Christian Kuehne

Natural Regeneration in Thinned Douglas-fir Stands in Western Oregon

ABSTRACT In response to interests by land management agencies to transform even-aged stands to structurally mimic old-growth forests, we evaluated whether thinning in 40- to 80-year-old Douglas-fir (Pseudotsuga menziesii) stands influenced amount and composition of advanced regeneration 5 to 7 years following treatment. We used data from two large-scale management experiments (Density Management Study and Young Stand Thinning and Diversity Study) conducted in western Oregon. Thinning focused on the removal of Douglas-fir, while maintenance of minor species was encouraged. Although both experiments showed higher tree regeneration after thinning, we found that variation in regeneration density was too high (3 orders of magnitude) to find statistical differences among thinning intensities. While seedlings of the major species, Douglas-fir and western hemlock (Tsuga heterophylla), were always present and dominated regeneration on nearly all sites, species trends were driven by high spatial and compositional variation throughout all units, treatments, and sites. Thinning increased the number of species within the regeneration layer. Hereby, species diversity was strongly related to overstory composition, suggesting seed source limitations for minor species. Hence, favoring rare species during thinning operations may be an effective method to increase regeneration species richness. Local conditions, as defined by overstory density appeared more influential than regional climate patterns in determining seedling densities. Shrub and grass competition did not prevent seedling establishment as their cover values were generally not as high as typically found in clearcuts in the region. The high variation in seedling density and species richness within the seedling stratum in the thinned stands may set the stage for development of structural complexity in even-aged Douglas-fir plantations.

Species-specific and generic biomass equations for seedlings and saplings of European tree species

AbstractBiomass equations are a helpful tool to estimate the tree and stand biomass production and standing stock. Such estimations are of great interest for science but also of great importance for global reports on the carbon cycle and the global climate system. Even though there are various collections and generic meta-analyses available with biomass equations for mature trees, reports on biomass equations for juvenile trees (seedlings and saplings) are mainly missing. Against the background of an increasing amount of reforestation and afforestation projects and forests in young successional stages, such equations are required. In this study we have collected data from various studies on the aboveground woody biomass of 19 common tree species growing in Europe. The aim of this paper was to calculate species-specific biomass equations for the aboveground woody biomass of single trees in dependence of root-collar-diameter (RCD), height (H) and the combination of the two (RCD2 H). Next to calculating species-specific biomass equations for the species available in the dataset, we also calculated generic biomass equations for all broadleaved species and all conifer species. The biomass equations should be a contribution to the pool of published biomass equations, whereas the novelty is here that the equations were exclusively derived for young trees.

A comparative study of physiological and morphological seedling traits associated with shade tolerance in introduced red oak (Quercus rubra) and native hardwood tree species in southwestern Germany

Northern red oak (Quercus rubra L.), a moderately shade-tolerant tree species, is failing to regenerate throughout its native North American range, while successful recruitment in Central Europe has been observed since its introduction. To examine whether comparative photosynthetic performance could explain the regeneration success of this non-native species in Central Europe, we compared the physiological and morphological seedling traits of red oak with three co-occurring tree species under three canopy types in southwestern Germany. Native species included a moderately shade-tolerant native oak (Quercus robur L.) and two shade-tolerant species (Acer pseudoplatanus L. and Carpinus betulus L.). The photosynthetic traits of non-native red oak seedlings were similar to those reported for this species in the native range, where shade-tolerant competitors readily outperform red oak under low light conditions. However, compared with native shade-tolerant species in Europe, red oak seedlings photosynthesized efficiently, especially under closed canopies and in small canopy gaps, exhibiting high photosynthetic capacity, low leaf dark respiration and leaf-level light compensation points that were similar to the more shade-tolerant species. The superior net carbon gain of red oak seedlings at low and moderate light levels was likely facilitated by high leaf areas and reflected by seedling dry masses that were greater than the observed native European species. A competitive advantage for red oak was not evident because relative height growth was inferior to seedlings of co-occurring species. In North America, the inability of seedlings to compete with shade-tolerant tree species in deeply shaded understories is central to the problem of poor oak recruitment. Our study suggests that the ability of non-native red oak to perform equally well to native shade-tolerant species under a variety of light conditions could contribute to the consistent success of red oak regeneration in Europe.