The Trouble With “Ecological Thinning”: When Good Intentions Meet a Drying Forest

I’ve spent enough time walking through the Southern Jarrah Forest to know when something feels off. The air changes. The ground changes. The sound changes. These forests are meant to breathe like lungs, cool, damp and steady within themselves, not rasp like an animal that’s been running too long.

So when people suggest that large-scale “ecological thinning”, prescribed with little consideration of site-specific context and nuance, is the solution to forest decline, that we just need to open the canopy, let in more light, and the forest will thank us, I can’t help but feel they’ve mistaken the patient for the disease. What they call a sickness, dense regrowth, heavy shade, increased groundwater uptake, is really the forest’s recovery response to a century of logging, burning and drying. Thinning it again to “fix” those symptoms is a bit like bleeding a patient to make them more resilient to blood loss.

I’m not against management. I’m against the kind that mistakes the forest’s attempt to heal itself for something that needs to be managed out of it.

The Climate We Think We Know

Let’s start where most of these arguments do, with the claim that rainfall hasn’t really declined, that we’re just returning to “normal” after an unusually wet 20th century.

It’s true that from roughly 1900 to the 1960s, the southwest of Western Australia experienced a wetter anomaly, a pluvial period, compared with some earlier centuries. Palaeo-records from tree rings in inland southwest show there were similar dry phases before European settlement, so it’s not wrong to say rainfall has fluctuated over time.

But what matters isn’t just how much it rains. It’s how fast things are changing, how coherent that change is across the region, and what kind of landscape it’s happening in.

Studies show that while inland southwest has experienced comparable dry spells in the past, the coastal southwest, which includes the Jarrah forest, is a bit of a different story. Here, the decline since the 1970s has been sharper, more prolonged, and more spatially consistent than anything recorded in the past seven centuries. It also matches the changes predicted by climate models that account for rising greenhouse gas concentrations.

In other words, yes, the climate has always varied, but this time it is doing it differently, and in a forest that is already fragmented and stressed from a century of logging, grazing, development and burning.

So when someone insists that rainfall “hasn’t really changed,” what they are usually looking at is a single gauge in an area where forest cover and evaporation have changed dramatically. They are reading a weather record, not an ecosystem. The forest experiencing this drying is no longer the vast, connected system it once was. It has been carved into fragments by clearing and roads, its soils have been compacted and stripped of organic depth, its groundwater systems disconnected, and its canopy thinned again and again. In the past, intact forests could buffer change; moisture recycled through the canopy, shade protected the soil, leaf litter had time to break down and feed the soil, and animals and plants could move to cooler refuges when they needed to. Now, that network is broken. The same level of drying that a connected forest once absorbed can push a fragmented one past its tipping point.

The Good Idea on Paper

Let’s give thinning its due. The argument goes like this: the forest is overstocked after years of regrowth following logging. Thousands of thin, spindly trees compete for too little water, nutrients and light. So we thin it out, remove some stems, and the rest will thrive. The survivors will be stronger, grow faster and resist drought better.

On paper, that logic is sound. It has been observed in plantation silviculture and in some semi-arid forests around the world. But forests are not spreadsheets, and the Jarrah is not a plantation. It is a complex, self-regulating system that depends on density for its own stability.

When you remove too much canopy, you don’t just alter competition for water and nutrients. You alter the entire operating environment of the forest itself.

The Forest’s Internal Plumbing

Think of the canopy as the forest’s plumbing and insulation combined. When rain falls, leaves catch part of it in a process called interception. Some of that intercepted water evaporates straight back into the air, but that is not waste. It cools the forest air, maintains humidity, and helps recycle moisture through transpiration and condensation. It is part of the forest’s built-in air-conditioning system.

When canopy cover is reduced, interception drops. At first, more rain reaches the ground, and this is where the argument for thinning usually begins: “Look, the soil is wetter and the groundwater is higher.” In the short term, it can be. But with less shade and less evaporative cooling above, the surface warms, wind moves through more freely, and moisture is lost faster from the soil. The extra water that once moved slowly through leaf litter and humus now evaporates directly from the surface. The forest swaps a buffered cycle of moisture recycling for a rapid pulse of wetting and drying.

In the Wungong catchment trials, sometimes cited as proof of thinning’s success, one sub-catchment did show a short-term increase in groundwater recharge. The other didn’t. Even in the “successful” site, the treatment had to be repeated every nine years, with ongoing control of understory to maintain the effect. Without follow-up, the canopy rapidly recovered and streamflow gains reduced.

