Malosma laurina is a chaparral shrub species that dominates coastal exposures of the Santa Monica Mountains of southern California. Between 2012 and 2016, protracted drought with half normal rainfall over a five-year period (175 mm), led to severe dieback, exceeding 60% whole plant mortality at some sites. The drought dissipated in 2017 with an annual rainfall that exceeded the 140-yr normal by 108 mm. As a result of this elevated annual rainfall in 2017, we were interested in documenting the potential for full recovery of Malosma laurina as a keystone species of coastal chaparral shrub communities. We tested the hypothesis of full recovery by comparing plant water relations measured in June 2015 to June 2017. Specifically, we determined if there was a significant increase in the 1) water status of dieback plants (ψpd, predawn water potential), 2) water transport efficiency of stem xylem (ks), 3) rate of photosynthesis (A), and 4) rate of stomatal conductance to water vapor diffusion (gs).
Results/Conclusions
We found significant increases in all physiological parameters measured between 2015 and 2017. Differences were consistently lower for dieback plants than hydrated control plants in 2015 but were not significantly different between control and dieback plants in 2017. Predawn water potentials were -1.8 MPa (± 0.05 SE, n = 7) during the drought of 2015 but -0.8 MPa in 2017 (± 0.08 SE, n = 7). Likewise, stomatal conductance and photosynthetic rates were much lower in dieback plants in 2015 (23.8 mmol m-2 s-1 and 2.01 µmol m-2 s-1, respectively) than in 2017 (166.5 mmol m-2 s-1 and 12.4 µmol m-2 s-1, respectively). Dieback was primarily caused by protracted drought predisposing M. laurina to fungal infection of its stem xylem. Thus, xylem specific conductivity of stem xylem (ks) was much lower in 2015 (0.50 ± 0.14 mg mm-1 kPa-1 s-1) than in 2017 (4.10 ± 0.54 mg mm-1 kPa-1 s-1). Taken together, these results suggest a good possibility for the full recovery of Malosma laurina, assuming few subsequent drought years and adequate time for new recruitment of juveniles to replace greater than 50% dieback of M. laurina adults in the coastal exposures of the Santa Monica Mountains.