High-elevation tree mortality shifts plant phenology in the Central Sierra Nevada
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Authors
North, Halina
Issue Date
2024
Type
Thesis
Language
en_US
Keywords
High Elevation , Plant Phenology , Tree Mortality
Alternative Title
Abstract
Large-scale tree mortality events are increasing in frequency and severity across the globe. Given that hotter and drier droughts are projected to continue with climate change, the cascading ecosystem level impacts of overstory canopy mortality may have unforeseen consequences on understory plant communities. Following a recent wave of drought-driven tree mortality events in the Central Sierra Nevada region, we investigated the potential impact of canopy cover loss at treeline on microsite conditions, and how this loss may impact the phenology of corresponding understory plant species. We asked: 1) Does a treeline mortality event create differing microsite conditions and influence plant community diversity? 2) is tree mortality altering understory plant phenology?, and 3) does the loss of canopy cover associated with tree mortality alter understory plant growth and morphology? To answer these questions, we monitored forb phenology weekly throughout the growing season, as well as annual diversity and PAR (photosynthetic active radiation) measurements, at 1m2 microsite plots under dead trees, live trees, and adjacent open areas at three high elevation study sites with varying levels of tree mortality in the Central Sierra Nevada in two drastically different climate years: 2022 and 2023. We found that microsite plots under dead trees had higher PAR values relative to those under live trees. Plants under dead trees showed earlier timing of key flowering phenological events relative to those under live trees. In addition to effects of tree canopy cover, date of first flowering and flowering duration were highly linked to site, temperature and snow depth, consistent with findings from many other high-elevation study systems. Furthermore, our study period covered two contrasting water years, and we observed diverging responses to weather on the species level as well as site- and year-level differences in phenology that were large relative to microsite differences. In the greenhouse we observed higher emergence rates for plants grown under a live shading treatment, though plants had higher total biomass in an open or no shade treatment. Monitoring changes in understory plant communities following large tree mortality events is important for understanding the effects on relationships with co-evolved pollinators and can provide insight to future changes in forest and plant community composition under novel climate conditions.