America's forests are in a race against climate change

The escalating threat of climate change, characterized by soaring temperatures and increasing drought, paints a grim future for America’s forests. 

A recent study led by researchers at UC Santa Barbara and University of Utah has found that while the majority of U.S. forests have the capacity to adjust to these harsh conditions, the rate of adaptation is not swift enough to escape the environmental stress.

The experts conducted a comprehensive investigation into the susceptibility of woodlands to severe drought conditions. The study, published in the journal Global Change Biology, was focused on mathematical models and data from both the U.S. Forest Service and plant physiologists. 

Good news and bad news

“We were concerned to find that forests were not changing fast enough to avoid increased water stress due to climate change,” said study lead author Greg Quetin, who serves as an assistant project scientist at the UCSB Department of Geography. 

Despite the concern, Quetin also emphasized the silver lining that lies ahead, stating that most forests in the continental U.S. contained enough functional diversity to increase their drought tolerance through shifts in species composition.

How forests adapt

Forests can adopt various strategies to adapt to drier conditions. Individual trees can modify their activities, physiology, and gene expression in response to the environmental changes they encounter. 

Moreover, species that are inherently drought-tolerant can gain dominance in the ecosystem. Evolutionary dynamics can also play a role in the form of natural selection, resulting in alterations in species composition, although the impact would likely be minimal over the next century given the long lifespan of trees.

Focus of the study

The team’s primary focus was to discern whether the traits and species present in the nation’s forests could allow them to acclimate to the impending climate changes without triggering widespread mortality. 

Quetin and his team analyzed data from the Forest Inventory and Analysis program – a comprehensive database maintained by the U.S. Forest Service that provides insights into the health and conditions of the country’s woodlands. They also leveraged information from the Xylem Functional Traits Database, containing measures of tree physiology and hydraulic traits.

Building on this robust foundation of data, the team devised a model simulating a forest’s reaction to heightened water stress, including processes such as photosynthesis, respiration, and growth, as well as plant stress. 

To understand how changes in leaf area could alleviate stress caused by changing environmental conditions, the researchers also integrated an optimization technique into the model.

Study co-author Professor Lee Anderegg noted: “All the data to date suggest that leaf area is just the biggest lever that individual trees can throw to manage water stress.” Therefore, forests situated in drier regions are expected to develop sparser canopies, while forests in wetter areas can sustain dense foliage.

What the researchers learned 

Upon running their model, the researchers found that approximately 88% of forests across the continental U.S. possess the requisite species diversity and trait variability to adjust to climate change, and these forests have already begun the transition. However, the speed of adaptation lagged behind the rate that the model indicated as necessary to prevent increased water stress and subsequent mortality.

Study co-author Professor Anna Trugman commented: “It’s concerning that we don’t see the required shifts that our model predicts need to happen.” Despite this, she remains hopeful, emphasizing that biodiversity could act as a significant buffer against the impacts of climate change on a given forest.

The rising concentrations of carbon dioxide in the atmosphere add a layer of complexity to the study. While higher levels of CO2 allow plants to decrease their pore size, thus limiting water loss and maintaining their photosynthesis rates, the drier atmosphere associated with a warming climate could increase water loss from leaves.

The energy costs involved in transporting this water cannot be overlooked either, as observed in a previous paper by the authors. 

Ongoing research 

Currently, the team is gathering data on the physiological changes in trees following climate-driven fires in Sequoia National Park. This will help to establish the extent to which trees can adjust their physiology, and to explore whether they can avert future water stress entirely through changes to their leaf area.

Forests are already demonstrating signs of change. As the atmosphere becomes drier, sparse canopies will become increasingly common, and the mix of species within woodlands will likely deviate from historical norms. 

These transformations will also impact the carbon storage capacity of forests, which currently sequester about 30% of human-made emissions. Recent findings from the group indicate a probable decrease in this capacity under climate change.

As we move forward, it becomes increasingly crucial to adopt management strategies that facilitate forest adaptation. Anderegg emphasizes the need to see forests not as static entities, but as dynamic systems that need to change in harmony with the shifting climate. 

This gradual transition could avert catastrophic events such as wildfires and widespread tree deaths, thereby protecting forests, wildlife, and nearby human communities.

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