High resistance to drought-induced cavitation in ferns of the coast Redwood forest

Background/Question/Methods

Ferns grow throughout the mesic range of the coast redwood forest ecosystem of Northern California.  Ferns in the southern ecosystem range experience greater climatic drought conditions every year due to less rainfall and higher evapotranspiration. We hypothesized that ferns from wetter, northern forests would therefore exhibit increased vulnerability to drought-induced cavitation relative to ferns from drier forests when seasonal leaf water potential declines.  During summer of 2010, we measured the midday leaf water potential in the field, frond allometry, and stipe vulnerability to cavitation curves in the laboratory of four dominant fern species in three forests locations along the north-south gradient of the redwood forest ecosystem. We sample two evergreen ferns, Polystichum munitum (Western sword fern) and Woodwardia fimbriata (giant chain fern), and two deciduous ferns, Pteridium aquilinum (bracken fern) and Adiantum aleuticum (five-finger fern).  

Results/Conclusions

All four ferns species maintained high leaf water potential between -0.18 and -1.0MPa during midsummer when evapotranspiration is highest and rainfall is lowest. Frond length and surface area increased slightly in ferns sampled from more northern sites for all species except W. fimbriata which exhibited greater frond length and area in the south. There were no significant intraspecific differences in vulnerability to cavitation evident in any of the species. However, the evergreen ferns exhibited higher resistance to cavitation (P. munitum P₅₀= -6.5 ± 2.3 SD; W. fimbriata P₅₀= -6.1 ± 1.1 SD) than the deciduous ferns (A. aleuticum P₅₀= -3.1 ± 0.27 SD; P. aquilinum P₅₀= -2.7 ± 0.39 SD). Our results suggest that redwood forest ferns maintain high water status despite their large leaf surface area that promotes high transpiration during midsummer drought conditions. Evergreen ferns exhibit greater resistance to cavitation (more negative P₅₀ values) likely because their xylem must supply water to the fronds year-round even if drought is intense, while deciduous ferns can shed their fronds when water availability drops and hydraulic conductivity decreases. Both drought tolerance strategies will promote fern survival in the redwood forest as drought conditions intensify with climate change.