Legacy forest structures in irregular shelterwoods differentially affect regeneration in a temperate hardwood forest

Forest Ecology and Management 454 (2019) 117650

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Legacy forest structures in irregular shelterwoods differentially affect regeneration in a temperate hardwood forest

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Jessica Wikle , Marlyse Duguid, Mark S. Ashton Yale School of Forestry & Environmental Studies, 195 Prospect Street, New Haven, CT 06511, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Acer rubrum Betula lenta New England Quercus rubra Reserve trees Stand dynamics

Foresters are faced with managing forests for a broad variety of societal demands such as wildlife habitat, aesthetics and site stability while still providing monetary value to landowners through timber harvest. Regeneration methods need to be developed that offer alternatives to traditional management for timber to help meet these goals. Regeneration responses in third-growth forest stands, or those following harvest of secondgrowth stands are not well-studied. Such forests are now widespread across temperate regions and may exhibit growth patterns different than those reported previously for regenerating second growth that originated on land abandoned from agriculture. There is a need for research that can lead to new methods or modification of existing practice. Our study examines legacy trees and the associated regeneration across a 25-year chronosequence of 34 irregular shelterwoods regenerated from second-growth forest in southern New England. We compared regeneration of the three most common regenerating tree species: red maple (Acer rubrum), black birch (Betula lenta), and red oak (Quercus rubra). We used regression analyses to examine changes through time in height growth of the tallest regeneration for each species as well as changes in total abundance. Similarly, regressions for annual height growth of regeneration in relation to legacy overstory basal area were compared for each species. Over time, self-thinning occurred in the regeneration of all focus species, but at different rates. Black birch thinned most dramatically through time, and the saplings that survived retained a high position in the canopy. Red oak self-thinned most slowly, and its tallest stems retained competitive height, although did not surpass black birch. As legacy overstory basal area increased, annual growth of red oak regeneration slowed. Stand dynamics patterns are therefore significantly different from those of second growth forests where oak is a more vigorous competitor in both number and growth as compared black birch and red maple. Our results suggest that resource managers need to both recognize these differences in stand development and consider the tradeoff between increasing legacy trees and decreases in growth rate of oak regeneration, as well as long-term effects of increased structure post-timber harvest.

1. Introduction Forest management faces growing pressures to evolve to meet a variety of societal demands. Managers are encouraged to employ strategies that seek multiple benefits from forests such as wildlife habitat, aesthetics, carbon storage and climate mitigation, recreational opportunities and clean drinking water; all while producing direct monetary benefits to landowners through harvesting timber and non-timber forest products (Ashton and Kelty, 2018; Knoot et al., 2010; Nyland, 1992; O’Hara, 2001). Forestry today aims to meet these demands by incorporating practices such as managing forests as complex systems (Fahey et al., 2018; Gustafsson et al., 2012; Puettmann et al., 2012), creating resistant and resilient forests by retaining a diversity of species

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and size classes (Ashton and Kelty, 2018; Brang et al., 2014), and designing timber harvests to emulate natural disturbance more closely by leaving biological legacies (Franklin et al., 2007; O’Hara and Ramage, 2013). Biological legacies include standing dead trees, woody debris, and living trees that originate from an earlier age class than the regenerating stand (Franklin et al., 2000). As many ecological forest management techniques involve leaving more standing trees following timber harvest, they often promote shade tolerant species by allowing less sunlight to reach the ground, thus limiting both regeneration and recruitment to the canopy of shade intolerant species (Schuler, 2004; Vickers et al., 2014). The uniform shelterwood is a traditional method of obtaining natural regeneration in temperate hardwood forests. Treatments gradually

Corresponding author. E-mail address: [email protected] (J. Wikle).

https://doi.org/10.1016/j.foreco.2019.117650 Received 19 July 2019; Received in revised form 17 September 2019; Accepted 21 September 2019 0378-1127/ © 2019 Elsevier B.V. All rights reserved.

