Question: How does form (leaf and trunk morphology) relate to function (tolerance of shade and nutrient storage) in trees?

Location: Los Toldos montane valley in NW Argentina.

Methods: We analysed the relationships amongst (1) ten vegetative and four reproductive traits across 40 tree species, (2) a distribution based measure of recruitment under shade and (3) a distribution based measure of recruitment over a soil fertility gradient.

Results: Ordinations revealed three main axes of species' morphological differentiation: (1) evergreen species had leaves with a lower specific leaf area, greater tensile resistance and slower decomposition rate, denser wood and thinner bark than deciduous species; (2) tall tree species that lack spines and are anemochorous were separated from short, spinescent and zoochorous species and (3) species were distinguished according to clonal growth, seed mass and pollination syndromes. Notably, species' recruitment under shade and over a soil fertility gradient were independent of each other, but both were correlated with species' scores along the first axis of morphological variation (tolerant species have attributes that favour resource conservation). Different sets of traits were correlated with recruitment under shade and over a soil fertility gradient when traits where assessed individually. Amongst shade tolerant species, recruitment under shade was negatively correlated with species' maximum height, suggesting differential responses to vertical gradients of light.

Conclusions: These results provide new evidence of integration between leaf and stem morphology which is consistent with an evolutionary compromise between high rate of resource acquisition and resource conservation. Generalizations about the functional value of individual morphological characteristics and of ‘strategies’ vary with the resolution of analyses.

Nomenclature: Zuloaga & Morrone (1999) for species; Anon. (2003) for families and orders.

Question: Can we recognize areas of high endemism and high endemic richness, using data from collections, and what are the ecological variables that best explain these areas?

Location: Peninsula of Baja California, Mexico.

Methods: We analysed the distribution of 723 endemic vascular plants species along the peninsula of Baja California and neighbouring islands distributed in 218 cartographic cells 15′ × 20′ in size. By means of a residual analysis, we identified areas of significantly high endemic species richness, and we calculated the degree of endemicity (or rarity) in each cell by giving to each species a weight factor inversely proportional to the land area it covers.

Results: Nine regions of high-endemicity and/or high endemic species richness were found.

Discussion and conclusions: The analyses of rarity and endemic species richness showed two contrasting scenarios: High endemicity values in oceanic and sky islands accounts for a high number of species with a restricted distribution, promoted most likely by genetic isolation and high environmental heterogeneity. High endemic richness along the peninsular coast is related to ecotonal transition along vegetation types. After correcting for collection effort (i.e. the number of specimens collected within a cell), we found the phytogeographic region and altitudinal heterogeneity to be the variables that best predicted endemic richness. Both high endemism and high endemic richness have distinct geographic patterns within our study region. The nine endemic regions provide elements for priority definitions in future conservation programs.

Question: Our study aimed at testing to what extent water and/or nutrients affect community composition in sub-alpine heath. We hypothesized that nutrient addition will have an overall positive effect on heath vegetation but water addition will have greater effects and will probably reinforce the effects of fertilization in drier habitat conditions.

Location: Monte Rondinaio, northern Apennines, Italy (44°08′ N; 10°35′ E, ca. 1850-1930 m a.s.l.).

Methods: Nutrients and water were added during five growing seasons in two communities (HV community, moister; EV community, drier) and the biomass of all species was estimated non-destructively by the point-quadrat method.

Results: Total above-ground biomass increased in both communities with fertilization due to increased graminoid biomass and decreased moss biomass, but was unaffected by irrigation. In the HV community species richness decreased as an effect of fertilization while in the EV community species richness was raised by irrigation.

Conclusions: Our study partly supported our hypothesis since water addition had a stronger effect in the drier community. However, this effect was not revealed by changes in above-ground biomass but rather by differing responses of individual species in the two communities.

Nomenclature: Aeschimann et al. (2004) for vascular plants with the exceptions of Vaccinium uliginosum ssp. microphyllum, henceforth called V. uliginosum (instead of Vaccinium gaultherioides Bigelow) and Festuca rubra L. ssp. commutata Gaudin, henceforth called F. rubra (instead of Festuca nigrescens Lam.); Frahm & Frey (1987) for bryophytes with the exception of Polytrichastrum alpinum (Hedw.) G.L. Smith instead of Polytrichum alpinum Hedw.

Question: What are the determinants of fine-scale plant spe-cies richness (SR)?

Location: Île-aux-Grues, Québec, Canada.

