Dan Binkley and Sarah Greene

Transcription

1 PRODUCTION IN MIXTURES OF CONIFERS AND RED ALDER: THE IMPORTANCE OF SITE FERTILITY AND STAND AGE Dan Binkley and Sarah Greene ABSTRACT: Red alder, a source of biologicallyfixed nitrogen, can enhance site fertility and growth of interplanted conifers. On infertile sites, we found mixed alder/conifer stands to have greatly increased rates of ecosystem production; gains in conifer production occurred after age 30 when conifer dominance developed. On fertile sites, mixed stand productivity did not exceed that of pure conifer stands; conifer production was impaired throughout stand development. INTRODUCTION Biologic nitrogen (N) fixation offers an alternative to the use of inorganic fertilizers for maintaining or enhancing site fertility. Yet despite considerable discussion and research, N fixation has not been commercially exploited in Pacific Northwest forestry. Recently, researchers have analyzed the biologic and economic potential of using red alder (Alnus rubra Bong.), a Nfixing species, to enhance conifer production (cf. Atkinson et al. 1979, Miller and Murray 1979); but divergent findings from various experimental stands have not allowed uniform endorsement of red alder's potential. Tarrant (1961) reported that a 27yearold mixed red alder/douglasfir [Pseudotsuga menzieii (Mirb.) Franc01 plantation at Wind River, Washington, produced double the total stand volume of an adjacent, pure conifer stand. In a similar study at Cascade Head, Oregon, Bernsten (1961) reported that the yield at age 30 of a naturally established red alder/conlfer stand was 20% less thap that of an adjacent conifer plot where alder had been thinned at age 8. The differences in response hetween these experimental sites have increased over the past 20 years (Miller and Murray 1978, Greene L/). The Wind River site is much less fertile than the Cascade Head site, supporting Miller and Murray's (1978) suggestion that the benefits of red alder would probably be greatest.on Ndeficient sites. DAN BINKLEY is an assistant professor and research associate at the School of Forestry and Environmental Studies of Duke University, Durham, N. Carolina (formerly graduate research assistant Dep. Forest Science, Oregon State University, Corvallis, Oreg.). SARAH GREENE is a research forester with the USDA Forest Service Pacific Northwest Forest and Range Exp. Stn., Corvallis, Oreg. 1/Greene, S. Growth and yield in 50yearold pure or mixed conifer and red alder stands at Cascade Head, Oregon. Manuscript in preparation. 112

