Soil heterogeneity in tree mixtures depends on spatial clustering of tree species

Heterogeneity in soil characteristics promotes and maintains coexistence between a diverse set of species. In forests, trees have species-specific impacts on soil abiotic characteristics and mixing of tree species is being promoted as a tool to ensure high levels of diversity and functioning. Yet, limited knowledge is available on the effect of tree species composition and spatial clustering on heterogeneity in soil characteristics. In this paper we derived heterogeneity of key characteristics of the leaf litterfall, the forest floor and the mineral topsoil (C, N and base cation concentration, C:N ratio and mass) in 53 plots of 7 different tree species compositions. We found that heterogeneity increased from the leaf litterfall, through the forest floor down to the mineral topsoil. Mixing tree species did not lead to an increased heterogeneity in the forest floor and topsoil compared to monocultures. However, we did find that mixed plots where conspecific trees stand in groups are more heterogeneous than plots where species are intimately mixed. Our results imply that heterogeneity in soil characteristics does not necessarily increase with tree diversity, but that within mixed stands the spatial organization of tree species should be considered in relation to the scale at which heterogeneity is desired.     

DOC and N<Subscript>2</Subscript>O dynamics in upland and peatland forest soils after clear-cutting and soil preparation

Forest clear-cutting followed by soil preparation means disturbance for soil microorganisms and disruption of N and C cycles. We measured fluxes of N₂O and dissolved organic carbon (DOC) in upland soil (podzol) and adjacent peat within a clear-cut forest catchment. Both soil types behaved in a similar way, showing net uptake of N₂O in the first year after the clear-cutting, and turning to net release in the second. The N₂O flux dynamics were similar to those of N content in logging residues, as reported from a nearby site. As organic matter is used in the food web of the decomposers, we attempted to explain the dynamics of N₂O uptake and release by measuring the concurrent dynamics of the low molecular weight (LMW) fraction and the aromaticity of DOC in a soil solution. The labile and most readily available LMW fractions of DOC were nearly absent in the year following the clear-cutting, but rose after two years. The more refractory high molecular weight (HMW) fraction of DOC decreased two years after the clear-cutting. The first year’s net uptake of N₂O could be accounted for by the growth of decomposer biomass in the logging residues and detritus from the degenerating ground vegetation, resulting in immobilization of nitrogen. Simultaneously, the labile, LMW fraction of DOC became almost completely exhausted. The low availability of the LMW fraction could retard the growth and cause the accumulated decomposer biomass to collapse. During the following winter and summer the fraction of LMW clearly increased, followed by increased N₂O emissions. The presence of LMW DOC fractions, not the concentration of DOC, seems to be an important controller for N₂O liberation after a major disturbance such as clear-cutting and site preparation. The complex connection between DOC characteristics, nitrification or denitrification merits further studies.     

Insights into the pathways of spread and potential origins of Dothistroma septosporum in Britain

Dothistroma needle blight (DNB) is a disease caused by two fungi, Dothistroma septosporum and Dothistroma pini, that has resulted in significant damage to pine forests worldwide. Analysis of 1194 British Dothistroma isolates revealed that only D. septosporum occurred in Britain; D. pini was not detected. The genetic diversity, population structure, and reproductive mode of D. septosporum in Britain were investigated using species-specific mating type markers and eleven microsatellite markers, revealing 382 multilocus haplotypes. Comparison of clustering methods (STRUCTURE, BAPS, DAPC) as well as spatial principal component analysis (sPCA) showed some differences between the methods but similar groupings. A clear north-south cline was found with attributes consistent with a native fungus. Other groups were most probably introduced, with one nearly exclusive lodgepole pine group exhibiting links with Canada. Evidence for the movement of specific multilocus haplotypes via nursery stock as well as across borders is provided and the implications discussed.     

Potential natural vegetation: validity and applicability in landscape planning and nature conservation

Abstract. Since the introduction of ‘potential natural vegetation’ (PNV) as a concept in vegetation science by Tüxen (1956), many PNV-maps with different scales have been made. Tüxen emphasized the great value of PNV-maps for different purposes in land use, landscape planning and nature conservation, in particular with regard to forestry, agriculture and landscape management. Different aspects are discussed in order to examine the validity and applicability of PNV-maps in landscape planning and nature conservation. PNV-maps are useful for the differentiation of natural and landscape units on a small scale (< 1 : 100 000). However, maps of the potential natural vegetation are less useful for purposes of detailed planning on larger scales (> 1 : 100 000). Problems arise, for example, from the often highly hypothetical character of the construction and the practice of taking remnants of ‘natural’ vegetation as a reference object for the PNV. With regard to the goals of modern landscape planning and nature conservation purposes (e.g. conserving biodiversity in the cultural landscape of Central Europe) the exact documentation of the actual real vegetation (ARV) on intermediate and large scales gives much more detailed information than a hypothetical PNV.