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centuries followed by land abandonment in the 19th century resulting in regional trends of even-aged forests (Abrams, 1992; Cogbill et al., 2002; Foster, 1992; Hall et al., 2002; Oliver, 1980). Current young forest stands are intentionally regenerated from the second-growth hardwood forests and after smaller scale disturbances (e.g. tornadoes, windstorms, logging). As succession and management proceed in forests of Eastern North America it is important to examine and compare these patterns and processes of stand dynamics in managed third-growth forests, as little to no work has yet examined these patterns. To examine third growth forests and the effects of legacy trees on regeneration we studied 34 irregular shelterwoods implemented to regenerate oaks that span a developmental time period from of 1 to 25 years since harvest and that represent a range of overstory structure. Specifically, we first identify how the regenerating third-growth stands change in structure and composition over time following harvest. Second, we examine whether more shade-tolerant or site-generalist, but less desirable timber species such as red maple (Acer rubrum L.) and black birch (Betula lenta L.) outcompete oak when individual canopy trees are retained. We base our species selection for comparisons on the fact that these three species are the dominant trees in the forest for our study area (Oliver, 1978; Ward et al., 1999) and are truly representative of a widespread forest region of northeastern North America (Eyre, 1980).

open up the forest canopy, affording shaded or sheltered growing conditions during the early stages of stand development, then with later tree removal, providing more sunlight to regenerating stands (Ashton and Kelty, 2018). Shelterwoods are intended to facilitate establishment of advance regeneration from a nearby seed source and provide adequate sunlight for its release and growth, while also limiting growth of some competing species (Brose, 2011; Crow, 1988; Hannah, 1987; Larsen and Johnson, 1998). However, after regeneration is established no legacy tree canopy remains for long-term structure (Ashton and Kelty, 2018). Oak species are of critical economic (Hanewinkel et al., 2013) and ecological value (McShea et al., 2007) to temperate hardwood forests. Given changes in climate, deer population, and land use disturbance history (Abrams, 2003; McDonald et al., 2002) and impacts of insects and disease (Brasier, 1996; Thomas et al., 2002) oak is declining as a component of future forests across eastern North American and Eurasia. Regeneration of oak in temperate hardwood forests can be difficult (Abrams and Nowacki, 1992; Crow, 1988; Loftis, 2004). Challenges include a decrease in large forest disturbances such as fire or land clearing since the establishment of the current second-growth oakhardwood forests (Moser et al., 1996; Nowacki and Abrams, 2008), increased deer populations (Kittredge and Ashton, 1995, 1990; McEwan et al., 2011) and past selective timber harvesting of canopy oak trees that eliminates the seed source and limits light to the forest floor (Schuler, 2004; Ward, 2007). Because of these challenges, uniform shelterwoods are commonly applied to oak-hardwood forests because the preparation and establishment treatments favor the build-up and development of oak (Quercus spp.) seedlings and their root development in the understory, before the complete removal of the canopy. The removal of the overstory after significant root development makes oak competitive in growth with both more shade-tolerant (e.g. Acer spp.) and intolerant hardwoods (e.g. Betula spp.) (Brose and Van Lear, 1998; Hannah, 1987; Loftis, 2004). There are caveats to this: studies the southern Appalachians found that the most successful oak regeneration begins as advance growth that exists in the forest understory, even before regeneration treatments start, and the preparation and establishment treatments of a uniform shelterwood are intended to release this regeneration (Loftis, 2004, 1990a, 1990b). In southern New England, oak regeneration and recruitment is strongly episodic, and not every masting event results in accumulation of successful advance regeneration in the understory (Frey et al., 2007). The oak that establishes as advance growth under the second-growth forest overstory is sparse and a true establishment cut is generally needed to achieve higher levels of oak regeneration (Frey et al., 2007; Smith and Ashton, 1993). Irregular shelterwoods offer an alternative to traditional shelterwood methods in that trees from multiple age classes are retained longterm (Ashton and Kelty, 2018). Retention of these trees has the potential to provide the light conditions necessary to meet regeneration goals while still leaving a stand that is varied in structure and composition. (Rosenvald and Lõhmus, 2008). Previous research has examined the use of irregular shelterwoods, primarily in shade-tolerant conifer and northern hardwoods of eastern Canada and Central Europe (Klopcic and Boncina, 2012; Raymond et al., 2009; Raymond and Bédard, 2017; Suffice et al., 2015), but no study to date has explored the application of irregular shelterwoods in oak-hardwood forests. In particular, the effects of the spacing, size, and species composition of legacy trees left standing post-harvest on the regenerating stand are not well understood. Further, most of the foundational work on how we understand regeneration and stand development in this system is based on studies of second-growth forests which originated after the removal of old-field pine from timber harvests or hurricanes in the early 1900’s (O’Hara, 1986; Oliver, 1978; Oliver and Larson, 1996; Oliver and Stephens, 1977). This phenomenon is relevant across much of eastern North America given forest clearance for settlement in the 17th and 18th