Methods: Elevation, soil organic matter, soil pH, irradiance, tree basal area (B A) and plant SR (herbs, shrubs, and trees) were determined in 100 contiguous 25 m² quadrats in a deciduous forest. Each variable was analysed for spatial autocorrelation using Moran's I. Path analysis was used to determine the effects of different variables on tree, shrub and herb SR in a hierarchical modelling approach.

Results: Most of the variables, except tree BA, PPFD (photo-synthetic photon flux density) and shrub SR, were positively autocorrelated at a scale of ca. 20 m or less. The path analyses explained ca. 11%–40% of the variance in plant SR; however, the model for shrub SR was not significant. Tree SR was posi-tively associated, but herb SR was negatively associated with tree BA. Tree SR had a positive influence on shrub SR, but herb SR remained unaffected by tree or shrub SR.

Conclusion: The positive association of tree BA and tree SR suggests that the data from the study site correspond to the left (ascending) portion of the SR-biomass relationship (un-dersaturated SR). The negative effect of tree BA on herb SR is direct and not mediated through reduced PPFD. High tree BA might cause high resource use, induce high litter production and affect soil properties, all of which might significantly affect herb SR. Several factors not considered here might influence fine-scale SR, such as interspecific interactions, fine-scale disturbances and heterogeneity (both spatial and temporal) in resources and abiotic conditions. Within-site variations of SR might be difficult to model with precision because of the relative importance of stochastic vs deterministic processes at this spatial scale.

Question: How does Alnus alnobetula colonization affect plant communities in sub-alpine grasslands undergoing land abandonment on a regional scale?

Location: Savoie, French Alps.

Methods: Variations within and between communities were monitored in 243 plots within three types of stands representing an A. alnobetula colonization chronosequence from grassland to dense vegetation. They were tested for species richness, species strategies and species distribution through multivariate analysis. Regional variations were assessed at three sites along a rain shadow gradient.

Results: We detected a convergence of the understorey communities along an A. alnobetula colonization gradient at all sites, associated with a reduction in species richness and the replacement of stress related strategists by competitor strategists. This convergence was unaffected by the rain shadow gradient although it was a key factor of plant distribution.

Conclusions: The dense cover and the soil properties provided by A. alnobetula stands buffered the heterogeneity of sub-alpine communities induced by a rain shadow gradient throughout the study region. It facilitated the development a homogeneous understorey community dominated by competitor related strategists while excluding grasslands stress adapted species through competition processes. The resulting community, although species-poor, has an interesting conservation value. Accordingly, as a ‘native invader’, A. alnobetula does not display typical traits of biological invaders. Future experimental research on the interactions between A. alnobetula and components of the understorey would be particularly interesting in a severe environment that promotes facilitation among plants.

Nomenclature: Kerguelen (1993) and Anon. (2004).

Question: Does shrub invasion at ecotones indirectly limit grass establishment by increasing mammalian seedling herbivory?

Location: Chihuahuan Desert, New Mexico, USA.

Methods: We tested the hypothesis that herbivore-related mortality of seedlings of the dominant perennial grass Bouteloua eriopoda would be highest in shrub-dominated portions of grassland-shrubland ecotones. We tested the hypothesis in two Chihuahuan Desert sites featuring similar shrub encroachment patterns but different shrub species, grass cover, and different abundances of small mammals. Within each site we transplanted B. eriopoda seedlings to grass-dominated, middle, and shrub-dominated positions of replicate ecotones during the time of year (mid-summer) when they would naturally appear and monitored seedling fates. We estimated population size/activity of putative small mammal herbivores.

Results: Seedlings were killed by mammals in greater numbers in shrubland than in grassland or middle ecotone positions at the site with large herbivore numbers. At the site with low herbivore numbers, most seedlings were killed in middle ecotone positions. The abundance patterns of herbivores did not parallel patterns of seedling herbivory across the ecotones or between sites.

Conclusions: Seedling herbivory is an important process and is related to vegetation composition, but the mechanisms underlying the relationship are not clear. We speculate that variation in small mammal foraging behavior may contribute to seedling herbivory patterns. Restoration strategies in the Chihuahuan Desert need to account for the abundance and/or behavior of native herbivores.

Nomenclature: Allred (1997).

Question: In November 2000, Chusquea culeou, a bamboo species dominating Andean forest understories in southern Argentina and Chile, massively flowered and died over a north-south distance of ca. 120 km. Because bamboo is the major forage for large herbivores in these forests, we examined the interactive influences of the bamboo die-off and herbivory by introduced cattle on understory and tree regeneration.

