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The recovery process of a Dutch heathland after fire is investigated. The study area, 12 m x 20 m, has been surveyed yearly between 1963 and 1993. Previous work has shown that a stationary Markov chain models the observed recovery process well. However, the Markov model fails to capture an important observation, the existence of a phase structure. The process begins deterministically, but small random (non-Markov) effects accumulate through time and at some point the process suddenly becomes noisy. Here we make use of the spatial information contained in vegetation maps to examine dynamics at a fine spatial scale. We find that the phases observed at a large spatial scale separate themselves out distinctly at finer spatial scales. This spatial information allows us to investigate hypotheses about the mechanisms governing deterministic versus noisy vegetation dynamics.
Fire is a constitutive ecological force in savanna ecosystems, but few studies have monitored its short-term effects on plant community dynamics. This study investigated changes in plant diversity in the South American savanna (Cerrado) after severe disturbance by fire. We monitored 30 permanent plots (10 m × 5 m) distributed in two Cerrado physiognomies (típico: more forested; ralo: grass-dominated), being 10 plots in the area disturbed by fire, and five in a preserved control area (undisturbed). From August 2010 to June 2011, we evaluated changes in species richness, abundance and composition of savanna vegetation. Monitoring started one week after the fire; disturbed plots were surveyed monthly, while control plots were surveyed every two months. We observed rapid reassembling in both physiognomies: plots affected by fire showed rapid increase in species richness and plant density during the first four months after the disturbance. Concerning species composition, disturbed plots in the cerrado típico tended to converge to control plots after one year, but each local assemblage followed particular temporal trajectories. A different pattern characterized cerrado ralo plots, which showed heterogeneous trajectories and lack of convergence between disturbed and control plots; the structure of these assemblages will likely change in next years. In conclusion, our results showed that fire significantly affected plant diversity in the two savanna physiognomies (cerrado típico and ralo), but also indicated that community reassembling is fast, with different dynamics between Cerrado physiognomies.
We used space-for-time substitution to obtain a directed successional sequence for subalpine meadow vegetation in the Swiss National Park. Since human impacts (e.g., domestic animal grazing) ceased in 1914, the successional processes documented are assumed to be autogenic in nature. The data consist of 59 permanent plots spanning almost 90 years, and include many spatial replications. An initial inspection of the individual time series revealed the existence of a variety of response patterns, which are described in the literature as representing different successional types. However, a closer inspection suggested that many of these series can be superimposed, as they are part of a much longer deterministic series. Linking the individual time series proved to be challenging. A heuristic approach produced results that differed depending on initial starting conditions. We therefore derived a deterministic algorithm to produce a unique solution. The resulting sequence largely confirmed the heuristic interpretation, suggesting a trend from early successional (post-grazing) grassland to pine invasion spanning about 400 years. This timespan is valid only for the climatic conditions near the treeline, and for plant species specific to the study site. Our results suggest that the various species temporal response models described in the literature may be artifactual, representing portions of underlying Gaussian responses. The data also indicate that species assemblages may persist for several decades with only minor fluctuations, only to change suddenly for no apparent reason.
We studied the early vegetation dynamics in former croplands (sunflower and cereal fields) sown with a low-diversity seed mixture (composed of 2 native grass species) in Egyek-Pusztakócs, Hortobágy National Park, East-Hungary. The percentage cover of vascular plants was recorded in 4 permanent plots per field on 7 restored fields between 2006 and 2009. Ten aboveground biomass samples per field were also collected in June in each year. We addressed two questions: (i) How do seed sowing and annual mowing affect the species richness, biomass and cover of weeds? (ii) How fast does the cover of sown grasses develop after seed sowing? Weedy species were characteristic in the first year after sowing. In the second and third year their cover and species richness decreased. From the second year onwards the cover of perennial grasses increased. Spontaneously immigrating species characteristic to the reference grasslands were also detected with low cover scores. Short-lived weeds were suppressed as their cover and biomass significantly decreased during the study. The amount of litter and sown grass biomass increased progressively. However, perennial weed cover, especially the cover of Cirsium arvense increased substantially. Our results suggest that grassland vegetation can be recovered by sowing low diversity mixtures followed up by yearly mowing. Suppression of perennial weed cover needs more frequent mowing (multiple times a year) or grazing.
