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August 7, 2000

EFFECTS OF PARTIAL-CUT TIMBER HARVESTING ON LANDBIRDS IN THE NORTHERN ROCKY MOUNTAINS

RICHARD L. HUTTO, Division of Biological Sciences, University of Montana, Missoula, MT 59812

JOCK S. YOUNG, Division of Biological Sciences, University of Montana, Missoula, MT 59812

Abstract: As part of the U.S. Forest Service Northern Region Landbird Monitoring Program, we investigated the short-term effects of partial-cut timber harvesting on bird abundances in mid-elevation conifer forests by conducting a single-season survey of landbirds on 37 uncut sites and 35 partial-cut sites within 3 National Forests in northwestern Montana and northern Idaho. Of 32 bird species that were detected on at least 13 sites, the abundances of 14 were significantly different between the cut and uncut sites. Six species (Pileated Woodpecker [Dryocopus pileatus], Common Raven [Corvus corax], Brown Creeper [Certhia americana], Winter Wren [Troglodytes troglodytes], Golden-crowned Kinglet [Regulus satrapa], and Townsend's Warbler [Dendroica townsendi]) were significantly more abundant in uncut sites and 8 species (Cassin's Vireo [Vireo cassinii], Mountain Chickadee [Poecile gambeli], Ruby-crowned Kinglet [Regulus calendula], Townsend's Solitaire [Myadestes townsendi], Orange-crowned Warbler [Vermivora celata], Western Tanager [Piranga leudoviciana], Chipping Sparrow [Spizella passerina], and Dark-eyed Junco [Junco hyemalis]) were significantly more abundant in partially cut sites. The fact that several species were negatively affected by relatively low-volume partial cutting in this and all other studies of similar cutting practices to date has clear management implications: some species need intact forests to reach their greatest abundances, and the same forests may provide the most suitable conditions for such species as well. For species that are relatively more abundant in partial-cut and other kinds of harvested forest types, there would seem to be little of management concern. Nonetheless, an important issue to resolve is whether those species do as well in forests produced by an "unnatural" form of disturbance (timber harvesting) as they do in the earlier successional forests that are born of fire.

The present study was conducted as part of the U. S. Forest Service (USFS) Northern Region Landbird Monitoring Program. This program was designed to include both long-term monitoring of bird populations independent of specific land-use practices, and short-term monitoring of bird populations in relation to specific management practices. The long-term monitoring is conducted across the region on a biennial basis, leaving the off years for more focused projects that address current information needs, especially as they relate to known management issues and potential population declines that may one day reveal themselves through the long-term monitoring efforts. The short-term monitoring conducted in relation to specific land-use practices is designed to (1) take advantage of the impressive level of site replication that is possible through the use of a sizeable field crew, and (2) place information into the hands of managers in a timely fashion so that they can avoid negative longer-term effects by managing the land pro-actively and in the context of "adaptive management" (sensu Haney 1996, Lancia 1996).

In 1997, we conducted an "off-year" study to examine the effects of partial-cut timber harvesting on bird abundance in mid-elevation, conifer forests on 3 National Forests in northwestern Montana and northern Idaho. Our specific objectives were: (1) to compare the abundances of diurnal landbird species in partial-cut timber harvest stands and uncut stands; (2) to compare vegetation structure and tree species composition between cut and uncut stands; and (3) to develop statistical models to help identify the vegetative components that might help explain any observed differences in bird abundance between cut and uncut stands.

METHODS

General Study Design.--The USFS Northern Region Landbird Monitoring Program involves all 13 National Forests in the region, but only 3 forests participated in the present study. These forests (Lolo, Kootenai, and Idaho Panhandle National Forests) comprise the northwestern part of the region and have considerable climatic influence from the maritime Pacific, especially to the west where mesic Cascadian forests are common (Peet 1988). The study was conducted in the mixed-conifer zone, where Douglas-fir (Pseudotsuga menziesii) is the dominant species and western larch (Larix occidentalis), ponderosa pine (Pinus ponderosa) and lodgepole pine (Pinus contorta) are the common associates. In the western portion, grand fir (Abies grandis) is important and may become dominant.

