Questions: How do fire frequency, tree canopy cover, and their interactions influence cover of grasses, forbs and understorey woody plants in oak savannas and woodlands?

Location: Minnesota, USA.

Methods: We measured plant functional group cover and tree canopy cover on permanent plots within a long-term prescribed fire frequency experiment and used hierarchical linear modeling to assess plant functional group responses to fire frequency and tree canopy cover.

Results: Understorey woody plant cover was highest in unburned woodlands and was negatively correlated with fire frequency. C₄-grass cover was positively correlated with fire frequency and negatively correlated with tree canopy cover. C₃-grass cover was highest at 40% tree canopy cover on unburned sites and at 60% tree canopy cover on frequently burned sites. Total forb cover was maximized at fire frequencies of 4–7 fires per decade, but was not significantly influenced by tree canopy cover. Cover of N-fixing forbs was highest in shaded areas, particularly on frequently burned sites, while combined cover of all other forbs was negatively correlated with tree canopy cover.

Conclusions: The relative influences of fire frequency and tree canopy cover on understorey plant functional group cover vary among plant functional groups, but both play a significant role in structuring savanna and woodland understorey vegetation. When restoring degraded savannas, direct manipulation of overstorey tree canopy cover should be considered to rapidly reduce shading from fire-resistant overstorey trees. Prescribed fires can then be used to suppress understorey woody plants and promote establishment of light-demanding grasses and forbs.

Nomenclature: Barkley (1986).

Questions: To what extent are the distributions of tropical rain forest tree ferns (Cyatheaceae) related to environmental variation, and is habitat specialization likely to play a role in their local coexistence?

Location: Lowland rain forest at La Selva Biological Station, Costa Rica.

Methods: Generalized linear (GLM) and generalized additive (GAM) logistic regression were used to model the incidence of four tree fern species in relation to environmental and neighbourhood variables in 1154 inventory plots regularly distributed across 6 km² of old-growth forest. Small and large size classes of the two most abundant species were modelled separately to see whether habitat associations change with ontogeny.

Results: GLM and GAM model results were similar. All species had significant distributional biases with respect to microhabitat. Environmental variables describing soil variation were included in the models most often, followed by topographic and forest structural variables. The distributions of small individuals were more strongly related to environmental variation than those of larger individuals. Significant neighbourhood effects (spatial autocorrelation in intraspecific distributions and non-random overlaps in the distributions of certain species pairs) were also identified. Overlaps between congeners did not differ from random, but there was a highly significant overlap in the distributions of the two most common species.

Conclusions: Our results support the view that habitat specialization is an important determinant of where on the rain forest landscape tree ferns grow, especially for juvenile plants. However, other factors, such as dispersal limitation, may also contribute to their local coexistence.

Nomenclature: Moran & Riba (1995).

Questions: Does grazing have the same effect on plant species richness at different spatial scales? Does the effect of spatial scale vary under different climatic conditions and vegetation types? Does the slope of the species-area curve change with grazing intensity similarly under different climatic conditions and vegetation types?

Location: Pastures along a climatic gradient in northeastern Spain.

Methods: In zones under different regimes of sheep grazing (high-, low-pressure, abandonment), plant species richness was measured in different plot sizes (from 0.01 to 100 m²) and the slope of the species-area curves was calculated. The study was replicated in five different locations along a climatic gradient from lowland semi-arid rangelands to upland moist grasslands.

Results: Species richness tended to increase with grazing intensity at all spatial scales in the moist upland locations. On the contrary, in the most arid locations, richness tended to decrease, or remain unchanged, with grazing due to increased bare soil. Grazing differentially affected the slope (z) of the species-area curve (power function S = c A^z) in different climatic conditions: z tended to increase with grazing in arid areas and decrease in moist-upland ones. β-diversity followed similar pattern as z.