A cell-grid method was used to survey seasonal changes in four herbaceous communities of a wooded pasture. Permanent plots consisted in 1-m2 grids subdivided into 100 cells of 1 dm2. In each cell of each plot and every month from May to September, we estimated dominance, and grazing occurrence of all species. Plant communities included an eutrophic grazed meadow, a temporary refused meadow, an underwood herb community and an oligotrophic lawn. Our results showed that seasonal changes in species composition were very strong and scale-dependent. Changes at plot scale were mainly driven by a seasonal shift. Changes at cell scale suggested high small-scale dynamics of species. Despite high changes at cell scale, the structure of the community did not change and local species richness did not show any trend. We found no correlation between the turnover at cell scale and cattle activity. We conclude that dynamics and internal species turnover of the community at fine scale and short time seem more driven by internal characteristics of the community than by disturbances induced by cattle. Furthermore, at seasonal scale, plant communities may be stable in their structure despite fluctuations in their texture.
Ad hoc decisions during fieldwork reduce the accuracy and reliability of vegetation maps. A method is proposed to objectify vegetation (thematic aspects) mapping (spatial aspects) for monitoring (temporal aspects). The most accurate and reliable description of the vegetation is a list of all plant species found within a plot. Therefore, the proposed method is an interpolation of a spatially representative sample of permanent plots combined with aerial photo interpretation. The method is objective because surveyors do not have to make decisions during fieldwork based on their personal judgement. Moreover, it is flexible, because the classification and interpolation methods can be adapted to specific views or needs depending on the aim of a study. The method was applied to an area in the north of the Netherlands in 1998. A sampling design with a density of 1 plot/ha was used, and interpolated with a perpendicular bisector. In 2002, the number of plots/ha was doubled. The influence of sample density on the mapping results was studied because it is an important decision to be made before fieldwork. Two plots/ha seem to be sufficient in order to obtain reliable information on patterns of plant species composition and vegetation types of the area, and their change over time. However, in patches where vegetation varies on a very small spatial scale this plot density was insufficient.
In recent decades, nitrogenous compounds, arising from various anthropogenic sources, have become significant components of precipitation and have been shown to have some profound effects on both species richness and dominance of some plant communities. To examine how nitrogen addition can affect the species composition of Central Great Plains prairie plant communities, we applied N fertilizer at five levels for each of five consecutive years at two sites in south-central Kansas with similar rainfall and temperature regimes. One site was a tallgrass prairie and the other was a sand prairie community. The treatments consisted of N additions at the rate of 0, 5, 10, 20, and 40 g N/m2 in the form of solid urea. Within permanent plots, we monitored annually species richness and evenness, and percentage cover by cool-season graminoids, N-fixing species, and annual/biennial species. All these measures varied considerably among years at both sites, but N treatment effects were evident at the sand prairie site only. At the sand prairie, in general species richness and percentage cover by legumes declined with N addition. Moreover, species were excluded non-randomly from N addition plots, with several species apparently particularly intolerant of N addition. The results reinforce a relationship, observed in Europe and the northern Great Plains, between N addition and plant biodiversity decline in grassland communities, and may point to a serious conservation concern for rare species under a chronic regime of N-enhanced precipitation.
