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This transverse section of an infected beech leaf shows that the diseased portion (left) is 249% thicker than the healthy portion of the leaf (right).

An Inside Look at Beech Leaf Disease

Beech trees provide food for animals, timber for wood products, and sustenance for beech drop plants, but they are under threat from beech leaf disease. The disease, first documented in 2012 in the Midwest, is associated with the nematode Litylenchus crenatae mccannii and is spreading rapidly throughout the central and northeast regions of North America. A team of scientists led by Craig Brodersen, professor of plant physiological ecology, and Leila Fletcher, postdoctoral associate, uncovered new insights on how the disease impacts leaves at the cellular level and provided a novel mechanistic explanation for the decline of the trees post-infection in a study published in Forest Pathology.

Brodersen first became interested in the disease after noticing the infected trees during a walk in the woods with his children.

“It’s a beautiful tree and an important part of the landscape. I wanted to do something with the tools we have to contribute to a better understanding of what the disease is doing to the tree. To come up with a solution, you have to come at it from as many angles as possible,” he says. 

The team studied leaves collected from beech trees at Yale-Myers Forest that hadn’t yet been infected by the nematode, and leaves from infected trees in New Haven’s West Rock Ridge State Park. The presence of the nematode influences the physical or hormonal regulation of leaf development, which leads to the distinctive dark green stripes on affected leaves, the research team says.

After comparing the leaf structures at the cellular level, the scientists also discovered that maximum photosynthetic rates were approximately 61% lower in symptomatic leaves and respiration rates increased as the percentage of affected leaf tissue increased. This combination causes a reduction of carbon assimilation capacity, potentially leading to tree mortality, the study found. 

“We hope this (research) leads to a better understanding of what the disease is doing to the tree and enables others to use that information to come up with new strategies to manage the disease,” Brodersen says. 

The Costs of Addressing Climate Change

How much will it cost to meaningfully reduce greenhouse gas emissions on a global scale? A study published in Science and co-authored by Matthew Kotchen, professor of economics, found that two of the world’s most prominent climate modeling approaches use differing measurements on the costs and benefits of emissions reductions. 

One is a “bottom-up” approach used by the United Nations Intergovernmental Panel on Climate Change (IPCC), in which the cost of reductions in emissions are inferred based on estimates of the direct financial expense. The other is the “top-down” approach used in integrated assessment models (IAMs) that is favored by economists. These address tradeoffs — how investment in greenhouse gas (GHG) mitigation comes at the expense of other potential investments. The costs of mitigation are considerably lower in the bottom-up approach used in a section of the 2022 IPCC’s Sixth Assessment Report than they are in standard economics-focused IAMs.

Kotchen says the study points to the critical question of which starting point is more helpful for understanding how costly it will be for societies to reduce emissions and the importance of gaining scientific consensus on the modeling calculations to form impactful policy.

“It is likely the IPCC estimates are overly optimistic about how little it will cost to reduce emissions, but the IAMs are also likely to be overly pessimistic,” says Kotchen, who co-authored the study with James Rising, assistant professor at the University of Delaware, and Gernot Wagner, climate economist at Columbia University. “The true costs are likely between these two extremes." 

How Climate Change Affects Flower Production in the Amazon

Forests in the western Amazon are among the wettest areas in the world, and climate change models predict they may face even warmer and wetter conditions. How these conditions will impact plant reproduction, which is essential in sustaining food webs and forest regeneration, is of particular importance in these forests because of the area’s high biodiversity and essential role in the global carbon cycle.

A study co-authored by Liza Comita, professor of tropical forest ecology and co-director of the Yale Center for Natural Carbon Capture, Simon Queenborough, senior lecturer and research scientist and director of the Yale Tropical Resources Institute, and Jason Vleminckx, a former postdoc in the Yale Institute for Biospheric Studies, analyzed a unique long-term dataset from Yasuni National Park in Ecuador of the flower production of 184 woody plant species from 2000-2018. The study, published in the journal New Phytologist, revealed a decline in community-level flower production that was correlated with an increase in temperature and relative humidity and a decrease in irradiance over the study period.

If these climate trends persist, they could significantly alter tree regeneration dynamics and reduce fruit resources of western Amazonia, posing a threat to the diverse animal and human populations that rely on these forests for sustenance, the researchers say. 

Scientists from Ecuadorian and U.S. institutions collaborated on the study. The research team is now conducting similar analyses to determine whether fruit production in this forest shows a similar trend.

Examining a Century-Old Urban Forest 

Urban forests provide a bevy of benefits. They sequester carbon, provide wildlife habitat, moderate local climates, and offer recreational and educational opportunities. Historically, however, there have not been many long-term studies on urban forests. Data collected from the Thain Family Forest, which the New York Botanical Garden (NYBG) has been stewarding for more than a century, has provided a rare opportunity for scientists from The Forest School at YSE to study a century of changes of its composition.

Using inventory data from 1937-2021, a team of researchers including Mark Ashton, Morris K. Jesup Professor of Silviculture and Forest Ecology, and Marlyse Duguid, Thomas J. Siccama Senior Lecturer in Field Ecology, documented changes in forest structure, diversity, and composition to analyze the health of the forest over time. 

The study, published in the Journal of Forestry, found that the forest has experienced significant change in species composition. Following the loss of eastern hemlock from an invasive insect, the forest increased its proportion and diversity of hardwood species such as oaks, hickories, red maple, black cherry, and American beech, and native species remained dominant. Despite the changes, forest canopy remained stable. 

With continued conservation and management, urban and rural forests may develop in similar patterns, encouraging regional approaches to forest management, the authors say.