Nährstoffentzüge durch die holz- Und biomassenutzung in wäldern. Teil 1: Schätzfunktionen für biomasse und nährelemente und ihre anwendung in szenariorechnungen

Translated title of the contribution: Export of nutrients from forest ecosystems by harvesting timber and biomass. Part 1: Functions for estimating tree biomass and nutrient content and their application for scenario analyses

Hans Pretzsch, Joachim Block, Jochen Dieler, Jürgen Gauer, Axel Göttlein, Ralf Moshammer, Julius Schuck, Wendelin Weis, Uwe Wunn

Research output: Contribution to journalArticlepeer-review

32 Scopus citations

Abstract

The background for this study is the prevailing increase of timber and biomass use from forest ecosystems and the resulting export of organic matter and risk of depletion of nutrients. In order to estimate and better manage such exports, functions for estimating tree biomass and nutrient content for Norway spruce, Scots pine, Douglas-fir, European beeche and Sessile oak were developed and coupled with an individual tree based growth model for scenario analyses. For biomass and nutrients analyses we harvested 852 trees in Rhineland-Palatinate (n=534) and Bavaria (n=318) in Germany. The sample sites cover poor to fertile site conditions and the trees represent young to mature age phases. Field measurement included size at the tree level (stem diameter, tree height, height to crown base) and the organ level (branch position, branch diameter and length). Biomass samples were taken from various tree compartments (stem, crown, branches twigs, bark, needles) separately for wood above and below 7 cm diameter at the smaller end, and also separately for sapwood and heart wood if necessary. They were used for ascertain the dry mass and element concentration of C, N, P, S, Ca, Mg, K, Al, Fe, Mn, Cu, Zn and B. The results were the basis for developing models which predict compartment-wise the biomass and nutrient for each species depending on dendrometric tree variables. We firstly and completely publish these functions which apply for a broad range of tree sizes (d1.3 = 8 to 75 cm, h = 10 to 40 m), differentiate between tree compartments, and are site specific in case of the nutrient concentrations. By coupling the functions with the growth simulator SILVA the effect of site conditions, tree species, and harvesting intensities was exemplary analysed by model scenario runs. The scenarios evaluated the mean annual biomass and nutrient export (in t·ha-1·yr-1) including all thinning and final harvest and revealed among others that (i) The substrate of the sites hardly affects the biomass export but the nutrient export increases from poor to fertile site due to the higher nutrient content in most tree compartments. (ii) On equal substrate increasing water supply raises both biomass and nutrient export by about 30% due to faster growth. (iii) Biomass and nutrient export is the lowest for pine and oak and highest for douglas-fir and spruce. Douglasfir stands out by high biomass growth but very low K, Ca, and Mg concentrations. Beech, in contrast, has medium growth but high K and Mg concentrations. (iv) The harvest intensity (compartments >7 cm of diameter at the smaller end vs. whole tree) increase the biomass export only by 10% to 25% but the nutrient export by 150% to 200% due to the high nutrient concentrations in the tree tops, branches, twigs, needles. (v) As the proportion of latter compartments decreases with tree size and stand age, nutrients exports are much lower per unit biomass in old compared with young stands. The discussion emphasizes the broad range of validity and flexibility of the biomass and nutrient models. They go beyond existing approaches as they base on tree size instead of the often flawed tree age, they use crown dimensions in addition to tree diameter, and they are site specific and thus better reflect the prevailing nutrient concentrations. How the biomass and nutrient export contribute to the calculation of input/output-balances at stand level and that already moderate exports can result in negative balances for e.g. Ca and critical levels of Mg and K is show exemplarily for a poor site and different harvesting intensities. Transfer of the results into forest planning and management will be achieved on three different ways. Biomass and nutrient content at tree and stand level will be compiled and edited in tables. A computer based decision support system is developed, which enables recalling the biomass and nutrient export depending on species, site conditions, stand development phase, and harvest intensity. Furthermore thematic digital maps with, among others, import, export and balances for the main nutrients will be provided for forest and landscape planning.

Translated title of the contributionExport of nutrients from forest ecosystems by harvesting timber and biomass. Part 1: Functions for estimating tree biomass and nutrient content and their application for scenario analyses
Original languageGerman
Pages (from-to)261-285
Number of pages25
JournalAllgemeine Forst- und Jagdzeitung
Volume185
Issue number11-12
StatePublished - 2014

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