That’s not an ecological miracle, that’s a hydrological treadmill. And remember: Wungong was never designed to measure biodiversity or long-term forest structure. It was a water-yield study. Its aim was more water in dams, not healthier ecosystems.

When Water Becomes Heat

In a healthy forest, the canopy acts as a thermal shield. Through transpiration, trees release water vapour from their leaves, cooling the air in much the same way that sweat cools the human body. When too much canopy is removed, that self-cooling system collapses.

The ground begins to heat, the air dries, and the vapour pressure deficit, which is essentially the drying power of the atmosphere, rises. Remaining trees must close their stomata more often to avoid losing water, which slows photosynthesis and compounds stress.

At the same time, the understorey begins to change. Many of the plant, fungal and invertebrate species that make up the shaded forest floor community are finely tuned to the stable humidity and filtered light of a closed canopy. When that buffer is lost and sunlight and wind penetrate more deeply, the conditions they evolved to survive in can disappear within a single season. Some species simply persist for a time and then fade out; others are replaced by more open-country or disturbance-tolerant plants that further alter the microclimate.

The result is a self-reinforcing shift. What was once a moist, shaded forest interior begins to behave more like a woodland or heath. It does not need to be a dramatic change in appearance to be consequential. Once that feedback of drying, heating and canopy loss starts, it becomes very difficult to reverse.

Soil, Fungi, and the Forest’s Hidden Economy

Jarrah forests grow on some of the most nutrient-poor soils on Earth. Their survival depends on running a closed internal economy where nothing is wasted. Every fallen leaf, branch and rotting root is an investment returned to the system. Fungi and microbes break it down, transfer nutrients through vast underground networks and feed the next generation of plants. In this way, the forest quietly funds its own survival.

When we remove timber, rake up fallen wood or burn off “fuel,” we are exporting capital from that economy. The nutrients locked in stems, leaves and charcoal are gone for good. The fungal and microbial networks that keep the system cycling are scorched or compacted. In an environment that evolved to conserve every atom of phosphorus and nitrogen, those losses do not just add up; they snowball.

It is not so different from a farmer who cuts and exports hay from a paddock year after year. Each load takes away nutrients that must be replaced to keep the soil productive. But unlike the farmer, a forest cannot wander down to the local agricultural store for a bag of superphosphate. Once those nutrients leave the system, they are not coming back.

After repeated mechanical disturbance and post-thinning burns, studies show that soil carbon and nitrogen decline, microbial diversity falls and infiltration rates change. The top layer can even become hydrophobic, meaning rain runs off instead of soaking in. With less organic matter to hold water, the soil dries faster and supports less life. It becomes a landscape living from paycheque to paycheque, always spending more than it earns.

In response, the forest does what it has always done when stressed: it throws up a dense understory of nitrogen-fixing pioneer plants such as acacias, daviesias and bossiaeas. These species are nature’s first responders, restoring fertility and shading the soil while the system rebuilds. Yet the very process that helps the forest recover is often treated as a problem by land managers, who see it only as “fuel” and seek to burn or clear it again. The result is a cycle of recovery and removal that gradually depletes both the nutrient bank and the diversity that sustains the forest.

Repeated and frequent burning carried out on top of thinning, through post-thinning burns and the broader prescribed burning program, amplifies the impact of thinning. Each fire interrupts the slow work of soil and fungi trying to stitch the system back together, resetting succession before stability can return. Instead of healing, the forest is pushed back a little further each time, trapped in an early-stage loop that begins to look less like recovery and more like decline.

From Forest to Woodland: A Gradual Unravelling

Jarrah forests depend on canopy closure to hold their internal climate together. The canopy is what keeps the air cool and damp, the soil shaded, and the humidity high enough for fungi and moisture-loving understory species to survive. When that cover opens too far, the forest starts to behave differently.

Studies have shown that once the leaf area index, the total leaf surface area relative to the ground, drops below a certain point, the system can no longer recycle its own moisture. The cooling and humidifying effects of transpiration fall away, and the forest begins to cross an invisible line toward something more open and dry.

You can already see it on exposed ridges and upper slopes where the canopy was thinned or logged. Jarrah there grows as smaller, multi-stemmed, stunted trees, more like mallee or heath than tall forest. It isn’t just a temporary growth stage. It is a structural change that signals the system has shifted to a new balance point, one that favours open woodland forms rather than closed forest.

