Drought as a Disturbance Regime

Severe drought has been implicated as a contributing factor to recent accelerated rates of tree mortality in the southeastern United States (Tainter et al. 1984; Starkey

Table 3.2 Comparison of rainfall between two sampling periods at Coweeta Hydrologic Laboratory near Otto, North Carolina, USA

Period 1984-1988 1989-1997

Years 5 9

Mean annual precipitation (mm) 1431 2010

Mean growing season precipitation (mm) 634 913

Relative to long-term mean (%) -14 +17 Number of consecutive growing season droughts and deficit range (%) 4 (-13 to -34)* 0 (-23 and -26)

aValues in parentheses for the number of consecutive growing season droughts represent the range of the deficit relative to the long-term mean. Adapted from Clinton et al., unpubl. data, 1999.

Table 3.3 Mortality patterns of the red oaks (Quercus rubra, Q. velutina, Q. coccinea) for the two sampling periods by watershed for stems > 10 cm dbh at Coweeta Hydrologic Laboratory near Otto, North Carolina, USA

South-Facing Watershed 2 North-Facing Watershed 18

Table 3.3 Mortality patterns of the red oaks (Quercus rubra, Q. velutina, Q. coccinea) for the two sampling periods by watershed for stems > 10 cm dbh at Coweeta Hydrologic Laboratory near Otto, North Carolina, USA

South-Facing Watershed 2 North-Facing Watershed 18

1983-1989

1990-1998

1983-1989

1990-1998

Dead stems (# ha-1)

8

2

38

16

Live stems (# ha-1)

24

25

77

70

Mortality (%)

25

7

33

19

Total dead wood biomass (kg ha-1)a

5615

8597

10777

14446

Total live wood biomass (kg ha-1)b

45592

55273

87801

88640

Biomass lost (%)c

11

14

11

14

Aboveground woody net primary

production (kg ha-1 yr-1)

10

1076

679

93

aThis is the sum of standing and fallen dead red oak stems for a given sampling period.

bThis represents total red oak wood standing crop.

aThis is the sum of standing and fallen dead red oak stems for a given sampling period.

bThis represents total red oak wood standing crop.

cThis is the percentage of the total for a given sampling period.

No adjustment for loss of wood density was made in the calculation of dead biomass. Adapted from Clinton et al., unpubl. data, 1999.

et al. 1989; Stringer et al. 1989; Clinton et al. 1993). This pulse of mortality may have a long-term impact on stand structure and function (Clark et al. 2002). The structural pattern associated with drought-induced mortality (i.e., standing-dead snags) implies that important types of microhabitats are not produced. For example, species such as pitch pine that require large openings (Barden and Woods 1976) commonly associated with large-scale, wind-induced mortality or wildfire are at a distinct disadvantage. In addition, the lack of a pulse addition of coarse woody debris, typical of wind-induced gap formation, may reduce regeneration opportunities for species such as sweet birch (Betula lenta) and eastern hemlock (Tsuga canadensis), whose regeneration strategies include "nurse logs" as fresh substrate for seed germination (Burns and Honkala 1990). This is not to say that other important classes of microhabitat are not produced. The standing-dead tree and the shade it casts are an important microhabitat for many organisms and processes (Franklin et al. 1987). Thus, effects of drought-induced mortality may have important influences on micro- as well as macro-level processes (Mueller-Dombois 1987).

We must also begin to assess ecosystem-level impacts of such climatic alteration of the forest structure. Canopy openings and shifts in species composition alter microclimatic factors such as light, temperature, and moisture (B. D. Clinton, unpubl. data, 1999) that regulate nutrient cycling processes. For example, the response of the nitrogen-fixing black locust (Robinia pseudoacacia) in large gaps and shifts in litter quality or decomposition rates of leaves of different species are two potential manifestations. The long-term importance of increasing our understanding of drought impacts on forest structure and function is central to anticipating the full impacts of predicted long-term climate change.

Acknowledgments Components of this work were funded by the National Science Foundation to the Coweeta Long-Term Ecological Research Program (Grant #9632854). We thank Jim Deal, Barry Argo, Sharon Taylor, and Susan Steiner for collection of the tree den-drometer band data. We thank the USDA Forest Service, Coweeta Hydrologic Laboratory, for the collection and management of the precipitation and streamflow data.

References

Barden, L. S., and F. W. Woods. 1976. Effects of fire on pine and pine-hardwood forests in the southern Appalachians. Forest Science 22: 399-403.

Bolstad, P. V., W. T. Swank, and J. M. Vose. 1998. Predicting southern Appalachian overstory vegetation with digital terrain data. Landscape Ecology 13: 271-283.

Burns, R. M., and B. H. Honkala. 1990. Silvics of North America. USDA Forest Service Agriculture Handbook 654, Volume 1. Conifers, 675 pp. Volume 2. Hardwoods, 877 pp.

Chang, S. J., J. D. Puryear, M. A. D. L. Dias, E. A. Funkhouser, R. J. Newton, and J. Cairney. 1996. Gene expression under water deficit in loblolly pine (Pinus taeda): Isolation and characterization of cDNA clones. Physiologia Plantarum 97: 139-148.

Chellemi, D. O., K. O. Britton, and W. T. Swank. 1992. Influence of site factors on dogwood anthracnose in the Nantahala Mountain Range of western North Carolina. Plant Disease 76: 915-918.

Clark, J. S., E. C. Grimm, J. J. Donovan, S. C. Fritz, D. R. Engstrom, and J. E. Almendinger. 2002. Drought cycles and landscape responses to past aridity on prairies of the northern great plains, USA. Ecology 83: 595-601.

