Showing papers in "Journal of Vegetation Science in 2002"

JUICE - software for vegetation classification

Abstract: The program JUICE was designed as a Microsoft® WINDOWS® application for editing, classification and analysis of large phytosociological tables and databases. This software, with a current maximum capacity of 30 000 releves in one table, includes many functions for easy manipulation of table and header data. Various options include classification using COCKTAIL and TWINSPAN methods, calculation of interspecific associations, fidelity measures, average Ellenberg indicator values, preparation of synoptic tables, automatic sorting of releve tables, and export of table data into other applications (word processors, spreadsheet programs or mapping packages). JUICE is optimized for use in association with TURBOVEG which is the most widespread database program for storing phytosociological data in Europe.

Equations for potential annual direct incident radiation and heat load

Abstract: Estimation of potential annual direct incident radia- tion has traditionally required numerical integration with simu- lation models. As an alternative, we present convenient equa- tions for use in spreadsheet, GIS, and database applications. Input variables are latitude, slope, and aspect. The equations apply to 0-60∞ north latitude, slopes from 0-90∞, and all aspects. By transforming aspect, the equations can also be applied as an index of heat load, symmetrical about a northeast to southwest axis.

Determination of diagnostic species with statistical fidelity measures

Abstract: Statistical measures of fidelity, i.e. the concentration of species occurrences in vegetation units, are reviewed and compared. The focus is on measures suitable for categorical data which are based on observed species frequencies within a vegetation unit compared with the frequencies expected under random distribution. Particular attention is paid to Bruelheide's u value. It is shown that its original form, based on binomial distribution, is an asymmetric measure of fidelity of a species to a vegetation unit which tends to assign comparatively high fidelity values to rare species. Here, a hypergeometric form of u is introduced which is a symmetric measure of the joint fidelity of species to a vegetation unit and vice versa. It is also shown that another form of the binomial u value may be defined which measures the asymmetric fidelity of a vegetation unit to a species. These u values are compared with phi coefficient, chi‐square, G statistic and Fisher's exact test. Contrary to the other measure...

Land-use history and fragmentation of traditionally managed grasslands in Scandinavia

Abstract: Plants associated with traditional agricultural landscapes in northern Europe and Scandinavia are subjected to drastic habitat fragmentation. In this paper we discuss species response to fragmentation, against a background of vegetation and land-use history. Recent evidence suggests that grassland-forest mosaics have been prevalent long before the onset of human agriculture. We suggest that the creation of infield meadows and outland grazing (during the Iron Age) increased the amount and spatial predictability of grasslands, resulting in plant communities with exceptionally high species densities. Thus, distribution of plant species in the present-day landscape reflects historical land-use. This holds also when traditional management has ceased, due to a slow response by many species to abandonment and fragmentation. The distribution patterns are thus not in equilibrium with the present habitat distribution. Fragmentation influences remaining semi-natural grasslands such that species density is l...

An international network to monitor the structure, composition and dynamics of Amazonian forests (RAINFOR)

Abstract: The Amazon basin is likely to be increasingly affected by environmental changes: higher temperatures, changes in precipitation, CO2 fertilization and habitat fragmentation. To examine the important ecological and biogeochemical consequences of these changes, we are developing an international network, RAINFOR, which aims to monitor forest biomass and dynamics across Amazonia in a co-ordinated fashion in order to understand their relationship to soil and climate. The network will focus on sample plots established by independent researchers, some providing data extending back several decades. We will also conduct rapid transect studies of poorly monitored regions. Field expeditions analysed local soil and plant properties in the first phase (2001–2002). Initial results suggest that the network has the potential to reveal much information on the continental-scale relations between forest and environment. The network will also serve as a forum for discussion between researchers, with the aim of stand...

Reproductive success in fragmented habitats: do compatibility systems and pollination specialization matter?

Abstract: This paper explores whether plant breeding system and pollination specialization influence the reproductive response of plants to habitat fragmentation. It is meaningful for conservation to predict a plant species' extinction risk. We found 25 studies in the literature assessing the effects of habitat fragmentation on either pollination or reproductive success of 46 plant species to answer the following questions: 1. Are pollination and reproductive success of self-incompatible species more likely to decline with habitat fragmentation than the pollination and reproductive success of self-compatible species? Although most of the species showed statistically significant negative effects, the pollination and reproduction of self-incompatible species were as likely to decline with fragmentation as those of self-compatible species. 2. Are pollination and reproductive success of specialist plants more affected than the pollination and reproduction of generalist plants? Comparisons of fragmentation-rela...

