Mind the gap: Mimicking nature in the managed forest

Maybe it was disease, or insects or wind, but the day finally came for the old-growth tree.

First came the pops and creaks as the wood fibers began to stretch and break on one side of the trunk and collapse on the other. Then the weight of the trunk began to shift in earnest. As momentum built, the sound gathered and rushed into a roar that culminated in a thud that shook the forest floor.

In its downward progress, the tree ripped branches from neighboring trees or toppled them completely, creating a long, jagged gap in the forest’s canopy. As the sounds faded and the leaves came drifting down, the forest floor was illuminated with sunlight (Photo 1) that eventually will bring plants, young trees, and wildlife eager to colonize this new largess of energy and space.

Multiply these gaps across the forest, add the effects of growth, decay, and renewal across the entire forest over many years, and the result is the complex world of the older forest. Snags, downed wood, multiple canopy layers, gaps and places of dense growth provide a range of habitat for plants and wildlife.

By contrast, nature has not yet run this course in the younger, managed forest. Trees often are closely spaced, with a single canopy layer and no gaps.

Gap created by a fallen old-growth tree
PHOTO 1. Gap created by a fallen old-growth tree. Note people standing on the far end of the tree. Photo: Allen Estep/DNR

To diversify the structure of these young stands and to increase revenue from thinning operations, forest managers may deliberately create gaps in the canopy by removing trees. Yet how close do these gaps come to mimicking nature? In 2015, the Washington State Department of Natural Resources (DNR) decided to find out.

A New Concept in Thinning

When the Olympic Experimental State Forest (OESF) was established on the western Olympic Peninsula in 1992, DNR was faced with a different kind of gap: between vision and reality. The vision was a forested landscape with openings and young, mature, and old-growth stands arranged in an irregular pattern, capable of supporting northern spotted owls and other native species. The reality was the second growth forest. Because of extensive clearcutting in the previous three decades, over half of the forests DNR managed in the OESF were structurally simple and less than 40 years old.

One way to address this challenge was to use variable density thinning. With this type of thinning, trees are removed in an irregular pattern: some areas are not thinned at all, some areas are gaps, and others are thinned to different densities. The idea is to put a single-canopy stand on the fast track to becoming habitat while also supporting healthy tree growth for revenue production.

Variable density thinning in the OESF was based in part on practical experience in how forests grow, and in part on the recommendations of forest scientists such as Andrew Carey from the US Forest Service Pacific Northwest Research Station. Carey recommended variable density thinning of second growth to better support populations of northern flying squirrels, a major prey species of northern spotted owls. He also incorporated this technique into “biodiversity pathways,” a landscape-level management approach for meeting multiple objectives that DNR later adopted as part of its agency-wide silvicultural approach.

The challenge for DNR was writing variable density thinning prescriptions for large areas. DNR instructed loggers to create half-acre gaps for every 10 acres of thinning. Loggers were asked to avoid thinning in sensitive areas (called “skips” because loggers skip those areas) and to retain certain species of trees. They also were given a target relative density that ranged between 35 and 50. The result was a stand that was thinned more heavily in some places than others. Techniques have been refined over the years, but the basic concepts have remained the same.

For patterning the gaps, DNR had little to go on. How common are they? What shape do they tend to be? Despite decades of forest research, the scientific literature was curiously silent on gap geometry in the old-growth forests of the Pacific Northwest. Without those answers, DNR’s success was hard to gauge. So DNR began a study appropriately named “Mind the Gap.”

The Study

For this study, DNR wanted to understand how the managed forest responds to gaps and how to make the gaps (size, shape, and frequency) resemble those found in older forests. The study was done in three parts: a look at the half-acre gaps created at least 10 years ago in western hemlock and Douglas fir stands, an analysis of gaps in mature and old-growth forests, and a test of a common gap shape and size in a timber sale. The end product would be refined prescriptions for creating gaps.

