Old, hollow trees may be the best climate shelter wild animals have

Trees protect wild animals from extreme heat and cold by keeping temperatures inside their cavities more stable than outside.

From Earth.com

Trees protect wild animals from extreme heat and cold by keeping temperatures inside their cavities more stable than outside.

Deep inside the trunk of an old tree, temperatures can remain surprisingly stable, even when the world outside is baking or freezing.

A new global analysis has found that tree cavities buffer their animal residents from extreme heat and cold almost everywhere scientists have looked, a discovery that could matter more than ever as the climate grows less predictable.

The study was led by Joy O’Keefe from the University of Illinois Urbana-Champaign.

Small temperature differences matter

From Oregon’s old-growth forests to Queensland’s eucalyptus groves, countless species depend on trees for shelter. Hollows, cracks, peeling bark: these small gaps in a tree’s structure double as homes.

That shelter is becoming more urgent by the year.

“We’ve seen mass mortality events in both birds and bats in recent years due to extreme heat,” O’Keefe said.

“Our meta-analysis finds that, on average, tree cavities can stay 3-4 degrees Celsius (5-7 degrees F) cooler than the outside air, with some larger or deeper cavities staying up to 15 degrees (27 degrees F) cooler.”

A few degrees might not sound dramatic, but when outside temperatures creep toward lethal levels, that gap could make a critical difference for animals sheltering inside.

The insulation cuts both ways too. On cold days, cavities run warmer than the outside air – a detail that can matter for animals during gestation, or for babies left alone in the nest while a parent forages.

Bats lead to a bigger question

O’Keefe and her collaborators have spent more than a decade studying how bats regulate their body temperature across different types of roosts, both natural and artificial.

That work already pointed to a clear pattern: tree cavities, peeling bark, and other natural crevices shield animals from temperature extremes far better than most artificial roosts, like the bat boxes conservationists often install.

Most of that research had focused on the eastern United States. But temperature extremes are hardly a regional problem.

The team started wondering whether tree cavities offer the same protection everywhere, not just in the forests they knew best.

Pulling together 65 years of data

To find out, conservation scientist and Illinois graduate Katrina Cotten teamed up with O’Keefe to run the first global literature review and analysis on how well tree cavities buffer temperature.

Cotten combed through the scientific record and pulled data from 36 relevant studies spanning 65 years and five continents.

Any study that measured the temperature difference between the inside and outside of a tree cavity made it into the analysis.

The pattern that emerged was remarkably consistent.

“This was a rare case where the data showed us exactly what we thought would happen,” O’Keefe said.

“Everywhere we looked, trees were great for buffering outside air temperature extremes.”

The gaps in the map

Consistency aside, the dataset has real blind spots.

Cotten notes that the available literature was heavily concentrated in the Global North, with no studies of tree cavity temperatures from South America or Asia, and none from Africa apart from South Africa.

That absence isn’t necessarily a sign that trees behave differently there. It’s more likely a sign of where research funding and attention have historically gone.

“There’s good reason to suspect that trees do the same things in those places, but it would be nice to study more types of trees and more types of environments,” Cotten said.

“It would be especially great to study trees in biodiversity hotspots, which also happen to be areas that are underrepresented in research.”

What this means for forest management

Despite the gaps, the researchers argue there’s already more than enough evidence to act on.

Protecting large or old trees, the ones most likely to have developed deep cavities over decades, looks like a straightforward way to shield tree-dwelling animals from a warming climate.

“Forest managers should focus on protecting and regenerating natural habitats for animals, especially in areas where you get heatwaves that can result in mass mortality events,” said co-author Reed Crawford, a postdoctoral researcher in O’Keefe’s group.

Crawford also points out where the usual substitutes fall short. Nest boxes and other artificial roosts, however well-intentioned, may not provide the same thermal protection as natural tree cavities.

“We know bat boxes almost always exceed outside air temperature, and then they don’t retain that heat at night,” he said.

“If people are compelled to use artificial roosts in their conservation plans, we need to improve their design to better replicate what’s happening in nature and try to keep those animals safe.”

For now, the message is fairly simple: an old tree, left standing, is doing more work than it looks like. Somewhere inside its trunk, an animal may be riding out a heatwave that would otherwise kill it.

The study is published in the journal Forest Ecology and Management.

