Spatial and Climate Analysis of Bigleaf Maple
Decline in Western Washington
Rachel DeCordoba
Background
Spatial Results
Adviser: Dr. Patrick Tobin
Climate Results
Bigleaf maple trees (Acer macrophyllum) are an iconic species in Western
Washington, but starting around 2010, they have undergone significant dieback,
marked by tree mortality, reduced/yellowing leaf growth, and crown thinning. The
Washington State Department of Natural Resources (DNR) has examined the
presence of Verticillium wilt, Xylella fastidiosa bacteria, Phytophthera (a root
pathogen), and Armillaria (a parasitic fungi) as possible causes of symptoms and
mortality, determining that none of the pathogens were significantly present in all
symptomatic trees. In 2014, WA DNR sampled 56 affected sites, recording
symptoms of both individual trees and stands.
Temperature trends varied seasonally for all three cities, as predicted. Precipitation
varied as well, with more deviation overall. The most significant increases in
precipitation occurred in Centralia and Portland, the two cities nearest to sites
showing smaller percentages in mortality. The years preceding the first reports of
Bigleaf maple decline (2010) show periods of both drought and above average
precipitation.
A healthy Acer macrophyllum leaf (left)
compared to one with significantly
stunted growth (right)
A sampled tree
showing symptoms of
crown dieback.
Methods
Two sets of data were collected and analyzed for this project. The first set consisted
of National Oceanic and Atmospheric Administration (NOAA) temperature and
precipitation data during 2000-2015 from three cities throughout the general area
of DNR sample sites: Everett, WA, Centralia, WA, and Portland, OR. Any especially
unusual or severe trends may indicate climate reasons for tree stress and
subsequent decline.
Discussion
Trees with larger DBHs were assumed to be older in age. Tree ages were fairly
diverse amongst the site samples, with a slightly greater population of younger
trees than older trees (A). Individual trees exhibited a wide range of severity in
crown dieback, with some trees showing an unaffected crown and others in severe
decline (B). Stands surrounding sample trees exhibited a wide range of crown
thinning as well, with higher likelihood of severe thinning as opposed to mild (C).
Stands surrounding sample trees exhibited low levels of mortality overall, usually
between 0-10% (D).
Acknowledgements
Many thanks to my adviser Dr. Patrick Tobin for his help with guiding the project,
introducing me to the wonders of R for spatial analysis (and other functions), and
mentoring me throughout the process.
Thank you to graduate student Riley Metz for her help with ArcMap and organizing
data to be used between programs.
Thank you to Amy Ramsey and her team at the Washington State Department of
Natural Resources for providing background information about Bigleaf maple
decline, data from multiple symptomatic tree surveys, and information about
previous studies.
The second consisted of analyzing WA DNR symptomatic tree and stand data, based
upon the parameters of stunted leaf growth, mortality, crown thinning, and
diameter at breast height (DBH). Locations of all 56 trees were plotted in ArcMap
and spatially analyzed using the program R. Spatially significant clusters of affected
trees could indicate pathogen or other biotic influence.
Looking specifically at the variables of stand mortality and stand reduced leaf size,
there is very little spatial correlation between location and each variable’s intensity,
especially as distance betweens sites increases (left). Using Ripley’s L function to
look for deviations from spatial clustering, the spacing of all 56 sample trees was
found to be significantly clustered, as noted by the solid black, which is outside of
the expectation under complete spatial randomness (grey area) (right).
A map showing locations of the 56 tree sites surveyed by The
Washington State Department of Natural Resources in 2014, with
red indicating higher levels of stand mortality. The three cities
from which climate data was obtained are denoted as well.
This study found that climate patterns do not appear to have an effect on tree
death, although analysis with R found that symptomatic trees are significantly
spatially clustered. There is a significant correlation between mortality and reduced
leaf size with location, but no significance with regards to symptom intensity. The
spatial clustering of affected trees indicates that the cause of decline could still be a
biotic factor, though further specific studies will be necessary in order to determine
which pathogen, if any, is responsible for widespread symptoms.
•Big Leaf Maple Tree Data. 2014. Raw data. Washington DNR.
•National Oceanic and Atmospheric Administration, National Centers for Environmental
Information. Climate Data: Monthly Totals. 2016. Raw data.
•Ramsey, Amy. Crown dieback in bigleaf maple sampled for Verticillium wilt. Digital image.
Washington State Department of Natural Resources. Mar. 2012. Web.
•Ramsey, Amy. Healthy (1) vs. symptomatic (2) bigleaf maple leaves. Digital image. Washington State
Department of Natural Resources. Mar. 2012. Web.