This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Studies in Avian Biology No. 16:41-45, 1994. INFLUENCE OF SITE QUALITY AND STAND DENSITY ON GOSHAWK HABITAT IN SOUTHWESTERN FORESTS RICHARD L. BASSETT, DOUGLAS A. BOYCE, JR., M. HILDEGARD REISER RUSSELL T. GRAHAM, AND RICHARD T. REYNOLDS Abstract. Current management guidelines for the Northern Goshawk (Accipitergentilis)in the Southwest call for a mosaic habitat consisting of approximately 10 percent of the forest area in grass-forb/ shrubs, 10 percent in 2.5-12.7 cm trees, 20 percent in 12.7-30.5 cm trees, 20 percent in 30.545.7 cm trees, 20 percent in 45.7-6 1.O cm trees, and 20 percent in 6 1.O cm and greater trees. This habitat mosaic was conceived as convenient categories to describe a generally balanced, ecologically sustainable, forest ecosystem. In reality, however, the vegetative structural percentages vary. We describe how differences in site quality and stand density affect vegetative structural stage percentage and forest age. Key Words: Accipitergentilis;forest regulation; Northern goshawk; site quality; stand density, SDI; vegetative structural stages. A goshawk scientific committee developed recommendations for managing the Northern Goshawk (Accipitergentilis) in the southwestern United States (Reynolds et al. 1992). These recommendations focused on developing and maintaining forest conditions to provide habitat for sustaining goshawks and their key prey species. The recommendations defined size, location, stand structure, woody debris, and soil condition requirements for nest, post-fledging family, and foraging areas. Stand structure included such properties as the proportion and distribution of six different diameter classesor vegetative structural stages(VSS), and number of large (>46 cm) trees, snags, and down logs per ha. Reynolds et al. (1992) recommended a mosaic of vegetative structural stages interspersed throughout the post-fledging family and foraging areas in small, less than 1.7 ha patches to form a balanced, ecologically sustainable, uneven-aged forest. Vegetative structural stageis a generalized description of forest structure and age based on the majority of trees in a specific diameter class within the forest (Table 1). The mosaic included patches ranging from grass-forb/shrub to old forests, with a high priority on sustaining as much as 40 percent of the area in mature and old forests. The recommended average proportion was about 10 percent of the goshawk management area (14,820 ha) in grass-forb/shrubs, 10 percent in 2.5-l 2.7 cm (1-5 in.) trees, 20 percent in 12.730.5 cm (5-12 in) trees, 20 percent in 30.545.7 cm (12-l 8 in) trees, 20 percent in 45.7-6 1.O cm (18-24 in) trees, and 20 percent in 61.0 cm (24 in) and greater trees. In this paper, we examine how differences in site quality and stand density influence VSS percentage and forest age. This paper also describes the impacts of varying VSS proportions within goshawk management areas to goshawk habitat and their key prey species. FOREST DEVELOPMENT Reynolds et al.% (1992) recommended VSS and forest mosaic can be approached by traditional, even-aged area regulation. Area regulation consists of dividing the forested post-fledging family and foraging areas into as many I 1.7 ha patches as there are years in the expected life of a forest (forest age) and regenerating an equal percentage of the forest each entry period (Smith 1986). The entry period could vary from 10 to 20 years depending upon existing forest conditions. Three basic principles apply to area regulation. First, structural stagesbecome important to sustain a forest over time, even where the desired condition is to have large, old trees. Second, new trees must be established at regular intervals