4.1. Immediate Windstorm Effects
The pattern of damage immediately following the 1974 windstorm at Dinsmore Woods was complex. After examining species-specific changes in the density in the different diameter classes and basal area, the pattern of damage seemed to be unrelated to specific tree size or species. For example,
Quercus spp. (oaks) which had a significant proportion of larger diameter (≥40 cm) trees prior to the storm appeared to be the species in the canopy most affected by the windstorm of 1974 with decreases in both density and basal area (
Table 1 and
Table 2;
Figure 5). In contrast to the oak species, the disturbance seemed to have had no immediate effect on
Fraxinus americana (white ash) and
Acer saccharum (sugar maple) in the canopy class (
Table 1 and
Table 2;
Figure 3 and
Figure 4). In 1973 however, the subcanopy density of sugar maple was higher than in the canopy. Immediately following the disturbance (1974), sugar maple density and basal area decreased due to the reductions in the number of individuals in both subcanopy diameter classes (10–19.9 and 20–29.9 cm) (
Table 3 and
Table 4;
Figure 3). White ash,
Celtis occidentalis (hackberry) and
Ulmus rubra (slippery elm) also had fewer individuals in the 20–29.9 cm diameter class immediately post-disturbance (
Figure 4,
Figure 7 and
Figure 8). The decreases in sugar maple, white ash, hackberry and slippery elm account for most of the overall decrease in subcanopy basal area post disturbance (
Table 4). Thus, our study indicated no clear relationship between tree size and immediate windstorm damage.
An explanation of the pattern we observed may be the variation in the intensity of the windstorm and the effects of trees being damaged by other trees toppling and breaking. Peterson [
10] compared two tornado events, that differed in intensity, on the same old growth forest and found differences in the extent of damage and recovery depended on the intensity of the disturbance. In the less intense windstorm event, Peterson [
10] found little change in either canopy species composition or diversity. Similarly, we found little change in diversity in the canopy and subcanopy immediately post-disturbance. In a study of two forests affected by windstorms, Webb [
26] indicated that indirect damage of trees by other falling trees may produce a pattern with no relationship to tree size. Canopy breakage and toppling may have contributed to the decline in the subcanopy of sugar maple and other species immediately post-disturbance at Dinsmore Woods. In a simulated hurricane blowdown on a temperate forest, it was found that larger overstory trees were uprooted whereas subcanopy and understory trees were crushed by falling trees [
27]. Canham et al. [
28] noted that moderate winds resulting from F0-F1 tornados may result in differences in the proportion of trees that are blown down. Although the tornado that affected Dinsmore Woods was rated an F5 tornado, the distance of the forest from the direct path of the tornado may have resulted in the forest being subjected to wind speeds that were variable and moderate in intensity. Additionally, other factors such as site topography may have further influenced the pattern of damage [
13,
14].
4.2. Delayed Windstorm Effects, Recovery and Regeneration
After the 1974 windstorm and the resurvey of the forest immediately following the windstorm, damaged and fallen trees were salvaged along the perimeter of the forest, but not from the interior where subsequent surveys were conducted [
17]. Also, no evidence of sprouting or deer browsing was observed during the course of the surveys. The residual or delayed effects of the storm were evident in the 1985 and 1994 surveys with declines in canopy and subcanopy total density and basal area (
Table 1,
Table 2,
Table 3 and
Table 4). These longer-term effects of the storm have had an impact on the recovery and regeneration of the forest. Held and Bryant [
17] noted trees that had been uprooted and damaged by the windstorm had generated single tree and multiple tree gaps and that many of the wind- damaged trees that remained standing had died and fallen in the period between 1974 and 1985. Based on the 1985 and 1994 surveys, the number of oak trees increased in the subcanopy diameter classes which was also reflected in small increases in oak basal area (
Table 4 and
Figure 5). The transition of oak saplings to smaller diameter oak trees of the subcanopy may have been a response to the continued formation of canopy openings/gaps post-disturbance and, in part, this transition may account for the disappearance of oak saplings after 1974. Ehrenfeld [
29] found evidence of slight increases in the number of oak saplings as well as lower canopy oak trees in response to canopy gaps. Similarly, Thomas-Van Gundy et al. [
30] reported increases in oak sapling importance values in response to canopy gaps and exclusion of browsers although the importance values overall remained low for oak spp. Oak in the understory can undergo periods of suppression [
31]. In a detailed study of saplings and understory trees in a mixed oak forest that had experienced a windthrow disturbance, Orwig and Abrams [
31] found that following the disturbance, all species including white oak increased in height in gaps as compared to control sites. Additionally, white oak saplings and small trees showed increases in radial growth in response to recent gap formation [
31]. The positive response (increased radial growth) of oaks to gaps or crown release may also explain the subsequent increase in the number of larger diameter trees in 1985 and 1994 in the canopy (
Figure 5) [
32,
33].
