Productivity of red alder in western Oregon and Washington

Transcription

1 From Biology of Alder Proceedings of Northwest Scientific Association Annual Meeting April 14-15, 1967 Published 1966 Productivity of red alder in western Oregon and Washington Red alder in western Oregon and Washington grows rapidly when young and outproduces Douglas-fir up to ages years on median sites of both species. Red alder readily responds to thinning. Its ability to add nitrogen to soil is important for site improvement over much of its natural range. Richard L. Williamson Forestry Sciences Laboratory Pacific Northwest Forest and Range Experiment Station Olympia, Washington This literature review on growth and yield of red alder (Alnus rubra Bong.) revealed very little past research in this commercially important species in the Pacific Northwest. Hopefully, the increasing interest in red alder management signals recognition that we sometimes stand to gain much more by managing this species rather than by routinely trying to eliminate it. Pure stands of red alder generally occur at lower elevations and where soil moisture is abundant - valley bottoms, moist flats, or lower slopes where soils frequently have restricted drainage. But, best stands occur on deep, well-drained soils of alluvial origin having abundant soil moisture (Powells, 1965). Pure stands extend upslope until excessive drainage restricts development. Red alder usually occurs in mixture with coniferous species- principally, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) on lower slopes; proportion of red alder decreases with increasing elevation above the valley floor. Red alder is a shade-intolerant species which relies upon major site disturbance for perpetuation. Otherwise, from general silvical principles, we would expect that pure stands would be gradually replaced by more tolerant species. Productivity of Mixed Stands It is difficult to assess productivity of red alder since it occurs as a minor component of mixed stands over much of its range. Only half of the estimated total volume (about 19 billion board feet) is in pure stands (Metcalf, 1965). A portion of McCleary Experimental Forest in southwest Washington is occupied by an even-aged, 60-year-old, alder-conifer mixture, typical of mixed stands in which alder is mature. Douglas-fir predominates with 49 percent of the volume (4,304 ft3); red alder contributes 22 percent (1,

2 fe). Western hemlock and western redcedar (Thuja plicata Donn) make up the other 29 percent (2, 707 ft 3 ). From age 45 to age 60, annual mortality for the red alder component equaled net growth (about 15 fe per acre). Thus, alder volume remained fairly static at about 1,800 fe per acre for 15 years. One can conclude that, for all practical purposes, net volume production ( 40 fe' m.a.i.) by red alder in this typical mixed stand ceased by age 45. Productivity of Pure Stands UNMANAGED STANDS Normal yield tables for red alder (Worthington et al., 1960) show that net growth. of well-stocked, natural stands is rapid (about 150 ft 3 per acre per year) when they are young, and decreases gradually to zero at age 90 on median site index 90. On this site, average yield is 4,940 ft 3 per acre at age 50, when rotation based on culmination of mean annual increment is attained. MANAGED STANDS Published results of three red alder studies indicate that thinning reduces total stand growth little, if any. Forty percent of the basal area was removed from a 26-year-old stand near McMurray, Wash., in a thinning from below (Lloyd, 1955). During the 2- to 8-year period after thinning, gross annual volume growth differed little between thinned and unthinned areas ( 133 and 128 fe per acre, respectively). No mortality occurred in the thinned stand, so net growth equaled gross growth. In contrast, extensive mortality in unthinned plots (35 ft 3 per acre per year) resulted in net growth of only 73 percent of gross growth. In a second study on Cascade Head Experimental Forest near Otis, Oregon, an 11-year-old alder-conifer mixture was heavily thinned to pure red alder on approximately an 8- by 8-foot spacing (Berntsen, 1961 ). Residual cubic volume (total stand) was only 39 percent of the volume in an adjacent unthinned pure alder stand, with which the thinned stand was to be compared. Bernsten does not say whether the thinning was essentially from below or from above, but he notes that some of the taller trees were cut. Substantial cutting from above, or possibly lower site index, is probable since the unthinned stand started with a 12-foot average height advantage over the thinned stand (36.5 feet vs feet). The initial advantage of the control stand is further apparent since it initially had more than eight times as much cubic volume, in trees 6 inches DBH and larger, as did the thinned stand (67 ft 3 vs.8 fe). As could be expected, during the first 5 years after thinning, total stand gross and net growth in the thinned stand was only 58 percent of that (266 fe per year) in the unthinned stand. For the remaining 15 years of observation, there was no appreciable difference (thinned was 97 percent of unthinned) in gross growth between the two stands (237 ft 3 v s. 246 ft 3 ). 288

