI in upland grasslands, but only during 3 of the 8 dates considered. Flooded plots from the upland community showed changes in species composition and Paspalum dilatatum, a C4 perennial, flood-tolerant grass, became dominant. This experiment shows that the pattern found at landscape scale of reduced NDVI variability in areas dominated by lowland grasslands, could not be explained by a greater resistance of this plant community to drought.