Where Climate Change and Invasive Species Intersect – the role of climate in green ash conservation

If you stand next to a stream or river in Vermont, you are likely amongst Green Ash trees. These trees are native to Vermont along with the majority of North America. While they prefer moist stream banks, they can be found in upland natural communities as well. Green ash trees have various ecological, economic, aesthetic, and cultural benefits.

The introduction of the Emerald Ash Borer (EAB), a green beetle that feeds on ash trunks, to North America in the early 2000s has significantly threatened and reduced green ash trees across the continent. It was first detected in Vermont in 2018 and has since been found in almost every county. While all ash trees are susceptible to EAB infestations, it appears green ash trees are specifically targeted.

“EAB is such a great and pervasive threat because it threatens a whole genus (all ash species) with little natural resistance to damage among North American ash,” explained Schaberg.

The EAB invasion has already killed over one hundred million ash trees across North America. Researchers and forest managers are working to find ways to protect the remaining green ash trees and preserve their many benefits. Both tree genetics and climatic variation have a role in predicting how trees will adapt to different climates and stressors. By studying green ash trees from different locations (with different genetic adaptations) grown together in one stand (a homogeneous environment), researchers can determine the role genetics have on adaptation – to and aside from environmental influences like climate. Then, researchers can use tree ring data to study how climatic variation impacts tree growth across the years.

In a new study led by Paul Schaberg (recently retired from the USDA Forest Service and UVM graduate faculty), researchers studied the growth and climate sensitivity of green ash trees from five different Plant Hardiness Zones at a Vermont test site (see figure 1). Researchers collected tree cores to determine the differences in growth trends over time. They measured basal area increment (BAI), a direct measure of wood production, which allows researchers to study growth trends. The research team expected trees from warmer climates to experience more shock to cold temperatures and snow. Researchers studied how differences in temperature, precipitation, and snow across the years were related to BAI growth over time.

Figure 1: Source codes, locations, and plant hardiness zones for the five green ash sources at the planting on the UVM campus.

Although there were only a few noticeable differences in the amount of growth between zones, the analysis of growth in relation to rainfall, temperature, and snow showed that trees from different zones reacted differently to changes in the climate. Trees from the warmest zone (PHZ 7) generally grew more, but these differences were not often statistically significant. For trees from all zones, growth increased during rainy years and decreased the year following a rainy year, possibly due to lingering effects. Similarly, high temperatures were correlated with less growth that year but with more growth the following year. Winter snow generally helped growth, but as expected, snow in spring or fall negatively affected trees from the warmest zones. Trees from the coldest zone (PHZ 3) only showed positive growth correlations with climate factors, had no lingering effects from previous years, and were not affected by snow. Over time, trees from the warmest zones grew more during periods of increasing temperature and rainfall.

Over time, trees from colder zones have grown at a steady or even slightly negative rate. The data cannot discern if this trend is due to the changes in temperature and precipitation. If so, the trend is likely to exacerbate as climate change worsens. Trees from warmer zones, on the other hand, showed steeper positive growth trends indicating their growth might benefit from modest increases in temperature and precipitation.

“The associations between climate and growth observed in our study are based on climate patterns seen to date,” shared Schaberg. “It is hard to predict how green ash growth will respond to future patterns that could subject trees to more extreme temperature and precipitation levels than those currently experienced. Nonetheless, our results support findings from similar studies with other species that indicate that genetic sources two hundred or more miles south of the stand’s location often grow better than local sources when climate extremes are not encountered.”