Analysis of Data. Analysis followed that given by Curtis et al. (1974) with modifications. Preliminary analyses using individual age class

Transcription

1 Height Growth and Site Index for Douglas-fir in High-Elevation Forests of the Oregon-Washington Cascades ROBERT O. CURTIS FRANCIS R. HERMAN DONALD J. DeMAR Abstract. Height growth and site index estimation curves were derived from stem analyses of 52 Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) trees. Their height growth pattern differs from lowland Douglas-fir, and corresponding differences in the pattern of volume growth must exist. Site index estimation procedures and growth information derived from lowland Douglas-fir are not applicable to these high-elevation forests. Forest Sci. 20: Additional key words. Measurement techniques, Pseudotsuga menziesii, volume growth. THE HIGH-ELEVATION FORESTS on the western slope of the Cascade Mountains have been little studied. Most of the area has been relatively inaccessible and considered of minor importance for commercial timber production. With improved road systems and intensified management, these forests have recently become much more important for timber production as well as recreational use. The need for information on their characteristics and productivity led us to undertake extensive stem analysis studies, beginning in One segment of our work deals with the height growth pattern and site index classification of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii). Though broadly classified as true fir-hemlock, these high-elevation forests contain substantial amounts of Douglas-fir, both as stands and as scattered trees or groups intermingled with other species. Douglas-fir is also frequently planted on cutover areas within this zone. The area corresponds to the Abies amabilis zone of Franklin and Dyrness (1973), which occurs on the western slope of the Cascade Range at elevations from about 3,000 to 5,000 feet ( m) in Oregon, to about 2,000 to 4,000 feet ( m) in northern Washington. The forests are of mixed species composition. Although some stands are even-aged, others are composed of two or more distinct age classes. The most abundant species are the tolerant Pacific silver fir and western hemlock, whose growth is often much influenced by past suppression and for which conventional site index procedures are probably inaccurate and possibly inapplicable. However, Douglas-fir is a relatively intolerant species, and when abundant, has usually developed under locally even-aged conditions without overhead competition. Conventional site index procedures which express productivity in terms of height attained at a standard reference age can therefore be applied. Data were obtained by stem analyses of 52 selected dominant Douglas-fir trees at locations in the Cascade Range between McKenzie Pass in central Oregon and Stevens Pass in northern Washington (Figure 1). An effort was made to sample all identifiable habitat types within this area, within the limitations imposed by The authors are Principal Mensurationist, Mensurationist, and Associate Mensurationist, Pacific Northwest Forest and Range Exp. Stn., Forest Service, U.S. Dep. of Agric., Portland, Oreg. Manuscript received Nov. 12, volume 20, number / 307

2 existing stand conditions and species com position. All locations represented unmanaged stands, mainly old growth. The phys ical difficulty and cost of stem analyses of very large trees necessarily limited sample size. (Existing young-growth stands represent a small fraction of the area and a very restricted distribution.) The distinction between height growth estimation and site index estimation made in this paper follows Curtis et al. (1974). Field Procedure Each study location was selected for ap parent uniformity in site and stand condi tions and was about one-fourth acre in size, within a stand or group of trees in which the dominants were judged to represent a single age class. The tallest dom inant of each species was felled. Sections were cut at stump height, 4.5 feet (1.3 m), (bh), and at intervals up the stem. Section lengths were usually 18 feet (5.5 m) within the merchantable portion of large trees and and shorter in the upper portion of the stem and in small trees. Heights were plotted over ages for the sections from each tree and interpolated heights obtained for each 10 years of age bh. (In this paper, age is based on num ber of rings present at bh.) These graphs were examined for evidence of abnormal growth patterns suggesting past stem damage, early suppression, or differences in age class. As expected of an intolerant species, the Douglas-fir data showed rela tively few such anomalies. The number of acceptable Douglas-fir trees for which sec tions were available was, for each of the tree ages indicated: Table 1 gives the distribution of trees by classes of height attained at age 100 (= H100). Stem analysis procedures are discussed in more detail by Herman and DeMars (1970) and in another paper in preparation. 1 Analysis of Data Analysis followed that given by Curtis et al. (1974) with modifications. Preliminary analyses using individual age class 1 Francis R. Herman and Donald J DeMars. Stem Analysis field and laboratory techniques for coniferous tree species. Manuscript in preparation.) 308 / Forest Science

3 regressions preceded fitting of conditioned regressions to pooled data. Equations were transformed before fitting by dividing by smoothed standard errors of estimate (SEE) from the individual age class regressions, to stabilize variances. Principal differences from the earlier procedure were use of a different height growth function and fitting of site index estimation curves in segments, to accommodate the wide range in ages and insure reasonable behavior at the extremes. H100 represents total height of an individual tree at age 100 bh, which is an estimate of site index or S100 (= the location mean of all such heights for the specified stand component). H represents total height at any specified age. Regressions were calculated using the variables (H ) and (H 4.5) in feet, in which subtraction of 4.5 (approx. 1.3 m) serves to place these scales on the same basis as age ( A ), measured from bh as origin. Height Growth Curves. Initial trials used two equations frequently found satisfactory for height growth curves: the exponential function (H 4.5) = a (l e b A ) c given by Prodan (1961), Beck (1971), and others; and the inverse polynomial (H 4.5) = A 2 /[a[ + ba + ca 2 ] previously used by King (1966) for low-elevation Douglas-fir. The first gave an excellent fit over the lower twothirds of the age range but diverged from the data at advanced ages. The second was satisfactory over most of the range but diverged somewhat at young ages. volume 20, number / 309

