In Un-Managed Forests, Site Preparation Is Needed Before Forest Thinning

I've been going back and looking at the forest thinning projects I worked on, or colleagues had, 5-20 years ago. I try to return every few years to check on some available sites. It turns out what I intended to happen has only partially come true.

In most of the forest thinning projects worked on during that time, there was alot of ironwood and sugar maple in the understory and midstory that had formed dense canopies. It was dark and shady below and nothing grew on the forest floor. We came in and knocked all the ironwood or maple down with chainsaws and herbicide. Sometimes it was hackberry, elm, dogwood, hickory, rusty blackhaw.

Sometimes I see that a certain desirable species, like white oak, are the only species left standing in the overstory, after the thinning. All the other overstory species were cut out. Going into these monoculture forests conjure up strange and eery feelings, as if something is missing. Today I always try to save some individuals from every species.

A monoculture isn't good for ecology and can likely lead to a total loss of the monoculture crop from disease or pest. They were doing this with Sitka spruce in southeast Alaska when I was working up there in 2002. After the old-growth mixed spruce/hemlock forests are clearcut, the Forest Service returns and thins out all the hemlock saplings so the next forest is pure spruce. The spruce is higher value at the sawmill was their reasoning. It's similar everywhere you find a forester conducting management. It's part of our training. The indigenous people managing the land previously left a large range of chestnut and other nut trees like walnut, pecan, & hickory. Seems to be normal human behavior.

The lack of fire in the woods led to an overcrowded and dark understory. This dark understory was a desert for wildlife that offered no food or cover. There were no oak seedlings or saplings ready to replace the oaks in the overstory. Our solution was to go in and whack all that stuff back so the ground would grow up with a diversity of grasses, forbs, shrubs, & oak seedlings.

Increased sunlight on the ground usually produces oak seedlings. At the same time we were taking out the undesirable and unhealthy trees from the overstory to improve spacing and allow more sunlight to reach the ground.

A lot of forest thinning has been conducted in Missouri in the last 20 years in scenario's similar to above.

Today, when I return, most of these woodlands have great looking, well-spaced overstory trees. That part of the thinning met our expectations. Many times I see too many trees were taken out. There is still open space in some of the canopy after all this time. The stand was thinned to heavily and too many trees were cut. This is largely due to the training we received to thin a woodland down to the 'B-line'.

The 'B-line' is the name of a curve on the Gingerich Stocking Chart. The chart tells us how much space in the forest is filled in. We need to know the number of trees/acre and the basal area/acre to use the stocking chart. Every forest inventory I work on I plot the stand of trees onto the chart. We can then better understand tree spacing and compare different stands of trees. If you've had a forest management plan developed, the stocking for each stand, along with the B-line, is reported in the plan.

When a forest is thinned to the B-line, there is some open space between the trees and they have adequate spacing to grow free of heavy competition. In 10 years the open space will have completely filled in and the stand will have theoretically grown to the 'A-line', which is a fully-stocked stand with no available growing space. Using this model puts the woodland on a 10 year re-entry thinning schedule. Every 10 years the stand is thinned down to the B-line.

It's been over 10 years and many of these stands I return to still have open growing space. Upon closer examination of how the Gingerich stocking chart was made along with seasoning experience, it became clear the model wasn't exactly predictive for Missouri's woodlands.

In many cases, cutting to the B-line was too aggressive, too much. It's taken more than 10 years to fill back in. In many cases we were thinning out half of the trees in these overcrowded stands, in order to get to the B-line. My experiences of thinning woodlands and returning to look has shown that reducing stocking to the B-line has many unintended consequences.

For one, taking out that many trees at one time is akin to a heavy wind disturbance in the community of plants and animals. It's a shocker to the system that takes time to recover from.

Another potential impact of thinning too heavily, is the significant amount of herbicide that is dosed into the ecosystem. The large dose added to the soil biome impacts soil biology, which is the ecosystem digester. The disturbed soil biology impacts plant biology. There have been projects that I returned to the year or two or three after the thinning and could see the reduced productivity from the herbicide. The first time I seen it clearly I was startled at the lackluster response of ground vegetation. The herbicide oftentimes diminishes the density and abundance of ground vegetation that is expected. I learned to identify herbicide stress to the ecosystem by returning to sites and monitoring outcomes, and working with specialists at the Plant Diagnostic Clinic at MU.

