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Overstory–understory biomass changes over a 35-year period in southcentral Oregon
Forest Ecology and Management 150 (2001) 267±277
Overstory±understory biomass changes over a 35-year period in southcentral Oregon James M. Peeka,*, Jerome J. Korola,1, Donald Gaya,2, Terry Hersheyb,3 a
College of Natural Resources, University of Idaho, Moscow, ID 83844, USA b Fremont National Forest, Lakeview, OR 97630, USA Received 29 February 2000; accepted 24 July 2000
Abstract Forest canopy development is known to in¯uence understory biomass relationships. An 85,268 ha area in southcentral Oregon was examined for changes in overstory canopy closure using 1953 and 1988 aerial and satellite imagery and a geographic information system. A negative exponential curve predicted a loss of approximately half of understory biomass over the 35-year interval. Reductions in understory biomass were most pronounced at higher elevations where growing conditions for conifers and canopy closure changes were most pronounced. The loss of understory biomass was related to declines in mule deer (Odocoileus hemionus) populations in the area. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Canopy closure; Forage biomass; Mule deer; Odocoileus hemionus; Oregon; Understory±overstory relationships
1. Introduction Major changes in forests in the northwestern US have occurred in this century (Covington et al., 1994; Henjum et al., 1994). These changes are attributed to timber harvest, ®re suppression, urbanization, and conversion to pasture and cropland. Changes attributable to ®re suppression typically involve ingrowth of conifers and replacement of shade-intolerant species
*
Corresponding author. Tel.: 1-208-885-7120; fax: 1-208-885-6226. E-mail address: [email protected] (J.M. Peek). 1 Present address: 605 Indian Hills Drive, Apt 2, Moscow, ID 83843, USA. 2 Present address: Mount Baker Ranger District, 2105 State Route 20, Sedro Woolley, WA 98284, USA. 3 Present address: Cobalt Ranger District, US Forest Service, Highway 93, Salmon, ID 83467, USA.
such as pines (Pinus spp.) with shade-tolerant species such as ®rs (Abies spp.). These changes typically reduce understory productivity as canopies progressively close and shade out understories (Jennings et al., 1999; Long, 1982). A series of relationships between overstory characteristics such as stand density and canopy closure have been correlated with understory biomass (Ffolliott and Clary, 1972). Canopy cover is the proportion of the forest ¯oor covered by the vertical projection of tree crowns (Jennings et al., 1999), and is quanti®ed by Crookston and Stage (1999) for use in stand development models. Canopy cover has been used primarily to de®ne cover for wildlife (Thomas, 1979), and is useful in de®ning forest attributes using remote sensing systems (Law and Waring, 1994; Leckie, 1990). Clary (1969) reported that percent crown cover determined from aerial photographs provided the most precise estimates of understory biomass production
0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 5 8 5 - 5
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when compared with timber basal area and pine needle litter depth. Pase (1958) reported an inverse curvilinear relationship between % ponderosa pine (Pinus ponderosa) crown cover and herbage production in South Dakota forests, which was extended by Reid (1964) to the entire range of this species. Mitchell and Bartling (1991) reported that canopy cover was a signi®cant predictor of understory biomass, best expressed by a nonlinear model such as that used by Jameson (1967). Consequences of increasing forest canopy closure to roe deer (Capreolus capreolus) in England have been documented by Loudon (1987), and Gill et al. (1996), but the magnitude of change and consequences for Rocky Mountain mule deer are not well documented. Mule deer occupy a variety of vegetative communities, including forests where understory biomass of well-developed shrubs and associated herbs is most abundant in early successional stages (Mackie et al., 1982). This situation exists along the western edges of the mule deer range east of the Cascades in the intermountain west. The situation is different from coastal rainforest conditions involving Columbia black-tailed deer (O.h. columbianus) and Sitka deer (O.h. sitkensis), because old-growth coastal forests provide understory biomass that is not available in the intermountain forests (Alaback, 1982; Stage, 1973). These interior forests were originally in¯uenced frequently by wild®re (Agee, 1995; Barrett et al., 1997; Ogle and DuMond, 1997), thus providing diversity in stand age and composition throughout the region. Mule deer have been declining in southcentral Oregon for at least 30 years (Salwasser, 1979). The ®nite rate of decline was determined by Peek et al. (1999) to be at the rate of 0.0921 per year. Causes for the decline were identi®ed by Salwasser (1979) as low-quality diet during late spring and fawning, that in¯uenced fawn survival, implicating quantity and quality of spring to early summer forage conditions. Mule deer population trends are in¯uenced by changes in their habitat at regional scales. Use of remote sensing tools to estimate ecological change over large areas facilitates assessment of population change. The purpose of this investigation is to estimate the magnitude of understory biomass change over a 35-year period in a representative area of southcentral
Oregon using a geographical information system (GIS), coupled with veri®cation by ground observation. We relate the ®ndings to mule deer population changes. 2. Study area An 85,268 ha area encompassing the Sprague River drainage was selected for study because it contained habitat used by deer during all seasons, could be well de®ned, and was representative of the region. Plant associations of the central Oregon pumice zone that includes the Sprague River drainage have been described by Volland (1985). Forest types included lodgepole pine (Pinus contorta), ponderosa pine and mixed conifer communities. A white ®r (Abies concolor) series was prominent on the study area in mixed conifer. A western juniper (Juniperus occidentalis) series occurred on drier sites. Manzanita (Arctostaphylos patula), antelope bitterbrush (Purshia tridentata), big sagebrush (Artemisia tridentata), and snowbrush ceanothus (Ceanothus velutinus) were common associated shrub species. Manzanita and snowbrush ceanothus were found in mixed conifer, ponderosa pine, and lodgepole pine communities, while antelope bitterbrush was found in western juniper, ponderosa pine and lodgepole pine communities. Big sagebrush was found primarily in western juniper and ponderosa pine stands. These species are generally shade-intolerant. Big sagebrush and antelope bitterbrush are less resistant to ®re than manzanita and snowbrush ceanothus. All are palatable to mule deer (Salwasser, 1979). Franklin and Dyrness (1973: p. 180) stress that ®re was originally an important in¯uence on vegetation within the ponderosa pine zone of Oregon and Washington. Fire suppression in the region became effective in the 1920s, as in much of the western US (Agee, 1995). Among expected consequences of efforts to eliminate ®re from the forests were increases in canopy closure which in turn caused reductions in understory species, especially shade-intolerants. These trends may be altered by increases in logging and grazing. Grazing pressures have been altered to a point where greater amounts of ungrazed grasses remain in meadows and riparian sites in this area today than prior to 1960.