Trends in savanna structure and composition along an aridity gradient in the Kalahari

Abstract: The Kalahari sand sheet occupies 2.5 million ha in southern Africa. It is an area with relatively similar deep aeolian soils, and a strong south to north gradient in rainfall, from 200 to 1000 mm mean annual precipitation (MAP) in the region studied. This provides an excellent basis for gradient studies at the subcontinental scale. This paper briefly reviews the literature on the vegetation of the Kalahari and describes the vegetation structure and composition at 11 new sites. There is a clear gradient in woody plant biomass (as indexed by basal area) from south to north. Above the minimum level of 200 mm MAP, the woody basal area increases at a rate of ca. 2.5 m2.ha−1 per 100 mm MAP. Mean maximum tree height also increases along the gradient, reaching 20 m at ca. 800 mm MAP. The number of species to contribute > 95% of the woody basal area increases from one at 200 mm to 16 at 1000 mm MAP. Members of the Mimosaceae (mainly Acacia) dominate the tree layer up to 400 mm MAP. They are replaced by ei...

Validity of Ellenberg indicator values judged from physico-chemical field measurements

Abstract: The relationship between mean Ellenberg indicator values (IV) per vegetation releve and environmental parameters measured in the field usually shows a large variation. We tested the hypothesis that this variation is caused by bias dependent on the phytosociological class. For this purpose we collected data containing vegetation releves and measured soil pH (3631 records) or mean spring groundwater level (MSL, 1600 records). The releves were assigned to vegetation types by an automated procedure. Regression of the mean indicator values for acidity on soil pH and the mean indicator values for moisture on MSL gave percentages explained variance similar to values that were reported earlier in literature. When the phytosociological class was added as an explanatory factor the explained variance increased considerably. Regression lines per vegetation type were estimated, many of which were significantly different from each other. In most cases the intercepts were different, but in some cases their slop...

Relative competitive performance of 63 species of terrestrial herbaceous plants

Abstract: There is growing evidence that plant and animal species are arranged in hierarchies of relative competitive performance. More work is needed to determine which plant traits best predict relative competitive performance. We there- fore measured relative competitive performance of 63 terres- trial herbaceous plant species using Trichostema brachiatum as a reference species (that is, phytometer or target species). The neighbour species came from a wide array of terrestrial vegetation types (e.g. rock barrens, alvars, old fields), and represented a wide array of growth forms (e.g. small rosette species such as Saxifraga virginiensis and large clonal graminoids such as Agropyron repens). The experiment was repeated with two pot sizes: large (control) and small (stress treatment). Relative competitive performance in large pots (controls) was highly correlated with that in small pots (stress treatment) (r = 0.90, p < 0.001). The hierarchy of relative competitive performance in the large pots was also highly correlated with the hierarchy in the small (stressed) pots (rs = 0.91, p < 0.001). Principal components analysis and multiple linear regression showed that plant size (measured by total biomass, above-ground biomass, below-ground biomass, canopy area, height and leaf area index) and leaf shape (mea- sured as length to width ratio, length, width) were the two characteristics that best predicted relative competitive perfor- mance (large pots, r2 = 0.55; small pots, r2 = 0.48).

Nurse effect of Bolax gummifera cushion plants in the alpine vegetation of the Chilean Patagonian Andes

Abstract: It has been proposed that in the harsh arctic and alpine climate zones, small microtopographic variations that can generate more benign conditions than in the surrounding environment could be perceived as safe sites for seedling recruitment. Cushion plants can modify wind pattern, tem- perature and water availability. Such modifications imply that cushion plants could act as 'nurse plants' facilitating the recruitment of other species in the community. This effect should be more evident under stressful conditions. We tested these hypotheses comparing the number of species that grow inside and outside Bolax gummifera cushions at two eleva- tions (700 and 900 m a.s.l.) in the Patagonian Andes of Chile (50 ∞S). At both elevations, and in equivalent areas, the number of species was registered within and outside cushions. A total of 36 and 27 plant species were recorded either within or outside B. gummifera cushions at 700 and 900 m a.s.l., respec- tively. At 700 m a.s.l., 33 species were recorded growing within cushions and 29 outside them, while at 900 m a.s.l. these numbers were 24 and 13 respectively. At both elevations there were significantly more species growing within than outside cushions, and the proportion of species growing within cushions increased with elevation. Thus there is a nurse effect of cushion plants and it is more evident at higher elevations. Shelter from wind and increased soil water availability seem to be the factors that increase plant recruitment within cushions.

