Sapling size influences shade tolerance ranking among southern boreal tree species

1 Traditional rankings of shade tolerance of trees make little reference to individual size. However, greater respiratory loads with increasing sapling size imply that larger individuals will be less able to tolerate shade than smaller individuals of the same species and that there may be shifts among species in shade tolerance with size. 2 We tested this hypothesis using maximum likelihood estimation to develop individual-tree-based models of the probability of mortality as a function of recent growth rate for seven species: trembling aspen, paper birch, yellow birch, mountain maple, white spruce, balsam fir and eastern white cedar. 3 Shade tolerance of small individuals, as quantified by risk of mortality at low growth, was mostly consistent with traditional shade tolerance rankings such that cedar > balsam fir > white spruce > yellow birch > mountain maple = paper birch > aspen. 4 Differences in growth-dependent mortality were greatest between species in the smallest size classes. With increasing size, a reduced tolerance to shade was observed for all species except trembling aspen and thus species tended to converge in shade tolerance with size. At a given level of radial growth larger trees, apart from aspen, had a higher probability of mortality than smaller trees. 5 Successional processes associated with shade tolerance may thus be most important in the seedling stage and decrease with ontogeny

Predation and Competition Within an Assemblage of Larval News (Triturus)

The impact of crested newts (Triturus cristatus) on the smaller-bodied palmate and smooth newts (T. helveticus and T. vulgaris) was studied during the larval stages using a combination of field and laboratory experiments. In pond enclosures T. cristatus larvae had no effect on the two smaller species over the first four weeks of development. By eight weeks, however, T. cristatus had achieved a size advantage which enabled it to eliminate T. helveticus and severely reduce T. vulgaris by predation. In laboratory trials under food-limited conditions, T. helveticus and T. vulgaris were slightly smaller when raised with T. cristatus, suggesting that this predatory effect was complemented by interspecific competition during early development. Predation of the smaller species started when T. cristatus reached a threshold size of c. 27 mm. No reciprocal effects on T cristatus growth or survival were observed. Although T. cristatus may be a significant predator of congeneric species in natural ponds, other factors, such as differences in microhabitat selection, higher-order predator-prey interactions, and the occasional desiccation of pond habitats may facilitate coexistence between the species