2. Methods 2.1. Study area and site description We conducted this research at Yale-Myers Forest in northeastern Connecticut, a 3213-ha research and demonstration forest. The forest consists of ridge and valley topography that ranges from 170 to 300 m above sea level. Average temperatures are 22.2° C in July and −3.3 °C in January with an average annual precipitation of 1206 mm y−1 (NOAA, 2019). The forest is located in southern New England; as a forest type it is in the transition zone between northern hardwoods and oak-hickory forest (Fralish, 2003). Two thirds of the forest area are actively managed for timber production. Since 1992, one or more stands of approximately 6–8 ha have been regenerated annually using irregular shelterwood systems. We used harvest records of Yale-Myers Forest to determine which stands had received an irregular shelterwood treatment and sampled all 34 shelterwoods that had occurred since 1992 (Fig. 1). To maintain the same site classification and productivity almost all the shelterwood sites were on similar glacial soils that would be considered ablation till of medium depth to bedrock, medium- to well-drained loams (Brookfield, Chatfield, Charlton) (NRCS, 2003). Only four shelterwood sites were on slightly different and would be considered to be of basal till origin (Woodbridge, Paxton-Montauk) (NRCS, 2003). These soils are considered in the middle range for site productivity for this region of southern New England. The original forest was cleared primarily for hayfields or grazing pastures. 2.2. Experimental design In each of the 34 shelterwoods, we randomly selected a point at least 50 m from any harvest boundary, to minimize edge effect. This point served as plot center for a 50-meter radius plot (0.785 ha) where we recorded the species and diameter at breast height (dbh = 1.37 m) for all trees with dbh > 15 cm. These trees comprised the original forest trees that remained after the regeneration harvest and represent the total legacy overstory. To further quantify the large structural legacies left behind in irregular shelterwoods we classified the large legacy overstory as trees with dbh > 40 cm. Our rationale for defining the bigger trees separately is related to the fact that they comprise the majority of the basal area left in the stand, serve as the major seed source, represent the largest and most important structural element for wildlife habitat, and avoid inclusion of large ingrowth in the older 2

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Fig. 1. Study Location. Map of the 34 shelterwoods used in the study, labeled by year the stand was regenerated. The Yale-Myers Forest is located in northeast Connecticut, in the northeast region of the United States.

(25 m2). In each sapling plot we measured dbh and height using a telescoping range pole for all tree species taller than 1.3 m and with dbh ≤ 15 cm. We gathered overstory data in the summers of 2015 and 2017 and seedling and sapling data in the summer of 2017.

harvests. To record regeneration, we installed three transects from plot center; one to the north, one to the southeast, and one to the southwest (Fig. 2). Each transect had four subplots; the first subplot was located 15 m from plot center, then the remaining subplots were spaced ten meters apart. If the furthest plot on a transect was close to a harvest boundary we included this effect as part of the shelterwood overstory treatment as it is representative of small stand sizes that occur across southern New England and the range of oak-hardwood forests due to undulating topography and small parcel size of many private landholdings. At all 12 subplots, we measured the diameter at root collar, and height (from ground to terminal bud) of seedlings in a 1.13-meter radius plot (4 m2). For our purposes, a seedling was defined as any tree species with a height of less than 1.3 m. At the first and third subplot on each transect, we measured a sapling plot with a radius of 2.82 m

2.3. Data analysis We used DBH to calculate basal area (BA) per tree (BA = πr 2) and then scaled and converted both overstory and regeneration data to values per hectare (overstory to m2/ha, and regeneration to stems per hectare). Large legacy overstory basal area for trees > 40 cm DBH ranged from 1.3 to 16.1 m2/ha (see Table S1 for more details on legacy overstory basal area). We calculated regeneration density by converting seedling and sapling counts to stems per hectare and combining for each plot. We calculated average height by taking the mean height of all 3