4 Discussion Height Growth Curves and Site Index Es timation Curves. EquationIandFigure2representaconventionalsystem of height growth curves describing the average pat 310 / Forest Science

5 tern of height development of trees which actually attain a specified height at spec ified age (site index). However, if height actually attained at index age is unknown and the objective is to estimate this from height measured at some age other than the index age, then the traditional pro cedure based on the height growth curve is less efficient than one using the regres sion (equations II and III) of site index on age and height (Curtis et al. 1974). For comparison, these last are shown in tra ditional height-over-age format in Figure 4, but a more natural mode of presentation is that in Figure 3. The two systems of curves are expected to coincide for the mean site index; to co incide at index age only for all other site indices; and for other ages, to diverge in a consistent manner as site index diverges from the mean value. Our curves (Fig. 4) approximate these conditions though not meeting them exactly. (Dahms 2 has 2 Walter G. Dahms. Gross yield of central Oregon lodgepole pine. (Manuscript in prepa ration.) since developed an alternative fitting pro cedure which insures that these conditions are met.) since developed an alternative fitting procedure which insures that these conditions are met.) Polymorphism of Height Growth Curves. Height growth curves corresponding to equation I are polymorphic, with age of culmination of current annual height increment varying from 27 years bh for H100 = 60, to 22 years bh for H100 = 160. Differences from simple propor tionality become more evident as age in-creases beyond the index age, but are not striking and are less pronounced than those found in similar analyses with some other species (Beck 1971, Carmean 1972). Of course, there may be differences associated with site or stand characteristics which are not adequately expressed by site index alone. Choice of Index Age. Analyses were based on an index age of 100 years bh (corre sponding to a total age of about 110), chosen on the bases of precedent and the belief that rotations in these high-elevation forests are likely to be relatively long. Over a span of several decades about volume 20, number / 311

6 index age 100 bh, there is little difference between the site index estimation curves and the height growth curves, and the latter are nearly proportional. For a limited range of possible rotation ages, height at rotation age should be nearly proportional to H100. Although we derived another site index es timation equation using index age 140 bh, the system of curves based on index age 100 bh should suffice for practical use for anticipated rotations in the range of per haps 80 to 150 years. Possible Sources of Bias. The height growth curves show sustained height growth to very advanced ages. However, the upward bend in the graphical curves at about age 260 seems unreasonable, and the data contain relatively few trees beyond this age. In addition to graphical curves based on all 52 trees, others were prepared for ages: (1) 0 to 250, using the 31 trees older than 250 years; (2) 0 to 100, using the 13 trees 90+ to 200 years; and (3) 0 to 300, using the 10 trees over 300 years. Curves from (1) and (2) differed little from those based on all 52 trees; those from (3) suggested that these few very old individuals differed somewhat from the average growth pattern, having more prolonged height growth. This could merely represent sampling variation. It could also reflect effects of recent climatic change (Franklin et al. 1971), or the possibility that the tallest in dividuals in very old stands may frequently have unusually prolonged height growth. If present, such effects would produce an 312 / Forest Science

7 upward warping of the curves, probably important only at the upper margin of the age range. Shifts in relative tree position over time (Dahms 1963) should have little effect on average curves within that portion of the age range in which there is substantial overlap in tree ages (100 to 250 yr in these data), but could introduce bias at younger ages. Such bias is probably present but cannot be evaluated with one sectioned tree of the species per location. We think its practical importance is small except at very young ages. Age and Site Index Estimates. Site index estimates for individual locations were cal culated for successive 10-yr measurements using both the site index estimation equa tion and traditional inverse estimation from the height growth equation. The site index estimation equation gave the better estimates of H100, and it was evident that es timates made for ages less than about 20 yr bh had little meaning (as expected from the low correlations between H and H100 at these ages, Figure 5). Better es timates might be possible using curves con structed from younger stands, based on a more carefully specified stand component identifiable in young stands. As yet, rel atively few such stands are available for study. Comparisons with Other Height Growth Curves. The principal previous height growth curves for Douglas-fir in the Pacific Northwest are those of McArdle et al. (1930, 1961) and King (1966). These were based on low-elevation Douglas-fir and differ considerably both from each other and from our new curves (Figures 6A and 6B). McArdle used the old guide curve volume 20, number / 313

