What Trees Should We Cut?

When we cut out invasives from an oak woods, some people imagine that we're "getting rid of bad species." But perhaps the workday leader or steward also marks some maples or even oaks to be cut. Are these "bad species"? 

Most of us have at least some feeling that "trees are good." We'd also readily agree that a healthy ecosystem is preferable to an ailing one. But we may be less clear about the  difference. A healthy ecosystem in central North America is a rare thing. It consists of thousands of mostly rare species of plants, animals, and other biota. All sites have already lost some of their diversity for a variety of reasons, but the main cause today is diminishing light due to lack of natural fire. The main reason we reduce the numbers of trees is to restore that needed light.  

First, some rules of thumb:

Easy decision: In a real prairie, cut all trees. Prairie can’t grow under trees. Only 1/100th

of 1% of the original prairie survives. It's so endangered and precious. No tree should be allowed to degrade a prairie remnant.

Medium decision: In an overgrown but original savanna, cut enough trees of all kinds to get enough sun to grow fuel for regular grassland burns. It may be best to cut some of all tree species, but try to save many young examples of the oak species best represented among the site’s oldest trees which are likely to be, in most cases, bur, white, or black oak. On a small site, it may be necessary to remove almost all fire-sensitive and aggressive species, such as maples, red cedars, basswoods, ashes, etc. to allow enough sunlight to grow grasses for fuel and re-establish a natural burn regime.

Harder decision: In an oak woodland, our burns will probably not be hot enough, under today’s controlled conditions, to kill mature invading trees. Thus we have to make hard choices. However unqualified and humble we are, there’s no alternative. If we want to conserve the vanishing oak woods biodiversity, we have to perform some "first aid" to save the species and re-establish natural processes. Then, in the longer run, we can hope that nature (including fire) will be able to make its own decisions.  

Consider the degraded oak woodland shown below. Judging from the oldest trees (some of which predate Euro-american settlement by more than a century) the “original” dominant trees here were bur and white oak.  Thus probably most of the biota that need conserving here (e.g. invertebrates, fungi, bacteria) are part of an ecosystem sunny enough for reproduction of bur and white oaks.

Specifically, what gets cut? First, of course, all the buckthorn (still green in this photo). Second, we’d cut at least 90% of the shadiest invaders including maple and basswood (yellowish wide leaves on the right).

We spare some elm in the wettest areas, where all the big elms seem to be. We cut most of the big cherry (which one study found to be the most invasive canopy tree of the region’s oak woodlands). Box elder mostly goes. We stopped cutting large ash years ago, because they all die of the emerald borer. But ash saplings won’t succumb to the borer for many years, and they make as much shade as buckthorn – so we cut the young ash.

Some people are surprised by the oaks we cut. See that skinny pole tree on the far left – the dark-barked, branchless, youngster with its top already in the canopy? That’s a red oak. They’re fast growing and survive a lot of shade. Yes, they’re oaks, which some people assume makes them holy. But after cherry, they’re often the major pest in a long-unburned bur or white oak woods. Neither bur nor white can grow under the dense shade of red oaks. We usually don’t cut the big ones (good idea? bad idea?), but the pole reds go into the bonfire with the buckthorn.

Nor do we seek to make the woods uniform. Especially in bur oak woods and savanna, trees on the original landscape were highly clumped (according to botanist John T. Curtis). As a result, woodland plants (and some animals) occupied the darker areas; distinctive savanna species thrived in moderate light conditions; and many prairie species lived in the parts with the fewest trees. This kind of diversity is way more complicated than we could make detailed plans for. But if we restore basic structure and processes (including fire and seed dispersal), the rest of the biodiversity can compete and evolve its way to rich resurgence.

Artificial edges are also special cases. As Joe Neumann put it, "Much of my tree thinning targets the edge where an oak woodland opens into a field. Black oaks fill in this border much more aggressively than other oak species. I remove about half of the black oaks. More can be removed later if necessary. I don't focus on the outer trees, since they are generally more open-grown and healthy. I create a savanna strip that boosts light levels and allows more wind to penetrate the interior. This should increase the intensity of the landscape fires so that over the longer-term, a healthier understory can develop along with a less crowded, new generation of oaks."

In this second photo, we’re looking at a bur and scarlet oak woodland that’s been burned for 28 years. The buckthorn was everywhere as dense as it still is along the edge (the dark green slash across the middle of the photo). We left all the “native” trees when we burned (top-killed) the buckthorn and herbicided its re-sprouts.

Is what we have here now healthy enough? The three big trees are bur oaks. But there are no sapling bur oaks in the grove (although there are hundreds where sunlight is ample, around the edges). The pole trees in the grove are red oak, cherry, hickory, and basswood. Should we have cut some of them earlier? Probably. Should we cut some now, so we don’t end up with a bizarre forest of shade survivors? Yes, it seems about time.

And then there's the photo on the right. When we're too timid about what to cut, a common result is that the "spared" pole trees soon blow over. In this case the pole tree was a bur oak. 

Should it have been cut when the other invasive and over-dense trees were removed here? Perhaps. Would it be fine now to just leave it as it is and let fire and fate decide its future? Perhaps.

But there's another good option, that might be shocking to some. Wait until winter, cut it off at the base, let it re-sprout next spring, cage it from the deer, and let it grow in the now-ample sunlight so that it takes on a more natural structure. An additional (and more shocking?) next step might also be to cut the bigger semi-pole tree next to it, another bur oak. With the re-sprout getting yet more light, there'd soon be at least one tree in this woodland that got its start in the brighter light needed by woodlands. I have to admit that I'd incline toward recommending that no-nonsense approach. Too few such trees exist in our oak woods. 

CREDITS: Thanks to Joe Neumann, Pete Jackson and Jeff Weisz for helpful edits and additions.