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Are There Significant Differences in Forest Resources On and Off Appalachian Trail Lands?

Figure 1. Appalachian National Scenic Trail showing HUC10 shell and area of focus (dark blue).
Figure 1. Appalachian National Scenic Trail showing HUC10 shell and area of focus (dark blue).

Background

The Appalachian National Scenic Trail (APPA) traverses mostly through temperate deciduous forest. But the trail also cuts through a handful of less common habitats such as grassy and heath balds, mountain bogs, cliffs and rock outcrops, talus slopes, and beech gaps. This resource review focuses on forest health, with a particular focus on how forests that occupy APPA land compares to forests on outside land. Our overall goal with respect to forest health is to assess the current status of forest resources along the Appalachian Trail using freely available data, and to create a process that will promote monitoring of those resources. The portion of the APPA that is the subject of this review is shown in Figure 1.

Lands owned and managed by the National Park Service (NPS) as part of the APPA measure approximately 270,000 acres, making APPA one of the largest parks in the east. In fact, a recent analysis by APPA staff found that the exterior boundary length (perimeter) of all lands that are Federally owned and administered by APPA is second only to Wrangell-St. Elias National Park in Alaska (the single largest NPS unit with a land area that is approximately the same as the combined size of Connecticut and Massachusetts). This long but narrow ribbon on the landscape helps to protect significant resources, but also exposes the APPA to potential threats along its entire length.

Worldwide, temperate deciduous forests have been highly altered and possess the highest index of human disturbance of any major biome (Hannah et al. 1995) as well as high fragmentation indices (Ritters et al. 2000). The eastern United States and the area surrounding the APPA is no exception, where temperate deciduous forests have been heavily used for timber, cleared for agriculture, or converted into towns and cities. In some locations, the APPA corridor may be the only remaining segment of protected land on the landscape, so while the corridor itself is protected from development, land use change and habitat fragmentation continue to affect the forested land surrounding the corridor. Other key concerns for forests in the Appalachian region include the impact of invasive exotic species, atmospheric deposition and ozone pollution, climate change, and visitor impacts (Dieffenbach 2011).

The statement above supports the commonly held notion that resources associated with the APPA are in better ecological health than resources on lands surrounding it. While the logic behind that belief is understandable, it is untested. Though there have been no attempts to show that APPA lands are generally healthier than surrounding lands, a recent study by Miller et. al. (2016) found that the condition of forests in eastern and mid-western parks are generally better at supporting diverse ecosystem services than similar forests that surround the parks. Multiple years of park forest health monitoring data were compared to Forest Inventory Analysis Program (FIA) data available from the U.S. Forest Service (O'Connell et. al. 2016). The current resource review is based on the approach used by Miller et. al. (2016), except that only publicly available FIA data from the Southern Blue Ridge Mountain subsection were used. A subsection is an area "…with similar surficial geology, lithology, geomorphic process, soil groups, subregional climate, and potential natural communities. Subsection boundaries usually correspond with discrete changes in geomorphology…" (ECOMAP 1993).

Data used in this analysis were further constrained to mid-sized hydrologic units that intersect the APPA land area. We call this the HUC10 shell (Figure 1). It is based on hydrologic units defined by the USGS at the fifth level of the Hydrologic Unit Code (HUC) system, with each being given a discrete 10-digit code (HUC10). For a complete description of the HUC10 shell, visit: http://science.nature.nps.gov/im/units/netn/reports/Resource Briefs/APPA_HUC10_Shell.cfm.

Methods

We analyzed recent data from the USFS FIA program, the nation's forest census. The FIA program collects and reports data on status and trends in forest area and location; the species, size, and health of trees; total tree growth and mortality; and forest land ownership. Data are collected from permanent plots at a scale of about 1 plot per 6,000 acres (2,400 ha) in Trail states. A subset of plots are analyzed for additional forest health variables, such as soil chemistry, vegetation structure and woody debris -- these data are collected on about 1 plot per 96,000 acres (39,000 ha).

Figure 2. Appalachian National Scenic Trail land area distribution within the HUC10 shell among subsections.

For the current review we identified all plots located on APPA owned lands using a Geographic Information System (GIS). Those plots were tagged and then analyzed using a series of queries developed by the Northeast Temperate Network to analyze FIA data. All plots originated from within the HUC10 shell and the Southern Blue Ridge Mountain subsection. We then compared data that originate from within the APPA land area to data from the nearby surrounding land area.

