Seeing the forest for the seeds: masting and mortality in dry forest and woodland ecosystems of the Rocky Mountains and Colorado Plateau
Date
2022
Authors
Wion, Andreas P., author
Redmond, Miranda D., advisor
Pearse, Ian S., committee member
Stevens-Rumann, Camille S., committee member
Rocca, Monique E., committee member
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Abstract
For forests to persist on the landscape, tree recruitment must keep pace with tree mortality. Larger, more frequent, and more severe disturbances have raised concerns about the capacity for water-limited forests and woodlands to recover under increasingly hotter climates. For most conifer species, seed availability is the fundamental prerequisite to new tree recruitment. In addition, most conifer species are also masting species, meaning seed production is highly variable among years and synchronous within a population. Masting creates boom and bust patterns of seed availability that shape forest dynamics. In many species, it remains unknown what drives masting or how the mosaic of climate and competition shape patterns of seed production across species ranges. This limits our ability to forecast forest demography across large spatial areas and under uncertain climate futures. This dissertation is an exploration into two key processes driving forest persistence and loss, mast seeding and tree mortality, in two pine species native to dry forests and woodlands of the Rocky Mountains and Colorado Plateau: piñon pine and ponderosa pine. In chapter one, I examine the spatiotemporal drivers of masting in piñon pine across its latitudinal distribution in Colorado and New Mexico. I demonstrate that masting was driven by favorable weather conditions acting during key phenological periods of cone development: initiation and pollination. Cone production was sensitive to the spatial variability in long-term climate - cooler and wetter sites produced more cones, more frequently than hotter and drier sites. In chapter two, I examined cone production in ponderosa pine across a large portion of this species' distribution between Arizona and Wyoming. In contrast to piñon pine, I found that individual-level factors like tree size, age, and stand density, were better predictors of cone production than long-term climate or annual weather. In chapter three, I examined patterns of cone production between these two species jointly and related them to large scale modes of climate variability, like the North American monsoon and the El Niño Southern Oscillation. I found similar patterns, where piñon pine cone production was strongly associated with climate while ponderosa pine was not. Range wide synchrony in masting was associated with the North American monsoon, and regional north-south anti-synchrony was associated with the phase of the El Niño Southern Oscillation. Collectively, these chapters highlight contrasting masting patterns among these two, co-occurring pine species of dry forests and woodlands of the Rocky Mountains and Colorado Plateau. I explore the consequences of these contrasting life history strategies for forest management in this region, as well as the potential impacts of a changing climate and disturbance regime on seed production. In the final chapter of this dissertation, I evaluated four drought metrics on their ability to correctly predict piñon pine die-off following a severe drought in 2018. The results highlight how some of these simple climate metrics can be used as an indicator of piñon pine die-off in future, hotter drought events, and I provide a simple framework for evaluating mortality risk at regional scales.
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Subject
dryland
seed production
western United States
forecast
climate
tree die-off