Environmental variation, vegetation distribution, carbon dynamics and water/energy exchange at high latitudes

Abstract: The responses of high latitude ecosystems to global change involve complex interactions among environmental variables, vegetation distribution, carbon dynamics, and water and energy exchange. These responses may have important consequences for the earth system. In this study, we evaluated how vegetation distribution, carbon stocks and turnover, and water and energy exchange are related to environmental variation spanned by the network of the IGBP high latitude transects. While the most notable feature of the high latitude transects is that they generally span temperature gradients from southern to northern latitudes, there are substantial differences in temperature among the transects. Also, along each transect temperature co-varies with precipitation and photosynthetically active radiation, which are also variable among the transects. Both climate and disturbance interact to influence latitudinal patterns of vegetation and soil carbon storage among the transects, and vegetation distribution appears to interact with climate to determine exchanges of heat and moisture in high latitudes. Despite limitations imposed by the data we assembled, the analyses in this study have taken an important step toward clarifying the complexity of interactions among environmental variables, vegetation distribution, carbon stocks and turnover, and water and energy exchange in high latitude regions. This study reveals the need to conduct coordinated global change studies in high latitudes to further elucidate how interactions among climate, disturbance, and vegetation distribution influence carbon dynamics and water and energy exchange in high latitudes.

Hyperdynamism in fragmented habitats

Abstract: Are the dynamics of most ecological processes fundamentally increased in frequency or magnitude in fragmented habitats? Hyperdynamism could alter a wide range of population, community, and landscape phenomena, and appears to be evident in fragmented tropical, temperate, and boreal communities. I suggest some potential causes and consequences of hyperdynamism, and argue that the responses of many species and ecological processes to habitat fragmentation can be understood in this context.

Carbon and nitrogen dynamics across a natural precipitation gradient in Patagonia, Argentina

Abstract: Both ecosystem carbon gain and nutrient availability are largely constrained by the magnitude and seasonality of precipitation in arid and semi-arid ecosystems. We investigated the role of precipitation on ecosystem processes along an International Geosphere Biosphere Programme (IGBP) transect in temperate South America. The transect consists of a contiguous precipitation gradient in the southern region of Argentinean Patagonia (44–45° S), from 100 mm to 800 mm mean annual precipitation (MAP) and vegetation ranging from desert scrub to closed canopy forest. Gravimetric soil water content tracked changes in seasonal and annual precipitation, with a linear increase in soil water content with increasing MAP. Above-ground net primary production (ANPP) increased linearly along the gradient of precipitation (ANPP = − 31.2 + 0.52 MAP, r2 = 0.84, p = 0.028), supporting the relationship that carbon assimilation is largely controlled by available water in these sites, and was in general agreement with regi...

Patterns of β-diversity in a Mexican tropical dry forest

Abstract: Patterns of β-diversity in a highly diverse tropical dry forest tree community are described; the contribution of environmental heterogeneity and distance to β-diversity was assessed. Significant differences in elevation, insolation, slope and soil water holding capacity (p < 0.01), variables related to water availability, were found among 8 30 m × 100 m transects laid along contrasting slopes of a system of three parallel microbasins. A gradient in elevation and insolation was found within north-facing transects, among 10 m × 10 m sites; south-facing transects showed an elevation gradient while crest transects showed a gradient in water holding capacity. In total 119 species were registered, with 27 to 64 species per transect, and 4 to 16 species per site. A large β-diversity was found among and within transects; two indices of β-diversity consistently showed a higher β-diversity within transects than among them. Among transects, 64% of the variance in species composition could be attributed to ...

