Northeastern Naturalist 308 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 22001188 NORTHEASTERN NATURALIST 2V5(o2l). :2350,8 N–3o1. 82 Avian Community Response to Short-rotation Aspen Forest Management Gina M. Jarvi1,*, Jessie L. Knowlton1,2, Colin C. Phifer1, Amber M. Roth3, Christopher R. Webster1, and David J. Flaspohler1 Abstract - In the upper midwestern US and parts of Canada, forests dominated by Populus tremuloides (Aspen) are increasingly being considered as a bioenergy feedstock for power plants. When used for bioenergy, these forests are harvested at much younger ages than when they are used for more traditional products, such as pulpwood and lumber. To better understand the potential consequences of a shift in shorter-rotation–harvest strategies on avian communities, we employed point counts to examine bird community composition in a chronosequence (10–45 y since harvest) of 12 coppiced, even-aged Aspen stands. Young (8–15 y old), middle (20–44 y old), and mature (45 y old) stands had no significant differences in species richness or relative abundance, but distinct avian community assemblages were associated with each stand-age class. Four bird species were significantly associated with a particular age class. Maintaining a wide range of Aspen stand-age classes in the landscape appears to be the best strategy for conserving a diverse bird community in this region. Introduction Meeting global commitments to reduce climate-change–causing gases will require significant de-carbonization of energy production. Biomass-energy production is one means to meet this goal, and it has replaced coal in some power plants in the US and elsewhere. In 2011, biomass energy accounted for 53% of renewable energy-resource use in the US, and is projected to increase by 30% in the next 20 y (Radloff et al. 2012). In the upper midwestern US and parts of Canada, forests dominated by Populus spp. (e.g., cottonwoods, P. balsamifera L. [Balsam Poplar], P. tremuloides Michx. [Aspen], and other species) have been managed for decades as a feedstock source for pulp and paper, but these species also provide feedstock for biomass-energy plants (Burns and Honkala 1990). Populus tremuloides Michx. (Aspen)-dominated forests are well suited for feedstock because this species grows fast and has clonal, vegetative reproduction (Becker et al. 2009). The area for Aspen biomass production would need to be large to produce a stable supply of this fuel (Becker et al. 2009, Cook and Beyea 2000). The state of Wisconsin has more than 25 biomass-power plants that produce almost 1/3 of the state’s renewable energy (USEIA 2017), with Aspen trees and other woody residuals often used as feedstocks (Radloff et al. 2012). Recommended rotation lengths for Aspen timber products are typically between 45 y and 60 y (Melorose et al. 2006), but Aspen grown for woody 1School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI 49931. 2Current address - Biology Department, Wheaton College, Norton, MA 02766. 3Department of Wildlife, Fisheries, and Conservation Biology and School of Forest Resources, University of Maine, Orono, ME 04469. *Corresponding author - gmtesta@mtu.edu. Manuscript Editor: Peter Paton Northeastern Naturalist Vol. 25, No. 2 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 2018 309 biomass-energy production may be harvested at intervals of 10–30 y—a significantly shorter harvest rotation (Burns and Honkala 1990). A continued shift towards bioenergy production will result in altered landscapes increasingly dominated by younger Aspen forests, which may impact some forest-dependent wildlife species including birds. Bird communities respond strongly to vegetation composition and structure at both the local and landscape scales (Cody 1985, James and Wamer 1982, MacArthur and MacArthur 1961, Willson 1974), and are often impacted by forest-management practices (Venier and Pearce 2005). Structural characteristics important to birds include vegetation height, vertical profile, horizontal patchiness, stem diameter and density, and the proportion of coverage by woody versus herbaceous plants (King and Schlossberg 2014). In boreal Aspen forests, differences in vegetation structure with stand-age class led to differences in the number and types of niches available to breeding birds, and thus, distinct bird communities within each age class (Hobson and Bayne 2000, James and Wamer 1982, Moskat and Szekely 1989). However, research in Aspen-dominated forests has produced conflicting results, with some studies documenting greater avian species richness in old stands (80–110 y; Hobson and Bayne 2000, Schieck et al. 1995) and others showing higher richness in young stands (<15 y; Roth 2012, Westworth and Telfer 1993). Changes caused by an increase in short-rotation Aspen management clearly have the potential to impact avian communities and their demography (Hobson and Bayne 2000, Westworth