J.M. Peek et al. / Forest Ecology and Management 150 (2001) 267±277
3. Methods Forest conditions in the 1953 and 1988 periods on the Sprague River Unit were analyzed using a GIS. The Winema National Forest provided thematic vegetation data for 1953 and 1988, and elevational data. The 1953 data included information on tree species, size/structure class, and canopy closure combined into one thematic layer. This layer was constructed by digitizing timber type maps drawn between 1950 and 1953. Species, size/structure class, and canopy closure data for 1988 were provided as three separate thematic layers. The 1988 data set was developed by Paci®c Meridian Resources, Portland, OR, from a 1987 Landsat Thematic Mapper scene. For both data sets, the minimum mapping unit was approximately 0.0625 ha. United States Geological Survey 7.5 min quadrangles named Applegate Butte, Calimus Butte, Fuego Mountain, S'Ocholis Canyon, Buttes of the Gods, and Cook Mountain were used in the analysis. The 1953 data were available in Map Overlay Statistical System (MOSS) format, and were converted to ARC-INFO coverages. These data were then recoded to create nine usable canopy coverage classes: water, rock and sparse vegetation, grass, developed and/or agricultural areas, shrubs, trees with less than 30% canopy coverage, trees with 31±70% canopy coverage, and trees with greater than 70% canopy coverage. The data were recoded a second time to create 16 classes (water, rock and sparse vegetation, grass, developed and agricultural, shrub, grand ®r (Abies grandis), red and silver ®r (Abies magni®ca, Abies amabilis), Douglas ®r (Pseudotsuga menziesi), ponderosa pine, lodgepole pine, mixed pine, mountain hemlock (Thuja mertensiana), incense cedar (Libocedrus decurrens), Englemann spruce (Picea engelmanni), western juniper, and mixed conifers). Finally, the data were converted to ERDAS 7.5 GIS ®le format for comparison with the 1988 data. The 1988 data were provided as PC-ERDAS 7.5 GIS ®les. The original canopy coverage classes were recoded to the nine classes listed above. Of a possible 128 species classes, 33 occurred in the study area. These were recoded to 16 classes to match the 1953 data. A six 7.5 min quadrangle subset described above was then cut from each of the four data sets. Each of the quadrangles was also cut from the canopy coverage data set for individual analysis. The elevational
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data were resampled into three elevational classes (<1425, 1425±1600, and >1600 m) that approximated mule deer winter, transition, and summer ranges, respectively, based on Salwasser (1979), Volland (1985) and C. Yee, pers. commun., Chiloquin Ranger District, 1994. The PC-ERDAS 7.5 module SUMMARY was used to overlay the various subsets to estimate changes that occurred between 1953 and 1988. The data were tabulated by thematic layer for each quadrangle of the canopy coverage data, and by the six-quadrangle subset of both canopy coverage and species type for each elevational class. An understory biomass-canopy cover regression developed by Jameson (1967) was then used to estimate forage biomass in the three elevational classes. The canopy cover value for each vegetative class in each elevational class was derived from the GIS analysis. Midpoint values for stands exhibiting <30% canopy cover (15%), for stands exhibiting 31±70% canopy cover range (50%) and stands exhibiting >70% canopy cover (85%) were assigned. The proportion of each vegetative type for the 1953 and 1988 data was multiplied by the estimated understory biomass derived from the regression. Understory biomass production was sampled after growth was completed in summer 1996. Canopy closure layer pixels were aggregated to create a layer with a minimum polygon size of 2.02 ha. This layer was intersected with the plant association layer to create vegetative strata. In April and May, three stands in each of seven vegetative strata were sampled from stands at <1425 m elevation. In the period June± September, four to seven stands in each of 16 vegetative strata were sampled at higher elevations. Stands were randomly selected from the GIS vegetative strata layer. The most ef®cient plot size and number of plots per stand were determined during initial vegetation sampling. Stands sampled in early June had 10 plots using multiple plot sizes (1.0 and 3.0 m2 for shrubs, 0.25 and 1.0 m2 for herbs). Current year vegetative growth was clipped, oven dried and weighed by vegetation class (grass, forb, shrub) by plot. Mean biomass production (g/m2) and variance were compared by plot to determine at what point they stabilized. By mid-June, data collections indicated that more than eight plots per stand did not appreciably decrease variance and did
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not change mean production. For grasses and forbs, 0.25 m2 plots had comparable variance to 1.0 m2 plots and mean production was similar for both plot sizes. Similarly, plot sizes of 1.0 and 3.0 m2 yielded similar variance and mean production values for shrubs. Stands were sampled thereafter with eight plots of 0.25 m2 for herbs and 1.0 m2 for shrubs. The biomass estimates were then allocated to the different canopy categories and stand types based on the proportions of each within the study area. 4. Results 4.1. Canopy closure Below 1425 m, sparsely vegetated areas changed very little (Fig. 1). Grasslands comprised 2.9% of the area below 1425 m in 1953 and 6.8% of the area in 1988, suggesting a real change. While 35.5% of this type remained in grassland in 1988, 25.4% changed to moderately-closed canopied forest, and 18.6% to open-canopied forest.
Shrub-dominated communities declined from 18.5% of the area below 1425 m in 1953 to 11.7% in 1988. These communities either shifted towards grassland (19.2%) or towards open-canopied forest (28.7%), while 35.5% remained in shrub-dominated communities. The shrub communities that were switching towards conifer-dominated stands were probably old burns or logged areas where conifers were re-establishing dominance. Open-canopied forests declined dramatically from 63.1% of the area below 1425 m in 1953 to 25.3% of the area in 1988. The major shift was towards moderately-canopied forest (64.3%). While some of the smaller shifts in the composition of 1953 communities may be questioned as being real or the subject of different classi®cation of the same stand, or sampling error, this change is large enough to be considered real. It also ®ts with predictions of how forests would change following prolonged protection from ®re. This complements the observed shift in moderatelycanopied forests from 13.8% in 1953 to 54.5% in 1988. Also, 71% of the moderately-canopied forest remained in that category in 1988, which, for the drier
Fig. 1. Changes in overstory canopy cover for the Sprague Unit, southern Oregon. T < 30% tree canopy closure less than 30%.
J.M. Peek et al. / Forest Ecology and Management 150 (2001) 267±277
forests is expected. Densely-canopied forests comprised less than 1% of the area below 1425 m that was sampled. Between 1425 and 1600 m elevation, sparse communities, grasslands, shrublands, and dense-canopied forests did not change appreciably by 1988 (Fig. 1). The major changes were from open-canopied forests in 1953 to moderately-canopied forests in 1988. The open-canopied forests comprised 64.9% of the area in 1953, and declined to 17.4% in 1988, with 84.2% of the change being a shift towards moderately-canopied forests. Moderately-canopied forests increased from 25.6% of the area in 1953 to 72.9% in 1988. Above 1600 m elevation, sparsely vegetated areas, grasslands, shrublands and densely-canopied forests all comprised under 1% of the area in 1953. The nonforested sites did not change appreciably by 1988, while a small change from 0.4 to 3.7% in close-canopied forests was evident. The dramatic shift was from open-canopied forest, 86.3% in 1953, to moderately-canopied forest, 85.9% in 1988. For all elevations combined, the most pronounced change from 1953 to 1988 was from open-canopied forest to moderately-canopied forest (Fig. 1). In 1953, 67.8% of the entire area was classi®ed as opencanopied, while in 1988, 69.8% of the area was classi®ed as moderately-canopied. The most pronounced change was at the highest elevation, where moisture and temperature gradients would most favor conifer growth, followed by the mid-elevation band, with the areas below 1425 m showing the least change. Species composition did not show the major changes that the canopy closure did (Table 1). Elevations below 1425 m were dominated by ponderosa
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pine in 1953 and 1988 with lesser amounts in the latter period. The major change was in the mixed conifer category, which changed from <1% in 1953 to 30.4% in 1988. The major shift to mixed conifer was from shrub, ponderosa pine and western juniper communities. The shift from western juniper to mixed conifer may be partially attributable to differences in stand classi®cations in 1953 and 1988. Sparsely vegetated areas increased about 4%, probably associated with the presence of recently logged areas or a result of the ®ner-scale mapping resolution of the 1988 information as compared to 1953. Sparsely vegetated areas shifted either to shrub, ponderosa pine or mixed tree communities. Shrubs shifted to sparsely vegetated areas or mixed conifer communities, again probably attributable to the resolution level of the mapping. Ponderosa pine stands either remained in that category or shifted to mixed conifer communities. Western juniper shifted to mixed conifer, ponderosa pine, or shrub communities. Mixed conifer stands in 1953 shifted primarily to ponderosa pine communities, which may be attributable to harvest and planting to ponderosa pine. The mid-elevation band of communities was dominated by ponderosa pine in both 1953 and 1988 (Table 1). Seventy-six percent of the area was in ponderosa pine in 1953, while 47.8% was in that type in 1988. Lodgepole pine stands were next in importance in 1953 and 1988. Mixed conifer increased from 2.4% in 1953 to 26.1% in 1988. Elevations of more than 1600 m showed trends similar to those of the mid-elevation band. The predominant community was ponderosa pine, in both 1953 and 1988, with hardly any change. White ®r,
Table 1 Changes in species composition for the Sprague Unit, southern Oregon Elevation 1425 m
Sparse Shrub Firs Ponderosa pine Lodgepole pine Juniper Mixed conifer
1425±1600 m
>1600 m
All elevations
1953
1988
1953
1988
1953
1988
1953
1988
0.038 0.156 0 0.623 0.154 0.028 0.002
0.08 0.117 0 0.356 0.136 0.006 0.304
0.013 0.055 0.002 0.76 0.125 0.022 0.024
0.032 0.055 0.002 0.478 0.169 0.002 0.261
0.004 0 0.045 0.567 0.043 0.337 0
0.014 0.017 0.031 0.548 0.101 0.207 0.074
0.018 0.074 0.008 0.6904 0.12 0.02 0.069
0.043 0.067 0.007 0.455 0.149 0 0.276
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mixed conifer stands would be expected to increase at these elevations, but they did not. Lodgepole pine stands increased from 4.4% in 1953 to 10.1% in 1988. The overall shifts for all elevations show a decline in ponderosa pine communities, small increases in sparsely vegetated sites, shrubs, white ®r, and juniper, and a fourfold increase in mixed conifer stands. Trends are consistent with predicted effects of ®re suppression policies, and what we observed directly, but sampling errors including changes in stand classi®cation undoubtedly contribute to some of the apparent change. Ponderosa pine communities that would have been maintained under a ®re regime would be expected to change to mixed stands as succession progressed, except on sites where this species was climax. Shifts in shrub communities towards mixed timber represent succession to juniper and ponderosa pine on dry sites and to mixed conifer on the more mesic sites where these communities originated from ®re and logging. The successional trends suggest that stands created by logging have balanced those stands created earlier by ®re that were succeeding towards forest. An evaluation of the comparability of GIS information from the Sprague Unit information can be made using a larger block from the Winema National Forest. A 341,500 ha block within the forest was compared with the Sprague Unit block. The shrub layer in the 1953 data includes sparse timber. Similar trends in forest canopy closure exist in both areas (Table 2). Forests with 26±70% canopy closure shifted from less than 30% of total land area in 1953 to over 50% in 1988. If the shrub layer and lowest tree canopy layer are combined for both units, then very similar trends
occur in both. This suggested that results from the Sprague Unit were applicable to the general area. 4.2. Understory biomass production 4.2.1. Estimates using the negative exponential curve There was less biomass at all elevations in 1988 than in 1953 (Table 3). The most pronounced reduction in understory biomass occurred at elevations greater than 1600 m. Growing conditions would be most favorable for conifers at the higher elevations, so this is entirely reasonable. The estimate of 7.3 g/m2 in 1988 is 34.4% of the estimate of 21.3 g/m2 in 1953 at elevations greater than 1600 m. The greatest shift in production was at the <30% canopy category, where 15.2 g/m2 was estimated in 1953 and only 4.0 g/m2 was estimated to be present in 1988. The 1425±1600 m elevation band biomass estimate of 9.9 g/m2 in 1988 is 44.1% of the 1953 estimate of 22.6 g/m2, the magnitude of reduction being similar to the higher band. However, understory biomass in 1988 at this elevation band was similar to that estimated at the higher elevation band. Canopy closure at the <1425 m band changed appreciably as well but enough area remained in nonforest communities and in the most open forest canopy category to cause a reduction to 76.2% of the 1953 estimate by 1988. The 1988 estimate at this elevation band was 2.4 times higher than at the 1425±1600 m band. The overall estimate for all elevations suggested that, by 1988, understory biomass production was 65.5% of the 1953 estimate. The greatest change came at the most open forest canopy level, which was
Table 2 Comparison of canopy closure from a 341,500 ha block of the Winema National Forest with the Sprague Unit information for the period 1953±1988 Stand type
Canopy cover (%)
a
Miscellaneous Shrubb Tree Tree Tree a
11±25 26±70 71±100
Winema Block 1953 (%)
1988 (%)
1953 (%)
1988 (%)
2.3 65.97 0.75 29 0
17.5 7.1 15.18 57.45 2.77
1.88 9.41 67.82 20.21 0.68
4.23 6.64 18.04 69.83 1.26
Includes water, rock, and sparse vegetation. 1953 data in Winema Block for shrubs includes ``sparse timber''.