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velutina Lam.) (Oliver, 1978). To examine regeneration through early stand development, we constructed generalized linear models (GLM) with regeneration density (stems/ha) of all stems ≥ 15 cm tall as a response variable and time since harvest as a predictor. Any seedling stems < 15 cm we deemed to be not established. Past studies on regeneration recruitment and establishment support this assumption (Frey et al., 2007; Kittredge and Ashton, 1990; Smith and Ashton, 1993). We ran GLMs for all regeneration together and for each of the three target species individually. Tests with the AER package (Kleiber and Zeileis, 2008) revealed the data to be overdispersed, so we used negative binomial distributions for regeneration GLMs, using the MASS package (Venables and Ripley, 2002) and rsq package (Zhang, 2018) to extract r-squared values. We used the lm.beta package (Behrendt, 2014) to calculate standardized coefficients for regressions. To examine how height growth changes over time, we constructed simple linear regression models with mean height of the most competitive individuals 0.5 m and taller for each species and all stems as response variables and age as a predictor. We used the plot() function in R to test model assumptions of linearity, and normality and homoscedasticity of residuals. We considered results statistically significant at p < 0.05. To assess effects of legacy overstory on growth, we used simple linear regression with legacy basal area as the predictor variable and a response variable of AGR (height/age), again of the best performing trees for each species per subplot. In addition, we tested for potential confounding effect on regeneration AGR and stand density from possible changes in amount of overstory basal area retained with stand age, with either more or less being held over time. We found that there was a wide variation in amount of legacy overstory held but that this was consistent through time with no confounding effects.

Fig. 2. Study Design: Overstory plot is 50 m in radius. Each transect has 12 seedling (10 cm to 103 cm height) plots with 1.13 m radius (4 m2). The first and third plots on each transect were also measured as sapling (0.1 cm dbh to 15 cm dbh) plots with a 2.82 m radius (25 m2).

regenerating stems, weighted by plot size. In these stands, it can be common both to have no regeneration before harvest, or to have persistent scattered advance regeneration of varying age comprising seedlings less than 30 cm tall and often less than 20 cm (Frey et al., 2007; Liptzin and Ashton, 1999; Smith and Ashton, 1993). In either case, substantial height increase occurs only after harvest. We therefore calculated annual growth rate (AGR) as height/time since regeneration harvest even though some regeneration was present prior to the regeneration harvest. We did this on both all regeneration and also for the most competitive individuals, which we designated as the two tallest trees for each species in each subplot. These tallest individuals are the best performers, and the most likely long-term survivors, and therefore more logically predict the future growth and development of the stand (Oliver and Larson, 1996). As self-thinning occurs as stands develop, the smaller trees succumb to competition while the largest tend to succeed into the canopy (Hunt, 1982; Westoby, 1984). We used R version 3.5 (R Core Team, 2018) to examine relationships between regeneration, legacy canopy, and time since harvest. We focused our analyses on the three most common regenerating hardwood species representative of the stand dynamic for this forest type: red maple, black birch, and red/black oak (Quercus rubra L.)/(Quercus

3. Results 3.1. Regeneration in early stand development Self-thinning among all stems took place as the stand matured, with total regeneration reducing at a steady rate (ß, B (SE) = −0.467, −1236 (433); r2 = 0.22, p = 0.005). Based on regressions the three species showed different thinning patterns; black birch density diminished through time from estimated means of over 25,000 stems/ha soon after harvest to less than 2500 stems/ha after twenty-five years; while the other two species exhibited no statistically significant trends (Fig. 3). In the first five years following regeneration harvest, black birch was present at more than double the density of either of the other two species; variations were 10,000–47,000 for birch stems/ha as compared to 200–18,000 for oak or maple stems/ha. After 20 or more years following harvest, all three species were present at similar