3 Net growth, for the latter period, was somewhat greater for the thinned stand (207 ft 3 vs. 178 fe), due to heavier mortality in the unthinned plot. In a third study, also on the Cascade Head Experimental Forest, a plot in a 21-year-old red alder stand was heavily thinned to an approximate spacing of 12 by 12 feet (Berntsen, 1962). This plot was compared with a nearby unthinned plot. Both plots were in stands containing a scattered overstory of 80-year-old Douglas-fir which were girdled during plot establishment. A photograph of stand conditions at the beginning of the study, and comparison of before-thinning cubic volumes for these two plots with that of another nearby pure alder plot which lacked the overstory, indicate that the overstory decreased stocking on the first two plots by about 20 percent. Before-thinning volumes also indicate that productivity of the thinned plot, without any treatment, would be about 12 percent less than that of the unthinned plot. After thinning, residual cubic volume in the thinned plot was 56 percent of that in the unthinned plot. Despite the indicated lesser inherent productivity, gross periodic annual growth of the thinned stand, for the first 5 years after treatment, was practically identical with ( 125 fe vs. 128 ft 3 ) that of the unthinned stand. For the entire 20-year-period, thinned stand gross increment ( 112 fe per year) averaged 95 percent of that for the unthinned stand (118 ft 3 ). Net increment for the 20-year period was identical for both stands due to low mortality in the unthinned stand. The thinnings reported by Berntsen were of moderate to severe intensity; approximately 36 percent volume removed in one case and 61 percent removed in the other. That post-treatment growth of the thinned stands, relative to that of the unthinned stands, did not decline, or declined for a short period only, indicates that their response was in fact substantial (contrary to the opinions of Berntsen, 1961 and 1962). This conclusion is in agreement with Lloyd's (1955) previously cited results. Thus, evaluation of published data indicates that thinning can increase productivity of red alder stands. Rapid early growth rate of red alder, as indicated above, sometimes influences land managers to consider retaining alder on a site rather than converting to a coniferous species. One of the Cascade Head studies (Berntsen, 1961) also compared growth of thinned and unthinned alder stands with that of an adjacent conifer stand (Douglas-fir and Sitka spruce (Picea sitchensis (Bong.) Carr.) which was thinned to an approximate 6- by 6-foot spacing at age 8. The conifer stand had less annual gross growth than either alder stand up to age 21, but it rapidly outgrew both alder stands thereafter. Cubic volumes of thinned and unthinned alder stands (3,000 and 4,400 fe, respectively) exceeded that of the conifer stand until ages 25 and 28, respectively. Thereafter, cubic volume of the conifer stand rapidly surpassed those of the alder stands (Fig. 1). Significance of Height Growth Height-age curves for red alder and Douglas-fir for median site indices 90 and 105, respectively, illustrate one major reason why red alder is so much 289

4 400 ALDER (UNTHINNED) "- CONIFER STAND AGE (YEARS) Figure 1. Volume per acre for pure alder (thinned and unthinned) and pure conifer stands at Cascade Head Experimental Forest. (Modified from Figure 5 of Berntsen (1961).) more productive at young ages (Fig. 2). Douglas-fir on this site index requires 8 years, on the average, to attain breast height and consequently would have no basal area (and therefore no volume) up to then. Red alder attains maximum height growth 2 or 3 years from seed, Douglas-fir only after 15 to 20 years. Height growth of red alder begins to decrease when Douglas-fir growth reaches its maximum (Fig. 2). This is when volume of Douglas-fir stands begins to overtake and rapidly surpass volume of red alder stands. Site Improvement Red alder's ability to add nitrogen to soil is particularly important to improve growth of associated species (Tarrant, 1961). This ability has particular significance over much of red alder's natural habitat in western Washington- gravelly stream bottoms and gravelly glacial outwash 290

5 RED ALDER DOUGLAS-FIR BREAST HEIGHT TOTAL AGE (YEARS) Figure 2. Height-age curves for red alder and Douglas-fir for 50-year site indices 90 (Worthington et al., 1960) and 105 (King, 1966), respectively. For Douglasfir, total age equals breast-height age+ 8. plains- because these sites are generally poor in nitrogen (Gessel, Stoate, and Turnbull, 1965). In summary, when we consider rotations of 30 to 35 years, red alder is a possible alternate species to manage in a region where foresters commonly think only in terms of managing its coniferous associates. It definitely is capable of responding to thinning and provides a natural, silvicultural means for increasing wood production on some nitrogen-deficient soils. It is clear that we need further study of net yields of red alder under intensive stand management. Existing studies have generally involved only a single thinning with a limited range of unreplicated treatments. 291

6 Literature Cited Berntsen, C. M Growth and development of red alder compared with conifers in 30-year-old stands. U. S. Forest Serv. Pacific Northwest Forest & Range Exp. Sta. Res. Pap p A 20-year growth record for three stands of red alder. U. S. Forest Serv. Pacific Northwest Forest & Range Exp. Sta. Res. Note p. Powells, H. A. [ed.] Silvics of forest trees of the United States. U.S. Dep. Agr. Handb p. Gessel, S. P., T. N. Stoate, and K. J. Turnbull The growth behavior of Douglas-fir with nitrogenous fertilizer in western Washington. Univ. Wash. Inst. Forest Prod. Res. Bull p. King, J. E Site index curves for Douglas-fir in the Pacific Northwest. Weyerhaeuser Co. Forest. Res. Center, Weyerhaeuser Forest Pap p. Lloyd, W. J Alder thinning progress report. U. S. Soil Conserv. Serv. Woodland Conserv. Tech. Note 3. 6 p. Metcalf, M. E Hardwood timber resources of the Douglas-fir subregion. Pacific Northwest Forest & Range Exp. Sta. U. S. Forest Serv. Resource Bull. PNW p. Tarrant, R. F Stand development and soil fertility in a Douglas-fir- red alder plantation. Forest Sci. 7: Worthington, N. P Normal yield tables for red alder. U.S. Forest Serv. Pacific Northwest Forest & Range Exp. Sta. Res. Pap p. 292