Clinton, B. D., L. R. Boring, and W. T. Swank. 1993. Canopy gap characteristics and drought influences in oak forests of the Coweeta Basin. Ecology 74: 1551-1558.

Critchfield, H. J. 1966. General Climatology, second ed., Englewood Cliffs, New Jersey: Prentice Hall.

Daughtrey, M. L., and C. R. Hibben. 1983. Lower branch dieback, a new disease of Northeastern dogwoods. Phytopathology 73: 365-365.

Dunbar, D. M., and G. R. Stephens. 1975. Association of two-lined chestnut borer and shoestring fungus with mortality of defoliated oak in Connecticut. Forest Science 21: 169-174.

Elliott, K. J., and W. T. Swank. 1994. Impacts of drought on tree mortality and growth in a mixed hardwood forest. Journal of Vegetation Science 5: 229-236.

Franklin, J. F., H. H. Shugart, and M. E. Harmon. 1987. Tree death as an ecological process. BioScience 37: 550-556.

Gram, W. K., and V. L. Sork. 2001. Associations between environmental and genetic heterogeneity in forest tree populations. Ecology 82: 2012-2021.

Hofacker, T. H., R. F. Fowler, L. Turner, K. Webster, and M. Reiffe. 1992. Forest insects and disease conditions in the United States 1991. USDA Forest Service, Forest Pest Management, AB-2S. 139 pp.

Hursh, C. R., and F. W. Haasis. 1931. Effects of 1925 summer drought on southern Appalachian hardwoods. Ecology 12: 380-386.

Lorio, P. L., and J. D. Hodges. 1977. Tree water status affects induced southern pine beetle attack and brood production. USDA Forest Service, Research Paper 20-135. Southern Forest Experiment Station, New Orleans, Louisiana. 7 pp.

McNulty, S. G., and W. T. Swank. 1995. Wood S13C as a measure of annual basal area growth and soil water stress in a Pinus strobus forest. Ecology 76: 1581-1586.

Mueller-Dombois, D. 1987. Natural dieback in forests. BioScience 37: 575-583.

Nilsen, E. T., and D. M. Orcutt. 1996. The Physiology of Plants Under Stress: Abiotic factors. New York: John Wiley and Sons. 689 pp.

Oak, S. W., C. M. Huber, and R. M. Sheffield. 1991. Incidence and impact of oak decline in western VA, 1986. USDA Forest Service, SEFES Resource Bulletin SE-123. 16 pp.

Pedersen, B. S. 1998. The role of stress in the mortality of midwestern oaks as indicated by growth prior to death. Ecology 79: 79-93.

Pedersen, B. S. 1999. The mortality of midwestern overstory oaks as a bioindicator of environmental stress. Ecological Applications 9: 1017-1027.

Smith, R. N. 1991. Species composition, stand structure, and woody detrital dynamics associated with pine mortality in the southern Appalachians. Masters thesis, University of Georgia, Athens, Georgia. 163 pp.

Staley, J. M. 1965. Decline and mortality of red and scarlet oak. Forest Science 11: 2-17.

Starkey, D. A., S. W. Oak, G. W. Ryan, F. H. Tainter, C. Redmond, and H. D. Brown. 1989. Evaluation of oak decline areas in the South. USDA Forest Service, Forest Protection Report R8-TR17.

Stringer, J. W., T. W. Kimmerer, J. C. Overstreet, and J. P. Dunn. 1989. Oak mortality in eastern Kentucky. Southern Journal of Applied Forestry 13: 86-91.

Swank, W. T., and D. A. Crossley, Jr. 1988. Forest Hydrology and Ecology at Coweeta. Ecological Studies 66, New York: Springer-Verlag.

Swift, L. W., Jr., G. B. Cunningham, and J. E. Douglas. 1988. Climatology and Hydrology. Pages 35-55 in W. T. Swank and D. A. Crossley, Jr., editors, Forest Hydrology and Ecology at Coweeta. Ecological Studies 66, New York: Springer-Verlag.

Swift, L. W., Jr., J. B. Waide, and D. L. White. 1990. Application of the Z-T extreme event analysis using Coweeta streamflow and precipitation data. Pages 13-18 in D. Greenland, and L. W. Swift, Jr., editors, Climate Variability and Ecosystem Response: Proceedings of a Long-Term Ecological Research Workshop. USDA Forest Service General Technical Report SE-65, Asheville, North Carolina.

Tainter, F. H., S. W. Fraedrich, and D. M. Benson. 1984. The effect of climate on growth, decline, and death of northern red oaks in the western North Carolina Nantahala Mountains. Castanea 49: 127-137.

Tainter, F. H., W. A. Retzlaff, D. A. Starkey, and S. W. Oak. 1990. Decline of radial growth in red oaks is associated with short-term changes in climate. European Journal of Forest Pathology 20: 95-105.

Vose, J. M., and W. T. Swank. 1994. Effects of long-term drought on the hydrology and growth of a white pine plantation in the southern Appalachians. Forest Ecology and Management 64: 25-39.

Wargo, P. M. 1977. Armillaria mellea and Agrilus bilineatus and mortality of defoliated oak trees. Forest Science 23: 485-492.

Wargo, P. M. 1996. Consequences of environmental stress on oak: Predisposition to pathogens. Annales des Sciences Forestieres 53: 359-368.

Waring, R. H. 1987. Characteristics of trees predisposed to die. BioScience 37: 569-574.

Wyckoff, P. H. 1999. Growth and mortality of trees in the southern Appalachian Mountains. Ph.D. dissertation, Duke University, Durham, North Carolina.

0 0

Post a comment