b
Sprague Unit
J.M. Peek et al. / Forest Ecology and Management 150 (2001) 267±277 Table 3 Understory biomass predictions, Sprague Unit, southern Oregon Elevation
Predicted biomass (g/m2) 1953
1988
<1425 m No canopya 15% canopy 50% canopy 85% canopy Total
16.96 14.16 0.08 0.04 31.24
15.05 5.70 3.04 0.01 23.80
1425±1600 m No canopy 15% canopy 50% canopy 85% canopy Total
6.81 14.57 1.42 0.03 22.61
2.03 3.91 4.05 0.04 9.98
>1600 m No canopy 15% canopy 50% canopy 85% canopy Total
0.31 19.37 1.61 0.02 21.31
2.37 1.57 4.78 0.17 7.32
All elevations No canopy 15% canopy 50% canopy 85% canopy Total
8.63 15.22 1.12 0.03 25.00
8.4 4.04 3.88 0.06 16.38
a No canopy includes sparsely vegetated nonforest communities, grasslands which include meadows and some riparian zones, shrub communities including sagebrush and bitterbrush-dominated rangelands as well as burned forest in seral shrub stage. The forests with a canopy closure of <30% were truncated to 15% canopy, 31±70% canopy to 50% canopy and >70% canopy to 85% canopy for this analysis. The Jameson (1967) prediction for ponderosa pine canopy, which should make the forage estimate conservative for higher elevations and all elevations, is used. The midpoint in canopy coverage for each category of canopy closure is assumed.
producing 15.2 g/m2 in 1953 and 4.0 g/m2 in 1988. The least change came in the nonforested communities, as expected. 4.2.2. Biomass estimates using ®eld data Individual stand biomass estimates were within the range of the predictions using the Jameson (1967) curve. Estimates for 0±25% canopy forests of ponderosa pine were 2.3±16.2 g/m2 (mean 9.3 g/m2), 25±55% canopy cover 0±9.2 g/m2 (mean 2.9 g/m2), >55% canopy cover 0±8.0 g/m2 (mean 1.1 g/m2). For
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big sagebrush stands clipped in summer, production estimates were 1.2±15.1 g/m2 (mean 8.2 g/m2). Dry meadow production estimates were 4.7±20.2 g/m2 (mean 12.5 g/m2). However, extrapolations to the study area produce estimates that averaged 22.3% of the predictions using the negative exponential curve (Fig. 2). For elevations of more than 1400 m, total production for 1953 was estimated at 7.07 and 31.96 g/m2 with the curve, and for 1988, 5.91 and 23.83 g/m2. The proportional decline in production between 1953 and 1988 using ®eld data was 16.4% as compared to 25.5% with the curve. For elevations between 1400 and 1600 m, the proportional decline was 52.0% using the ®eld calculations and 51.8% using the curve. At the highest elevation range, the proportional decline was 53.3% using ®eld data and 56.6% using the curve. For all elevations, the proportional decline was 49.6% using ®eld data and 34.5% using the curve. The differences in proportional decline using the two methods increased as elevations increased. 5. Discussion The data set represents a series of transitions from the 1953 to the 1988 period, but since the GIS analysis consists of a total enumeration of parameters within the six-quadrangle area, no transition probabilities were calculated. If ecological conditions warranted, a projection of potential succession could be investigated using a Markov analysis (Facelli and Pickett, 1990; Waggoner and Stephens, 1970). However, our judgment is that the conditions and trends portrayed in the 1953±1988 period were unlikely to continue. Also, stands were probably at or near maximum canopy closure at the 60% level. First, logging, pathogens, and ®re are likely to perturb the forests suf®ciently over the next 25 years to make any projections unreliable. The Lone Pine ®re of 1992 burned 27,000 acres of National Forest land and 3000 acres of private lands within the Sprague Unit analysis area. Our analysis was con®ned to a portrayal of change over the 1953±1988 period and estimation of the effects of those changes on understory biomass. Jameson (1967) considered the nonlinear model as a general model for overstory±understory relationships, subsequently con®rmed by Mitchell and Bartling
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Fig. 2. Production estimates by canopy and cover type using ®eld data from Gay (1998). JUOC Ð western juniper, PIPO Ð ponderosa pine, PICO Ð lodgepole pine, and mixed Ð mixed conifers.
