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

Authors: BLOODWORTH, KATHRYN - University Of North Carolina Greensboro; KOMATSU, KIMBERLY - University Of North Carolina Greensboro; Porensky, Lauren; Reinhart, Kurt; VAARRE-LAMOUREUX, KAYSA - University Of North Carolina Greensboro; WILCOX, KEVIN - University Of North Carolina Greensboro; KOERNER, SALLY - University Of North Carolina Greensboro

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Drought frequency and intensity are increasing in the northern Great Plains, USA. The effects of drought on plant community composition (i.e. the relative abundances of different plant species) are routinely assessed. However, the effects of drought on plant functional traits like leaf area, leaf thickness, and leaf dry matter content are less commonly studied. These traits are good indicators of drought tolerance or avoidance, and can help us understand ecosystem responses to drought. We manipulated drought and grazing intensity in a field experiment and simultaneously performed an observational study of natural drought at two mixed-grass prairie rangeland sites in Montana and Wyoming. In the field experiment, multiple intensities of drought treatments were implemented for two years and recovery from drought was monitored for three years. We measured plant community shifts and community-level functional traits during and after experimental drought. To further assess the effects of drought, we measured plant functional traits of five focal plant species at each site across four years. Compared to the average precipitation across these four years, precipitation ranged 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, site-specific shifts and no appreciable shift in community-level functional traits. In contrast, natural variability of precipitation through time was associated with shifts in plant functional traits of individual focal species. During droughts, species exhibited drought avoidant or tolerant traits such as greater leaf thickness and lower leaf area. Our study indicates that plant communities and community-level trait studies may under-estimate trait changes by not including species-level trait responses to drought.

Technical 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 responses 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 two years and recovery from drought was monitored for three 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 four years. Compared to the average precipitation across these four years, precipitation ranged 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 area. 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.