Plant biomass production, soil chemical and microbial parameters, microbial processes of C and N cycle and gases emissions were studied in soils at two types of grasslands (wet meadows). Both sites are situated in the Czech Republic: (1) a nutrient poor sedge meadow on organic soil (Z) and (2) a mesotrophic sedge-sweet grass meadow on mineral soil (H). Eutrophication was simulated by the application of NPK fertilizer to selected permanent plots in 2006 and 2007 in amounts of 9 kg N + 4 kg P ha −1 year −1 (low dose) and 45 kg N + 20 kg P ha − year − (high dose). After two years of fertilizer application, we observed an increase in net aboveground plant production (about 9–12 kg ha −1 year − ) connected with an increase in shoot:root ratio in fertilized plots of both sites, with more pronounced changes in oligotrophic sedge meadow. Total CO 2 efflux from the ecosystem measured in situ was significantly higher at fertilized plots as well as increase in total soil respiration in case of sedge meadow, but we found no significant effect of fertilization on CO 2 efflux from the system at mesotrophic site. Surprisingly, other parameters, like soil microbial biomass C and N content, the rates of respiration, denitrification, nitrification, nitrogen mineralization and nitrogen assimilation were not affected by fertilization. In conclusion, an interesting finding is that despite non significant impact on aboveground component there were significant responses in belowground part which suggest that belowground processes may be suitable early warning signals. Peaty oligotrophic soil seems to be more sensitive to nutrient addition than mineral soil. However, final effect of fertilization on ecosystem C balance stays unknown and longer study is necessary to draw explicit conclusion.
Grassland ecosystems in the Carpathian Basin may be particularly vulnerable to current and predicted changes in precipitation, and ecosystem responses to potential effects of water are not well understood. To examine how water addition can affect the species composition and structure, and CO 2 -flux of a Central European natural steppe plant community, grassland monoliths were irrigated for three consecutive years at Gödöllő, from 2002 through 2004. The loess grassland studied by ex situ is a characteristic plant association of Hungary and similar vegetation can be found in other temperate regions. The treatment consisted of spray irrigation during night-time only in the growing season as well as aboveground biomass removal twice per year. Interannual and intraannual dynamics of species richness, Shannon Diversity, percentage cover, and different functional groups (monocots/dicots; plant life forms; social behaviour types; C 4 /C 3 plants), and Net Ecosystem CO 2 Exchange in treated and untreated permanent plots, were studied simultaneously. To measure NEE and water vapour at stand level a self-developed, portable, non-destructive open chamber system (d=60cm) was used. The majority of the examined parameters varied considerably among years at both irrigated and control, but concerning carbon fluxes water addition effects were evident in dry periods only. At the treated plots, in general species richness, Shannon Diversity, the number of plant life forms and social behaviour types, the ratio of dicots and C 4 plants declined with addition of water. Our study proved that decline in species richness and Shannon diversity is not necessarily followed by the reduction of stand physiological (synphysiological) processes.
We conducted a field study to determine whether species composition and environmental relationships were scale dependent among the strata of a temperate deciduous forest. We compiled tree basal area, woody understory density, field layer cover and three environmental variables from 378 permanent plots in ten 7–386 ha sites in the 294,455 ha Shawnee National Forest, southern Illinois, USA. The effect of changing sample foci (area of inference) without changing the extent (geographic space) on the relationship between species composition and environmental factors was quantified using Non-metric Multidimensional Scaling (NMDS) and Procrustes analysis separately for each stratum. Species composition-environment relationships showed a separation of sites based on their geographic location and bedrock. Species data collected from these heterogeneous and hierarchically structured habitats across scales exhibited varying degree of specialization depending on the habitat. NMDS showed that the species composition-environmental relationships were dependent on the sample foci and stratum investigated with the number of environmental variables related to the vegetation ranging from two-three at the landscape scale and zero-three at the site scale. Procrustes analysis showed that the species composition of individual sites was most frequently related to the physiographic division that the site occurred in rather than the landscape (58%, 18 of 30 comparisons). This relationship was less frequent in the tree stratum than the lower strata. The results of this study are consistent with the hierarchical continuum concept in that the recognition of species-environment relationships depends upon the scale of analysis. From a management perspective it is important to consider these scale relationships to manage the physiographic divisions and the sites within them depending on conservation issues and the priorities of local land managers.