After reviewing the available FIA data, the Southern Blue Ridge Mountain (M221Dc) subsection (Figure 1) had sufficient data to support a comparison of forest dynamics on and near APPA. The number of plots per subsection inside the entire APPA land area ranged from 0 to 39, while only 6 subsections had 10 or more plots. By comparison, the aforementioned 6 subsections where 10 or more plots were found contained 204 to 574 plots when the spatial area was expanded to the HUC10 shell. Consequently, the comparison between forest resources on APPA land to nearby resources, based on FIA data, is restricted to just the Southern Blue Ridge Mountain subsection. Of all the subsections that intersect the APPA land area, the Southern Blue Ridge Mountain subsection (Figure 2) is the second largest, exceeded only by the Northern Blue Ridge Mountain subsection.

The FIA data used in this, as well as other NETN derived resource reviews, are spatially swapped and fuzzed. Swapping, as defined by the FIA program (O'Connell et. al. 2016), "…consists of exchanging the plot coordinates for a small number of similar plots within the same county. Swapping only occurs on private forested plots and depends on the region of the country. Between 0 and 10 percent of the forested plots are randomly selected for swapping with plots from the remaining data for a total swapping of between 0 and 25 percent. The primary criterion for swapping is based on a measure of ecological similarity. Plots with the smallest ecological difference are swapped. The variables for swapping (e.g., x and y coordinates, forest type group, and stand size) vary by region. This induces enough uncertainty as to the actual property owner to satisfy the legal requirements without introducing an unacceptable amount of error in the population estimates computed for analyses…" Fuzzing, also defined by the FIA program, "…consists of randomly relocating most plot latitude and longitude coordinates within one-half mile of their actual coordinates, with the remainder relocated up to 1 mile. This means that the actual plot location is generally masked within a 500-acre area…" As a result, a portion of the plots thought to be on APPA lands may not be in reality. Conversely, some plots thought to be off APPA lands may actually be on. Despite the deliberately introduced imprecision, the comparison of FIA data on APPA land to nearby land is still meaningful because the swapping and fuzzing process is random and designed to minimize differences that would render useless the type of analysis NETN has undertaken.

Figure 3. Appalachian National Scenic Trail land ownership distribution within the HUC10 shell and Southern Blue Ridge subsection.

Land Ownership

Within the Southern Blue Ridge subsection, nearly the entire APPA land area passes through lands administered by the U.S. Forest Service. This is not an unusual circumstance, as much of the APPA land area passes through other Federal lands, particularly in the south where several large National Forests are located. While the overall distribution of government versus privately owned lands throughout the subsection is probably close, two very small portions of the Southern Blue Ridge subsection APPA land area are not within a National Forest. These areas are not represented in Figure 3 because none of the randomly distributed FIA plots, where the landowner information is identified, are in those segments. Despite the known inaccuracy related to land ownership within the APPA land area, the overall conclusion is that within the Southern Blue Ridge subsection approximately 31% of lands are privately held versus 100% Federal ownership within the APPA land area, and that the APPA land area represents approximately 2.2% of the overall Southern Blue Ridge subsection within the HUC10 shell. With such a small portion of the Southern Blue Ridge subsection being protected, the ability of the APPA land area to effectively enhance the health of the region's forest resources is limited.

Species Composition

Tree species diversity is greater outside the APPA land area (85 species total) with "other species" accounting for 43% of observations compared to 19% inside the land area (26 total species; Figure 4). In general, however, the relative abundance of the ten most common tree species is similar on and outside the APPA land area (Figure 4). The four most common species (red maple, yellow poplar, chestnut oak, and sourwood) comprise 53% of all observations made on APPA Land, and 41% observed off APPA land.

(a) Within the limits of the Appalachian Trail land area.
(b) Outside the APPA land area but within the HUC10 Shell.
Figure 4. The 10 most common tree species (> 10 cm DBH) shown as a % of all individuals observed in the Southern Blue Ridge Mountain subsection.
Figure 5. Forest structure threshold ranges for assessing ecological health.

Forest Structure

Diverse forest structure helps to enhance biological diversity and sustain key ecosystem function. Mature and late successional stages are valued to be ecologically important because sensitive species are generally more dependent on their structural features than younger forests. Standing dead trees (snags) and coarse woody debris (CWD), discussed later, are important structural features of forest stands that provide habitat for wildlife and fungi.

