Heloisa S. Miranda

Carbon Dioxide Uptake by an Undisturbed Tropical Rain Forest in Southwest Amazonia, 1992 to 1993

Measurements of carbon dioxide flux over undisturbed tropical rain forest in Brazil for 55 days in the wet and dry seasons of 1992 to 1993 show that this ecosystem is a net absorber of carbon dioxide. Photosynthetic gains of carbon dioxide exceeded respiratory losses irrespective of the season. These gains cannot be attributed to measurement error, nor to loss of carbon dioxide by drainage of cold air at night. A process-based model, fitted to the data, enabled estimation of the carbon absorbed by the ecosystem over the year as 8.5 ± 2.0 moles per square meter per year.

REMOTE MEASUREMENT OF ENERGY AND CARBON FLUX FROM WILDFIRES IN BRAZIL

Temperature, intensity, spread, and dimensions of fires burning in tropical savanna and slashed tropical forest in central Brazil were measured for the first time by remote sensing with an infrared imaging spectrometer that was designed to accommodate the high radiances of wildland fires. Furthermore, the first in situ airborne measurements of sensible heat and carbon fluxes in fire plumes were combined with remote measurements of flame properties to provide consistent remote-sensing-based estimators of these fluxes. These estimators provide a means to determine rates of fuel consumption and carbon emission to the atmosphere by wildland fires as required for assessments of fire impacts on regional air pollution or global emissions of greenhouse gases. Observed fires developed complex fire-line geometry and thermal structure, even as average whole-fire temperatures varied little. Flame temperatures sometimes exceeded 1600 K along the leading edge of actively spreading fire lines, yet >90% of the radiant energy from observed fires was associated with temperatures of 830–1440 K. Fire in a partially slashed forest encompassed a high-intensity flaming front and a trailing reach of residual combustion extending 400 m. Fire fronts in tropical savanna typically formed with little depth and a high proportion of their radiant flux density associated with high temperatures due to low levels of residual combustion. Measured fires had such low and variable radiance compared with that of a blackbody of comparable temperature as to preclude the use of fire radiance at a single wavelength as a measure of fire intensity or temperature. One-half of the radiant flux density from a measured savanna fire was associated with values of a combined emissivity–fractional-area parameter <0.091 m2/m2; for a slash fire this fraction was associated with values <0.37 m2/m2. Observations reported here show wildland fires to be so complex and dynamic as to require frequent high-resolution measurements over their course and duration in order to specify their effects in the environment; an understanding of global fire impacts may require such measurements over a large sample of individual fires.

On the delineation of tropical vegetation types with an emphasis on forest/savanna transitions

Background: There is no generally agreed classification scheme for the many different vegetation formation types occurring in the tropics. This hinders cross-continental comparisons and causes confusion as words such as ‘forest’ and ‘savanna’ have different meanings to different people. Tropical vegetation formations are therefore usually imprecisely and/or ambiguously defined in modelling, remote sensing and ecological studies. Aims: To integrate observed variations in tropical vegetation structure and floristic composition into a single classification scheme. Methods: Using structural and floristic measurements made on three continents, discrete tropical vegetation groupings were defined on the basis of overstorey and understorey structure and species compositions by using clustering techniques. Results: Twelve structural groupings were identified based on height and canopy cover of the dominant upper stratum and the extent of lower-strata woody shrub cover and grass cover. Structural classifications did not, however, always agree with those based on floristic composition, especially for plots located in the forest–savanna transition zone. This duality is incorporated into a new tropical vegetation classification scheme. Conclusions: Both floristics and stand structure are important criteria for the meaningful delineation of tropical vegetation formations, especially in the forest/savanna transition zone. A new tropical vegetation classification scheme incorporating this information has been developed.

