Shifts in plant functional composition following long-term drought in grasslands

Authors: GRIFFIN-NOLAN, R - Colorado State University; Blumenthal, Dana; COLLINS, S - University Of New Mexico; FARKAS, T - University Of New Mexico; HOFFMAN, A - Colorado State University; MUELLER, K - Cleveland State University; OCHELTREE, T - Colorado State University; SMITH, M - Colorado State University; WHITNEY, K - University Of New Mexico; KNAPP, A - Colorado State University

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2019
Publication Date: 6/1/2020
Citation: Griffin-Nolan, R.J., Blumenthal, D.M., Collins, S.L., Farkas, T.E., Hoffman, A.M., Mueller, K.E., Ocheltree, T.W., Smith, M.D., Whitney, K.D., Knapp, A.K. 2020. Shifts in plant functional composition following long-term drought in grasslands. Journal of Ecology. 107:2133-2148. https://doi.org/10.1111/1365-2745.13252.
DOI: https://doi.org/10.1111/1365-2745.13252

Interpretive Summary: Functional traits can provide unique insights into plant performance. A diversity of plant traits within a community (i.e., functional diversity) can increase the stability of ecosystem functioning, such as plant productivity, in response to drought. The duration of drought is expected to increase with rising global temperatures; thus, understanding both the response of functional diversity to long-term drought and the corresponding effect that has on ecosystem function will be critical for predicting drought sensitivity. Here, we experimentally reduced growing season precipitation by 66% across six temperate grasslands for four years and measured changes in several metrics of functional diversity. Plant traits related to leaf carbon, water, and nutrient economics were measured for species cumulatively representing ~90% plant cover. We observed significantly higher functional diversity following long-term drought for four of the six sites, Community-level drought strategies (assessed as community-weighted trait means) largely shifted from drought tolerance to drought avoidance and/or escape strategies, as evidenced by lower leaf-level drought tolerance (assessed via turgor loss point), higher specific leaf area (leaf area/mass), and higher leaf nitrogen content. Lastly, the sensitivity of plant productivity to drought was positively correlated with functional diversity, suggesting that functional diversity increases grassland resistance to drought. Our results highlight the importance of considering both the diversity and weighted means of functional traits within plant communities as their collective effect may either stabilize or enhance ecosystem sensitivity to drought.

Technical Abstract: 1. Functional traits can provide unique insights into plant performance at the community scale. A diversity of plant traits within a community (i.e., functional diversity; FD) can increase the stability of ecosystem functioning, such as aboveground net primary production (ANPP), in response to climate extremes. Further complexity arises, however, when FD itself responds to environmental change. The duration of climate extremes, such as drought, is expected to increase with rising global temperatures; thus, understanding both the response of FD to long-term drought and the corresponding effect that has on ecosystem function will be critical for predicting ecosystem sensitivity to climate change. 2. Few experiments have assessed both the response and effect of FD and community-weighted trait means (CWMs) within the context of climate extremes at broad spatial and temporal scales. Here, we experimentally reduced growing season precipitation by 66% across six temperate grasslands for four years and measured changes in FD, CWMs, and phylogenetic diversity (PD). Plant traits related to leaf carbon, water, and nutrient economics were measured for species cumulatively representing ~90% plant cover. 3. We observed significantly higher FD following long-term drought for four of the six sites, independent of changes in PD. Community-level drought strategies (assessed as CWMs) largely shifted from drought tolerance to drought avoidance and/or escape strategies, as evidenced by higher community-weighted leaf turgor loss point, specific leaf area, and leaf nitrogen content. Variable responses were largely attributable to mortality/senescence of the dominant species and/or species re-ordering. Lastly, the sensitivity of ANPP to drought was positively correlated with several indices of FD, suggesting the effect of FD is to increase grassland resistance to drought. 4. Synthesis: We find that high FD in temperate grasslands increased ecosystem resistance to extreme drought. While long-term drought largely increased FD further, community drought tolerance may be decreased due to dominant species loss. Our results highlight the importance of considering both the diversity and weighted means of functional traits within plant communities as their collective effect may either stabilize or enhance ecosystem sensitivity to drought.