Question: Does the understorey vegetation of Norwegian boreal forests change in relation to broad-scale, long-term changes?

Location: Norway.

Methods: Permanently marked 1-m² vegetation plots from 17 monitoring reference areas in forests dominated by Picea abies (11 areas, 620 plots) and Betula spp. (six areas, 300 plots) were analysed twice, at the start in 1988–1997 and 5 yr later (1993–2002). Species subplot frequency data were analysed separately for each area by univariate and multivariate statistical methods; 5-yr changes in single species abundances, species number per plot and species composition were tested.

Results: Two distinct patterns of change were found: 1. Abundance of several vascular plant species decreased in SE Norwegian Picea forests, most noticeably of species with a preference for richer soils, such as Oxalis acetosella. 2. Abundance of many bryophyte species as well as bryophyte species number per plot increased in forests of both types over most of Norway.

Conclusions: The pattern of vascular plant changes is probably a time-delayed response of long-lived, mainly clonal, populations to acidified soils resulting from deposition of long-distance airborne pollutants. The pattern bryophyte changes, with reference to the close link between climatic conditions for growth and abundance changes for Hylocomium splendens established in previous demographic studies, is related to climatic conditions favourable for bryophyte growth. We conclude that many forest understorey plants are sensitive indicators of environmental change, and that the concept used for intensive monitoring of Norwegian forests enables early detection of changes in vegetation brought about by broad-scale, regional, impact factors.

Nomenclature: Lid & Lid (1994) for vascular plants; Frisvoll et al. (1995) for bryophytes (except Polytrichastrum G.L. Sm. which is not recognized as distinct from Polytrichum Hedw.); Krog et al. (1994) for lichens. Several taxa are treated collectively.

Abbreviation: DCA = Detrended Correspondence Analysis.

Question: Is the seed bank in dune slacks during the whole successional range mainly composed of early successional species or does it vary according to the changing vegetation?

Location: Belgium, western part of the coast.

Methods: We investigated the soil seed bank composition using a seedling germination method in a chronosequence of 20 dune slacks, ranging in age from five to 55 yr.

Results: Both seed density and species richness in the seed bank were very low in the first successional stages and increased with age, mainly as a result of increasing seed production. The similarity between seed bank and vegetation was higher in older slacks. A comparison of characteristics between seed bank and vegetation showed that the seed bank was, to a large extent, composed of later successional species. Occurrence patterns of individual species also showed that seeds become incorporated in the soil after the species has established in the vegetation.

Conclusion: The seed bank is not likely to be the driving force for successional changes in the vegetation, and successional changes rely on dispersal. Some early successional species persist in the seed bank, but generally only in low numbers. The results also confirm that most typical dune slack species do not form persistent seeds, so that re-establishment from the seed bank is not to be expected when the species has disappeared from the vegetation.

Question: Do soil water content and/or soil nitrogen (N) content and/or soil phosphorus (P) content affect the biomass of Vaccinium myrtillus and V. vitis-idaea in a sub-alpine heath?

Location: Dolomites, northern Italy, 1800 m a.s.l.

Methods: We determined above-ground and below-ground biomass of the shrubs at three sites, each on a different substrate type. At each site, we determined soil N- and P-contents. We also determined leaf water potential (ψ_l), N- and P-concentrations in plant tissues and litter, as well as δ¹³C and δ¹⁵N in mature leaves.

Results: V. myrtillus biomass was highest at the silicate site, V. vitis-idaea biomass was highest at the carbonate site. Both shrubs had low biomass at the peat site, possibly due to a toxic effect of waterlogging in wet soils. For both species, pre-dawn ψ_l indicated optimal hydration and midday ψ_l did not show any sign of water stress. Water use efficiency (WUE) did not differ among sites for any species. Whole-plant nutrient concentrations showed that, with increasing biomass, N was diluted in V. myrtillus tissues while P was diluted in V. vitis-idaea tissues. Foliar N-concentration was higher overall for V. myrtillus. Foliar P-concentration in V. myrtillus peaked at the silicate site. Foliar N : P ratios suggested that V. myrtillus was primarily P-limited and V. vitis-idaea primarily N-limited.

Conclusions: Water content affected the distribution of the two shrubs in a similar way, higher P-availability in the soil enhanced V. myrtillus rather than V. vitis-idaea.