to sustain the desired structural stages in a forest through time. Third, forests are dynamic. Trees regenerate and grow at different rates and die at different ages, resulting in a forest that is constantly changing over time (Oliver and Larson 1990). Four variables that affect VSS distribution and forest age are: (1) length of stand establishment period, (2) site quality, (3) stand density, and (4) tree longevity. STAND ESTABLISHMENT Length of stand establishment varies by species, regeneration method (natural or planting), amount and kind of forest floor disturbance, and climatic variation. The observed stand establishment period (or years in VSS 1) for ponderosa pine (Pinus ponderma) forests can range from 15 years on a highly productive site to 30 years on 41 42 STUDIES IN AVIAN BIOLOGY NO. 16 TABLE 1. VEGETATNESTRUCWRALSTAGFLS AND pine seedlings and 4 large reserve trees (trees > 46 cm dbh) were simulated over a 320-year period at SDIs of 113 and 157. Four reserve trees Vegewere allowed to die (snag creation) at stand age tative 30, and thereafter thinning from below, to the stnlcDiameters’ tura1 specified SDI, was allowed at 20-year intervals, stage Forestdescription (cm) starting at stand age 40 years. 1 O-2.5 Grass-forb/Shmb Generally, seeding/sapling and young trees Opening have a faster diameter growth rate than mature 2 2.5-12.7 Seedling/Sapling and old trees (Table 2). The time it took for a 12.7-30.5 3 Young tree to move through one VSS ranged from 11 4 30.545.7 Mid-aged to 59 years for 25% maximum SDI, and from 5 Mature 45.7-61.0 11 to 95 years for 35% maximum SD1 depending >61.0+ 6 Old on the site quality and VSS. Also, it takes longer ’ Tree diametermeasuredat I .4 m abovegroundlevel. to grow through a stage on the low productivity sites than on high productivity sites; the excepa poor site. Generally, south- to west-facing, drytion was VSS 3 on average and high sites. er slopes with shallow soils require the longest seedling establishment period, north-facing, more mesic sites with deeper soils require the least STAND DENSITY time. Under similar topographic and soil conStand density influences tree diameter growth. ditions, sites where annual precipitation is usuOn sites with the same site quality, tree diameter ally less require more time than when moisture growth will vary with different management inis more plentiful. tensities. For example, the number of years in VSS 4, 5, and 6 are longer for the higher stand SITE QUALITY density (35% SDI) than for the lower stand denSite quality influences tree growth and varies sity (25% SDI). The growth simulator model greatly in the Southwest. Site quality denotes the showed no differences in number ofyears for VSS relative productivity of a site for a particular tree 2. The seedling establishment period for low (30 species (Ford-Robertson 197 1). Factors that inyears), average (20 years), and high (15 years) fluence site quality include soil characteristics, site quality was assumed to be equal for the two mineral composition, slope, aspect, microclidensities. mate, and tree species (Daniel et al. 1979). Site Stand density and site quality also influence index is useful to help quantify site quality, and VSS percentage and the time required to achieve refers to the average height of dominant and cothe desired forest structure for goshawks and their dominant trees in a stand at an arbitrarily chosen prey species (Tables 3 and 4). One desired forest age. Minor (1964) developed site index curves structure condition is to maintain 40% of the for ponderosa pine in northern Arizona, meagoshawk post-fledging family and foraging areas suring age at 1.4 meters above ground level using in VSS 5 and 6 to sustain moderate to high popa chosen base age