Steven H. Sharrow

Carbon and nitrogen storage in agroforests, tree plantations, and pastures in western Oregon, USA

Pastures store over 90% of their carbon and nitrogen below-ground as soil organic matter. In contrast, temperate conifer forests often store large amounts of organic matter above-ground in woody plant tissue and fibrous litter. Silvopastures, which combine managed pastures with forest trees, should accrete more carbon and nitrogen than pastures or timber plantations because they may produce more total annual biomass and have both forest and grassland nutrient cycling patterns active. This hypothesis was investigated by conducting carbon and nitrogen inventories on three replications of 11 year-old Douglas-fir (Pseudotsuga menziesii)/perennial ryegrass (Lolium perenne)/subclover (Trifolium subterraneum) agroforests, ryegrasss/subclover pastures, and Douglas-fir timber plantations near Corvallis, Oregon in August 2000. Over the 11 years since planting, agroforests accumulated approximately 740 kg ha–1 year –1 more C than forests and 520 kg ha–1 year–1 more C than pastures. Agroforests stored approximately 12% of C and 2% of N aboveground compared to 9% of C and 1% of N above ground in plantations and less than 1% of N and C aboveground in pastures. Total N content of agroforests and pastures, both of which included a nitrogen-fixing legume, were approximately 530 and 1200 kg ha–1 greater than plantations, respectively. These results support the proposition that agroforests, such as silvopastures, may be more efficient at accreting C than plantations or pasture monocultures. However, pastures may accrete more N than agroforests or plantations. This apparent separation of response in obviously interrelated agroecosystem processes, points out the difficulty in using forest plantation or pasture research results to predict outcomes for mixed systems such as agroforests.

Effects of fire, ash, and litter on soil nitrate, temperature, moisture and tobosagrass production in the rolling plains.

Removal of litter by burning or clipping in tobosagrass communities increased soil temperature and the rate of nitrogen mineralization. Ash had no effect on either of these soil properties in 1972, but did appear to stimulate production in 1974. With adequate soil moisture, the higher soil temperatures on burned or clipped plots stimulated plant growth and concomitantly reduced soil moisture and nitrates. By contrast, suboptimal soil temperatures on control plots limited plant growth, even though soil nitrate and moisture were ample. During dry years, soil moisture is the limiting plant growth factor and burning has no beneficial effects.

Effect of grazing by sheep on the quantity and quality of forage available to big game in Oregon's Coast Range.

Effects of sheep grazing in Douglas-fir (Pseudotsuga menziesii) plantations in Oregons Coast Range were studied in 1981 through 1983 to determine the impact of grazing on big game habitat. Biomass, dry matter digestibility, and crude protein content of forage present on grazed vs. ungrazed areas were determined in October and March both years. Sheep grazing reduced total current years phytomass of browse and forbs (p<.10) in October. October phytomass of graminoids was not affected by grazing. Forage from grazed areas in October generally had higher crude protein levels and dry matter digestibility than forage from ungrazed areas in October. Few differences in either crude protein or dry matter digestibility of forage from grazed vs. ungrazed areas were evident in March. However, a greater quantity (p<.10) of new, succulent forage was generally present in grazed areas compared to ungrazed areas. These data suggest that sheep grazing can improve big game forage supply in Oregons Coast Range by improving forage quality in the fall and by increasing the quantity of high quality forage in the spring.

Sheep grazing as a silvicultural tool to suppress brush.

The possibility of using livestock as a biological agent to control unwanted ground vegetation in Pacific Northwest coniferous forests has been discussed for over 50 years. However, little quantitative information has yet been published documenting the efficacy of livestock in suppressing brush and other ground vegetation in commercial Douglas-fir (Pseudotsuga menziesii) plantations. Therefore, a study was conducted in 1981 and 1982 to evaluate the potential for using herded sheep to control competing vegetation in Douglas-fir plantations in Pacfic Northwest coniferous forests. Three 4to 6-year-old plantations were grazed once each year during the May to September grazing season. Estimates of current years growth present in October, both inside and outside a livestock exclosure on each study plantation, were used to evaluate the effects of grazing. In general, utilization of brush by sheep was moderate to heavy, except in the spring of 1982, when brush was lightly utilized. Sheep grazing effectively reduced (p<0.01) both total understory plant growth and brush net current years growth on all plantations. Reduced brush biomass on grazed areas was associated with greater Douglas-fir diameter growth in 1981-82 and 1982-83. By 1985, trees in grazed areas were 5% taller (p<0.0S) and 7% greater in diameter (p<0.01) compared to ungrazed controls. Our data and observations suggest that sheep may be effectively used as a biological control agent for brush control in coastal Douglas-fir forests.