Abbreviations: SOM = Soil organic matter; WUE = Water use efficiency.

Question: What is the relative influence of forest continuity, environmental differences and geographical context on vegetation and species richness in ancient and recent forests?

Location: Himmerland and Hornsherred in Denmark.

Methods: Lists of forest species from deciduous forests were subjected to CCA with variation partitioning to quantify the relative amount of variation in species composition attributable to historical, present geographical and environmental variables. GLM was used to estimate the importance of the variables to species richness.

Results: The importance of temporal forest continuity in one region was negligible but was considerable in the other. The variation in species composition explained by geographical, environmental and historical variables showed little overlap in both regions, particularly at the fine scale.

Conclusions: This paper does not support the idea that differences in the flora between ancient and recent forests is mainly caused by environmental differences. Furthermore, species richness seemed unaffected by isolation and forest connectivity.

Nomenclature: Hansen (1981).

Abbreviations: GLM = Generalized linear model; TVE = Total variation explained.

Question: In the population dynamics of four understorey shrub species (Hp, Hydrangea paniculata Sieb. et Zucc.; Lu, Lindera umbellata Thunb. var. membranacea (Maxim.) Momiyama; Ms, Magnolia salicifolia (Sieb. et Zucc.) Maxim.; Vf, Viburnum furcatum Blume ex Maxim.), (1) What is the relative importance of seedling regeneration versus vegetative growth? (2) Can these shrubs persist stably for a long time in the understorey? (3) What kind of variation in demographic features is observed among these shrubs?

Location: 780m a.s.l., north-eastern Japan.

Methods: Population dynamics were analyzed by using stage-classified matrix models. Models were mainly constructed from five years stem-census data, including current-year seedlings and sprouts.

Results: Current-year sprouts emerged every year in every species. Current-year seedlings emerged every year in Lu and Vf, but densities were very low. In every species, population growth rate (λ) was close to the equilibrium value 1.0 and no statistical difference was found among species. The stable stage-distribution predicted from the matrix model was similar to the observed distribution for Lu, Ms and Vf, but much different for Hp. Elasticity matrix was also similar among Lu, Ms and Vf, but was quite different for Hp.

Conclusions: Lu, Ms and Vf were considered as climax shrubs that can regenerate and maintain their population stably in the understorey, even if canopy gaps form infrequently. Hp is a pioneer shrub that require more frequent formation of canopy gaps for long-term persistence in the understorey.

Abbreviations: Hp = Hydrangea paniculata; Lu = Lindera umbellata var. membranacea; Ms = Magnolia salicifolia; SL = Current-year seedling; SP = Current-year sprout; Vf = Viburnum furcatum.

Question: Are soil properties and topographic variation related with palm (Arecaceae) species composition and distribution patterns? If so, are species distribution patterns consistent across sites?

Location: 100–200 m a.s.l, non-inundated Amazonian rainforest, NE Peru.

Methods: One 0.65-ha line transect divided into 5 m by 5 m subunits was inventoried for all palm individuals at each site. Soil samples were collected, and topography was measured.

Results: A total of 56 palm taxa were recorded. Floristic similarity among transects clearly corresponded with similarity in soil cation content when species abundances were taken into account, but less so when only presence-absence data were used. Taxon-wise distribution analyses were done for the 37 most abundant palm taxa. Quite a few of these taxa proved not to be randomly distributed along the transects, but instead were clearly more abundant in some topographic positions than in others. However, the consistency of the distribution patterns across study sites proved to be rather low, and many of the palm taxa showed different distribution patterns at different sites.

Conclusions: The ambiguity in distribution patterns across study sites may partly be due to the complexity of topography as a measure of ecological conditions, and the probability that it is related to the variation in different environmental variables at different sites.

Nomenclature: Henderson (1995, 2000); see also ^{Material and Methods} section.

Questions: 1. How does species richness of recipient communities affect Reynoutria invasion? 2. How does Reynoutria invasion change host community structure? 3. Are there any differences in habitat preferences among three closely related Reynoutria taxa? 4. How does the genetic structure of Reynoutria populations change along the course of a river?

Location: River Jizera basin, north Bohemia, Czech Republic.

