Tonje Økland

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.

Species response curves along environmental gradients. A case study from SE Norwegian swamp forests

Abstract Vegetation science has relied on untested paradigms relating to the shape of species response curves along environmental gradients. To advance in this field, we used the HOF approach to model response curves for 112 plant species along six environmental gradients and three ecoclines (as represented by DCA ordination axes) in SE Norwegian swamp forests. Response curve properties were summarized in three binary response variables: (1) model unimodal or monotonous (determinate) vs. indeterminate; (2) for determinate models, unimodal vs. monotonous and (3) for unimodal models, skewed vs. symmetric. We used logistic regression to test the influence, singly and jointly, of seven predictor variables on each of three response variables. Predictor variables included gradient type (environmental or ecocline) and length (compositional turnover); species category (vascular plant, moss, Sphagnum or hepatic), species frequency and richness, tolerance (the fraction of the gradient along which the species occurs) and position of species along each gradient. The probability for fitting a determinate model increased as the main occurrence of species approached gradient extremes and with increasing species tolerance and frequency and gradient length. Appearance of unimodal models was favoured by low species tolerance and disfavoured by closeness of species to gradient extremes. Appearance of skewed models was weakly related to predictors but was slightly favoured by species optima near gradient extremes. Contrary to the results of previous studies, species category, gradient type and variation in species richness along gradients did not contribute independently to model prediction. The overall best predictors of response curve shape were position along the gradient (relative to extremes) and tolerance; the latter also expressing gradient length in units of compositional turnover. This helps predicting species responses to gradients from gradient specific species properties. The low proportion of skewed response curves and the large variation of species response curves along all gradients indicate that skewed response curves is a smaller problem for the performance of ordination methods than often claimed. We find no evidence that DCA ordination increases the unimodality, or symmetry, of species response curves more than expected from the higher compositional turnover along ordination axes. Thus ordination axes may be appropriate proxies for ecoclines, applicable for use in species response modelling. Abbreviations: GAM = Generalized Additive Model; GLM = Generalized linear model; HOF = Huisman-Olff-Fresco; LNMDS = Local Non-metric Multidimensional Scaling. Nomenclature: Lid & Lid (1994); Frisvoll et al. (1995); Krog et al. (1994), except for Polytrichastrum G.L.Sm., which is not recognized as distinct from Polytrichum Hedw. Several groups of related taxa were treated collectively.

Population Biology of the Clonal Moss Hylocomium Splendens in Norwegian Boreal Spruce Forests. II. Effects of Density

1 Hylocomium splendens is a perennial clonal moss in which new modules may emerge annually by ramification of the current mature segment or by regeneration, the time-delayed development from dormant buds. Demographic information on 14 776 growing points was obtained by following 119 Norwegian boreal spruce forest populations for 5 years. The dry weight of each mature segment was estimated from morphological field measurements. 2 The mean size of mature segments was positively correlated with the density (cover) of the bryophyte layer; a density increase from 0 to 100% was associated with a doubling in mean size. This relationship is attributed to the improved moisture conditions, and hence to prolonged periods with positive net photosynthesis, in dense stands. Mean size was not significantly related to the density of Hylocomium splendens growing points, suggesting that interactions in bryophyte stands are mainly with neighbours regardless of species. 3 The frequency of regeneration declined with increasing density, notably when the effects of segment size were corrected for. Negative density-dependence of regeneration is considered to be due to reduction of light-induced bud initiation in dense stands. 4 The size-corrected mortality (termination and loss) rate was not related to density. Danger of burial in the moss carpet, causing about one tenth of all terminations and losses, increased strongly with density and affected c. 5% of all growing points at high densities. 5 At high densities, Hylocomium splendens populations are not regulated by a negative relationship between size and density, but by a combination of reduced branching (ramification and regeneration) and increased danger of burial in the bryophyte carpet. However, such high densities are infrequent in the forest floor (affecting less than 10% of the area investigated). It is suggested that differences in history due to finescale disturbance and microenvironmental variation are reflected in size differences of individual Hylocomium splendens segments, and that this is the major factor that keeps boreal forest bryophyte populations at low densities.