That kind of change is hard to reverse. Once the leaf litter, soil carbon and organic matter are lost, the microclimate dries and the seed banks of shade-loving understory species disappear. The feedbacks reinforce themselves: hotter soils, faster evaporation, lower humidity, less seedling survival and further canopy loss. With each dry summer or management disturbance, the forest drifts a little further from what it once was.

It is sometimes said that this is simply the forest “adapting” to a changing climate, but that isn’t adaptation in the sense of resilience, it is adaptation through contraction. A forest becoming a woodland is not proof of flexibility; it is a symptom of limits being reached.

The Insect Effect

There’s a quieter risk that rarely enters the thinning debate: insects.

Warmer, drier conditions speed up insect life cycles and expand their ranges. Most insects are cold-blooded, so their metabolism and reproduction are controlled by temperature. A few degrees’ difference can mean two breeding cycles a year instead of one. Warmer winters also increase the survival of larvae and pupae that would normally be killed by cold, so each new season begins with a higher starting population.

Add drought stress to the mix, and the trees themselves change. When water is scarce, trees close their stomata to conserve moisture and slow their growth. Their internal pressure drops, and their ability to produce and move defensive chemicals like tannins and resins declines. What was once a well-defended fortress becomes soft timber. Bark borers, leaf miners and longicorn beetles that would have caused minor damage under normal conditions can now reproduce faster, live longer and move more easily between weakened hosts.

In an intact forest, the canopy and understory act as a kind of natural pest management system. Shade keeps temperatures lower and humidity higher, slowing the insects’ metabolism. Moist litter supports fungi and nematodes that infect larvae. Predatory insects, spiders and parasitic wasps patrol the shaded understory and leaf litter, keeping populations balanced.

Thin the forest, and that balance falls apart. The extra sunlight and wind dry out the microhabitats that predators and pathogens depend on. Many beneficial insects decline because they lose shade, moisture, and nectar sources. Meanwhile, pest species get a longer breeding season and a landscape full of stressed hosts. The result is not an outbreak overnight but a slow, steady erosion of balance, the kind that leaves trees looking healthy one year and skeletal five years later.

The Paradox of “Resilience”

“Resilience” is the word that gets thrown around most when people defend large-scale thinning. The argument goes that by reducing stress on individual trees, we make the forest as a whole more resilient. But resilience is not just about keeping a few trees alive through the next drought; it is about maintaining the forest’s capacity to regulate itself, to recover, and to hold its diversity together through change.

A forest’s real resilience lies in its complexity: the overlap of its canopy layers, the variety of age classes, and the patchwork of damp gullies and shaded understory that let species retreat and recolonise when things go wrong. Strip that away in the name of efficiency and neatness, and you might make some trees temporarily “healthier,” but you make the system itself weaker.

Nature already has a thinning mechanism. It is called self-thinning, and it works on a slow timescale. Some trees die, others grow, and the gaps they leave are small, scattered, and buffered by the intact canopy around them. Each loss becomes part of the patchwork, creating microhabitats and maintaining overall continuity. The system absorbs the shock.

Mechanical thinning is not the same thing. It removes biomass suddenly, opens up large continuous areas, and compacts the soil under heavy machines that were originally designed for logging. Access tracks and extraction zones carve through the forest, creating wide wounds rather than the small scars of natural mortality. In a stressed, drying landscape, those open wounds are harder to heal. They dry faster, lose their litter, and erode their microclimate before recovery can even begin.

The difference extends beyond structure and into genetics. When drought kills trees naturally, it does so selectively. The survivors tend to be those with the right physiological traits: deeper roots, efficient water use, tougher leaf tissues. Over time, that process builds a forest genetically tuned to the local climate. Mechanical thinning interrupts that process. Selection is not based on adaptation but on reach and convenience. The operator’s grapple, not natural fitness, decides which trees remain. In some cases, that means removing individuals that might actually be best suited to the hotter, drier climate we are moving into.

There is also the physical risk of exposure. A closed forest shares the load of wind and weather. Open it up, and individual trees face the full force of storms they were never shaped to withstand. Thinning can leave tall trees suddenly isolated and more likely to fall, further reducing canopy cover and accelerating the shift toward openness.

So yes, a drought might naturally kill a portion of trees in a dense stand, but the survivors will have proven their resilience under real pressure. They will maintain the canopy connection, hold the humidity, and shelter the understory. In the long term, that is how resilience actually builds: not through mechanical neatness, but through ecological endurance.

