Forest Ecology and Management 304 (2013) 110–120
Contents lists available at SciVerse ScienceDirect
Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco
Diversifying the composition and structure of managed, late-successional forests with harvest gaps: What is the optimal gap size? Christel C. Kern a,⇑, Anthony W. D’Amato b, Terry F. Strong a,1 a b
US Forest Service, Northern Research Station, 1831 Hwy 169 East, Grand Rapids, MN 55744, USA Department of Forest Resources, University of Minnesota, 1530 North Cleveland Avenue, Saint Paul, MN 55108, USA
a r t i c l e
i n f o
Article history: Received 4 March 2013 Received in revised form 17 April 2013 Accepted 18 April 2013
Keywords: Uneven-aged management Selection system Tree recruitment Shrub interference Microclimatic extremes Sugar maple
a b s t r a c t Managing forests for resilience is crucial in the face of uncertain future environmental conditions. Because harvest gap size alters the species diversity and vertical and horizontal structural heterogeneity, there may be an optimum range of gap sizes for conferring resilience to environmental uncertainty. We examined the impacts of different harvest gap sizes on structure and composition in northern hardwood forests of the upper Great Lakes region, USA using a robustly-designed experiment consisting of six harvest gap sizes replicated 12 times with over a decade of repeated measurements. We found that germinant (trees < 0.15 m tall) densities were greatest in year 2 (71.6 stems m2) but were present in all measurement years (0, 2, 6, and 12 years post-harvest), suggesting a continuous influx of tree seedlings on site. Sapling (>0.6 m tall) densities were greatest (3.9 stems m2) at the end of the study period (year 12) and were composed primarily of three species (Acer saccharum, Fraxinus americana, Ostrya virginiana) already present on site. The 6 and 10 m diameter (dia.) harvest gaps had, on average, the minimum number of saplings (2 stems m2) needed to fill dominant positions in small canopy openings, but, by year 12, small gaps were closed by crown extension of mature edge trees. Medium (20 m dia.) to large harvest gaps (30 and 46 m dia.) had fewer saplings than small gaps. The understory layer of medium and large gaps were dominated by shrubs (Rubus sp.) that dampened growth of species such as F. americana. Consequently, gap-filling from the sapling layer would not occur in the near future, and, from a management perspective, would be considered a regeneration failure within larger gaps. Our study suggests that the optimum gap size does not exist and using harvest gaps to create resilient forests with structural and compositional diversity alone is challenging on productive habitats. In particular, medium to large gaps will likely require microsite specific placement of desired advance regeneration or additional treatments (e.g., vegetation control) to cultivate a dense sapling layer in an acceptable timeframe after harvest. Published by Elsevier B.V.
1. Introduction Managing forests for resilience is crucial in the face of uncertain future environmental conditions (Millar et al., 2007). Resilient forests maintain essential functions and sustain a range of ecosystem goods and services despite changing climate and disturbances. For instance, variation in species and within-stand structural complexity provides a range of species traits and growing conditions that enhances the capacity of an ecosystem to respond to a natural disturbance. Thus, in managed forests, ecosystem resilience can be achieved through creative harvest prescriptions and reforestation practices that influence the variation of species composition and stand structure (Drever et al., 2006). ⇑ Corresponding author. Tel.: +1 218 326 7134; fax: +1 218 326 7123. 1
E-mail address:
[email protected] (C.C. Kern). Retired.
0378-1127/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.foreco.2013.04.029
One example of a management practice that can be utilized to restore or maintain resilience is harvest-created canopy gaps. Harvest gaps create vertical and horizontal heterogeneity in stand structure and diversity through recruitment of less abundant species (Gray and Spies, 1996). Harvest gaps have been proposed as a component of natural disturbance-based management by emulating the frequency, distribution, and size of canopy gaps that result from natural disturbance at various stages of stand development (Coates and Burton, 1997; Franklin et al., 2007). Harvest gaps are also inherent outcomes from previous partial harvests (Eyre and Zillgitt, 1953), especially in uneven-aged management where the goal is to maintain multiple age classes within one stand. For example, in selection system silviculture, individual or group removal of dominant and co-dominant trees creates growing space for new age class recruitment and development among older age classes (Smith et al., 1997).
