The spatial pattern at alpine tree line may be part of a feedback process in which wind plays a central role. The basic aspects of such a feedback were embedded in a cellular automaton. Spatial metrics of the patterns generated by this simulation and those of observed patterns at a windy tree line site were ordinated using Principal Component Analysis. Only the simulations that included a directionally weighted feedback fell close to the observed sites in ordination space. MANOVA indicated that the directionally weighted feedback is most important in structuring the tree line pattern, but that random hotspots for establishment and the overall steepness of the environmental gradient from forest to tundra in space also have an effect. The importance of wind in determining feedback with the spatial pattern of a canopy indicates that non-linear reactions to climatic change are likely.

Nomenclature: Lesica (2002).

Abbreviation: CA = Cellular automaton.

The role of the propagule bank in aquatic plant maintenance was studied in two riverine wetlands. Four sites were selected, characterized respectively by flooding, drying up, both disturbances operating, and neither operating. Our hypothesis was that recolonization after drying up would mostly involve seeds and buds from the propagule bank, whereas recolonization after floods would mostly involve rhizomes. Dry sites were characterized by a high density of seeds, and a high similarity between seed species and established vegetation. Unspecialized fragments remaining in wet parts of the sediment probably also contribute to species maintenance. Species maintenance in sites subjected to flooding was highly dependent on deeply anchored rhizomes, as indicated by the strong floristic similarity between species that occur in the established vegetation and rhizomes in the bank. Regeneration of the community after scouring floods also involved seeds, some species being able to flower under water. When scouring flooding and drying up were superimposed, regenerative strategies exhibited in the bank did not simply result from the ‘addition’ of the two disturbance effects. When the disturbances did not occur too closely together in time, species were able to survive either by: (1) producing many propagules under aquatic conditions or (2) coping with the temporal variability by producing several types of propagules.

Nomenclature: Lambinon et al. (1992); Wiegleb & Kaplan (1998) for Potamogeton species and Corillon (1975) for Characeae.

Many grasslands and arable fields have been abandoned in Europe in the last decades. So far, however, ecologists have not been able to establish one general, unifying theory for successionally induced changes in species composition following abandonment. In fact, the course of succession seems to be unique for each site and year. Here we focused on the analysis of plant functional traits in order to detect general trends in trait responses to succession which would prove independent of site characteristics. We studied permanent plot series of 14 grassland sites in SW Germany that had been abandoned for 25 yr. Species composition as well as the course of succession varied significantly among the grasslands.

Response to succession was analysed for the following traits: ‘plant height’, ‘canopy structure’, ‘specific leaf area’ (SLA), ‘storage organ’, ‘vegetative spread’, ‘plant persistence’, ‘seed bank longevity’, ‘start of flowering’, ‘duration of flowering’ and ‘seed mass’. We compared a univariate with a multivariate approach. In the univariate approach, attributes of each trait were analysed separately employing GLM whereas in the multivariate approach all traits were handled together in NMS. Both analyses indicated similar trait responses to succession. There was a significant increase in the proportion of species characterized by the following attributes: plant height > 0.6 m, leaves distributed regularly along the stem, vegetative spread > 100 mm, start of flowering later than May, duration of flowering 1–2 months and seeds of either low or high mass.

Abbreviations: GLM = General Linear Model; NMS = Non-metric Multidimensional Scaling.

Nomenclature: vegetation types according to Rennwald (2000).

Reproducibility of vegetation measurements is critical for large-scale or long-term studies, where numerous observers collect data, but past studies have questioned reproducibility of some techniques. Five methods of evaluating understory composition were appraised for reproducibility among six observers in two forest types in south-central Alaska: ocular estimates in quadrats, overall community species rank and cover estimates, nested rooted frequency, horizontal-vertical profiles, and pin drop (systematic points). One forest type was selected to represent structure of coastal communities, another to represent structure of interior Alaska communities. Three general methods of evaluating reproducibility were considered: standard deviations (precision among observers), components of variance (percentage of total variance attributable to observers), and analysis of variance (significance of observer variance). Observer variances were generally similar among techniques and significant in most cases. No technique stood out as being more reproducible than others. Features of techniques other than reproducibility may be more important when selecting a technique. Management decisions based on vegetation cover data should consider the observer errors involved as well as biological significance.

Abbreviations: CV = Coefficient of variation; HV = Horizontal-vertical; SD = Standard deviation.