Management for Management’s Sake

Whenever I raise these points, someone eventually says, “Well, we have to do something.”

Fair enough. But that assumes doing something is inherently better than thinking first. If thinning had been clearly demonstrated to restore natural forest structure and enhance biodiversity under today’s climate, there would be no argument. But it has not been.

Right now, the justifications rest on a patchwork of hydrological studies, extrapolated plantation logic, and well-intentioned assumptions. The data on long-term ecological outcomes simply do not exist yet, and the results we do have show that site, scale, and context matter enormously.

In some overstocked stands with little understory diversity, targeted, careful thinning might help reduce stress and stimulate regeneration. But what we are seeing on the ground now is not targeted, careful, or minimal. It is heavy machinery working through complex native ecosystems, removing large trees alongside small, and followed by burns that leave blackened, compacted soils and stripped understories.

That is not ecological restoration. That is management for management’s sake, activity mistaken for progress.

The Feedback Nobody Mentions

There is another piece to this puzzle that often gets overlooked. Canopy loss does not only change the forest’s internal climate; it also influences the regional one.

A continuous forest acts as part of the water cycle itself. Through transpiration, trees release vast amounts of water vapour into the atmosphere. That moisture helps form clouds, recycle rainfall, and regulate temperature. The process is especially important in summer, when the forest effectively cools itself and the air above it through evaporation.

When large areas of canopy are removed, that recycling loop weakens. Less water vapour enters the atmosphere, fewer clouds form, and more sunlight reaches and heats the ground. The exposed soil and leaf litter absorb that energy during the day and release it back as heat at night, raising local temperatures and lowering humidity. Over time, this shifts the surface-energy balance toward more warming and less rainfall.

So while thinning is often justified as a way to build resilience against a drying climate, the cumulative loss of canopy over large areas can actually intensify the very drying it is meant to reduce. This is not speculation or theory. It is the same basic physics that governs why an asphalt carpark feels hotter than a shaded forest floor. The difference is that, in this case, the heat feedback plays out across landscapes instead of streets.

The Real Question

The more I see of this so-called “ecological thinning,” the more I find myself asking: what exactly are we trying to restore?

If the answer is the pre-logging forest, we have already lost too many variables, rainfall patterns, soil moisture, landscape connectivity, and old-growth structure, for that to be realistic.

If the goal is “forest health,” then we need to ask: health for what? For water yield? For timber growth? For biodiversity? Because those are not the same things, and they do not always move in the same direction.

And if the goal is simply to act, to demonstrate that something is being managed, to keep the machines moving and the budgets flowing, then perhaps the honesty should start there.

Closing the Loop

This is not a call to lock the gates and walk away, nor is it a claim that thinning is always wrong. It is a call for humility, precision, and honesty.

The Jarrah forest is not failing because it is too dense. It is failing because we keep managing it as though it is still the 1950s, as though climate, soil, and ecology are constants that can be adjusted by cutting or burning. We do not live in that world anymore. The margins are thinner, the feedbacks sharper, and the cost of getting it wrong is much higher.

Thinning can have value in the right place and at the right scale. But prescribing a treatment to be applied across large areas without regard for site-specific nuance or ecological context is not management, it is standardisation. What is being carried out under the banner of “ecological thinning” in many places does not match what is described in the Forest Management Plan. The plan speaks of light-touch methods that retain canopy and habitat trees, yet what I am seeing on the ground are heavy machines, broad clearances, and even large habitat marri trees being taken down while smaller, straighter jarrah are left standing.

Once a management approach becomes entrenched, it is hard to change. Bureaucratic momentum sets in, equipment is purchased, budgets are allocated, and soon the method becomes the objective. That is why it is so important to get it as close to right as possible before it is locked into policy. Nothing will ever be perfect, and some trial and error is inevitable, but we should start with the best understanding we can, and build adaptability into the process from the beginning. History shows that once a practice becomes routine, flexibility is the first thing lost.

If we are going to thin, then let us do it as the plan promised: with restraint, with proper baseline data, and with monitoring that extends beyond timber metrics to include soil, fungi, water, and wildlife. Let us do it in a way that can stand up to scrutiny, not as a gesture to appear active or decisive.

Because this is not about being anti-management. It is about being pro-forest, and that means recognising that sometimes the most ecological act is patience.

A forest is not something you fix. It is something you listen to.