Odd Eilertsen

Canonical Correspondence Analysis with variation partitioning: some comments and an application

. This study presents an alternative treatment of data from a comprehensive vegetation study in which the main gradient structure of boreal coniferous forest vegetation in southern Norway was investigated by ordination techniques. The data sets include vegetation samples of different plot sizes, supplied with measurements of 33 environmental explanatory variables (classified in four groups) and nine spatial explanatory variables derived from geographical coordinates. Partitioning the variation of the species-sample plot matrices on different sets of explanatory variables is performed by use of (partial) Canonical Correspondence Analysis. Several aspects of vegetation-environment relationships in the investigation area are discussed on the basis of results obtained by the new method. Generally, ca. 35% of the variation in species abundances are explained by environmental and spatial variables. The results indicate support for the hypothesis of macro-scale topographic control over the differentiation of the vegetation, more strongly so in pine than in spruce forest where soil nutrients play a major role. Towards finer scales, the primary topographical and topographically dependent factors lose importance, and vegetational differentiation is more strongly affected by the accumulated effects of the vegetation (including the tree stand) on soils, shading, litter fall, etc. The fraction of variation in species abundance explained by significant environmental variables was found to be ca. twice as large as the fraction explained by spatial variables. The fraction of variation explained by the supplied variables differed between data sets; it was lower for cryptogams than for vascular plants, and lower for smaller than for larger sample plots. Possible reasons for these patterns are discussed. Some methodological aspects of CCA with variation partitioning are discussed: improvements, necessary precautions, and the advantages over alternative methods.

Acid deposition and its effects in China: an overview

Acid rain is an increasing environmental problem in China. At present SO2 emission is about 20–22 million tons. However with a growing number of large power plants the long-range transport of air pollutants is expected to increase. The highest acid deposition is near the emission sources. Wind-blown, alkaline soil dust is important in neutralizing the acidity of the emissions, especially in large parts of northern China. In the south, where alkaline soil dust contributes less to acid neutralization, the annual pH in precipitation was below 4.5 at monitoring stations in several provinces and as low as 4.1 in some urban areas. Total sulfur deposition has been estimated to be about 10 g S m−2 year−1 in heavily exposed areas. Negative effects on forests, including die-back, have been reported for relatively small areas near large cities. Since large, regional surveys have not been carried out, there are large uncertainties about effects on a regional level. The high concentrations of gaseous pollutants, especially within and near the cities, are likely to have severe effects on human health as well as on materials and vegetation. Several field and laboratory studies, as well as computer simulations, indicate that acidification of soil and soil water has occurred in the past few decades. This has probably caused elevated concentrations of toxic aluminum in soil water. At present, the toxic effect of Al is likely to be counteracted by high concentrations of calcium at many places. The Chinese authorities have recognized air pollution and acid rain as serious environmental problems, however, there are difficulties in implementing effective measures to reduce the problems. With respect to ecological effects we lack a comprehensive regional overview of the extent of the acid deposition problem in China. Such information is necessary before effective countermeasures can be developed.

Data manipulation and gradient length estimation in DCA ordination

The effects of different kinds of data manipu- lation on gradient length estimation by non-linear rescal- ing (as in DCA ordination) are evaluated by considering the first axis in DCA ordinations of 11 field data sets from four investigations. Gradient length estimates are de- pendent on the range of the abundance scale; the more the scale favours the quantitative aspect (abundance) of the data over the qualitative aspect (presence), the longer the DCA axes. The gradient length estimate decreases when infre- quent species are deleted. A new formula is proposed to replace the option for downweighting of rare species in DCA, as the option presently available has some undesir- able properties. Some implications for interpretation of gradient length estimates by non-linear rescaling in gen- eral (and in DCA in particular) and for comparison of gradient length estimates between studies, are discussed. The potential of non-linear rescaling of gradients for estimation of P3 diversity is emphasized.

Dynamics of understory vegetation in an old-growth boreal coniferous forest, 1988 -1993

. Understorey vegetation changes in a South Norwegian old-growth coniferous forest were studied between 1988 and 1993 in 200 1-m2 vegetation plots. Our aims were to quantify the amount of between-year compositional change, and to elaborate the environmental basis for long-term vegetation change, including the previously identified gradient structure with a major gradient related to topography (and soil nutrient status and soil depth) and a minor gradient reflecting paludification and canopy coverage. Species richness (yearly mean and cumulative species number) and change in species richness differed between vascular plants and cryptogams, and between forest types. The number of vascular plant species decreased in pine forest in dry years; bryophyte species number increased in spruce forest. Statistically significant vegetation change, as tested by constrained ordination (CCA) with time as the constraining variable, is demonstrated for most one-year periods and for the five-year period in most forest types. Vegetation change along identified gradients, measured as plot displacement along DCA ordination axes, also occurred. The magnitude of year-to-year vegetation change was related neither to forest type nor to one-year period; different responses to climatic and environmental change were observed in each forest type. The largest average displacement observed, from medium-rich spruce forest towards poor spruce forest, was interpreted as a long-term trend. Humus-layer pH decreased by ca. 0.25 units from 1988 to 1993, most strongly in medium-rich spruce forest where exchangeable Ca decreased and Al and Mn increased strongly. Our study supports the hypothesis that vascular plants show a long-term and broad-scale response to soil acidification. Change in bryophyte composition is linked to some very long growing-seasons. Detailed analysis of short-term vegetation dynamics enhances the interpretation of long-term changes and stresses the complementarity of univariate and multivariate methods in the analysis of vegetation change.

On the relationship between sample plot size and beta diversity in boreal coniferous forests

Nested sample plots of three sizes (16, 1, and 1/16 sq. m) from three different studies of Norwegian coniferous forests have been subjected to DCA ordination using the same choice of options. At each sample plot size, species quantities are recorded as frequency in 16 subplots. Beta diversity, measured as length of the first DCA axis, invariably increased upon lowering of sample plot size. The same applied to the eigenvalues of the axes. This is explained as a consequence of the weakening of structure in the data matrices when the fine-grained patterns of the vegetation are emphasized.

Integrated Monitoring Program on Acidification of Chinese Terrestrial Systems (Impacts) — A Chinese-Norwegian Cooperation Project

A 5-year Chinese-Norwegian research project was launched in the autumn of 1999. Forested sites for intensive studies are or will be established in the Chongqing municipality and in Guizhou, Hunan and Guangdong provinces in southern China. Previous studies have shown that harmful effects of acid deposition are likely to be most severe in this region. The research and monitoring sites shall give information about acidification mechanisms and effects on vegetation in order to improve policy oriented acidification models and critical load estimates as well as function as interdisciplinary training centers for acid rain research. Furthermore, the project shall improve the basis for developing an efficient regional acid rain monitoring system. At one site in Guizhou and one in Chongqing, research on soil and soilwater chemistry has been ongoing for several years. The forest at these sites appears to show symptoms of reduced vitality. The sensitivity of Chinese forests to acidification is uncertain and will be focused. Decision-makers should get an improved basis for optimal mitigation measures through the project.

The Nordic Vegetation Survey - concepts and perspectives

Abstract. The background of the Nordic Vegetation Survey is outlined and the concepts for this international collaboration project are reviewed. The project includes, so far, institutions in Denmark, Faeroes, Finland, Norway and Sweden. We are setting up a common basis for a vegetation database, with a Nordic species list, exchange of data and joint revisions of important Nordic vegetation types. Progress in the Nordic Vegetation Survey is described and the future perspectives indicated.