In terms of the subcanopy, white ash continued to decline in number post-disturbance, but showed some evidence of recovery in 1994 with slight increases in both density and basal area (
Table 3 and
Table 4). Some of this recovery may be due to saplings moving into the subcanopy and, like oak, may partially account for the disappearance of saplings in the 1994 and subsequent surveys. Canopy level ash trees continued to decline in density and basal area over a 20-year period following the windstorm (
Table 1 and
Table 2;
Figure 4). These declines are consistent with wind-damaged trees dying and falling. However, recent increases in 2004 and 2014 in density and basal area of white ash exceed pre-disturbance levels resulting in greater importance of white ash in the canopy compared to oak spp. and sugar maple (
Figure 6). Merrens and Peart [
34] found that ash saplings and smaller ash trees in a site that was damaged by a 1938 hurricane continued to maintain increased growth rates (radial growth) relative to a control site nearly 45 years after the disturbance. Furthermore, the increased growth rate exhibited by ash was significantly greater than other canopy species present at the site and, because of this sustained growth response, the hurricane disturbance seemed to favor ash compared to other species [
34]. Over the 40-year period at Dinsmore Woods, the continued growth of white ash has resulted in a shift from mostly smaller diameter trees (<40 cm diameter
) to larger (≥40 cm diameter) trees in the canopy (
Figure 4).
Sugar maple has maintained a continual ingrowth of trees into the subcanopy over the 40-year period of monitoring. Immediately post-disturbance, there was a decrease in the number of sugar maple stems in the subcanopy, but by 1985 the number of these smaller diameter trees had reached pre-disturbance levels (
Figure 3). The effect of gap openings as a result of windstorm damage may explain the increase of smaller diameter trees of sugar maple between 1974 and 1985. Sugar maple saplings show increased height and radial growth in response to canopy gaps [
35,
36,
37]. In contrast, the transition to the larger diameter classes of the canopy has been inconsistent for this species and may further reflect its adaptation to shade. Sugar maple can undergo long periods of growth suppression followed by release in response to canopy openings [
38] or thinning [
39]. Abrams and Scott [
40] found that sugar maple growth was slower in an old growth forest compared to forests that had been logged. The continued presence and dominance of large white ash and oak trees in the canopy at Dinsmore Woods may be restricting growth of sugar maple into the canopy.
At Dinsmore Woods, prior to the 1974 disturbance, hackberry and slippery elm were important components of the subcanopy. The density and basal area of hackberry was reduced post-disturbance showing some recovery by 2014, but not to pre-disturbance levels (
Table 3 and
Table 4). In contrast to hackberry, slippery elm showed little change in density and basal area between 1973 and 1974. However, since 1994 slippery elm has declined in importance in the subcanopy and was not recorded in 2014 (
Figure 8). Although both species are fast growing, slippery elm saplings may be more dependent upon larger canopy gaps for growth and regeneration than hackberry [
41].
In summary, over the 40-year recovery period, the canopy remains an Oak-Ash-Maple system. However, there has been shifts between the three species with fewer but larger diameter oaks and an increase in importance of white ash while sugar maple in the canopy is composed of smaller diameter trees when compared to the other two species. In contrast, sugar maple has increased in importance in the subcanopy not by an actual increase in abundance, but due to the decline in abundance and basal area of the other major species of the subcanopy: oak, white ash, hackberry and slippery elm. This change is reflected in the decline in subcanopy diversity.
The continued decline in the diversity of the subcanopy as well as the sapling class at Dinsmore Woods suggests that the longer-term significant effect of the 1974 disturbance was on the forest understory. The change in the understory post-disturbance appears to be affecting forest regeneration, specifically the transition of seedlings to the sapling class and saplings to the subcanopy. Various studies and reviews have emphasized that the understory may have a significant, but not completely understood effect on forest dynamics and resilience [
3,
20,
42].