(1991). However, Arnold (1953) reported that logging in ponderosa pine forests produced increases in annual grasses and forbs and perennial forbs, but slight decreases in perennial grasses 5 years after logging. Arnold (1953) also noted that logging roads, skid trails, and slash reduced understory growth. Thus, estimates of understory biomass change apply to areas where roads, trails and slash were not present. In the mixed conifer forest of northeastern Oregon, Young et al. (1967) reported a hump-shaped curve that correlated percent shrub cover and overstory crown cover following logging, while herbaceous plants showed declines from open to moderate to heavy shade. Since the understories of mixed conifer forests on the Sprague area consisted more of shade-intolerant species, their cover was expected to follow the negative curvilinear relationship. A stand density index correlation developed by Moore and Deiter (1992) produced a curve similar to that of Jameson (1967). However, Ratliff et al. (1991) demonstrated that, in California oak-woodland, different overstory species affect herbage understory, and Scanlon (1992) produced different curves related to distance from eucalyptus and mesquite trees, suggesting that the curves may vary by type of plant community. Use of the Jameson (1967) curve to project understory biomass change over the 35-year time span assumes that slash has decomposed suf®ciently to
not be a major effect on ground cover, that roads and logging trails are not signi®cantly altering the relationships, and that shrub cover follows the predicted pattern. Untreated slash and soil compaction from roads and skid trails would also suppress understory production in logging areas. Thus, if the assumptions are not correct, the net effect is to reduce understory production more than predicted, with the exception that shrub cover may be underestimated in the mid-canopy ranges and overestimated in the lowcanopy range. If the negative exponential curve that portrays biomass relative to canopy coverage is widely applicable, then understory biomass production should decrease exponentially as a stand develops and canopy cover increases. An indirect evaluation of the ef®cacy of the Jameson curve can be made using the COVER extension of the stand prognosis model. The prognosis model (Stage, 1973; Wykoff et al., 1982), allows use of current stand conditions to predict future stand growth. The COVER extension predicts development of tree crowns and understory vegetation (Moeur, 1985). Crookston and Stage (1999) provide an estimate of canopy cover that accounts for crown overlap which is de®ned as the percentage of the ground area that is directly covered with tree crowns. The prognosis model variant for southcentral Oregon and northeastern California was used to describe a range
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of stand ages and corresponding changes in canopy coverage, using a ``default'' stand, wherein 750 trees per hectare were speci®ed as being initially established, and 100% survival occurred. Two ponderosa pine stands, with site indices of 50 and 90, and three white ®r stands with site indices of 50, 80, and 110 were used. The canopy closure in each model stand was examined at 10-year intervals out to 150 years. The projected canopy cover on stand age essentially is the mirror image of the negative exponential curve. All stands reach maximum canopy closure at 50±70 years after establishment. The white ®r stands reach higher canopy closures than the ponderosa pine stands as expected (Law and Waring, 1994). These stands represent a wide range of growing conditions and show essentially the same curves. The analysis tends to support use of the negative exponential curve across a variety of stand types. Gilmore and Seymour (1996) reported similar relationships in balsam ®r (Abies balsamea) leaf area and growth ef®ciency. Roberts et al. (1993) considered both tree-level and stand-level dynamics to be involved. Redistribution of nutrients and light from understories in open-canopy stands of shade intolerant conifers to overstory leaf area increases as tree canopies increased and incident light and moisture to understories declined would cause a decline in forest understories, especially where shade-intolerant species are involved. The predicted understory biomass using the negative exponential curve falls within the range of individual stand estimates obtained in the ®eld, but are higher than the ®eld estimates for the entire study area. Four of ®ve comparisons show the predicted biomass being higher than the mean biomass estimate based on ®eld data. The 1996 water year (October±September) for Lakeview was 51.8 cm as compared to the longterm average of 37.4 cm, indicating the ®eld data were taken during an above-average growing season. We conclude that the predicted biomass was higher than what actually existed, that substantial variation between stands and sites and ¯uctuations in growing conditions make broad-scale assessments of vegetation biomass change dif®cult. Much of this area was probably dominated by oldgrowth ponderosa pine before logging activities began (Henjum et al., 1994). Logging was common in the 1920s, but ponderosa pine probably dominated the
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area as old-growth forest as late as 1936 (Wall, 1972). By the early 1940s, extensive cutting from Bend, OR, south had created large areas of pine second growth (Cowlin et al., 1942). The proportion of forest dominated by ponderosa pine had considerably declined by the time of the 1953 surveys used in this report. A combination of high-grading old-growth pine, ®re suppression, and encroachment of mixed forest was in progress in 1953, continuing through 1988, and is a common forest trend throughout the west (Hungerford et al., 1991; Mutch, 1993; Covington et al., 1994). The original old-growth forest likely contained substantial understory vegetal development, if it was frequented by ground ®res that maintained the savanna-like nature of the forest. It is likely under a frequent ®re regime that understories were dominated by bunchgrass until some time in the late 1800s, when overgrazing by livestock may have caused shrubs to proliferate. These open forests with bunchgrass understories may not have provided high-quality habitat for mule deer, but the successional changes following heavy grazing would have created a more diverse forage supply for this species. Keay and Peek (1980) reported that mule deer made heavy use of the more open, recently burned stands dominated by ponderosa pine in the upper Selway River of Idaho, and this probably applied to the southcentral Oregon region as well. However, Salwasser (1979) concluded that deer were not abundant prior to 1850 in this region, and remained at low numbers through the early 1900s. Mule deer began to increase around 1915, probably because of increased shrublands attributable to prior heavy grazing, coupled with subsequent reductions in domestic livestock, and reductions in predation (Salwasser, 1979), which may have been a typical pattern across the west (Peek and Krausman, 1995). Shrublands have since continued to mature and senesce as well across the western ranges (Urness, 1990), and such is undoubtedly the case in this area, the result being a progressive decline in mule deer populations over the past 30 years. Acknowledgements This work was supported by a consortium of agencies and organizations interested in southcentral Oregon mule deer, including the Fremont National Forest,
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Winema National Forest, National Council for Air and Stream Improvement, Bureau of Land Management, Mule Deer Foundation, Oregon Department of Fish and Wildlife, Oregon Hunters Association, and the University of Idaho, College of Natural Resources. We thank Andy Peavy for providing the GIS coverage of the Winema National Forest. Ron Anglin, Don Leckenby, Ralph Opp, and Chris Yee provided information and answers to our questions throughout the study. Suggestions of anonymous reviewers improved the manuscript. This is University of Idaho, College of Natural Resources Experiment Station publication 916. References Agee, J.K., 1995. Fire Ecology of Paci®c Northwest Forests. Island Press, Washington, DC, 493 pp. Alaback, P.B., 1982. Dynamics of understory biomass in Sitka spruce-western hemlock forests of southeast Alaska. Ecology 63, 1932±1948. Arnold, J.F., 1953. Effect of heavy selection logging on the herbaceous vegetation in a ponderosa pine forest in northern Arizona. J. For. 51, 101±105. Barrett, S.W., Arno, S.F., Menakis, J.P., 1997. Fire episodes in the inland northwest (1540±1940) based on ®re history data. U.S. Forest Service General Technical Report INT-GTR-370. 17 pp. Clary, W.P., 1969. Increasing sampling precision for some herbage variables through knowledge of the timber overstory. J. Range Manage. 22, 200±201. Covington, W.W., Everett, R.L., Steele, R., Irwin, L.L., Daer, T.A., Auclair, A.N.D., 1994. Historical and anticipated changes in forest ecosystems of the Inland West. J. Sust. For. 4, 13±63. Cowlin, R.W., Briegleb, P.A., Moravets, F.L., 1942. Forest resources of the ponderosa pine region of Oregon and Washington. US Forest Service Miscellaneous Publication 490, 99 pp. Crookston, N.L., Stage, A.R., 1999. Percent canopy cover and stand structure statistics from the Forest Vegetation Simulator. US Forest Service General Technical Report RMRS-GTR-24. Fort Collins, CO. Facelli, J.M., Pickett, S.T.A., 1990. Markovian chains and the role of history in succession. Trends Ecol. Evol. 5, 27±31. Ffolliott, P.F., Clary, W.P., 1972. A selected and annotated bibliography of understory±overstory vegetation relationships. University of Arizona Agricultural Experiment Station Technical Bulletin 198, 33 pp. Franklin, J.F., Dyrness, C.T., 1973. Natural vegetation of Oregon and Washington. US Forest Service General Technical Report PNW-8, 417 pp. Gay, D., 1998. A test of the southcentral Oregon mule deer habitat suitability index model. University of Idaho Master of Science Thesis, Moscow, ID.
Gill, R.M.A., Johnson, A.L., Francis, A., Hiscocks, K., Peace, A.J., 1996. Changes in roe deer (Capreolus capreolus L.) population density in response to forest habitat succession. For. Ecol. Manage. 88, 31±41. Gilmore, D.W., Seymour, R.S., 1996. Alternative measures of stem growth ef®ciency applied to Abies balsamea from four canopy positions in central Maine. U. S. A. For. Ecol. Manage. 84, 209±218. Henjum, M.G., Karr, J.R., Bottom, D.L., Perry, D.A., Bednarz, J.C., Wright, S.G., Beckwitt, S.A., Beckwitt, E., 1994. Interim Protection for Late-Succession Forests, Fisheries, and Watersheds: National Forests East of the Cascade Crest, Oregon and Washington. The Wildlife Society, Bethesda, MD, 245 pp. Hungerford, R.D., Harrington, M.G., Frandsen, W.H., Ryan, K.C., Niehoff, G.J., 1991. In¯uence of ®re on factors that affect site productivity. US Forest Service General Technical Report INT280, pp. 32±50. Jameson, D.A., 1967. The relationship of tree overstory and herbaceous understory vegetation. J. Range Manage. 20, 247±249. Jennings, S.B., Brown, N.D., Sheil, D., 1999. Assessing forest canopies and understory illumination: canopy closure, canopy cover and other measures. Forestry 72, 59±73. Keay, J.A., Peek, J.M., 1980. Relationships between ®res and winter habitat of deer in Idaho. J. Wildlife Manage. 44, 372±380. Law, B.E., Waring, R.H., 1994. Combining remote sensing and climatic data to estimate net primary production across Oregon. Ecol. Appl. 4, 717±728. Leckie, D.G., 1990. Advances in remote sensing technologies for forest surveys and management. Can. J. For. Res. 20, 464±483. Long, J.N., 1982. Productivity of western coniferous forests. In: Edmond, R.L. (Ed.), Analysis of Coniferous Forest Ecosystems in the Western U.S. HS/IBP Synthesis Series, Vol. 14. Hutchinson Ross, Stroudsburg, PA. Loudon, A.S.I., 1987. The in¯uence of forest habitat structure on growth, body size and reproduction in roe deer (Capreolus capreolus, L.). In: Wemmer, C.M. (Ed.), Biology and Management of the Cervidae. National Zoological Park, Front Royal, VA, pp. 559±567. Mitchell, J.E., Bartling, P.N.S., 1991. Comparison of linear and nonlinear overstory±understory models for Ponderosa pine. For. Ecol. Manage. 42, 195±204. Moeur, M., 1985. COVER: a user's guide to the CANOPY and SHRUBS extension of the Stand Prognosis Model. US Forest Service General Technical Report INT-190, 49 pp. Moore, M.M., Deiter, D.A., 1992. Stand density index as a predictor of forage production in northern Arizona pine forests. J. Range Manage. 45, 267±271. Mutch, R.W., 1993. Unnatural stand density and species composition: affecting the resilience of ponderosa pine in a ®re environment. In: Proceedings of Forest Health in the Inland West. Department of Forest Resources, University of Idaho, Moscow, ID, pp. 39±40. Ogle, K., DuMond, V., 1997. Historical Vegetation on National Forest Lands in the Intermountain Region. USDA Forest
J.M. Peek et al. / Forest Ecology and Management 150 (2001) 267±277 Service, Rocky Mountain Research Station, Ogden, UT, 129 pp. Pase, C.P., 1958. Herbage production and composition under immature ponderosa pine stands in the Black Hills. J. Range Manage. 11, 238±243. Peek, J.M., Krausman, P.R., 1995. Grazing and mule deer. In: Krausman, P.R. (Ed.), Rangeland Wildlife. Society for Range Management, Denver, CO, pp. 183±192. Peek, J.M., Korol, J.A., Dennis, B.C., 1999. A review and analysis of habitat relationships and mule deer populations in southcentral Oregon. Unpublished report on ®le, Fremont National Forest, Lakeview, Oregon. 52 pp. Ratliff, R.D., Duncan, D.A., Westfall, S.E., 1991. California oak-woodland overstory species affect herbage understory: management implications. J. Range Manage. 44, 306±310. Reid, E.H., 1964. Forage production in ponderosa pine forests. In: Proceedings of Society of American Foresters, Denver, CO, pp. 61±64. Roberts, S.D., Long, J.N., Smith, F.W., 1993. Canopy strati®cation and leaf area ef®ciency: a conceptualization. For. Ecol. Manage. 60, 143±156. Salwasser, H., 1979. The ecology and management of the Devil's Garden Insterstate Deer Herd and its range. Ph.D. Dissertation, University of California (Berkeley).