of 100 years. For example, a ulations of key prey species. These older age site index of 70 (a tree 21.3 m [70 ft] tall at 100 classesmaintain the most speciesat an abundant years of age at 1.4 m above ground level) is conpopulation level (10 of 12 species found in ponsidered about average for ponderosa pine in the derosa pine forests: i.e., woodpeckers, chipSouthwest. Thus, the length of time required for munks, tassel-eared squirrels) (Reynolds et al. trees in each VSS is a function of site quality and 1992). Since forest stands reach VSS 5 and 6 from stand density (Table 2). 30 to 70 years earlier in stands with lower density A forest growth simulator model (Edminster (25% SDI), the desired forest structure could be et al. 199 1) was used to project diameter growth maintained for a longer period of time. Olderper decade (Table 2) for two key stand densities: aged stands (> 200 years) are also more frequent(1) Stand density index (SDI) 113 (25% of maxly used as goshawk nest sites. Because nest sites imum SD1 450 for ponderosa pine); and (2) 157 have a higher density of large trees, these areas SD1 (35% of maximum SDI). The first level is should be managed for even higher density stands considered to be the onset of competition be(43% SDI). Actual VSS percentage varies from the rectween trees, whereas the second is the lower limit of full site occupancy (Long and Daniel 1990). ommended lo- 1O-20-20-20-20 (Reynolds et al. Stand density index is the number of trees at an 1992). For example, the VSS percentage for a low quality site with 25% SD1 is 12-9- 18- 17-23average stand density of 25.4 cm (10 in) (Daniel et al. 1979, Lilliholm et al., this volume). 2 1 and for a high quality site with 25% SDI, VSS During modeling, the growth of 500 ponderosa is 9-6-22-l 9-22-22 (Table 3). THEIR DWETERS SITE QUALITY AND GOSHAWK et al. HABITAT--Bassett 43 DIA~~ETER GROWTHANDNUMBEROFYEARSIN EACHVEGETATNESTR~~~~RALSTAGE TABLE 2. ESTIMATED (VSS) FORDIFFERENTQUALJTYPONDEROSAPINESITESANDSTAND DENSITIE~INTHESOIJTHWEST Stand density’ Dieter Site quality’ @owth per decade (cm) 25% Max SD1 Approxima;S;ars 35% Max SD1 in each 25% Max SD1 35% Max SD1 30 20 15 30 20 15 1 LOW Average High 2 LOW Average High 4.32 6.86 9.40 4.32 6.86 9.40 24 15 11 24 15 11 3 LOW Average High 3.68 4.83 4.15 3.05 4.32 4.24 48 37 37 58 41 42 4 Low Average High 3.48 4.06 4.83 2.79 3.25 3.63 44 38 32 55 47 42 5 Low Average High 2.59 3.63 3.99 1.60 2.44 2.91 6 LOW Average High 1.52 2.08 2.52 1.27 1.70 2.08 59 42 38 -3 95 63 51 -3 Seedling estabishment period Seedling establishment period ’ Site quality (Minor 1964). Low = 50 SI (SI = site index - dominant tree height at 100 years), Average = 70 SI, High = 90 SI. 2 Stand density index (SDI) is the number of trees of average stand diameter of 25.4 cm. 25% max SD1 = onset of competition; lower limit of full site occupancy. 6 depends on the selected forest age. ’ Years in VSS 35% max SD1 = ponderosa pine is closer to 200 years or less (Pearson 1950, White 1985, Covington and Moore 1991). Life expectancy for Engelmann spruce (Picea engelmannii) ranges from 250-450 years (Alexander and Shepperd 1990). The life expectancy of the typical tree would be more appropriate to set targets for sustaining forests then the age of the oldest tree. Tree species and TREE LONGEVITY Tree longevity influences forest life expectancy and forest age required to achieve desired forest structure. The lifespan of trees varies within and between species. For example, the oldest known living ponderosa pine tree in the Southwest was found to be 742 years old (Swetnam and Brown 1992), whereas the average life expectancy of most DLNVIETER GROWTH,YEARSIN VEGETATIVE STRUCXWRAL TABLE 3. ESTIMATED STAGE,ACCUMULATED AGE, AVERAGE,ANDHIGHQUALITYSITES