Methods: Nine 0.25 km² plots were chosen along the river. Within each plot all main habitat types were determined and sampled using the Braun-Blanquet scale to determine the invasibility of various communities. The patches invaded by Reynoutria taxa and surrounding Reynoutria-free vegetation in the same habitat type were sampled as relevé pairs to compare the composition of invaded and non-invaded vegetation. In addition, to characterize the genetic structure of Reynoutria populations along the river, 30 samples from different clones were collected.

Results and conclusions: 1. The species richness of communities has no influence on the success of Reynoutria invasion in the area studied. The combination of environmental conditions and propagule spread is more important to the invasion success than the number of species in the host community. 2. Reynoutria invasion greatly reduces species diversity. 3. R. japonica invaded more habitat types than R. sachalinensis and R. × bohemica. The hybrid R. × bohemica outcompetes the parental taxa at sites where both taxa co-occur. 4. Isozyme analysis revealed phenotype variability in the hybrid in contrast to the parental taxa. Different hybrid phenotypes are distributed randomly on the middle and lower reaches of the River Jizera; one of them dominates and the other three occur occasionally. This pattern supports the hypothesis that sexual reproduction occasionally occurs within Reynoutria taxa.

Nomenclature: Ehrendorfer (1973).

Question: How does the frequency of heathland fire events affect population growth rates of two woody shrub species, Ulex gallii and U. minor?

Location: Dry heathland on the south coast of England, UK.

Methods: The population dynamics of U. gallii and U. minor were modelled at each phase of the heathland cycle – pioneer, building, mature and degenerate – using periodic matrix products to investigate the response to different fire regimes.

Results: Population growth rates of both Ulex species declined under annual burning. Initially, as the time between burns increased, population growth rate increased for both species. Maximum population growth rates for each Ulex species were achieved under a 16-yr fire return interval. Fire return intervals > 16 yr resulted in declining population growth rates.

Conclusions: A species-specific critical fire frequency can be predicted, the minimum fire return intervals permitting persistence were 4 yr for U. minor and 3 yr for U. gallii. These patterns are similar to those reported for a range of woody plant species within savanna environments.

Nomenclature: Stace (1997).

Question: What is the effect of vertical seed distribution (i.e. seeds on soil surface or buried) and the relative importance of vertical and horizontal seed distribution (i.e. seeds near established vegetation or near bare soil) for seedling emergence?

Location: Patagonian arid steppe.

Methods: To evaluate the effect of vertical seed distribution on seedling emergence, four sowing treatments were used in field and greenhouse experiments. Vertical vs. horizontal seed distribution and the occurrence of seedlings were analysed in the field. Effects of trampling and mechanical drilling on seedling emergence were investigated experimentally in the field.

Results: In field and glasshouse experiments buried seeds showed a higher emergence and larger seedling size than seeds lying on the soil surface. The observational field study indicated that natural emergence was not associated with particular microsites or plant cover (horizontal distribution), but that most seedlings emerged from buried positions (vertical distribution). Buried seeds represented less than 10% of the sampled seed cohort but account for almost 60% of the seedlings. The trampling/drilling experiment showed that sheep trampling and mechanical drilling increased seedling emergence compared to non-treated controls.

Conclusions: The population dynamics of Bromus pictus is strongly constrained by the vertical position of seeds, which largely affects emergence, and seed horizontal distribution, which largely affects seedling survival. Our results showed that sheep trampling after seed dispersal may be a low-input technique for increasing grass recruitment.

Nomenclature: Cabrera (1971).

Questions: Are species richness and species abundances higher in the presence of tidal creeks? Do species richness and species abundances vary with plot size?

Location: Intertidal plain of Volcano Marsh, Bahia de San Quintin, Mexico.

Methods: We analysed vegetation patterns in large areas (cells) with tidal creeks ( creek) and without (−creek). We surveyed vegetation cover, microtopography, habitat type, and distance to creeks in nested plots of five sizes, 0.1, 0.25, 1, 2.5, and 10 m².

Results: Species richness, frequency, cover, and assemblages differed between ±creek cells. Richness tended to be higher in creek cells, and cover and frequency of individual species differed significantly between ±creek cells. We found consistent patterns in vegetation structure across plot sizes. We encountered 13 species that occurred in 188 unique assemblages. The most common assemblage had six species: Batis maritima, Frankenia salina, Salicornia bigelovii, S. virginica, Salicornia spec. and Triglochin concinna. This assemblage occurred in ±creek cells and at all spatial scales. Of the most common assemblages all but one were composed of multiple species (3–9 species/plot).

Conclusions: The persistence of vegetation patterns across a 100-fold range in spatial scale suggests that similar environmental factors operate broadly to determine species establishment and persistence. Differences in assemblage composition result from variation of frequency and cover of marsh plain species, particularly Suaeda esteroa and Monanthochloe littoralis. The recommendation for restoration of Californian salt marshes is to target (and plant) multi-species assemblages, not monocultures.

Nomenclature: Hickman (1993).

Question: Is there a difference in plant species and life form composition between two major patch types at a biome transition zone? Are subordinate species associated with different patch types at the shortgrass steppe - Chihuahuan desert grassland transition zone? Is this association related to differences in soil texture between patch types and the geographic range of associated species?

Location: central New Mexico, USA.

Methods: Patches dominated by either Bouteloua gracilis, the dominant species in the shortgrass steppe, or Bouteloua eriopoda, dominant species in the Chihuahuan desert grasslands, were sampled for the occurrence of subordinate species and soil texture within a 1500-ha transitional mosaic of patches.

Results: Of the 52 subordinate species analysed, 16 species were associated with B. gracilis-dominated patches and 12 species with B. eriopoda-dominated patches. Patches dominated by B. gracilis were richer in annual grasses and forbs, whereas patches dominated by B. eriopoda contained more perennials forbs and shrubs. Soils of B. gracilis-dominated patches had higher clay and lower rock contents compared with soils of B. eriopoda-dominated patches. Differences in species characteristics of the dominant species as well as differences in soil texture between patch types contribute to patch-scale variation in composition. The association of species to patch types was not related to their geographic range and occurrence in the adjacent biomes.

Conclusions: Patch types at this biome transition zone have characteristic life-form and species composition, but species are associated to patch types due to local constraints, independently from their affinity to the adjacent biomes.

Nomenclature: Anon. (1999).

Abbreviation: SNWR = Sevilleta National Wildlife Refuge.

Question: Is tree regeneration in canopy gaps characterized by chance or predictable establishment.

Location: Coastal scarp forests, Umzimvubu district, Eastern Cape Province, South Africa.

Methods: Estimation of richness of gap-filling species across canopy gaps of different size. Data are compared with regeneration under the canopy. Probability of self-replacement of gap forming species is calculated.

Results: Forest area under natural gap phase was 7.8%, caused mostly by windthrow (54%). The abundance and average size of gaps (87.8 m²) suggests that species diversity may be maintained by gap dynamics. However, only four of 53 gap-filler species displayed gap size specialization and these were pioneer species. An additional 13 species were more common in larger gaps but there was no gradient in composition of gap-filler species across gap size (p = 0.61). Probabilities of self-replacement in a gap were low (< 0.3) and common canopy species were equally abundant in gaps and the understorey. Species composition in gaps showed no pattern of variation, i.e. was unpredictable, which suggests absence of a successional sequence within tree-fall gaps. There was also only a slight increase in species richness in gaps at intermediate levels of disturbance.

Conclusions: Coastal scarp forest appears not to comprise tightly co-evolved, niche-differentiated tree species. Unpredictable species composition in gaps may be a chance effect of recruitment limitation of species from the species pool. Chance establishment slows competitive exclusion and may maintain tree diversity in these forests. These data suggest that current levels (⩽ 3 gaps per ha) of selective tree harvesting may not cause a reduction in species richness in this forest.

Nomenclature: Arnold & de Wet (1993).

Abbreviation: IDH = Intermediate disturbance hypothesis.

Question: What are the main pathways of long-term stand development in forest ecosystems on oligotrophic and acidic sandy soils?

Location: Nine forest reserves at different locations in The Netherlands; all ageing Pinus sylvestris forests that are no longer managed and where massive regeneration of broad-leaved species is often reported.

Methods: Agglomerative cluster analysis was used to define structural classes from forest reserve data. Sequences of structural classes, representing different trajectories of stand development, were constructed with the aid of a process based gap model.

Results: Four main pathways of stand development could be distinguished. Three pathways are linked to gap dynamics, and lead towards dominance of Betula, Quercus or Fagus. They differ in light availability for regeneration and/or seed tree availability. The fourth pathway comprises of development patterns after major disturbances.

Conclusions: The new methodological approach, combining the empirical strength of forest reserve data with the predictive ability of a process based model, made it possible to detail and quantify insights into structure and dynamics of forests on poor sandy soils. Some factors not included in the study can substantially influence pathways, especially those where Quercus and Fagus potentially play an important role.

Abbreviations: Be = Betula pendula and Betula pubescens; Fa = Fagus sylvatica; MP_p = Model prediction for a simulation plot p; OV_p,t = output vector at time t of MP_p; Pi = Pinus sylvestris; Qu = Quercus robur and/or Q. petraea; SC = Structural class; SCO = Structural class object.

Nomenclature: De Langhe et al. (1988).

Question: Are flower production and associated phenological variables (onset, end, duration, and three measures of flowering synchrony) randomly distributed in space or, alternatively, is there a neighbourhood structure (spatial autocorrelation) in the values of these variables? To which extent does spatial autocorrelation affect the correlation tests between phenological traits?

Location: A tree savanna reserve in Southeastern Brazil (22°15′ S, 47°08′ W).

Methods: The flowering season of Chromolaena odorata was followed for all (96) individuals in a completely mapped area of 3000 m². The phenological traits were estimated by counting flower heads in anthesis on individual plants every seven days for 14 weeks.

Results: Flowering time was unimodally distributed, but with different peak dates depending on whether individual flower heads or plants were counted. Three phenological traits and canopy closure above the plants showed some degree of spatial autocorrelation, which caused loss of up to 35% of degrees of freedom in nine of 36 correlation tests. Such a decrease in the degrees of freedom resulted in loss of significance for correlations in three pairs of variables.

Conclusions: We hypothesize that the spatial autocorrelation in phenological traits between C. odorata neighbours may be driven by genetic similarity among neighbouring plants and/or spatial structuring of environmental factors. Because location and distance between samples may affect their statistical independence, we suggest that spatial autocorrelation should be taken into account in future studies of plant phenology, e.g. by using effective sample size in statistical tests.

Abbreviations: SA = Spatial autocorrelation; S_AG= Augspurger's (1983) synchrony index; S_M = Marquis' (1988) synchrony index; S_EL-K = English-Loeb & Karban's (1992) synchrony index.

In his recent essay, Ch. Körner (2003) suggested that we need to clarify and narrow the meaning of limitation and stress. Current definitions and usage of these terms are not in any great need of revision. Limiting the use of ‘stress’ to only extreme situations is inappropriate because sometimes stress is indeed weak. What is needed is to use the terms within a comparative framework, a framework based on comparisons along gradients and among taxa. Recent studies on the factors controlling diversity have used such a comparative framework, and have increased our understanding of the relative roles of various types of stress. Redefining and narrowing general terms that relate to common processes would limit our ability to make comparisons and generalizations, and this should be avoided.

Körner's recent paper provided an excellent initial discussion of the term ‘limitation’ in ecology. I accept most of his arguments but would argue that there are some circumstances in applied ecology where the concept of limitation can be useful; these are (1) where there is a deficiency of a factor that prevents growth, and (2) where there is a need to manage an ecosystem so that a limitation is enforced.

A recent perspective paper offered by Körner essentially argued that ‘limitation’ and ‘stress’ are functionally useless terms for ecology except perhaps within limited contexts such as plant physiology or agriculture. We strongly disagree, and to this end argue that, although stress is not as precise as other concepts in ecology and is probably more difficult to apply to communities than to individuals, if ecologists want to communicate in a meaningful and interesting way about the distribution and abundance of species, we have to use multi-purpose terminology that allows us to scale from reductionistic, strictly quantifiable levels of analysis to more general conceptual levels. Here, we revisit the main arguments presented against these concepts and use three lines of counter-argument to support our conclusion that limitation and stress are necessary concepts for organizing and integrating general ecological inquiry. We discuss (1) the role of interactions between individuals in changing the limitation experienced by a species, (2) the importance of delineating whether stress is being applied to individuals or to the community, and (3) the evolutionary argument that fitness is never perfect since even adapted species are likely limited to some degree by the environment.

A reply is presented on the comments by Marrs, Weiher, and particularly Lortie et al. on an earlier Forum paper. The main point is that adapted alpine plants are not stressed, which follows, i.a., from their productivity which is equal to that in tropical systems when the length of the growing season is taken into account. Another point is that individual-based and community-based considerations should not be confused.