Density Matters

tree diameters © D. Schmitz
Diameter difference between two 15 yr-old Ponderosa pine trees planted at different densities (5×5 ft. and 15×15 ft.).

For many years, forestry researchers around the world have studied the effects of competition for sunlight, moisture and space on tree and shrub growth. These researchers have developed a set of guiding principles that can be applied to practically every plant species. Before establishing a new stand, foresters should analyze the effort they are able to invest in future management for the desired stand structure. They should consider whether they want to plant densely and thin out later or plant sparsely now, so the trees have enough room to grow and survive into adulthood. Experienced foresters determine how their chosen planting strategy will affect the form of the tree and their ability to remove weeds in the future. The long term effects of competition should also be considered. Various density recommendations could produce good timber quality or diverse wildlife habitat.

Twenty five years ago, at a clear cut in the McDonald Dunn Forest, OSU faculty and students planted Douglas fir at various densities to compare tree growth, tree survival, and stand vigor. I recently visited these experimental stands with a group of restoration ecologists who gathered to discuss the merits of different planting strategies. As we toured the plots, it became strikingly apparent that planting density directly affects tree health and stand vigor. After 25 years, the trees planted at five foot spacing were small in diameter. The canopy was totally closed. Tree crowns were small and crowded; the pencil thin branches were dead. Nothing else was growing on the ground, not even moss. We agreed that even if the stand was thinned right now, these trees would not survive because they had been stressed their whole lives by lack of light, moisture and space.

The second site, with eight foot spacing, had a few understory forbs. It was difficult but possible to walk through the stand. The canopy was closed but trees had bigger crowns than the closely spaced trees. Tree limbs were larger in diameter but dead. If this stand was thinned, the remaining trees would survive but their form had been compromised by the close spacing. However, the cost to thin this stand would be high because it would not be offset by the production of marketable timber. Furthermore, the necessary machinery would damage the existing trees, yet hand labor would be cost prohibitive for most landowners.

In contrast, the trees that were growing in the 25 foot spacing looked healthy. They had limbs that were alive and canopies that were full. Small and large native shrubs grew in the understory and forbs were abundant on the forest floor. The canopy was closed except for a few holes, which allowed a little sunlight into the stand. There weren’t a lot of weeds competing with the forbs or shrubs. Evidence of wildlife activity was present.

Six months later, Marvin Gilmour, a local farmer and one of our Directors, brought in two “tree cookies”. The cookies came from two ponderosa pines that were both planted fifteen years ago near each other in similar soils. One tree was planted at a spacing of 15 by 15 feet. The other was planted at a spacing of five by five feet. You can count the same number of rings on each tree. What is totally amazing is the diameter size difference between the two (13” and 3.5” diameters, respectively).

Since moving to Oregon, I have realized that the silviculture principles are the same wherever you go, regardless of species. Not surprisingly, the outcomes we saw in OSU’s experimental Douglas fir stands were similar to the red pine (Pinus resinosa) and quaking aspen (Populus tremuloides) stands I studied in my silvics class in Wisconsin. Even in the garden, we apply these same principles. Vegetable planting density charts abound and every seed packet has recommended spacing to assure healthy plants. Site objectives should be taken into consideration when determining planting density. However, no matter where you are or what you are growing, the competition for sun, moisture and space in densely planted stands affects the vigor of plants.