and Telfer 1993). We examined bird community composition and species’ abundances in young (8-15 y), middle (20–44 y), and mature (45 y) Aspen stands in northeastern Wisconsin, which is an area with active biomass plants (USEIA 2017). We hypothesized that avian abundance, species richness, and evenness would increase with stand age due to the greater structural complexity and wider variety of niches available as forests matured (King and Schlossberg 2014). We also hypothesized that more regionally rare and threatened species would occur in mature stands because this age class has been greatly diminished due to logging that occurred post-Euro-American settlement (Mladenoff et al. 1993). We also hypothesized that differences in habitat structure between stand ages would lead to distinct avian communities within each stand-age class because of differences in habitat needs between species. If a bird species prefers a specific Aspen forest-age class, then alterations to the age-class distribution could have important impacts on the overall bird community composition, including rare, threatened, or declining species (Venier and Pearce 2005). Information on the impact of local-level stand-age patterns on bird communities is needed to inform forest-management planning for sustaining both biodiversity and associated ecosystem services (Niemi et al. 1998, Venier and Pearce 2005). Field-site Description We used ArcGIS layers provided by the Wisconsin Department of Natural Resources to select 12 coppiced, even-aged, no-retention, Aspen forest-stand sites ranging from 8 to 45 y post-harvest in Vilas and Oneida counties in Wisconsin (Fig. 1). We randomly selected a subset of sites from these data layers, then Northeastern Naturalist 310 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 2018 Vol. 25, No. 2 screened them for accessibility, and excluded sites that were too difficult to reach, were not contiguously shaped (i.e., narrow riparian strips), or were scheduled for harvest during the study period. We divided study sites into 3 age classes: young (8–15 y), middle (20–44 y), and mature (45 y), for a total of 4 study sites per age class. Selected sites were ≥16.2 ha in size and ≥5 km from other sites. The study area had a mean monthly precipitation of 74 mm and a mean temperature of 19 oC during the birds’ breeding season (NOAA 2016). The soils were dominated by sandy loams, loamy sands, Rubicon sands, and Rubicon complexes, and were generally moderately well drained to excessively well drained (Soil Survey Staff 2013). All study stands regenerated naturally following harvest. Although our sites were all no-retention stands, some residual trees (sometimes called “legacy trees”) did remain due to harvesting guidelines. Methods Avian community sampling We randomly located 3–12 point-count stations within each of the 12 study sites, based on site area; each station was ≥200 m from other stations to minimize the probability of double-counting individuals. We also placed stations 200 m from roads and stand edges to minimize the impacts of edge effects (Sisk et al. 1997). There was a cumulative total of 30 point-count stations per age class. Only one observer conducted all surveys to reduce observer bias. Point counts consisted of a 5-min survey within a 25-m fixed radius to reduce differences in bird detectability among stand ages that varied greatly in vegetation density. The observer waited 1 Figure 1. (A) Map of the study area and 12 study plots (4-letter codes) of 3 Aspen stand age classes where we conducted point counts in Vilas and Oneida counties in northern Wisconsin. Examples of typical (B) young (8–15 y), (C) middle (20–44 y), and (D) mature (45 y) Aspen forest stands. Photograph © C.C. Phifer. Northeastern Naturalist Vol. 25, No. 2 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 2018 311 min upon arrival at each station to allow birds to settle before initiating the survey and did not count birds flying overhead. Each station was surveyed twice: once between 21 May and 3 July 2015 and again between 3 July and 16 July 2015. Except in cases of inclement weather (strong wind and heavy rain), we conducted point counts from sunrise through 10:00 AM. In most cases, we surveyed all point-count stations at a given site within 1 morning. Due to frequent heavy rain early in the season, the first round of point counts took longer to complete than anticipated, which resulted in the rather late second round. This situation may have influenced our results because birds were more likely to have been moving in and out of the study area during this post-breeding dispersal period. The second round of surveys was done in the reverse order of the first round to avoid a temp oral bias. Vegetation surveys We conducted vegetation surveys by subsampling 20 point-count stations per age class. At each selected station, we sampled 2 points: one 10 m NW and one 10 m SE