STAND MANAGEMENT COOPERATIVE

Articles of Interest

Wind Stability of Douglas-fir Plantations*

Jeremy Wilson, Ph.D. Thesis, University of Washington, CFR, 1998

Managed forest landscapes of the coastal Pacific Northwest are undergoing a transition from dominance by naturally regenerated second-growth stands to Douglas-fir plantations. In most plantations, even-aged seedlings of a single species are planted in a regular pattern. This practice may limit variation of tree sizes, making plantations susceptible to stagnation and/or instability. Severe windstorms that damage forests have been common in the Pacific Northwest. Hurricane-force windstorms hit the coast of Washington and Oregon every 20 years on average. Damage caused by a 1921 storm was concentrated on the Washington coast. The storm is estimated to have blown down 23 to 28 million cubic meters of timber. A 1962 storm blew down an estimated 38 million cubic meters of timber in western Oregon and Washington (Wiley 1965, Lynott and Cramer 1966, Henderson et al. 1989).

Data from repeatedly measured experimental plots were used to compare tree size variation and stand stability measures between naturally regenerated and planted stands. The data were pieced together from portions of three, large repeat measurement studies in the Pacific Northwest. Data sets were combined to increase sample size and to attempt to cover similar ranges of Douglas-fir densities and stand ages in both stand types. A list of studies used is provided in Table 1.

Table 1: Experimental studies providing data for this analysis.

Study	Agency/Reference	Locations	Plots
Experimental Plot 703 (EP703)	BC Ministry of Forests (Stone 1994, Zens in prep)	Vancouver Island, B.C.	82
Levels of Growing Stock (LOGS)	USFS-PNW (Curtis and Marshall 1986, Curtis et al. 1997)	Oregon and Washington	9
Stand Management Cooperative (SMC)	Stand Management Cooperative (SMC)	Oregon and Washington	18
Misc.	Cooperative (Collier and Haukaas, 1996 Wilson 1998 )	Washington	2

Naturally regenerated stands tend to develop greater variation of tree sizes compared to plantations (Wilson 1998). Limited size variation in plantations makes them more susceptible to developing high height-to-diameter ratios (H/D same units) in the dominant trees (Figure 1). The H/D of a tree is a relative measure of stability under wind and snow loads. As H/D increases trees become increasingly unstable (Cremer et al. 1982, Becquey and Riou-Nivert 1987, Lohmander and Helles 1987).

Figure 1: Predicted dominant H/D vs dominant height of unthinned plantations ((ht – bh) / d = 63.72588 + 1.20446 * (ht – bh - 19.06629) + .00715 * (iddf), adj. r2=.771, p < .0000). Line labels (numbers on far right) correspond to initial Douglas-fir density. Broken lines correspond to H/D trends for individual plots.

Average H/D of dominant trees in plantations can be lowered through reduced planting densities or early thinning. The higher the initial density the shorter the period during which thinning can effectively lower future H/D values (Figure 2).

Figure 2: Early H/D trends from Haney, B.C. nelder plot data. Numbers to the right of legend lines represent initial Douglas-fir density (tph) in particular arcs. (Data from Smith 1983, Reukema and Smith 1987).

Thinning requirements in dense plantations make their management less flexible than the naturally regenerated stands they replace. Stands planted at higher densities develop greater H/D values and have a limited stand height window during which thinning can improve future stand stability. Stands planted at lower densities do not require thinnings to maintain stand stability. Thinnings can be optionally incorporated at any time during the rotation if specific timber markets are good or a certain stand structure is desired. If wider spacings raise weed competition or wood quality concerns (e.g., large branches, juvenile wood core, and low percentage latewood) species such as western hemlock and western red cedar, that appear to have a minimal impact on future dominant Douglas-fir stability, can be interplanted with Douglas-fir to augment overall stand density.

References:
Becquey, J. and P. Riou-Nivert. 1987. L’existence de zones de stabilite des peuplements. Consequences sur la gestion. Revue Forstiere Francaise 39:323-334.

Collier, R. and J. Haukaas. 1996. Stand Management Cooperative: Database Report. Stand Management Cooperative, College of Forest Resources, University of Washington. 25 p.

Cremer, K.W., C.J. Borough, F.H. McKinnell, and P.R. Carter. 1982. Effects of stocking and thinning on wind damage in plantations. N. Z. J. For. Sci. 12:244-268.

Curtis, R.O. and D.D. Marshall. 1986. Levels-of -growing -stock cooperative study in Douglas-fir: Report no. 8-The LOGS Study: Twenty-year results. USDA Forest Service Research Paper. PNW-356. 113 p.

Curtis, R.O., D.D. Marshall, and J.F. Bell. 1997. LOGS a pioneering example of silvicultural research in coast Douglas-fir. J. For. 95(5):19-25.

Henderson, J.A., D.H. Peter, R.D. Lesher, and D.C. Shaw. 1989. Forested plant associations of the Olympic National Forest. USDA Forest Service, Regional Ecology Technical Paper 001-88.

Lohmander, P. and Helles, F. 1987. Windthrow probability as a function of stand characteristics and shelter. Scand. J. For. Res. 2:227-238.

Lynott, R.E. and O.P. Cramer. 1966. Detailed analysis of the 1962 Columbus Day windstorm in Oregon and Washington. Monthly Weather Review 94:105-117.

Reukema, D.L. and J.H.G. Smith. 1987. Development over 25 years of Douglas-fir, western hemlock, and western redcedar planted at various spacings on a very good site in British Columbia. USDA Forest Service Research Paper, PNW-381, 46 p.

Smith, J.H.G. 1983. Graphical summaries and data on the U.B.C. Research Forest spacing trials (57-5) to age 26 and evaluations of results to date. Dept. of Forest Resource Management. Faculty of Forestry. 79 p.

Stone, J.N. 1994. Extensive studies of fertilizing and thinning coastal Douglas-fir and western hemlock: An installation report. B.C. Ministry of Forests, FRDA Report 227. 116 p.

Wilson, J.S. 1988. Wind stability of naturally regenerated and planted Douglas-fir stands in coastal Washington, Oregon, and British Columbia. Dissertation. University of Washington. 160 p.

Wiley, K.N. 1965. Effects of the October 12, 1962 windstorm on permanent growth plots in southwest Washington. Weyerhaeuser Forestry Paper No. 7, 13 p.

Zens, S. in prep. Structure and dynamics of Douglas-fir and western hemlock responses to reductions in density. Dissertation. Dartmouth University

* This work is part of an analysis that evaluated changing wind risk at the stand, landscape, and regional scales (Wilson 1998). The analysis was supported by the Landscape Management Project a cooperative effort between the University of Washington and the USFS PNW Research Station.

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