Oak Trees’ Drought Resilience is Rooted in Microbes

The oak tree is majestic, long-lived, and adds to the natural beauty and ecological health of woodlands. These woody sentinels also serve as living records of climatic and environmental history. One of the key adaptations that researchers can track is the trees’ microbiome, which is crucial for their resilience.

However, tree-associated microbiota’s responses to stressors such as drought, nutrient stress, and pests and diseases remain poorly understood, particularly in long-lived plants like oak trees. This motivated James McDonald, a microbial ecologist at the University of Birmingham, and his colleagues to track changes in the microbiome over a two-year period under various conditions.

In a recent study published in Cell Host & Microbe, the researchers found that oak trees maintained relatively stable microbial communities with subtle shifts in response to drought stress.¹ They observed an increased abundance of Actinobacteriota, which are linked to drought tolerance, and other bacterial and fungal genera, suggesting that the oak trees can recruit beneficial organisms under stressful conditions. These changes could help researchers identify additional bacterial biomarkers as trees adapt to climate change.

First, McDonald and his colleagues characterized the baseline microbiota of 144 oak trees by sampling leaf, inner bark, and root tissues. They observed that each area had a distinct microbial profile. Then, the team set up an experimental woodland plot to manipulate various environmental challenges: abiotic stressors, including drought and nutrient limitation, applied alone; and biotic stressors, including the introduction of bacteria and beetle larvae, applied in combination with the abiotic treatments.

For this, the setup involved rain exclusion shelters around some trees to simulate drought. Meanwhile, the team ringbarked another group, which involved stripping bark around the circumference of the tree to impair water and nutrient flow. Lastly, the researchers introduced bacteria and beetle larvae associated with acute oak decline, a disease that causes wounds, to a subset of trees from each group. The researchers hypothesized that microbial communities would fluctuate within the 40-year-old trees in response to these environmental challenges.

The researchers collected samples at various time points over a two-year period and used DNA sequencing to assess microbiome changes in the leaves, stems, and roots. “The scale of our study is pretty unique because we were able to study a large number of trees in the same area, where you have the same soil and the same amount of sunshine and wind and so on, and manipulate their conditions to overlay these different types of environmental stress,” said Sandra Denman, a study coauthor and plant pathologist at Forest Research in a statement.

Contrary to expectations, both above- and below-ground microbial communities remained largely stable throughout the experiment. Only prolonged drought at the final timepoint produced minor shifts in the root microbiome, including an increase in drought-associated Actinobacteriota and other bacterial and fungal taxa. This subtle microbial change may serve as a biomarker for drought conditions, raising the question about other microbial markers that arise under stress.

Building on these results, the researchers plan to explore the molecular mechanisms behind microbe-driven resilience and extend their work to older trees across different locations. McDonald added, “We should start to think about how changes in climate and environmental perturbation might influence not just disease severity, but also biogeochemical cycles, and the role that trees play in carbon sequestration,” in a press release.