from the point-count station, for a total of 40 vegetation-survey points per age class. At each of these points, we recorded foliage-height class (James and Shugart 1970, Tinoco et al. 2013), canopy closure, and the number of stems >2 cm DBH per m2 . We measured foliage-height class with an extendable height pole to record the presence of foliage within a 15-cm radius of the pole at 0–1 m, 1–2 m, 2–5 m, 5–10 m, 10–15 m, and >15 m. We measured canopy closure with a spherical densiometer. We chose these vegetation metrics because structural diversity, canopy openness, and density of vegetation influence bird community composition (Cody 1985, James and Wamer 1982, MacArthur and MacArthur 1961, Willson 1974). Statistical analyses We classified bird species as a conservation priority if they were listed as a species of concern (Rosenberg et al. 2016). To assess differences in bird community structure among Aspen stands of different ages, we calculated mean unadjusted relative abundance for each bird species and species richness (S), Shannon’s diversity index (H '; H '= -Σpi log pi), and Shannon’s evenness (E; E = H ' / ln[S]) across the 4 sites in each Aspen stand-age class. These data were not normally distributed and did not meet parametric assumptions; thus, we used Kruskal-Wallis tests to determine if there were differences in species richness or abundance among Aspen stand-age classes. We used maximum counts from the 2 rounds of point counts for analysis because we were interested in the community as a whole within each site. We used ANOVA to test for significant differences in vegetation characteristics among stand ages (i.e., canopy closure, number of stems >2 cm diameter, and foliage height class index) because the data were normally distributed. To evaluate avian community associations with Aspen stand age and habitat characteristics, we conducted a nonmetric multidimensional scaling (NMS) ordination in PC-Ord v6.08 (McCune and Grace 2002). NMS is an iterative optimization method that attempts to place n samples on k axes so that the rank order of the distances between samples agrees with the rank order of the original distances in the data matrix, with “stress” being a measure of the final lack of agreement in these 2 sets Northeastern Naturalist 312 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 2018 Vol. 25, No. 2 of ranks (McCune and Grace 2002). We used maximum abundance for each species as our main species matrix, and the program’s “slow and thorough” autopilot defaults: Sorensen (Bray–Curtis) distance and 250 iterations of the real data with 250 runs of random values (McCune and Grace 2002). Our environmental matrix (secondary matrix) included age class, canopy closure, and foliage-height–class index (FHCI) 0–1 m, FHCI 1–2 m, FHCI 2–5 m, FHCI 5–10 m, FHCI 10–15 m, and FHCI >15 m. To complement the NMS analysis, we also used PC-Ord (v6.08) to conduct a species indicator analysis to identify species that might be more strongly associated with a particular stand-age class (Dufrene and Legendre 1997, McCune and Grace 2002). This analysis calculates an indicator value from 0 to 100 based on the frequency and abundance of species, with 100 being complete coincidence to a single age class. We tested these values for significance using a Monte Carlo test (McCune and Grace 2002). We conducted all statistical tests in R version 3.3.0 (R Core Team 2016) unless otherwise noted. Results Bird abundance We detected a total of 972 individual birds representing 53 species from 180 point counts. The most abundant species was Seiurus aurocapilla L. (Ovenbird; 13.1% of all observations), followed by Vireo olivaceus L. (Red-eyed Vireo; 12.5%) and Setophaga pensylvanica L. (Chestnut-sided Warbler; 11.2%). Estimated avian relative abundance was not significantly different among age classes (H ' = 0.27, P = 0.87; Table 1). Bird richness, diversity, and evenness Species richness, Shannon’s diversity, and evenness were not significantly different among stand age classes (H ' = 1.07, P = 0.58), (H ' = 0.80, P = 0.67), (H ' = 1.25, P = 0.54), respectively (Table 1). Of the 53 species observed, only Vermivora chrysoptera L. (Golden-winged Warbler) and Empidonax minimus Baird (Least Flycatcher) were conservation priority species. Vegetation We found no significant differences in the number of stems >2 cm diameter/m (F = 2.52, P = 0.09) or in foliage height (F = 0.09, P = 0.92) among the 3 age classes (Table 2). However, we did find that canopy closure differed significantly among Table 1. Average avian species richness, evenness, and abundance for 3 Aspen stand-age classes with 4 sites each in Oneida and Vilas Counties, WI. Sites did not differ in species richness or abundance by age class. Aspen stand age (y) Species richness Evenness Abundance Young (8–15 y) 33.5 ± 0.33 0.84 176.5 ± 1.2 Middle (20–44 y) 28.0 ± 0.22 0.76 151.0 ± 1.5 Mature (45 y) 26.0 ± 0.49 0.80 