We selected 37 study sites in uncut forest stands, and 35 sites in partial-cut stands within the Douglas-fir and Grand fir forest types. Study sites were chosen by queries of USFS databases based on design criteria, as well as on-site inspection of many sites. Both cut and uncut sites were selected from a subset of the USFS database that included mixed-conifer stands of immature and mature sawtimber (not poletimber) with the largest portion of basal area greater than 12.7 cm (5.0 in) diameter-at-breast-height (dbh) made up of Douglas-fir or grand fir, and within habitat types (Pfister et al. 1977) ranging from more mesic Pseudotsuga menziesii/Linnea borealis to drier Abies lasiocarpa/Xenophylum tenax.

A protocol was also developed to control for stand age and mean diameter of trees. All sites were between 50 and 120 years old and had mean tree diameters ranging from 25-43 cm (10-17 in). To avoid selecting old-growth stands, sites also contained fewer than 10 trees per acre greater than 50 cm (20 in) dbh. Age ranges were intended to correct for the large differences in site productivity across the geographical extent of the study area to maintain similar stand structures throughout the study. Study sites were located adjacent to secondary and tertiary roads, and were easily accessible on foot from the roadway. Sites were at least 2 km apart, and were at least 200 m in width, providing 100 m of suitable habitat on either side of a transect route and long enough (at least 800 m) for 3 bird survey points to be positioned 200 m apart.

Treated sites had undergone partial-cut harvest activity at some time between 1985 to 1995. Partial-cut harvest methods were restricted to single-tree sanitation and/or salvage cutting, and in a few cases, thinning. The reduction of tree volume in treated stands relative to adjacent untreated stands was relatively minor but still apparent from aerial photographs. Nonetheless, there was still considerable overlap in canopy cover between cut and uncut stands (Figure 1).

Bird Survey Methods.--The bird counts followed recommendations discussed by Ralph et al. (1995) and methods described by Hutto et al. (1986). We used 6-7 observers who participated in a one-week training session prior to the formal field work. A single observer conducted a 10-minute point count at each of the 3 sampling points within a site. Points were visited 3 or 4 times (4 visits on the Kootenai National Forest and 3 visits elsewhere) between mid-May and mid-July. All birds seen or heard within the count period were recorded, noting species, number of individuals, and distance to the bird(s). Field observers generally began counts about 15 min after sunrise (after the pre-dawn chorus), and completed counts between 0630 and 1100 hrs (MST). Counts were not conducted on days with continual rain or high winds.

Because woodpecker numbers are typically low in point count studies, we instructed observers to record woodpeckers detected while walking between points (outside the formal 10-min sample period), and to link any detections with the nearest point. Sample sizes of most woodpecker species would have been too low to analyze without these extra data, so these supplemental data were included in the analyses. Specifically, 22 of the 40 Pileated Woodpeckers (Dryocopus pileatus) recorded within 100 m were from outside the sample period; 26 of 66 Northern Flickers (Colaptes auratus), 28 of 58 Hairy Woodpeckers (Picoides villosus), and 21 of 50 Red-naped Sapsuckers (Sphyrapicus nuchalis) were likewise added by this protocol. We assumed that observer bias was no different for these data than for typical point-count data. The main bias would result from an unequal amount of time spent on each site, but survey durations were independent of treatment.

Any comparison of bird detections between vegetation types that differ in vegetation density must involve a consideration of potential biases resulting from differential detectability of bird songs. Sound is attenuated by vegetation, so the results of the present study may be biased by the greater likelihood of hearing bird songs in the more open, partial-cut sites. To explore this potential bias, we examined profiles of detection distances (up to 100 m) in all uncut sites versus all partial-cut sites, for all species combined as well as for each common species with sufficient data. We compared the profiles statistically using the median test as well as the Kolmogorov-Smirnov Z test, which compares the shapes of the distributions.

Methods of Characterizing Vegetation.--After each visit to a point, vegetation data were collected at a plot centered 50 m from the point at which bird data were recorded in a direction that differed from one visit to the next so that the 4 samples were evenly spaced around the bird survey point. After standardization through training, observers made ocular estimates of the following variables within a 30-m-radius circle: (1) the average height of the tree canopy layer; (2) the percent cover of canopy trees (larger than saplings); (3) the percent cover of sapling trees (between 5 and 10 cm dbh); (4) the percent cover of seedling trees (<5 cm dbh); (5) the percent cover of tall shrubs (multi-stemmed woody plants greater than 1 m tall); (6) the percent cover of low shrubs (less than 1 m tall); (7) the percent cover of grasses and forbs; and (8) tree species composition, as estimated by the proportionate makeup of each tree species in the overstory canopy. In addition, all snags within the 30-m-radius circle were counted, and for each we recorded the tree species, dbh, height, percent bark remaining, and whether it had a broken top or excavated cavities. Live tree density was sampled by counting the number of large-dbh (>40 cm), medium-dbh (10-40 cm), and small-dbh (<10 cm) trees within 11.3 m of the same vegetation point. A size class index was obtained by assigning the numbers 3, 2, and 1, respectively, to trees in each of these size classes and averaging across all points and visits within the site.