Conclusions: Results confirm that the impact of grazing on plant species richness are spatial-scale dependent. However, the effects on the species-area relationship vary under different climatic conditions. This offers a novel insight on the patterns behind the different effects of grazing on diversity in moist vs. arid conditions reported in the literature. It is argued that the effect of spatial scale varies because of the different interaction between grazing and the intrinsic spatial structure of the vegetation. Variations in species-area curves with grazing along moisture gradients suggest also a different balance of spatial components of diversity (i.e. α- and β-diversity).

Nomenclature: Bolòs et al. (1993).

Questions: 1. Which habitats have the highest degree of invasion? 2. Do native species-rich communities have also a high degree of invasion? 3. Do the patterns of association between native and alien species richness vary between habitats.

Location: Catalonia region (NE Spain).

Methods: We conducted a large regional analysis of 15 655 phytosociological relevés to detect differences in the degree of invasion between European Nature Information System (EUNIS) habitats representative of temperate and Mediterranean European areas.

Results: Alien species were present in less than 17 % of the relevés and represented less than 2% of the total number of species per habitat. The EUNIS habitats with the highest alien species richness were arable land and gardens followed by anthropogenic forb-rich habitats, riverine and lakeshore scrubs, southern riparian galleries and thickets and trampled areas. In contrast, the following habitats had never any alien species: surface running waters, raised and blanket bogs, valley mires, poor fens and transition mires, base-rich fens, alpine and sub-alpine grasslands, sub-alpine moist or wet tall-herb and fern habitats, alpine and sub-alpine scrub habitats and spiny Mediterranean heaths. There was a unimodal relationship between the mean native and mean alien species richness per EUNIS habitat with a high number of aliens in habitats with intermediate number of native species and a low number of aliens at both extremes of the native species gradient. Within EUNIS habitats, the relationship was positive, negative or non-significant depending on the habitat type without any clear pattern related to the number of native species. Alien species richness was not related to plot size, neither between habitats nor within habitats.

Conclusions: The analysis emphasised that the habitats with a higher degree of invasion were the most disturbed ones and that in general habitats rich in native species did not harbour less invaders than habitats poor in native species.

Objective: Treeless meadows and parks are widespread but poorly understood features of the montane vegetation of the western USA. These communities frequently form reversed treelines where grassy valleys occur below forested slopes above. Our purpose was to assess the environmental correlates of such treelines, as well as patterns in the composition and diversity of grasslands and forest margins in the Valles Caldera National Preserve.

Location: Valles Caldera National Preserve (35°50′–36°00′ N, 106°24′–106°37′ W, 2175–3150 m), Jemez Mountains, New Mexico, USA.

Methods: We conducted a gradient analysis based on 200 nested quadrats on transects crossing reversed treelines and spanning the compositional heterogeneity of grasslands. We used cluster analysis and non-metric multidimensional scaling to assess relationships between compositional variation and environmental variables.

Results: We found strong, highly significant relationships of the vegetation to gradients in slope inclination, soil texture, moisture, nutrient availability, and nighttime minimum temperatures. Reversed treelines are most strongly associated with shifts in the thermal regime, exhibit weaker relationships with soil texture and nutrient content, and show no relationship with gravimetric soil moisture. Gradients in aspect, soil moisture, and annual mean temperature are associated with compositional variation within grasslands and forest margins.

Conclusions: Lower nightly minimum temperatures and fewer consecutive frost-free days resulting from cold-air drainage may prevent tree seedling establishment in valley bottoms via photo-inhibition, tissue damage, or frost heaving. Finetextured soils may also impede tree seedling establishment in valley bottoms. These findings lay the groundwork for experimental and physiological tests of these potential causes of these reversed treelines.

Nomenclature: Hartman & Nelson (2005); Anon. (2006)

Question: What characteristics of local biotic neighbourhood is the best proxy of competitive effects experienced by plants in a herbaceous community: (1) total above-ground biomass, (2) root mass or (3) relative above-ground abundance of selected species?

Location: Grassland at ca. 1100 m a.s.l. in the Krkonoše Mts., northern Czech Republic.