C.C. Kern et al. / Forest Ecology and Management 304 (2013) 110–120
Ideally, the size of harvest gaps can be manipulated to influence tree diversity within a given management approach. Larger gaps facilitate establishment of more trees intolerant to shade than smaller openings (Ricklefs, 1977; Denslow, 1980). This was the premise for the development of selection system silviculture. Single-tree selection was developed in shade tolerant European silver fir (Abies alba) forests to perpetuate this species in multi-aged stands, and group selection was developed to maintain a component of Norway spruce (Picea abies) and diversify this forest type (Puettmann et al., 2009). Today, harvest gaps are a key component of close-to-nature approaches in other forest types, such as European beech (Fagus sylvatica L.) (Naaf and Wulf, 2007; Madsen and Hahn, 2008). In North American forests, harvest gap size has been important to tree recruitment, diversity, and productivity of managed mixed species, multi-aged forests (Gray and Spies, 1996; Webster and Lorimer, 2002; Holladay et al., 2006). However, outcomes of various harvest gap sizes are not simple to predict because tree recruitment is influenced by more factors than gap size alone (Brokaw and Busing, 2000), including proximity to seed source, suitability of microsite, presence of advanced or pre-gap regeneration, intensity of local browsing pressure, and abundance of competing vegetation (Schutz, 1997; Dietze and Clark, 2008; Falk et al., 2010; Bolton and D’Amato, 2011). Thus, the long-term efficacy of using harvest gap size to influence tree diversity remains unclear. The northern hardwood forests of the US and Canada are an example of a forest type in which managers often desire to maintain tree diversity and structural heterogeneity while sustaining a variety of forest goods and services. The predominant management approach for this forest type is the single-tree selection system (Jacobs, 1987), which, after decades of application, has led to declines in tree species diversity (Niese and Strong, 1992; Neuendorff et al., 2007) and regional homogenization of forest conditions (Schulte et al., 2007). As a result, in the northeastern US, group and patch selection has been developed to maintain a component of shade intolerant trees in this forest type (Leak and Filip, 1977; Leak, 1999; Kelty et al., 2003). In the western Great Lakes region, a single-tree/group selection hybrid approach has emerged to recruit trees mid-tolerant to shade, such as yellow birch (Betula alleghaniensis), into the predominately shade tolerant sugar maple (Acer saccharum) stands (Erdmann, 1986; Nyland, 1987). Specifically, single-tree removals are interspersed with group removals that create small gaps for release and thinning and medium gaps for new age class development, respectively. Although, in northern hardwoods in the northeastern US, long-term studies have indicated that mid-tolerant trees occupy at least one-third of the basal area 40–60 years after large gap creation (Leak, 1999; McClure et al., 2000). Recent observations in northern hardwood forests of western Great Lakes suggest that seedlings may not recruit in larger gaps because of elevated deer browsing (Matonis et al., 2011). In light of these findings and the suite of emerging objectives related to increasing forest adaptation potential, there is great need for long-term evaluations of the effectiveness of different harvest gap sizes at restoring and maintaining forest resilience in mixed species, multi-aged forests. In this study, we asked the following questions: How does gap size influence tree seedling recruitment over time? Once seedlings are established, how does gap size influence tree sapling composition and diversity over time? And, more broadly, how does gap size influence mode of gap-filling and future canopy composition? We addressed these questions with a robustly-designed experiment with a decade of repeated measurements of regenerating trees across six harvest gap sizes replicated 12 times. We hypothesized that tree recruitment would occur shortly after harvest and would include a combination of pre-harvest advance regeneration and post-harvest recruits. We also hypothesized that shade tolerant