Fires in the cerrado, the Brazilian savanna

The cerrado is the largest area savanna formation in South America, and originally covered approximately 25% of the Brazilian territory. The savanna forms of cerrado are characterized by a ground layer of grasses, small palms, shrubs, and trees. During the wet season there is a high production of biomass that dries as the dry season progresses, favoring the occurrence of fire. For thousands of years, natural fires, during the wet season, and anthropogenic fires, during the dry season, coexisted in the cerrado region. suggesting that fire, together with the seasonality of rainfall and the poor nutrient soils, is one of the determinants of the cerrado vegetation form. Cerrado fires can be characterized as surface fires, consuming basically the fine fuel of the herbaceous layer. Most species of the herbaceous layer are highly resistant to fire and resprout a few days after a fire. For open form s of cerrado, nine months after a fire, dead biomass (grasses+litter) represents 65% of the total, suggesting that it is possible to have a sustained fire, even during the short dry spells of the next rainy season. Th e woody vegetation presents several adaptive characteristics, such as thermal insulation o f live tissues and the presence of underground organs that store water and nutrients. However, fire frequency and time of fire affects the structure and species composition of cerrado’s woody plant communities. In this chapter, we present a review of cerrado fire ecology, with emphasis on fire behavior, changes in the structure and composition of the vegetation, and its effects on water use and carbon flux.

SEASONAL AND DEPTH VARIATION OF SOIL MOISTURE IN A BURNED OPEN SAVANNA (CAMPO SUJO) IN CENTRAL BRAZIL

The soil water regimes of two areas of open savanna (campo sujo) near Brasilia, Brazil, were monitored between August 1999 and November 2000. Each area was subjected to a different fire regime. Soil water content was measured to a depth of 3.6 m, using a neutron probe. The profile storage at the end of the 1999 and 2000 dry seasons was very similar despite a difference in dry season duration and large differences in rainfall in the preceding wet seasons, indicating that the vegetation is conservative in its water use. In the last two months of the dry season, the water content of the upper 0.6 m of the soil profile did not decrease further, suggesting that the vegetation had used all of the available water in this layer. The seasonal variation in soil water storage to a depth of 3.6 m was 403 mm, 65% of which occurred below 1 m. The wet and dry season evaporation rates were estimated to be 2.4 mm/d and 1.6 mm/d, respectively, but for a month after fire, before regrowth started, the evaporation rate was less than 0.5 mm/d.

Effects of fire regimes on herbaceous biomass and nutrient dynamics in the Brazilian savanna

This study explores the long-term effects of fire treatments on biomass and nutrient pools in an open savanna from Central Brazil. Treatments included early, middle and late dry season burns every 2 years, a middle dry season burn every 4 years, and protection from fire on five 4-ha plots. We quantified aboveground biomass of graminoids and forbs/sub-shurbs, and their nutrient concentrations and stocks in both dry and wet seasons, and below-ground biomass down to 30-cm depth. We found strong differences between wet and dry season, with biomass and nutrient concentrations being highest in the wet season, across all fire treatments. Fire treatments had significant effects on plant nutrient stocks and root distribution, although total biomass was not affected. Concentrations of the most volatile nutrients (N, S, K and P) were higher in the herbaceous aboveground biomass of the quadrennial and the unburnt plots, suggesting that increases in fire frequency would reduce the amount of nutrients in aboveground biomass and increase the concentration of fine roots at the soil surface. Results highlight the role of fire in maintaining community dynamics in the Brazilian savanna. Overall, the quadrennial burn appears to be the optimal fire regime in open Cerrado vegetation.

Effects of prescribed fires on the survival and release of seeds of Kielmeyera coriacea (Spr.) Mart. (Clusiaceae) in savannas of Central Brazil

Effects of prescribed fires on survival and release of seeds of the woody species Kielmeyera coriacea (Clusiaceae) were investigated in two plots of cerrado sensu stricto, a savanna vegetation of Central Brazil. The first plot was burnt in June, at the beginning of the dry season, and the second in August, in the middle of the dry season. Seed survival was measured after fire in both areas and related to internal and external fruit temperatures measured during the June fire. The proportion of open fruits per individual of K. coriacea was also assessed at two-week intervals. Maximum external temperatures during fire (393 to 734oC) were strongly reduced inside the fruits (61 to 63oC). Before the June fire, the majority of the fruits were closed in both plots. Most fruits in the June plot opened within two weeks following the burning while, in the same period, most fruits remained closed in the August plot. Fifteen days after the prescribed fire in the August plot most fruits opened, as observed in the June plot. No germination was observed in seeds from closed fruits collected before the fire, while those from fruits that were closed during the burning showed a high mean germination rate (June = 79 ± 12%; August = 69 ± 14%). The results indicate that fruits of K. coriacea are good insulators for seeds during fires and that seed release is anticipated independently of the burning season.

