Distribution of black spruce versus eastern larch along peatland gradients: Relationship to relative stature, growth rate, and shade tolerance

The relative abundance and performance of black spruce (Picea mariana) and eastern larch (Larix laricina) were quantified along fertility gradients in two peatlands in northern Wisconsin. We hypothesized that: (i) the distribution of these two dominants should correspond to the portions of the gradient where each has an advantage in stature and (or) rate of height growth relative to its principal competitor; that (ii) larch should have a higher rate of height growth on wetter, more minerotrophic sites, based on the costs and benefits of its deciduous habit; and that (iii) spruce should be more shade tolerant and increase in abundance during succession. Our findings largely support these hypotheses, and (at the hydrologically stable site) provide the first circumstantial evidence for the competitive sorting of woody species along a natural gradient. The incongruity of interspecific differences in height and growth at one of the two sites is evidence for an historic change in the environment there, involving a shift in hydrology around 1925.