Nomenclature: Hultén (1968); Viereck & Little (1972); NRCS PLANTS website:  http://plants.usda.gov/

Description of the spatial structure of vegetation is the first step in generating hypotheses regarding underlying processes. It is also important to take into account variation in spatial structure between different localities to be able to distinguish between life history traits and the influence of environment on population processes. Correlogram analysis was used to examine spatial recruitment and mortality patterns of Aspidosperma polyneuron, an emergent tree species, at Reserva Municipal de Santa Genebra (SG), a forest fragment in southeast Brazil. The spatial structure of individuals was compared with two areas with similar physiognomy, one in the same forest and another at Mata do Ribeirão Cachoeira (RC), a semi-deciduous forest located ca. 14 km away. Seedling, sapling and adult mortality occurred in clumps, while seedling recruitment occurred mostly in one region of the area. The spatial pattern varied among the three study areas, and the differences were higher between fragments than within SG. The adults showed a negative correlation in space with saplings and seedlings in one of the SG areas, which could be a consequence of intraspecific competition, or the influence of density or distance dependent natural enemies. However, this pattern was not observed in the other areas. The results suggest that the mechanisms governing the spatial population dynamics of A. polyneuron vary among nearby and similar localities.

Nomenclature: Carvalho (1994); Lorenzi (1992).

Abbreviations: bd = Basal diameter; RC = Mata do Ribeirão Cachoeira; SG = Reserva Municipal de Santa Genebra.

In this paper we present a new approach to the simultaneous analysis of a species composition data set, an environmental gradient data set and a functional attribute data set. We demonstrate its advantages in terms of statistical modelling including model development and assessment as well as subsequent prediction. Our method is applied to a set of case data deriving from experimental wetland microcosms including 20 species, 12 treatment combinations and a classification of species into functional groups. Acknowledging that lack of independence between samples and over-interpretation of data may lead to overly optimistic assessment of model performance, we use cross-validation with different subsets of data to obtain realistic model performance measures. It is shown that although the outcome of the wetland experiment is predictable in terms of experimental treatments and taxonomic species, the functional groups cannot be used to explain the variation in species frequencies in the experiment. We compare the method with recently published approaches to the functional analysis of vegetation data, and discuss its applied perspectives.

Nomenclature: Tutin et al. (1964–1980).

We used classification tree analysis to develop a climate-based distribution model for Fagus crenata forests in Japan. Four climatic variables judged likely to affect the distribution of the species (summer and winter precipitation, minimum temperature of the coldest month and Kira's warmth index) were chosen as independent variables for the model. Latitudinal and longitudinal information was also used to examine effects of spatial autocorrelation on the model. The climatic factors associated with the distribution of the forests were analysed using a classification tree to devise prediction rules. Predicted areas of high probability for forest occurrence lay mainly on the Sea of Japan side of northern Honshu and southern Hokkaido. This is consistent with actual forest distribution. Some areas with high predicted probabilities of F. crenata forest occurrence were beyond the current natural northern range limits of these forests. Since these areas were widely scattered, it was assumed that the species has been hindered from colonizing them due to dispersal limitations. Deviance-weighted scores, used to compare magnitudes of the contributions of predictor variables, revealed winter precipitation as the most influential factor, followed by the warmth index, the minimum temperature of the coldest month and summer precipitation. Attempts were made to generate ecological explanations for the effects of the four climatic factors on the distribution of F. crenata forests.

Abbreviations: CA = Classification accuracy; DWS = Deviance weighted score; JMA = Japan Meteorological Agency; MER = Misclassification error rate; TMC = Minimum temperature of the coldest month; NSNE = National Survey on the Natural Environment; OE = Omission error; PRS = Summer precipitation; PRW = Winter precipitation; RMD = Residual Mean Deviance; WI = Warmth index.

Nomenclature: Ohwi & Kitagawa (1992).

Large-scale disturbances, notably fire and grazing, structure grass and shrubland dynamics in semi-arid environments. We studied early post-fire succession in two burned grasslands, one unburned grassland, and one shrubland near the burned area. We observed three processes: (1) establishment of a ‘phantom’ community comprised of fugitive species. Although transient, these species increase diversity and recharge the seed bank before the next disturbance; (2) regeneration of the original community by persistence of resprouter species and by auto-replacement; (3) early stages of invasion by seedlings of the shrub Fabiana imbricata, which germinate next to shrubland and create new F. imbricata patches. Weed invasion was principally due to the ruderal exotic species Verbascum thapsus from the nearby road verge and by rapid increase of Rumex acetosella cover, another exotic species present before the fire. Although post-fire climatic conditions are particularly important in semi-arid environments, succession depends greatly on the regeneration strategies and dispersal abilities of the species present in the burned area. The phantom community occurs only at the first stage of succession when there is little competition for resources. We could call this process ‘the race for occupation of the area’. The second stage, when competition for resources becomes progressively more important, could be called ‘the effort to maintain space’.

Abbreviation: MRPP = Multi response permutation procedures.

Nomenclature: Correa (1969–1999).