8 method, with little data below age 40; King s curves, prepared from internode measurements, should be a more reliable expression of the actual course of growth. McArdle used average height of dominants and codominants; King used average heights of the largest 20 percent (by diam eter) from a group of 50 stems, at time of measurement; while we used heights of the tallest tree per one-fourth acre in mixed, old-growth stands. Since shape of height growth curves derived by regression is not independent of the index age used (Heger 1973), we also prepared graphical curves using index age 50 bh, as in King (1966). These differed only slightly from our curves based on index age 100 bh and do not affect the comparison in Figure 6. These different systems represent de velopment over time of slightly different stand components. Some differences would also be expected from differences in method of construction, and possibilities of bias exist in each. However, it seems very unlikely that these could account for differences in curve shape of the magnitude shown. We conclude that our trees do have a different growth pattern, characteristic of upper-slope sites with their lower tem peratures, higher rainfall, and shorter grow ing season. Compared with our new curves, McArdle s site index curves can be ex pected to overestimate site index of old stands and to underestimate that of young stands (also true to a lesser extent of King s). Slower height growth in youth and more prolonged height growth in later life must be accompanied by corresponding differ ences in patterns of volume and basal area growth. Lacking volume or basal area growth data for these stands, we can make no direct estimates of corresponding patterns of volume growth. However, the probable nature of such differences can be inferred indirectly. If a common relationship of cubic volume to stand height is as sumed for our stands and for those described in Tables 1 and 3 in McArdle et al. (1961), and if the trend of height over age indicated by equation I for the same height at total age 100 is substituted for McArdle s height growth curve, the compari son shown in Figure 7 is obtained (for McArdle s site index 110). Since much of the Douglas-fir at high elevations is in mixed stands rather than the pure stands decribed by McArdle et al., the relationship of stand volume to height probably differs and the specific numerical values obtained are probably biased. The comparison nevertheless il lustrates that differences in shape of the 314 / Forest Science

9 height growth curve must be associated with corresponding differences in the volume increment curve, and that McArdle s yield estimates cannot be safely applied to upper-slope Douglas-fir of the same numerical site index. Application. The new curves should be used for Douglas-fir within the geographical and elevational zone represented by the basic data. The height growth curves (equation I) are the proper basis for arranging stand volume or volume increment in age sequence by sites when preparing yield tables, and for comparing height growth patterns with other species or localities. Volume yields for a species are closely associated with stand height and although the relationship is curvilinear in young stands, it approaches linearity with increasing age (and heights). Hence, heights at an index age approximating probable rotation age(site indexes) provide a means of stratifying stands into productivity classes even in the absence of applicable volume 20, number / 315

10 yield tables. Here, the appropriate estimate is given by equations II and III. For stand ages from about 80 to 200 yr, there is little practical difference between estimates using equations II and III and those obtained by the traditional procedure of inverse estimation using the height growth equation. However, differences become important at younger ages and are appreciable for ages over 200. The equations are based on trees selected as the single tallest undamaged dominant Douglasfir per one-fourth acre. In application to oldgrowth stands having widespread top damage, the undamaged dominant requirement necessarily takes precedence over any specific area standard, and definition of site trees as leading dominants should be adequate for stand classification in oldgrowth forests. The area estimate of site index is then the mean of estimates of H100 made for several well-distributed leading dominants, exclusive of trees with major top damage or indications of early injury or suppression. Literature Cited Beck, Donald E Height-growth patterns and site index of white pine in the southern Appalachians. Forest Sci 17: Carmean, Willard H Site index curves for upland oaks in the Central States. Forest Sci 18: Curtis, Robert O., Donald J. DeMars, and Francis R. Herman Which dependent variable in site index-height-age regressions? Forest Sci 20: Dahms, Walter G Corrections for a possible bias in developing site index curves from sectioned tree data. J Forest 61: Franklin, Jerry F., and C. T. Dyrness Natural vegetation of Oregon and Washington. Pac Northwest Forest & Range Exp Stn USDA Forest Sery Gen Tech Rep PNW-8, 417 p., William H. Moir, George W. Douglas, and Curt Wiberg Invasion of sub-alpine meadows by trees in the Cascade Range, Washington and Oregon. Arctic & Alpine Res 3: Heger, L A method of constructing site index curves from stem analyses. Forest Chron 44: Effect of index age on precision of site index. Can J Forest Res 3 (1) :1 6. Herman, Francis R., and Donald J. DeMars Techniques and problems of stem analysis of old-growth conifers in the Oregon Washington Cascade Range. In Tree-ring analysis with special reference to Northwest America. J. H. G. Smith and J. Worral, eds. Univ B C Fac Forest Bull 7, p King, James E Site index curves for Douglas-fir in the Pacific Northwest. Weyerhaeuser Forest Pap 8, 49 p. McArdle, Richard E., and Walter H. Meyer The yield of Douglas-fir in the Pacific Northwest. U S Dep Agric Tech Bull 201, 64 p., Walter H. Meyer, and Donald Bruce The yield of Douglas-fir in the Pacific Northwest. U S Dep Agric Tech Bull 201 (rev.), 74 p. Prodan, Michail Forstliche Biometrie. BLV Verlagsgesellschaft. 431 p., Munich, Bonn, Vienna. (English translation published as Forest Biometrics p. Pergamon Press.) About This File: This file was created by scanning the printed publication. Misscans identified by the software have been corrected; however, some mistakes may remain. 316 / Forest Science