Disturbance as a factor in breaking dormancy and enhancing invasiveness of African grasses in a Neotropical Savanna

The Cerrado is threatened by wildfires and invasive species. We aimed to evaluate in laboratory conditions whether temperature fluctuation at the soil surface, resulting from the absence of vegetation due to fire, can affect the germination of Urochloa decumbens and U. brizantha, two invasive African grasses. Seeds of both species were submitted to simulations: 1) temperature during fire at 1cm belowground (F); 2) temperature fluctuation at 1cm belowground without vegetation cover for a month (TF); 3) (F) + (TF); 4) control at 25ºC. After treatments, seeds were put to germinate at 25ºC for 40 days. We had four replicates per treatment and three temporal replicates. We compared germination percentage and the mean germination time among treatments using ANOVA. The treatments TF and F+TF had the highest germination values for both species. The results showed that fire per se could not stimulate seed germination, however, they suggest that a disturbance that produces a pattern of temperature fluctuation is able to break dormancy and enhance seed germination and, consequently, increase the invasiveness of the study species. Vegetation gaps resulting from disturbance may become new sites of invasion. This information is important for making management decisions regarding the control of these species.We thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/DGU 227/2010) for financial support. M.J.B. acknowledges the support from the programme FORESTERRA ERA-Net (Medwildfirelab, PCIN-2013-140-C04-03) and PROMETEO II (Desestres es/2014/038)

The effects of an exotic palm on a native palm during the first demographic stages: contributions to ecological management

Biological invasions have negative impacts on native biota and consequently on biodiversity. In patches of Atlantic Forest in Brazil, the Australian palm tree Archontophoenix cunninghamiana has become invasive, whereas the endemic palm Euterpe edulis is threatened with extinction. The two species occupy parts of the same functional niche within the forests, which raises a question: Does the invasive species interfere with the recruitment of the native one when the two co-occur? We compared the initial demographic stages of these palms, and assumed that A. cunninghamiana would present better performance (higher rates of germination and seed viability) and would feature allelopathic properties that could influence the recruitment of the native species. We investigated indirect and direct allelopathic interference, respectively, by watering E. edulis seeds with aqueous leachate solutions of A. cunninghamiana fruits and leaves and by conducting combined germination experiments. The leachate solutions neither inhibited germination nor affected the size of E. edulis seedlings. In the direct interference experiments, depulped A. cunninghamiana seeds had higher viability and germination rates than did E. edulis seeds. In E. edulis, exposure to A. cunninghamiana seeds did not affect germination nor seedling development but slightly decreased germination speed. In conclusion, A. cunninghamiana presented no significant allelopathic impediment to E. edulis establishment. However, because A. cunninghamiana seeds are usually depulped when dispersed by birds, the potential of the species to establish itself in the community surpasses that of E. edulis. We propose management strategies to enhance E. edulis performance and to restrict the spread of A. cunninghamiana

The Year 2017: Megafires and Management in the Cerrado

The year 2017 was a megafire year, when huge areas burned on different continents. In Brazil, a great extension of the Cerrado burned, raising once more the discussion about the “zero-fire” policy. Indeed, most protected areas of the Cerrado adopted a policy of fire exclusion and prevention, leading to periodic megafire events. Last year, 78% of the Chapada dos Veadeiros National Park burned at the end of the dry season, attracting media attention. Furthermore, 85% of the Reserva Natural Serra do Tombador burned as a result of a large accumulation of fuel caused by the zero-fire policy. In 2014, some protected areas started to implement the Integrate Fire Management (IFM) strategy. During 2017, in contrast to other protected areas, the Estação Ecológica Serra Geral do Tocantins experienced no megafire events, suggesting that a few years of IFM implementation led to changes in its fire regime. Therefore, we intended here to compare the total burned area and number of fire scars between the protected areas where IFM was implemented and those where fire exclusion is the adopted policy. The use of fire as a management tool aimed at wildfire prevention and biodiversity preservation should be reconsidered by local managers and environmental authorities for most Cerrado protected areas, especially those where open savanna physiognomies prevail. Changing the paradigm is a hard task, but last year’s events showed the zero-fire policy would bring more damage than benefits to Cerrado protected areas

Impact of the conversion of Brazilian woodland savanna (cerradão) to pasture and Eucalyptus plantations on soil nitrogen mineralization

The Brazilian savanna (Cerrado) has been extensively converted to croplands, pastures and forestry plantations, and the deforestation frontier continues expanding. Land conversion may cause critical changes in soil functioning, yet very little is still known about the impact of Cerrado conversion on nutrient cycling and soil fertility. Here, we addressed this knowledge gap by investigating the effects of the woodland cerrado (cerradão) conversion into pastures and Eucalyptus plantations on nitrogen availability and mineralization potential, considering a wide range of spatial and temporal variability due to soil depth, site conditions, and seasonal variation. For three sites in São Paulo state and each of the target land cover types, we assessed the total N and inorganic N (NH4-N and NO3-N) pools, potentially mineralizable nitrogen (PMN) and soil urease activity in the first 2 m of the soil profile. Cerrado conversion to either pastures or Eucalyptus plantations significantly reduced NH4-N, while NO3-N showed similar values in Cerrado and Eucalyptus and lower values in pastures. We found a consistent pattern of lower N mineralization in the uppermost soil layers associated to Cerrado conversion, with decreases in PMN rate and urease activity. The soil below 30 cm depth showed no relevant changes. Considering the first 30 cm of the soil profile, the reduction in the stocks of inorganic N (NH4-N + NO3-N) ranged from ~14% for the conversion to Eucalyptus to ∽20% for the conversion to pasture. The impact of land conversion on N cycling surpassed the influence of the spatial (between-site) and seasonal variation. Overall, the results indicate a decline in available N and overall soil fertility due to Cerrado conversion, which could further increase N limitation in the Cerrado region, increase fertilization needs for future exploitation, and compromise the recovery of Cerrado in case of land abandonment or restoration.The research was funded by Conselho Nacional de Desenvolvimento Científco e Tecnológico – CNPq (Ciência Sem Fronteiras Project – Grant numbers 400758/2014-8; 313732/2014; 0305253/2015-8; and 208405/2017-8). GSB was supported by a doctorate scholarship from CAPES. SB and RLP acknowledge financial support from the project DRYEX2 (CGL2017-89804-R), funded by the Spanish Ministry of Science, Innovation and Universities

