Increased [CO₂] is expected to alter global precipitation patterns, with the intensity of heavy precipitation events increasing. This study examined a 25% increase in seasonal (winter and summer) precipitation on the physiology of Dasylirion leiophyllum (C₃ shrub) and Bouteloua curtipendula (C₄ grass) in the Chihuahuan Desert. During a five year period (2001-2006) soil moisture, daily carbon gain (A_net), photosynthesis at saturating light (A_sat), stomatal conductance (g_s), leaf nitrogen [N], and total non-structural carbohydrates were measured and best predictors of A_sat evaluated. A_sat was controlled more by environmental factors than biochemistry since 90% of A_sat was explained by g_s in D. leiophyllum and 76% of A_sat was explained by g_s in B. curtipendula. Indeed, both natural precipitation and supplemental precipitation altered A_net and A_sat. Following a wet year, A_sat increased in both species. Following a dry year or in average years, supplemental winter precipitation led to increased or maintained A_sat and A_net in the summer. Following multiple wet years, increased winter and summer precipitation decreased A_sat as a result of limited soil nitrogen. Winter soil moisture was less effective at predicting A_sat in both species than summer soil moisture. The ability of precipitation history to predict A_sat was more dependent on annual precipitation in D. leiophyllum, but on supplemental seasonal precipitation in B. curtipendula. Leaf [N] controlled A_sat to a greater degree in grasses than shrubs. When all physiological parameters were examined together, differences due to annual and seasonal precipitation were driven by leaf [N] and soluble sugar and starch concentrations.