Evaluating Southern Appalachian Forest Dynamics without Eastern Hemlock: Consequences of Herbivory by the Hemlock Woolly Adelgid

Andrew G. Birt, Yu Zeng, Maria D. Tchakerian, Robert N. Coulson, Charles W. Lafon, David M. Cairns, John Waldron, Weimin Xi, Szu-Hung Chen, Douglas A. Street

Department of Biological and Health Sciences, Texas A&M University-Kingsville, Kingsville, USA.
Department of Environmental Studies, University of West Florida, Pensacola, USA.
Department of Geography, Texas A&M University, College Station, USA.
Knowledge Engineering Laboratory, Department of Entomology, Texas A&M University, College Station, USA.
Southern Research Station, US Forest Service, Alexandria Forestry Center, Pineville, USA.

DOI: 10.4236/ojf.2014.42014                

Abstract

Eastern hemlock (Tsuga canadensis Carriére) and the Carolina hemlock (Tsuga caroliniana Engelmann) are ecologically important tree species in eastern North America forests that are currently threatened by the hemlock woolly adelgid (HWA, Adelges tsugae Annand, Hemiptera: Adelgidae). HWA has spread rapidly from its original introduction site into new areas. Once present, HWA kills its hosts over a period of 4 to 10 years leading to a phenomenon that is known scientifically and colloquially as hemlock decline. To date, quarantine, chemical management, and biocontrol efforts have failed to curb the spread of the HWA. As such, forest management efforts are now being redirected towards developing an understanding of the effects of hemlock removal on vegetation dynamics, changes in forest composition, and changes in ecosystem function. In this study, we parameterize a spatially explicit landscape simulation model LANDIS II for a specific forested region of the southern Appalachians. Parameterization involves defining the life-history attributes of 37 tree species occupying 11 ecological zones and is based on knowledge of: current vegetation composition data, recent historic management and fire regimes, and life-history traits of each species. The parameterized model is used to explore a simple scenario of catastrophic hemlock mortality likely to occur as a result of HWA herbivory. Our results emphasize that hemlock is an important foundation species. When hemlock is removed from the system, forest composition changes considerably with a greater presence of shade intolerant pine and oak species. Additionally, hemlock removal leads to a period of transient, relatively unstable vegetation dynamics as the forest communities restructure.

 

Keywords

Hemlock; Forest Dynamics; LANDIS II; Hemlock Woolly Adelgid; Southern Appalachian

Table 1.

Received November 21^st, 2013; revised January 3^rd, 2014; accepted January 21^st, 2014

Figure 2.

Average difference in species abundance between scenarios with hem- lock and without hemlock for each ecological zone (except Spruce-Fir zone). Gray bars show the original (baseline) abundance of hemlock. Black bars show the average change in abundance of all species without hemlock.

tion to the abundance of hemlock in the baseline scenario.

In Figure 2, ecological zones marked with an asterisk represent 95% of the study area. Figure 3 provides a more de- tailed view of species abundance within these three ecological zones. In simulations without hemlock, the abundance of most species increase between 25% to 74%. Together, Figures 2 and 3 show that the greatest change in species abundance occurs in

Figure 3.

Comparison of species abundance between forest landscapes with and without hemlock. A. in Acidic Cove; B. in Xeric Pine-Oak Heath and Oak Heath; C. Mesic Oak-Hickory. Note that only species that have greater than 20% abundance in any ecological zone are included. Gray bars show the original (baseline) abundance of hemlock. Black bars show the average change in abundance of all species without hemlock.

Mesic zones (Acidic Cove and Mesic Oak-Hickory zones) with least change in Xeric zones (Xeric Pine-Oak Heath and Oak Heath). In the Acidic Cove and Xeric Pine-Oak Heath and Oak Heath ecological zones, hemlocks are mostly replaced by 4 species (white pine, chestnut oak, red oak, and yellow poplar). In contrast, the Mesic Oak-Hickory ecological zone shows relatively uniform increases in abundances across 13 species. Figure 3 also illustrates differential response to hemlock re- moval for two important understory shrubs, great rhododendron and mountain laurel. In all three ecological zones, great rhodo- dendron shows very little change in abundance between simu- lations with and without hemlocks. In contrast, in all ecological zones, mountain laurel increases by 74%, 22% and 91% in the Acidic Cove, Xeric Pine-Oak Heath and Oak Heath, and Mesic Oak-Hickory ecological zones respectively.

Figures 4-6, illustrate the temporal pattern of vegetation dy- namics in simulations with and without hemlock in the three major ecological zones. In simulations with hemlock, the rela- tive changes of each species follow a smoother trajectory and

reach a stable state more quickly than in the non-hemlock sce- nario. In scenarios without hemlock, the abundance of each species fluctuates over greater amplitudes and for longer time periods before reaching a stable state. This phenomenon is related to the structure and parameterization of the model. By removing hemlock from the system, the relative competitive abilities of the remaining species change and radically alter the vegetation dynamics. The extent to which hemlocks exert a stabilizing effect on the forest landscape is illustrated in Figures 4-6. In addition to this general trend, the figures also illustrate that, for any species, fluctuations in abundance occur over relatively long time-periods (between 200 and 400 years).

