Species Specific Mortality

Mortality patterns during severe drought are often species specific (Tainter et al. 1984; Starkey et al. 1989; Clinton et al. 1993; Elliott and Swank 1994). For example, Clinton et al. (1993) found that the species most susceptible to drought-related mortality were members of the red oak group (particularly Quercus coccinea) and Carya spp. This pattern of mortality was observed across the southeastern region during the mid- to late 1980s (Starkey et al. 1989; Stringer et al. 1989; Oak et al. 1991). The same pattern of mortality was observed in other studies at Coweeta. B. D. Clinton et al. (unpubl. data, 1999) examined tree mortality on two opposing (north- and south-facing) mixed hardwood watersheds in the Coweeta Basin. The period of study covered 18 years and was generally split between an extremely dry period (1984-1988) and a period of above-average precipitation (1989-1997; table 3.2). During the dry period, annual precipitation averaged 20% less than the long-term (60+ years) mean, and, during the wetter period, precipitation averaged 12% above the long-term mean (table 3.2). In the study of Clinton et al., mortality varied considerably between watersheds and within species. On south-facing watershed 2 for the period 1983-1989, the highest mortality rates by species ranked Carya spp. > Q. velutina = Oxydendrum arboreum = Acer rubrum; in 1998, mortality rates for that watershed ranked Cornus florida > A. rubrum > Liriodendron tulipifera = Carya spp. = O. arboreum. On north-facing watershed 18, mortality rates ranked C. florida > Q. prinus > A. rubrum > Q. velutina > Carya spp. > O. arboreum. Even though watershed 2 was subject to the same meteorological variation over the sampling period, mortality rates were less significant for watershed 2 than for watershed 18. For example, aboveground woody net primary productivity (ANPP) for watersheds 2 and 18 for the period 1983-1998 were 3.4 and 2.1 Mg ha-1 yr-1, respectively. The lower productivity for watershed 18 resulted from high rates of mortality following the earlier drought. For the oak species, variation in rates of mortality were considerable (table 3.3).

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