Vegetation–environment relationships and classification of the seasonal savannas in Venezuela

The seasonal savannas dominated by the C₄ grasses of the genus Trachypogon, are widespread in northern South America. In Venezuela, they extend from the central lowland Llanos to intermediate elevations in the Coastal and Andean Mountains and to the Guiana Plateau (Gran Sabana) in the south. This study aims to classify these savannas and to understand the environmental factors that determine their composition and structure. Thirty-seven sites were sampled, plant cover and density were measured in 376 one square meter quadrats and the importance value index (IVI) was calculated. Climate data were obtained from climatological stations and soil properties were analyzed. The results were classified by clustering and TWINSPAN. Floristic and environmental data were ordered with the canonical correspondence analysis (CCA).Trachypogon savannas are heterogeneous, both floristically and environmentally. An altitudinal gradient of 2000 m separates the intermediate elevation Coastal Mountains (Type I) and the Guiana savannas (Type II) from the lowland Llanos. In the lowlands, sparsely covered and treeless communities on extremely oligotrophic and sandy soils (Type IV) differ structurally and floristically from woody savannas (Type III). The floristics and structure of the savannas respond directly or indirectly to elevation and water and nutrient availability. These responses were evidenced by: (a) floristic composition, as Neotropical lowland savanna species merged with those of the Andean sub-páramo in Type I savannas or with those from the Guianan flora in Type II savannas. (b) Species richness, which ranged from an average of 13.8 per site in sandy Type IV savannas to 38.9 per site in Type I intermediate elevation savannas. (c) Community physiognomy, which changes from an homogeneous herbaceous matrix with dispersed trees in Type III savannas to a shrubby Type I savannas to “treeless” Types II and IV savannas. (d) Plant cover which varied from 93.8% in the fertile Type I savannas to around 80% in the sandy oligotrophic savannas of Types II and IV. Each savanna type is related to one of the major geomorphological regions of Venezuela.     

Bird community composition in an actively managed savanna reserve, importance of vegetation structure and vegetation composition

The mosaic of trees, shrubs and open grassland in mesic African savannas is highly dynamic and strongly influenced by mammal herbivory and fire. We investigated the bird fauna in four different savanna habitats to help assess the impacts of vegetation change on this component of faunal diversity. Birds were censused, plant species were identified and vegetation structure was measured in four different vegetation types (Acacia nilotica woodland, Acacia nigrescens woodland, broadleaf thicket and open grassland) in the Hluhluwe-Umfolozi Park in northern KwaZulu Natal, South Africa. Multivariate ordination analyses were used to determine the relative importance of vegetation structure and floristic composition in defining bird assemblages. The bird communities of the grasslands, the acacia woodlands, and the broadleaf woodlands were clearly separated on the first axis of the detrended canonical correspondence analysis (DCCA). Canopy cover and foliage height diversity (FHD) were strongly correlated with the first axis of DCCA, possibly reflecting a secondary successional series from grassland to woodland, known as bush encroachment. Floristic composition (based on presence–absence data only) seemed to be less important for bird community composition than vegetation structure. The results indicate that changes in vegetation structure, caused by bush encroachment, could cause concomitant changes in bird community composition.     

CO2 Exchange in soil algal crusts occurring in the trachypogon savannas of the Orinoco Llanos,Venezuela

CO₂ exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities to savanna woodlands. A maximum CO₂ flux of 0.207 mg m^-2 s^-1 was measured in the crusts of the Guanipa savannas, while in the other examined crusts (0.035–0.105 mg m^-2 s^-1) the flux was similar to values reported for terrestrial algae. The CO₂ flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic efficiencies of the crusts were compared. The activation energy calculated for the CO₂ fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12 kcal mole^-1), whereas they were evident in the other crust studied. An optimum CO₂ incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the basis of the results and characteristics of the habitats.     

