COS 202-6 - Recent trends in urban tree health are explained by variation in historical investment across neighborhoods

Wednesday, August 17, 2022

4:45 PM – 5:00 PM EDT

Location: 512E

Coauthor(s)

JM

Jason C. Maxfield

Portland State University, United States

Presenting Author(s)

IZ

Ingrid Zoll

Reed College, United States

Coauthor(s)

TC

Tyler Camp

Portland State University, United States
SG

Sandhya Gunarathne

Portland State University, United States
IA

Idowu Ajibade

Portland State University, United States
KM

Kevan Moffett

Washington State University, United States
TR

Todd Rosenstiel

Portland State University, United States
VS

Vivek Shandas

Portland State University, United States
KG

Kate Gregory

Portland State University, United States
AB

Axcelle Bell

Portland State University, United States
LW

Lea Wilson

Washington State University Vancouver, United States
GP

Garett Pignotti

Portland State University, United States
KB

Kyle Blount

Portland State University, United States
FR

Fernanda Ribeiro

Portland State University, United States
AR

Aaron Ramirez

Reed College, United States

Background/Question/Methods

Tree health presents a critical tipping point for urban ecosystem function that drives both positive ecosystem services–when trees are healthy–and negative disservices via poor health or mortality. Therefore, understanding the relationship between tree health and the abiotic, biotic, and social factors that determine it increases understanding of urban ecosystem function. Recent studies have revealed inequitable distributions of urban trees; generally, tree canopy correlates with higher socio-economic status and levels of public investment. However, the link between these factors and tree health is not well understood.

We hypothesized that abiotic, biotic, and social factors interact to explain patterns of tree health in US cities. To test this, metrics of tree health and physiology were assessed for mature individuals of the 4 most common large tree species in Portland, OR, USA: Acer macrophyllum, Acer platanoides, Pseudotsuga menziesii, and Thuja plicata. During summer 2021, sampling was completed across eight neighborhoods with contrasting levels of investment and abiotic conditions. Importantly, this study took place following the 2021 “heat dome” event in the US Pacific Northwest, allowing evaluation of both acute and chronic tree stressors. Following sampling, time-matched, 4-band satellite imagery (Planetscope, 3m) was obtained for each tree, and correlated with ground-based measurements.

Results/Conclusions

Our study investigates the effects of disparate investment in green infrastructure and management, and resulting heterogeneity in abiotic conditions (temperature and air quality) on individual tree health in a U.S. city with high levels of historical discrimination at the neighborhood scale: Portland, OR.

Significant variation was observed for long-term crown damage (missing canopy, branch dieback), acute foliar scorch (caused by the heat dome event), as well as physiological parameters like chlorophyll fluorescence (leaf-level stress), and stomatal conductance (water relations) across neighborhoods. Across species, foliar scorch damage in response to the heat event varied significantly, with western redcedar experiencing much higher rates of damage than other species. Field observations also revealed different rates of active watering for trees across neighborhoods, as well as significant variation in stressful abiotic conditions (e.g., air temperatures). Satellite-derived vegetation indices (NDVI, NDRE) correlated significantly with multiple tree health attributes, including acute foliar scorch, allowing for a city wide assessment of heat dome impacts on Portland’s urban forest canopy. These findings provide a foundation for expanded understanding of the spatial variation in tree health in cities and the complex socio-ecological factors that drive it.