Fig. 3. Changes in density of red maple, black birch, and red oak through time. Black line indicates significant relationship. Black birch density reduced significantly through time (GLM, B, ß (SE) −0.09095, −0.00005923 (0.024: r2 = 0.16, p = 0.0001). Both red oak and red maple show no significant trends (r2 = 0.01 and 0.03, respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4

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Fig. 4. Top Panels A, B, and C show the average height of most competitive individuals: Red maple (ß, B (SE) = 0.652, 0.409 (0.095); r2 = 0.426, p = 0.0003); black birch, (ß, B (SE) = 0.824, 0.414 (0.051); r2 = 0.680. p = 0.0000); red oak (ß, B (SE) = 0.838, 0.348 (0.051); r2 = 0.701, p = 0.0001). Panels D, E, and F show average height of all stems of each species by plot. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

legacy canopy increased (ß, B (SE) = −0.016, −0.001 (0.009); r2 = 0.0003, p = 0.929).

estimated mean densities based on regressions; varying between about 5000 and 11,000 stems/ha. 3.2. Changes in height through time

4. Discussion Overall, regenerating stems increased in height with time in a linear fashion (ß, B (SE) = 0.777, 0.109 (0.016); r2 = 0.6, p = less than 0.001). For the most competitive individuals in each species, black birch grew at a faster rate than red oak or red maple (Fig. 4); regression equations predict black birch achieving heights of 10.8 m, red maple 9.7 m, and red oak 7.7 m by 25 years after regeneration harvest. When we examined all stems, plot data for each species showed a different growth pattern, with only black birch increasing in height in a linear fashion. Red oak plot data showed average height stayed below one meter for the first 20 years, before showing a sharp increase in height. Red maple height diverged, with some harvests showing steady height increase, while others remained below a meter in height (Fig. 4).

In this study we shed light on how the third generation of temperate hardwood forests are developing following timber harvests of second growth. In particular we have focused on oak given its significance to the ecology and economics of these forests. We also highlight specific effects of legacy forest structure on oak regeneration annual to the other dominant hardwood tree species following harvest. Our results show post-harvest tree species dominance is still shifting after twenty-five years. However, it is still too early to determine if third-growth stands in New England will differ in species composition from oak-dominated second-growth stands. We affirm that in regenerating stands there are noticeable differences across species, particularly regarding height growth; and we suggest there are differences in establishment and growth of oak between second and third generation forests. Lastly, and most importantly, we find that increasing the retained legacy of the original overstory trees has a negative effect on the growth rate of red oak, a negligible effect on black birch, and benefits growth of red maple. Our findings demonstrate that legacy overstory trees affect the growth rate of each target species (red maple, black birch, and red oak) differently; increasing legacy overstory limits oak growth more than the other species studied, shifting the competitive edge toward birch and red maple.

3.3. Legacy effects on regeneration Each of the three species exhibited different responses to amount of legacy overstory (Fig. 5). Red oak regeneration showed growth limitation with increase in overstory basal area (ß, B (SE) = −0.405, −0.022 (0.011); r2 = 0.164, p = 0.062), and we found no red oak stems taller than 0.5 m in our plots when legacy canopy increased above 10 m2/ha. Red maple annual height growth increased with increasing legacy canopy (ß, B (SE) = 0.491, 0.031 (0.110); r2 = 0.241, p = 0.009). Black birch exhibited no changes in annual growth rate as 5

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Fig. 5. Annual height growth (height/age) of two tallest stems of each species in each plot in response in increasing legacy basal area. Red maple (ß, B (SE) = 0.49077, 0.03108 (0.1103); r2 = 0.2409, p = 0.00934), black birch (ß, B (SE) = −0.016078, −0.000811 (0.009059); r2 = 0.000258, p = 0.929), red oak (ß, B (SE) = −0.40482, −0.02234 (0.01128); r2 = 0.1639, p = 0.0616. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