A major consequence of the 1974 windstorm at Dinsmore Woods may have been to help establish by 1985 a dense understory layer consisting of
Asimina triloba (pawpaw) and
Lindera benzoin (spicebush) (
Table 5 and
Table 6). Spicebush and pawpaw are clonal species and shade tolerant. Both species, however, are also responsive to openings in the canopy that may enhance their productivity, growth and reproduction [
43,
44,
45]. Hosaka et al. [
46] suggested that the growth habit of pawpaw may take advantage of temporal variations in light. Canopy and subcanopy gaps formed after the windstorm may have also enhanced sexual reproduction in both species resulting in fruits and seeds that were dispersed throughout the forest by animals, which contributed to the further spread of these species. Both of these species may affect regeneration of tree species by competing for resources. Shotola et al. [
47] reported large increases in the density of pawpaw over a 17-year period in an old growth forest in Indiana and this could affect tree regeneration. The cause of the pawpaw expansion in this Indiana forest may have been due to insect defoliation of the canopy. Similarly, in an eight-year study of an old growth
Fagus-Acer forest in Ohio, substantial increases in density of
Asimina and
Lindera were found and it was speculated that both species may negatively affect other woody species regeneration in the forest [
42]. A recent study reported on an effort to control pawpaw growth in second growth hardwood stands in Missouri because of its potential negative effect on tree regeneration [
48]. Baumer and Runkle [
49] found that tree seedling stems (including sugar maple and white ash) not under pawpaw were taller and older compared to stems under pawpaw. As part of this study, Baumer and Runkle [
49] did a manipulative experiment to explore the nature of the interactions between pawpaw and sugar maple seedlings. Their results indicated combined aboveground and belowground competition seemed to negatively affect seedling biomass. The negative impact on seedling growth by pawpaw and possibly spicebush may affect the seedling to sapling transition and partially explain the decline over time in the densities of white ash, oak spp. slippery elm and hackberry saplings at Dinsmore Woods.
Further confounding the effects of pawpaw and spicebush on tree regeneration was the increase in abundance of sugar maple saplings in 1985 and 1994. Shading by sugar maple saplings may also have affected the transition from seedling to sapling of other species such as oak and white ash [
42,
47,
50]. Canham [
35] found that lateral growth of branches of sugar maple saplings was increased in response to light gaps which would benefit the plant. However, Canham [
35] further indicated that the more important effect of this response may be the shading of smaller less shade tolerant tree species after gap formation thereby reducing competition.
Since 1994, two other factors may be affecting tree regeneration at Dinsmore Woods. In the 1994 survey we observed the presence of
Alliaria petiolata (garlic mustard) in the forest. However, at that time it was not as extensively present as it was observed in the 2014 survey (Personal Observation). Beginning in 1995, a study was conducted at Dinsmore Woods to determine the effects of prescribed burns on garlic mustard and the understory [
51]. A series of burns were completed over a 3-year period. The prescribed or controlled burns in that study consisted of a small number of plots localized within the forest and did not overlap with plots in our subsequent surveys. The results indicated the repeated prescribed burns had no effect on garlic mustard abundance but did have a negative effect on woody seedlings (≤5 cm diameter) on upland sites within the forest [
51]. The persistence of garlic mustard in the forest may be contributing to further decline in woody seedling richness and sapling diversity. In a short-term study, Stinson et al. [
52] found that woody seedling abundance and Shannon diversity decreased with increasing garlic mustard cover in a New England forest. Among the tree species negatively affected by garlic mustard were sugar maple and white ash. Stinson et al. [
52] further suggested that the species-specific response to garlic mustard removal may be due to plant–mycorrhizae interactions and/or shading. In an earlier study, Stinson et al. [
53] experimentally demonstrated that garlic mustard negatively affected arbuscular mycorrhizal fungi (AMF) colonization of sugar maple and white ash seedlings and thereby affected seedling growth. In a subsequent field study, AMF colonization of sugar maple seedlings was reduced in forest patches invaded by garlic mustard [
54].
Finally, in the 2014 survey,
Lonicera maackii (Amur honeysuckle) was in significant abundance in the understory at Dinsmore Woods (
Table 5 and
Table 6). This invasive species can negatively affect tree seedling density and species richness over time [
55,
56]. Recently, Cameron et al. [
57] conducted a study examining the effects of
Lonicera and other environmental factors on sugar maple regeneration in forests of Southwestern Ohio including sites that are near Dinsmore Woods. Their results indicated that the effect of
Lonicera depended on the life stage of sugar maple. Just as important, this study showed that other environmental factors such as soil and topography may be affecting seedling to sapling transition and sapling to tree transition in sugar maple, factors which also may be affecting tree regeneration in general at Dinsmore Woods.