Acacia trees as keystone species in Negev desert ecosystems

Abstract: The only trees in most of the Negev desert are 3 native Acacia species. We tested the hypothesis that they act as keystone species as a result of the improved soil conditions under their canopies. Furthermore, because many Acacia populations suffer high levels of mortality due to water stress, we tested whether trees in high mortality populations had diminished effects on plant species and soil quality under their canopies. We show that plant species diversity beneath the tree canopies is higher than in the surrounding areas. There was also a clearly identifiable suite of species with higher occurrence under the trees. Plant species composition differed significantly between high and low mortality sites. However, there was higher species diversity in high mortality sites and under trees with higher water stress. Soil nutrient content was higher under the trees than in the open areas, especially under larger trees and trees with higher water status. The results indicate that there is a combination...

Structure and composition of edges next to regenerating clear-cuts in mixed-wood boreal forest

Abstract: We investigated the spatial and temporal pattern of edge influence on primary (forest structure) and secondary (understory structure and composition) responses at forest edges next to regenerating clear-cuts in Populus-dominated boreal forest in Alberta, Canada. We used before/after-harvest comparisons to determine whether there were significant ef- fects of edge creation in the first and second year after harvest. We also investigated 5- and 16-yr-old edges to determine whether their structure and composition were significantly different from interior forest. Distance of edge influence (20 - 60 m) was not extensive compared to findings from studies conducted in other forest types. In the first two years after harvest, there were significant primary responses to edge creation including increased tree mortality, snag breakage and amount of downed coarse woody debris; as well as secondary responses including prolific vegetative regeneration of Populus spp. and a decrease in total shrub cover. Significant edge effects were still detectable 5 and 16 yr after edge creation, despite the rapid regeneration of Populus- dominated forest in the adjacent clear-cuts. These older edges showed evidence of past primary and secondary responses, including a more het- erogeneous canopy, along with additional changes in under- story composition. At both younger and older edges the dis- tance of edge influence was greater for secondary responses (up to 60 m) than for primary responses (10 - 20 m). We hypothesize that secondary responses to edge creation start later than primary responses and that the distance of edge influence is greater for secondary (vs. primary) responses.

The species pool concept applied to forests in a fragmented landscape: dispersal limitation versus habitat limitation

Abstract: The species pool concept has been used as a theo- retical framework for understanding local community rich- ness. A significant problem in putting the concept into practice is the lack of methods for determining the size of the species pool. We tested the hypothesis that species composition of recent forests is primarily determined by the species composi- tion of neighbouring older forests against the null-hypothesis that species are a random sample of the species occurring in the study area. Forest plant species composition of recently established fragments was significantly correlated with spe- cies composition in neighbouring older forests (i.e. the local species pool). When older forest within a neighbourhood of 1000m radius is considered, seed dispersal sources can be found for 91% of the flora in the recent forests. For an individual fragment, dispersal is a much more important deter- minant of species presence than the environment, with an average of 46% of the total pool excluded from local pools by dispersal limitation and only 8% excluded by environmental limitations. The species richness of recent forests is on aver- age 23% of the local species pool. Several hypotheses are proposed for this low percentage, such as asymmetric compe- tition due to the early successional state or the limited coloni- zation period.

Declining plant diversity: empty niches or functional shifts?

Abstract: The evidence is compelling. The planet is losing plant species and populations at a rate too fast to allow a case by case evaluation of the consequences for ecosystem functioning and human need. With limited time and resources available how can we recognize the problems likely to arise from plant losses and then decide the priorities for action? Ecologists with very different skills and experience have been attracted to this field of enquiry and over the last decade the subject has become the focus of heated debate (Loreau et al. 2001). Sufficient time has now passed for the architecture of this debate to be revealed and my purpose here is to comment on its progress and to recommend a change in direction. I will focus on the effects of diversity on productivity because it is this aspect of the debate that has attracted most attention.

Are nutrient availability and acidity-alkalinity gradients related in Sphagnum-dominated peatlands?

