No abstract

1. Against a backdrop of rising temperature, large portions of the western United States are experiencing fewer, larger and less frequent precipitation events.How such temporal 'repackaging' of precipitation alters the magnitude and timing of seasonal maximum gross primary productivity (GPP max ) remains unknown.Addressing this knowledge gap is critical, since changes to GPP max magnitude and timing can impact a range of ecosystem services and management decisions.2. Here we used a field-based precipitation manipulation experiment in a semi-arid mixed annual/perennial bunchgrass ecosystem with mean annual precipitation ~384 mm to investigate how temporal repackaging of a fixed total seasonal precipitation amount impacts seasonal GPP max and its timing.3. We found that temporal repackaging of precipitation profoundly influenced the seasonal timing of GPP max . Many/small precipitation events advanced the seasonal timing of GPP max by ~13 days in comparison with climatic normal precipitation. Conversely, few/large events led to deeper soil water infiltration, which delayed the timing of GPP max by up to 16 days in comparison with climatic normal precipitation, and altered end-of-season community composition by increasing the diversity of shallow-rooted annual plants. While GPP max magnitude did not differ across precipitation treatments, it was positively correlated with the abundance and biomass of deeper-rooted perennial bunchgrasses. The sensitivity of plant growth, biomass accumulation and plant life histories to the timing and magnitude of precipitation events and the resulting temporal patterns of soil moisture regulated ecosystem responses to altered precipitation patterns. 4. Our results highlight the sensitivity of semi-arid grassland ecosystem to the temporal repackaging of precipitation. We find that already-observed and modelforecasted shifts toward few/large precipitation events could drive significant delays in the timing of peak productivity for this ecosystem. Adaptive land management frameworks should consider these findings since shifts in peak ecosystem productivity would have major implications for multiple land user communities. Additional research is needed to better understand the role of | 647 Functional Ecology ZHANG et Al.

Transitions from grass to woody plant dominance, widely reported in arid systems, are typically attributed to changes in disturbance regimes in combination with abiotic feedbacks, whereas biotic mechanisms such as competition and facilitation are often overlooked. Yet, research in semi‐arid and subhumid savannas indicates that biotic interactions are important drivers in systems at risk for state transition. We sought to bridge this divide by experimentally manipulating grass‐on‐shrub and shrub‐on‐shrub interactions in early and late stages of grassland–shrubland state transition, respectively, and to assess the extent to which these interactions might influence arid land state transition dynamics. Target Prosopis glandulosa shrubs had surrounding grasses or conspecific neighbours left intact or killed with foliar herbicide, and metrics of plant performance were monitored over multiple years for shrubs with and without grass or shrub neighbours. Productivity of small shrubs was enhanced by grass removal in years with above‐average precipitation, a result not evident in larger shrubs or during dry years. Proxy evidence based on nearest neighbour metrics suggested shrub–shrub competition was at play, but our experimental manipulations revealed no such influence. Competition from grasses appears to attenuate the rate at which shrubs achieve the size necessary to modify the physical environment in self‐reinforcing ways, but only during the early stages of shrub encroachment. Our results further suggest that at late stages of grassland‐to‐shrubland state transitions, shrub–shrub competition will not slow the rate of shrub expansion, and suggest that maximum shrub cover is regulated by something other than density‐dependent mechanisms. We conclude that grass effects on shrubs should be included in assessments of desert grassland state transition probabilities and rates, and that desertification models in arid ecosystems that traditionally focus on disturbance and abiotic feedbacks should be broadened to incorporate spatial and temporal variations in competitive effects. A plain language summary is available for this article.