Landscape-scale shifts in structural stage distributions of forests occur due to natural disturbances including fire and pest/pathogen outbreaks (Dale et al. 2001, Weed et al. 2013). Changes in disturbance regimes are a key reason for monitoring structural change over time. Along the APPA, even if forests are less impacted than the surrounding region, it is possible that an increasing frequency of disturbances in the surrounding area may have a disproportionately strong influence on future conditions.

(a) Within the limits of the Appalachian Trail land area.
(b) Within the HUC10 Shell.
Figure 6. Percentage of plots within each successional stage along the Appalachian National Scenic Trail in the Southern Blue Ridge Mountain subsection.

We compare our results to what might occur under natural disturbance regimes, thereby providing an indicator of altered disturbance regimes throughout the subsection. The result of this comparison is captured in the "green," "yellow," or "red" forest structure ecological integrity (EI) matrix above (Figure 5). Figure 6a shows that approximately 41% of the plots on APPA lands are late successional, placing them comfortably in the "Good" category for forest structure. However, while the forest found on APPA land may be good from a forest structure perspective, that characterization is only true for approximately 2% of the Southern Blue Ridge Mountain subsection (Figure 3). Knowing how far the late successional forest extends beyond the limits of APPA owned land would be valuable information for land managers who may be targeting adjacent lands for acquisition or protection, but is not currently known.

Basal Area

Live basal area (LBA) is a measure of the volume of live woody biomass (i.e., trees) on the landscape and is a key part of measuring tree growth rate. LBA is influenced by the dominant structural stage of the forest and may vary in response to different stressors. Because LBA is a measure of live trees only, LBA may decrease following a harvest or die-off. For lands like the APPA, which are protected from harvest, a decrease in LBA would presumably be a result of natural mortality, disease, or some form of disturbance (e.g., storm events, fire, etc.).

Figure 7. Appalachian National Scenic Trail Live Basal Area (LBA; m²/ha), On APPA land (Blue) and Off APPA land (Red).

Average LBA is not significantly different on and off APPA land (177.25 m²/ha vs. 273.81 m²/ha, respectively; )(148.41 m²/ha; Kruskal-Wallis rank sum test, p=0.532; Figure 7). An older forest structure may be expected to have larger trees, leading to greater basal area, but that is not what we find in this subsection, where the off-APPA land portion has a lower composition of late successional forest (Figure 6) and greater live basal area (Figure 7).

Similarity in LBA suggests that processes such as natural mortality, disease, or other types of damage are similar between the two areas.

Figure 8. Appalachian National Scenic Trail snag gauges: (a) snags as % of medium-large live tree density (On APPA); (b) snags as % total live tree density (On APPA); (c) medium-large snags/hectare (On APPA); (d) snags as % of medium-large live tree density (Off APPA); (c) snags as % total live tree density (Off APPA); (f) medium-large snags/hectare (Off APPA).

Snags

Caution

Dead wood, in the form of standing dead trees (snags) is an important structural feature of forest stands that provides habitat for arthropods, herpetofauna (amphibians and reptiles), small mammals, fungi, and cavity-nesting birds. Large diameter snags typically persist longer than smaller diameter snags (Morrison and Raphael 1993, Garber 2005) and provide habitat for a greater number of vertebrate species (Cline et al. 1980, DeGraaf and Shigo 1985). Certain types of silviculture, land management and hazard tree removal can reduce the quantity or quality of these features. Thoughtful land management, however, can maintain or enhance snags (Keeton 2006). This metric assesses the density of snags in relationship to live tree density and volume.

Density of snags in mature and late-successional stands varies substantially across ecosystems and with site conditions (Tyrrell et al. 1998). However, positive relationships between live and dead tree density and volume can be used to indicate expected snag levels (Sippola et al. 1998, Ferguson and Archibald 2002, Stewart et al. 2003).

A thorough review of scientific studies (Nillson et al. 2003; Goodburn and Lorimer 1998; Garber 2005; and, Spetich et al. 1999) suggests that snag density should be at least equal to 10% of overall and medium-large live tree density. Subsections with 10% or greater density are considered "Good," those with less than 10% but 5 or more med-large snags per hectare are "Caution," and subsections with fewer than 5 med-large snags per hectare are "Significant Concern" (Figure 8).

