Habitats of North Carolina
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General Hardwood Forests
Ash Forests
General Description The overwhelming environmental factor defining this habitat is the threat posed by the Emerald Ash Borer (Agrilus planipennis). This introduced species was first detected in North Carolina in 2012 and is now bringing the same massive devastation to our native species of Oleaceae -- Ash species and Fringetree -- as it has already done in the Northeast (see Wagner and Todd, 2016). These plants, along with their specialized herbivores, are now threatened with extirpation, not only at the local level but across the entire state.

Ashes (Fraxinus sp.), along with oaks, hickories, and maples, are major components of the vast network of hardwood forests that extend across North Carolina. However, they now face catastrophic collapse: all of our native species of Ash are threatened by the Emerald Ash Borer (Agrilus planipennis), an exotic beetle that has killed millions of Ash trees since it was first detected in North America in 2002 (see Threats and Trends below). Along with the Ashes themselves, Wagner and Todd (2016) have identified close to 100 species of herbivorous Arthropods that specialize on (Fraxinus) species and whose fate, consequently, is intimately tied to that of their host plants. At least one fungus, Botinellus meruloides, has a close mutualistic relationship with one of the those insects, an aphid Prociphilus fraxinifolii) that feeds on the roots of Ash trees, is now listed by the IUCN as globally Vulnerable.

The Ashes themselves are found under a wide range of habitat conditions. White Ash (Fraxinus americana), Biltmore Ash (Fraxinus biltmoreana), and Small's Ash (Fraxinus smallii) are major constituents of dry-to-mesic upland stands of hardwoods, particularly on rich soils. Green Ash (Fraxinus pennsylvanica) is a common to dominant member of Piedmont alluvial forests and brownwater floodplains in the Coastal Plain. Carolina Ash (Fraxinus carolina), Pumpkin Ash (Fraxinus profunda), and Swamp White Ash Fraxinus pauciflora) are found in even more hydric habitats, including both brownwater and blackwater swamps. About the only hardwood habitats not occupied by one species of Ash or another are dry, acidic uplands, including felsic ridges, sandhills, and barrier islands, and forests growing on peatland soils.

All species of Fraxinus are treated here as habitat factors but only White Ash and Small's Ash are included as determining species, since they occur in both uplands and bottomlands, although not in deeply flooded swamps. The other species are treated in other more specific types of habitats.

For all of the Insect members of this group, the presence of one or another of the Ash species is the critical habitat factor. Although specialists on (Fraxinus), most of these species feed on all members of this genus, with some also feeding on other members of our native Oleaceae, including Fringe Tree (Chionanthus virginicus); that species is also included as a member of this habitat group since it generally co-occurs with Ash species; has an overlapping set of specialist herbivores; and may also be threatened by the Emerald Ash Borer.

The Emerald Ash Borer itself, as a non-native species, is not included as a defining species although -- like the native Ash symbionts -- it is capable of feeding on all species of Ash, as well as other members of the Lauraceae. The overwhelming nature of the threat posed by this species is the main unifying environmental feature of this habitat, outweighing the differences in substrates, moisture, and other abiotic factors that separate the plant members of this habitat. Although the Ashes could be treated simply as environmental factors rather than as Determining Species, they have more conservation significance when considered as part of this habitat than they would as members of more the more general hardwood forest habitats associated with those various abiotic factors.

Abiotic Factors Geographic Regions: Lower Coastal Plain to High Mountains. USDA Hardiness Zones: 6-8. Landform: ridges, slopes, bottomlands, wet flats, and swamps. Slope Aspect: north, east, south, and west, also flat. Soil Moisture: Dry to hydric. Soil texture: loamy, silty,or mucky. Soil pH: acdic to circumneutral. Soil Nutrient Content: rich for most terrestrial sites to poor in blackwater swamps. Microclimate: cool to warm; dry to humid. Hydrological Features: streamheads on ridges and slopes, swamps in Outer Coastal Plain. Flood Frequency: essentially never flooded on ridges to permanently flooded in swamps. Flood Duration: nonexistent to permanent. Fire Frequency: uncommon on ridges, rare in swamps. Drought Frequency: uncommon in uplands, rare in swamp forests. Ice Storm Damage: high in the mountains, low in the Outer Coastal Plain. Wind Storm Damage: moderate. Insolation: canopies are well insolated, lower strata are deeply shaded.