111
trees would dominate small and large gaps, because these trees would tolerate the low resource conditions under close proximity of mature edge trees in small gaps and under the thick shrub layers anticipated to develop in the large gaps. Therefore, we hypothesized that medium gaps would harbor a dense, diverse sapling layer with greatest potential of all gap sizes to diversify the future forest canopy. 2. Materials and methods 2.1. Study site The study ecosystem is a 136 ha second-growth, northern hardwood forest located on the Chequamegon-Nicolet National Forest in northern Wisconsin, USA (N45°560 , W88°590 ). Similar to many forests in the region, the study ecosystem regenerated after exploitive timber harvesting during the early twentieth century. Before study installation (1994), the forest had no recent management and was estimated to be 60 years old and in the stem exclusion stage of stand development (Oliver and Larson, 1996). In 1993, an initial field reconnaissance was conducted to determine the homogeneity of composition, structure, and physiography of the study area. The inventory resulted in delineation of seven uniform areas of which four were selected randomly for study. The inventory also described the initial site conditions. Sugar maple (A. saccharum) dominated the site but 12 other trees species were identified. Tree (4 cm diameter at breast height [DBH]) basal area ranged from 20 to 39 m2 ha1 and tree density ranged from 89 to 285 trees ha1. The forest canopy was closed with an occasional small canopy gap created by recent single-tree blowdown. The topography is a hummocky kame-kettle complex with some cradles and knolls created from tip-up mounds (resulting from past canopy tree blowdowns). Soils are primarily Padus sandy loam (coarse-loamy, mixed, frigid Alfic Haplorthods) with inclusions of Padus-Pence sandy loam (sandy, mixed, frigid Entic Haplorthods) (USDA Natural Resource Conservation Service, 2005). There was a thick forest floor layer across most of the study site suggesting little presence or impact of invasive earthworms on these areas, as has been documented in other northern hardwood forests in the region (Loss et al., 2013). The habitat type is considered nutrient rich, mesic and well suited for sugar maple growth and classified as Acer-Tsuga/Dryopteris (ATD) according to a habitat type classification system by Kotar et al. (2002). ATD is one of several mesic habitat types (e.g., Acer-Tsuga/Maianthemum [ATM] and Acer/Osmorhiza-Caulophyllum [AOCa]) supporting northern hardwood forests of the western Great Lakes region. ATM and AOCa habitat types typically are more species rich, have more species less tolerant to shade, and pose more management options to influence species composition than ATD habitat types, which are heavily dominated by sugar maple (Kotar et al., 2002). 2.2. Study design The experimental design was a randomized complete block with sub-sampling. There were two levels of replication, block and replicate within block. Three replicates of each of six circular gap sizes (0 [reference area], 6, 10, 20, 30, and 46 m diameter gaps) were randomly assigned in each of four adjacent blocks. Experimental gaps were created by dormant-season timber harvesting in 1994 (two blocks) and 1995 (two blocks). The resulting design included 12 reference areas (0.4047 ha square, uncut patches) and 56 experimental gaps (four marked gaps were not cut). Openings were measured in 1997 and 2008 from gap center to dripline in cardinal and sub-cardinal directions (eight total radii) (Table 1).