Location: Lanín National Park, Argentina.

Methods: Permanent plots, in and outside livestock exclosures, were installed in a Nothofagus dombeyi forest in patches of flowered and non-flowered C. culeou. Plots were monitored over four years for changes in understory composition and tree seedling densities and heights.

Results: After the C. culeou die-off, new establishment of N. dombeyi was low, both with and without herbivory. Livestock alone directly increased N. dombeyi seedling mortality through physical damage. However, tree seedling browse ratings and height growth were interactively affected by bamboo flowering and herbivory; unfenced plots in flowered bamboo patches had the shortest seedlings, highest browse ratings, and lowest tree seedling annual growth rates. Understory cover was higher where livestock were excluded, and this effect was intensified in the patches of flowered bamboo. Neither herbivory nor bamboo flowering resulted in major changes in species composition, with the exception of Alstroemeria aurea

Conclusion: Effects of livestock on N. dombeyi regeneration were contingent on flowering of C. culeou Prior to introduction of livestock, N. dombeyi regeneration was probably successful beneath canopy gaps during windows of opportunity following bamboo die-off, but now livestock impede tree regeneration. Herbivory during bamboo withering periods also produces more open understories, particularly affecting palatable heliophyllous herb species such as Alstroemeria aurea The results underscore the importance of assessing herbivore impacts on tree regeneration during relatively short periods of potential tree regeneration immediately following rare bamboo flowering and die-off.

Nomenclature: Correa (1969-1984).

Question: What environmental factors and physiological traits determine the juvenile survival rates of 11 dipterocarp species (Dipterocarpaceae) co-occurring in a Bornean tropical rain forest?

Location: Tropical northwestern Borneo.

Methods: In 248 quadrats, distributed over 12 ha, the juvenile (height< 2.0 m) survival of 11 shade-tolerant dipterocarp species was monitored for one year. The probability of survival for each species was expressed as a logistic function of plant height and three environmental factors: canopy openness, soil water potential and conspecific individual density. The correlation between survival under shaded conditions and several physiological traits of juveniles was tested by normal Pearson correlation and a phylogeny based manner using independent contrasts.

Results: Seven species had increased survival under more open canopies, one species had increased and two species had decreased survival at drier sites. Six species had higher survival as the density of conspecifics increased, whereas two species had lower survival. The magnitudes of the effects for the three environmental factors were similar. However, the interspecific difference in survival was largely determined by a dependence of small seedlings on the light environment. Survival under low light was correlated positively with root dry mass and negatively with total leaf area of juveniles.

Conclusions: Responses of seedlings to the fine-scale gradient of light availability were considerably different among the 11 species. Species that suffered increased mortality under low light had a set of structural traits that would potentially allow rapid growth under bright conditions. Differential performance along the light gradient may contribute to the stable coexistence of these species.

Nomenclature: Newman et al. (1996, 1998).

Questions: Density dependence is thought to restrict exponential growth as well as give rise to size structure in populations. Size hierarchy in trees from tropical dry deciduous forests is studied to ask (1) whether nature of competition is symmetric or asymmetric and (2) what is the self thinning trajectory under a natural gradient of tree density.

Location: Western India.

Methods: Density was measured as the number of trees in 10-m radius circular plots (n = 96) and size was measured at DBH. Size variation was evaluated by the Gini coefficient (n = 1239 trees).

Results: Size inequality between neighbours decreased with density but in a non-linear manner. In the backdrop of existing theory this indirectly suggests that competitive interactions may be symmetric over a ‘depletive’ resource such as below-ground water (rather than a ‘pre-emptive’ resource such as light), which is very plausible in a semi-arid environment. The self thinning coefficient derived from the relationship between stem diameter and density (γ ≈ −1/4), is higher than expected from existing models of allometric plant growth (γ ≈ −1/3) which are based on above-ground interactions alone. Seen in conjunction, these results suggest that above-ground structures, such as stem size, do not adequately represent the outcome of competitive interactions when below-ground resources, such as water, may be more important under semi-arid conditions.

Conclusions: The non-linear relationship between size inequality and density indicates that there exists a density threshold beyond which investment in above-ground biomass becomes sluggish in semi-arid, deciduous forests. Since current allometric models do not incorporate below-ground biomass for trees, these data suggest that a more comprehensive allometric growth model may have higher predictive power and wider applicability.