Biotic Structure Key Species: the presence of Fraxinus species or Chionanthus is a requirement. Key Threat: Emerald Ash Borer. Vegetation Type: Closed canopy forests dominated by broadleaf, deciduous species in the uplands and bottomlands and by a mix of Cypress and hardwoods in coastal swamp forests; the forest structure varies widely depending on the type of forest occupied by the different species of Fraxinus. Organic Shelter, Foraging, and Nesting Structures: woody debris and leaf litter are abundant; snags and fallen logs are also becoming -- temporarily at least -- major features of EAB-attacked forests

Co-evolved Species Groups Phagic and Competitory Symbioses:
Fraxinus species-Chionanthus virginicus // Agrilus subcinctus-Ceratomia undulosa-Capraita sexmaculata-Copivaleria grotei-Dynastes tityus-Manduca jasminearum-Marmara fraxinicola-Olceclostera angelica-Palpita magniferalis-Plagodis kuetzingi-Podosesia aureocincta-Prociphilus fraxinifolii-Sphinx chersis-Sphinx franckii-Sphinx kalmiae-Sympistis chionanthi-Xyloryctes jamaicensis

Mutualistic Symbioses:
Plant // Fungi Mycorrhizal Symbioses:

Fraxinus species // Durandiella fraxini-Mycosphaerella effigurata-Mycosphaerella fraxinicola-Piggotia fraxini
Insect // Fungi Trophic-Protection Symbioses
Prociphilus fraxinifolii // Boletinellus merulioides

Note that the Emerald Ash Borer is not included as a symbiont of our species of Ash, despite the fact that it feeds solely on members of the Ash family. In this case, however, there is no long history of co-adaptation between this invasive species and our native Ash species -- there is not any true symbiosis since that requires a long history of co-adaptation between the host species and their predators. Instead, we place the Emerald Ash Borer in the Exotic Invaded Habitat, which we reserve for species and habitats that are mutually alien to one another.

Determining Species
sciNamecomNameg_ranks_rankmod_s_rankprob_of_extirpation
APHIDS
Prociphilus fraxinifoliiSNR (S1S2)
BEETLES
Dynastes tityusEastern Hercules BeetleSNR (S1S2)
Xyloryctes jamaicensisUnicorn Beetle, Rhinoceros BeetleSNR (S1S2)
Agrilus subcinctusSNR (S2S3)
Capraita sexmaculataSNR (S2S3)
FUNGI
Durandiella fraxini SNR
Mycosphaerella effigurataSNR
Mycosphaerella fraxinicola (= Piggotia fraxini)SNR
Boletinellus merulioidesSNR (S1S2)
Dendrothele macrodensSNR (S1S2)
HARDWOODS
Fraxinus smalliiSmall's AshGNRS4S1S20.11107
Fraxinus americanaWhite Ash, American AshG5S5S1S30.03699
MITES
Aceria fraxiniSNR
MOTHS
Sphinx chersisGreat Ash SphinxG4G5SHS10.33330
Sphinx franckiiFranck's SphinxG4SUS10.33330
Copivaleria groteiGrote's SallowG5S4S1S20.11107
Manduca jasminearumAsh Sphinx MothG4G5SUS1S20.11107
Marmara fraxinicolaGNRS2S3S1S20.11107
Palpita illibalisInkblot Palpita MothGNRS2S3S1S20.11107
Papaipema furcataAsh Tip Borer MothGNRS1S2S1S20.11107
Plagodis kuetzingiPurple Plagodis MothG5S3S4S1S20.11107
Ceratomia undulosaWaved SphinxG5S5S2S30.01230
Olceclostera angelicaThe AngelG5S3S4S2S30.01230
Palpita magniferalisSplendid Palpita MothGNRS4S5S2S30.01230
Podosesia aureocinctaBanded Ash Clearwing MothGNRSUS2S30.01230
Sphinx kalmiaeLaurel SphinxG5S3S4S2S30.01230
Sympistis chionanthiFringe-Tree SallowG5SUS2S30.01230
SHRUBS
Chionanthus virginicusFringe TreeG5S5S2S30.01230
Nr = Number of Ranked Species = 24
Ner = Number of Extant, Ranked Species = 17
Nv = Number of Historic and Extirpated Species = 0
Nar = Number of Species at Risk of Extirpation (State rank > S5) = 17
Nss = Number of Secure Species (State Rank = S5) = 0
Pss = Proportion of Secure Species (Nss/Ner) = 0.00000
ENE = Expected Number of Extirpations (Sum of PE) = 1.56718
Average PE (ENE/Ner) = 0.09219
Habitat Risk Index = (Nar+Nv) x Average PE = 17 x 0.09219 = 1.56723

Estimated Risk to the Determining Species All of our Ash species are highly threatened with extirpation from the state over the next ten years, whatever their current distribution and abundance -- the main factors used in the past to assign the state ranks for these species. In place of the state ranks currently assigned by the Natural Heritage Program, shown in the s_rank column, we substitute a set of much higher ranks. For the Ash species themselves, we use a range rank of S1S3 for the species previously ranked as S5, and S1S2 for the species with more restricted ranges. For the herbivorous insects, we follow the estimates of endangerment given by Wagner and Todd (2016), assigning state ranks as follows: Very High Endangerment Risk = S1; High Endangerment Risk = S1S2; High to Moderate Endangerment Risk = S2S3; Moderate Endangerment Risk = S3.