We evaluate the potential influence of disturbance on the predictability of alpine plant species distribution from equilibrium-based habitat distribution models. Firstly, abundance data of 71 plant species were correlated with a comprehensive set of environmental variables using ordinal regression models. Subsequently, the residual spatial autocorrelation (at distances of 40 to 320 m) in these models was explored. The additional amount of variance explained by spatial structuring was compared with a set of functional traits assumed to confer advantages in disturbed or undisturbed habitats. We found significant residual spatial autocorrelation in the habitat models of most of the species that were analysed. The amount of this autocorrelation was positively correlated with the dispersal capacity of the species, levelling off with increasing spatial scale. Both trends indicate that dispersal and colonization processes, whose frequency is enhanced by disturbance, influence the distribution of many alpine plant species. Since habitat distribution models commonly ignore such spatial processes they miss an important driver of local- to landscape-scale plant distribution.

Nomenclature: Adler et al. (1994).

Abbreviation: DEM = Digital elevation model.

Competition and facilitation may occur simultaneously in plant communities, and the prevalence of either process depends on abiotic conditions. Here we attempt a community-wide approach in the analysis of plant interactions, exploring whether in a semi-arid environment positive or negative interactions predominate and whether there are differences among co-occurring shrub species. Most shrubs in our plot exerted significant effects on their understorey communities, ranging from negative to positive. We found a clear case of interference and another case where the effect was neutral, but facilitation predominated and the biomass of annuals under most shrubs in our community was larger than in gaps. Effects on soil water and fertility were revealed as the primary source of facilitation; the build-up of soil organic matter changed soil physical properties and improved soil water relations. Facilitation by shrubs involved decoupling of soil temperature and moisture. Sheltering from direct radiation had an effect on productivity, but significant differences in understorey biomass did not parallel understorey light environment. A positive balance of the interaction among plants, essentially mediated by changes in soil properties, is the predominant outcome of plant interactions in this semi-arid community.

Nomenclature: Castroviejo et al. (1986–2001).

Abbreviations: A = photosynthetic rate; Fv/Fm = photochemical efficiency of photosystem II; gs = leaf conductance to water vapour; OM = organic matter; PAR = photosynthetic active radiation; Ψ_PD = Predawn water potential.

To assess seed bank persistence of target species in endangered flood-meadows (alliances Cnidion and Molinion), we investigated established vegetation and soil seed bank of 46 plots for 3 yr and 2 yr, respectively. As traits of seed persistence we calculated various continuous indices that refer to the frequency and abundance of species in above-ground vegetation and at different soil depths. Furthermore, we tested the significance and soundness of easily observed traits such as maximum seed density per plot and seed attributes (mass, size and shape) as predictors of soil seed bank features.

In linear regression, SAI, the seed accumulation index, showed the best agreement (R² = 0.64) with the seed longevity index that was derived from the database by Thompson et al. (1997) for a set of 115 species. The second best predictor (R² = 0.39) of the seed longevity index was maximum seed density per plot in the lower soil layer (5–10 cm). Depth distribution indices and seed attributes showed weaker but still significant relations. The dynamic character of flood-meadows was reflected by a large proportion of species with a strong tendency to accumulate seeds in the soil relative to their importance in above-ground vegetation. Most of these species have a ruderal strategy, exploiting gaps after flood disturbances, while the dominants of flood-meadows tended to have short-lived seed banks. Compared to other grassland types, a relatively large proportion of rare and endangered target species can be expected to form long-term persistent seed banks. However, only under marginal conditions that facilitate seed survival in the soil (e.g. fallow) are these persistent seed banks likely to contribute to restoration.

We conclude that easily observed traits of persistence such as seed weight, size and shape do not meet the accuracy needed in scientific and practical applications. Thus, there is a crucial demand for further seed bank studies in poorly investigated habitats and of rare species.

Nomenclature: Wisskirchen & Haeupler (1998).

Abbreviation: SAI = Seed accumulation index.

The shrub Buxus sempervirens and the trees Abies alba and Fagus sylvatica have recently recolonized old-pastures in the Central Pyrenees. We mapped all live and dead individuals (> 1.3 m tall) in a large forest plot in Ordesa Valley to examine the importance of density-dependent processes during recolonization. Biotic interactions were inferred from changes in horizontal structure and the influences of neighbours on tree survival. Buxus differentially influenced establishment and survival of tree species, thereby controlling future canopy composition and spatial structure. The rapidly invading Abies formed denser patches on elevated sites less occupied by Buxus, whereas Fagus preferentially established within shrubs. Abies reached densities which led to intense intraspecific competition and high mortality rates among saplings. Self-thinning in Abies led to smaller numbers of regularly spaced survivors, and greater relative dominance of Fagus. Disregarding intraspecific effects and abiotic environment, Abies survival was significantly lower under Buxus shrubs, which suggests that the spatial location and abundance of Abies was constrained by the location of Buxus. Fagus survival was not related to Buxus density, but remained significantly lower in denser Abies patches. The higher mortality of Fagus in denser Abies patches, and the resulting spatial segregation of the species, reflects asymmetric interspecific competition. Inhibition from Buxus shrubs and intraspecific competition prevent invading Abies from dominating and may therefore help in maintaining a mixed Abies-Fagus stand.