Effects of Fire on Seedling Diversity and Plant Reproduction (Sexual vs. Vegetative) in Neotropical Savannas Differing in Tree Density

Little is known about the effects of fire on the structure and species composition of Neotropical savanna seedling communities. Such effects are critical for predicting long‐term changes in plant distribution patterns in these ecosystems. We quantified richness and density of seedlings within 144 plots of 1 m² located along a topographic gradient in long‐unburned (fire protected since 1983) and recently burned (September 2005) savannas in Brazil. These savannas differ in tree density and canopy cover. Sites along the gradient, however, did not differ in species composition prior to the fire. In recently burned savannas we also evaluated the contribution of vegetative reproduction relative to sexual reproduction by quantifying richness and density of root suckers. Finally, we tested seed tolerance to pulses of high temperatures—similar to those occurring during fires on the soil surface and below—of five dominant savanna tree species. Seedlings were more abundant and diverse in unburned than in burned savannas. Seedling species composition differed among unburned and burned savannas probably reflecting early differences in root: shoot biomass allocation patterns. In recently burned savannas, root suckers were more abundant and diverse than seedlings. Relatively long exposures (>10 min) of temperatures of 90 °C reduced seed germination in all studied species suggesting a negative effect of fire on germination of seeds located at or aboveground level. Because vegetative reproduction contributes more than sexual reproduction in burned environments, frequent fires are likely to cause major shifts in species composition of Neotropical savanna plant communities, favoring clonally produced recruits along tree density/topographic gradients.     

Holocene global warming and the origin of the Neotropical Gran Sabana in the Venezuelan Guayana

Aim The assumedly anomalous occurrence of savannas and forest–savanna mosaics in the Gran Sabana – a neotropical region under a climate more suitable for tropical rain forests – has been attributed to a variety of historical, climatic, and anthropogenic factors. This paper describes a previously undocumented shift in vegetation and climate that occurred during the early Holocene, and evaluates its significance for the understanding of the origin of the Gran Sabana vegetation. Location A treeless savanna locality of the Gran Sabana (4°30′–6°45′ N and 60°34′–62°50′ W), in the Venezuelan Guayana of northern South America, at the headwaters of the Caroní river, one of the major tributaries of the Orinoco river. Methods Pollen and charcoal analysis of a previously dated peat section spanning from about the Pleistocene/Holocene boundary until the present. Results Mesothermic cloud forests dominated by Catostemma (Bombacaceae) occupied the site around the Pleistocene/Holocene boundary. During the early Holocene, a progressive but relatively rapid trend towards savanna vegetation occurred, and eventually the former cloud forests were replaced by a treeless savanna. Some time after the establishment of savannas, a marked increase in charcoal particles indicates the occurrence of the first local fires. Main conclusions The occurrence of cloud forests at the Pleistocene/Holocene boundary contradicts the historical hypothesis according to which the Gran Sabana is a relict of the hypothetical widespread savannas that have been assumed to have dominated the region during the last glaciation. The first local fires recorded in the Holocene were on savanna vegetation, which is against the hypothesis of fire as the triggering factor for the establishment of these savannas. Climate change, in the form of global warming and a persistently drier climate, emerges as the most probable cause for the forest–savanna turnover.     

Submillennium-scale migrations of the rainforest-savanna boundary in Colombia: C wiggle-matching and pollen analysis of core Las Margaritas

