The Trees’ Influence on Soil Development. Nutrient Cycling Strategies of Trees. Fundamental Knowledge for synecological Forest Ecosystem Management.

Abstract

The influence of various tree species and -genera on an initially homogenous soil substrate has been analysed and compared. The homogeneity of the site is due to a common loessial soil substrate, a narrow topographical and climatical range, the prehistoric lowland forest cover and an equivalent subsequent history with clearance of the indigenous vegetation, followed by reafforestation around the turn of the century. The trees were located at random and soil samples were taken, half way between the trunk and the drip lines at four locations and at three depths within the soil A-horizon. The four sample locations around each tree were pooled and the three depths at 0-1 cm; 7-8 c; 14-15 cm were subsequently analysed for: pH; cation exchange capacity; contents of the base cations (Ca; Mg; K; Na); phosphorus; and soil organic matter, specifically carbon, nitrogen and the humic matter fractions, including an elemental analysis (C, N and H) of the humic fractions. For each depth level, these soil parameters (except elemental contents of the humic fractions) were subjected to statistical analyses (Principal Component Analysis, P.C.A.; Cluster Analysis, C.A.), which grouped individual trees into two related groups and one sign ificantly different group according to their influence on the soil. Analyses of Variance established hypotheses with regard to group specific nutrient cycling strategies. A correlation analysis of all soil parameters and a regression analysis of two soil parameters aided with the interpretation of the ordination results. An elemental analysis of the humic fractions showed diffrenences in structure and the elemental allocation, dependent on the phanerophyte input and soil depth.

The first two P.C.A. derived groups, New Zealand Indigenous Trees (Dacrydium cupressinum, Prumnopitys ferruginea, P. taxifolia, Nothofagus fusca, N. solandri, N. truncata, Plagianthus regius, Sophora microphylla, Kunzea ericoides) and Exotic Deciduous Trees (Acer pseudoplatanus, Aesculus hippocastanum, Fagus sylvatica, Fraxinus excelsior, Quercus canariensis, Q. robur, Q. rubra, Tilia europaea, Ulmus procera, Ulmus x hollandica) have in common a strategy that increases the nutrient storage capacity and which favours the cycling of nutrients in the soil medium. Individual vigours in the cycling of nutrients and the creation of nutrient storage acpacity towards one or the other soil humus type are indicated by the ordinations, specifically the C.A. The histograms, the correlation and regression analyses of soil parameters demonstrate,the presence of specific strategic and amplitudinal distinctions in creating storage capactiy and the cycling of nutrients, that, based on site homogeneiety are independent from the quality of the mineral substrate. Trees from both groups show convergences and specifc inherent contributions towards the genesis of soil nutrient storage capacity and the cycling of nutrients.

A third group is significantly different from the indigenous and deciduous groups and includes the following trees: Pinus radiata, Pseudotsuga menziesii, Larix decidua, Eucalyptus regnans. A tight clustering in the ordinations, comparison with the mineral substrate reserves, the analyses of variance, soil parameter correlation and -regression analyses demonstrate a common strategy based on the extraction of nutrients from the soil. Nutrient extraction strategeis facilitate survival in harsh environments of their natural habitats. These trees have in common an adaptation to recurring events of disturbance, often caused by fire, which is fuelled by a litter load that resists decomposition. An ecological term describes these as Pyrophanerophytes (Fire Trees). Fre trees tend to form stands of little species diversity. The common transplantation of fire trees to forest-soil systems, that were based on nutrient cycling and consequently attained large soil nutrint stores will have the following consequences: 1) The growth of fire trees is faster than in their natural habitats. 2) A de-coupling of the nutrient cycling process ensues. 3) Pyrophanerophyte litter leachates and root exudates lead to the effective decomposistionof clay minerals and deminishes mineral nutrient reserves. 4) Soil organic nutrient stores may be decomposed and serve as a source of N (de-coupling of the nitrogen cycle) 5) The storage decrease due to the loss of soil organic matter diminishes the cation exchange capacity and the nutrient storage capacity. 6) Artificial fertilisation will rapidly lose effectiveness. 7) The resulting potential for soil acidification is considerable and the soil buffer capacity may rapidly degrade to the aluminium / iron buffer range. 8) The loss of soil organic matter and biological activity may lead to a compacted physical soil structure.

These effects result in a rapid degeneration to a podzol and are the main reasons why evolution did not lead to the dominance of firetree species, since fire trees would naturally eliminate themselves by rapidly changing the soil state. Fire trees are alien to a New Zealand oceanic-temperate forest ecosystem.Widespread commercial forestry, based on pyrophanerophytes (fire trees) is unsustainable. The strategies of trees from the indigenous or exotic deciduous forests show a common ecosystem strategy, based on the creation and maintenance of nutrient storage capacity and the cycling of nutrients in the soil medium, which provides the basis for a long-term potential for yield and the sustainable management of productive forests.