Ferns living on the edge: Differential traits for survival during California’s historic drought

Background/Question/Methods

In 2014, California experienced the worst drought within the last 1,200 years.  Ecological comparisons among eight fern species in the Santa Monica Mountains of southern California provided insight into differential mechanisms of drought survival and physiological performance during this extreme event.  Among the eight species of ferns examined, we identified five life history traits relative to drought survival: 1) evergreen dehydration tolerant, 2) evergreen dehydration sensitive, 3) winter cold deciduous, 4) summer drought deciduous, and 5) summer desiccation tolerant (resurrection ferns).  Although species co-occurred in the same general habitat, each species preferentially occupied specific microsites, over a narrow range.  We monitored seasonal changes in water potential (ψ_min) and chloroplast function (F_v/F_m), assessed seasonal changes in tissue water relations using PV-curves (leaf-turgor loss points, Ψ_tlp), estimated capacity for foliar water uptake and leaf hydrophobicity (measuring the contact angle made by epidermal water droplets), correlated the mechanical strength of fern stipes and pinnae to dehydration tolerance (using an Instron Mechanical Testing Machine), measured photosynthetic and transpiration rates in situ (using a Li-6400XT gas-exchange system plus carbon stable isotope analysis), and estimated vulnerability to xylem cavitation.

Results/Conclusions

Evergreen ferns displayed the greatest disparity in seasonal dehydration, with riparian species persistently above -2 MPa but upland evergreen species repeatedly below -8 MPa. This disparity in seasonal dehydration corresponded to seasonal shifts in osmotic potentials at the turgor loss point (osmotic adjustment), with upland species achieving a Ψ_tlp of -5.7 MPa compared to -1.6 MPa for riparian species. Chlorophyll fluorescence (F_v/F_m) tracked seasonal water potential (r² = 0.86). Mechanical strength of stipes did not correlate with dehydration tolerance (r² = 0.005) whereas mechanical strength of pinnae strongly correlated (r² = 0.54). Our estimates of foliar water uptake correlated with minimum seasonal water potentials (r² = 0.71) and hydrophobicity of the leaf surfaces (r² = 0.52).  Species inhabiting waterfalls had the lowest foliar water uptake, presumably because it is not adaptive in these microsites.  Gas exchange rates varied three-fold among species. Water use efficiencies (both instantaneous and integrated values) were highest for species inhabiting sheer rock faces and lowest for waterfall species. Taken together, observed differences among fern species likely result from niche segregation and life history traits. We predict differential survival among fern species with the onset of a mega-drought currently forecast for California.