Nomenclature: Champion & Seth (1968).

Questions: Is the occurrence of vine species in neotropical rain forests primarily determined by properties of the forest (environmental factors), by properties of the trees (tree species or tree size) or are vines randomly distributed?

Location: Maya Biosphere Reserve, Guatemala.

Methods: In five 1-ha plots that span variation from unlogged forest to forest impacted by recurrent human disturbance we recorded the presence of all climbing vine species on every tree. The presence of all free standing vine species and 11 environmental variables were recorded in 100-m² subplots. The relationship of host tree diameter and host tree identity on single tree vine species richness was investigated by GLM modelling. Partial redundancy analyses were used to partition the variation in vine species composition on two sources: environmental factors and tree species identity.

Results: Single tree vine richness increased with increasing host tree DBH and differed significantly among host species. For climbing vines, the ratio of variation in subplot presence explained by tree species and by environmental variables was ca. 4:1 (in the most disturbed logged plots slightly lower), for free standing vines this ratio varied from 1:2 in the most disturbed logged plots to 9:1 in reserve plots, while a ratio of ca. 1:1 was found for all plots analysed together.

Conclusion: Different tree species have different probabilities of being infested by vines. Vines see both the forest and the trees; the environment is more important in earlier developmental stages, properties of individual trees become more important from the time vines start to climb.

Nomenclature:

Question: Is primary dormancy required for seed bank persistence of Leucopogon species? How does the distinction between dormancy and persistence affect our understanding of seed bank dynamics in fire-prone regions?

Location: Sclerophyllous plant communities, Sydney region, southeastern Australia.

Methods: Seed bank longevity of three morphophysiologically dormant species was ascertained using seed burial trials. Seeds of each species were buried in situ in replicate mesh bags and retrieved annually over three years. Laboratory germination trials and embryo growth measurements over time were used to establish in which order each component of dormancy was overcome, and whether seed banks persisted after the loss of primary dormancy.

Results: All species had long-term persistent seed banks, with estimated half-lives between 3.5 and 5.5 years. Physiological dormancy was broken by stratification prior to embryo growth, during the first year of burial. Seeds were able to germinate after retrieval, in light and at warm temperatures. Only low levels of germination occurred in situ over three years of burial, and did not increase over time. During germination trials, embryos remained underdeveloped unless seeds germinated, indicating that seeds were committed to germination once embryo growth was initiated.

Conclusions: Long-lived seed banks are an important trait of plants from fire-prone communities, but cues observed to promote germination in situ are not necessarily the ones that break dormancy. A failure to distinguish between these two factors can divert attention away from mechanisms actually controlling dormancy and seed bank dynamics.

Questions: A multiple plot design was developed for permanent vegetation plots. How reliable are the different methods used in this design and which changes can we measure ?

Location: Alpine meadows (2430 m a.s.l.) in the Swiss Alps.

Methods: Four inventories were obtained from 40 m² plots: four subplots (0.4 m²) with a list of species, two 10m transects with the point method (50 points on each), one subplot (4m²) with a list of species and visual cover estimates as a percentage and the complete plot (40 m²) with a list of species and visual estimates in classes. This design was tested by five to seven experienced botanists in three plots.

Results: Whatever the sampling size, only 45–63% of the species were seen by all the observers. However, the majority of the overlooked species had cover < 0.1%. Pairs of observers overlooked 10–20% less species than single observers. The point method was the best method for cover estimate, but it took much longer than visual cover estimates, and 100 points allowed for the monitoring of only a very limited number of species. The visual estimate as a percentage was more precise than classes. Working in pairs did not improve the estimates, but one botanist repeating the survey is more reliable than a succession of different observers.

Conclusion: Lists of species are insufficient for monitoring. It is necessary to add cover estimates to allow for subsequent interpretations in spite of the overlooked species. The choice of the method depends on the available resources: the point method is time consuming but gives precise data for a limited number of species, while visual estimates are quick but allow for recording only large changes in cover. Constant pairs of observers improve the reliability of the records.

Nomenclature: Aeschimann et al. (1996).

Questions: How does invasion affect old-field seed bank species richness, composition and density? How consistent are these effects across sites? Does the soil seed bank match vegetation structure in old-fields?

Location: Menorca, Balearic Islands, Spain, western Mediterranean basin.

