Effects of Drought on Overstory Tree Species Growth Rates

Observed values of species-specific basal area growth rates vary considerably at Coweeta Hydrologic Laboratory (figure 3.2). Because of the wide range in tree diameters, data are expressed on a relativized basis, annual basal area growth (cm2)

Coweeta Hydrologic Laboratory

Figure 3.2 The relative basal area increment (cm2tree growth per cm tree diameter) of two selected species at Coweeta Hydrologic Laboratory near Otto, North Carolina. Each connected line represents a single tree over the measurement period. The deciduous red maple (Acer rubrum) and the evergreen white pine (Pinus strobus) exhibit wide variation of relative basal area increment between trees.

Figure 3.2 The relative basal area increment (cm2tree growth per cm tree diameter) of two selected species at Coweeta Hydrologic Laboratory near Otto, North Carolina. Each connected line represents a single tree over the measurement period. The deciduous red maple (Acer rubrum) and the evergreen white pine (Pinus strobus) exhibit wide variation of relative basal area increment between trees.

per diameter of the tree (cm). The species measured at Coweeta range from the greatest relative growth rate in Acer rubrum (see figure 3.2) to the lowest rate in Quercus prinus (data not shown). Quercus rubra, Q. coccinea, and the subcanopy species Oxydendrum arboreum are intermediate along with the combined "other" species. The understory Rhododendron maximum has widely varying growth rates, which likely depends on light availability, whereas the evergreen Pinus strobus (see figure 3.2) in watersheds 1 and 17 exhibits some of the greatest growth rates at Coweeta. Acer rubrum exists on a variety of sites with a wide range in moisture availability, whereas the Quercus species predominate on dry sites except for Quer-cus rubra.

A condensed summarization of the relativised growth rate data indicates that oaks maintained more consistent growth rates during dry and wet conditions, whereas white pine was more sensitive to moisture availability and hence exhibited a growth decline (figure 3.3; McNulty and Swank 1995). This observed difference between oaks and pines suggests that the oaks may be more deeply rooted than the white pines. Hence, the oaks were drawing from a deeper soil water resource that was more available during periods of precipitation decline.

1986 1988 1990 1992 1994 1398 1908 2000 2002

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