Impact of changing water availability on plant photosynthetic performance in Big Bend National Park, Texas.

Since water is the primary limiting factor for gorwth and establishment of desert plants in arid ecosystems, they are sensitive to the magnitude and timing of rainfall events. The Hadley Climate Model predicts an increase of 25% for both summer and winter precipitation at Big Bend National Park, located in the Chihuahuan Desert. Previous research has shown that different photosynthetic pathways confer different photosynthetic advantages depending on the timing of rainfall. C3 plants often photosynthesize during cooler temperatures, and, thus, use winter rainfall, while C4 plants are active during warmer summer periods and use summer rain. CAM plants are often photosynthetically active year-round and respond to summer and winter rain events. This study examined how predicted changes in the timing and magnitude of precipitation for Big Bend National Park affected the photosynthetic performance of the dominant plants in the sotol-grassland community: Dasylirion leiophyllum (Sotol; C3), Opuntia phaeacantha (Brown-spine prickly pear; CAM), and Bouteloua curtipendula (Side-oats grama; C4). Gas exchange measurements were taken at 36 plots in the field to assess each species response to four experimental manipulations: no water addition, winter water addition, summer water addition, and summer and winter water addition. Gas exchange measurements were also taken in growth chambers to measure seedling response to four different drought treatments during winter and summer temperatures. After the first year, our research supported the evidence that C3 are winter active, C4 plants are summer active, and CAM plants are active all year, but there was no significant correlation between increased rainfall and increased photosynthetic performance