We present the 11?150-cal-yr-long pollen record Laguna Las Margaritas (3°23′N, 73°26′W; 290 m altitude), located at a site sensitive to climatic change near the transition from the Amazonian rainforest to the savanna of the Llanos Orientales in Colombia. In the 10-m-long core nine AMS ¹⁴C bulk samples show ages from 9760±60 to 854±36 BP and provide initial time control. Thirty-one additional AMS ¹⁴C samples of selected macrofossils provide time control from 6250 to 4050 cal BP; for this interval precise time control was obtained by ¹⁴C wiggle-match dating. From 11?150 to 9100 cal BP, grass savanna dominated the landscape while gallery forest along the drainage system was poorly developed. Water availability was lower than today and the length of the dry season longer. From 9100 to 7330 cal BP gallery forest expanded pointing to wetter conditions. From 7330 to 5430 cal BP savanna was increasingly replaced by forest, but with alternating abundance of both vegetation types. From 5430 to 2500 cal BP forest and wooded savanna dominated the western Llanos Orientales suggesting high precipitation rates. Expansion of Mauritia palm forest at 2500 cal BP reflects increasing water availability and stagnant water environments. Increasing savanna between 2500 and 1000 cal BP may represent a combined natural and anthropogenic signal. Development from open savanna to forest during the Middle Holocene is synchronous with a decreasing caloric seasonality, and a southward migration of the Intertropical Convergence Zone, suggesting that the large-scale climatic and vegetational change in the Colombian savannas is precession-forced. High-frequent migration of the savanna-rainforest boundary started around 6000 cal BP and continued at least to 3000 cal BP. Precipitation regimes in northwest and northeast South America seem opposite: dry conditions in the Colombian savanna area seem to reflect a La Niña setting of the climate system; wet conditions reflect an El Niño setting. Δ¹⁴C fluctuations reflect changes of solar activity and we tested the hypothesis that Δ¹⁴C fluctuations correspond to climatic and vegetational change. For the interval 6250 to 4050 cal BP, we applied ¹⁴C wiggle-match dating (WMD), i.e. matching a series of radiocarbon ages in ¹⁴C BP from the sediment, with the dendrochronology-based calibration curve, and we also wiggle-matched the pollen record vs. the Δ¹⁴C record. In this first attempt in a neotropical ecosystem, we could not find unambiguous support that changes in solar activity did trigger climatic and vegetational change in the savannas of the Llanos Orientales.     

Quantitative classification and ordination of forest communities in Pangquangou National Nature Reserve

Quantitative analysis of ecological relationships between vegetation and the environment has become an essential means in the field of research of modern vegetation ecology. In this article, based on data from 84 quadrates, forest communities in this reserve were investigated using TWINSPAN, DCA and DCCA. The results will be helpful in the construction and development of Pangquangou National Nature Reserve. Using TWINSPAN, the forest communities were classified into seven types. The distribution pattern of vegetation reflects the comprehensive influence of environments. The results of DCA and DCCA clearly reflect the relationship between the pattern of forest communities and environmental gradients. The ordination result of DCCA indicates that altitude is more important than other environmental factors because the change of altitude gradient will lead to changes in the temperature and humidity gradients. The first of the DCA ordination axes indicates the humidity gradient, and the second indicates the temperature gradient. All these results show that the main factors restricting the distribution of communities in this reserve are temperature and humidity. The ecological meaning of the ordination axis in DCCA is much clearer than that in DCA, and the species-environment correlation of DCCA is more obvious than DCA. The first DCCA axis indicates the altitude gradient among the communities, while the second is the gradient in aspect and slope among the communities. DCCA ordination can simultaneously express similarities of species and environment. Therefore, the quadrat location in the DCCA ordination figure is much closer than in the DCA.     

Community composition and soil properties in northern Bolivian savanna vegetation

Abstract. Direct gradient analysis (Canonical Correspondence Analysis) of northern Bolivian savanna vegetation revealed correlations between the composition of plant communities and physical and chemical soil properties. Cover/abundance values for 193 species from 27 sites were related to data on eight soil factors. A water-regime variable and the percentage of sand and silt were correlated with the first axis of the species-environment biplot and explained most of the variation in community composition. Along this axis, species and sites of flood-plain vegetation were separated from sites not affected by flooding rivers. The second axis of the biplot was correlated with soil-chemical variables, namely extract-able phosphate, base saturation, organic carbon, pH, and effective cation exchange capacity. Part of the variation in community composition can be explained by these soil nutrient variables. Grassland communities were separated from woody vegetation along the soil nutrient gradient, and floodplain communities of white-water rivers from those of clear-water rivers. The results of the gradient analysis indicate that the soil texture-moisture gradient is the prime factor determining the variation in the floristic composition of the savanna communities examined, and that, in addition, the soil nutrient gradient accounts for some of the variation.     