Abstract: Gradients in acidity-alkalinity and nutrient availability were studied in 2 Sphagnum-dominated peatlands on the southeastern Italian Alps. Decreasing concentrations of most mineral elements (Ca2+, Mg2+, Mn2+, Al3+ and Si4+) in pore water indicated a progressively lower influx of mineral-soil water from the slightly minerotrophic conditions in the peatland margins to ombrogenous conditions in the central part of the peatlands. This was paralleled by decreasing concentrations of ash, bulk density, Ca, Fe and, partly, Mn in the peat. The nutrient gradient, as defined by pore water concentrations of N and P, was largely independent of the acidity-alkalinity gradient: NO3− and PO43− had similar concentrations throughout the gradient, whereas NH4+ concentrations increased with increasing pore-water pH. In contrast, the peat nutrient gradient coincided with the acidity-alkalinity gradient, with total concentrations of N and P decreasing from the margin to the centre. Bryophytes and vascular plants had d...

Niche characteristics of Danish woody species as derived from coenoclines

Abstract: A main floristic coenocline is extracted from a Danish forest vegetation data set with Detrended Correspondence Analysis. Huisman-Olff-Fresco (HOF) modelling of 262 species occurrences relative to site scores along the coenocline is compared with Gaussian and Generalized Linear Modelling. HOF models allow for responses of variously skewed forms, which may result from interspecific interactions. Most species have symmetric (60%) or skewed (29%), and only few species have flat or monotone response curves. This provides evidence for the generality of unimodal plant species response curves, provided the gradient in question comprises a sufficient amount of compositional turnover. The Gaussian niche width is correlated with density of optima and probability of occurrence of the species. Detailed analysis of HOF estimates for 28 woody species indicates that early succession species such as Pinus sylvestris and Populus tremula have wider niches than middle to late successional canopy species such as Aln...

Plant diversity, biogeography and environment in Iberia: Patterns and possible causal factors

Abstract: We associated patterns of plant diversity with pos- sible causal factors by considering 93 local regions in the Iberian Peninsula and Balearic Islands with respect to biogeo- graphy, environmental favourability, and environmental hetero- geneity, and their relationship with measured species diversity at four different scales: mean local species richness standard- ized at a grain of 100 m2, total species richness in a community type within a region (regional community richness), mean compositional similarity, and mosaic diversity. Local regions in biogeographic transition zones to the North African and Atlantic floras had higher regional commu- nity richness and greater mosaic diversity than did non-transi- tional regions, whereas no differences existed in mean local species richness or mean compositional similarity. Mean local species richness was positively related to environmental fa- vourability as measured by actual evapotranspiration, but negatively related to total precipitation and temporal heteroge- neity in precipitation. Mean local species richness was great- est in annual grassland and dwarf shrubland communities, and on calcareous bedrock types. Regional community richness was similarly related to actual evapotranspiration and total precipitation, but in addition was positively related to spatial heterogeneity in topography and soil water holding capacity. Mean compositional similarity decreased with increasing spa- tial heterogeneity and temperature seasonality. Mosaic diver- sity, a measure of complexity, increased with increasing local and regional richness. We hypothesize that these relationships can be explained by four ecological and evolutionary classes of causal factors: numbers of individuals, intermediate environments, limits to adaptation, and niche variation. These factors operate at various scales and manifest themselves in various ways. For example, at the site level, apparently processes that increase the number of individuals increase mean local species richness, but at the level of the entire region no such effects were found.

Responses of grassland ecosystems to precipitation and land use along the Northeast China Transect

Abstract: The Northeast China Transect (NECT) has been used to study how water availability influences the composition of plant functional types, soil organic matter, net primary production, trace gas flux, and land-use patterns. We discuss relations of plant species number, soil C and N and above-ground biomass with a precipitation gradient and interactions with land-use practices (grassland fencing, mowing and grazing), on the basis of data from the west part of NECT. The results indicate: 1. The above-ground biomass of grassland communities has a linear relationship with precipitation under three land-use practices, while plant species number, soil C, and total soil N have linear relationships with precipitation under fencing and mowing; under grazing the relationships are non-linear. 2. Plant species number, soil C and total soil N have strong linear relationships with above-ground biomass under both fencing and mowing, while they seem to have non-linear relationships under grazing. 3. Land-use practic...

