Many grassland ecosystems are characterized by their diverse set of species and corresponding variability phenological patterns, but the drivers of those phenological patterns remain 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 observed phenology in these systems. 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 patterns of water-use efficiency could decouple P50 from phenology. In order to begin evaluating 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 system. We measured maximum leaf hydraulic conductance (Ksat), P50, pre-dawn and mid-day leaf water potentials (ψpre and ψmid, respectively), 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), but this relationship was driven primarily by functional group differences rather than a continuous distribution along a common spectrum. Graminoids showed greater resistance to hydraulic failure (p = 0.003) but lower Ksat (p = 0.003) than the 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 ψmid than forbs across the growing season (p = 0.002), suggesting that graminoids are adapted to operate at lower leaf water potentials. Phenological patterns were not explained by leaf-level drought tolerance traits or Ksat, but an interaction between drought tolerance and rooting depth (estimated with water isotopes) did explain the timing of leaf senescence in this system (p = 0.02). Our results show distinct differences in hydraulic traits between graminoids and forbs, but suggest that understanding the phenological patterns in grasslands will require an understanding of both leaf-level drought tolerance and rooting depth.