For the first part of the study, DNR compared aerial photos taken before thinning to those taken recently and took detailed field measurements. Results are still preliminary. But generally speaking, and despite a lack of site preparation and planting, the forest had surged into the gaps. Nearly 90 percent of the gaps measured were occupied by trees. Western hemlock averaged 1,400 to 2,100 stems per acre. One gap had as many as 3,600 stems per acre, which is many more than the surrounding forest (Photo 2 and Graph 1). Gaps also saw recruitment (establishment) of Douglas fir, Sitka spruce, western redcedar, and Pacific silver fir, albeit in lower numbers. Height growth in the gaps ranged from 16 inches per year for silver fir, hemlock and redcedar to a robust 30 inches per year for Douglas fir. Shrubs were seldom dominant, easing fears that gaps would create “brush holes” in the forest.

Young western hemlock trees growing in a gap
PHOTO 2. Young western hemlock trees growing in a gap. Photo: John Stanek/DNR

What about gap shape? When gaps were first created, DNR feared that wind would gather speed across the opening and slam into the trees on the windward side, pushing them to the ground. It did happen. But it happened only in a quarter to a third of the gaps, and gaps only expanded a tenth to a quarter of an acre. And some tree crowns along the edge widened into the gap by as much as three feet, seemingly in response to increased sunlight.

To study the naturally-created gaps in older forests, DNR analyzed light detection and range (LiDAR) data and followed up with field verification. With LiDAR, lasers mounted on a small airplane are used to take measurements of the forest and ground. From these measurements, DNR creates a canopy surface model, which is essentially a topographic map of the top of the tree canopy, and a digital elevation model, which provides the contours of the ground. Between the two, one can determine the location, size, and shapes of gaps.

Western hemlock regeneration within the gaps.
GRAPH 1 . Western hemlock regeneration within the gaps.

But what is a gap? Is it a place where one tree fell or several? Is it bare ground or can it be filled with young trees? If several gaps seem to be connected by thin spaces between trees, is that actually one gap? And how do you quantify the shape of gaps? Nature is messy and seldom obliges with something as straightforward as a square.

To solve the first problem, DNR applied filters to the data. For example, the gap had be a certain size and the difference in height between young trees in the gap and the overstory (Figure 1) had to fall within a defined range.

Delineating a canopy gap in an old-growth forest
FIGURE 1. Delineating a canopy gap in an old-growth forest in the OESF. The size, shape, and number of gaps varies depending 
depending on the lower height threshold.

The second problem was tricky. Consider the shape in Figure 2. How long is it? One could measure across the points that seem the farthest apart, but which two points?

To solve this challenge, project researchers wrote a computer program to determine gap length. The program measures the distance between every point that describes the outer edge of the shape. That exercise creates a dense spider web of lines. Then, the program uses those measurements to find the shortest path between the two points farthest from each other (Figure 2).

Determining gap length.
FIGURE 2: Determining gap length.

Analysis of the older forest continues. In the meantime, the team took advantage of a planned variable density thinning in a 40-year-old western hemlock stand to test the most prevalent gap shape seen so far in the older forest: long and skinny. The team instructed loggers to create 20 rectangular gaps and, for comparison, 20 circular gaps ranging in size between one eighth and one quarter acre and randomly distributed across the stand. Growth in and along the edges of the rectangular gaps will be compared to growth in the round gaps and a thinned area with no gaps. The first post-treatment measurements will be taken later this year.

Mind the Gap

So far, canopy gaps have been an ingenious way to balance revenue production and ecological values in the OESF, also called the learning forest. The trees removed to create the gap generate revenue and the gap itself supports ecological values by enriching the structure of the stand. And although the gap will eventually fill in with trees, chances are other gaps will be created through thinning or natural forces as DNR works toward a more complex forest.

Can the gaps be more effective? This study will continue to probe that question. More complete results will be shared as DNR continues to mind the gap in the OESF.

By  Cathy Chauvin, DNR writer, editor; and Daniel Donato, Ph.D., DNR research scientist.  For questions about this study, contact: daniel.donato@DNR.wa.gov [article originally published in The Learning Forest e-Newsletter.]