Ferns

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What do you think of when you picture ferns? Perhaps you think of a shady forest floor or Jurassic Park. While many ferns thrive in tropical shady habitats, ferns are not restricted to places with lots of water. Some fern species thrive in dry ecosystems like the desert or chaparral, which experience long seasonal droughts every year. In the plant physiological ecology lab, we investigate how these ferns can survive seasonal drought, and what makes these ferns different from most other ferns.

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Ferns have diverse water use strategies

In the last decade, southern California experienced an unprecedented chronic drought accompanied by high temperatures. Our lab examined the seasonal water utilization traits of eight fern species in the Santa Monica Mountains surrounding Pepperdine. These eight fern species coexist side by side in the chaparral understory. So far, we have found incredible variation in the way that these ferns use water to survive. Their diverse water use strategies likely reduce competition for this precious resource, allowing them to share the same habitat. Our research on seasonal water use patterns in these eight fern species is published in the American Journal of Botany¹.

We have found some ferns that are highly dehydration tolerant, remaining green throughout the long summer drought. For example, Dryopteris arguta (pictured above) can survive water potentials (water status) surpassing -8 MPa, making it one of the most dehydration tolerant ferns ever reported in the literature. Some ferns “escape” the drought through dormancy, dropping dead leaves in the summer and growing new ones in the wet season.

Resurrection Ferns

Perhaps the most exciting survival strategy is that of the resurrection ferns. These ferns become completely desiccated (dried out) during the summer drought. When it rains, the “dead” leaves uncurl and come back to life! Here are two resurrection ferns after only 0.2″ rain. You can already see the shorter leaves uncurling and turning green.

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Most resurrection plants are small and non-vascular (e.g., mosses), and resurrection happens quickly because the whole plant gets wet at once. However, ferns are vascular plants, so resurrection ferns have the added challenge of restoring hydraulic flow through the vascular system. During her graduate research, Dr. Holmlund and her collaborators used targeted irrigation experiments to determine how water moves through a resurrecting fern. This research showed that whole-plant recovery is largely driven by root pressure, a phenomenon where the roots push water into the stem and leaves². Root pressure has been often observed in other plants (such as grasses), but our project was the first to document the importance of root pressure in resurrection fern recovery.

Collaborating with Drs. Anna Jacobsen and Brandon Pratt at CSU Bakersfield (former Pepperdine students!), Dr. Holmlund used high resolution micro-computed tomography (microCT) to visualize the movement of water through ferns while they were drying out and resurrecting. This technique allows us to see where the water is moving inside the plant without cutting the plant open.

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We found that the stem vascular system in Pentagramma triangularis experiences reversible desiccation, with the xylem (water-conducting) tissue being the first to dry out and the last to rehydrate³. Instead of dying, the living cells compress as they dry out and expand while they rehydrate. Furthermore, we used fluorescence microscopy to characterize the chemical composition of various tissues in the stem. We found that the xylem conduits are likely more flexible than xylem in most plants, allowing these conduits to bend without breaking during desiccation³.

Future Projects

There are so many unanswered questions about ferns in the chaparral! For example, the Woolsey Fire burned the Santa Monica Mountains (including Pepperdine’s campus) in 2018. How will the ferns recover post-fire? Also, climate change models predict longer and more severe droughts in the future. Will the chaparral ferns tolerate more extreme water deficit? Or will some species become locally extinct? Are resurrection ferns best adapted to survive longer droughts by hibernating in the desiccated state?

Pepperdine students interested in fern research are encouraged to email Dr. Holmlund about research opportunities in the plant physiological ecology lab. See also the literature cited list below for peer-reviewed research articles on the ferns around Pepperdine’s campus.

Literature Cited:

1. Holmlund H.I.*, V.M. Lekson*, B.M. Gillespie*, N.A. Nakamatsu*, A. M. Burns*, K.S. Sauer*, J. Pittermann and S.D. Davis. 2016. Seasonal changes in tissue water relations for eight species of ferns during historic drought in California. American Journal of Botany 103(9): 1607-1617.
2. Holmlund H.I., S.D. Davis, F.W. Ewers, N.M. Aguirre, G. Sapes, A. Sala, J.Pittermann. 2020. Positive root pressure is critical for whole-plant desiccation recovery in two species of terrestrial resurrection ferns. Journal of Experimental Botany 71(3): 1139-1150.
3. Holmlund H.I., R.B. Pratt, A.L. Jacobsen, S.D. Davis, J. Pittermann. 2019. High‐resolution computed tomography reveals dynamics of desiccation and rehydration in fern petioles of a desiccation‐tolerant fern. New Phytologist 224(1): 97-105.

*undergraduate research student