Snag scores within the APPA land area (Figures 8a, b, c) envelope are similar to off the APPA land area (Figures 8d, e, f). The only notable difference is that the number of medium-large snags/hectare is slightly higher on APPA lands (Figures 8c, f) and scored within the "good" range. Overall, the density of snags on or off the APPA land area is lower than optimal and both areas fall into the "caution" range.

Figure 9. (a) Coarse woody debris as % of live tree volume (On APPA land); (b) Coarse woody debris as % of live tree volume (Off APPA land); (a) Coarse woody debris volume (On APPA land); (b) Coarse woody debris volume (Off APPA land).

Coarse Woody Debris (CWD)

Caution

As with snags, fallen coarse woody debris (CWD) is an important structural feature of forest stands that provides important habitat for arthropods, herpetofauna, birds, small mammals, and fungi (DeGraaf and Rudis 1986, Petranka et al. 1994). This metric assesses the volume of CWD in relationship to live tree density (Figures 9a & 9b), and total volume (Figures 9c & 9d).

CWD continues to be used by wildlife as it breaks into smaller pieces over time, thus CWD volume can be a useful indicator of habitat availability. However, because CWD volume varies between successional stage, site condition, and across ecosystems (Tyrrell et al. 1998), precise threshold values for CWD volume do not exist. Based on available recommendations, subsections with CWD greater than 15% of live tree volume (LTV) are "Good," 5 - 15% is "Caution," and less than 5% is "Significant Concern" (Figures 9a & 9b). Based on CWD volume alone, Müller and Bütler (2010) report that a range of 20 - 50 m²/ha is optimal (Figures 9c & 9d).

CWD volume as a percentage of LTV on APPA land (7.1 % of live tree volume , Figure 9a) is less than off APPA land (9.7 % of LTV).

Mortality

Good

Annual mortality rate (% stems/year) provides a quantitative metric of forest health and vitality, and is calculated by comparing tree density observations during one event to the next.

Figure 10.(a) Overall mortality expressed as % of stems that die annually (a) on APPA land and (b) off APPA land.

Tree growth rates can decline in response to environmental factors or anthropogenic (human-caused) stress, and tree mortality is often preceded by some years of reduced tree growth (Ward and Stephens 1997, Pedersen 1998, Dobbertin 2005). Decreased growth or elevated mortality rate in trees of a particular species can indicate a particular health problem for that species, such as the effects of acid deposition on sugar maple decline (Duchesne et al. 2003, Hyink and Zedeker 1987); while altered vital rates for multiple species across a region may indicate a regional environmental stress (Steinman 2004, Dobbertin 2005) or a change in regional disturbance regimes (Dale et al. 2001).

Typical mortality rates vary by site, structural stage, tree species, tree size, and crown class (i.e., tree position in the canopy). For this reason, mortality rates are calculated for species as well as by subsection. Annual tree mortality rates in old-growth forest range from 0.3% to 1.6% (Busing 2005, Runkle 2000, Woods 2000). We rely on this range to say that a mortality rate equal to or less than 1.6% is "Good," and greater than 1.6% is considered "Caution." A threshold for "Significant Concern" has not been established.

Mortality data on APPA land from the Southern Blue Ridge Mountains subsection are very limited, with only a single individual being reported as dying during the interval between the two most recent survey cycles, and that individual was not one of the most common species. This lack of mortality data is a product of a small dataset, and should not be interpreted to mean that no trees die on APPA land. Despite this, data outside the APPA land area are abundant and useful. While interpreting data only from outside the land area is a departure from the other metrics considered in this review, it is worthwhile to note one species that exhibits a relatively high mortality rate, far exceeding the 1.6% value used to assess the health of the resource. Figure 11 shows off APPA land mortality rates for 9 of the 10 most common species (left to right) found on APPA land area (one species, striped maple, is omitted because no dead individuals were detected during the analysis cycle). Eastern hemlock, at 14.91% has the highest mortality rate, and while there are no corroborating data to support this, the high rate may be due to hemlock woolly adelgid, a very destructive invasive insect present in the eastern US (Ellison et al. 2005). Given its high mortality rate off APPA land, it makes sense to closely monitor eastern hemlock trees on APPA land for the presence of hemlock woolly adelgid

Figure 11. Appalachian National Scenic Trail Off APPA mortality rate by species for 9 of the 10 most common species found on APPA land. (Note: only 9 species are listed because the mortality rate for striped maple, the 7th most abundant species inside the APPA land area, was 0.)