Based on these modified state ranks, the Average Probability of Extirpation for these species is equivalent under our model to a state rank of S2.

Estimated Risk to the Co-evolved Species Groups All of the Determining Species of this habitat are involved in co-evolved complexes, whose strong connection to Ash species and Fringetrees make them highly vulnerable to extirpation due to the depredations of the Emerald Ash Borer. Comments made concerning the habitat as a whole apply just as well to these co-evolved species groups.

Estimated Security of the Habitat The state ranks of S5 previously assigned to several of the Ash species indicate that the hardwood forests they occupy are widespread and occur in large blocks or multitudes of smaller but well-connected units. In the face of the adverse environmental events to which they have been long-adapted, these factors would be enough to give these habitats a high degree of security. None of these factors, however, give the Ashes and associated species any defense against an exotic species to which they have no adaptations and for which their are no long-evolved ecological controls. If anything, the factors that once gave this habitat a high degree of security now work in favor of the Emerald Ash Borer, allowing it to quickly spread across the state, with few, if any barriers to its movements. If there are any significantly isolated patches of this habitat, they may persist for a while, but their long-term prognosis would still be very uncertain.

This overall lack of security for any of the Determining Species of this habitat is reflected in the PSS value of zero.

Index of Habitat Imperilment In absence of any counter-effect due to PSS, the HRI for this habitat is determined mainly by the Expected Number of Extirpations, which itself reflects the number of Species at Risk and the Average Probability of Extirpation. Both values are relatively high in the case of this habitat, producing an Expected Number of Extirpations of nearly 2. The resulting value of HRI places it in our Tier 2 Level of Conservation Concern (5 ≥ HRI > 0.5), meriting a high degree of conservation efforts.

Identified Risks The overwhelming threats to this habitat and its species are the depredations by the Emerald Ash Borer (Agrilus planipennis). In Europe, a newly emergent fungal pathogen of Ash, Hymenoscyphus fraxineus (=pseudoalbidus), is having a similar devastating impact to forests containing European Ash (see Pautasso et al., 2013) and could have similar impacts here if it makes the passage to North America.

Observed Trends The Emerald Ash Borer was first detected in North Carolina in Granville County in 2013. With no effective way to control its spread (quarantining efforts met with no success), this species has now spread across the entire Piedmont and Mountain regions of the state and is continuing to move eastward into the Coastal Plain (NC Forest Service, accessed 2022). Within the infected regions, massive kills of mature trees are taking place, both within natural areas and residential areas where this species was often planted as a shade tree.

Distribution Map
Distribution As shown on the map, Ash-containing forests occur across the entire state. The relatively low number of counties showing a high representation of the species in this habitat largely reflects the scarcity of Insect surveys, particularly for moths, which make up the largest proportion of the species in this habitat.

Major Conservation Reserves No areas are safe from invasion by the Emerald Ash Borer. Large tracts of Ash forests may even be more at risk than small, well-separated units.

Priority Areas for Surveys and Conservation Protection
Stewardship and Management Recommendations Currently there are no methods that have proven effective in stopping the spread of the Emerald Ash Borer or eliminating it once detected within a stand. Individual trees can be protected by injecting a systemic insecticide, but that method can be applied only to a limited number of trees and has the consequence of killing the specialized, native herbivores associated with Ash species. The USDA is currently rearing several parasitic wasps that are specialists on the EAB in its native range in East Asia; there are four sites in North Carolina where these species are now being released (see North Carolina Forest Service, 2022). It is still too early to tell, however, if these control measures will prove to be effective in slowing or halting this infestation.

References North Carolina Forest Service. 2022. Emerald Ash Borer Frequently Asked Questions. Available online at: https://www.ncforestservice.gov/forest_health/fh_eabfaq.htm .

Pautasso, M., Aas, G., Queloz, V. and Holdenrieder, O., 2013. European Ash (Fraxinus excelsior) dieback – a conservation biology challenge. Biological Conservation 158:37-49.

Wagner, D.L. and Todd, K.J., 2016. New ecological assessment for the emerald ash borer: a cautionary tale about unvetted host-plant literature. American Entomologist, 62(1), pp.26-35. Available online at: https://academic.oup.com/ae/article/62/1/26/2194528

Updated on 2022-07-14 18:32:51