Quantitative classification and ordination of forest communities in Pangquangou National Nature Reserve

Quantitative analysis of ecological relationships between vegetation and the environment has become an essential means in the field of research of modern vegetation ecology. In this article, based on data from 84 quadrates, forest communities in this reserve were investigated using TWINSPAN, DCA and DCCA. The results will be helpful in the construction and development of Pangquangou National Nature Reserve. Using TWINSPAN, the forest communities were classified into seven types. The distribution pattern of vegetation reflects the comprehensive influence of environments. The results of DCA and DCCA clearly reflect the relationship between the pattern of forest communities and environmental gradients. The ordination result of DCCA indicates that altitude is more important than other environmental factors because the change of altitude gradient will lead to changes in the temperature and humidity gradients. The first of the DCA ordination axes indicates the humidity gradient, and the second indicates the temperature gradient. All these results show that the main factors restricting the distribution of communities in this reserve are temperature and humidity. The ecological meaning of the ordination axis in DCCA is much clearer than that in DCA, and the species-environment correlation of DCCA is more obvious than DCA. The first DCCA axis indicates the altitude gradient among the communities, while the second is the gradient in aspect and slope among the communities. DCCA ordination can simultaneously express similarities of species and environment. Therefore, the quadrat location in the DCCA ordination figure is much closer than in the DCA.     

Buffering the savanna: fire regimes and disequilibrium ecology in West Africa

According to contemporary ecological theory, the mechanisms governing tree cover in savannas vary by precipitation level. In tropical areas with mesic rainfall levels, savannas are unstable systems in which disturbances, such as fire, determine the ratio of trees to grasses. Precipitation in these so-called “disturbance-driven savannas” is sufficient to support forest but frequent disturbances prevent transition to a closed canopy state. Building on a savanna buffering model we argue that a consistent fire regime is required to maintain savannas in mesic areas. We hypothesize that the spatiotemporal pattern of fires is highly regular and stable in these areas. Furthermore, because tree growth rates in savannas are a function of precipitation, we hypothesize that savannas with the highest rainfall levels will have the most consistent fire pattern and the most intense fires—thus the strongest buffering mechanisms. We analyzed the spatiotemporal pattern of burning over 11 years for a large subset of the West African savanna using a moderate resolution imaging spectroradiometer active fire product to document the fire regime for three savanna belts with different precipitation levels. We used LISA analysis to quantify the spatiotemporal patterns of fires, coefficient of variance to quantify differences in peak fire dates, and center or gravity pathways to characterize the spatiotemporal patterns of the fires for each area. Our analysis confirms that spatiotemporal regularity of the fire regime is greater for mesic areas that for areas where precipitation is lower and that areas with more precipitation have more regular fire regimes.     

Composition and species diversity of pine-wiregrass savannas of the Green Swamp,North Carolina

Fire-maintained, species-rich pine-wiregrass savannas in the Green Swamp, North Carolina were sampled over their natural range of environmental conditions and fire frequencies. Species composition, species richness, diversity (Exp H′, 1/C), and aboveground production were documented and fertilization experiments conducted to assess possible mechanisms for the maintenance of high species diversity in these communities. Although savanna composition varies continuously, DECORANA ordination and TWINSPAN classification of 21 sites facilitated recognition of 3 community types: dry, mesic, and wet savannas. These savannas are remarkably species-rich with up to 42 species/0.25 m² and 84 species/625 m². Maximum richness occurred on mesic, annually burned sites. Aboveground production, reported as peak standing crop, was only 293 g · m^?2 on a frequently burned mesic savanna but was significantly higher (375 g · m^?2) on an infrequently burned mesic site. Production values from fertilized high and low fire frequency sites were equivalent. Monthly harvest samples showed that savanna biomass composition by species groups did not vary seasonally, but within groups the relative importance of species showed clear phenological progressions. The variation in species richness with fire frequency is consistent with non-equilibrium theories of species diversity, while phenological variation in production among similar species and the changing species composition across the moisture gradient suggest the importance of equilibrium processes for maintenance of savanna diversity.     