112
C.C. Kern et al. / Forest Ecology and Management 304 (2013) 110–120
Table 1 Number of treatment replicates and their mean size (mean [standard error]) at two and 12 years after harvest at the Divide Canopy Gap Study located in a northern hardwood forest on the Chequamegon-Nicolet National Forest, Wisconsin, USA. Gap size (diameter) No. of replicates
0m 12
6m 11
10 m 11
20 m 10
30 m 12
46 m 12
Diameter (m)
Year 2 Year 12
– –
4.7 (0.2) 3.5 (0.7)
8.2 (0.6) 5.0 (0.6)
17.8 (0.7) 15.7 (0.7)
28.8 (0.6) 25.9 (1.5)
45.2 (0.7) 45.0 (0.6)
Area (m2)
Year 2 Year 12
– –
21.9 (3.9) 13.9 (8.7)
62.8 (9.6) 25.9 (10.0)
268.6 (25.9) 203.4 (24.1)
699.3 (30.1) 592.3 (65.0)
1729.3 (54.0) 1708.6 (36.2)
Diameter: Height
Year 2 Year 12
– –
0.2 (0.0) 0.1 (0.1)
0.3 (0.1) 0.2 (0.1)
0.7 (0.1) 0.6 (0.1)
1.2 (0.1) 1.0 (0.1)
1.9 (0.1) 1.9 (0.1)
In 1997, gap area ranged from 9.9 m2 to 1986 m2 (gap diameter to tree height ratio 0.1 to 2.2). By 2008 (hereafter ‘‘year 13’’), most small gap openings were closed and similar to the forest matrix conditions, while large gaps remained open. In addition to gap creation, the four blocks were also thinned (except for the reference areas) in 1994–1995 following first harvest entry guidelines in Erdmann (1986), a Lake States management publication for converting even-aged northern hardwood forests to uneven-aged stand conditions. To improve residual stand vigor, this first thinning entry (an ‘‘improvement cut’’) removes trees anticipated to die from self-thinning, disease, or damage or that have minimal potential to gain economic value before the next harvest entry (approximately 20 years). The thinning reduced the forest matrix density to a mean basal area of 23 m2 ha1 (range: 15–33 m2 ha1), while reference areas remained uncut at 31.1 m2 ha1 (range: 27–37 m2 ha1). In 1997, deer exclosures were added to the study design on a subset of the experimental units (one reference area, 20 m gap and 46 m gap per block; 12 total exclosures) and were maintained for 4 years. The exclosures were constructed with 1-m tall rabbit guard fence. The resulting exclosure enclosed vegetation in the shape of a ‘+’ sign with 3.6-m wide arms in each cardinal direction. The exclosures were designed to reduce costs and were believed to
deter deer from dwelling in a confined area. The deer exclosures were not a significant factor in any of our analyses. In particular, we were not able to determine if the exclosures were effective at reducing browse and browsing effects were insignificant outside the exclosure or if the exclosures were ineffective and browsing effects were ubiquitous inside and outside the exclosures. Thus, these results are not presented. 2.3. Field sampling Permanent sample points were arrayed in four transects radiating in cardinal directions from gap centers to 5–10 m into the adjacent forest matrix (the ‘‘forest-gap transect’’; see Fig. 1 in Janowiak et al. (2008)). Number and spacing of sampling points per transect varied by gap size. In the control, 30 m, and 46 m gaps, there were 24, 17, and 24 sample points, respectively, each spaced 5.5 m apart. In the 6 m, 10 m, and 20 m gaps, there were 9, 13, and 17 points, respectively, each spaced 3.7 m apart. Two plot sizes were used to estimate density and size of woody species at each permanent sampling point. A quadrat (1 1 m) plot was used to collect data on tree seedlings 0.6 m tall and all shrubs, and a circular (1.8-m radius) plot was used to collect tree sapling (>0.6 m tall) data. Individual species were tallied by height
Fig. 1. Mean density, richness, and evenness of woody stems by gap size over time for three height size classes in a northern hardwood forest on the Chequamegon-Nicolet National Forest, Wisconsin, USA. Y-axis labels are located at the top of each column of graphs and each row of graphs represents a different height size class (labeled at the far left).