low for the first 20 years of growth, followed by a steep increase in height growth (Fig. 4). However, when we narrowed our analysis to only the tallest two stems in each plot, red oak height was closer to that of the other two species although still shorter by 2–3 m. These tallest two stems are those most likely to recruit into the canopy, as codominant and intermediate oaks that are not released often die before reaching canopy level (Oliver and Larson, 1996; Ward and Stephens, 1994, 1993) and most oaks that make it to the dominant canopy position have achieved that status early in life (Oliver, 1978; Zenner et al., 2012). While these stems are the most likely to reach the canopy, they have not yet achieved a dominant canopy position, and may not without intervention such as crop tree release treatments (Ward, 2008; Ward and Stephens, 1994). Red maple persisted in forest stands regardless of canopy position and stand age. This success may be partially attributable to red maple’s ability to stump sprout as well as establish as advance regeneration from seedling origin (Beck and Hooper, 1986). Conversely, the height growth of black birch increased at a steady rate through time as density reduced drastically, indicating that the tallest black birch saplings are surviving, and the remaining stems are succumbing to competition. Black birch does not stump sprout prolifically, if at all, as compared to red maple, and when it does it produces fewer slower growing stems (Keyser and Loftis, 2013). However, it does regenerate prolifically from seed both on exposed mineral soils after canopy disturbances (Carlton and Bazzaz, 1998; Smith and Ashton, 1993), and as advance regeneration in small gaps after partial canopy disturbance on the organic surface (Orwig and Foster, 1998), both of which are conditions that occur preceding and following irregular shelterwood harvests that lead to its early dominance. At year twenty oak had parity in numbers with black birch and red maple but was still shorter in height than the other two species by 2–3 m. Taken together, this suggests that there is still potential for a significant oak component in the future canopy, although the height discrepancy may be cause for concern. Regenerating oak stands without fire on the landscape, and with larger deer populations has led to an increase in abundance of red maple (Abrams, 1998; Alderman et al., 2005; Hutchinson et al., 2008) and black birch (Kittredge and Ashton, 1990, 1995) that puts oak at a competitive disadvantage.

4.1. Abundance and height in early stand development Although there is high density of black birch and red maple stems in young stands, we found that by 20 years post-harvest, numbers of black birch, red oak, and red maple were comparable. Because of this, oak accounts for a higher proportion of total stems per hectare in older stands, so stands that appear to be dominated by black birch at a young age can still retain a significant oak component. Past studies of forest development in southern New England for second-growth have shown similar differential rates of self-thinning; oak tends to persist through time while the competing black birch and red maple are abundant soon after harvest but then self-thin dramatically (Hibbs, 1983; Oliver, 1978; Ward et al., 1999). Based on abundance alone, these similarities between development of second-growth forests and early development of third-growth forests indicate that these forests may be on track to develop into oak dominated canopies. Despite significant shifts in abundance through early stand development, the height discrepancy between the tallest stems of black birch, red maple, and red oak was consistent through 25 years post-harvest, meaning species growth rates did not change. After 25 years, oak saplings are still growing in the shade of black birch and red maple stems as well as the legacy canopy, which appears to be a limiting factor in its ability to ascend to the canopy. This is consistent with other studies comparing oak establishment in competition with faster growing more moisture-loving species such as tulip poplar in the Central and Southern Appalachians (Brose et al., 1999; Brose and Van Lear, 1998; Loftis, 1990a, 1990b). Conversely, stem analysis reconstruction of mature second-growth oak stands in southern New England, including some in close proximity to our study sites, derived beneath old field pine with no legacy structure (Oliver 1978), found black birch and red maple overtopping the tallest oaks before 25 years of stand development, with oaks successfully ascending to the canopy as black birch and maple dropped back. Other studies have shown that red oak of third-growth forest stands does not ascend into the canopy until 30 years or longer following regeneration (Liptzin and Ashton, 1999; Zenner et al., 2012). Without the use of fire (Brose and Van Lear, 1998) or herbicides (Loftis, 1990a, 1990b) to promote oak from other hardwood competition during early phases of stand development, it is still difficult to predict oak's future. And the use of fire and herbicides, for different reasons, are socially, legally and logistically difficult to apply across an increasingly fragmented and urbanized region (Ashton and Kelty, 2018). These conflicting snapshots of early stand development therefore highlight a need for more long-term research on whether oaks can successfully represent the future canopy of the next generation forest. We show that when including all regenerating stems, oak followed a pattern of delayed height growth, with average stem height remaining