ANDPER~ENTOFLANDXAPEINEACHVSSFORPONDEROSAPINEONLOW, MAX SDI)’ WHERE STAND DENSITY IS 113 SD1 (25% Vegetative structure stages Site quality vss 1 vss 3 vss 4 vss 5 VSS 6’ Diameter growth/decade Years (Act-years)’ % in VSS 0 30 (30) 12 4.32 24 (54) 9 3.68 48 (102) 18 3.48 44 (146) 17 2.59 59 (205) 23 1.52 55 (260) 21 Average(70 SI) Diameter growth/decade Years (Acc-years)3 % in VSS 0 20 (20) 10 6.86 15 (35) 8 4.83 37 (72) 18 4.06 38 (110) 19 3.63 42 (152) 21 2.08 48 (200) 24 High (90 SI) Diameter growth/decade Years (Act-years)” % in VSS 1: (15) 9 l?$) 6 3;‘7;63) 22 3;.8:95) 19 3;‘;l933) 22 3;$70) 22 Low (50 SI) vss 2 ’ Maximum stand density index for ponderosa pine is 450. 1 Number of years in VSS 6 is determined by selecting a growth period that is approximately 3 Number of years in VSS and accumulated years. 20% of forest age. STUDIES 44 TABLE 4. ESTIMATED DWTER GROWTH, IN AVIAN YEARS IN VEGETATIVE OF LANDSCAPE IN EACH VSS FOR PONDEROSA AND PERCENT WHERE STAND DENSITY IS 157 SD1 (35% MAX SDI)l NO. 16 BIOLOGY PINE STAGE, ACCUMULATED AGE, AVERAGE, AND HIGH QUALITY SITES STRUCIWZAL ON Low, Vegetative structural stages Site quality vss 1 vss 2 vss 3 vss 4 vss 5 vss 6’ Diameter growth/decade Years (Act-year@ % in VSS 30 (30) 9 4.32 24 (54) 7 3.05 58 (112) 18 2.79 55 (167) 17 1.60 95 (262) 29 1.27 65 (327) 20 Average (70 SI) Diameter growth/decade Years (Acc-years)3 % in VSS 0 20(20) 9 6.86 15 (35) 7 4.32 41 (76) 18 3.25 47 (123) 20 2.44 63 (186) 27 1.70 44 (230) 19 High (90 SI) Diameter growth/decade Years (Acc-years)3 % in VSS 0 15(15) 7 9.40 11 (26) 6 4.24 42 (68) 21 3.63 42(110) 21 2.97 51 (161) 25 2.08 39 (200) 20 Low (50 SI) 0 ’ Maximum stand density index for ponderosa pine is 450. 2 Numberof Yean in VSS 6 is determined by selecting a growth period that is approximately 20% of forest age. 3 Number of years in VSS and accumulated years. longevity must be considered when considering an older forest age. By decreasing or increasing forest age, VSS percentage is changed. For example, if the forest age on the low productive site was lowered from 327 to 250 years, the VSS percentage would change from 9-7-18-17-29-20 to 12-10-23-2233-0 (Table 4). VSS 6 may not be achievable if a 327-year forest age is not ecologically sustainable. This should not influence goshawks, given the average life expectancy of ponderosa pine and that sites with low productivity (SI I 50) account for only 2.3% (N = 4 national forests) of ponderosa pine stands in the Southwest (USDA 1993). Planning for forest ageslessthan 200 years, however, could negatively impact goshawks and their prey populations. Regardless of site quality or stand density index (25% or 35%), forests less than 200 years will not provide for the older classes(VSS 5 and 6) (Tables 3 and 4). Suitable goshawk nesting habitat is commonly composed of older trees (>200 years) in the VSS 5 and 6. Prey species like the Red-naped Sapsucker (Sphyrupicus nuchalis) and Williamson’s Sapsucker (Sphyrapicus thyroideus) would lose 66% of the forest structure conditions that maintain high populations (Reynolds et al. 1992). Forests without VSS 5 and 6 would also not provide the large snags that are used by other nesting-cavity prey species. TABLE 5. STRUCTURAL APPROXIMATE PERCENT RV EACH VEGETATIVE SUSTAINING GOSHAWK HABITAT Not all structural stagesare equally important for the goshawk and its prey species,but all structural stages are equally important for a forest to become established and to sustain itself from the grass-forb/shrub stage (VSS 1) with seedlings through the old forest stage (VSS 6). The traditional area even-aged method of regulating a forest can be applied successfully to sustain a forest with the mosaic of VSS that will meet the habitat needs of