The success of invasive species has been attributed to the ability to displace other species by direct competition. We studied growth and possible competition between the two macrophyte species Elodea nuttallii and E. canadensis, because the former has been observed to replace the latter in the field. Additional experiments were conducted in aquaria with mixed plantings of Elodea species. Species growth was measured and competitive abilities of each species determined by applying the reciprocal yield model to mean plant weight and length. In monocultures the growth rates of the two species were similar, while in mixtures the growth rate of E. canadensis was significantly lower than that of E. nuttallii. E. canadensis was more sensitive to intraspecific than to interspecific neighbours, whereas E. nuttallii was indifferent to the presence of neighbours. Differential growth characteristics of Elodea species can explain the displacement of E. canadensis by E. nuttallii under eutrophic field conditions.

Nomenclature: Tutin et al. (1980).

Abbreviation: RGR = Relative growth rate.

Composition and density of the soil seed banks, together with seedling emergence in the field, were examined on Svalbard. 1213 soil samples were collected from six dry-mesic habitats in three regions representing various stages of colonization from bare moraines to full vegetation cover and spanning a range of typical nutrient and thermal regimes. Of the 165 vascular plant species native to Svalbard, 72 were present as mature plants at the study sites and of these 70% germinated seed. Proglacial soil had 12 seedlings per m², disturbed Dryas heath 131, intact Dryas heath 91, polar heath 715, thermophilic heath 3113, and a bird cliff 10437 seedlings. Highest seed bank species richness was at the thermophilic heath (26 species). Seedlings of 27 species emerged in the field, with fewer seedlings in disturbed habitats (60 seedlings per m²) than in intact Dryas heath (142), suggesting that an absence of ‘safe sites’ limited seedling establishment in disturbed habitats. Measurement of seedling emergence in the field increased awareness of which species are able to germinate naturally. This may be underestimated by up to 31% if greenhouse trials alone are used, owing partly to unsuitability of greenhouse conditions for germination of some species and also to practical limitations of amount of soil sampled. Most thermophilic species failed to germinate and some species present at several sites only germinated from the thermophilic heath seed bank, suggesting that climate constrains recruitment from seeds in the High Arctic.

Nomenclature: Elven & Elvebakk (1996).

Forest herbs differ greatly in their capacity to recolonize secondary forests established on former agricultural land. We investigated whether interspecific differences in recruitment or growth can account for differences in colonization success. Seeds and adults of two species with limited colonization capacities (Anemone nemorosa and Primula elatior) and two species with high colonization capacities (Ranunculus ficaria and Geum urbanum) were introduced in ancient and recent forest sites within the Muizen forest (Belgium). At all sites, half of the plots were also cleared of above-ground vegetation.

Seedling establishment was similar in recent and ancient forest sites. However, both the number and subsequent survival of seedlings of the two hemicryptophytes (G. urbanum and P. elatior) were significantly lower when vegetation cover was present, while seedling number and survival of the two geophytes (A. nemorosa and R. ficaria) were less influenced by vegetation cover. Adult performance of P. elatior and G. urbanum was significantly better in the nutrient enriched recent forest stands and in the absence of vegetation cover. Performance of A. nemorosa and R. ficaria was not affected by any of these factors. We conclude that the extent of seed limitation is probably the main cause for the different colonization capacities of the species investigated.

Nomenclature: Lambinon et al. (1998).

Abbreviations: GLM = General linear model; PPI = Plant performance index.

In applying randomization tests to hierarchical cluster analyses, we have noted a potentially misleading result: within a significant group, linkages are often identified as significant even when species are randomly distributed among the group's sites. We demonstrate this through a cluster analysis of a constructed matrix with two groups of 20 sites that share no species, and within each group species are randomly distributed among sites. A randomization test identified both of the groups and all linkages within them as significant, while the same test found all linkages non-significant in the cluster analysis of a matrix containing just one of the two groups of 20 sites. In general, a non-random distribution of species within a data set shortens linkages relative to distances in null distributions derived from randomized versions of the data. This confounds efforts to identify significant sub-groups within a significant group. However, the significance of sub-groups possibly could be tested by comparing linkage distances to a null distribution derived from the randomization and clustering of a sub-matrix containing only the sites within the larger group. In essence, this comparison tests the null hypothesis that within the significant group, sites represent random assemblages of species. When applied to actual data sets, an approach involving sequential randomization tests could allow the evaluation of all nodes in a classification, increasing the utility of randomization tests and strengthening the interpretation of groups produced by cluster analysis.