Element concentrations in the boreal forest moss Hylocomium splendens: variation related to gradients in vegetation and local environmental factors

This study aims at assessing the suitability of Hylocomium splendens as a biomonitor for heavy metal deposition and contributing to the understanding of mineral nutrient uptake mechanisms in forest floor bryophytes. Concentrations of Ca, Mg, K, Cd, Cu and Pb were determined in Hylocomium splendens segment samples collected at 196 permanent vegetation plots from eleven monitoring areas, where the vegetation-environment relationships had previously been thoroughly studied. Element concentrations in Hylocomium splendens were related to environmentally interpreted vegetational gradients, cover of understorey vascular plants, and local environmental conditions, including element concentrations in humus, soil moisture and tree impact, by means of correlation analyses performed separately for each area. A combined test was performed for each Hylocomium splendens variable over all eleven areas, showing that concentrations of Ca, Cd and Mg in Hylocomium splendens were strongly correlated with the main vegetational gradient and with pH and concentrations of Ca and total N in humus, indicating considerable uptake of nutrients by Hylocomium splendens from water that has been in contact with humus. These relationships were less strong in the climatically most humid areas. Concentrations of all elements were more or less strongly correlated with one or more variables reflecting tree influence (tree density as measured by basal area and two crown influence indices) reflecting that throughfall precipitation, modified by leaching from the canopy, partly accounts for input of elements, especially for K, Cd and Mg. Only concentrations of Pb, Cd and Cu were significantly correlated, negatively, with vascular plant cover, indicating that supply of elements by leaching from understorey vascular plant foliage is negligible. The concentration of Cu in Hylocomium splendens was only weakly correlated, and the concentration of Pb unrelated to vegetational gradients and most environmental conditions. The results indicate that atmospheric deposition accounts for most of the input of Pb, confirming the suitability of Hylocomium splendens as a biomonitor for this element. For elements such as Cd local environmental conditions should be carefully considered, even when data from regional moss surveys are to be interpreted.

Single-tree influence on understorey vegetation in a Norwegian boreal spruce forest

A model for tree influence on understorey vegetation in boreal spruce forests is developed according to the principles of ecological field theory. Single-tree influence at a point in space was modelled as the product of two factors: (1) the size (diameter at breast height) of the tree relative to that of the largest tree encountered in the area; a parameter specifies the weight given to large trees, and (2) the points distance from the centre of the stem (one parameter specifies the zone of influence of a tree in crown radius units, another parameter specifies the relative weight given to closeness to, versus distance from, the tree stem). The total tree influence at the point is calculated from single-tree influence by a multiplicative model. Optimal choice of model parameters was found by maximizing the eigenvalue of a constrained ordination (CO) axis, obtained by use of the tree influence index as the only constraining variable. Two CO methods. Redundancy Analysis (RDA) and Canonical Correspondence Analysis (CCA), were applied to one vegetation data set with two subsets from a boreal spruce forest in SE Norway. The eigenvalue of CO axes constrained by the optimized tree influence index differed between the two CO methods, amounting to 9-10% of total inertia with RDA and 3% with CCA. The higher eigenvalue-to-total-inertia ratio with RDA was interpreted as due mainly to the low species turnover (low β-diversity) along the tree influence gradient, as demonstrated by the monotonic responses of a majority of species to tree influence. The linear species response model in RDA was therefore more appropriate in this case than the unimodal model of CCA. Vascular plants and cryptogamic species differed with respect to optimal parameters in the tree influence model. These differences were in accordance with throughfall precipitation and time in hydrated state as the most important determinants of cryptogamic species performance, and soil moisture and incoming radiation as the most important factors for vascular plants. Some applications of the tree influence model are proposed.

PLANT SPECIES COMPOSITION OF BOREAL SPRUCE SWAMP FORESTS: CLOSED DOORS AND WINDOWS OF OPPORTUNITY

Previous studies point to biogeographic (i.e., evolutionary and demographic) and ecological (i.e., habitat differentiation and disturbance) processes as the most important causes of spatial variation in species richness and species composition (occurrence and abundance). We examined patterns of variation in vascular plant and bryophyte species composition among 150 1-m2 plots distributed semi-randomly over 11 Norwegian boreal swamp-forest localities. Swamp forests are species-rich islands in an otherwise species-poor forest landscape. For each plot, 53 environmental variables were recorded. By using Canonical Correspondence Analysis (CCA), we found that ∼20% of the explainable variation in species composition was due to swamp-forest affiliation, in addition to the ∼35% that was due to environmental differences between swamp-forest localities. The uniqueness of the species composition of each swamp forest was also emphasized by analyses of compositional dissimilarity. Plots were significantly more dissimilar if situated in different swamp forests than if situated in the same swamp forest, after environmental differences had been corrected for. The lack of any significant relationship between compositional dissimilarity and geographical distance or swamp-forest area indicated that this pattern was not mainly due to recent successful dispersal and establishment events. We argue that the distinctness of swamp forests, in particular, those richer in species and soil nutrients, is due to a combination of factors among which randomness in establishment in gaps (“windows of opportunity”) and persistence of established clonal species are important. Furthermore, we argue that the probability for successful recruitment may have been higher in previous time periods than it is today. The unique combination of important determinants of the species composition in boreal swamp forests supports the view that there exists a diversity of explanations for diversity, and that these, to a large extent, are system and/or area specific.