4.2. Legacy effects on regeneration Our results show that for red oak to compete successfully with red maple and black birch through early stand development, legacy structures should be retained judiciously, and as legacy basal area increases past 10 m2/ha, oak may not reach a height of 0.5 m or greater, even over 25 years. This finding directly conflicts with recent emphasis on 6

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Declaration of Competing Interest

partial canopy retention driven by demands for increased structural and age-class diversity within newly regenerating stands. Other studies in the bottomland hardwood of the Mississippi delta (Oliver et al., 2005), the southern Appalachians (Atwood et al., 2011), and the Ozark Mountains (Miller et al., 2006) have examined overstory retention in oak regeneration treatments. These studies have found, similarly, that overstory retention limits height growth of many species, but for oak more than others. In our study red oak was far more limited in annual height growth (AGR) for the best performers than red maple or black birch with increase in legacy overstory basal area (Fig. 5). Using AGR for individuals that had the best height growth demonstrated that black birch was insensitive to the increase in shade created by legacy trees; while red maple AGR increased with the increasing shade. Both are able to thrive under the same conditions that are limiting the growth of oak, giving them potentially an important competitive advantage in the long run. Black birch is classified as more shade tolerant but faster growing in both partial shade and high light conditions (Burns and Honkala, 1990) as compared to mid-tolerant red oak (Burns and Honkala, 1990; Schuler and Miller, 1995; Stringer and Loftis, 1999). Conversely, oak decreases in shade tolerance as it ages and rarely colonizes gaps successfully (Johnson et al., 2002). These responses also illustrate some of the limitations of shade tolerance classifications. Black birch is able to respond to crop tree release, with the best responses in the sapling and small pole size classes (Ward, 2007). This plasticity sets it up well to respond to the release created by shelterwood removal cuts. Red maple is able to both persist for long periods of time in the forest understory as advanced regeneration, and respond rapidly to release particularly from sprouts (Hart et al., 2012; Hibbs, 1982; Tift and Fajvan, 1999); these traits as well as fire suppression are contributing to its increasing presence in northeast forests (Abrams, 1998). Red maple and red oak both have been shown to decrease in incremental or periodic growth rate when overstory basal area increases (Atwood et al., 2011; Vickers et al., 2014), contrary to our findings, where red maple AGR was higher as overstory basal area increased. However, other studies on shade tolerance would suggest red maple to be as shade tolerant and as much of a generalist as black birch (Ashton et al., 1999; Krueger et al., 2009; Lorimer, 1984), which supports results from our study. Finally, overstory effects are not static; while the legacy canopy is diverse, oak is the dominant species, and these legacy oak trees, though large (> 40 cm dbh) are capable of responding to release by dramatically expanding their canopies up to 88 percent (Miller et al., 2006) following a shelterwood harvest. They will therefore provide increasing shade to the regenerating stand beneath over time, potentially disproportionately effecting oak regeneration growth over the more shade tolerant competition.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We would like to begin by acknowledging that this research took place on the traditional land of the Nipmuc and Mohegan Peoples. We acknowledge the support in logistics and infrastructure provided by the Yale School Forests and its staff. This research would not have been possible without funding from the Kohlberg Donohoe Research Fellowship, the Yale School of Forestry and Environmental Studies Summer Fund, and the Carpenter-Sperry Funds. We are grateful for assistance in the field from Chase Ammon, Romy Carpenter, Louis Evans, Juliana Hanle, Serena Lian, Gabrielle Marion, Gabriel Oltean, and Austin Dziki. We would like to thank the Ashton and Duguid Lab Groups for feedback, comments, and support throughout the course of this study. Lastly, we thank two anonymous reviewers who helped improve the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.foreco.2019.117639. References Abrams, M., 1992. Fire and the development of oak forests – in eastern North America, oak distribution reflects a variety of ecological paths and disturbance conditions. Bioscience 42, 346–353. https://doi.org/10.2307/1311781. Abrams, M.D., 2003. Where has all the white oak gone? Bioscience 53, 927–939. https:// doi.org/10.1641/0006-3568(2003)053[0927:WHATWO]2.0.CO;2. Abrams, M.D., 1998. The red maple paradox. Bioscience 48, 355–364. https://doi.org/10. 