Tree Growth

Figure 12. (a) Overall growth rate expressed as annual % increase in DBH on APPA land; (b) Overall growth rate expressed as annual % increase in DBH off APPA land.

Tree growth rate provides a quantitative metric of tree health and vitality, and is calculated from repeated bole DBH measurements. Like all other forest health metrics for the APPA, growth rate is calculated from available FIA data.

Growth rates can decline as trees mature, or in response to environmental factors or anthropogenic stress (Ward and Stephens 1997, Pedersen 1998, Dobbertin 2005). Declines may also indicate a particular health problem for that species, such as sugar maple decline (Duchesne et al. 2003, Hyink and Zedeker 1987), or may indicate a regional environmental stress (Steinman 2004, Dobbertin 2005).

Typical growth rates vary by site, stand structural stage, and by tree species, size and crown class (i.e., tree position in the canopy). For this reason, growth rates are reported both by subsection ( Figure 12a & b) and by species (Figure 13). Ecological thresholds for growth rates calculated from existing FIA data are not currently available.

Figure 13 shows the growth rates for the 10 most common species found on APPA land. The species by species comparisons appear to be generally consistent with the overall growth rates shown in Figure 12. Yellow-poplar and sweet birch exhibit growth rates off APPA land that are substantially higher than on APPA land. Both species are considered to be early successional, meaning that they colonize areas quickly following a disturbance. Growth rate data are not available for eastern hemlock and striped maple.

Figure 13. Annual growth rate by species expressed as % increase in DBH for the 10 most common species found on APPA land.

Regeneration

This metric assesses the quantity and composition of tree regeneration in the forest understory using available FIA data.

The quantity and species composition of regeneration will impact future forest canopy structure and composition. Regeneration can be affected by a variety of disturbances including air pollution, climate change, and deer browse. Figure 14 shows seedling density (seedlings per acre) on APPA land area (blue - 10 most abundant species) and off APPA land area (red). For most species, the density of seedlings off APPA land area is equal to or higher than on APPA land area. Given its importance in determining future forest composition and their sensitivity to stressors,regeneration will be extremely important for APPA resource managers to continue monitoring. Although this work suggests that the composition and amount of regeneration is generally similar on and off APPA lands, continued evaluation is needed to determine whether regeneration among certain forest types is adequate for future desired conditions.

Figure 14. Appalachian National Scenic Trail seedling density by species.

Conclusions

Forests on the APPA harbor important cultural and natural resources that are valuable to visitors, wildlife, and other ecosystem services. It is imperative that we monitor APPA forests as they change over time and space to better understand how natural and human causes shape these resources. Factors such as development, visitation, invasive species, and climate change, for example, are a few of the main influences for which monitoring can help evaluate their relative impacts and assist in planning and prioritizing mitigation strategies. By virtue of its size, however, the vast forest resources of APPA are financially and logistically unfeasible to monitor using typical ground-based I&M methods. Hence the purpose of this resource review was to examine the utility of using freely available data collected by the USFS FIA program as a tool to evaluate the status and health of the vast forest resources located on and near the APPA. The utility of these data for monitoring forest resources on the APPA are currently limited to the Southern Blue Ridge Mountain subsection.

In general, our assessment found only subtle differences between the forest resources on and off APPA lands among the suite of metrics evaluated. When there were observed differences they appear to be small or inconclusive possibly due to small sample size. In no instance did any of the metrics suggest that the forest resources found in one location are superior to those from the opposing location. Tree growth and regeneration are two metrics where resources off APPA appear to differ from those on APPA. This could be a consequence of the APPA land area forest consisting of more late successional forest, because of logging practices, or it could be that some other mechanism is at work. Where possible, assessments of forest health should consider utilizing FIA data and data collected in park-based vegetation monitoring programs along the APPA (e.g., SHEN and GRSM). The latter of which is expected to have a much better representation of the forest area than FIA. For subsections lacking park-based vegetation monitoring programs FIA data may provide the single best option for long term assessment of their forest resources. NETN will continue to evaluate products for their ability to characterize forest and other natural resources along the APPA.

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