And another potenital impact that is becoming a greater risk, in some cases, invasive species are waiting to exploit the ecosystem while it is in shock and hasn't fully recovered. A landowner had some forest thinning done to the B-line about 10 years ago and recently called me. Where the forest was thinned had filled in with Japanese honeysuckle, bush honeysuckle, multiflora rose, and wintercreeper. He's upset about it and looking for ideas to control the invasive species. I got upset too cause this wasn't part of the plan. I hadn't done the thinning here but I had elsewhere. We were both sick about it and feeling defeated.

Around 2012 I had started carrying a chainsaw and doing thinning work by hand, I got a glimpse of the future. It made me stop and re-consider everything. In less than 10 minutes though I was back to work. The pressure of running a business favored status quo. What I saw eventually helped me understand that thinning in some forests requires site preparation.

We were conducting forest thinning on a property adjacent to a woodland that had been thinned about 3-5 years ago on a state owned Conservation Area. I was co-workers with the folks who managed the Conservation Area and knew the details for that project. It was the same hill with a property boundary dissecting it. It was the same forest.

We were thinning out a very thick understory and midstory of sugar maple in the 2"-14" diameter range, that were growing below high quality white oak in the overstory. Nothing was growing below the sugar maple because of the dense shade it casts. Our goal was to get oak seedlings and a diversity of other plants to fill in the ground flora after the thinning work. The same goal and methods that guided the work on the neighboring Conservation Area.

What I saw across the boundary, and what stopped me, was when I saw on the Conservation Area that the understory had filled in with ironwood. At the time, the ironwood was still young, about 4'-8' tall. It was a pure sea of ironwood. When I walked into it I couldn't see the bases of any of the trees around. The ironwood was very thick. Just a few years ago it was pure sugar maple. Forest management work transformed it from a pure maple to pure ironwood understory.

We continued doing the work convincing myself to ignore what I saw. I hadn't heard anyone else ever mention anything like this. We were doing good work restoring health and vitality to woodlands, I didn't want to rain on the parade. Alot of funding was going into doing this work and my career survived on it for years. I might have mentioned what I saw to a few colleagues, but it never went anywhere.

After a few years of ignoring what I saw and conducting thinning on thousands of acres, a client who was over-run with a sea of ironwood growing below white oak asked for help. The ironwood was up to 12' tall on this project. Below the larger ironwood shrubs was a dense carpet of ironwood seedlings under 12" tall, A thinning would release those and there would be no-change on the ground.

We decided to conduct a mild burn, before the thinning, in an attempt to control the ironwood. We burned under mild conditions so flame heights were generally under 12" and the fire moved slowly. Removing all the short ironwood seedlings allowed other ground flora to grow. We could now be more confident that the thinning would not result in a return of the ironwood sea, as long as we cut all the ironwood.

The fire did such a great job of removing all the ironwood seedlings under 12" tall and some of the larger ones, that we decided to burn again. We let one burn season pass and burned the next season. What we saw was a reduction of many of the smaller woody stemmed ironwood up to 1/2" diameter. Some of the larger ironwood had died and many weren't looking as healthy.

We liked the results so well that we waited a season and then burned again. This time many of the ironwood up to 2" diameter were falling out including more of the larger ones. In this stand of trees we decided it was time to thin. Ironwood is now a minor component, which is great!

At the same time we started doing the same process in other stands. We altered things like burn intensity, frequency, season, etc. to test and learn.

I've been applying this approach and appreciate the results much better. The first mild burn is a gentle return of fire. Instead of leaving tree scorch and sterilized soil, a partial layer of leaf litter is consumed, and many of the smallest woody stems perish.

The key in all this is 'mild-intensity' fire. If the fire is leaving scorch on trees, it's too hot, and unintended consequences will arrive. The fire is too hot if all of the leaf litter, the 'O-horizon' in the soil profile, has been consumed. The O in O-horizon stands for organic. In our woodlands the O-horizon is very thin and easily destroyed.

Mild burns are slow and boring. If an escape does happen, catching it is easy. An experienced practitioner can burn a small area of few acres alone. I started mild burns because I was too scared to burn under hot conditions, for fear of escape. I noticed the results and realized they are much healthier for the ecosystem than the hot burns.

