Identification of suites of traits that explains drought resistance and pheonological patterns of plants in a semi-arid grassland community

Authors: OCHELTREE, T - Colorado State University; MULLER, K - Cleveland State University; CHESUS, K - Colorado State University; Lecain, Daniel; Kray, Julie; Blumenthal, Dana

Submitted to: Physiological Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2019
Publication Date: 1/13/2020
Citation: Ocheltree, T.W., Muller, K.M., Chesus, K., Lecain, D.R., Kray, J.A., Blumenthal, D.M. 2020. Identification of suites of traits that explains drought resistance and pheonological patterns of plants in a semi-arid grassland community. Physiological Ecology. 192:55-66. https://doi.org/10.1007/s00442-019-04567-x.
DOI: https://doi.org/10.1007/s00442-019-04567-x

Interpretive Summary: Grasslands contain many plant species that vary in phenology, the timing of growth and reproduction. It is unknown, however, whether water-use strategies are coordinated with phenology. In semi-arid grasslands, soil moisture availability is a major driver of plant productivity, and interspecific differences in drought tolerance may contribute to the varied phenology of those species. The ability to resist hydraulic failure in leaves, estimated as the leaf water potential when leaf hydraulic conductance is reduced by 50% of maximum (hydraulic safety), may explain phenological patterns in grasslands, as species showing less hydraulic safety may only be active when soil moisture is readily available. However, differences in rooting depth or water-use efficiency could also influence phenology, and decouple it from hydraulic safety. To evaluate the impact of soil moisture and plant drought tolerance in driving plant phenology, we compared drought tolerance and growth characteristics of 10 species from two plant functional groups (forbs and graminoids) in a semi-arid grassland. We found a trade-off between hydraulic safety vs. efficiency (maximum leaf hydraulic conductance) among this set of species, driven primarily by functional group differences: graminoids showed greater hydraulic safety but lower hydraulic efficiency than forbs. There were no differences in maximum photosynthetic rates between the functional groups, but grasses also displayed lower mid-day water potential than forbs across the growing season. These results suggest that graminoids are adapted to operate at lower leaf water potentials. Phenological patterns were explained by an interaction between drought tolerance and rooting depth (estimated with water isotopes), with greater hydraulic safety associated with later season activity only among shallow-rooted species. We suggest that understanding the phenological patterns in grasslands requires an understanding of both leaf-level drought tolerance and rooting depth.

Technical Abstract: Grasslands contain many plant species with a variety of phenological patterns, but whether water-use strategies are coordinated with phenology remains unclear. In semi-arid grasslands, soil moisture availability is a major driver of plant productivity, and interspecific differences in drought tolerance may contribute to the varied phenology of those species. The ability to resist hydraulic failure in leaves, estimated as the leaf water potential when leaf hydraulic conductance is reduced by 50% of maximum (P50), helps explain the distribution of tree species across broad climatic gradients. This trait may also explain phenological patterns in grasslands, as species showing less resistance to hydraulic failure may only be active when soil moisture is readily available. However, differences in rooting depth or water-use efficiency could decouple P50 from phenology. To evaluate the impact of soil moisture and plant drought tolerance in driving plant phenology, we measured 10 species from two plant functional groups (forbs and graminoids) in a semi-arid grassland. We measured maximum leaf hydraulic conductance (Ksat), P50, pre-dawn and mid-day leaf water potentials, seasonal patterns of gas exchange, plant and soil water isotopes, and several metrics of plant phenology that encompass flowering and leaf emergence/senescence. We found a trade-off between hydraulic safety vs. efficiency among this set of species (p = 0.04), driven primarily by functional group differences: graminoids showed greater resistance to hydraulic failure (p = 0.003) but lower Ksat (p = 0.003) than forbs. There were no differences in maximum photosynthetic rates between the functional groups (p = 0.28) and, combined with the fact that Ksat was lower in graminoids, this functional group operated at lower mid-day water potential than forbs across the growing season (p = 0.002). This suggests graminoids are adapted to operate at lower leaf water potentials. Phenological patterns were explained by an interaction between drought tolerance and rooting depth (estimated with water isotopes), with lower P50 associated with later season activity only among shallow-rooted species (p = 0.02). We suggest that understanding the phenological patterns in grasslands require an understanding of both leaf-level drought tolerance and rooting depth.