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Western Swordferns’ Water Absorption Differences Likely Due to Leaf Structure

Polystichum munitum (Dryopteridaceae) varies geographically in its capacity to absorb fog water by foliar uptake within the redwood forest ecosystem

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Researchers found that western swordferns from the center of the north-south redwood range had the highest capacity for foliar uptake, or water absorption through leaves. Photo by photogjim2
Researchers found that western swordferns from the center of the north-south redwood range had the highest capacity for foliar uptake, or water absorption through leaves. Photo by photogjim2

The Swordfern: The Unexplored Link in Redwood Hydrology

In coast redwood ecosystems, many plants capture water in a surprising way. In addition to drawing water from their roots, certain species also absorb water through their leaves. This process, called foliar uptake, helps plants stay hydrated through California’s hot, dry summer months. As fog rolls in from the coast, plants can capture the moisture that hits their leaves—moisture that would otherwise evaporate or drip to the forest floor.

Researchers in the Department of Integrative Biology at the University of California, Berkeley, wondered if plants in different redwood forests shared the same capacity for foliar uptake and drought-relieving benefits from fog. They decided to investigate foliar uptake in western swordfern, an integral species in the redwood ecosystem. The plant, with its broad emerald fronds, can absorb more than twice as much water by foliar uptake compared to other plants in the redwood forest.

The researchers set out to study the ferns throughout the coast redwood range spanning seven old-growth redwood forests, from Prairie Creek Redwoods State Park in the north to Big Basin Redwoods State Park in the south.

In midsummer, which is peak fog season, the team collected fronds from each of the seven sites. They collected samples at dusk (between 8 and 9 p.m.), which is when forests are most likely inundated with coastal fog. Using a device called a Scholander pressure chamber, the team was able to measure each frond’s water potential, which indicates a plant’s level of hydration. (Leaves usually have a higher water potential compared to the atmosphere, causing water to travel from root to leaf, where it evaporates.)

Through a multistep process of weighing, wetting, then towel drying and reweighing the fronds, researchers were able to assess how much water was absorbed by foliar uptake. And by measuring fern cover at each site, the team was able to scale the results obtained from the sample fronds. In the end, the team found that ferns from the center of the north-south redwood range had the highest foliar uptake capacity. Fronds from most sites (besides Big Basin) increased their water content by 4 to 5 percent after being misted, while the fronds from the more centrally located Grove of Old Trees in Sonoma County saw an 11 percent increase in water content. Interestingly, ferns from Big Basin failed to absorb any water.

The researchers also found that larger ferns grow up north, in Prairie Creek Redwoods, while the smallest ferns grow at Big Basin, the southernmost site. This result is intuitive, since previous studies show that ferns grow to gargantuan proportions in the north, where the annual rainfall is much higher.

Overall, when it comes to foliar uptake, the results showed that western swordferns vary significantly across the region. Since the researchers extracted the fronds from their environment, it seems likely that any difference in foliar uptake has to do with leaf structure and not environmental factors. The results of this study show that there is much we don’t know about foliar uptake in the redwood ecosystem. By continuing to study this phenomenon, we might better understand the effects of drought on coast redwood forests across their range. In turn, this information can help land managers make informed decisions to help protect redwoods in the face of climate change.


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