I camped at the edge of this dam in southern New Mexico over the summer. The water that you see would not be impounded at the surface if it were not for this dam – nor would be the mosquitoes, odd odor, or recreational boaters. This part of the state would instead be a dry grassland. Why have grasses been so successful in these arid areas? They haven’t always – only for the last 15-20 million years have grasses dominated. Perhaps caused by changing climates, many grasses have evolved a different type of photosynthesis (C4) which uses water more efficiently, especially at high temperatures. These benefits are especially important in desert environments. Grasses also have uniquely shaped stomata (the small pores on the leaf surface that allow carbon dioxide in, and water out) which can open and close rapidly, also allowing finer control of water loss in variable environments.
So will we see more grasses in the future? Climate change projections are forecasting a hotter and dryer environment in many places. The answer is complicated – the advantage of grasses with C4 photosynthesis is lessened when carbon dioxide levels in the atmosphere also increase. More resources are available for all plants, not just the efficient ones, and some complicated biochemistry also shifts in all kinds of plants. The future is hard to predict!