Population biology of the clonal moss Hylocomium splendens in Norwegian boreal spruce forests. 4. Effects of experimental fine-scale disturbance

We studied demographic effects of density reduction by fine-scale disturhance on the perennial clonal moss species Hylocomium splendens in a field experiment in an SE Norwegian boreal forest. Half the bryophyte cover was removed from 10 plots in a fine-scaled pattern. Size of ca 800 mature segments (estimated from morphological field measurements) and demographic changes in this experimental population and an unmanipulated reference population were followed from 1993 to 1995. The mean size of mature segments decreased in both populations during the study, due to an extended drought period in 1994. The number of mature segments in the unmanipulated population remained almost unchanged. Branching rates and population growth rates were significantly higher in the experimental population. Rates of ramification were higher than in the unmanipulated population the first year after treatment, and rates of regeneration from fragments (in the gaps) and intact shoot chains were higher both in the first and second year after treatment. We consider enhanced light penetration deeper into the moss carpet as the main cause of the observed differences between the populations. The number of Hylocomium splendens growing points in the experimental population reached 68% of the pre-treatment number in two years. Mean segment size decreased more strongly in the experimental than in the unmanipulated population. Further size reductions are expected. because segments produced by regeneration are smaller than a populations average segment size. Our study demonstrates that the resilience of a population after disturbance depends on the populations current size structure, the frequency and severity of disturbance. and environmental conditions. Experimental disturbance also influenced sporophyte production: in 1995 the frequency of sporophytes was more than ten times as high in the experimental population than in the unmanipulated reference population. Stimulation of fertile branch production (and gametangia) by enhanced radiation, and higher probability of successful fertilization, e.g., due to amelioration of the microclimate, may be involved. Like ramification frequency, sporophyte production in Hylocomium splendens is positively dependent on the size of the source segment, and a well-defined reproductive threshold, a minimum resource (size) level for successful reproduction, seems to occur.

Species richness in boreal swamp forests of SE Norway: The role of surface microtopography

Abstract Question: What is the relative importance of environmental gradients and surface microtopography (variation in vertical level within sampling units) for fine-scale plant species richness in Picea abies swamp forests? Location: 11 swamp forests in SE Norway. Methods: We recorded species richness (number of species of vascular plants, mosses, Sphagnum and hepatics), depth to water table, soil base status and vertical range (microtopographic relief) in 2400 microplots, (each 1/16 m2), in 150 1-m2 plots in the 11 swamp forests. Relationships between species richness and environmental predictors were modelled by GLMM. Results: Moss and hepatic species richness increased with increasing microtopographic relief, most strongly for wet acid sites, in which similar trends were also found for Sphagnum. Relief was a poor predictor of vascular plant species richness. Conclusions: Microtopographic relief is a good predictor of local species richness in Picea abies swamp forests, partly because larger vertical variability means higher within-plot habitat diversity with respect to the wet-dry gradient, and partly because qualitatively new microhabitats associated with steep slopes are added in drier sites. The relationship between species richness and microtopographic relief is context dependent, differing in complex ways among species groups and among sites with different environmental conditions.

Regional variation of change in crown density in spruce forests: a comparative study of data from three projects in southern Norway

Recent forest decline, characterized by premature needle loss and a yellowing of older needles, has been reported from several areas throughout Europe and North America (Johnson & Taylor 1989, Prinz et al. 1987). No single explanation has been identified (Hinrichsen 1986, Wellburn 1994), but acid rain (Ulrich 1983), excess nitrogen deposition (Nihlgard 1985) and general stress (Rehfuess 1989) have been suggested among the primary causes. Public concern about possible forest dieback has also led to increased monitoring efforts in the Nordic countries, but the relationship between forest conditions and airborne pollutants in this region is still a controversial issue (Abrahamsen et al. 1994, Nellemann & Frogner 1994, Strand 1995b, Thomsen et al. 1995, Nellemann 1996). This is not surprising, since altitude, latitude, exposure and other environmental factors must be expected to exert a strong influence on tree vitality in climatically exposed forests at high latitudes (Tveite 1987).