the goshawk and its key prey species. Of 12 goshawk prey species found in ponderosa pine forests, openings (VSS 1) are of no importance to 5 prey species(i.e., sapsuckers, tasseleared squirrel), and important to 1 prey species (cottontail) for maintaining high populations. For only 1 prey species (tassel-eared squirrel) found in the ponderosa pine forests younger-aged forests (VSS 3) are important, and only when larger, STAGE (VSS) AND FOREST AGE THAT CAN BE EXPECTEDTO OCCUR IN GOSHAWK POST-FLEDGINGFAMILY AND FORAGING AREAS OF AVERAGE SITE QUALM FOR PONDEROSAPINE FOREST TYPE AND MANAGEMENT INTENSITIES(REYNOLDS ET AL. 1992) Percent in each vegetative structural stage Management intensity management* Minimal Moderate Intensive No I Forest age vss 1 10 9 10 10 vss 2 vss 3 10 13 10 8 80 20 19 18 vss 4 0 17 17 17 vss 5 vss 6 0 20 20 21 0 21 24 26 Unthinned ponderosa pine stand at Fort Valley Experimental Forest (Reynolds et al. 1992). An untbinned yield model, never grew beyond VSS 3 with a 200-year forest age (Ronco et al. 1985, Edminster et al. 1991). stand, using GENGYM (ye.-) 200 233 204 194 growth and SITE QUALITY AND GOSHAWK older trees are available for nesting and seed sources (Reynolds et al. 1992). Under varying management options, VSS percentages never attain lo- 1O-20-20-20-20. The youngest forest age (shorter time span) occurs under intensive management; oldest forest age (longest time span) occurs under minimal management (Table 5). Minimal management level is characterized by trees that are significantly competing with one another. When management intensities are at moderate and intensive levels, trees develop without significant competition. Without management, however, unthinned ponderosa pine stands of average site quality are unlikely to grow beyond the young forest structural stage (VSS 3) even after 200 years (Ronco et al. 1985, Edminster et al. 1991). To provide the desired forest conditions of large old trees over 40% of the goshawk management area and small forest openings for prey species and tree regeneration, entry periods for management activities (i.e., harvesting, fire, etc.) would need to be about every 20 years for the moderate level. An expected level of management intensity (stand density) must be determined prior to establishing the desired VSS proportions and forest age. The recommended 10-10-20-20-20-20 VSS percentage is now being considered as a hardand-fast rule by those implementing and reviewing timber sale projects. However, the 10-10-2020-20-20 distribution was intended to describe approximate percentages of each VSS throughout the post-fledging family and foraging areas to sustain suitable goshawk habitat (Reynolds et al. 1992). The achievable VSS percentage should be determined by considering existing local factors that inlluence forest establishment and growth, expected management intensity, and tree longevity. ACKNOWLEDGMENTS We would like to thank the following reviewers:W. M. Block, R. M. Jeffers,M. Johnson,M. J. Larson, R. J. Lilieholm, C. 0. Minor, and M. L. Morrison. LITERATURE CITED ALEKANDER, R. R., AND W. D. SHEPPERD. 1990. Engelmann spruce. Pp. 187-203 in R. M. Bums and B. H. Honkala (tech. coords.), Silvics of North America, vol. 1: Conifers. USDA Forest Service, Agriculture Handbook 654, Washington, DC. COVMGTON, W. W., AND M. M. MOORE. 199 1. Changes in forest conditions and multiresource yields from ponderosa pine forests since European settle- HABITAT--Bassett et al. 45 ment. Water Resources Operations, Final Report, Salt River Project, Phoenix, AZ. DANIEL, T. W., J. A. HELMS, AND F. S. BAKER. 1979. Principles of silviculture. McGraw-Hill, New York, NY. EDMINSTER, C. B., H. T. MOWRER, R. L. 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