Drought response syndromes, microclimates and vulnerability of a woodland community during severe drought

Background/Question/Methods

One of the most severe droughts on record occurred in our study region in California during 2013 and 2014 and may extend further into 2015. To explore the effects of this drought on physiological, population and community dynamics, we looked at the in situ drought response of dominant tree species in oak and mixed oak/conifer woodlands in three field sites across a regional climate/drought gradient and across hill to channel microenvironmental gradients within sites. We measured physiological and demographic responses of grey pine (Pinus sabiniana), California juniper (Juniperus californica), valley oak Quercus lobata and blue oak (Quercus douglasii) (all species in one site, Quercus species across all three sites), with the goal of relating species-specific suites or syndromes of drought response strategies to differential recent performance under extraordinarily dry conditions. Three of our four study species are high biodiversity value endemics to the California Floristic Province and our results may help anticipate possible future species trajectories under projections of increasing climate water deficits in the region.  

Results/Conclusions

Hydraulic vulnerability curves differed significantly between species at individual sites, but not within species across sites or between microsites. Sapwood specific conductivity (K_S), leaf specific conductivity (K_L) and percent loss of conductivity (PLC) at native (minimum observed field predawn) water potentials differed between species at individual sites and within species across sites. At our site where hilltop and channel oaks appeared to be extracting water from different sources based on stemwater isotope values (shallow soil water versus deeper groundwater), native K_L and K_S also differed between the microsites, with higher values associated with individuals near the channel than the hilltop. Depending on the hydrologic structure of individual sites, local spatial position on the landscape related to groundwater access may help to maintain hydraulic function during times of very low precipitation.