辽东栎林不同层植物沿海拔梯度分布的DCCA分析

 为研究北京东灵山辽东栎（Quercus liaotungensis）不同层植物的分布与环境因子之间的关系,对东灵山辽东栎林进行了植物群落学调查。沿 海拔梯度在乔木层、灌木层和草本层各调查119个样方,同时调查了每个样方的环境因子如海拔、坡度和坡位。物种数据采用二元属性数据,对 非数值环境因子进行赋值。对东灵山辽东栎林和各环境因子进行去趋势典范对应分析（Detrended canonical correspondence analysis, DCCA）,结果表明：DCCA 第一轴基本上反映了植物群落所处环境的海拔的梯度变化,第二轴基本上反映了坡位的梯度变化。同时发现辽东栎林 大多数植物分布在低海拔区域。乔木层的物种分布比较均匀。草本层的物种分布重叠率是最大的,说明草本植物的生境相似性很高。低海拔区 域的生物多样性要高于高海拔区域的生物多样性。结合环境因子间的相关关系和DCCA排序结果,发现海拔是影响东灵山辽东栎林物种分布的主 要环境因子。     

四川茂县土地岭大熊猫走廊带植被恢复格局及其与干扰的关系

通过样带调查和TWINSPAN、DCCA分析,从植物种、植物群落及其多样性与环境关系方面,研究了岷江上游土地岭大熊猫走廊带恢复植被的干扰状况。结果表明：应用TWINSPAN分类,并结合优势种组成、干扰状况分析及DCCA排序,可将植被划分为6个群落类型,同时划分出响应型、迟钝型、中度干扰忍耐型和重度干扰忍耐型4类干扰响应的植物类型。以样方物种和以样方多样性指数的DCCA分析结果基本一致,物种及群落的空间分布呈明显的聚集格局,反映其与环境因子间的密切关系。DCCA排序图上,海拔差、坡形、与公路距离、坡度及道路条数对群落和物种分布有明显的影响,与干扰相关性最大的坡度、样地道路数目、与公路间的距离3个因子反映了植被的干扰梯度。干扰对土地岭恢复植被影响显著,干扰降低了群落的物种多样性,同时阻碍了演替进程。     

East with the Night: Longitudinal Migration of the Orinoco Goose (Neochen jubata) between Manú National Park,Peru and the Llanos de Moxos,Bolivia

We report on the intra-Amazonian migration of a pair of Orinoco Geese (Neochen jubata) from Manú National Park, Peru. The species is Critically Endangered in Peru, so a major aim of the study was to aid conservation planning by learning the wet season location of the country''s last known breeding population. We captured a breeding pair on October 27, 2010, and fitted the birds with Microwave Telemetry, Inc. GPS/Argos satellite PTT''s. The pair migrated ∼655 km from Manú National Park to the Llanos de Moxos, Bolivia (Dept. of Bení) in a predominantly longitudinal migration, reaching their final destination on December 23, 2010. Major movements (>5 km per time period) were almost exclusively at night and were undertaken with and without moonlight. Foraging areas used at stopovers in the Llanos de Moxos were remarkably limited, suggesting the importance of grazing lawns maintained by the geese and other herbivores, possibly including cattle. Orinoco Geese are resident in the Llanos de Moxos year-round, so the Manú geese represent a partial migration from the Bení region. We hypothesize that cavity nest limitation explains the partial migration of Orinoco Geese from the Llanos de Moxos.     