2 We hoped consideration of age and site fertility (specifically, N status) would produce a clearer picture of the scope and limits of red alder as a tool for maintaining or enhancing site productivity. We took advantage of the thorough stand records for Cascade Head and calculated estimates of aboveground net primary production for pure conifer, mixed alder/conifer, and pure alder stands through 46 years of stand development. We then combined Cascade Head estimates with ones for three other locations to synthesize a pattern of alder effects in conifer plantations as a function of site fertility and stand age. SITE DESCRIPTIONS Each of the four sites discussed herecascade Head, Wind River, Skykomish River, Washington and Mount Benson, British Columbiacontains a pure conifer stand paired with an adjacent mixed stand of red alder and conifers. The Cascade Head site also includes a pure alder stand. Douglasfir is the major conifer in all stands, with a substantial component of Sitka spruce (Picea sitchensis (Bong.) Carr.] and western hemlock [Tsuga heterophylla (Raf.) Sarg.] occurring only at Cascade Head. Two sites (Wind River and Mount Benson) are infertile Site Class IV, and two (Cascade Head and Skykomish River) are fertile Site Class I or 11 (table 1). Cascade Head Agricultural land, abandoned in 1925, naturally seeded in with mixtures of red alder, Douglasfir, Sitka spruce, and western hemlock. Thinnings between 1935 and 1937 produced stands of pure conifer, alder/conifer, and pure alder; the pure conifer stand had grown with red alder for about 8 years before thinning. Growth and yield information for these stands (stems > 6 cm diameter at breast height (d.b.h.)) is provided by Bernsten (1961) and Green&/; Franklin et al. (1968) evaluated soil properties, and Tarrant et al. (1969) measured aboveground litterfall. The thinning operation that established these plots reduced the density of conifer stems in the pure conifer plot far below that of the mixed alder/ conifer plot (table 1). A portion of the response differences between these stands (especially mortality) is therefore attributable to a stocking difference and not to the presence of alder.a/ Wind River A 20mwide firebreak of red alder was planted within a 2yearold Douglasfir plantation. Tarrant (1961) summarized stand growth, yield, and nutrition at age 27, and Miller and Murray (1978) updated the growth and yield information at age 48. Soil properties were described by Tarrant and Miller (1963). The mixed alder/conifer stand had a much higher total stem density (table 1); however, Douglasfir mortality between ages 27 and 48 was greater without alder (1,000 stems/ha) than with alder (840 stems/ha) (calculated from Tarrant 1961 and Miller and Murray 1978). Skykomish River Twoyearold Douglasfir seedlings were machine planted in a pasture in 1958; and red alder seedlings, naturally established about 1958, were hand cleared from half the plantation between 1962 and Ecosystem biomass, production, and nutrient content are described by Binkley (1983). The total stocking of these two plots was similar; the addition of alder stems was matched by a proportionate reduction in Douglasfir stems (table 1). In this situation, the effect of alder on the conifers was due to interspecific competition rather than increased stem density. Mount Benson A portion of a 1958 Douglasfir plantation contained naturally seeded red alder. The pattern 'of alder establishment did not follow any trend in soil physical properties and was probably restricted by available seed source. Ecosystem descriptions are also reported by Binkley (1983). As at Wind River, the mixed alderlconifer plot at Mount Benson had much higher stocking than the pure conifer plot. The very open canopy of the pure conifer plot indicated considerable underutilization of site resources. METHODS Biomass regression equations were applied to stem diameter data from each stand to obtain estimates of aboveground biomass and net primary production. At Cascade Head, 0.2ha permanent plots were measured 7 times over 50 years; and regression equations (described below) were applied to individual tree tallies and summed for stand totals. For Wind River at age 27, we applied regression equations to stand average d.b.h. and diameter increment (from Tarrant 1961) and multiplied by the number of stems to obtain stand totals; for age 48, we used similar calculations broken down into six average diameter classes provided by Miller and Murray (1978). Estimates for 23yearold stands at Skykomish River and Mount Benson were calculated by applying regression equations to individual stem tallies (Binkley 1983). /Binkley, D. The importance of stocking and relative densities in managing mixed stands of Douglasfir and red alder.. Manuscript in review. 113