Methods: We monitored seed germination in soils from old fields that were both uninvaded and invaded (legacy effect) by the annual geophyte Oxalis pes-caprae. We also added O. pes-caprae bulbs to uninvaded soils to test O. pes-caprae interference with seedling emergence (competitive effect). We compared species composition in the seed bank with that of the vegetation.

Results: Species richness in the seed bank and in the vegetation was not significantly different between invaded and uninvaded areas. Uninvaded areas did not have larger seed banks than invaded areas. More seedlings, especially of geophytes, emerged when O. pes-caprae bulbs were added to the soil. Species similarity between invaded and uninvaded areas was higher in the seed bank (74%) than in the vegetation (49%). Differences in species composition were as important as differences among sites. The degree of species similarity between the seed bank and the vegetation was very low (17%).

Conclusions: Despite invasion by O. pes-caprae not affecting species richness, the variation in the seed bank species composition in invaded and uninvaded areas, and the differences between the seed bank and the mature vegetation, highlights that even if the invader could be eradicated the vegetation could not be restored back to the exact composition as found in uninvaded areas.

Nomenclature: De Bolòs et al. (1990).

Question: Chronic stress events are defined as disturbance events that exceed the lifespan of the dominant plant species, fluctuate in intensity and lack abruptness or physical destruction of biomass. Can the effects of chronic stress events be measured on vegetation communities? Did two chronic stress events, the removal of a tide gate and a four year drought, cause a temporary or permanent shift in the vegetation communities of a tidal marsh?

Location: Tidal marsh in southeastern United States.

Methods: Change in species composition and dominance and community change on a landscape level salinity gradient were measured between time periods ranging from four months to seven years to construct a statistical baseline reference community at freshwater, oligohaline, and mesohaline sections of a tidal marsh. Statistical shifts in the plant community were defined as changes in the plant community that fell outside of the defined baseline reference community.

Results: Plant community changes outside of the reference community occurred in 13 out of 378 community comparisons. Removal of the tide gate had a greater effect on interstitial salinity levels than the drought and was most intense in the oligohaline marsh, where between 20 to 45% of the freshwater/oligohaline community types permanently converted to oligohaline community types. However, community shifts in the freshwater and oligohaline marsh induced by the drought were temporary, lasting from 1 to 3 years. Neither chronic stress event permanently altered the mesohaline plant communities.

Conclusion: The effects of chronic stress events could be detected; an extended historical record of vegetation change (18 years) was necessary to identify community shifts outside of a reference condition of the community and to determine if those shifts were permanent or temporary.

Nomenclature: Godfrey & Wooten (1979, 1981).

Questions: What is the accuracy and reliability of the commonly used random soil sampling methodology for predicting seedling density, species richness and composition of the emerging seedling community?

Location: Lake Kraenepoel, western Belgium.

Methods: We compared density, species composition and observed and rarefactioned species richness of the seedling community emerging on a soft water lake bed exposed after drainage with the seedling community germinating in the laboratory from random soil samples in the same plots.

Results: Seedling density did not differ between the two methods and there was a significant correlation between seedling density on the exposed lake bed and in the soil samples. This indicates that future seedling density can be reliably predicted based on soil sampling, in particular for the most abundant species. The most frequently occurring and abundant species among the seedlings in the soil samples were also the most frequent and abundant species germinating on the exposed lake bed. In contrast, species richness was much higher on the exposed lake bed than in the soil samples, and this difference was still significant for annual species after correction for differences in sampling intensity by rarefaction. We found no correlation between the number of species retrieved by the two methods. Although seedlings of rare and target species emerged on the lake bed, random soil sampling clearly failed to detect seeds of most of these species.

Conclusions: Random soil sampling at a commonly used intensity and using the standard germination conditions can accurately predict future total seedling density and the density of the most abundant species. However, the method is not reliable for predicting the probability of establishment of populations of uncommon species. When executing a seed bank study, sampling intensity and germination conditions need to be adapted to the nature and the level of detail of the research question to be answered.

The recent Forum contribution by Grime (2006) contrasts the MacArthur/Diamond assembly-rule approach to studying plant communities with the study of environmental trait gradients. Both are valid and useful. In doing so, Grime declares that the assembly rules model, in which negative interactions between plants act with limiting similarity to cause local trait divergence, is “not supported by empirical study of plant communities”. This is, he says, the agony of community ecology. I show that there is now abundant evidence for assembly rules, and no agony.

Attention is drawn to a source citing some 90000 papers in vegetation science and related fields published before 1995. A subject index is provided.