162.0 ± 0.4 Northeastern Naturalist Vol. 25, No. 2 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 2018 313 age classes (F = 4.58, P = 0.01) with stands averaging 88% (SE ± 2.67%), 93% (2.39%), and 97% (0.76%) canopy closure in young, middle, and mature stands, respectively (Table 2). Avian communities The NMS ordination resulted in a 2-dimensional solution with a final stress of 6.12 and final instability criterion of <0.0001, which is indicative of a “good ordination with no real risk of drawing false inferences”, suggesting a true ecological gradient (McCune and Grace 2002). Axis 1 accounted for 68.8% of the variance and was most strongly associated with mid-story canopy structure (foliage height classes 5–10 m [r = 0.725, r 2 = 0.526] and 10–15 m [r = -0.513, r 2 = 0.263]) (Fig. 2). Axis 2 represented 26.2% was related to high canopy structure (foliage height class >15 m [r = -0.264, r2 = 0.901]) and percentage of closed canopy (r = 0.03, r 2 = 0.819) (Fig. 2). The indicator analysis and associated Monte Carlo test revealed that out of 53 observed species, 4 species were strongly associated with a specific age class (Table 3). Pipilo erythrophthalmus L. (Eastern Towhee; indicator value = 87.5, P = 0.009), Oreothlypis ruficapilla Wilson (Nashville Warbler; indicator value = 66.7, P = 0.047), and Zonotrichia albicollis Gmelin (White-throated Sparrow; indicator value = 87.5, P = 0.02) were most strongly associated with the young Aspen stands, while Sphyrapicus varius L. (Yellow-bellied Sapsucker; indicator value = 85.7, P = 0.02) was most strongly associated with mature Aspen stands. Table 2. Summary of differences in vegetation characteristics among vegetation survey plots in 12 Aspen forest stands of 3 age classes in Vilas and Oneida counties, WI, with ANOVA results shown. Values with same letter within a row are not significantly different. Stand age: Yo = young (8–15 y), Mi - middle (20–44 y), and Ma = mature (45 y). Res. = residual n (by stand age) Mean (SE) (by stand age) Parameter Yo Mi Ma Yo Mi Ma F df P Res. Canopy closure (%) 41 40 39 88 (2.67)A 93 (2.39)AB 97 (0.76)B 4.58 2 0.01 117 Stem >2 cm dbh/m2 20 20 20 6.4 (0.98)A 4.2 (0.78)A 6.9 (1.14)A 2.52 2 0.09 57 Foliage-height index 40 40 39 0.66 (0.02)A 0.68 (0.03)A 0.68 (0.03)A 0.09 2 0.92 116 Table 3. Indicator-analysis results (only significant values shown) for bird species in 12 Aspen forest stands of 3 age classes in Oneida and Vilas counties, WI. The highest abundance value for each species across age classes is indicated by an asterisk (*). A higher indicator value (0–100) indicates greater fidelity to an age class. Significance was tested using a Monte-Carlo test. Stand age: Yo = young (8–15 y), Mi - middle (20º44 y), and Ma = mature (45 y). I = indicator value. Aspen stand age Indicator analysis Common name Scientific name Yo Mi Ma I P Age class Eastern Towhee Pipilo erythrophthalmus 4.3* 0.2 0.0 87.5 0.01 Young Nashville Warbler Oreothlypis ruficapilla 6.9* 4.7 0.2 66.7 0.05 Young White-throated Sparrow Zonotrichia albicollis 13.2* 0.8 0.0 87.5 0.02 Young Yellow-bellied Sapsucker Sphyrapicus varius 0.2 0.0 2.8 85.7 0.02 Mature Northeastern Naturalist 314 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 2018 Vol. 25, No. 2 Discussion As predicted, we found that the bird communities in each Aspen stand-age class were distinct from one another. However, these differences appear to only be related to differences in canopy closure and not to other structural features of the vegetation. Contrary to our expectations we found no statistically significant differences in avian relative abundances, species richness, or evenness among stand-age classes. While we acknowledge that our study was limited in time and scope, these results suggest that young Aspen-dominated stands may be an important habitat for many bird species. Although not old growth, Aspen forests 80–100 y of age are mature Aspen forests in North America and can also support large, diverse, breeding bird communities, especially those requiring large-diameter trees, snags, Figure 2. Non-metric multidimensional scaling (NMS) ordination plot of 12 Aspen forest stands and 53 bird species. NMS ordination resulted in a two-dimensional solution with a final stress of 6.12, with both axes explaining 95% of the variance. Open circles are individual bird species, and closed shapes represent individual study sites of young, middle, and mature Aspen age-classes. Shapes closer together represent sites that are more similar in terms of bird species composition and abundance. Vector codes are: CC = canopy closure, FH5-10 = 5–10 m foliage-height class, FH10-15 = 10–15 m foliage-height class, and FH15+ = >15 m foliage-height class. Northeastern Naturalist Vol. 25, No. 2 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 2018 315 or downed woody material (Scheick et al. 1995). Given the importance of both mature and young Aspen