3 ~~ Table 1 Site and stand characteristics for four study sites Characteristic cascade Head Wind River Skykomish River Mt. Benson Location Central coastal Oregon Southwestern Wash in gton Elevation, m Northwestern Washington Eastern Vancouver Is land Precipitation, cdyr Douglasf ir site index, m at 50 yr Establishment year (approx.) Stocking, trees/ha Conifer only (age) 1,650 (24) Conif er/alder 2,300/1,550 1,400 (27) 1,100/1,550 1,860 (23) 1,600/ (23) 540/2,200 Total Soil N, kg/ha W i tho ut a 1 der 13,000 (to 80 cm With alder 14,200 in 1966) 3,200 (to 80 cm 4,200 in 1959) 4,650 (to 60 cm 5,570 in 1980) 1,560 (to 60 cm 2,380 in 1980) Anaerobic available N index, Pg/g 015 cm Without alder 115 With alder RESULTS AND DISCUSSION All regression equations were obtained from the literature except those for red alder, which were developed on t>e basis of data from Mount Benson (Binkley 1983). Because all the equations may provide biased estimates, our calculated values can be considered only as "ballpark" figures for stand comparisons. The Douglasfir and western hemlock equations were taken from Gholz et al. (1979); annual leaf production was assumed to be 20% (Douglasfir) and 28% (western hemlock) of total leaf biomass (Gholz et al. 1979). Because no equations were available for Sitka spruce, we arbitrarily modified the Douglasfir equations by subtracting 35% to account for the lower density of spruce boles. We used the Mount Benson red alder equations for all sites. But because both Cascade Head and Wind River alders exceeded the size of the trees used in deriving the Mount Benson equations, we reduced our estimates of alder canopy biomass by 20% for large (stems > 25 cm d.b.h.) trees, judging that the equations overestimated canopy biomass on these two sites by about 20% on the basis of maximum stand canopy biomass for red alder reported by Zavitkovski and Stevens (1972). Stem and branch production at Cascade Head was calculated as the average annual difference between sampling times; at other sites, these estimates were calculated on the basis of 5year diameter increment and biomass of stems and branches from the 5 years' previous growth as predicted by regression equations. Production estimates for Cascade Head included tree mortality; no mortality estimates were available for the other sites. Our detailed analysis of the Cascade Head site revealed striking differences among stands. Aboveground tree net primary production (leaf, stem, and branch growth plus mortality) rapidly peaked in the red alder stand, reaching a plateau at about 12 Mg hal yr'l between ages 15 and 25 (fig. 1). However, high mortality began at 20 (fig. 2), resulting in a gradual decrease in the rate of tree biomass accumulation (fig. 3). Aboveground net primary production of the mixed alder/conifer and pure conifer stands was similar throughout stand development (fig. 1); however, mortality was much greater in the more densely stocked mixed stand (fig. 2). This difference led to a much greater accumulation of aboveground tree biomass in the pure conifer stand by age 50 (fig. 3). Before age 20, the pure alder stand was more productive and accumulated more tree biomass than either of the stands with conifers. After that time, the pure conifer stand outperformed the mixed stand, which exceeded the pure alder stand. The patterns in production and tree biomass in the 23yearold Skykomish River stands were similar to those at Cascade Head at the same age (fig. 4). Production for these fertile sites consistently exceeded the average value reported by Grier,(1979) for the Douglasfir forest zone. Though Sykomish River was more productive than Cascade Head, at both sites red alder failed to increase total stand production or biomass. The proportional decrease in conifer production was greater at Cascade Head, where alder stocking was much higher. 114