Sheep as a Biological Control Agent for Tansy Ragwort

Tansy ragwort (Senecio jacobaea) is a biennial weed commonly found on forest and pasture lands in the maritime regions of the Paciflc Northwest. Pyrrolizidine alkaloids in tansy ragwort, when consumed by most types of livestock, produce progressive and irreversible liver damage. Sheep, however, appear immune to these alkaloids. To evaluate the possibility of using sheep to suppress tansy ragwort in cattle pastures, 100 plants were marked and their status followed during 1977 and 1978 in pastures grazed by cattle alone and in pastures grazed by both cattle and sheep. Total tansy ragwort mortality did not differ between pastures. However, the cause of mortality did differ. Mortality on the cattle-grazed pasture was predominately due to completion of the plants biennial life cycle (blooming and seed set), while most plant mortality on the sheep plus cattle pasture appeared to be the result of grazing. The data suggest that sheep may be used as a biological control agent to suppress tansy ragwort populations by reducing their ability to

Defoliation frequency and intensity effects on pasture forage quality.

Both quantity and quality of pasture forage produced generally varies with frequency and intensity of plant defoliation. However, intensity and frequency of defoliation have rarely been evaluated simultaneously. The objective of this study was to quantify forage quality response to simultaneous changes in defoliation treatments over a range of values likely to occur in short-duration grazing systems. Effects of defoliation treatments on forage digestibility (DMD), crude protein content (CPC), crude protein yield, and digestible dry matter yield were evaluated on a perennial ryegrass (Lolium perenne, L.)-subclover (Trifolium subterraneum L.) hill land pasture growing on a Ultic Haploxeroll soil near Corvallis, Oregon. Treatments consisted of all possible combinations of 4 defoliation intervals (clipped every 7, 21, 35, or 49 days) and 3 stubble heights (High-70, Medium-55, or Low-40 mm of stubble remaining after defoliation) applied during the 1980, 1981, and 1982 growing seasons. Digestible dry matter yield increased with increasing defoliation interval. With the exception of DMD in 1980, both digestibility and CPC of the forage produced decreased linearly as the period between defoliation events increased. Crude protein content increased linearly as stubble height increased, while forage digestibility was comparatively insensitive to changes in stubble height. Forage quality was generally adequate on all treatments to meet the needs of most classes of livestock.

Forage Standing Crop and Animal Diets under Rotational vs. Continuous Grazing

Effects of 5-paddock rotational grazing and continuous grazing were monitored in 1977 and 1978 on an annual grass-subclover (Trifolium subterraneum) pasture. More forage was available to livestock under rotational grazing than under continuous grazing during the midspring through late spring period. However, grazing management had little effect upon forage intake by Romney ewes and their lambs during this period. Live weight gains of ewes and lambs were higher under rotational compared with continuous grazing in the spring, perhaps due to an observed increase in subclover, a highly nutritious feed, in diets of sheep grazing rotationally. In contrast to the spring green-feed period, live weight gains of ewes under rotational grazing were lower than those under continuous grazing during the summer dry-feed period. Poor ewe performance on rotationally grazed pasture during the summer period apparently reflects reduced opportunity for dietary selectivity and, therefore, a lower quality diet compared with that available to ewes on continuously grazed pasture.

