Gábor Ónodi

Grazing Effects on Vegetation Composition and on the Spread of Fire on Open Sand Grasslands

We studied the effects of sheep [Ovis aries (L.)] and rabbit [Oryctolagus cuniculus (L.)] grazing on the spread of induced fire on an open sand grassland community in the Hungarian Plain. Patches of open sand grassland were grazed by sheep in April and by sheep and rabbit in May of 2003. Half of each patch was burned in July. Canopy cover of the litter and vascular plant species, species number, plant height, burnt area, and the speed of fire-spread were estimated in 1 × 1 m quadrates. The burnt area was significantly smaller with late sheep grazing, while the speed of fire-spread decreased significantly due to rabbit grazing compared to that of the control. Plant height was significantly decreased by early and late sheep grazing, while rabbit grazing resulted in significantly lower canopy cover values of vascular plants compared to the control. Early sheep grazing resulted in overcompensation of the canopy cover of vascular plants. Species number was not affected by the grazing treatments. This 1-year experiment demonstrated that moderate grazing has no short-term effects on the species diversity of the semi-arid open sand grassland. Furthermore, late spring grazing decreased the spread of fire on the grassland portions of the community; thus it may inhibit the burning of large areas of the semi-arid forest-steppe.

Rabbit Grazing as the Major Source of Intercanopy Heterogeneity in a Juniper Shrubland

Semi-arid shrublands are prone to wildfires especially if there are no firebreaks separating shrubs to reduce the spread of fire. The accumulated grass vegetation is a potential combustion source which also contributes to the spread of wildfires. We present a field study conducted in a semi-arid community, where the fire risk is high and the grass density surrounding the Juniperus communis shrubs is highly varied. We tested simultaneously the relative significance of the main factors contributing to plant cover around shrubs: allelopathy, shading, and herbivory. Grass cover was compared around junipers which were either currently occupied by rabbits or were unoccupied. We visually estimated plant cover and measured the Normalized Difference Vegetation Index. Both sides of the shrubs were sampled at various distances to see if allelopathy and/or shading reduced grass density. The allelopathy effect was also tested by lettuce bioassay. We found that the vegetation was open only around occupied junipers and that the effect of occupation decreased by distance from the shrub. The spatial distribution of plant cover showed that, although we could detect significant allelopathic effect of juniper by bioassay, neither the shading nor the allelopathy were as important as herbivory. We also compared the rabbit diet composition with plant availability for each occupied patch and found that the consumed species were under represented due to selective grazing. Our findings indicate that local herbivore activity could affect the spatial heterogeneity of combustible material, therefore, moderate rabbit grazing can be considered as a management tool to reduce the spread of fire.

Comparing the accuracy of three non-destructive methods in estimating aboveground plant biomass

Aboveground plant biomass is one of the most important features of ecosystems, and it is widely used in ecosystem research. Non-destructive biomass estimation methods provide an important toolkit, because the destructive harvesting method is in many cases not feasible. However, only few studies have compared the accuracy of these methods in grassland communities to date. We studied the accuracy of three widely used methods for estimation of aboveground biomass: the visual cover estimation method, the point intercept method, and field spectroscopy. We applied them in three independent series of field samplings in semi-arid sand grasslands in Central Hungary. For each sampling method, we applied linear regression to assess the strength of the relationship between biomass proxies and actual aboveground biomass, and used coefficient of determination to evaluate accuracy. We found no evidence that the visual cover estimation, which is generally considered as a subjective method, was less accurate than point intercept method or field spectroscopy in estimating biomass. Based on our three datasets, we found that accuracy was lower for the point intercept method compared to the other two methods, while field spectroscopy and visual cover estimation were similar to each other in the semi-arid sand grassland community. We conclude that visual cover estimation can be as accurate for estimating aboveground biomass as other approaches, thus the choice amongst the methods should be based on additional pros and cons associated with each of the method and related to the specific research objective.

Synergistic effects of the components of global change: Increased vegetation dynamics in open, forest-steppe grasslands driven by wildfires and year-to-year precipitation differences

Climate change and land use change are two major elements of human-induced global environmental change. In temperate grasslands and woodlands, increasing frequency of extreme weather events like droughts and increasing severity of wildfires has altered the structure and dynamics of vegetation. In this paper, we studied the impact of wildfires and the year-to-year differences in precipitation on species composition changes in semi-arid grasslands of a forest-steppe complex ecosystem which has been partially disturbed by wildfires. Particularly, we investigated both how long-term compositional dissimilarity changes and species richness are affected by year-to-year precipitation differences on burnt and unburnt areas. Study sites were located in central Hungary, in protected areas characterized by partially-burnt, juniper-poplar forest-steppe complexes of high biodiversity. Data were used from two long-term monitoring sites in the Kiskunság National Park, both characterized by the same habitat complex. We investigated the variation in species composition as a function of time using distance decay methodology. In each sampling area, compositional dissimilarity increased with the time elapsed between the sampling events, and species richness differences increased with increasing precipitation differences between consecutive years. We found that both the long-term compositional dissimilarity, and the year-to-year changes in species richness were higher in the burnt areas than in the unburnt ones. The long-term compositional dissimilarities were mostly caused by perennial species, while the year-to-year changes of species richness were driven by annual and biennial species. As the effect of the year-to-year variation in precipitation was more pronounced in the burnt areas, we conclude that canopy removal by wildfires and extreme inter-annual variability of precipitation, two components of global environmental change, act in a synergistic way. They enhance the effect of one another, resulting in greater long-term and year-to-year changes in the composition of grasslands.