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Scanlon, J.C., 1992. A model of woody-herbaceous biomass relationships in eucalypt and mesquite communities. J. Range Manage. 43, 75±80. Stage, A.R., 1973. Prognosis model for stand development. US Forest Service Research Paper INT-137, 32 pp. Thomas, J.W. (Ed.), 1979. Wildlife Habitats in Managed Forest: The Blue Mountains of Oregon and Washington. US Forest Service Agricultural Handbook, Vol. 553. 512 pp. Urness, P.J., 1990. Livestock as manipulators of mule deer winter habitats in northern Utah. US Forest Service General Technical Report RM-194, pp. 25±40. Volland, L.A., 1985. Plant associations of the central Oregon pumice zone. US Forest Service Paci®c Northwest Region R6Ecol-104-1985, Portland, OR, 138 pp. Waggoner, P.E., Stephens, G.R., 1970. Transition probabilities for a forest. Nature 225, 1160±1161. Wall, B., 1972. Log production in Washington and Oregon: a historical perspective. US Forest Service PNW-42. Wykoff, W.R., Crookston, N.L., Stage, A.R., 1982. User's guide to the Stand Prognosis Model. US Forest Service General Technical Report INT-133, 112 pp. Young, J.A., Hedrick, D.W., Keniston, R.F., 1967. Forest cover and logging Ð herbage and browse production in the mixed coniferous forest of northeastern Oregon. J. For. 65, 807±813.
Overstory±understory biomass changes over a 35-year period in southcentral Oregon James M. Peeka,*, Jerome J. Korola,1, Donald Gaya,2, Terry Hersheyb,3 a
College of Natural Resources, University of Idaho, Moscow, ID 83844, USA b Fremont National Forest, Lakeview, OR 97630, USA Received 29 February 2000; accepted 24 July 2000
Abstract Forest canopy development is known to in¯uence understory biomass relationships. An 85,268 ha area in southcentral Oregon was examined for changes in overstory canopy closure using 1953 and 1988 aerial and satellite imagery and a geographic information system. A negative exponential curve predicted a loss of approximately half of understory biomass over the 35-year interval. Reductions in understory biomass were most pronounced at higher elevations where growing conditions for conifers and canopy closure changes were most pronounced. The loss of understory biomass was related to declines in mule deer (Odocoileus hemionus) populations in the area. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Canopy closure; Forage biomass; Mule deer; Odocoileus hemionus; Oregon; Understory±overstory relationships
1. Introduction Major changes in forests in the northwestern US have occurred in this century (Covington et al., 1994; Henjum et al., 1994). These changes are attributed to timber harvest, ®re suppression, urbanization, and conversion to pasture and cropland. Changes attributable to ®re suppression typically involve ingrowth of conifers and replacement of shade-intolerant species
*
Corresponding author. Tel.: 1-208-885-7120; fax: 1-208-885-6226. E-mail address: [email protected] (J.M. Peek). 1 Present address: 605 Indian Hills Drive, Apt 2, Moscow, ID 83843, USA. 2 Present address: Mount Baker Ranger District, 2105 State Route 20, Sedro Woolley, WA 98284, USA. 3 Present address: Cobalt Ranger District, US Forest Service, Highway 93, Salmon, ID 83467, USA.
such as pines (Pinus spp.) with shade-tolerant species such as ®rs (Abies spp.). These changes typically reduce understory productivity as canopies progressively close and shade out understories (Jennings et al., 1999; Long, 1982). A series of relationships between overstory characteristics such as stand density and canopy closure have been correlated with understory biomass (Ffolliott and Clary, 1972). Canopy cover is the proportion of the forest ¯oor covered by the vertical projection of tree crowns (Jennings et al., 1999), and is quanti®ed by Crookston and Stage (1999) for use in stand development models. Canopy cover has been used primarily to de®ne cover for wildlife (Thomas, 1979), and is useful in de®ning forest attributes using remote sensing systems (Law and Waring, 1994; Leckie, 1990). Clary (1969) reported that percent crown cover determined from aerial photographs provided the most precise estimates of understory biomass production
0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 5 8 5 - 5
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when compared with timber basal area and pine needle litter depth. Pase (1958) reported an inverse curvilinear relationship between % ponderosa pine (Pinus ponderosa) crown cover and herbage production in South Dakota forests, which was extended by Reid (1964) to the entire range of this species. Mitchell and Bartling (1991) reported that canopy cover was a signi®cant predictor of understory biomass, best expressed by a nonlinear model such as that used by Jameson (1967). Consequences of increasing forest canopy closure to roe deer (Capreolus capreolus) in England have been documented by Loudon (1987), and Gill et al. (1996), but the magnitude of change and consequences for Rocky Mountain mule deer are not well documented. Mule deer occupy a variety of vegetative communities, including forests where understory biomass of well-developed shrubs and associated herbs is most abundant in early successional stages (Mackie et al., 1982). This situation exists along the western edges of the mule deer range east of the Cascades in the intermountain west. The situation is different from coastal rainforest conditions involving Columbia black-tailed deer (O.h. columbianus) and Sitka deer (O.h. sitkensis), because old-growth coastal forests provide understory biomass that is not available in the intermountain forests (Alaback, 1982; Stage, 1973). These interior forests were originally in¯uenced frequently by wild®re (Agee, 1995; Barrett et al., 1997; Ogle and DuMond, 1997), thus providing diversity in stand age and composition throughout the region. Mule deer have been declining in southcentral Oregon for at least 30 years (Salwasser, 1979). The ®nite rate of decline was determined by Peek et al. (1999) to be at the rate of 0.0921 per year. Causes for the decline were identi®ed by Salwasser (1979) as low-quality diet during late spring and fawning, that in¯uenced fawn survival, implicating quantity and quality of spring to early summer forage conditions. Mule deer population trends are in¯uenced by changes in their habitat at regional scales. Use of remote sensing tools to estimate ecological change over large areas facilitates assessment of population change. The purpose of this investigation is to estimate the magnitude of understory biomass change over a 35-year period in a representative area of southcentral
Oregon using a geographical information system (GIS), coupled with veri®cation by ground observation. We relate the ®ndings to mule deer population changes. 2. Study area An 85,268 ha area encompassing the Sprague River drainage was selected for study because it contained habitat used by deer during all seasons, could be well de®ned, and was representative of the region. Plant associations of the central Oregon pumice zone that includes the Sprague River drainage have been described by Volland (1985). Forest types included lodgepole pine (Pinus contorta), ponderosa pine and mixed conifer communities. A white ®r (Abies concolor) series was prominent on the study area in mixed conifer. A western juniper (Juniperus occidentalis) series occurred on drier sites. Manzanita (Arctostaphylos patula), antelope bitterbrush (Purshia tridentata), big sagebrush (Artemisia tridentata), and snowbrush ceanothus (Ceanothus velutinus) were common associated shrub species. Manzanita and snowbrush ceanothus were found in mixed conifer, ponderosa pine, and lodgepole pine communities, while antelope bitterbrush was found in western juniper, ponderosa pine and lodgepole pine communities. Big sagebrush was found primarily in western juniper and ponderosa pine stands. These species are generally shade-intolerant. Big sagebrush and antelope bitterbrush are less resistant to ®re than manzanita and snowbrush ceanothus. All are palatable to mule deer (Salwasser, 1979). Franklin and Dyrness (1973: p. 180) stress that ®re was originally an important in¯uence on vegetation within the ponderosa pine zone of Oregon and Washington. Fire suppression in the region became effective in the 1920s, as in much of the western US (Agee, 1995). Among expected consequences of efforts to eliminate ®re from the forests were increases in canopy closure which in turn caused reductions in understory species, especially shade-intolerants. These trends may be altered by increases in logging and grazing. Grazing pressures have been altered to a point where greater amounts of ungrazed grasses remain in meadows and riparian sites in this area today than prior to 1960.