Methods of analysis.--Using site as the sample unit, bird abundances in cut and uncut stands were compared using Poisson regression for all species observed on at least 10 of the 72 sites. In addition, Poisson regression was used with points as a sample units to examine multivariate relationships between vegetation structure (see above and Table 1) and bird abundance for species that occurred on at least 20 sites. Poisson regression is appropriate for count data of uncommon events, such as the occurrence of a particular species at a point, where there are many points with zeroes and the rest with one or a few individuals (McCullagh and Nelder 1989).

Variables considered for entry into a multivariate model were those that a univariate test indicated had a potentially significant effect, with P < 0.15. Multivariate models were then built using backward stepwise selection. We chose the best 4 to 6 variables as a set for all possible subsets, to help discover alternative models. Our intention was to explore the data and discover the variables that were best associated with each species. The p-values may not be reliable estimates of true values, but indicate relative effects of variables in this data set. For this reason, variables are listed in order of statistical importance, without quantitative values.

To explore whether certain types of snags may be key in determining numbers of cavity nesters, we created new variables to represent the density of 8 subsets of snags. These were based on characteristics thought to be important to the building of cavity nests (McClelland 1977): (1) all snags with dbh > 20 cm; (2) all snags with dbh > 40 cm; (3) larch snags with dbh > 40 cm; (4) snags with dbh > 40 cm and less than 100% bark intact; (5) snags with dbh > 40 cm and broken tops; (6) snags with dbh > 40 cm and height > 5 m; (7) larch snags with dbh > 20 cm and height > 5 m; and (8) snags > 20 cm dbh and 5 m high with less than 100% bark intact. The average dbh of snags over 20 and over 40 cm were also used as variables potentially associated with the abundance of each cavity-nesting species.

RESULTS

Vegetation Differences.--By design, the mean canopy cover was greater for uncut (48%) than for partial-cut sites (30%) (Table 1). There were also significantly more large trees (dbh > 40 cm dbh; P < .001) and medium trees (10-40 cm dbh; P = 0.008) on the uncut sites. The density of small (non-canopy) trees, however, did not differ between the cut and uncut sites. In fact, no component of understory vegetation was significantly different between cut and uncut sites (although the tall shrub cover averaged one-third higher on uncut sites).

The sites were dominated by Douglas-fir, which accounted for about half of the estimated canopy cover, with a somewhat smaller percentage on treated sites (Table 1). Ponderosa pine, western larch, and lodgepole pine accounted for most of the rest of the canopy, and an additional 13% was contributed by the mesic species--western redcedar (Thuja plicata), western hemlock (Tsuga heterophylla), and grand fir. Canopy cover estimates on 14 of the 72 sites were dominated by ponderosa pine (6 treated sites), lodgepole pine (5 sites), and either cedar or hemlock (3 sites).

The main difference in tree species composition between the cut and uncut sites was the component of ponderosa pine, which made up approximately 20% of the canopy on partial-cut sites, but only 5.4% on uncut sites (P = 0.004; Table 1). Most of the compensatory reverse trend was made up by less coverage of Douglas-fir on cut (mean = 43.3%) than on uncut (mean = 52.7%) sites. There was also twice as many snags on uncut than on cut sites (P = 0.001; Table 1). This is a disproportionately large difference compared with the differences in tree density (48% higher on uncut sites) or canopy cover (62% higher on uncut sites).

