View through conifer tree tops onto the forest below.

Research

DRIVERS OF TREE DEMOGRAPHY IN RESPONSE TO CHANGING ENVIRONMENTS AND SHIFTING DISTURBANCE REGIMES

Tree mortality from drought and many other compounding biotic and abiotic factors is increasing across the globe. In the northwestern US, more frequent and severe droughts are expected in the coming decades. Understanding how forest communities are going to respond to drought and other environmental stress and disturbance in the future will be essential for effective predictions of changes to forest functioning, and management applications.

Old growth Douglas Fir and Port Orford Cedar grove in the Six Rivers National Forest in northern California.

TREE-MYCORRHIZAL FUNGAL ASSOCIATIONS

Mycorrhizal fungi associate with more than 90% of terrestrial vascular plants on Earth, and are fundamental to nutrient cycling, carbon storage, and plant productivity across diverse ecosystems. Symbiotic associations with mycorrhizal fungi can improve tree resistance and resilience to drought and other stressors by improving nutrient acquisition and water uptake. Tree traits, such as root architecture, are also fundamentally tied to these important functions, and have been documented to vary in response to their mycorrhizal associations.

Mycorrhizal associations play an essential role in drought responses, tree demography, and the capacity of forests to perform important ecosystem services like supporting biodiversity and sequestering carbon. My work has characterized the taxonomic and functional composition of mycorrhizal fungal communities associating with tree species in sites from northern Washington to northern California.

Left: The hartig net of ectomycorrhizal fungi on roots of western hemlock in southwest OR. Center: Fungal mycelium in rhizosphere of western white pine in northwest CA; Right: Fungal mycelium in rhizosphere of Pacific silver fir in central OR.

TRAIT-BASED APPROACH TO STUDYING TREE DEMOGRAPHY

The use of traits instead of taxonomic information offers a more mechanistic approach to understand how species respond to their abiotic and biotic environment. Frameworks that evaluate plant strategies for resource acquisition and allocation often apply an economic lens - where plants can either exhibit a conservative (and typically slow-growing) strategy, or an acquisitive (and fast-growing) strategy. However, field studies frequently find that trait-growth relationships do not align so neatly with the theory. I have measured tree leaf and root traits in a long-term research forest to test whether these weak trait-growth relationships are due to the frequent exclusion of belowground traits and biotic interactions that are known to have important effects on tree performance.

Left: Foliage of noble fir in central OR; Right: Root sample of Pacific dogwood in northwest CA.