Growing-space relationships in young even-aged northern hardwood stands based on individual-tree and plot-level measurements

Abstract

et al. 2005). This relationship is visible in a wide range of speciesspecific crown-width models (e.g., Bragg 2001, Bechtold 2003) when the resulting crown-width to stem diameter ratios are plotted against dbh. Similar trends emerge from the canopy cover estimates for mixed northern hardwoods presented by Leak et al. (1987, Appendix Tables 17–19) and are supported by size-density relationships portrayed by traditional density management diagrams (Reineke 1933, Drew and Flewelling 1979), i.e., those assuming a self-thinning exponent close to 1.6. By contrast, if self-thinning were directly proportional to basal area (BA), the exponent would be 2, suggesting that stand BA remains constant at full stocking. A declining ratio between crown and stem size is consistent with a self-thinning exponent that is less than 2. The ranking of species by shade tolerance in relation to growing space requirements may change as stand development progresses. In a recent comparison of size-density relationships across numerous species, Lhotka and Loewenstein (2008) reported multiple self-thinning trajectories and noted that comparisons of growing-space utilization among species may be valid only for a given stage of stand development, i.e., at a common average stand diameter. Some evidence indicates shade-tolerance characteristics are most strongly expressed when trees are young (Kneeshaw et al. 2006), suggesting that differences in crown-stem allometry associated with shade tolerance might also be highest early in stand development. Our study was designed to investigate species differences in growing-space occupancy at the individual-tree level and to combine those results with observations of groups of trees measured on larger plots. To do so, we sampled three young even-aged northern hardwood stands in the stem-exclusion stage of stand development. We hypothesized a positive relationship between crown-stem allometry and species shade-tolerance characteristics, where more shadetolerant species occupy more growing space per unit stem diameter. We also hypothesized that the ratio of crown size to stem size would decline with stand age or average tree size independent of species. Methods Study Site These data were collected from three Adirondack northern hardwood stands established by shelterwood-method seed cutting at the Huntington Wildlife Forest near Newcomb, New York (44 E 00 N, 74 E 13 W). The climate is cool, moist, and continental; average annual precipitation is 1,010 mm, and the mean annual temperature is 4.4°C (Shepard et al. 1989). The study stands were located on gently to modestly sloping terrain. Soils at all three sites are Spodosols in the Beckett series. These are deep, moderately well drained, strongly acid, and moderately coarse-textured soils with a fragipan. Prior to seed cutting, the stands supported well developed northern hardwood communities 200–300 years in age dominated by sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia Ehrh.), with lesser amounts of yellow birch (Betula alleghaniensis Britton), white ash (Fraxinus americana L.), and black cherry (Prunus serotina Ehrh.). Dense American beech understories were controlled by mist-blowing of herbicide followed by individual stem injection of larger unmerchantable American beech trees (Sage 1987). The mist-blowing treatment was not selective and greatly reduced advance regeneration and trees up to approximately 5 m tall of all species (Sage 1987). Controlled hunting reduced local deer populations from 10 down to 5 deer km , a level deemed necessary for securing desirable reproduction in this region (Kelty and Nyland 1981, Sage et al. 2003). Shelterwood seed cutting left primarily sawtimber-sized sugar maple trees at a wide spacing (20–50% canopy cover) in all the stands. Data were collected for this study when these stands had developed for 19 (youngest stand), 24 (mid-aged stand), and 29 (oldest stand) years following seed cutting. At this time, 9, 20, and 19 years had elapsed since the final overstory removal in each stand. Species composition of the reproduction was similar in all three stands, with varying proportions of yellow birch and sugar maple and lesser amounts of American beech and white ash (Ray et al. 1999). The older stands had widely scattered individuals of black cherry and quaking aspen (Populus tremuloides Michx.), with crowns extending above the general level of the main canopy. Scattered pin cherry (Prunus pennsylvanica L.f.) were still of this stature in the youngest stand (19 years) but had begun to die off in the older ones (24 and 29