stands for many bird species, maintaining a wide variety of Aspen stand ages appears to be the best strategy for conserving bird diversity at the landscape scale. Over time, the structural characteristics of Aspen-dominated forests change rapidly due to the relatively fast growth and senescence of Populus spp. compared to many other temperate forest trees (Rytter 2006). As a result, the physical characteristics used by breeding birds to choose appropriate habitat differ greatly, even between forest stands a decade apart in age. In effect, although the plant community composition may change little over decades as these forests mature, birds appear to be assessing structural changes that make aging forests more or less appealing as breeding habitat. We found that canopy structure in the mid- and upper forest levels (5–15+ m tall) and the percentage of tree crown closure had the greatest influence on avian communities in terms of stand selection. These variables are associated with forest-stand development processes that provide habitat structure and complexity (Goetz et al. 2007, MacArthur 1964). Aspen-dominated forests often have a moderate complexity of live and dead vegetation when young, low complexity as they approach maturity, and then a high complexity in old (>75 y) forests due to gaps created by large trees dying and falling (Schieck et al. 1995). The mature, 45-y–old stands we studied, however, possibly were at the low-complexity stage, which may explain the lack of observed differences in abundance and richness with stand age in our study;a similar lack of complexity may have been the cause of the low species-richness in forests 50–65 years post-harvest reported in other studies (Hobson and Bayne 2000, Schieck et al. 1995). In boreal bird communities, the greatest species diversity and mature-forest specialist species were not observed until forests were over 75 y post-harvest (Schieck and Song 2006). There are few Aspen stands as old as 75 y in this region because of short-rotation management history and the inherently short lifespan of Aspen trees; thus, this study may have underestimated avian use of mature forest stands. Nashville Warbler, White-throated Sparrow, and Eastern Towhee were indicator species for young Aspen stands. All 3 species require high habitat structural complexity, light levels, shrub density, and woody stem density (e.g., Lowther and McWilliams 2011, Schill and Yahner 2009). Yellow-bellied Sapsucker was an indicator of mature Aspen stands, but surprisingly, no other cavity nesters were more abundant in different aged stands. Residual trees may have a disproportionately strong influence on the avian community, particularly on those species requiring the presence of large live trees or snags, such as woodpeckers (Hobson and Schieck 1999). For instance, Colaptes auratus L. (Northern Flicker) is known to inhabit young forests as long as residual trees of an appropriate size are present (Kirk et al. 1996). Yellow-bellied Sapsuckers may be attracted to mature Aspen forests because they have a nesting preference for Fomes-infected Aspen trees (Kilham 1971). Northeastern Naturalist 316 G.M. Jarvi, J.L. Knowlton, C.C. Phifer, A.M. Roth., C.R. Webster, and D.J. Flaspohler 2018 Vol. 25, No. 2 Conclusions Aspen-dominated forests historically existed as a heterogeneous mosaic of stand ages because of natural disturbances such as fire and wind, in addition to individual tree death (Morissette et al. 2002). Expansion of woody-biomass harvesting for bioenergy could further homogenize and truncate the age distribution of aspen stands in parts of the Upper Midwest. Our study was limited in time and scope, but the relatively distinct avian assemblages associated with each age class and lack of significant differences in abundance and diversity in our data suggest that landscapes with heterogeneous distributions of stand age may support greater overall avian abundance and diversity. We recommend a rotational harvest to create a balanced “portfolio” of forest ages on the landscape. This strategy could be attained using the “triad approach” of forest management, which attempts to balance intensive forest management, protected forest, and mid-intensity ecological forestry on a landscape (Seymour and Hunter 1999). This management strategy would likely benefit birds and other taxa, including mammals (Roy et al. 1995), while at the same time meeting the need for woody biomass and timber. Acknowledgments We thank the Michigan Technological University’s Summer Undergraduate Research Fellowship, the National Science Foundation Partnerships in Research and Education (grant number 124344) and the International Research Experience for Undergraduates programs (grant number DEB-1019928), and the US Department of Agriculture McIntire–Stennis Program for providing financial support for this project. 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