Simulated post-fire temperature affects germination of native and invasive grasses in cerrado (Brazilian savanna).

Background: Although fire is an important factor in determining cerrado vegetation, information about its effect on seed banks is sparse. Cerrado fires are rapidly moving surface fires with low residence time, producing only short-term heating of the uppermost centimetres of the soil. However, the reduction in vegetation cover and deposition of ashes increases the daily amplitude of soil temperature by as much as 35 °C. Aims: To assess the effect of post-fire daily soil temperatures on the germination of one alien and nine native grasses. Methods: Seeds were stored at alternating temperatures of 45 ºC/10 ºC (10 h/14 h) for 7 d or 30 d, simulating two different storage times in the soil seed bank before the onset of the rainy season. Germination was monitored over 30 d. Results: The variation in temperature had a significant effect on the rate of seed germination in some species, either enhancing it (Aristida setifolia) or reducing it (Schizachyrium sanguineum). Increased storage time reduced the viability of S. sanguineum and Echinolaena inflexa. The invasive Melinis minutiflora had the highest germination rate and it also showed the best toleration of post-fire conditions (45 ºC/10 ºC) after 7 d, with significant reduction in the germination time after 30 d. Conclusions: Fire seems to have a significant effect in the early life of cerrado grasses. Some native species responded positively to temperature oscillation, suggesting that they should be better prepared to compete with alien species after a fire, with more of their seeds germinating and/or at a more rapid rate.

Biological activity in Cerrado soils: evaluation of vegetation, fire and seasonality effects using the “bait-lamina test”

AimsThe Cerrado, a South American savanna, is considered a priority for conservation. In this case study, we assessed soil feeding activity as a way to improve understanding of the ecosystem functioning, in order to support and refine conservation strategies.MethodsSoil feeding activity was assessed using the bait-lamina method under different environmental conditions: in the dry and rainy seasons, in burned and unburned areas, and under native and invasive grasses.ResultsFeeding activity was significantly reduced after fire, but recovered to pre-fire levels with the rains. Activity increased significantly during the rainy season in both areas, being more pronounced in the unburned area. The highest feeding activity was observed under the invasive grass (Melinis minutiflora). Feeding activity declined with soil depth and was affected by season and fire.ConclusionsSeasonality was the most important factor affecting the feeding activity of soil organisms, followed by the fire history and the extant vegetation. Although this method does not allow distinguishing between feeding activity of different organisms, it can provide valuable insights into differences in soil functioning due to changes in environmental conditions.

EFEITO DE ALTAS TEMPERATURAS NA GERMINAÇÃO DE SEMENTES DE Plathymenia reticulata Benth. E Dalbergia miscolobium Benth.

Fire is common in the Cerrado occurring during the dry season when many species fruit or disperse the seeds. However, little is known about the fire effect on seed germination. This study aimed to investigate the effect of high temperatures on seed germination of Dalbergia miscolobium Benth. and Plathymeniareticulata Benth., common species of the Cerrado and with potential use in recovery of degraded areas and restoration of permanent preservation areas. Seeds were collected in August 2011, weighed, separated into three size classes and exposed to three temperatures: 80 °C, 100 °C and 150 °C for 2 and 5 minutes. After the treatments, the seeds were germinated. Exposure to high temperatures had different effects in germination of species. Regardless of size class, the seed exposure up to 100 °C for 2 and 5 minutes did not alter germination of D. miscolobium seeds (91%). However, after exposure to 150 °C only the seeds with higher mass germinated, but with a significant reduction in germination (53%; p<0.01). Exposure to 80 °C for 2 and 5 minutes and to 100 °C for 2 minutes did not affect P. reticulata germination (71%). No seed germinated after exposure to 150 °C. The low resistance of the seeds to high temperatures can significantly affect the recruitment of new individuals after the occurrence of Cerrado fires.