Reduction in primary production followed by rapid recovery of plant biomass in response to repeated mid-season droughts in a semiarid shrubland

The frequency and severity of extreme weather events, including droughts, are expected to increase due to the climate change. Climate manipulation field experiments are widely used tools to study the response of key parameters like primary production to the treatments. Our study aimed to detect the effect of drought on the aboveground biomass and primary production both during the treatments as well as during the whole growing seasons in semiarid vegetation. We estimated aboveground green biomass of vascular plants in a Pannonian sand forest-steppe ecosystem in Hungary. We applied non-destructive field remote sensing method in control and drought treatments. Drought treatment was carried out by precipitation exclusion in May and June, and was repeated in each year from 2002. We measured NDVI before the drought treatment, right after the treatment, and at the end of the summer in 2011 and 2013. We found that the yearly biomass peaks, measured in control plots after the treatment periods, were decreased or absent in drought treatment plots, and consequently, the aboveground net primary production was smaller than in the control plots. At the same time, we did not find general drought effects on all biomass data. The studied ecosystem proved resilient, as the biomass in the drought-treated plots recovered by the next drought treatment. We conclude that the effect of drought treatment can be overestimated with only one measurement at the time of the peak biomass, while multiple within-year measurements better describe the response of biomass.

Within-generation and transgenerational plasticity in growth and regeneration of a subordinate annual grass in a rainfall experiment

Precipitation changes may induce shifts in plant species or life form dominance in ecosystems, making some previously subordinate species abundant. The plasticity of certain plant functional traits of these expanding subordinate species may be one possible mechanism behind their success. In this study, we tested if the subordinate winter annual grass Secale sylvestre shows plasticity in growth and reproduction in response to altered environment associated with field-scale rainfall manipulations (severe drought, moderate drought, and watering) in a semiarid grassland, and whether the maternal environment influences offspring germination or growth in a subsequent pot experiment. Compared to control plots, S. sylvestre plants grew 38% taller, and produced 32% more seeds in severe drought plots, while plants in watered plots were 17% shorter, and had 22% less seeds. Seed mass was greatest in severe drought plots. Plants growing in drought plots had offspring with enhanced juvenile shoot growth compared to the progeny whose mother plants grew in watered plots. These responses are most likely explained by the decreased cover of previously dominant perennial grasses in severe drought plots, which resulted in wetter soil compared to control and watered plots during the peak growth of S. sylvestre. We conclude that the plasticity of this subordinate annual species in response to changing environment may help to gain dominance with recurring droughts that suppress perennial grasses. Our results highlight that exploring both within-generation and transgenerational plasticity of subordinate species may lead to a better prediction of changes in plant species dominance under climate change.

The potential of common ragweed for further spread: invasibility of different habitats and the role of disturbances and propagule pressure

The infilling of existing suitable habitats within a landscape after establishment is of critical importance for the final outcome of a plant invasion, yet it is an often overlooked process. Common ragweed, Ambrosia artemisiifolia, is an invasive annual species in Europe causing serious problems due to its highly allergenic pollen and as an agricultural weed. Recent studies have modelled the broad-scale distribution of the species and assessed future invasion risk, but for predicting the expected outcome of ragweed invasion we also need a mechanistic understanding of its local invasion success. We conducted a field experiment to investigate the invasibility of eight common non-arable habitat types and the role of soil disturbance in central Hungary, in the hot spot of ragweed invasion in Europe. Seed addition alone resulted in negligible amount of ragweed biomass, except for sites where disturbance was part of the present management. Soil disturbance alone resulted in ragweed at those few sites where ragweed seeds were present in the seed bank, related to farming in recent decades. When disturbance and seed addition were combined, ragweed emerged in all habitat types and reached high biomass in all habitat types except for closed forests. As our experiment showed that most habitat types have high invasibility when disturbed, we conclude that ragweed has a high potential for further spread, even in this heavily infested region. Management should focus on preventing seed dispersal and eradicating establishing populations where ragweed is still absent, while reducing soil disturbance may be needed to avoid ragweed emergence in infested sites. This latter may require a reconsideration of land-use practices in infested regions.