Environmental benefits and sustainable land-use options in the Jambi transect, Sumatra

Abstract: Loss of environmental services provided by forests is a non-linear process in Jambi Province, Sumatra, Indonesia. Intermediate-intensity land-use types in the form of complex agroforests have maintained global environmental benefits under a sustainable and profitable land use regime. Conver- sion to tree crop monocultures, however, poses a challenge to the environmental stakeholders and an opportunity from to stakeholders in the private economy. We quantified environ- mental indicators, as well as profitability and sustainability of a range of existing and possible production systems. Criteria and indicators were used at plot to landscape scales, taking into account local, national and global perspectives. Agro- nomic sustainability and profitability were assessed at plot level as they are of primarily local concern, while environ- mental services of forests, such as plant species and functional type richness, carbon stocks, greenhouse gas emissions, and trans-boundary haze, which are of national and global con- cern, were assessed at landscape level. Quantitative trade-offs and complementarities were analysed between global envi- ronmental benefits and local profitability. The current trend towards simplification of the complex agro-ecosystems and inherent loss of environmental services of forests is driven by profitability. The sequence in which environmental services of forests are lost is: standing carbon stocks, biodiversity, and low or negative greenhouse gas emissions.

The effects of Sasa and canopy gap formation on tree regeneration in an old beech forest

Abstract: We examined the response of tree seedling emergence and survival to the dieback of Sasa and canopy gap formation in an old-growth forest near Lake Towada, northern Japan. Synchronous death of Sasa occurred in 1995. We established four types of sampling sites differing in forest canopy conditions (Closed or Gap) and Sasa status (Dead or Live). Gap-Dead sites had the highest light levels and the greatest fluctuation in soil temperatures. The death of Sasa alone facilitated the emergence (Acer japonicum, Fagus crenata, Fraxinus lanuginosa, and Tilia japonica) and survival (Acanthopanax sciadophylloides, F. crenata, F. lanuginosa, Kalopanax pictus, and Sorbus commixta) of species with a seedling bank strategy. Cercidiphyllum japonicum grew at all sites at a higher density than other species, but survived well only in Gap-Dead sites. This behaviour was associated with a seed rain strategy. The additive effects of Sasa death and canopy gap formation promoted seedling emergence of pioneer tree species (...

Seed bank spatial pattern in a temperate secondary forest

Abstract: Seed bank spatial pattern was studied in a secondary forest dominated by Fagus sylvatica and Betula celtiberica in the Urkiola Natural Park (N Spain). Soil samples were taken every 2 m in a regular grid (196 points) and divided into two fractions (0-3 cm and 3-10 cm deep). The viable seed bank was studied by monitoring seedling emergence for ten months. The effect of different factors on seed bank composition and patterning was analysed using constrained ordination as a hypothesis testing tool. Furthermore, the existence of spatial autocorrelation was evaluated by geostatistical analysis. Seed density was high, 7057 seed.m−2, with a few species dominating. Species composition in the various layers were significantly correlated. The seed bank showed significant spatial structure, which was partially explainable by the spatial structure of the canopy and understorey vegetation. Spatial clumping from 0-8 m was observed in seed bank density and composition, mainly due to the pattern of two abundant t...

Natural regeneration of trees in urban woodlands

Abstract: We studied tree regeneration, a key process for the existence of urban woodlands. We hypothesized that, besides the usual biological factors, anthropogenic ones (fragmentation, wear, pollution etc.) determine the regeneration success of tree species in urban woodlands. To test this hypothesis, within an observational setting, we collected data from 30 urban woodlands in the cities of Helsinki and Vantaa, Finland. We defined the number of living saplings (30‐200cm in height) as an indicator of regeneration success and used regression analysis to test different factors as independent variables. The results showed that different tree species responded differently to urban pressure. The regeneration of Picea abies decreased with increasing fragmentation of the forest landscape, whereas for the other most common (deciduous) species, regeneration increased. Wear, measured as total path area per study site, had a negative effect on regeneration success. An a posteriori examination of the data suggested ...