Bird Abundance.--Sixty-nine bird species were detected within 100 m of an observer, including 53 species that were detected on at least 2 sites (Table 2). The abundances of 14 of the 32 species detected on at least 10 sites were significantly different between cut and uncut sites, with 6 species (Pileated Woodpecker, Common Raven [Corvus corax], Brown Creeper [Certhia americana], Winter Wren [Troglodytes troglodytes], Golden-crowned Kinglet [Regulus satrapa], and Townsend's Warbler [Dendroica townsendi]) being significantly more abundant in uncut sites and 8 species (Cassin's Vireo [Vireo cassinii], Mountain Chickadee [Poecile gambeli], Ruby-crowned Kinglet [Regulus calendula], Townsend's Solitaire [Myadestes townsendi], Orange-crowned Warbler [Vermivora celata], Western Tanager [Piranga leudoviciana], Chipping Sparrow [Spizella passerina], and Dark-eyed Junco [Junco hyemalis]) significantly more abundant in partial-cut sites (Table 2).

Vegetation associations.-We used multiple regression to identify variables that could help explain differences between cut and uncut stands. Abundances of most bird species were related to several continuous vegetation variables in the multiple regression analyses and, on average, about 3 variables were retained in the multivariate models for each species (Table 3). For example, differences in canopy cover may have caused differences in abundances of Dark-eyed Junco, Townsend's Warbler, Chipping Sparrow, Mountain Chickadee, American Robin (Turdus migratorius), and Golden-crowned Kinglet because there was no longer a significant effect of treatment (cut or uncut site) after controlling for canopy cover, while canopy cover was still significant after controlling for treatment (Table 2). On the other hand, the greater proportion of ponderosa pine in the cut sites may have been the more important treatment effect for Western Tanager, Cassin's Vireo, and possibly Chipping Sparrow because there was no longer a significant effect of treatment after controlling for the proportion of ponderosa pine (PIPO), while PIPO was still significant after controlling for treatment (Table 2). If so, this would be a good example of how the choice of tree species to harvest may have an effect on the resulting bird community. The abundance of the Swainson's Thrush (Catharus ustulatus) was not correlated with canopy cover, but was correlated with shrub cover and the proportion of ponderosa pine/western larch in the canopy, which probably combined to produce a nearly significant treatment effect.

Snag density.--There was very little evidence for an effect of snag density or size on the abundance of any bird species. Many species had significant univariate relationships with snag density, probably due to its strong correlation with live tree density, but these relationships were negative. The variety of snag variables produced similar results. The only cavity-nesting species whose abundances were positively associated with any of the snag variables were the Chestnut-backed Chickadee (Poecile rufescens) and Brown Creeper. The creeper was positively associated with "snags over 40 cm", which was still significant after controlling for canopy cover.

Detectability Bias.--Counter to what one might expect, the mean detection distance in uncut stands was actually higher than in cut stands (58.3 m vs. 56.9 m), but the two distributions did not differ significantly in central tendency (median test, P = 0.48) or shape (Kolmogorov-Smirnov Z, P = 0.84; Figure 1). A comparison based on data from all species combined may have masked important differences for individual species, but only 7 of the 30 most common species showed any indication of a significant difference, and only one of these (Townsend's Warbler) had a significantly greater mean detection distance in the cut sites. Moreover, two of the most difficult species to detect at greater distances (Golden-crowned Kinglet and Brown Creeper) were still more abundant in uncut forests, which is opposite what a detectability bias might be expected to produce. Therefore, we doubt that the results presented herein are an artifact of a detectability bias.

DISCUSSION

Four of the six bird species that were more abundant in uncut than in partial-cut forest sites (Brown Creeper, Winter Wren, Golden-crowned Kinglet, and Townsend's Warbler) have been reported to be affected negatively by partial cutting in nearly every other study of such effects in the Rocky Mountains (Hejl et al. 1995). More recent studies of partial cutting in conifer forests of the Rocky Mountains (Hutto and Young 1999) and Pacific Northwest (Hansen et al. 1995, Hansen and Hounihan 1996, Beese and Bryant 1999, Chambers et al. 1999) also show negative effects on the same bird species. The level of replication at the stand level is a noteworthy characteristic of the present study; the size of our field crew enabled us to monitor 72 stand-level sites across three National Forests. This, combined with similar results published elsewhere, give us confidence that the significant negative treatment effects we report are not artifacts of sampling error. These universal findings are of potential management concern because a good deal of mature forest is likely to be harvested in the future.

