Aims In arid grasslands forbs represent a large component of species diversity and provide a key resource for pollinators and consumers. However, low abundances and high temporal variability make it challenging to successfully predict forb presence and abundance from 1 yr to the next. In this study we: (1) characterize patterns of semi‐arid forb diversity and abundance over time; (2) determine the relative importance of direct vs indirect (via grass competition) effects of precipitation on forb richness and abundance; and (3) separate the effect of precipitation timing on forb community composition from the effects of precipitation amount and season. Location Semi‐arid grassland, Sevilleta National Wildlife Refuge, Socorro County, central New Mexico, USA. Methods We analysed forb dynamics along two 400‐m long line‐intercept transects in response to inter‐annual and seasonal precipitation variability and abundance of perennial grasses using a unique 20‐yr species composition data set from ungrazed native grassland in the northern Chihuahuan Desert. Results Forb richness and cover were significantly positively associated with precipitation across seasons. Total richness was high across the time series but substantially lower within any given year. A direct, positive effect of precipitation overrode any potential negative, indirect effect via grass competition. Although aggregate forb responses were strongly linked with precipitation, individual species responses were highly variable and generally not linked with either precipitation timing or season. Conclusions In the hotter, drier climate predicted by meteorological models for the US southwest, forbs may be negatively impacted if winter moisture decreases and monsoon precipitation becomes more variable. On the other hand, the flexibility exhibited by forbs in germination and establishment throughout the growing season may help buffer some common species to increased inter‐annual precipitation variability.

Global environmental change is altering temperature, precipitation patterns, resource availability, and disturbance regimes. Theory predicts that ecological presses will interact with pulse events to alter ecosystem structure and function. In 2006, we established a long-term, multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric nitrogen (N) deposition, and increased winter precipitation on plant community structure and aboveground net primary production (ANPP) in a northern Chihuahuan Desert grassland. In 2009, a lightning-caused wildfire burned through the experiment. Here, we report on the interactive effects of these global change drivers on pre- and postfire grassland community structure and ANPP. Our nighttime warming treatment increased winter nighttime air temperatures by an average of 1.1 °C and summer nighttime air temperature by 1.5 °C. Soil N availability was 2.5 times higher in fertilized compared with control plots. Average soil volumetric water content (VWC) in winter was slightly but significantly higher (13.0% vs. 11.0%) in plots receiving added winter rain relative to controls, and VWC was slightly higher in warmed (14.5%) compared with control (13.5%) plots during the growing season even though surface soil temperatures were significantly higher in warmed plots. Despite these significant treatment effects, ANPP and plant community structure were highly resistant to these global change drivers prior to the fire. Burning reduced the cover of the dominant grasses by more than 75%. Following the fire, forb species richness and biomass increased significantly, particularly in warmed, fertilized plots that received additional winter precipitation. Thus, although unburned grassland showed little initial response to multiple ecological presses, our results demonstrate how a single pulse disturbance can interact with chronic alterations in resource availability to increase ecosystem sensitivity to multiple drivers of global environmental change.

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