Methods: We implanted two phytometer species, Anthoxanthum alpinum and Festuca rubra, into a mountain grassland, and examined their response to local variation in (1) total above-ground biomass, (2) root mass at three soil depths, and (3) relative abundance of individual species above-ground.

Results: Performance of both phytometer species was determined much more consistently by the mass of neighbouring roots and by species composition of neighbours than by the total above-ground biomass. The two phytometer species showed different responses to these parameters. The most important relationships were (1) negative relationship between performance of Anthoxanthum and mass of neighbouring roots at 0–3 cm, (2) positive relationship between performance of Festuca and mass of neighbouring roots at 3–6 cm, and (3) negative relationship between performance of Festuca and relative abundance of Festuca in the neighbourhood.

Conclusions: Neighbouring root mass and above-ground species composition are better determinants of biotic interactions than total above-ground biomass of neighbours in the studied mountain grassland. However, the relationships found are not necessarily due to variation in competitive intensity but can be due to other hidden factors as well, e.g. local availability of resources.

Question: Invasion of woody species into grasslands is a global phenomenon. This is also topical in semi-natural temperate grasslands that are no longer profitable for agricultural management. Trees and grasses interact through harsh root competition, but below-ground processes have been neglected in the dynamics of semi-natural grasslands. Trees are thought to have a competitive advantage in resource-rich and heterogeneous soils. We tested whether soil resource quantity and heterogeneity differ between paired temperate semi-natural grasslands and forests (former grasslands), and whether this was caused abiotically by varying soil depth or biotically by fine roots.

Location: Thin-soil calcareous alvar grasslands with over-grown parts (young Pinus sylvestris forests) in W. Estonia.

Methods: The quantity and spatial heterogeneity of soil resources (moisture and nutrients), soil depth, and root parameters (mass, length and specific length) were measured in 1-m transects of 11 samples in 26 paired grasslands and forests. The quantity and heterogeneity of soil resources were compared between vegetation types and related to soil depth and root parameters.

Results: Soil resources were lower and more heterogeneous in forests than in grasslands. The invasion of woody species was enhanced abiotically by deeper soil. Root mass was larger in the forests, but root length was longer in the grasslands. Both root mass and specific root length were more heterogeneous in the forests. Forest root length was negatively correlated with transient soil moisture patches and positively correlated with more persistent nutrient-rich patches. No such relationship was found in grasslands.

Conclusions: Abiotic soil heterogeneity (local deep-soil patches) supports woody species invasion, but the trees themselves also biotically make soils more heterogeneous, which further enhances woody species invasion. Large trees use soil resources patchily, making soils biotically poorer and more heterogeneous in resources. The dynamics of temperate semi-natural grasslands are strongly linked to below-ground ecological processes, and high soil heterogeneity can be both the cause and the outcome of woody species invasion.

Questions: For eucalypt savanna in northeast Australia subject to multi-year rainfall deficits this paper asks whether (1) dominant tree species (Ironbarks, Boxes) are more drought susceptible than the sub-dominant Bloodwoods; (2) whether soil moisture is beyond wilting point in surface soil layers but available at depth; (3) soil conditions (moisture availability and texture) are related to tree death during drought; (4) the root systems of the Boxes and Ironbarks are shallower than the Bloodwoods; and the survivors of drought within species have deeper root systems than those that died.

Location: Central Queensland, Australia.

Methods: Patterns of tree death between eucalypt species were compared from field data collected after drought. Soil conditions during drought were described and compared with patterns of tree death for the Ironbark Eucalyptus melanophloia, The basal area and orientation of coarse roots were measured on upturned trees after broad-scale tree clearing, and compared between species, and between live and dead trees with tree size as a covariate.

Results: Drought-induced tree death was higher for dominant Ironbark-Box than for sub-dominant Bloodwoods. During a moderate to severe drought in 2004, 41% of 100 cm deep sub-soils had soil matric potential less than −5600 kPa. The drought hardy Bloodwoods had a greater root basal area and particularly so for vertical roots compared to the drought sensitive Ironbark-Box. Within species there was no significant difference in root basal area characteristics between trees that were recently killed by drought and those that remained relatively healthy. Surface soil moisture availability was lower where tree densities were high, and tree death increased as surface soil moisture became less available. Tree death was also greater as the clay content of sub-soils increased.

Discussion: The study suggests species with roots confined to upper soil layers will suffer severe water stress. The results strongly indicate that root architecture, and the way it facilitates water use during drought, is important for the relative dominance of the tree species. Patchiness in drought-induced tree death seems to be at least partially a product of heterogeneity in sub-soil conditions and competition for soil moisture.

Nomenclature: Because of the complex nomenclature of the taxa to be treated here vernacular names as elucidated in Table 1 will be used (see also Henderson 2002).

Question: What are the edge effect responses of epiphytic lichen communities in Mediterranean Quercus pyrenaica forest?

Location: Central Spain.

Methods: We established ten transects perpendicular to a road dissecting a well conserved remnant of Q. pyrenaica forest into two sections. Transects extended from the forest/road edge to 100 m into the forest. Data were collected from seven plots in each transect at different distances from the edge. Variables were grouped into stand scale variables (distance to edge, number of trees per plot, mean diameter per plot, irradiance) and tree scale variables (diameter and height of sampled trees, aspect of the sampled square and relative height of the square). We used General Mixed Linear Models and constrained ordination techniques to test the hypothesis that the spatio-temporal heterogeneity of light and water controls the occurrence of lichens and bryophytes along the edge-interior gradient in the Q. pyrenaica forest.

Results: Microclimatic parameters vary in a non-linear way; edge and interior stands showed the most divergent and extreme values. Although the micro-environment within Mediterranean forests is heterogeneous, interior conditions are apparently suitable for the performance of some specific forest epiphytes. Consequently, species richness does not show significant differences along the gradient. Total epiphytic cover increases towards the forest interior, but distance to the edge together with other predictors at the tree scale (aspect and height of the square) are the most relevant predictors for the composition and structure of these communities.

Conclusions: Composition and structure of epiphytic communities in a Mediterranean semi-deciduous forest are affected by the edge between the forest and the road constructed. Since some extremely rare lichens only occur at interior stands, the conservation of these threatened elements requires urgent conservation measures because well preserved and unmanaged forests in the Mediterranean region are very rare.

Nomenclature: Hafellner & Türk (2001); Bisby & Rostov (2005).

Questions: In a system of five annual plant species restricted to nest-mounds of the ant Lasius flavus in a perennial grassland: 1. Are the population dynamics influenced by ant disturbance? 2. Is the survival of the annuals at the scale of the whole grassland possible under the observed conditions of disturbance dynamics? 3. Which phases in the annuals' life cycle and patch types contribute most to population growth?

Location: Boreč hill, northern Czechia, 50°31′ N, 13°59′ E, 446 m a.s.l.

Methods: Local population dynamics of the annuals were analysed separately for five patch types that differed in the proportion of bare soil. Vitality rates were assessed directly in the field, but also in a garden experiment, during 2000–2001 and 2001–2002. Population dynamics at the scale of the whole grassland was analysed with a megamatrix approach, combining patch dynamics of the nest-mounds with patch-specific population dynamics. Contributions of different phases and patch types to growth rate were estimated by elasticity analysis.

Results: Nest-mounds differed in the percentage of bare soil. Increasing moss cover significantly reduced germination and seed production of all studied annuals and decreased their population growth rates (λ). Although successional processes dominated over ant disturbance, populations of all species could survive well (λ≫ 1) in the grassland according to the 2000–2001 megamatrix dynamics. Based on the dynamics from the following period, two species would not survive in a long-term perspective due to random environmental variation. Whereas the A-A transition (adult plants originating from adults of the previous year) had the highest elasticity under open conditions and 'good period' demography, the importance of persistent seeds increased under reverse conditions. This, however, differed among species.

Conclusions: Ant-disturbance was shown to be critical for the population survival of five annual species in the studied grassland. The fate of the annual populations in the grassland system also depends on random environmental variation, which may override the effect of ant activity.

Questions: 1. Do disturbances by harvester ants (Messor barbarus L.) affect soil properties? 2. Do they alter seed distribution? 3. Do they show a different species composition? 4 Are these changes related to seed size (length and weight)?

Location: Mediterranean grasslands in central Spain, near Madrid.

Methods: We recorded autumn seed banks and spring vegetation in middens, trunk trails and controls. Soil properties were also measured in middens and controls. The effect of seed weight and length was analysed after transforming data into phylogenetically independent contrasts.

Results: Soil under middens is more silty and higher in potassium, organic matter and pH. Seed density and seed bank species richness increased greatly in middens, while vegetation species richness was significantly lower in comparison to control samples. Abundance changes in this disturbance type were positively correlated with seed weight, both in seed bank and vegetation. In contrast, we only detected a slight increase in bare ground on foraging trunk trails, with no clear effects on seed bank and vegetation composition.

Conclusions: Midden development is a mechanism that generates heterogeneity which favours the survival of certain large-seeded species mainly predated by ants in Mediterranean grasslands. This effect may partially neutralize the dominance of small seeded species expected from the seed predation process.

Nomenclature: Castroviejo (1986–2003), except taxa yet to be covered, for which Tutin et al. (1964–1980) is followed.

Question: We tested whether (1) the change in composition and structure of whole plant communities after fire is directly related to regeneration of the dominant tree species in the canopy; (2) the change in structure and composition of plant communities several years after fire decreases with the proportion of obligate seeders and (3) the proportion of obligate seeders in plant communities increases with the dryness gradient.

Location: Catalonia (NE Spain).

Methods: We measured floristic differences between burned and long-since burned sites in eight vegetation types across a climate gradient. We compared 22 sites burnt in 1994 in paired plots with 22 sites that had not been burnt since the 1940s. In each site we placed plots in burned and long-since burned areas, where we identified the presence and abundance of all plant species.

Results: When the tree canopy recovers, structure and composition of the vegetation also return to the long-since burned community; when tree canopy does not recover, composition of the post-fire community varies compared to the long-since burned one. A higher proportion of obligate seeders in the pre-fire community promotes quicker regeneration of the original community. The proportion of obligate seeders increased along the dryness gradient.

Conclusions: Regeneration of plant communities after fire depends on the vegetation type before the fire. Regeneration increases when the dominant tree or shrub species persists after fire and with a higher proportion of obligate seeders in the pre-fire community. The proportion of obligate seeders varies along the dryness gradient, which suggests that vegetation in drier areas (when seeders are more abundant) recovers earlier than in moister areas.

Nomenclature: Bolós et al. (1990).

Question: Vertical zonation schemes are widely used in biodiversity studies with vascular epiphytes as a tool to capture spatial distribution patterns, the one most commonly used was proposed by Johansson more than 30 years ago. Does a survey of the epiphytes found on larger trees really yield a representative sample of the local community?

Location: Lowland rainforest of the San Lorenzo Crane Plot, Republic of Panama.

Methods: A complete census of the vascular epiphytes on all trees > 1 cm DBH in 0.4 ha of undisturbed lowland forest was analysed with both cluster and discriminant analysis to detect groupings of epiphyte species.

Results: Six different groups of species were detected, five of them preferring different substrates on larger trees (as defined by (1) the height above ground at the attachment site, (2) the diameter of the substrate and (3) the occurrence on stem vs branches/twigs) and resembling to some extent the original Johansson zones. A sixth group of epiphytes, comprising ca. 10% of all taxa, was almost always found on small diameter stems and branches of trees with small DBH at lower and intermediate heights within the forest.

