Diego F. Steinaker

EFFECTS OF CLIMATIC VARIABILITY ON FACILITATION OF TREE ESTABLISHMENT IN NORTHERN PATAGONIA

Facilitation of tree establishment by nurse shrubs, which ameliorate otherwise unfavorable microenvironmental conditions, is a widely studied phenomenon. However, relatively little is known about how facilitative influences change in relation to interannual climatic variability. In northern Patagonia, Argentina, we examined influences of potential nurse shrubs on the establishment of the conifer Austrocedrus chilensis and assessed the significance of those influences to establishment during years of contrasting climate. We also experimentally investigated the effects of nurse shrubs and different water availability on tree seedling emergence and survival. A strong spatial association of Austrocedrus juveniles with shrubs, both beneath shrub canopies and near shrub canopies, indicates that shrubs favorably influence tree regeneration and that in some habitats and time periods nurse plants appear to be required for successful tree seedling establishment. Protection from direct sunlight was the main factor c...

BELOWGROUND LITTER CONTRIBUTIONS TO NITROGEN CYCLING AT A NORTHERN GRASSLAND-FOREST BOUNDARY

Forest expansion at the northern edge of the Great Plains is associated with increased availability of soil nitrogen (N). Studies of N dynamics typically focus on above- ground litter production, but in semiarid ecosystems, fine-root production greatly exceeds shoot production. We explored the contribution of root and shoot litter to N cycling in adjacent grassland and aspen (Populus tremuloides) forest at the northern edge of the Great Plains. We used a new approach to measure N inputs from root litter production: we combined root productivity data from minirhizotron images with N content data from several root diameter and color classes. We also measured the production and N content of above- ground litter. The novel contribution of our study comes from the simultaneous measure- ments of above- and belowground productivity and N input from litter production in adjacent forest and grassland habitats. Aboveground litter production was threefold greater in forest than in grassland (330 vs. 136 g-m-2.yr-1), but fine roots accounted for 80-90% of total litter production. As a result, total production was not significantly different between hab- itats, and the N contribution from total litter production was surprisingly similar between grassland (16.8 g.m-2.yr-1) and forest (17.1 g.m-2.yr-1). Thus in spite of great differences between habitats in aboveground litter production, N inputs from total litter production cannot explain the higher availability of N in forest soils. However, we found differences between habitats in root litter quality (forest, 1.14% N; grassland, 0.81% N, as well as in the seasonal and vertical distribution of root production. Grassland root production was significantly greater than forest root production in early summer in the top 20 cm of soil. Conversely, forest produced more root length at the end of the growing season in deeper soil layers (>50 cm). These differences may increase available N in forest soils, but this increase is not attributable to differences in total litter production between habitats.

Fine root dynamics of trembling aspen in boreal forest and aspen parkland in central Canada

Abstract• Fine root responses to potential climate change are relatively unknown in spite of their central role in ecosystem functioning.• We quantified fine root length, production, and turnover in boreal forest and aspen parkland of central Canada because the future climate of the boreal site is expected to be similar to the current climate of the parkland site.• Root depth distribution and turnover were similar between sites. Fine root mass was 4× greater at the parkland site and root length was 10× greater. Accordingly, the ecosystem level fine root: leaf mass ratio was 1.6 in the boreal site compared to 4.3 in the parkland site. On a per tree basis, however, fine root biomass was similar between sites due to the higher stem density of the parkland site.• The parkland site had a greater proportion of very fine roots (62% of the fine roots were < 0.1 mm in diameter) compared with the boreal site (82% of the fine roots were between 0.1–0.5 mm in diameter).• These differences indicate a large-scale shift towards increased root allocation at the parkland site associated with decreasing water availability and earlier successional stage.