Differential responses of three grasses to defoliation, water and light availability.

Abstract

Grazing not only modifies the structure and functioning of grasslands, it also changes micro-environmental conditions that alter the availability of resources. The aim of this study was to analyze the response of grasses with different photosynthetic pathways (C3/C4), growth forms (prostrate/erect), and grazing responses (increaser/decreaser) to defoliation and resource availability. In a greenhouse, we performed a factorial experiment with three factors: defoliation, light, and water and three species: Axonopus affinis (C4 prostrate, increaser), Coelorachis selloana (C4 erect, decreaser), and Bromus auleticus (C3 erect, decreaser). We measured the relative growth rate (RGR), biomass assignment, and specific leaf area. The RGR of both C4 species was affected by light availability, while the decreaser C3 did not respond to any factor. Biomass allocation to leaves and stolons changed with the interaction between light and water in the C4 prostrate species (increaser). In the C4 erect grass (decreaser), biomass allocation was more affected by defoliation under low levels of light and water. Low light availability and defoliation reduced the assignment to leaves, while the allocation to rhizomes increased. Species-specific responses to resources availability that are modified by grazing were related to photosynthetic pathway, growth form, and grazing responses. Biomass allocation was related to strategies to avoid and/or tolerate grazing. The investment to leaves was limited by light and water availability in prostrate species, while in erect grasses it was controlled by defoliation and water availability. Our results highlight the importance of species responses to changes in resource availability associated to grazing regimes.