Labile soil organic carbon loss in response to land conversion in the Brazilian woodland savanna (cerradão)

Conversion of native vegetation to agriculture may change the carbon cycle by reducing carbon soil storage capacity and increasing CO2 emissions. We aimed to comparatively assess the impact of land use change on labile pools and dynamics of soil organic carbon (SOC) in two land uses (Urochloa pastures and Eucalyptus forestry) relative to the native reference ecosystem (Brazilian woodland savanna, the cerradão), as a function of soil depth and season. For three replicated study sites, each of them including a control area of the native vegetation (Cerrado) and two land uses (Pasture, Eucalyptus), we sampled soil from 0 to 2 m depth in both dry and wet seasons. We quantified dissolved organic carbon (DOC) and microbial biomass carbon (MBC), estimated the microbial quotient (MBC/SOC) and DOC/SOC ratio, and evaluated C dynamics by assessing soil basal respiration and the metabolic quotient (qCO2). Compared with Cerrado, DOC, MBC and MBC/SOC decreased in both Pasture and Eucalyptus. Differences between land uses vanished below 30 cm soil depth. Seasonality affected most analyzed variables, with lower values for DOC, DOC/SOC and qCO2, and slightly higher values for MBC and MBC/SOC in the wet season. In the dry season qCO2 increased in the Eucalyptus topsoil as compared to Cerrado, suggesting higher stress in the microbial community and/or lower decomposition efficiency in Eucalyptus. Overall, our results show that cerradão conversion to pastures and Eucalyptus plantations negatively affects labile pools and dynamics of SOC, with the effects surpassing a strong spatial and seasonal variability in the soil response to land conversion.The Conselho Nacional de Desenvolvimento Científco e Tecnológico (CNPq) funded this research through a CNPq-Ciência Sem Fronteiras Project (Proc. 400758/2014-8) and fellowships granted to G.S.B. (Proc. 208405/2017-8) and V.R.P. (Proc. 305253/2015-8). The Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) granted a doctorate scholarship to G.S.B. SB and RLP also acknowledge financial support from the Project DRYEX2 (CGL2017-89804-R), funded by the Spanish Ministry of Economy and Competitiveness

How can an invasive grass affect fire behavior in a tropical savanna? A community and individual plant level approach

Some invasive grasses have been reported to change fire behavior in invaded plant communities. Urochloa brizantha is an aggressive invasive grass in the Brazilian Cerrado, an ecosystem where fire is a common disturbance. We investigated the effects of U. brizantha on fire behavior in an open Cerrado physiognomy in Central Brazil. Using experimental burnings we compared fire behavior at both the community and the individual plant level in invaded (UJ) and non-invaded (NJ) areas burned in July. We also assessed the effect of fire season in invaded areas by comparing July (UJ) and October (UO) burnings. We evaluated the following variables: fuel load, fuel moisture, combustion efficiency, maximum fire temperature, flame height, and fire intensity. Additionally, we evaluated the temperatures reached under invasive and native grass tussocks in both seasons. Fuel load, combustion efficiency, and fire intensity were higher in NJ than in UJ, whilst flame height showed the opposite trend. Fuel amount and fire intensity were higher in October than in July. At the individual plant level, U. brizantha moisture was higher than that of native species, however, temperatures reaching ≥600 °C at ground level were more frequent under U. brizantha tussocks than under native grasses. At the community level, the invasive grass modified fire behavior towards lower intensity, lower burning efficiency, and higher flame height. These results provide essential information for the planning of prescribed burnings in invaded Cerrado areas.We thank the Fundação Grupo Boticário de Proteção à Natureza (Termo de Parceria No 0106_2011_PR), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/DGU 227/2010), and the Ministry of Education (PHB2009-0073-PC) and of Science and Innovation (FEEDBACK Project; CGL2011-30515-C02-01) of the Spanish Goverment for financial support

Elevated compositional change in plant assemblages linked to invasion

Alien species are widely linked to biodiversity change, but the extent to which they are associated with the reshaping of ecological communities is not well understood. One possible mechanism is that assemblages where alien species are found exhibit elevated temporal turnover. To test this, we identified assemblages of vascular plants in the BioTIME database for those assemblages in which alien species are either present or absent and used the Jaccard measure to compute compositional dissimilarity between consecutive censuses. We found that, although alien species are typically rare in invaded assemblages, their presence is associated with an increase in the average rate of compositional change. These differences in compositional change between invaded and uninvaded assemblages are not linked to differences in species richness but rather to species replacement (turnover). Rapid compositional restructuring of assemblages is a major contributor to biodiversity change, and as such, our results suggest a role for alien species in bringing this about