Stock type performance in addressing top-down and bottom-up factors for the restoration of indigenous trees

Abstract

Using planted trees to restore the attributes of natural forest is especially challenging when disturbances are still effective. Chronic browsing by large herbivores can act as such a chronic disturbance curtailing natural regeneration and potentially altering successional trajectory of forest. In the context of herbivore overabundance encountered in many regions of the world, plantation strategies must thus address both the top-down pressure exerted by consumers on planted trees and the bottom-up control related to competition for resources. In this paper, we explore whether selection of competition-adapted balsam fir (Abies balsamea L.) seedling stock types (small, 110cm3 container; medium; 200cm3; or large, 350cm3) could be used together with the management of white-tailed deer (Odocoileus virginianus) populations in order to lower the effect of local competition as well as minimizing browsing on seedlings. When the top-down pressure from herbivores is low or absent, we hypothesize that height and diameter growth as well as survival will be proportional to the initial size and biomass of seedlings. Inversely, in plantations exposed to deer, the apparency hypothesis predicts that herbivores are most likely to feed on taller, more obvious seedlings. Overall, we predict that medium stock size seedlings will outperform small and larger ones as they offer the best size compromise to withstand competition while maintaining a minimum level of apparency in the establishment phase. After 3 growing seasons, the height and diameter of medium stock size seedlings (48.6±0.7cm and 1.06±0.05cm, respectively) were similar to large ones (51.7±1.1cm, p=0.12 and, 1.22±0.05cm, p=0.07) that had been almost twice their biomass at the onset of plantation. The overall browsing occurrence was under 10% for all stock types exposed to browsing, yet the relative risk of being browsed increased by almost 20% for seedlings 30cm vs. 60cm at the end of the previous growing season. Mortality rate was unrelated to the browsing regime (p=0.14) but overall, medium stock seedlings performed slightly better (2.9±0.3%) than both small (7.0±0.2%, p=0.10) and large ones (10.5±0.4%, p=0.03). Based on the prominent effect of bottom-up control over top-down control in our experimental plantation, we conclude that choosing a size-adapted stock can optimize the cost of the restoration scheme following herbivore population reduction.