历山自然保护区猪尾沟森林群落植被格局及环境解释

应用TWINSPAN、DCA和DCCA,从植物种,植物群落与环境的生态关系方面,研究历山自然保护区猪尾沟森林群落的植被分布格局,并给予合理的环境解释。结果如下：（1）采用TWINSPAN数量分类方法,将植被划分为9个群落类型。（2）对于特定的研究区域猪尾沟,制约森林群落类型,植物种分布格局的主要因素是海拔梯度,即水、热两个环境因子。（3）DCCA排序图明显反映出排序轴的生态意义,第一轴基本上突出反映了各植物群落所在环境的海拔梯度,即热量因素,沿第一轴从左到右,海拔逐渐升高,植物群落或植物种对热量要求降低;第二轴主要表现了各植物群落或植物种所在环境的坡度,坡向,即水分和光照因素,沿第二轴从下到上,坡度渐缓,坡向渐向阳。     

The impact of shrub encroachment on savanna bird diversity from local to regional scale

Aim  Evidence is accumulating of a general increase in woody cover of many savanna regions of the world. Little is known about the consequences of this widespread and fundamental ecosystem structural shift on biodiversity.
Location  South Africa.
Methods  We assessed the potential response of bird species to shrub encroachment in a South African savanna by censusing bird species in five habitats along a gradient of increasing shrub cover, from grassland/open woodland to shrubland dominated by various shrub species. We also explored historical bird species population trends across southern Africa during the second half of the 20th century to determine if any quantifiable shifts had occurred that support an ongoing impact of shrub encroachment at the regional scale.
Results  At the local scale, species richness peaked at intermediate levels of shrub cover. Bird species composition showed high turnover along the gradient, suggesting that widespread shrub encroachment is likely to lead to the loss of certain species with a concomitant decline in bird species richness at the landscape scale. Finally, savanna bird species responded to changes in vegetation structure rather than vegetation species composition: bird assemblages were very similar in shrublands dominated by Acacia mellifera and those dominated by Tarchonanthus camphoratus .
Main conclusions  Shrub encroachment might have a bigger impact on bird diversity in grassland than in open woodland, regardless of the shrub species. Species recorded in our study area were associated with historical population changes at the scale of southern Africa suggesting that shrub encroachment could be one of the main drivers of bird population dynamics in southern African savannas. If current trends continue, the persistence of several southern African bird species associated with open savanna might be jeopardized regionally.     

Floristic composition,plant species abundance,and soil properties of montane savannas in the Gran Sabana,Venezuela

Floristic composition, species abundance, and soil properties were studied in slope, flat and disturbed savannas in the northern part of the Gran Sabana, Venezuela. All savannas presented shallow soils (<30 cm depth) with high content of sand and low content of clay. In general, the soils were poor in nutrients and strongly acidified. The major difference between the soils was the content of the stony fraction, which was significantly higher (P<0.05) in the slope savannas than in the flat savannas. A total of 57 dicot, 42 monocot, and 7 fern species were recorded in all studied savannas. In the flat and slope savannas predominated the monocot species, while in the disturbed savanna predominated the dicots. The families with the largest number of species were Poaceae (19.8%), Cyperaceae (13.2%), Asteraceae (10.4%) and Melastomataceae (8.5%). The number of species in the flat savannas was higher than that of the slope savannas. The lowest plant species richness was associated to slope savannas and their high content of stony fraction of the soils. The highest floristic similarity was found between slope savannas, and the lowest between disturbed savanna and slope savannas. The most abundant life forms in the studied savannas were perennial (42.4%) and annual (24.5%) herbs, followed by suffruticoses (16.0%) and shrubs (12.3%), and the less frequent was lianas (4.7%). The disturbed savanna showed the higher richness and diversity index. Trachypogon plumosum (Poaceae) was the most abundant species in all studied savanna.     

