Heterogeneity decreases as time since fire increases in a South American grassland

Questions: Disturbances change the fundamental properties of grasslands on different spatio-temporal scales. Uruguay is part of the Río de la Plata grasslands, and 60% is occupied by native grasslands dominated by perennial species. In plant communities dominated by tall tussock grasses, patchy and asynchronous field burns are a traditional practice among ranchers. We asked: how do the structural characteristics of vegetation vary in patches with different time since the last fire? Location: Grassland of the Eastern Hills, Uruguay. Methods: We selected 18 grazed sites in order to obtain a spatial chronosequence with four age categories since the last fire: 6, 18, 30, and more than 60 months before sampling. Plant composition, species richness, coverage of each species, bare soil, and standing dead biomass were determined in plots of 25 m2. We used nonmetric multidimensional scaling (NMDS) and the multiresponse permutation procedure (MRPP) to determine differences in community composition, and the ANOVA or the Kruskal–Wallis test to compare structural variables between patches of different burning ages. Results: Patches of different burning age had different species compositions. Species richness, Shannon diversity index, evenness, and bare soil decreased, whereas plant coverage, standing dead biomass, and vegetation strata increased as time since the last fire increased. Conclusions: Our study confirmed occasional and localized field-burns as major driver of vegetation change and structural diversity in a grazed native grassland dominated by a tall tussock grass. On a larger scale, we observed the coexistence of patches in different successional stages and differences in species composition between patches belonging to early stages. These grasslands require asynchronous burning of patches to generate structural changes that maximize both the spatial and temporal heterogeneity.Agencia Nacional de Investigación e InnovaciónComisión Sectorial de Investigación CientíficaInstituto Nacional de Investigación AgropecuariaComisión Académica de PosgradoInter-American Institute for Global Change Researc

Plant quality and primary productivity modulate plant biomass responses to the joint effects of grazing and fertilization in a mesic grassland

Questions: Human activities are increasing the density of domestic grazers and global nutrient loads, modifying the main determinants of vegetation community dynamics. Grazing (top-down control) and nutrient availability (bottom-up control) may interactively modify plant biomass, which is particularly important in grasslands devoted to livestock production. Here, we aim to understand the interactive effects of grazing and fertilization on grassland plant biomass. We hypothesized that the joint effects of nutrient addition and domestic grazing on above-ground plant biomass are not additive, but they modify each other through changes in ground-level light, leaf nutritional quality, above-ground net primary productivity (ANPP), and below-ground plant allocation. Location: Flooding Pampa (Buenos Aires, Argentina). Methods: We carried out a factorial experiment of grazing exclusion and fertilization with nitrogen, phosphorus, and potassium + micronutrients during ​several years in a mesic grassland devoted to livestock production. Results: After four years, grazing reduced live above-ground plant biomass by 52%, and when combined with fertilization this reduction was 70%. Nutrient addition in the grazed grassland increased ANPP and leaf nutrient concentration. These changes in turn intensified grazing pressure and cattle’s plant consumption. By contrast, fertilization did not produce any significant effect on plant biomass or ANPP inside the exclosures, where ground-level light was low. A structural equation model revealed that the increase in ANPP fostered above-ground and reduced below-ground plant biomass. Conclusions: This is the first study conducted in the Pampas grasslands that evaluated the effect of cattle grazing and fertilization on plant communities under field conditions over several years. Grazing and nutrient addition synergistically controlled grassland plant biomass, as the reduction in above-ground biomass by cattle consumption was greater in fertilized plots. Our results provided empirical evidence that leaf nitrogen and ANPP modulated plant biomass dynamics in grasslands devoted to livestock production in the context of increased nutrient loads in terrestrial ecosystems.Fil: Campana, María Sofía. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; ArgentinaFil: Yahdjian, María Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentin