The forest floor cracks under relentless summer heat. Leaves curl inward, conserving what little moisture remains. To the casual observer, the woodland appears silent. But beneath the surface, an urgent conversation is taking place. It’s a sophisticated emergency response system that transforms individual trees into a cooperative community fighting for survival.

Mycorrhizal networks—the underground fungal threads connecting tree roots—have captured public imagination as the “wood wide web”. But the most remarkable aspect of these networks reveals itself during times of scarcity. When drought strikes, tree communication shifts from routine maintenance to crisis mode, demonstrating that forests are resilient communities that share, sacrifice, and survive together.

The emergency alarm system

When soil moisture drops, trees detect the threat early through hormonal changes in their roots and immediately begin broadcasting warnings. The primary messenger is abscisic acid (ABA), the “stress hormone” of the plant world. As drought intensifies, trees release this chemical signal through their roots into the soil and fungal networks, creating a cascade of preparation throughout the forest.

But chemical signals aren’t the only language trees speak during drought. Recent discoveries reveal that trees also use electrical impulses transmitted through mycorrhizal networks, a phenomenon researchers liken to a plant nervous system. While chemical warnings can take hours or days to spread, electrical signals travel at centimeters per second, providing neighbors with rapid alerts.

Trees also release volatile organic compounds (VOCs), which are airborne chemicals that neighboring trees can detect and respond to. One tree’s distress becomes everyone’s early warning, triggering defensive responses like closing stomata to conserve water, even before those neighbors experience the full impact of drought themselves.

Sharing when there’s nothing to share

Perhaps the most counterintuitive aspect of tree communication during drought is what happens next: trees begin sharing their dwindling resources. This seems to defy logic. Why would a water-stressed tree give away precious carbon or water to its neighbours?

Research on interior Douglas fir provides compelling answers. Studies have shown that when trees are connected by mycorrhizal networks, water transfer between individuals increases precisely when drought conditions worsen. In controlled experiments, Douglas fir seedlings with access to water through their roots transferred that water to drought-stressed neighbors via fungal networks. The facilitation of seedling establishment through mycorrhizal networks appeared to increase as water stress intensified.

Carbon transfer tells a similar story. Even when photosynthesis slows during drought, carbon continues to move through the network. This exchange operates on both altruism and enlightened self-interest: keeping neighbors alive maintains the integrity of the network itself. A dead tree means a broken connection, reduced fungal health, and ultimately less resilience for everyone.

The evidence shows measurable benefits! Carbon gains of just 2-3% through mycorrhizal networks have been associated with fourfold increases in seedling establishment. In a drought, such margins can mean the difference between survival and death.

The role of mother trees

Not all trees contribute equally during drought. Old-growth trees play an outsized role in drought resilience because of one key advantage: deep roots. Forest ecologist Dr Suzanna Simard calls these “mother trees”. 

While young seedlings might access water only in the top meter of soil, century-old trees send roots many meters down, reaching moisture reserves that persist even when surface soils turn to dust. These ancient trees don’t hoard their access to deep water. Through hydraulic redistribution, large trees lift water from deep soil layers at night and release it into shallower soils where younger trees can access it.

The mycorrhizal networks associated with these hub trees act as conduits for this deep water, distributing it to dozens of shallow-rooted neighbors. Research in Douglas fir forests found that old trees facilitate the establishment of conspecific seedlings, particularly under drought conditions. Remove these mother trees, and the entire network’s drought tolerance collapses.

This is why logging practices that remove the largest, oldest trees dismantle the forest’s emergency response infrastructure.

When different species share a crisis

Drought communication isn’t limited to trees of the same species. The mycorrhizal networks that connect Douglas firs also link them to paper birch, ponderosa pine, and other species sharing the same forest. This cross-species cooperation becomes especially critical during water scarcity because different species employ complementary strategies.

Some trees, like ponderosa pine, have evolved as drought-avoiders with rapid stomatal closure at the first sign of water stress. Others maintain photosynthesis longer but at a higher water cost. When connected by networks, these different strategies benefit the community. Trees with deeper roots share water with shallow-rooted species.

Studies have demonstrated that defoliated Douglas fir trees transfer photosynthetic carbon to neighboring ponderosa pine seedlings through mycorrhizal networks. It’s a remarkable example of one stressed tree supporting another species entirely. This interspecies generosity helps explain why diverse forests handle drought better than monocultures. Different species provide redundancy, complementary resource access, and varied stress responses that stabilise the entire community.

During severe drought, trees also coordinate strategic sacrifices. Self-pruning of branches, coordinated leaf drop, and even the death of some individuals can reduce overall canopy water demand, allowing the network as a whole to persist.

When networks fail

There are limits to what cooperation can achieve. When drought becomes severe enough that multiple trees simultaneously enter crisis mode, the support system can be overwhelmed. Mycorrhizal fungi themselves need water to function, and when soil moisture drops below critical thresholds, the physical connections between trees begin to break down.

This is when forest die-offs occur: not because individual trees fail, but because the entire networked community reaches a breaking point together. Human activities accelerate this breakdown. Logging fragments mycorrhizal networks. Soil compaction crushes the delicate fungal threads. Monoculture plantations lack the species diversity that provides network resilience.

Building drought-resilient forests

Understanding how trees communicate during drought should fundamentally change how we approach reforestation. The implications are clear: we need to plant for networks, not just tree counts.

This means several practical shifts. Species diversity matters enormously. Mixing drought-tolerant and drought-sensitive species, combining deep-rooted and shallow-rooted trees, creates the redundancy and complementary strategies that enhance network resilience.

Protecting old trees during any planting operation is critical. They are the communication hubs and water access points that allow young trees to survive establishment. Reforestation projects should identify and protect existing large trees as anchor points for new networks.

Soil health determines network health. This means minimising soil disturbance during planting, avoiding compaction, and potentially inoculating seedlings with mycorrhizal fungi adapted to drought conditions. As a fee for services, mycorrhizal networks retain about 30% of the sugar that trees photosynthesise.

Spatial design matters. Isolated patches of trees struggle more during drought than connected corridors where mycorrhizal networks can extend continuously. Planting in patterns that facilitate network formation optimises the cooperative benefits while minimizing competitive costs.

The final word

What trees reveal during drought challenges our fundamental assumptions about nature. We’ve long viewed forests through a competitive lens: trees fighting for light, water, and nutrients. But the underground conversations during water scarcity tell a different story.

Trees facing drought don’t face it alone. They send warnings, share resources, coordinate responses, and sometimes sacrifice themselves to keep the network alive. Old trees become emergency water sources for the young. Different species compensate for each other’s weaknesses. The forest survives not despite scarcity, but through a sophisticated system of mutual support that activates precisely when conditions are most dire.

This understanding should reshape our forest management and restoration practices, as we do at EcoMatcher. We understand that when we plant a tree, we’re not adding an individual organism to a landscape. We’re adding a voice to an ancient conversation, a node to a resilient network, a participant in a community that has evolved to survive together.

As droughts intensify with climate change, the forests that will persist are those with strong communication networks intact. By protecting old-growth trees, planting diverse species, maintaining soil health, and designing connected landscapes, we can help forests build the underground infrastructure they need to weather the dry years ahead.