Discussion and Conclusions

Removal of hemlock leads to an increase in the abundance of the remaining 36 species in all ecological zones. This general increase in species abundance is approximately proportional to the baseline abundance of hemlock in each ecological zone and

Figure 4.

Species abundance in Acidic Cove.

Figure 5.

Species abundance in Xeric Pine-Oak Heath and Oak Heath.

Figure 6.

Species abundance in Mesic Oak-Hickory.

leads to a substantial change in the forest landscape. The simu- lation results highlight two important dimensions to this distur- bance. The first involves the long-term, stable dynamics of the landscape and the types of species that are expected to become dominant in the absence of hemlock. As Figure 3 shows, the removal of hemlock from the system generally leads to an in- crease of shade intolerant species such as pines and oaks. The second dimension of this change involves the temporal fluctua- tions in forest composition that may occur as the forest re- sponds to this disturbance (Figures 4-6). Catastrophic removal of hemlock from the system leads to a restructuring of forest composition that occurs over 200+ years. In forests without hemlock, short term fluxes in species abundance are much greater. This relative instability illustrates the importance of models such as LANDIS II for forest planning, especially for forests with diverse communities of interacting species. With- out the benefit of integrative models such as LANDIS II, it is tempting to interpret forest patterns in a single dimension (for example by looking at current landscape composition and con- figuration). An important contribution of models such as LANDIS II is to illustrate the link between individual species ecology, and the short term and/or long term vegetation dy- namics that may arise under different forest management and/or disturbance scenarios.

In the broadest sense, these results show that hemlock is a dominant, foundation species that significantly affects forest composition and structure. Through a combination of longevity and large size, it inhibits the establishment of shade-intolerant species such as oak and pine. The model results also suggest that this dominance is exacerbated by suppressed fire regimes. In comparison to pine and oak species, hemlocks are relatively fire intolerant. Fires create gaps in forest canopies, and light conditions suitable for the growth and establishment of shade- intolerant pine and oak species. However, our results show that nearly all species (not just shade intolerant pine and oaks) in- crease as a result of hemlock removal. It is likely that this gen- eral increase in species abundance, drives increased compete- tion, and contributes to the long term instability of vegetation dynamics in simulations without hemlock.

The results of this analysis largely concur with other studies. For example, Nuckolls et al. (2009) proposed that, over short temporal scales, hemlocks will either be replaced by a mix of advanced regeneration and early successional tree species such as red maple, black birch, and yellow poplar, or great rhodo- dendron. Although, in our study, mountain laurel increases in abundance, great rhododendron does not. Heard and Valente (2009) studied a catastrophic hemlock decline in the mid-Ho- locene using fossilized pollen, and found increases in maple, birch, beech, and oak. HWA hemlock decline is already being compared to the impact of chestnut blight on American chest- nuts between 1900 and 1940. In line with our results for hem- lock, Day and Monk (1974) studied the changes in species dy- namics following chestnut mortality in a southern Appalachian watershed and found that in most cases, chestnuts were re- placed by multiple species, rather than a single co-dominant species. They also suggest that these large scale disturbances may also have allowed “pest” species such as great rhododen- dron and mountain laurel to establish and spread.

In our future work, we intend to use the model to explore scenarios under more realistic patterns of hemlock mortality, and to investigate management scenarios with the potential to mitigate hemlock decline. Given the destabilizing effect of hemlock removal in this study (which uses the simplest as- sumption of catastrophic, instantaneous hemlock mortality), it is important to investigate how different spatio-temporal pat- terns of hemlock decline may affect short and long-term vege- tation dynamics. Our current model structure provides the op- portunity to explore such scenarios, and ongoing, independent research to measure and characterize the current pattern of Hemlock decline will provide the inputs necessary to address this issue (Clark et al., 2012). Similarly, the diversity of species, and the complex social and ecological function of forests in this area suggest that the model can be used to address forest man- agement options with potential to mitigate the effects of Hem- lock decline. For example, the HWA resistant Chinese hemlock (Tsuga chinensis (Franch.) E. Pritz.) (Tredici and Kitajima, 2004) has been proposed as a direct replacement for native hemlocks. Similarly, modification of the current fire suppres- sion regime has been proposed as a method to control the ex- pansion of mountain laurel and possibly increase the dominance of fire-tolerant species such as oaks and pines. Both mitigation efforts could be assessed by building upon the parameterization of LANDIS II outlined in this paper. Furthermore, they can be tested by using a specific culturally, socially, and ecologically important landscape (i.e., the Grandfather Ranger District).

Acknowledgements

This research is funded by US Forest Service through USDA Forest Service cooperative agreement SRS-10-CA-11330129- 039. We thank Mr. William Flatley for providing the dendro- logical fire data and constructive discussions.

References

Conflicts of Interest

The authors declare no conflicts of interest.

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