Patch dynamics integrate mechanisms for savanna tree–grass coexistence

Many mechanisms have been suggested to explain the coexistence of woody species and grasses in savannas. However, evidence from field studies and simulation models has been mixed. Patch dynamics is a potentially unifying mechanism explaining tree–grass coexistence and the natural occurrence of shrub encroachment in arid and semi-arid savannas. A patch-dynamic savanna consists of a spatial mosaic of patches. Each patch maintains a cyclical succession between dominance of woody species and grasses, and the succession of neighbouring patches is temporally asynchronous. Evidence from empirical field studies supports the patch dynamics view of savannas. As a basis for future tests of patch dynamics in savannas, several hypotheses are presented and one is exemplarily examined: at the patch scale, realistically parameterized simulation models have generated cyclical succession between woody and grass dominance. In semi-arid savannas, cyclical successions are driven by precipitation conditions that lead to mass recruitment of shrubs in favourable years and to simultaneous collapse of shrub cohorts in drought years. The spatiotemporal pattern of precipitation events determines the scale of the savanna vegetation mosaic in space and time. In a patch-dynamic savanna, shrub encroachment is a natural, transient phase corresponding to the shrub-dominated phase during the successional cycle. Hence, the most promising management strategy for encroached areas is a large-scale rotation system of rangelands. In conclusion, patch dynamics is a possible scale-explicit mechanism for the explanation of tree–grass coexistence in savannas that integrates most of the coexistence mechanisms proposed thus far for savannas.     

Stocks and dynamics of carbon in trees across a rainfall gradient in a tropical savanna

In this study, systematic variation in tree morphology across a rainfall gradient in Australia's tropical savanna biome and its implications for carbon stocks and dynamics were quantified. The aim was to support efforts to manage fire regimes to increase vegetative carbon stocks as a greenhouse gas mitigation strategy. The height of trees for a given trunk diameter declines with decreasing rainfall from 2000 to 300 mm and increasing dry season length across the Australian savanna biome. It is likely that increasing dry season length is the main driver of this decline rather declining rainfall per se. By taking account of the response of total basal area to rainfall and soil type, stand structure, and tree height and diameter relationships, the carbon stocks in live trees were estimated to decline from about 34 t ha^?1 in the wetter savannas to 6 t ha^?1 in the drier savannas. These values are broadly consistent with field‐based estimates. Because of the declining ratio of height to trunk diameter, trees of a given diameter in drier regions will be more likely to be killed by fires of a given intensity than trees in wetter regions. Thus single fires of given intensity are likely to have a greater proportionate impact on live tree carbon stock in drier savannas, but a much greater absolute impact in wetter savannas due to the greater total carbon stock. Projected decreases in early wet season rainfall under climate change scenarios, despite projections of little change in total precipitation in northern Australia, may lead to decreased carbon stock in live trees through two mechanisms: a reduction in total basal area and decreases in tree height for given trunk diameters.     

Savannas after afforestation: Assessment of herbaceous community responses to wildfire versus native tree planting

Afforestation and fire exclusion are pervasive threats to tropical savannas. In Brazil, laws limiting prescribed burning hinder the study of fire in the restoration of Cerrado plant communities. We took advantage of a 2017 wildfire to evaluate the potential for tree cutting and fire to promote the passive restoration of savanna herbaceous plant communities after destruction by exotic tree plantations. We sampled a burned pine plantation (Burned Plantation); a former plantation that was harvested and burned (Harvested & Burned); an unburned former plantation that was harvested, planted with native trees, and treated with herbicide to control invasive grasses (Native Tree Planting); and two old-growth savannas which served as reference communities. Our results confirm that herbaceous plant communities on post-afforestation sites are very different from old-growth savannas. Among post-afforestation sites, Harvested & Burned herbaceous communities were modestly more similar in composition to old-growth savannas, had slightly higher richness of savanna plants (3.8 species per 50-m²), and supported the greatest cover of native herbaceous plants (56%). These positive trends in herbaceous community recovery would be missed in assessments of tree cover: whereas canopy cover in the Harvested & Burned site was 6% (less than typical of savannas of the Cerrado), the Burned Plantation and Native Tree Planting supported 34% and 19% cover, respectively. By focusing on savanna herbaceous plants, these results highlight that tree cutting and fire, not simply tree planting and fire exclusion, should receive greater attention in efforts to restore savannas of the Cerrado.