In the previous post, we talked about spatial patterning in older forests and why it’s important. In a nutshell, older trees in older forests that historically experienced frequent fire are often arranged in highly variable mixtures of clumps, individuals and openings, rather than in uniform stands of evenly spaced trees. These spatial patterns can have a significant influence on fire behavior, the growth and diversity of trees and understory vegetation, wildlife habitat, interception and retention of rainfall and snow, and many other forest ecosystem processes. If you’re interested in creating more fire-resistant forests, in regenerating shade intolerant species like pine and oaks, and in increasing overall plant and wildlife diversity, spatial patterning is something you may want to think about. In this article, we’ll look first at how complex spatial patterns develop in “natural,” unmanaged forests, and then talk briefly about how and why they might be encouraged or emulated in managed forests. In particular, can we emulate the developmental processes that create spatial patterning in older forests earlier in the the life of a stand? If so, what are they?
Development of spatial patterns
In western forests, fire is a major driver of spatial patterning. Fire behavior is influenced by fuels, weather and topography, and these factors combine in endless ways to create spatial variability. Historically, many dry forests experienced frequent light surface fires that mostly underburned, killing smaller trees and leaving larger trees intact. Here and there small patches of trees killed by bark beetles or root disease would flare up, resulting in the creation of canopy gaps. Such forests often contained a mosaic of clumps, randomly arranged individual trees and small openings. Forests that were a bit wetter or cooler, or both, often experienced a mix of surface fire and higher intensity fire, typically resulting in larger patches of fire-killed trees and hence, larger openings.
Fire-killed patch of trees creating opening. Tiller complex fire 2002, photographed 2012.
Bark beetles, root disease and windthrow often kill small patches of trees and create openings of variable sizes and shapes. Frequently these disturbances interact with each other and with fire to create and maintain complex spatial patterns. For example, a small patch of beetle- or root disease-killed trees is a good candidate for blowdown in a windstorm, or is an area of heavy fuels to be consumed in the next wildfire. Over time, with frequent fire, new gaps are created and old ones fill in, creating a mosaic of small patches with considerable diversity in the sizes and ages of clumps and gaps.
Patch of beetle-killed trees, destined to become a canopy gap.
How often have you seen a clump of three or more large trees growing together? Was there a good seedbed in that very spot many decades or hundreds of years ago, while the surrounding area was covered in brush? Or is the clump simply the result of random distribution of tree seeds many years ago? Who knows, but studies of natural regeneration show quite a bit of variability in the distribution of tree seedlings after wildfire, variability that results in spatially complex forests down the road.
Soils also play an important role in spatial patterning. Sometimes openings are found in forests where there are patches of soil that are particularly shallow and don’t support much tree growth, or there are variations in soil texture, with some soils supporting primarily grasses and other soils more favorable for tree growth.
“Living stump” illustrates below-ground connection of trees via root-grafting. Although the tree on left’s life support mechanisms above ground were eliminated, sufficient resources were supplied below ground to heal over a major wound.
We typically think of trees as individual organisms, but a growing body of evidence suggests that trees are connected below ground. One familiar example of this is the “living stumps” that result from root grafting. Recent research has shown that clumps of trees may be interconnected underground via fungal hyphae, facilitating the exchange of nutrients, carbon, and water. In a study of a dry Douglas-fir forest in British Columbia, researchers created a map of one such belowground network. One of the trees on their study site was linked to 47 other trees! If trees are indeed linked below ground and share resources, trees growing together in clumps could be thought of, in a sense, as cooperators as much as competitors. This is more likely the case in older stands of trees than in young forests. However, much more research is needed before we can draw any firm conclusions about the degree of competition versus resource sharing among trees in older forests.
To summarize, fire, insects, disease, soils, regeneration processes, and underground linkages interact to produce spatially complex forests. And while some spatial patterns may be semi-permanent (such as those due to soils), in most cases they change over time. An opening fills with trees, while fire or root disease creates a new opening somewhere else. A fire thins out a clump of trees, while a group of younger trees grows and develops interlocking crowns, forming a new clump. Change is the rule.
Increasing spatial complexity in managed stands
Let’s say you’re managing a forest with primarily younger (<40 years) or middle aged (40-100 year old) trees. The forest regenerated naturally (i.e., was not planted), was logged one or more times in the past, and is fairly dense. Trees are not evenly spaced but there are not many gaps or openings and few obvious clumps. This describes the condition of many small woodland forests in southern Oregon.
One simple way of increasing spatial complexity in such forests is through thinning for quality and vigor. The basic idea is to focus on retention of healthy, vigorous trees and without getting too hung up on tree spacing. That’s not to say density management is unimportant, but adhering rigidly to a pre-determined spacing rules (e.g., 15’ x 15’) in thinning could lead you to cut a healthy, high quality tree in favor of a less vigorous, low quality tree. Fixed spacing works best in plantations where the trees are already pretty uniformly spaced; for natural stands where trees are variably spaced, a more flexible approach makes sense. Just retaining the best trees, even when they are spaced close together, will increase the spatial variability or complexity of your stand. It also will likely improve growth individual tree growth rates, vigor, and resistance to pests and drought.
Stand thinned to retain more vigorous trees, accentuating natural clumpiness and small canopy gaps.
Taking it to the next level, you could deliberately seek to create gaps and openings while retaining tree clumps or promoting clumpiness. Why would you do this? Here are four possible reasons among many:
1) Canopy gaps and openings provide opportunities for regeneration of shade-intolerant species and maintenance of early successional vegetation;
2) These openings break up contiguous fuels layers which contribute to high severity fire;
3) Spatial diversity contributes to habitat diversity, e.g., through creation of edge habitat and promoting the development of patches of early successional vegetation; and
4) Vertical canopy diversity (multi-story stand structure), an important feature of older forests, can still be provided, but by separating groups by their canopy strata.
OK, so you want some gaps and clumps. Figuring out suitable targets is a big challenge: How many gaps? How big are they? What shape are they? How many clumps? How many trees in clumps? What is the desired density? And so forth…. One approach is to use reference conditions, that is, the historic density, amount and distribution of gaps and clumps. Determining reference conditions for an individual site is very challenging, but research in progress in southern Oregon will begin to provide reference condition information for Klamath-Siskiyou forests in the near future. Another approach is to “work with what you’ve got” moving toward increased spatial complexity without necessarily having a specific target in mind. “Variable density thinning” is a good approach for this, and is the topic of the next post in this series.
All this is not to say that there’s something wrong with uniform, spatially simple stands. If timber production is your main objective, it probably makes sense to have a stand of relatively evenly spaced trees. That way, trees occupy all or nearly all of the growing space and overall stand growth is maximized. Forests of evenly spaced trees also can be pretty fire-resistant if there are not a lot of surface and ladder fuels. As always, what type of stand structure is “best” depends a lot on your objectives.
