LAUSR

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.