The O-horizon is subdivided into classes of plant & leaf litter and various stages of decomposition. This layer contains a hydrogen battery that is exchanging hydrogen atoms throughout the ecosystem to maintain overall ecosystem health. If the humic layer does get destroyed, it can be rebuilt in several years, but recovery is largely dependent upon severity of disturbance. The humic acid battery is only one of many hidden services that the O-horizon provides to the ecosystem.

Post-burn results show a partially intact leaf litter layer.

Fine tuning our senses to what is going on in the O-horizon improves our ability to apply controlled burning and manage on the ecological level. Before a burn, I monitor the leaf layer for humidity. A mild burn will be done while the lower layers of the leaf litter and the O-horizon are damp, but the fluffy top layer of leaves is dry and crispy.

When I do forest surveys to examine trees & ecology, I check on the O-horizon. Over time the senses get more tuned into what is going on down there. Patterns about what is happening down there and what is being seen up there above the soil, start to be revealed.

For instance, old-field cedar groves rarely have much of an O-horizon. In Ozark landscapes with large hills and thin soils, the O-horizon on the northern facing slopes will generally be greater in depth and quality than on the southern facing slope. Much nuance is found when comparing various sites. It's become a useful way to understand site productivity & ecological health.

In my practice, it's become standard to prescribe 3-4 mild-intensity controlled burns before any thinning. After the mild burning has tended to a desirable cast of characters on the ground, it becomes time to thin the smaller understory trees & shrubs. Removing the undesirables there, such as ironwood or sugar maple, allows more sunlight to reach the ground flora, which has now been tended to and ready for a release.

After the first thinning that addresses the smaller trees, it's good to burn a few more times to further develop the ground vegetation and understory.

After the understory starts to look desirable, it's time to have a second thinning, this time in the midstory and overstory. The burning will likely have already reduced some of the unhealthy trees in the canopy, so some of the thinning work is already done. Now that the ground and understory are stocked with desirable species, the canopy thinning releases them.

The above prescription has two-entries, where a chainsaw is used two different times to reduce trees. Ideally, going through after every burn and removing a few trees, is more gentle and can be more easily integrated than widespread forest thinning on a limited entry schedule. In the early years the chainsaw is used to remove mostly smaller trees & shrubs to improve spacing & remove poor health. It's a quick pass-through so only a few trees are cut here and there. It's light work so that once the growing season flushes it will be hard to see that any trees were cut. Gentle is the way. Each pass builds so that after a few passes it becomes noticeable that tending has been occurring.

An important consideration about burn scheduling: much advice is to burn every so many years. Here I have advised to burn every other year for a few years. These schedules are helpful for crafting an intention, but we have to go and check on what's happening out there to decide if burning is appropriate. Sometimes the ecosystem takes longer to recover and we need to provide more rest between burns.

Sometimes the ecosystem recovers so well we can burn two years in a row. That's fine, but avoid burning three years in a row. The reason I prescribe every other year is because the results compound on the last burn if done soon enough. If we compound results too much the disturbance can become great. In other words, burning three years in a row can stress the ecosystem out too much. Too frequent of mild burns compound stress to the same or greater levels than hot burns.

In the beginning, before thinning, mild burning needs to be frequent. If too many years pass between mild burns, each burn will be as if it was the first. Mild burns are easy to recover from, that is the goal, and so the site returns to it's path rather quickly after only one burn. Another burn is needed rather soon to build on the work of the first burn. A third burn builds more, and so on.

After we've done some burning and thinning, the woodland becomes more 'restored' and we can conduct burning less often. Once the ecosystem is at a desirable stocking of desirable species, the burns are used to maintain rather than transform. There may even come a time when a long fire-free period is desired to allow the ecosystem to grow more woody shrubs and trees, and less ground vegetation. It largely depends on our goals and current conditions.

Site preparation for forest thinning has come about out of necessity. It's a practice that allows us to focus more on the ecological consequences of forest management work. We can still grown banging high quality sawlogs. It's not really a trade-off. Mild-intensity burns don't generally degrade timber quality.

Site preparation for thinning allows for all-around better results.