RESPONSE OF OLDER TREES TO THINNING

LOCAL RESEARCH BRIEF
By Max Bennett, Extension Forester, OSU Extension Service

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How do older Douglas-fir trees respond to thinning?  In many areas of interior southern Oregon, extensive wildfires more than a century ago and subsequent fire exclusion have resulted in dense, relatively even-aged stands dominated by Douglas-fir, with overstory trees in excess of 100 years in age.  Some of these stands have been thinned to reduce the fuel hazard, increase individual tree vigor, improve resistance to insect and drought, and generate commercial timber.  However, initial densities in these stands are often very high and there have been concerns that the trees left after the thinning could suffer from reduced growth, shock, or windthrow.

In the late 1990s, local consulting forester Marty Main installed 15 permanent plots in one such dense Douglas-fir stand on BLM lands near Anderson Butte. (The stand is visible on the ridgeline south of Medford.)  Following installation of the plots, BLM thinned the stand heavily to create a “defensible fuel profile zone”, an area where fuels have been reduced to slow the spread of a fire, but an overstory canopy remains. 

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The stand likely originated after a 19th century wildfire, perhaps one set by miners or early settlers, and it contained Douglas-fir trees ranging from 85 to 150 years old, with most trees in the 90-120 year range.  Pre-treatment stand density was very high, with basal areas of 300-450 ft2/acre, far higher than the range that would typically be recommended for such a site.  The thinning reduced the density to about 100-120 ft2/acre.  The trees left after thinning were dominants and co-dominants, the healthiest and most vigorous in the stands, and generally the largest as well.

Several years later a crew from OSU under the direction of Extension Forester Max Bennett re-measured the permanent plots to see how the trees responded to the treatment.  The crew measured the heights and diameters of the trees, and took increment cores to measure the radial growth since thinning.   From these measurements the basal area growth of each tree was calculated.  We then compared the basal area growth of the trees before and after thinning.  If the growth was faster after thinning, we concluded that the trees “released”, i.e., they responded positively to the treatment.

What did we find out?

  • More than 9 out of 10 trees responded positively, that is, they grew faster after the thinning than before.  The average ratio of basal area growth after treatment to before treatment was about 2.5 to 1, but there was wide variability in the response among the trees.  (The figure below shows increment cores from four trees in one of the plots.)
  • Two of the 78 trees in the plots died following treatment, and there was no blowdown.
  • Trees in the higher productivity stands grew on average more than twice as much in basal area as trees in the lower productivity stands.  But, growth of trees in the lower productivity stands accelerated more after treatment than trees in the more productive stands.  Why?   Most trees in the low productivity stands were barely growing before treatment (you had to look closely to see the growth rings), so the small increase in actual growth translated into a large relative increase.  Conversely, trees in the more productive stands were growing much better before treatment, so although they grew faster in absolute terms after treatment, the relative increase was not as great.
  • Traditional measures of tree vigor such as crown ratio and radial growth were not very strong predictors of tree growth after thinning. 
  • The was no apparent relationship between the amount of basal area reduction around individual trees or the post-treatment density around individual trees.  In other words, the results suggest that on an individual tree basis, older Douglas-fir trees may respond equally well to a wide range of density reduction or retained basal areas.

Management Implications

after thin compareOur findings suggest that older (80- 150 yrs) Douglas-fir trees growing in dense, even-aged stands in southwestern Oregon can respond to stand density reduction with increased diameter and basal area growth, but the magnitude of this response is highly variable and difficult to predict on an individual tree basis.  We observed positive growth responses for individual trees in stands with high initial densities that were heavily thinned to a uniform stand density of around 100-120 ft2/acre.  Despite concerns about reductions in growth, thinning shock, and blowdown, there was little mortality, no windthrow, and less than one in 10 trees grew more slowly after treatment. It should be noted that while the pre-treatment stands, as a whole, were dense and experiencing competition-related morality, residual trees were the most vigorous dominants and co-dominants available, with pre-treatment live crown rations averaging 1/3 or greater in all but one stand.  Had the trees selected for retention been of lower initial vigor, the results would likely have been much different.  Presumably the increased growth of the residual trees equates to improved vigor and resistance to stressors, such as drought and perhaps insect attack.  Trees that are growing in diameter and are more vigorous contribute to ecological forestry objectives in older stands.  In general, the positive growth response of these older trees provides managers with additional options for stand management. 

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