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3. Methods Forest conditions in the 1953 and 1988 periods on the Sprague River Unit were analyzed using a GIS. The Winema National Forest provided thematic vegetation data for 1953 and 1988, and elevational data. The 1953 data included information on tree species, size/structure class, and canopy closure combined into one thematic layer. This layer was constructed by digitizing timber type maps drawn between 1950 and 1953. Species, size/structure class, and canopy closure data for 1988 were provided as three separate thematic layers. The 1988 data set was developed by Paci®c Meridian Resources, Portland, OR, from a 1987 Landsat Thematic Mapper scene. For both data sets, the minimum mapping unit was approximately 0.0625 ha. United States Geological Survey 7.5 min quadrangles named Applegate Butte, Calimus Butte, Fuego Mountain, S'Ocholis Canyon, Buttes of the Gods, and Cook Mountain were used in the analysis. The 1953 data were available in Map Overlay Statistical System (MOSS) format, and were converted to ARC-INFO coverages. These data were then recoded to create nine usable canopy coverage classes: water, rock and sparse vegetation, grass, developed and/or agricultural areas, shrubs, trees with less than 30% canopy coverage, trees with 31±70% canopy coverage, and trees with greater than 70% canopy coverage. The data were recoded a second time to create 16 classes (water, rock and sparse vegetation, grass, developed and agricultural, shrub, grand ®r (Abies grandis), red and silver ®r (Abies magni®ca, Abies amabilis), Douglas ®r (Pseudotsuga menziesi), ponderosa pine, lodgepole pine, mixed pine, mountain hemlock (Thuja mertensiana), incense cedar (Libocedrus decurrens), Englemann spruce (Picea engelmanni), western juniper, and mixed conifers). Finally, the data were converted to ERDAS 7.5 GIS ®le format for comparison with the 1988 data. The 1988 data were provided as PC-ERDAS 7.5 GIS ®les. The original canopy coverage classes were recoded to the nine classes listed above. Of a possible 128 species classes, 33 occurred in the study area. These were recoded to 16 classes to match the 1953 data. A six 7.5 min quadrangle subset described above was then cut from each of the four data sets. Each of the quadrangles was also cut from the canopy coverage data set for individual analysis. The elevational
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data were resampled into three elevational classes (<1425, 1425±1600, and >1600 m) that approximated mule deer winter, transition, and summer ranges, respectively, based on Salwasser (1979), Volland (1985) and C. Yee, pers. commun., Chiloquin Ranger District, 1994. The PC-ERDAS 7.5 module SUMMARY was used to overlay the various subsets to estimate changes that occurred between 1953 and 1988. The data were tabulated by thematic layer for each quadrangle of the canopy coverage data, and by the six-quadrangle subset of both canopy coverage and species type for each elevational class. An understory biomass-canopy cover regression developed by Jameson (1967) was then used to estimate forage biomass in the three elevational classes. The canopy cover value for each vegetative class in each elevational class was derived from the GIS analysis. Midpoint values for stands exhibiting <30% canopy cover (15%), for stands exhibiting 31±70% canopy cover range (50%) and stands exhibiting >70% canopy cover (85%) were assigned. The proportion of each vegetative type for the 1953 and 1988 data was multiplied by the estimated understory biomass derived from the regression. Understory biomass production was sampled after growth was completed in summer 1996. Canopy closure layer pixels were aggregated to create a layer with a minimum polygon size of 2.02 ha. This layer was intersected with the plant association layer to create vegetative strata. In April and May, three stands in each of seven vegetative strata were sampled from stands at <1425 m elevation. In the period June± September, four to seven stands in each of 16 vegetative strata were sampled at higher elevations. Stands were randomly selected from the GIS vegetative strata layer. The most ef®cient plot size and number of plots per stand were determined during initial vegetation sampling. Stands sampled in early June had 10 plots using multiple plot sizes (1.0 and 3.0 m2 for shrubs, 0.25 and 1.0 m2 for herbs). Current year vegetative growth was clipped, oven dried and weighed by vegetation class (grass, forb, shrub) by plot. Mean biomass production (g/m2) and variance were compared by plot to determine at what point they stabilized. By mid-June, data collections indicated that more than eight plots per stand did not appreciably decrease variance and did
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not change mean production. For grasses and forbs, 0.25 m2 plots had comparable variance to 1.0 m2 plots and mean production was similar for both plot sizes. Similarly, plot sizes of 1.0 and 3.0 m2 yielded similar variance and mean production values for shrubs. Stands were sampled thereafter with eight plots of 0.25 m2 for herbs and 1.0 m2 for shrubs. The biomass estimates were then allocated to the different canopy categories and stand types based on the proportions of each within the study area. 4. Results 4.1. Canopy closure Below 1425 m, sparsely vegetated areas changed very little (Fig. 1). Grasslands comprised 2.9% of the area below 1425 m in 1953 and 6.8% of the area in 1988, suggesting a real change. While 35.5% of this type remained in grassland in 1988, 25.4% changed to moderately-closed canopied forest, and 18.6% to open-canopied forest.
Shrub-dominated communities declined from 18.5% of the area below 1425 m in 1953 to 11.7% in 1988. These communities either shifted towards grassland (19.2%) or towards open-canopied forest (28.7%), while 35.5% remained in shrub-dominated communities. The shrub communities that were switching towards conifer-dominated stands were probably old burns or logged areas where conifers were re-establishing dominance. Open-canopied forests declined dramatically from 63.1% of the area below 1425 m in 1953 to 25.3% of the area in 1988. The major shift was towards moderately-canopied forest (64.3%). While some of the smaller shifts in the composition of 1953 communities may be questioned as being real or the subject of different classi®cation of the same stand, or sampling error, this change is large enough to be considered real. It also ®ts with predictions of how forests would change following prolonged protection from ®re. This complements the observed shift in moderatelycanopied forests from 13.8% in 1953 to 54.5% in 1988. Also, 71% of the moderately-canopied forest remained in that category in 1988, which, for the drier
Fig. 1. Changes in overstory canopy cover for the Sprague Unit, southern Oregon. T < 30% tree canopy closure less than 30%.