Conclusions: Applying pre-established zonation schemes may lead to misleading results in biodiversity studies with epiphytes. Important aspects of spatial distribution patterns may be missed, and the determination of relative species abundances may carry a strong quantitative and qualitative bias when analyses rely completely on epiphytic plants found on larger trees.

Nomenclature: For flowering plants D'Arcy (1987), for ferns Lellinger (1989) and Croat (1978), for filmy ferns compare also Zotz & Büche (2000). Encyclia aemula (=Prosthechea aemula) was treated as a separate species following Dressler (1993). A complete species list was published by Zotz (2004). Voucher specimens are deposited in the herbarium of the Smithsonian Tropical Research Institute, Panama (Tupper Center).

Objective: To present a non-classificatory technique of map representation of compositional patterns of vegetation as no two plant species assemblages are completely alike and gradations often occur. Variation is depicted as continuous fields instead of classes.

Location: Murnauer Moos, Bavaria.

Methods: The study combined vegetation ecology and remote sensing methods. The gradual representation of compositional patterns was based on techniques of ordination and regression, instead of mapping class fractions. The floristic field data were collected in relevés and subjected to three-dimensional non-metric multidimensional scaling (NMS). The reflectance information corresponding to plots was gathered from remotely sensed imagery with a high spectral resolution. Reflectance values in numerous wavelengths were related to NMS axes scores by partial least squares regression analysis. The regression equations were applied to the imagery and yielded three grey-scale images, one for each ordination axis. These three images were transformed into a red, green, and blue colour map with a specific colour for each position in the ordination space. Similar colours corresponded to similar species compositions.

Results: Compositional variation was mapped accurately (R² = 0.79), using continuous fields. The results took account of various types of stand transitions and of heterogeneities within stands. The map representation featured relatively homogeneous stands and abrupt transitions between stands as well as within-stand heterogeneity and gradual transitions.

Conclusions: The use of NMS in combination with imaging spectroscopy proved to be an expedient approach for nonclassificatory map representations of compositional patterns. Ordination is efficiently extended into the geographic domain. The approach in abandoning pre-defined plant communities is able to reconcile mapping practice and complex reality.

Nomenclature: Wisskirchen & Haeupler (1998) for phanerogams; Smith (1980) for mosses; Rennwald (2000) for syntaxa.

Question: What determines the balance between the cover values of vascular plants, lichens and mosses in dry calcareous grassland communities?

Location: Western Estonia.

Methods: A five-year (2001–2005) study was conducted in a dry calcareous grassland. The cover of mosses, lichens and vascular plants and all moss species was recorded in permanent plots. Vascular plants were cut in half of the plots. Data from a nearby weather station were used to calculate mean values of different weather parameters and a summer moisture index for the study years.

Results: Significant differences in cover values between years were found. The fluctuations of total moss cover and the cover of the dominating moss species Ctenidium molluscum followed changes in annual precipitation. Both cover values were highest in years with high precipitation. The cover change of vascular plants was best characterized by the moisture index of the growth period (three summer months). Summers with high moisture indexes facilitated vascular plant and lichen growth. Annual precipitation and the cover of mosses had a negative influence on the cover of vascular plants. The cutting of vascular plants did not have a significant effect on moss and lichen cover.

Conclusions: 1. On dry calcareous grasslands the growth of mosses is enhanced by high annual precipitation, while the growth of vascular plants and lichens is influenced rather by the high summer moisture index. The cover of vascular plants is inhibited by the large moss cover. 2. Mowing of vascular plants does not influence the cover of mosses and lichens.

Nomenclature: Ingerpuu et al. (1998) for bryophytes; Kukk (1999) for vascular plants; Trass & Randlane (1994) for lichens.

Trees on small islands formed when a new hydroelectric dam was filled are said to have been killed by leaf-cutter ants that were no longer regulated by their armadillo predators. This top-down hypothesis is questioned and an alternative bottom-up hypothesis is proposed: the trees died because their roots were flooded.