PS 38-57 - Direct and legacy effects of dry winter on photosynthesis in a semi-arid grassland

Abstract: In the semi-arid grasslands of southern Arizona, annual precipitation is divided between warm-season (June-October) convective rainfall associated with the North American monsoon and cool-season (November-May) frontal storms. Cool season precipitation amounts show clear declines in historical data and model projections. While the ecological consequences of warm-season precipitation variability have been intensively studied in this ecosystem, plant functional type specific productivity responses to cool-season precipitation as well as the potential for legacy effects of cool-season precipitation on subsequent warm-season productivity are less understood.

We conducted a field manipulation experiment to investigate how cool-season precipitation variability impacts a semi-arid mixed annual/perennial bunchgrass ecosystem by making repeated measurements of ecosystem gross primary productivity (GPP) and canopy cover. We applied three cool-season precipitation treatments based on the local rainfall history: dry (5^th quantile), normal (50^th quantile), wet (95^th quantile), while holding warm-season precipitation constant at its normal value. We predicted that lowering cool-season precipitation would reduce cover and GPP both within the cool season and the subsequent warm season.

Consistent with our prediction, cool-season precipitation strongly affected cool-season GPP, with dry winter plots showing 55% less, and wet winter plots 75% greater GPP than normal winter plots. In contrast with our prediction that cool-season water limitation would constrain subsequent warm-season productivity, dry winter plots had 19% greater GPP in the subsequent warm season than the normal winter plots. Likewise, canopy cover of warm-season annuals was greatest in dry-winter plots. The counterintuitive legacy effect of cool-season water limitation enhancing subsequent warm-season GPP and annual plant cover might be explained by carry-over effects of cool-season precipitation on warm-season nutrient availability. Specifically, we are now exploring whether dry winters slowed litter decomposition and/or plant nutrient uptake, resulting in greater plant-available nitrogen during the subsequent warm season. We will present the results of ongoing analysis of relationships among winter precipitation, litter decomposition, soil inorganic nitrogen, plant cover and GPP.

Our results show how this semi-arid grassland is responding to drier winters within the cool growing season, and they reveal surprising legacy effects of winter drought on subsequent summer productivity. This work highlights the importance of the less well-studied cool growing season and its impact, both direct and indirect, on the annual ecosystem carbon budget.