Most of the bird species that were significantly more abundant in the partial-cut stands (Western Tanager, Dark-eyed Junco, Chipping Sparrow, Cassin's Vireo, Mountain Chickadee) were still relatively common in the uncut sites, and may be of little management concern. However, other species that we detected in partial-cut forests were either not detected at all (Calliope Hummingbird [Stellula calliope], Rufous hummingbird [Selasphorus rufus], and Black-headed Grosbeak [Pheucticus melanocephalus]), or were detected much less frequently (Orange-crowned Warbler, Ruby-crowned Kinglet) in uncut forests. The same pattern has been documented previously for one or more of these species and for Williamson's Sapsucker (Sphyrapicus thryoideus), Olive-sided Flycatcher (Contopus cooperi), and MacGillivray's Warbler (Oporornis tolmiei) (Hansen et al. 1995, Beese and Bryant 1999, Chambers et al. 1999, Hutto and Young 1999). The latter species would seem to be of little management concern as well, except that we do not know if the bird species in these partial-cut forest patches are doing as well as their relative abundance in harvested forests suggests, or if they are attracted to harvested forests that provide them with visual cues for settling without providing habitat that is otherwise suitable, thereby acting as "ecological traps". Brown-headed Cowbirds (Molothrus ater) are, in fact, much more likely to occur in partially cut than in uncut forests (Table 2; Hutto and Young 1999) and the presence of this nest parasite may contribute to the creation of unsuitable but otherwise attractive habitat.

Single-tree, partial-cut timber harvesting does not change forest structure as much as clearcutting, so virtually all conifer forest bird species that we detected occurred in both uncut and partial-cut stands. The differences in bird abundance that did occur, however, suggest that regional bird populations may be strongly affected when summed across a new landscape of increasing partial-cut forestry (Thompson et al. 1995). The reduction of closed-canopy forests and associated species is not the only concern. As "friendly" as partial cutting appears to be on the surface, we need to recognize that such treatment cuts short or eliminates the earliest naturally produced post-fire successional stages that are necessary for the maintenance of species (e.g., Black-backed Woodpecker [Picoides arcticus]) that are relatively restricted to the standing dead trees associated with post-fire disturbance (Hutto 1995). Nonetheless, although clearcuts are not suitable for those bird species that may depend on fire-killed trees in severely burned forests, clearcuts are heavily used by shrub-nesting species such as Dusky Flycatcher (Empidonax oberholseri), Warbling Vireo (Vireo gilvus), and MacGillivray's Warbler. These species have been shown elsewhere (Hutto and Young 1999) to be much less common in partial-cut than in clearcut forests, which may be why we did not find them to be relatively common in our partial-cut sites. Widespread replacement of clearcuts by partial-cut harvesting regimes may, therefore, further reduce the amount of habitat for a variety of early-successional bird species beyond that already reduced by fire suppression.

ACKNOWLEDGMENTS

The Northern Region of the U.S. Forest Service was the primary source of support for this project. Potlatch Timber Company; Plum Creek Timber Company; Montana Fish, Wildlife and Parks; and the Bureau of Land Management also contributed financial support, access to lands, and field crews. We extend special thanks to John Hoffland, who hired, trained, and coordinated data collection by the field workers. We also appreciate the efforts of field workers who participated in the study.

LITERATURE CITED

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Table 1. Means and standard deviations of each vegetation variable on 37 unlogged (control) and 35 partial-cut (treated) sites. The sign indicates the direction of a statistically significant effect of the partial-cut treatment. Capitalized variable codes used in Table 3.

Table 2. Abundance and occurrence of each bird species on 37 unlogged and 35 partial-cut sites that were surveyed in 1997. All species detected on at least two sites are listed in taxonomic order. P values from Poisson regressions are presented for species that occurred on >10 sites. The sign associated with an effect is given in parentheses when p 0.05.

Table 3. Variables retained in the multivariate regression models for each species (nemonic codes given in Table 1). Models were obtained by stepwise backward elimination in Poisson regression. Variables in parentheses would be included if cutoff were P < 0.10. Also shown is the proportion of total variation explained by each model, as estimated by an analysis of deviance (analogous to an R² value). Species are listed in phylogenetic order, and varaibles are listed in descending order of statistical importance in descending order of statistical importance.

FIGURE LEGEND

Figure 1. Frequency distribution of detection distances (m) to individual birds within

(A) uncut sites (mean = 58.3; n = 2687) and (B) partial-cut sites (mean = 56.9; n = 2786).