Inherently slow growth in two Caribbean epiphytic species: A demographic approach

Abstract: In relation to the drought-prone and nutrient-poor habitat, vascular epiphytes are routinely referred to as inherently slow-growing plants, although actual evidence is rare. To test this notion we measured in situ growth of the understorey orchid Aspasia principissa and the tank bromeliad Vriesea sanguinolenta, and, for the latter species, also the growth under favourable conditions in the greenhouse. Using growth analysis we show: (1) that in an intraspecific comparison, small to intermediate individuals yield the highest relative growth rates (RGR) in situ: A. principissa: 1.6 10−3 d−1; V. sanguinolenta: 3.3 10−3 d−1; (2) that the bromeliad reaches maximum size after ca. 15 yr, while the orchid needs at least 20 yr; and (3) small V. sanguinolenta plants exhibit a highly plastic growth response to favourable conditions in the greenhouse, reaching an almost 10-fold increase in RGR. In spite of a substantial increase in growth under more favourable conditions, our results are consistent with the n...

Invasibility of a coastal strip in NE Spain by alien plants

Abstract: The alien plant species and the ecological factors that facilitate their invasion to a coastal strip in the Baix Camp region (Tarragona, NE Spain) were studied. A detailed inven- tory of the area showed that 20% of the plant species, most of them from the American Continent, were aliens, many of which were strongly invasive. At the habitat level, the relationships between the invasive behaviour and a number of autecological, ecological and habitat variables were analysed by means of logistic analyses. In the best model five variables were related to alien plant invasions: climatic affinity, disturbance of the invaded habitat, route of introduction, soil moisture during summer, and life form. To elucidate the environmental factors that could be responsible for alien plant invasions at a regional scale, the number of alien taxa and the environmental features at 13 localities of the western Mediterranean area were analysed. The warm lowlands, subjected to low annual rainfall and high population density, exhibited the greatest reception capacity for alien plants. It is concluded that various specific environmental fea- tures - high mean temperatures, the abundance of riparian systems, the existence of biotopes with favourable water bal- ance which harbour a great richness of alien species and the high human pressure to which the enclave is subjected, com- bined with the ecological requirements of the alien species and certain species attributes (biotype) can interact at different scales and have caused the over-representation of alien plants.

Recent fire regime characteristics and potential natural vegetation relationships in Spain

Abstract: In heavily altered landscapes, where vegetation is not natural and where people are the main source of ignitions, relationships between fire occurrence and climate conditions may be unclear. The objective of this study was to evaluate to what extent territories with similar Potential Natural Vegeta- tion (PNV) in peninsular Spain differ in their forest fire characteristics. From 1974 to 1994, more than 174 000 fires occurred. We used (1) the Spanish data base of forest fires, (2) a PNV map and (3) a land use map. Separate fire characteris- tics, based either on the number of fires occurred or the area burned, were obtained for each of the ca. 5000 grid-cells (10 km ∞ 10 km) into which peninsular Spain is divided in the UTM projection. Also, meteorological conditions at the time of fire ignition, cause of ignition and present forest cover were referred to the same grid-cells as external factors potentially determinant of fire occurrence. The relationships between fire regime characteristics and PNV units were explored with Principal Components Analysis (PCA). The role of the three sets of external factors in the fire characteristics was evaluated with Redundancy Analysis (RDA). Groups of similar PNV types were clearly segregated, suggesting a gradient of fire characteristics. Higher fire incidence (higher frequencies and spatial incidence of fires, but lower proportions of grid-cells affected by large fires) was associated with Atlantic, warm territories with deciduous forests as PNV. Intermediate fire frequency and rotation period, but with a higher relative inci- dence of medium and large fires occurred in Mediterranean PNV units, dominated by sclerophyllous oak forests. Low fire frequency and long rotation periods, with strong seasonal and yearly variability occurred for PNV units in the cold uplands (Fagus, Pinus, Abies, Juniperus) or in the semi-arid, shrubby PNV units. The cause of ignition best explained the patterns of forest fire characteristics, followed by weather conditions. Our results indicate that, even in human influenced re- gions, climate and soil conditions exert control on the result- ing forest fire characteristics, as indicated by the high segrega- tion of the PNV types. However, the role of man was crucial in shifting the patterns of fire incidence. This was so that highest fire incidence occurred in regions that, otherwise, would be expected to have a much lower one, thus posing a serious threat for such areas. PNV maps, by providing a phytogeo- graphical framework for characterizing forest fires, could be valuable tools for applying research results to forest fire man- agement policies, taking properly into account the underlying determinant factors.