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forests is expected. Densely-canopied forests comprised less than 1% of the area below 1425 m that was sampled. Between 1425 and 1600 m elevation, sparse communities, grasslands, shrublands, and dense-canopied forests did not change appreciably by 1988 (Fig. 1). The major changes were from open-canopied forests in 1953 to moderately-canopied forests in 1988. The open-canopied forests comprised 64.9% of the area in 1953, and declined to 17.4% in 1988, with 84.2% of the change being a shift towards moderately-canopied forests. Moderately-canopied forests increased from 25.6% of the area in 1953 to 72.9% in 1988. Above 1600 m elevation, sparsely vegetated areas, grasslands, shrublands and densely-canopied forests all comprised under 1% of the area in 1953. The nonforested sites did not change appreciably by 1988, while a small change from 0.4 to 3.7% in close-canopied forests was evident. The dramatic shift was from open-canopied forest, 86.3% in 1953, to moderately-canopied forest, 85.9% in 1988. For all elevations combined, the most pronounced change from 1953 to 1988 was from open-canopied forest to moderately-canopied forest (Fig. 1). In 1953, 67.8% of the entire area was classi®ed as opencanopied, while in 1988, 69.8% of the area was classi®ed as moderately-canopied. The most pronounced change was at the highest elevation, where moisture and temperature gradients would most favor conifer growth, followed by the mid-elevation band, with the areas below 1425 m showing the least change. Species composition did not show the major changes that the canopy closure did (Table 1). Elevations below 1425 m were dominated by ponderosa
271
pine in 1953 and 1988 with lesser amounts in the latter period. The major change was in the mixed conifer category, which changed from <1% in 1953 to 30.4% in 1988. The major shift to mixed conifer was from shrub, ponderosa pine and western juniper communities. The shift from western juniper to mixed conifer may be partially attributable to differences in stand classi®cations in 1953 and 1988. Sparsely vegetated areas increased about 4%, probably associated with the presence of recently logged areas or a result of the ®ner-scale mapping resolution of the 1988 information as compared to 1953. Sparsely vegetated areas shifted either to shrub, ponderosa pine or mixed tree communities. Shrubs shifted to sparsely vegetated areas or mixed conifer communities, again probably attributable to the resolution level of the mapping. Ponderosa pine stands either remained in that category or shifted to mixed conifer communities. Western juniper shifted to mixed conifer, ponderosa pine, or shrub communities. Mixed conifer stands in 1953 shifted primarily to ponderosa pine communities, which may be attributable to harvest and planting to ponderosa pine. The mid-elevation band of communities was dominated by ponderosa pine in both 1953 and 1988 (Table 1). Seventy-six percent of the area was in ponderosa pine in 1953, while 47.8% was in that type in 1988. Lodgepole pine stands were next in importance in 1953 and 1988. Mixed conifer increased from 2.4% in 1953 to 26.1% in 1988. Elevations of more than 1600 m showed trends similar to those of the mid-elevation band. The predominant community was ponderosa pine, in both 1953 and 1988, with hardly any change. White ®r,
Table 1 Changes in species composition for the Sprague Unit, southern Oregon Elevation 1425 m
Sparse Shrub Firs Ponderosa pine Lodgepole pine Juniper Mixed conifer
1425±1600 m
>1600 m
All elevations
1953
1988
1953
1988
1953
1988
1953
1988
0.038 0.156 0 0.623 0.154 0.028 0.002
0.08 0.117 0 0.356 0.136 0.006 0.304
0.013 0.055 0.002 0.76 0.125 0.022 0.024
0.032 0.055 0.002 0.478 0.169 0.002 0.261
0.004 0 0.045 0.567 0.043 0.337 0
0.014 0.017 0.031 0.548 0.101 0.207 0.074
0.018 0.074 0.008 0.6904 0.12 0.02 0.069
0.043 0.067 0.007 0.455 0.149 0 0.276
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mixed conifer stands would be expected to increase at these elevations, but they did not. Lodgepole pine stands increased from 4.4% in 1953 to 10.1% in 1988. The overall shifts for all elevations show a decline in ponderosa pine communities, small increases in sparsely vegetated sites, shrubs, white ®r, and juniper, and a fourfold increase in mixed conifer stands. Trends are consistent with predicted effects of ®re suppression policies, and what we observed directly, but sampling errors including changes in stand classi®cation undoubtedly contribute to some of the apparent change. Ponderosa pine communities that would have been maintained under a ®re regime would be expected to change to mixed stands as succession progressed, except on sites where this species was climax. Shifts in shrub communities towards mixed timber represent succession to juniper and ponderosa pine on dry sites and to mixed conifer on the more mesic sites where these communities originated from ®re and logging. The successional trends suggest that stands created by logging have balanced those stands created earlier by ®re that were succeeding towards forest. An evaluation of the comparability of GIS information from the Sprague Unit information can be made using a larger block from the Winema National Forest. A 341,500 ha block within the forest was compared with the Sprague Unit block. The shrub layer in the 1953 data includes sparse timber. Similar trends in forest canopy closure exist in both areas (Table 2). Forests with 26±70% canopy closure shifted from less than 30% of total land area in 1953 to over 50% in 1988. If the shrub layer and lowest tree canopy layer are combined for both units, then very similar trends
occur in both. This suggested that results from the Sprague Unit were applicable to the general area. 4.2. Understory biomass production 4.2.1. Estimates using the negative exponential curve There was less biomass at all elevations in 1988 than in 1953 (Table 3). The most pronounced reduction in understory biomass occurred at elevations greater than 1600 m. Growing conditions would be most favorable for conifers at the higher elevations, so this is entirely reasonable. The estimate of 7.3 g/m2 in 1988 is 34.4% of the estimate of 21.3 g/m2 in 1953 at elevations greater than 1600 m. The greatest shift in production was at the <30% canopy category, where 15.2 g/m2 was estimated in 1953 and only 4.0 g/m2 was estimated to be present in 1988. The 1425±1600 m elevation band biomass estimate of 9.9 g/m2 in 1988 is 44.1% of the 1953 estimate of 22.6 g/m2, the magnitude of reduction being similar to the higher band. However, understory biomass in 1988 at this elevation band was similar to that estimated at the higher elevation band. Canopy closure at the <1425 m band changed appreciably as well but enough area remained in nonforest communities and in the most open forest canopy category to cause a reduction to 76.2% of the 1953 estimate by 1988. The 1988 estimate at this elevation band was 2.4 times higher than at the 1425±1600 m band. The overall estimate for all elevations suggested that, by 1988, understory biomass production was 65.5% of the 1953 estimate. The greatest change came at the most open forest canopy level, which was
Table 2 Comparison of canopy closure from a 341,500 ha block of the Winema National Forest with the Sprague Unit information for the period 1953±1988 Stand type
Canopy cover (%)
a
Miscellaneous Shrubb Tree Tree Tree a
11±25 26±70 71±100
Winema Block 1953 (%)
1988 (%)
1953 (%)
1988 (%)
2.3 65.97 0.75 29 0
17.5 7.1 15.18 57.45 2.77
1.88 9.41 67.82 20.21 0.68
4.23 6.64 18.04 69.83 1.26
Includes water, rock, and sparse vegetation. 1953 data in Winema Block for shrubs includes ``sparse timber''.
b
Sprague Unit
J.M. Peek et al. / Forest Ecology and Management 150 (2001) 267±277 Table 3 Understory biomass predictions, Sprague Unit, southern Oregon Elevation
Predicted biomass (g/m2) 1953
1988
<1425 m No canopya 15% canopy 50% canopy 85% canopy Total
16.96 14.16 0.08 0.04 31.24
15.05 5.70 3.04 0.01 23.80
1425±1600 m No canopy 15% canopy 50% canopy 85% canopy Total
6.81 14.57 1.42 0.03 22.61
2.03 3.91 4.05 0.04 9.98
>1600 m No canopy 15% canopy 50% canopy 85% canopy Total
0.31 19.37 1.61 0.02 21.31
2.37 1.57 4.78 0.17 7.32
All elevations No canopy 15% canopy 50% canopy 85% canopy Total
8.63 15.22 1.12 0.03 25.00
8.4 4.04 3.88 0.06 16.38
a No canopy includes sparsely vegetated nonforest communities, grasslands which include meadows and some riparian zones, shrub communities including sagebrush and bitterbrush-dominated rangelands as well as burned forest in seral shrub stage. The forests with a canopy closure of <30% were truncated to 15% canopy, 31±70% canopy to 50% canopy and >70% canopy to 85% canopy for this analysis. The Jameson (1967) prediction for ponderosa pine canopy, which should make the forage estimate conservative for higher elevations and all elevations, is used. The midpoint in canopy coverage for each category of canopy closure is assumed.
producing 15.2 g/m2 in 1953 and 4.0 g/m2 in 1988. The least change came in the nonforested communities, as expected. 4.2.2. Biomass estimates using ®eld data Individual stand biomass estimates were within the range of the predictions using the Jameson (1967) curve. Estimates for 0±25% canopy forests of ponderosa pine were 2.3±16.2 g/m2 (mean 9.3 g/m2), 25±55% canopy cover 0±9.2 g/m2 (mean 2.9 g/m2), >55% canopy cover 0±8.0 g/m2 (mean 1.1 g/m2). For
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big sagebrush stands clipped in summer, production estimates were 1.2±15.1 g/m2 (mean 8.2 g/m2). Dry meadow production estimates were 4.7±20.2 g/m2 (mean 12.5 g/m2). However, extrapolations to the study area produce estimates that averaged 22.3% of the predictions using the negative exponential curve (Fig. 2). For elevations of more than 1400 m, total production for 1953 was estimated at 7.07 and 31.96 g/m2 with the curve, and for 1988, 5.91 and 23.83 g/m2. The proportional decline in production between 1953 and 1988 using ®eld data was 16.4% as compared to 25.5% with the curve. For elevations between 1400 and 1600 m, the proportional decline was 52.0% using the ®eld calculations and 51.8% using the curve. At the highest elevation range, the proportional decline was 53.3% using ®eld data and 56.6% using the curve. For all elevations, the proportional decline was 49.6% using ®eld data and 34.5% using the curve. The differences in proportional decline using the two methods increased as elevations increased. 5. Discussion The data set represents a series of transitions from the 1953 to the 1988 period, but since the GIS analysis consists of a total enumeration of parameters within the six-quadrangle area, no transition probabilities were calculated. If ecological conditions warranted, a projection of potential succession could be investigated using a Markov analysis (Facelli and Pickett, 1990; Waggoner and Stephens, 1970). However, our judgment is that the conditions and trends portrayed in the 1953±1988 period were unlikely to continue. Also, stands were probably at or near maximum canopy closure at the 60% level. First, logging, pathogens, and ®re are likely to perturb the forests suf®ciently over the next 25 years to make any projections unreliable. The Lone Pine ®re of 1992 burned 27,000 acres of National Forest land and 3000 acres of private lands within the Sprague Unit analysis area. Our analysis was con®ned to a portrayal of change over the 1953±1988 period and estimation of the effects of those changes on understory biomass. Jameson (1967) considered the nonlinear model as a general model for overstory±understory relationships, subsequently con®rmed by Mitchell and Bartling
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Fig. 2. Production estimates by canopy and cover type using ®eld data from Gay (1998). JUOC Ð western juniper, PIPO Ð ponderosa pine, PICO Ð lodgepole pine, and mixed Ð mixed conifers.
