Co-limitation by nitrogen and water constrains allocation response to drought in deciduous and evergreen shrubs in a semi-arid ecosystem

Semi-arid regions worldwide are expected to experience reduced precipitation with future climate change, in addition to increased intensity of precipitation events, heightening the pulsed nature of water availability in these systems. Shifting biomass allocation is a plastic response of species to altered resource availability, and optimal allocation theory predicts plants should increase allocation to roots when water is limiting to growth. However, optimal allocation could be influenced by other factors, such as plant nutrient status and the seasonality of precipitation. Here, we evaluated the allocation response of eight common shrub species (four deciduous, four evergreen) from the Mediterranean climate region of Southern California to experimental manipulation of nitrogen and water availability, as well as seasonal drought. Evergreen species increased allocation to roots in response to low water availability to a greater degree than deciduous species, and also showed less of a decline in total biomass with water limitation, consistent with the predictions of optimal allocation theory. Deciduous species were more limited by nitrogen than evergreen species, requiring them to maintain high allocation below-ground regardless of water availability. Evergreen species were also able to equalize root allocation in response to early- versus late-season drought, while deciduous species responded non-optimally, reducing allocation to roots when early-season soil moisture was plentiful, resulting in the most strongly reduced growth by late-season drought. These experiments demonstrate how optimal allocation theory can be used to predict species responses to environmental changes in semi-arid systems, but should account for patterns of co-limitation and seasonality of resources.