COS 27-1 - Population-level drought responses in northern oak fern (Gymnocarpium dryopteris)

Background/Question/Methods

Ferns serve as an invaluable indicator species for soil moisture and distribution patterns of other plant groups. Northern oak fern (Gymnocarpium dryopteris), a common fern in the interior of British Columbia, Canada, has received little research attention despite its prevalent use in the forest resource sector as an indicator species for moisture. To understand the impact of water deficits on oak fern, we sampled in two different ecoregions in interior British Columbia, a wet-cool ecoregion and a dry-warm ecoregion. We recorded population density and soil moisture content at each site. We then translocated 25 ferns from each site to the University of Northern British Columbia campus where we assessed acclimation to drought, by measuring the size and abundance of stomata using microscopy and NIS Elements software. We subjected the ferns to an 18-day period of drought, and recorded the response to stress by measuring the photosynthetic efficiency and vigour of the oak fern populations over time. We analyzed the measured data to determine significant difference between sites and ecoregions using Kruskal-Wallis tests, ANOVA and GLMs, and correlated changes over time to environmental conditions.

Results/Conclusions

The ferns growing in the wet-cool region were distributed more widely and were less drought-accustomed, as evidenced by larger and more abundant stomata, than those from the dry-warm region. The higher general incidence of soil moisture across the wet-cool region led oak ferns to encroach on more mesic sites, where there was enough moisture for survival, but less competition from larger fern species occupying hydric sites. The consistently higher moisture deficit within the dry-warm ecoregion drove the overall population of oak ferns to wetter pockets, which was a form of drought avoidance not observed in the wet-cool region. During the drought experiment, oak ferns from both populations were significantly reliant on soil and air moisture to maintain photosynthetic efficiency, and < 20% of the population remained healthy after 18 days. The stress response of both populations followed a similar pattern; differences in stomatal size and abundance did not result in differences in drought tolerance. We propose that a critical water stress threshold for oak ferns occurred when the soil moisture reached 15% combined with a relative air humidity below 75%, after which lower water potential likely caused stomatal closure and lower photosynthetic rates across both populations of oak fern.