(1991). However, Arnold (1953) reported that logging in ponderosa pine forests produced increases in annual grasses and forbs and perennial forbs, but slight decreases in perennial grasses 5 years after logging. Arnold (1953) also noted that logging roads, skid trails, and slash reduced understory growth. Thus, estimates of understory biomass change apply to areas where roads, trails and slash were not present. In the mixed conifer forest of northeastern Oregon, Young et al. (1967) reported a hump-shaped curve that correlated percent shrub cover and overstory crown cover following logging, while herbaceous plants showed declines from open to moderate to heavy shade. Since the understories of mixed conifer forests on the Sprague area consisted more of shade-intolerant species, their cover was expected to follow the negative curvilinear relationship. A stand density index correlation developed by Moore and Deiter (1992) produced a curve similar to that of Jameson (1967). However, Ratliff et al. (1991) demonstrated that, in California oak-woodland, different overstory species affect herbage understory, and Scanlon (1992) produced different curves related to distance from eucalyptus and mesquite trees, suggesting that the curves may vary by type of plant community. Use of the Jameson (1967) curve to project understory biomass change over the 35-year time span assumes that slash has decomposed suf®ciently to
not be a major effect on ground cover, that roads and logging trails are not signi®cantly altering the relationships, and that shrub cover follows the predicted pattern. Untreated slash and soil compaction from roads and skid trails would also suppress understory production in logging areas. Thus, if the assumptions are not correct, the net effect is to reduce understory production more than predicted, with the exception that shrub cover may be underestimated in the mid-canopy ranges and overestimated in the lowcanopy range. If the negative exponential curve that portrays biomass relative to canopy coverage is widely applicable, then understory biomass production should decrease exponentially as a stand develops and canopy cover increases. An indirect evaluation of the ef®cacy of the Jameson curve can be made using the COVER extension of the stand prognosis model. The prognosis model (Stage, 1973; Wykoff et al., 1982), allows use of current stand conditions to predict future stand growth. The COVER extension predicts development of tree crowns and understory vegetation (Moeur, 1985). Crookston and Stage (1999) provide an estimate of canopy cover that accounts for crown overlap which is de®ned as the percentage of the ground area that is directly covered with tree crowns. The prognosis model variant for southcentral Oregon and northeastern California was used to describe a range
J.M. Peek et al. / Forest Ecology and Management 150 (2001) 267±277
of stand ages and corresponding changes in canopy coverage, using a ``default'' stand, wherein 750 trees per hectare were speci®ed as being initially established, and 100% survival occurred. Two ponderosa pine stands, with site indices of 50 and 90, and three white ®r stands with site indices of 50, 80, and 110 were used. The canopy closure in each model stand was examined at 10-year intervals out to 150 years. The projected canopy cover on stand age essentially is the mirror image of the negative exponential curve. All stands reach maximum canopy closure at 50±70 years after establishment. The white ®r stands reach higher canopy closures than the ponderosa pine stands as expected (Law and Waring, 1994). These stands represent a wide range of growing conditions and show essentially the same curves. The analysis tends to support use of the negative exponential curve across a variety of stand types. Gilmore and Seymour (1996) reported similar relationships in balsam ®r (Abies balsamea) leaf area and growth ef®ciency. Roberts et al. (1993) considered both tree-level and stand-level dynamics to be involved. Redistribution of nutrients and light from understories in open-canopy stands of shade intolerant conifers to overstory leaf area increases as tree canopies increased and incident light and moisture to understories declined would cause a decline in forest understories, especially where shade-intolerant species are involved. The predicted understory biomass using the negative exponential curve falls within the range of individual stand estimates obtained in the ®eld, but are higher than the ®eld estimates for the entire study area. Four of ®ve comparisons show the predicted biomass being higher than the mean biomass estimate based on ®eld data. The 1996 water year (October±September) for Lakeview was 51.8 cm as compared to the longterm average of 37.4 cm, indicating the ®eld data were taken during an above-average growing season. We conclude that the predicted biomass was higher than what actually existed, that substantial variation between stands and sites and ¯uctuations in growing conditions make broad-scale assessments of vegetation biomass change dif®cult. Much of this area was probably dominated by oldgrowth ponderosa pine before logging activities began (Henjum et al., 1994). Logging was common in the 1920s, but ponderosa pine probably dominated the
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area as old-growth forest as late as 1936 (Wall, 1972). By the early 1940s, extensive cutting from Bend, OR, south had created large areas of pine second growth (Cowlin et al., 1942). The proportion of forest dominated by ponderosa pine had considerably declined by the time of the 1953 surveys used in this report. A combination of high-grading old-growth pine, ®re suppression, and encroachment of mixed forest was in progress in 1953, continuing through 1988, and is a common forest trend throughout the west (Hungerford et al., 1991; Mutch, 1993; Covington et al., 1994). The original old-growth forest likely contained substantial understory vegetal development, if it was frequented by ground ®res that maintained the savanna-like nature of the forest. It is likely under a frequent ®re regime that understories were dominated by bunchgrass until some time in the late 1800s, when overgrazing by livestock may have caused shrubs to proliferate. These open forests with bunchgrass understories may not have provided high-quality habitat for mule deer, but the successional changes following heavy grazing would have created a more diverse forage supply for this species. Keay and Peek (1980) reported that mule deer made heavy use of the more open, recently burned stands dominated by ponderosa pine in the upper Selway River of Idaho, and this probably applied to the southcentral Oregon region as well. However, Salwasser (1979) concluded that deer were not abundant prior to 1850 in this region, and remained at low numbers through the early 1900s. Mule deer began to increase around 1915, probably because of increased shrublands attributable to prior heavy grazing, coupled with subsequent reductions in domestic livestock, and reductions in predation (Salwasser, 1979), which may have been a typical pattern across the west (Peek and Krausman, 1995). Shrublands have since continued to mature and senesce as well across the western ranges (Urness, 1990), and such is undoubtedly the case in this area, the result being a progressive decline in mule deer populations over the past 30 years. Acknowledgements This work was supported by a consortium of agencies and organizations interested in southcentral Oregon mule deer, including the Fremont National Forest,
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Winema National Forest, National Council for Air and Stream Improvement, Bureau of Land Management, Mule Deer Foundation, Oregon Department of Fish and Wildlife, Oregon Hunters Association, and the University of Idaho, College of Natural Resources. We thank Andy Peavy for providing the GIS coverage of the Winema National Forest. Ron Anglin, Don Leckenby, Ralph Opp, and Chris Yee provided information and answers to our questions throughout the study. Suggestions of anonymous reviewers improved the manuscript. This is University of Idaho, College of Natural Resources Experiment Station publication 916. References Agee, J.K., 1995. Fire Ecology of Paci®c Northwest Forests. Island Press, Washington, DC, 493 pp. Alaback, P.B., 1982. Dynamics of understory biomass in Sitka spruce-western hemlock forests of southeast Alaska. Ecology 63, 1932±1948. Arnold, J.F., 1953. Effect of heavy selection logging on the herbaceous vegetation in a ponderosa pine forest in northern Arizona. J. For. 51, 101±105. Barrett, S.W., Arno, S.F., Menakis, J.P., 1997. Fire episodes in the inland northwest (1540±1940) based on ®re history data. U.S. Forest Service General Technical Report INT-GTR-370. 17 pp. Clary, W.P., 1969. Increasing sampling precision for some herbage variables through knowledge of the timber overstory. J. Range Manage. 22, 200±201. Covington, W.W., Everett, R.L., Steele, R., Irwin, L.L., Daer, T.A., Auclair, A.N.D., 1994. Historical and anticipated changes in forest ecosystems of the Inland West. J. Sust. For. 4, 13±63. Cowlin, R.W., Briegleb, P.A., Moravets, F.L., 1942. Forest resources of the ponderosa pine region of Oregon and Washington. US Forest Service Miscellaneous Publication 490, 99 pp. Crookston, N.L., Stage, A.R., 1999. Percent canopy cover and stand structure statistics from the Forest Vegetation Simulator. US Forest Service General Technical Report RMRS-GTR-24. Fort Collins, CO. Facelli, J.M., Pickett, S.T.A., 1990. Markovian chains and the role of history in succession. Trends Ecol. Evol. 5, 27±31. Ffolliott, P.F., Clary, W.P., 1972. A selected and annotated bibliography of understory±overstory vegetation relationships. University of Arizona Agricultural Experiment Station Technical Bulletin 198, 33 pp. Franklin, J.F., Dyrness, C.T., 1973. Natural vegetation of Oregon and Washington. US Forest Service General Technical Report PNW-8, 417 pp. Gay, D., 1998. A test of the southcentral Oregon mule deer habitat suitability index model. University of Idaho Master of Science Thesis, Moscow, ID.
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