113
C.C. Kern et al. / Forest Ecology and Management 304 (2013) 110–120
class (0.6–2.1 m tall, >2.1 m tall–4 cm diameter at breast height (DBH), and >4–12 cm DBH in circular plots). In addition, the tallest sapling or ‘‘dominant’’ individual of each tree species was identified in each circular plot and then measured for DBH and total height. Nomenclature follows PLANTS Database (2009). Measurements were made during mid-summer (late June–early August) over four survey periods: pre-harvest (1994 [two blocks]–1995 [two blocks]; ‘‘year 0’’ hereafter) and post-harvest in 1997 (‘‘year 2’’ hereafter), 2000 (two blocks) or 2001 (two blocks) (’’year 6’’ hereafter), and 2007 (’’year 12’’ hereafter) with one exception. Raspberry (Rubus idaeus) densities were only collected in year 12. 2.4. Data analysis Generalized linear mixed models were used to examine the influence of gap size on tree seedling recruitment and sapling development. Gap size was treated as a fixed effect and subset to include only permanent sample points within the reference areas and within opening driplines as measured in 1997. Replicate (e.g., an individual gap or reference area within a block) means were considered random subsamples within each block and were weighted by the number of permanent sample points. Models were checked for normality, residual variation, and outliers. Analyses were performed using the PROC MIXED and the PROC GLIMMIX procedures in SAS Institute Inc. (2008). A few species (A. saccharum, Fraxinus americana, Ostrya virginiana, and R. idaeus) dominated the understory. As such, infrequent species were classified as ‘‘Other trees’’ or ‘‘Other shrubs.’’ Similarly, the majority of individual stem heights were noted in the two smallest height size classes, 0.6 m tall were combined into one size class. The analysis approach for each component of the tree seedling and sapling community examined is described below. 2.4.1. Tree reproduction Size of dominant saplings across gap sizes were analyzed by species (A. saccharum, F. americana, O. virginiana) using repeated measures. Serial correlation was modeled with compound symmetry structure after comparing various repeated measure models with Akaike’s Information Criterion (AIC). Repeated measures were also used to analyze the effect of density and diversity of all tree species by height size class. Diversity responses included species richness and evenness (see calculations in Magurran (1988)) and were transformed (lognormal or square-root) to meet model assumptions. Because survey year and survey yeartreatment were significant in all models of density and diversity by height size class, models were also run by survey year to isolate treatment effects at each time step. Post-hoc pair-wise comparisons (Bonferroni correction P < 0.003) were made when group differences were detected (P < 0.05). To test for tree (>0.6 m tall) compositional differences among gap sizes and survey years, we used multi-response permutation procedure (MRPP) and indicator species analysis (ISA) in PC-Ord v5.31 (McCune and Medford, 2006) using both abundance and presence/absence data. MRPP is a nonparametric test for group differences that are greater than expected by chance and accommodated our unbalanced design. Post Hoc pair-wise comparisons (with Bonferroni correction P < 0.003) were made when group differences were detected (P < 0.05). Group differences were then analyzed with ISA for species associations using indicator values that ranged from no indication (0) to perfect group indication (1 0 0). Species were considered significant indicators of a group when P < 0.05 based on a Monte Carlo test using 1000 permutations (McCune and Grace, 2002).
2.4.2. Relationships between trees and shrubs The effect of shrub abundance, gap size, and their interaction on tree growth was analyzed using an ANCOVA. Growth responses of dominant saplings were the absolute difference of replicate means between years 2 and 12 in DBH (log-transformed), height, and height:DBH (hereafter Ht:Dia). Year 2 growth responses were used as covariates and means of raspberry cover in years 2, 6, and 12 (log-transformed) represented the effects of dominant shrubs. Gap size was represented with actual mean gap diameters in year 2. Growth models were compared among full- and sub-models (gap size and raspberry abundance, gap size alone, or raspberry alone) then selected based on the corrected AIC. All models within two delta units of the best approximating model (lowest AIC) were interpreted. Parameter coefficients and fit (R2) were used to interpret model effects and suitability. The relationship between the density of trees and shrubs were analyzed by including life form (tree or shrub) in the model. The data were subset to year 12, because raspberry densities were only available that year. Post-hoc pair-wise comparisons (with Tukey’s HSD) were made when group differences were detected (P < 0.05). 3. Results 3.1. Tree density, diversity, and composition over time Tree reproduction changed significantly in density, richness, and evenness over time (Fig. 1 and Table 2). Maximum mean densities followed a temporal dynamic reflective of regeneration development with maximum densities of shorter height classes occurring in early years and maximum mean densities of taller height classes occurring in later years (Fig. 1 and Table 2). For example, shorter stature ‘germinants’ (trees < 0.15 m tall) maximized mean density in year 2 at 71.6 stems m2, while slightly taller ‘seedlings’ (trees 0.15–0.6 m tall) maximized mean density in year 6 at 14.2 stems m2 and even taller ‘saplings’ (trees > 0.6 m tall) maximized at 3.9 stems m2 in year 12. Richness followed a similar height class by time pattern. Generally, germinants (