2307/1313374. Abrams, M.D., Nowacki, G.J., 1992. Historical variation in fire, oak recruitment, and postlogging accelerated succession in central Pennsylvania. Bull. Torrey Bot. Club 119, 19–28. https://doi.org/10.2307/2996916. Alderman, D.R., Bumgardner, M.S., Baumgras, J.E., 2005. An assessment of the red maple resource in the northeastern United States. North. J. Appl. For. 22, 181–189. https:// doi.org/10.1093/njaf/22.3.181. Ashton, M.S., Kelty, M.J., 2018. The Practice of Silviculture: Applied Forest Ecology. John Wiley & Sons. Ashton, P.M.S., Yoon, H.S., Thadani, R., Berlyn, G.P., 1999. Seedling leaf structure of New England maples (Acer) in relation to light environment. For. Sci. 45, 512–519. https://doi.org/10.1093/forestscience/45.4.512. Atwood, C.J., Fox, T.R., Loftis, D.L., 2011. Effects of various silvicultural systems on regeneration in mixed hardwood stands of the southern Appalachians. J. Sustain. For. 30, 419–440. https://doi.org/10.1080/10549811.2011.541020. Beck, D.E., Hooper, R.M., 1986. Development of a southern Appalachian hardwood stand after clearcutting. South. J. Appl. For. 10, 168–172. https://doi.org/10.1093/sjaf/10. 3.168. Behrendt, S., 2014. lm.beta: Add Standardized Regression Coefficients to lm-Objects. Brang, P., Spathelf, P., Larsen, J.B., Bauhus, J., Boncčìna, A., Chauvin, C., Drössler, L., García-Güemes, C., Heiri, C., Kerr, G., Lexer, M.J., Mason, B., Mohren, F., Mühlethaler, U., Nocentini, S., Svoboda, M., 2014. Suitability of close-to-nature silviculture for adapting temperate European forests to climate change. Forestry (London) 87, 492–503. https://doi.org/10.1093/forestry/cpu018. Brasier, C.M., 1996. Phytophthora cinnamomi and oak decline in southern Europe. Environmental constraints including climate change. Ann. For. Sci. 53, 347–358. https://doi.org/10.1051/forest:19960217. Brose, P., Van Lear, D., Cooper, R., 1999. Using shelterwood harvests and prescribed fire to regenerate oak stands on productive upland sites. For. Ecol. Manage. 113, 125–141. https://doi.org/10.1016/S0378-1127(98)00423-X. Brose, P.H., 2011. A comparison of the effects of different shelterwood harvest methods on the survival and growth of acorn-origin oak seedlings. Can. J. For. Res.-Rev. Can. Rech. For. 41, 2359–2374. https://doi.org/10.1139/X11-143. Brose, P.H., Van Lear, D.H., 1998. Responses of hardwood advance regeneration to seasonal prescribed fires in oak-dominated shelterwood stands. Can. J. For. Res. 28, 331–339. https://doi.org/10.1139/x97-218. Burns, R.M., Honkala, B.H., 1990. Silvics of North America: Volume 2. Hardwoods. United States Department of Agriculture (USDA), Forest Service, Agriculture Handbook 654. Carlton, G.C., Bazzaz, F.A., 1998. Regeneration of three sympatric birch species on experimental hurricane blowdown microsites. Ecol. Monogr. 68, 99–120. https://doi. org/10.2307/2657145.

4.3. Management implications Forest managers most importantly may be able to manipulate overstory conditions and legacy structure in favor of oak in irregular shelterwoods by keeping legacy basal area low. To do this only a few large trees per hectare can be left, or more numerous smaller trees can be grouped together to reduce the edge effect. Intermediate interventions such as cleaning release treatments (Ashton and Kelty, 2018) around competitive oak tree saplings may help close the height gap between oak and its competitors as well. Secondly, forest stands continue to experience shifts in species composition and canopy dominance many years after regeneration (Oliver and Larson 1996), and an examination of 25 years of growth, while valuable, is not enough to determine with confidence the future composition of third generation forests after regeneration harvest. Longer term monitoring is needed. Lastly, based on comparison with the literature on second growth forest development, oak in third generation forest lags in competitive ability compared with its species associates. 7

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