Tree planting pattern effects on forage production in a Douglas-fir agroforest

Resource sharing among agroforestry system components, as expressed by spatial patterns along interfaces between components, is a crucial factor in both understanding present systems and in designing new agroforestry applications. A study of the spatial pattern of forage production surrounding 9–10 year old Douglas-fir trees in a agrosilvopastoral plantation near Corvallis, Oregon, was conducted during 1988 and 1989. Transects of plots were clipped both between trees (tree/tree) and between trees and open pastures (tree/pasture). Best-fit regression models relating forage production to distance from trees (tree/tree R2 = 0.87; and tree/pasture R2 = 0.89) were combined into a single prediction model. Observed forage production increased rapidly with increasing distance from trees during the initial 4 m. Trees had little effect on forage production beyond 4.5 m (approximately 2 canopy diameters) from the nearest tree. Predictions of different combinations of tree density and planting pattern indicated a strong interaction between density and pattern with highly aggregated plantations better able to maintain forage production at high tree densities.

Plant-soil-water relations in forestry and silvopastoral systems in Oregon

Plant-soil-water relations of a silvopastoral system composed of a Douglas-fir (Pseudotsuga menziesii) timber crop, subterranean clover (Trifolium subterraneum) as a nitrogen-fixing forage, and tall fescue (Festuca arundinacea) as a forage crop were investigated near Corvallis, Oregon, during 1983–1986. Treatments included all possible combinations of two tree-planting patterns (trees planted 2.4 m apart in a grid, and groups of five trees spaced 7.6 m between clusters) and two grazing/understory management systems (agroforests were seeded to subclover and grazed by sheep; forests were unseeded and ungrazed).Mean twig xylem water potential (XWP) for Douglas-fir trees ranged from −0.3 to −1.5 MPa on forest plots and from −0.3 to −1.2 MPa on agroforest sites. Pre-dawn and sunset XWP were more negative for forest than for agroforest plots during dry summer periods. Midday XWP was similar for both agroforest and forest plots on all dates. Soil water content at 50–100 cm depth was greater under agroforest plots as compared to forest plots in 1984, but not in 1985 (unusually dry spring). Average foliage nitrogen content of tree needles was 1.54% vs. 1.43% for agroforests vs. forests, respectively.Our data are consistent with the hypotheses that: (1) grazing of understory vegetation may reduce water stress of trees during dry periods by reducing transpirational water use by the forage plants; and (2) nitrogen-fixing vegetation combined with grazing increases nitrogen uptake of associated trees. However, neither mean foliar N nor average XWP differences experienced by trees in agroforest versus forest plantations were sufficient to have an effect on tree growth. Our data demonstrate that it is possible to produce a second crop (i.e. forage grazed by sheep) in timber plantations without reducing the growth of the main tree crop.

Sheep grazing effects on coastal Douglas fir forest growth : a ten-year perspective

Abstract Interest in using livestock as a biological control agent to suppress unwanted vegetation in conifer plantations has expanded rapidly in the last 10 years. Additional information concerning the silvicultural implications of livestock grazing, particularly the effects of browsing and competition suppression on timber tree growth, are needed if grazing is to be widely adopted as a forest management tool. Tree diameter and height growth were measured during 1981–1990 for ungrazed and grazed tree stands in a coastal Oregon Douglas fir ( Pseudotsuga menziesii ) forest. Grazed stands were intensively used by a herded flock of 700–900 sheep for 3–4 days each May and August in 1981 and 1982. Understory vegetation phytomass and its utilization by sheep was evaluated using a before-and-after technique in 1981 and 1982. Sheep removed 28% and 64% of new tree lateral branches in 1981 and 1982, respectively. The major effect of browsing, however, appeared to be removal of terminal leaders which reduced 1990 Douglas fir tree height by 61 cm and diameter at breast height (dbh) by 1.9 cm for each terminal removed. Sheep browsed terminal leaders of 38% and 77% of grazed-plantation trees in 1981 and 1982, respectively. Grazing proved very effective in reducing red alder ( Alnus rubra ) establishment and growth. Total tree basal area in 1990 was similar for grazed and ungrazed stands. However, alder trees contributed over 45% of the tree basal area present on ungrazed stands compared to only 19% on grazed stands. Vegetation control by sheep, without associated browsing of terminal leaders, increased 1990 Douglas fir height by 16% and dbh by 34%. The net effect of grazing, reflecting the negative impacts of browsing together with the positive effects of reduced competing vegetation, was to increase the 1990 Douglas fir height by 6% and dbh by 22% on grazed compared to ungrazed timber stands.