SITE‐SPECIFIC CHANGES IN THE BARK TEXTURE OF QUERCUS FALCATA MICHX. (SOUTHERN RED OAK)

A method of measuring bark texture is described. This method is used to show that bark roughness in Quercus falcata Michx. (southern red oak) increases from mesic to xeric sites. This change does not appear to be explained by variability in tree size and age, or by changes in the relative abundances of varieties falcata and pagodaefolia. We suggest that it may be due to postulated relationships between bark texture (or the associated property of outer bark thickness) and tree fire resistance, nutrient conservation or susceptibility to pathogens. DIFFERENCES in bark texture (rough versus smooth) may affect tree heat balance (Derby and Gates, 1966), nutrient cycling (Voigt, 1960) and growth of bark organisms (Carmer, 1975) and hence be of potential adaptive significance. However, serious testing of the adaptive role of bark texture cannot take place until a method exists whereby differing degrees of roughness can be quantified. In this study, such a method was devised to test the hypothesis that bark roughness in Quercus falcata Michx. (southern red oak), a tree species with notoriously variable bark (Harlow and Harrar, 1968), varies with site moisture status. We had three reasons for examining textural variability as a function of site moisture. First, visual observation seemed to indicate that trees on drier sites had rougher bark. Second, we could postulate ways in which different bark textures might be selected for on wet and dry sites. Third, Q. falcata can be legitimately divided into two varieties (Ware, 1967), variety pagodaefolia which is restricted to wet and mesic sites, and variety falcata which occurs in dry or mesic stands. Since var. pagodaefolia is said to have smoother bark than var. falcata (Harlow and Harrar, 1968) we were interested to see whether the changing proportions of the two varieties from wet to dry sites might be responsible for a shift from smooth to rough bark in the species as a whole. MATERIALS AND METHODs-Four stands (all located within 125 km of the upper Texas coast; Fig. 1) were chosen as being representative of 1 Received for publication 7 September 1978; revision accepted 31 January 1979. Supported in part by the National Park Service. We thank B. Chabot, P. L. Marks, and D. R. Streng for helpful comments on the manuscrint. different moisture regimes. The stands were selected on the basis of their position on a reciprocal averaging ordination (Hill, 1973), including 54 southeast Texas stands, performed by Marks and Harcombe (1978). The ordination had clearly shown changes in vegetation which correlated well with site physical properties, especially soil texture and slope position, implying that moisture is an important underlying environmental gradient. Stand location and descriptive data are summarized in Table 1. The stands are numbered 1-4 in order of increasing moisture. Note especially the shift in stand dominance from species characteristic of dry uplands to species characteristic of wet-mesic sites and the corresponding decrease in percent sand. The changing relative abundance of the varieties, from 100% falcata at site 1, to 100% pagodaefolia at site 4 (Glitzenstein, 1979) is also indicative of a change in site moisture status (Caplenor et al., 1968). Randomly located transects were walked through each site. Bark micro-relief of southern red oaks encountered along each transect was measured with an instrument described by Yarranton (1967). His instrument consists of an aluminum ring of known inner diameter which is divided into two semicircles, so that it can be easily attached to and removed from a tree. At regular intervals along the ring (2.5 degrees on our instrument) the distance from the tree trunk to the ring is recorded. These measurements are used to create an estimate of trunk circumference. Circumference is also determined by tape measure. The latter estimate is lower than the former because the tape does not penetrate furrows in the bark. The difference between the two measures, standardized to unit circumference (and multiplied by 1,000 for convenience) is the measure of bark roughness (BR). Symbolically, BR = 1,000 (Ca C,)/C,, where C a is circumfer-