1 Comments

anon6789@lemmy.world · 3 pts · 1d

Some thoughts/quotes from the full journal article.

In addition to protection from temperature, it is also critical shelter from other environmental factors like storms and flooding, which is also being exacerbated by climate change.

However, forested areas supporting cavity-bearing trees are in decline globally due to anthropogenic stressors. Crowther et al. (2015) estimated that ~15 billion trees are lost annually, mainly in tropical regions. Additionally, large trees, which typically support the most cavities, are especially at risk of being lost to anthropogenic disturbance.

I'm suspecting that is due to us having already removed so many trees from the global north over the centuries. Losing so many in tropical regions will probably have an increased impact on wildlife if those areas continue to be hotter and hotter.

While it is widely thought that tree cavities buffer outside air temperatures, only one study has aggregated literature on this topic (Maziarz et al., 2017). Further, no studies have statistically quantified the universality of this pattern and the global extent to which it has been studied.

That's moderately surprising no other studies have been done. Seems like it could have been an easy paper for a group to research. While it seems a fairly obvious conclusion, it never hurts to document things, especially with science denialism seemingly on the rise. It would probably also be helpful to know if this effect works better/worse in different areas or with different species.

We identified 36 relevant studies dating from 1958 to 2024 (see Supplementary Reference List), which were unevenly distributed across the globe, seasons, tree families, and wildlife species of interest. Studies were concentrated in North America (22% of studies), Australasia (42%), and Europe (22%), with 6% in Africa and 8% in Latin America (Fig. 2). Tree temperature studies most often occurred during summer months (Fig. 3A).

Only 20 studies reported tree species or family, but 21 tree families were represented overall. The most represented families were Myrtaceae (myrtle family, n = 9 studies), Salicaceae (willow family, n = 5 studies), and Betulaceae (birch family, n = 4 studies). The remaining 18 families were represented in 2 or fewer studies. Work was focused on taxa comprising various cavity users: bats (n = 15), birds (n = 11), other mammals (n = 9), and arthropods (n = 1; Fig. 3B).

It was uncommon for tree cavities to overheat in summer or to reach sub-freezing temperatures during winter. This global review shows that tree cavities could act as essential climate refugia for cavity-using animals. However, published studies have mainly concentrated on bats and other endotherms, while providing data on only a few tree species, mainly in the Global North.

If animals can find shelter in tree cavities that buffer outside heat extremes by 4°C (7 degrees F), they may be protected from heat stress so long as outside air temperature remains below 44°C (111F). Animals with access to large trees with deep hollows may be effectively buffered from even higher temperature extremes (e.g., (O’Connell and Keppel, 2016)). However, it is important to note that tropical species may have lower heat tolerance than organisms which inhabit more variable thermal environments in the temperate zone (Pollock et al., 2020). Thus, small amounts of environmental warming due to climate change could pose challenges for animals even if they inhabit stable microhabitats like tree cavities.

Through this review, we have identified significant knowledge gaps regarding tree microclimates. For many of the papers included in this review, studying tree microclimates was not the primary objective, as they often focused on wildlife and factors such as heterothermy, torpor, reproductive condition, nesting mortality, and roost selection; this meant we lacked data on factors like tree thickness or cavity depth, which would have been informative for our analyses. We see a critical need for rigorous study of how tree cavity temperatures respond to differing levels of solar radiation, humidity, and outside air temperature. Buffering capacities of trees vary by species, structure (e.g., height, diameter, cavity size, solar exposure), and decay stage (Grüebler et al., 2014, Johnson and Lacki, 2013, O’Connell and Keppel, 2016). However, many studies fail to consider or report these key characteristics, which makes it challenging to assess patterns across different systems.

Thermal and bioenergetic models of tree cavity microclimates are a promising tool that can substantially reduce the need for extensive field work when we have adequate data on the thermal properties of the specific tree cavity (Velander et al., 2023). These models can further be extended to artificial roost structures to screen potential designs for temperature suitability and comparability to tree cavities before they are deployed on the landscape (Bakken and O'Keefe, 2025). Gathering rigorous data on the tree (species, bark thickness, wood density, height, solar exposure) and cavity architecture (e.g., opening size, depth, humidity) will be essential for informing these models.

I've seen previous reporting of older artificial animal housing designs having difficulty coping with increased average temps, so this type of modeling could improve artificial housing needed in areas that have already lost their suitable trees.