Mixed effects of drought on species‐level traits and plant composition in the United States mixed‐grass prairie

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Abstract Precipitation variability is increasing globally, leading to increases in drought occurrence and intensity which will have complex consequences for ecosystem dynamics. The effects of drought on plant communities are routinely assessed. However, plant functional traits (community‐ and species‐level) may allow us to gain a greater understanding of the complexity of ecosystem shifts in response to changing precipitation regimes. We manipulated drought and grazing intensity in a field experiment and simultaneously performed an observational study of natural drought to assess plant species composition and functional trait shifts at a community‐ and species‐level in two mixed‐grass prairie rangeland sites in Montana and Wyoming, USA. Multiple intensities of drought treatments were implemented for 2 years and recovery from drought was monitored for 3 years. We measured plant community shifts and community‐level functional traits during and after drought implementation using plant community metrics, community weighted means and functional dispersion. To further assess the effects of drought, we measured plant functional traits of five focal plant species at each site across 4 years with precipitation ranging from 34% below average to 15% above average in Montana and from 30% below average to 17% above average in Wyoming. We found that the plant communities at both sites were tolerant of drought, with the plant community having only subtle and occasional site‐specific shifts and no appreciable shift in community‐level functional traits. In contrast, variability of precipitation through time was associated with shifts in plant functional traits of individual species. Specifically, species exhibited increased drought avoidant or tolerant traits such as greater leaf thickness and lower leaf dry matter content and smaller leaf area. Synthesis . Our study indicates that plant communities and community‐level trait studies may be under estimating trait changes by not including species‐level trait responses to drought.

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