Land and Watershed Management

Regenerating Douglas-fir in multi-aged stands

The recent emphasis on managing Pacific Northwest Douglas-fir forests for multiple objectives, including wildlife habitat and old-forest aesthetics, has created a renewed interest in managing stands with trees of multiple age classes. While some types of commercial thinning in even-aged second-growth Douglas-fir forests can produce characteristics of uneven-aged stand structure, long-term development of multiple cohorts requires successful establishment and continued growth of conifer regeneration. It is well documented that growth and survival of Douglas-fir regeneration in the understory require at least a moderate amount of sunlight (40% or more full sun). However, belowground resources (soil water and nutrients) also are vital to success of this regeneration, and belowground competition from overstory trees and understory vegetation in multi-aged stands are not well understood. This study was designed to learn how these types of root competition affect Douglas-fir regeneration.

Where small canopy gaps were created by a previous variable-density thinning treatment, Douglas-fir has regenerated in the available sunlight. Over time, crowns of overstory trees will expand to fill these gaps, and the vigor of saplings will decline.

Research approach

The study took place in three stands of recently thinned 60- to 80-year-old Douglas-fir on gravelly glacial-outwash soils on Fort Lewis Military Reservation near Tacoma, WA. To evaluate the effects of root competition on naturally regenerated Douglas-fir saplings, we installed 48 plots where two different types of belowground competition were either present or excluded.

On a subset of the plots, tree root competition was excluded by digging a trench around the plot perimeter. On another subset of plots, the effect of understory root competition was excluded by clipping all understory vegetation back to ground level every two weeks (without leaves this vegetation provided virtually no root competition). By comparing plots with or without each type of competition, we evaluated the effects of each on saplings and the soil.

Study findings

For Douglas-fir saplings growing in the partially shaded understory of a thinned Douglas-fir stand:

• Root competition from overstory trees reduced Douglas-fir sapling height growth by 29% and diameter growth by 22% during the study. Roots from understory vegetation had no detectable effect on sapling growth.
• Root competition from overstory trees also had a negative effect on sapling foliar production, bud formation, bud size, and retention of older needles. However, morphology of individual needles (length, thickness, and distance between needles) was influenced by shade, not by root competition.
• Soil water content in mid- to late-summer was significantly reduced by roots of overstory trees; competing understory vegetation had a smaller negative effect on soil water.
• Although foliar nitrogen levels were generally low for the saplings, sapling growth did not increase in those treatments where foliar N concentration was increased. Thus, in a partially shaded understory environment, N was not the most growth-limiting resource: growth of Douglas-fir saplings was more limited by light and water availability.
• In the low light of the understory (less than 30% full sunlight), understory vegetation provided relatively little competition. Management of this vegetation would likely produce little benefit.
• Stand thinning provided enough understory light and soil moisture for Douglas-fir seedling establishment, but their continued development depends on future treatments to further reduce the density of the overstory. This could be in the form of thinning or creating gaps around young trees.

Stem growth was greater for the saplings that had no competition from roots of overstory trees.

In addition to the effects of root competition, the amount of available sunlight was positively related to sapling height growth.

A summary of the information on this page is available for download as a one-page pdf document.

There are two journal articles describing the results of this study. The first one, in Canadian Journal of Forest Research, describes belowground competition effects on sapling growth, soil water, and sapling N status. The second one, in the journal New Forests, describes how sapling morphology was affected by belowground competition. If you cannot access the full articles from these links, you can request copies by emailing Warren Devine.