4 ALDER STAND ALDERICONIFER STAND CONIFER STAND 15t r AGE Figure 1. Aboveground tree net primary production (leaf, stem, and branch growth plus mortality) for the three stand types at Cascade Head. RA = red alder, DF = Douglasfir, SS = Sitka spruce, WH = western hemlock. Contribution of each species represented by area between curves; uppermost signifies cumulative total. 3'51 ALDER STAND I ' ' ' I ALDER/CONIFER STAND rr CONIFER STAND I I AGE Figure 2. Aboveground tree mortality for the three stand types at Cascade Head. ALDER STAND ALDER/CONIFER STAND CONIFER STAND I c 0 CI, c IO0 n " AGE Figure 3. Aboveground tree biomass [the intergral of figure 1 minus mortality (figure 2)] for the three stand types at Cascade Head. 115

5 Patterns in production and tree biomass were similar for the infertile Mount Benson and Wind River sites at age 25 (fig. 4). In both cases, red alder had little effect on Douglasfir growth, but its presence more than doubled total stand production and biomass. The current Douglasfir production rate at age 48 at Wind River was about 50% greater where red alder was present; however, the increasing dominance of Douglasfir had not yet resulted in a substantial increase in Douglasfir biomass. Where red alder was absent, total ecosystem production was near the bottom of the range reported for Douglasfir (Grier 1979). The precise interactions of red alder and conifers in mixed stands will of course vary with sitespecific conditions, especially relative stocking densities and dominance. Our comparisons suggest some general trends we think will apply to a broad range of sites. The pattern emerging from our analysis of ecosystem production indicates that mixed stands of red alder and conifers on fertile Site Class I or IT areas would not be expected to exhibit higher rates of aboveground net primary production than pure conifer stands. Indeed, greater mortality in mixed stands (especially at high densities) may lead to a decrease in accumulated stand biomass at maturity. Conifer production would probably be much less in mixed stands than in pure conifer stands. In contrast, total ecosystem production in mixed stands on infertile Site Class IV areas may be more than twice that in pure conifer stands at all stages of stand development. Conifer production in mixed stands may be largely unaffected during the early stages, with gains appearing as the stand nears maturity. The timing of conifer canopy dominance and expansion should determine the period of increased conifer growth in mixed stands on infertile sites. Early emergence of conifer canopies should result in early gains in conifer growth. This general sitefertility pattern is consistent with the common occurrence of low growth responses to N fertilization in fertile Site I or I1 stands and high responses in Site IV stands (Miller and Fight 1979). Our results underscore the necessity, as noted by Miller and Murray (1979), for stratifying stands according to fertility and for considering complete rotation intractions of species to provide a mechanistic, comprehensive understanding of the potential usefulness of biologically fixed N in maintaining and increasing site productivity. ACKNOWLEDGMENTS This paper was made possible through the foresight and efforts of USDA Forest Service personnel, including C. Bernsten, J. Booth, L. Issac, W. Meyer, R. Miller, M. Nance, and R. Tarrant, in establishing, maintaining and measuring the experimental stands at Cascade Head, Oregon and Wind River, Washington. We thank Phil'Sollins, Joe Means, and Connie Harrington for their helpful reviews, and Carol Perry for technical editing. Paper 1700, Forest Research Laboratory, Oregon State University, Corvallis, OR FERTILE FERTILE 30 lid3ra r SKYKOMISH CASCADE HEAD," AGE 23 AGE 24 AGE52 25 MT. EFNSON WINOR~VER AGE 23 AGE 21 AGE 48 I SKYKOMISH CASCADE HEAD AGE 23 AGE 24 AGE 52 MTBENSON WINO RIVER PGE 23 AGE 27 AGE 48 Figure 4. Aboveground tree net primary production (NPP) and tree biomass for (A) fertile and (B) infertile sites. LITERATURE CITED Atkinson, W. A., B. T. Bomann, and D. S. DeBell Crop rotation of Douglasfir and red alder: a preliminary biological and economic assessment. Botanical Gazette Supplement 140: S102SlO7. Binkley, D Ecosystem production in Douglasfir plantations: interactions of red alder and site fertility. Forest Ecology and Management 5: Bernsten, C. M Growth and Development of red alder compared with conifers in 30yearold stands. USDA Forest Service Res. Pap. PNW38, Pac. Northwest For. and Range Exp. Stn., Portland, OR. Franklin, J. F., C. T. Dyrness, D. G. Moore, and R.F. Tarrant Chemical soil properties under coastal Oregon stands of alder and conifers. p In Biology of alder (J. M. Trappe, J. F. Franklin, R. F. Tarrant, and G. H. Hansen, eds.). USDA Forest Service,, Pac. Northwest For. and Range Exp. Stn., Portland, OR. 116

6 Gholz, H. L., C. C. Grier, A. G. Campbell, and A. T. Brown Equations for estimating biomass and leaf area of plants in the Pacific Northwest. Research Paper 41, Forest Research Laboratory, Oregon State University, Corvallis, OR Grier, C.C Productivity assessment of Pacific Northwest forests. pp In Forest fertilization conference (S: P. Gessel, R. M. Kenady, and W. A. Atkinson, eds.). Institute of Forest Research Contribution 40, University of Washington, Seattle. Miller, R. E., and R. D. Fight Fertilizing Douglasfir forests. USDA Forest Service Gen. Tech. Rep. PNW83, Pac. Northwest For. and Range Exp. Stn., Portland, OR. Miller, R. E., and M. D. Murray The effects of red alder on growth of Douglasfir, p In Utilization and management of alder (D. G. Brizs, D. S. DeBell, and W. A. Atkinson, eds.). USDA Forest Service Gen. Tech. Rep. PNW70, Pac. Northwest For. and Range Exp. Stn., Portland, OR. Miller, R. E., and M. D. Murray Fertilizer versus red alder for adding nitrogen to Douglasfir forests of the Pacific Northwest, p In Symbiotic nitrogen fixation in the management of temperate forests (J. C. Gordon, C. T. Wheeler, and D. A. Perry, eds.). Forest Research Laboratory, Oregon State Univ., Corvallis. Tarrant, R. F Stand development and soil fertilityin a Douglasfir/red alder plantatlon. For. Sci. 7: Tarrant, R. F., K. C. Lu, W. B. Bollen, andj. F.. Franklin Nitrogen enrichment of two forest ecosystems by red 'alder. USDA Forest Service Res. Pap. PNW76, Pac. Northwest For. and Range Exp. Stn., Portland, OR. Tarrant, R. F., and R. E. Miller Soil nitrogen accumulation beneath a red alder Douglasfir plantation. Soil Science Society of American Proceedings 27: Zavitkovski, J., and R. D. Stevens Primary productivity of red alder ecosystems. Ecology 53: