Scientific progress in plant ecology is at risk of being obscured by increasing ignorance of major works in the field. The driving force seems to be the twin seductions of novelty and crowd psychology. I illustrate this tendency with three examples from plant community ecology that span the past thirty years of ecological research. I offer, as one solution, the concept of a canon: a short list of essential books that we assume all students and co-workers have read, a short list that summarizes the wisdom of the discipline. A canon can be likened to DNA, be it in music, art, or science, as it carries forward through time the key ideas that have worked in the past. Without a canon, there is no memory of past achievement, no context for appreciating current work, and no way of judging the quality of newer productions. I suggest 20 essential books (the short canon), and 22 complementary readings, for a total of 42 books needed in any young professional's library on plant ecology.

Question: How do functional types respond to contrasting levels of herbage use in temperate and fertile grasslands?

Location: Central France (3°1′ E, 45°43′ N), 870 m a.s.l.

Methods: Community structure and the traits of dominant plant species were evaluated after 12 years of contrasted grazing and mowing regimes in a grazing trial, comparing three levels of herbage use (high, medium and low).

Results and Conclusions: Of 22 measured traits (including leaf traits, shoot morphology and composition, phenology), seven were significantly affected by the herbage use treatment. A decline in herbage use reduced individual leaf mass, specific leaf area and shoot digestibility, but increased leaf C and dry matter contents. Plants were taller, produced larger seeds and flowered later under low than high herbage use. Nine plant functional response types were identified by multivariate optimization analysis; they were based on four optimal traits: leaf dry matter content, individual leaf area, mature plant height and time of flowering. In the high-use plots, two short and early flowering types were co-dominant, one competitive, grazing-tolerant and moderately grazing-avoiding, and one grazing-avoiding but not -tolerant. Low-use plots were dominated by one type, neither hardly grazing-avoiding nor grazing-tolerant, but strongly competitive for light.

Nomenclature: Tutin et al. (1993).

Abbreviations: BE = Beginning of flowering period; DI = Digestibility; IT = Height at top of inflorescence; LA = Individual leaf area; LCC = Leaf carbon concentration; LDM = Leaf dry mass; LDMC = Leaf dry matter content; LFM = Leaf lamina fresh mass; LNC = Leaf nitrogen concentration; ME = Flowering plant height, highest leaf elongated; MH = Flowering plant height, highest leaf not elongated; NG = Number of growing green leaves; NM = Number of mature green leaves; PRT = Plant functional response type; RA = Leaf:Shoot dry matter ratio; SLAF = Specific leaf area (fresh mass based); SLAD = Specific leaf area (dry mass based); SM = Seed mass.

Questions: Does distance decay exist in an old-growth neotropical forest? Is this distance decay stronger than expected due to environmental heterogeneity alone? At what spatial scales are distance decay manifested?

Location: La Selva Biological Station, Costa Rica, Central America.

Methods: An index of distance decay is applied appropriate for small quadrats (the probability of encountering a conspecific tree) to a grid of 1170 0.01-ha plots. A null model is provided that accounts for environmental heterogeneity.

Results: Significant, but weak, distance decay is found. After correcting for known patterns of environmental heterogeneity, the distance decay almost disappears, except for fine spatial scales.

Conclusions: These results are inconsistent with models that predict distance decay at all spatial scales. However, biological processes leading to distance decay may be more relevant and detectable at scales broader than this study. Research utilizing objectively-located samples over much broader scales is necessary to evaluate the generality and magnitude of distance decay.

Question: How does fire affect the aggregation patterns of trees in a species-poor oak woodland?

Location: East-central Minnesota, USA.

Methods: More than 10 000 trees with DBH > 2 cm (comprising more than 11 000 stems) were monitored in a 16-ha grid on an annual basis from 1995–2001 in a species-poor temperate woodland. Different portions of the grid experienced different frequencies of controlled burns. Aggregation indices were calculated for individual species and individual size classes within species. A community-wide aggregation index was also calculated for different burn units. Spatial data were managed, and many of the aggregation indices calculated using a GIS ArcInfo™ (ESRI).

Results: Fire initially increased clumping, although repeated fires reduced it, a finding that suggests a corollary to the intermediate-disturbance hypothesis, the corollary stating that intermediate levels of disturbance are expected to maximize community-wide patterns of aggregation. Analyses also showed that all species are aggregated at small scales, that the degree of aggregation of a stem type (species or size) declines with distance from individual stems, that the degree of aggregation of large stems is usually less than that of small stems, and that rare species are more aggregated than common species. Findings from this study are consistent with those from similar studies in other temperate and tropical forests, woodlands, and savannas.

Conclusion: The spatial patterns of trees in this woodland are dynamic, continually changing in response to the relative strengths of the often opposing forces of competition, which tends to reduce clumping, and disturbance, which, at low and intermediate frequencies, tends to increase it.

Nomenclature: Anon. (1991).

Question: How does changing resource availability induced by fertilization and defoliation affect seedling establishment and mycorrhizal symbiosis in a subarctic meadow?

Location: 610 m a.s.l., Kilpisjärvi (69°03′ N, 20°50′ E), Finland.

Methods: A short-term full-factorial experiment was established, with fertilization and defoliation of natural established vegetation as treatments. Seeds of two perennial herbs Solidago virgaurea and Gnaphalium norvegicum were sown in natural vegetation and their germination and growth followed. At the final harvest we measured the response in terms of arbuscular mycorrhizal (AM) colonization, biomass and nitrogen concentration of the seedlings and the established vegetation.

Results: Germination rate was negatively affected by defoliation in the unfertilized plots. The shoot biomass of S. virgaurea seedlings was reduced by the defoliation and fertilization treatments, but not affected by their interaction. In G. norvegicum, the germination rate and the seedling shoot biomass were negatively correlated with moss biomass in the plots. In the established plants the arbuscular colonization rate was low and defoliation and fertilization treatments either increased or did not affect the colonization by AM fungi. In the seedlings, the colonization rate by AM fungi was high, but it was not affected by treatments. Both seedlings and established plants were colonized by dark septate fungi.

Conclusions: Reduction of plant biomass by herbivores can have different effects on seedling growth in areas of high and low soil nutrient availability. The weak response of AM colonization to defoliation and fertilization suggests that AM symbiosis is not affected by altering plant resource availability under the conditions employed in this study.

Abbreviations: AM = arbuscular mycorrhizal; DS = dark septate.

Nomenclature: Hämet-Ahti et al. 1998 (vascular plants).

Question: How do the relative frequencies of plant traits (clonality, growth form, seed weight, diaspore morphology) vary during the life cycle and how does this affect regeneration?

Location: Alpine meadow and heath communities at Kilpisjärvi, sub-Arctic Finland.

Methods: Control plots and three treatments were used to measure relative species abundances for five life cycle stages: standing vegetation, seed rain, seed bank and seedlings emerging in gaps and in closed vegetation.

Results: The relative frequencies of plant traits varied between the life cycle stages. The meadows were dominated by weakly clonal herbs, small or intermediate seeds and unappendaged diaspores, while the heaths were dominated by clonal dwarf shrubs, small seeds and fleshy fruits. In the meadows, species with small seeds dominated during the seed rain and in the seedling stage in gaps, while species with intermediate seeds dominated the seed bank and the seedling stage in closed vegetation. Species with unappendaged diaspores dominated throughout the life cycle. In the heaths, seed bank and seedling stage were practically absent.

Conclusions: The observed differences in plant trait spectra between life cycle stages indicate that important environmental factors differ among the stages. Small seeds are advantageous for dispersal, whereas intermediate seeds have a greater probability of germinating and establishing in closed vegetation. Appendages facilitate dispersal, whereas unappendaged diaspores favour seed burial. Although the plant growth form spectrum largely reflects environmental constraints during the regeneration cycle, information on seed weight and diaspore morphology improves our knowledge of the relative importance of morphological adaptations of sexual structures in different stages during the life cycle.

Nomenclature: Hämet-Ahti et al. (1998).

Question: Is the facilitative effect of nurse shrubs on early recruitment of trees mediated by a ‘canopy effect’ (microclimate amelioration and protection from herbivores), a ‘soil effect’ (modification of soil properties), or both?

Location: Two successional montane shrublands at the Sierra Nevada Protected Area, SE Spain.

Method: Seedlings of Quercus and Pinus species were planted in four experimental treatments: (1) under shrubs; (2) in open interspaces without vegetation; (3) under shrubs where the canopies were removed; (4) in open interspaces but covering seedlings with branches, mimicking a shrub canopy.

Results: Both effects benefited seedling performance. However, microclimatic amelioration due to canopy shading had the strongest effect, which was particularly pronounced in the drier site. Below-ground, shrubs did not modify soil physical characteristics, organic matter, total N and P, or water content, but significantly increased available K, which has been shown to improve seedling water-use efficiency under drought conditions.

Conclusions: We propose that in Mediterranean montane ecosystems, characterised by a severe summer drought, pioneer shrubs represent a major safe site for tree early recruitment during secondary succession, improving seedling survival during summer by the modification of both the above- and below-ground environment.

Nomenclature: Castroviejo et al. (1986–2001) for Quercus and Pinus species, and Molero-Mesa et al. (1992) for shrub species.

Question: What are the relative influences of human impact, macroclimate, geographic location and habitat related environmental differences on species composition of boreal epiphytic macrolichen communities?

Location: Troms county in northern Norway.

Methods: Detrended Correspondence Analysis revealed the main gradient structure in lists of epiphytic macrolichen species from deciduous forests. By Canonical Correspondence Analysis with variance partitioning, the relative amount of variance in macrolichen species composition attributable to human impact, macroclimate, spatial context and environmental differences was quantified.

Results: There was no significant effect of human impact on species composition of epiphytic macrolichens of deciduous forests. Macroclimate was the most important factor determining epiphytic macrolichen communities, which were also strongly influenced by ecological differences such as forest stand properties.

Conclusions: Epiphytic macrolichen communities are determined by a macroclimatic gradient from the coastline to the interior of central north Norway. In marked contrast, the species composition of epiphytic macrolichen communities seems to be unaffected by human impact in the study area, where air pollution was marginal.

Nomenclature: Santesson et al. (2004); Lid & Lid (1994).

Question: Are tree saplings in wooded pastures spatially associated with specific nurse structures or plants that facilitate tree sapling survival?

Location: Wooded pastures in the Jura Mountains, Switzerland.

Methods: In two sites, 73 km apart, we sampled 294 plots of 4 m², systematically distributed on 1 ha. We recorded number and height of all established Picea abies saplings (> 1 a of age and up to 40 cm in height), and visually estimated cover of rocks, shrubs, tree stumps, overhanging tree branches and unpalatable plant species.

Results: Despite differences in site characteristics, we found overall positive effects of cover of unpalatable plants, rocky outcrops and tree stumps on the density of Picea saplings. Plots with tree stumps and higher cover of rocky outcrops and unpalatable plants were more likely to contain Picea saplings.

Conclusions: Unpalatable plants, rocky outcrops and tree stumps seem to form safe sites for Picea saplings in this grazed system, improving their establishment and survival. Our findings support the idea that associational resistance drives the dynamics of wooded pastures, but experimental evidence for this hypothesis is still required.

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

Question: Does the seed bank filter annual plant composition and determine cover at the species level?

Location: 510 m a.s.l., central Spain.

Methods: Seven transects and 136 quadrats were established in a semi-arid gypsum system. Seed bank samples were collected in each quadrat in September. The community was sampled the following April. For each quadrat we measured slope, microslope, landform, elevation, perennial cover and crust cover. Seed bank was estimated using the direct emergence method in glasshouse. Relationship among seed bank and annual community was assessed by Mantel correlations. Above-ground cover for the five most abundant species was modelled with GLMs.

Results: Seed bank density was the best predictor for annual community cover; perennial cover and landform were also included in the model. Species composition between September seed bank and April annual community cover was also highly related according to the Mantel test. This relationship was constant, even when the effect due to other abiotic (landform, microslope) or biotic (perennial cover, crust cover) parameters were partialled out. Microslope, elevation and seed bank density were the best parameters to predict spring cover of the five most abundant species.

Conclusions: Above-ground and below-ground community compartments are strongly related in terms of abundance and species composition. This relationship is filtered by several environmental factors (e.g. perennial cover, landform, microslope) that exert a strong control at community and individual levels. Our results support the hypothesis that annual community performance is affected by seed bank pattern.

Nomenclature: Tutin et al. (1964–1980) and Castroviejo et al. (1986–2005), except for Chaenorhinum reyesii (C. Vicioso & Pau) Benedí.

Abbreviation: GLM = General linearized model.

Question: According to Whittaker's proposal, ecologists have traditionally viewed β-diversity as the ratio between γ-diversity and average α-diversity. More recently, an alternative way of partitioning diversity has been ‘rediscovered’ for which β-diversity is obtained as the difference between γ-diversity and average α-diversity. This additive way of partitioning diversity has rapidly become a very popular framework for hierarchical diversity decomposition at various spatial scales. The question for this study is: Can we highlight any relation between these two ways of partitioning diversity, or do these methods really capture different facets of spatial turnover in species composition?

Methods: First the properties that a diversity measure should possess for enabling additive decomposition into α-, β-, and γ-components are reviewed. Next, attention is drawn to the relationships between additive and multiplicative diversity decomposition.

Results: It is shown that the additive model is closely related to its multiplicative counterpart through a simple logarithmic transformation.

Conclusions: Contrary to the current assumption, both methods for partitioning diversity are not as different as they appear. Hence, the supposed superiority of additive diversity partition over multiplicative diversity decomposition is largely unjustified.

Question: What are the tempo and mode of long-term succession and of demographic processes in an alpine community, especially: tenacity, transition patterns, predictions, growth of individual cushion plants, cyclic succession, spatial patterns?

Location: A low-alpine mixed cushion/turf/snow-tussock / shrub community in southeastern New Zealand.

Methods: The distribution of seven plant cover-types was recorded at 1024 fixed points in an 8 m × 8 m plot at approximately decade intervals for 50 years. The diameters of eight Donatia novae-zelandiae cushions were monitored.

Results: The process was essentially first-order Markovian. There was a change in transition frequencies about 1980. The tenacity of the two major cover types – cushion and turf – was high, but that of cushion decreased about 1980 as some of its area was taken over by turf. The original informal prediction of 1955 that the cushion / turf would increase proved to be correct, probably because of paludification of the site. A prediction of 1987 made from observed transitions that cushion would dominate over turf has proved untrue because of a change in the transition probabilities in the 1980s, of unknown cause. There is a ten-fold range in diameter growth rates among the eight cushions measured, but the mean rate of 5.3 mm.a⁻¹ is similar to that reported from other alpine and arctic sites. As cushions aged, turf colonized their centres, and in two cases new cushions colonized into this turf: consistent with cyclic succession. The pattern of transitions was compatible with a general interpretation of cyclic succession, but not definitive.

Conclusions: Change is slow in this alpine community, and tenacity high. The change in transition frequencies about 1980, the invasion of individual cushions, and the decrease in spatial autocorrelation all suggest that cushions established on the site as a result of the clearance of woody vegetation after 1400 AD. Paludification may be causing some loss of tussock grass. A tendency for the cushions to break up, and the shallow peat accumulated below them, may indicate that they are the first generation of cushions on the site. Though these cushions are breaking up, other cushions are establishing, and cushions will continue to be an important part of the vegetation dynamics which may be part of a cyclic succession.

Nomenclature: Mark & Adams (1995) unless the author is explicitly indicated.

Questions: This paper examines the long-term change in the herbaceous layer of semi-arid vegetation since grazing ceased. We asked whether (1) there were differences in the temporal trends of abundance among growth forms of plants; (2) season of rainfall affected the growth form response; (3) the presence of an invasive species influenced the abundance and species richness of native plants relative to non-invaded plots, and (4) abundance of native plants and/or species richness was related to the time it took for an invasive species to invade a plot.

Location: Alice Springs, Central Australia.

Methods: Long-term changes in the semi-arid vegetation of Central Australia were measured over 28 years (1976–2004) to partition the effects of rainfall and an invasive perennial grass. The relative abundance (biomass) of all species was assessed 25 times in each of 24 plots (8 m × 1 m) across two sites that traversed floodplains and adjacent foot slopes. Photopoints, starting in 1972, were also used to provide a broader overview of a landscape that had been intensively grazed by cattle and rabbits prior to the 1970s. Species' abundance data were amalgamated into growth forms to examine their relationship with environmental variation in space and time. Environmental variables included season and amount of rainfall, fire history, soil variability and the colonization of the plots by the exotic perennial grass Cenchrus ciliaris (Buffel grass).

Results: Constrained ordination showed that season of rainfall and landscape variables relating to soil depth strongly influenced vegetation composition when Cenchrus was used as a covariate. When Cenchrus was included in constrained ordination, it was strongly related to the decline of all native growth forms over time. Univariate comparisons of non-invaded vs impacted plots over time revealed unequivocal evidence that Cenchrus had caused the decline of all native growth form groups and species richness. They also revealed a contrasting response of native plants to season of rainfall, with a strong response of native grasses to summer rainfall and forbs to winter rainfall. In the presence of Cenchrus these responses were strongly attenuated.

Discussion: Pronounced changes in the composition of vegetation were interpreted as a response to removal of grazing pressure, fluctuations in rainfall and, most importantly, invasion of an exotic grass. Declines in herbaceous species abundance and richness in the presence of Cenchrus appear to be directly related to competition for resources. Indirect effects may also be causing the declines of some woody species from changed fire regimes as a result of increased fuel loads. We predict that Cenchrus will begin to alter landscape level processes as a result of the direct and indirect effects of Cenchrus on the demography of native plants when there is a switch from resource limited (rainfall) establishment of native plants to seed limited recruitment.

Nomenclature: Albrecht et al. (1997).

Question: Do rectangular sample plots record more plant species than square plots as suggested by both empirical and theoretical studies?

Location: Grasslands, shrublands and forests in the Mediterranean-climate region of California, USA.

Methods: We compared three 0.1-ha sampling designs that differed in the shape and dispersion of 1-m² and 100-m² nested subplots. We duplicated an earlier study that compared the Whittaker sample design, which had square clustered subplots, with the modified Whittaker design, which had dispersed rectangular subplots. To sort out effects of dispersion from shape we used a third design that overlaid square subplots on the modified Whittaker design. Also, using data from published studies we extracted species richness values for 400-m² subplots that were either square or 1:4 rectangles partially overlaid on each other from desert scrub in high and low rainfall years, chaparral, sage scrub, oak savanna and coniferous forests with and without fire.

Results: We found that earlier empirical reports of more than 30% greater richness with rectangles were due to the confusion of shape effects with spatial effects, coupled with the use of cumulative number of species as the metric for comparison. Average species richness was not significantly different between square and 1:4 rectangular sample plots at either 1- or 100-m². Pairwise comparisons showed no significant difference between square and rectangular samples in all but one vegetation type, and that one exhibited significantly greater richness with squares. Our three intensive study sites appear to exhibit some level of self-similarity at the scale of 400 m², but, contrary to theoretical expectations, we could not detect plot shape effects on species richness at this scale.

Conclusions: At the 0.1-ha scale or lower there is no evidence that plot shape has predictable effects on number of species recorded from sample plots. We hypothesize that for the mediterranean-climate vegetation types studied here, the primary reason that 1:4 rectangles do not sample greater species richness than squares is because species turnover varies along complex environmental gradients that are both parallel and perpendicular to the long axis of rectangular plots. Reports in the literature of much greater species richness recorded for highly elongated rectangular strips than for squares of the same area are not likely to be fair comparisons because of the dramatically different periphery/area ratio, which includes a much greater proportion of species that are using both above and below-ground niche space outside the sample area.

A new database is described, named EcoPlant, that currently holds data from 6432 phytosociological relevés of French forests. The aim of the database is to investigate and model the distribution of forest plant species, and their response to ecological factors, i.e. their ecological niche. Unlike other similar databases, a full environmental description is stored with the floristic data of each site. The climatic data at the site are included, and access to direct climatic variables is possible through geographic information system (GIS) modelling. Precise data on the soil are also stored for each site (soil profile and horizon descriptions, physical and chemical analyses of soil samples). The database is designed to enable linkages to existing soil, floristic or plant-trait databases.