EAGER: How genetic variation in riparian trees influences stream succession and ecosystem function

Rivers are generally very old, carving through canyons and valleys over millennia. Because new streams and rivers are rare, little is known about how they form. One place we can learn more is in volcanic landscapes. After catastrophic volcanic eruptions, water from springs and snowmelt carve fresh stream channels. We can study the organisms that colonize these new streams, both along the river's edge and living in the water. The trees that line rivers and streams provide shade, shelter, and crucial inputs of leaves and wood to aquatic ecosystems. Trees help shape the stream itself by stabilizing banks, creating meanders, and halting erosion. Studying the establishment of trees along new streams is important because they can influence many other organisms and the ecosystem services they provide (e.g., water filtration and the breakdown of materials). We know very little about how these processes first begin, or whether different types of trees can engineer different types of streams. Just as there is great variation and diversity among humans, there is similar genetic variation within each species of tree, including for some tree species, differences between males and females.

This research addressed the influences of plant genetics in willow on in-stream ecosystem function, the succession of in-stream communities, and channel dynamics across four headwater streams at Mount St. Helens. Since the presence of riparian trees has the ability to influence channel dynamics and stream morphology, the genetics of colonizing trees may structure the entire stream network, extending genes-to-ecosystems research into evolutionary geomorphology. In trees that separate male and female flowers onto different individuals, this genetic difference may influence how streams develop since male and female trees allocate resources differently. This research has increased our understanding of successionary processes in streams and the influences of plant genetics on the development of stream ecosystems in a post-eruption volcanic landscape.

We published 16 journal articles during the period of this grant. We found that male and female willow shrubs colonize riparian habitats differentially (females colonize closer to the wetted edge of streams on the Pumice Plain, LeRoy et al. 2020). Female willow shrubs had lower %N, and higher C:N in summer leaf litter than males and female leaves decomposed more slowly once in the stream (LeRoy et al. 2020). Female willow shrubs were more infected by stem-boring herbivores, which increased branch death and summer litter inputs (Minsavage-Davis et al. 2023). These differences in combination support the idea that female willows are more likely to drop summer leaf litter into streams, providing altered organic matter inputs for a longer duration. We further explored the influences of willow plant sex on leaf litter chemistry and found significant influences of plant sex on nitrogen and a suite of plant chemicals using time-of-flight mass spectrometry (Ramstack Hobbs et al. 2022). Stem-boring herbivore-attacked litter (collected as summer leaf fall) had significantly higher %N, and lower C/N and condensed tannins (CT), than leaves that were unattacked and senesced in the autumn. We found that decomposition rates differed between female leaves compared to female flowers for willows, that macroinvertebrates colonized flowers at higher rates, and that overall communities of macroinvertebrates differed on leaves compared to flowers (Garthwaite et al., 2021). In addition, we found that female willows tended to have higher concentrations of mite galls, fungal rust infections, and tent caterpillar masses across the Pumice Plain of Mount St. Helens (Minsavage-Davis et al. 2023). An undergraduate student led an extensive review paper on plant sex influences on riparian communities and ecosystems (Scheuerell & LeRoy 2023). We established an experiment examining open/closed willow canopies on in-stream communities and ecosystem functions. Organic matter processing rates were significantly influenced by both the watershed and the open/closed canopy treatment (LeRoy et al. 2023).

We provided science outreach to a variety of audiences. We have given talks as part of lecture series in prisons, worked with high school students in the Mount St. Helens Institute's Summer Ecology program, participated in GeoGirls - a middle school program with USGS, worked with Upward Bound's first-generation college-bound program, hosted and gave a keynote address to 400 K-12 students for annual on-campus watershed ecology events, and have engaged with Evergreen alumni through annual events. PI LeRoy collaborated on an artist book, and a new 15-minute video for the Mount St. Helens Visitor Center to play on repeat.

We provided research experiences to 36 undergraduate students and 26 are co-authors on journal articles. PI LeRoy was awarded the Presidential Award for Excellence in Science, Mathematics, and Engineering Mentoring in 2021 and named a River Warrior by the Lewis Pugh Foundation in 2023. Former undergraduate students have gone on to recieve NSF Graduate Research Fellowship and Presidential Management Fellowship awards. 

Last Modified: 09/25/2023
Modified by: Carri J Leroy