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





Dead stems (# ha-1)





Live stems (# ha-1)





Mortality (%)





Total dead wood biomass (kg ha-1)a





Total live wood biomass (kg ha-1)b





Biomass lost (%)c





Aboveground woody net primary

production (kg ha-1 yr-1)





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.


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