William A. Hoffmann

Ecological thresholds at the savanna-forest boundary: how plant traits, resources and fire govern the distribution of tropical biomes.

Fire shapes the distribution of savanna and forest through complex interactions involving climate, resources and species traits. Based on data from central Brazil, we propose that these interactions are governed by two critical thresholds. The fire-resistance threshold is reached when individual trees have accumulated sufficient bark to avoid stem death, whereas the fire-suppression threshold is reached when an ecosystem has sufficient canopy cover to suppress fire by excluding grasses. Surpassing either threshold is dependent upon long fire-free intervals, which are rare in mesic savanna. On high-resource sites, the thresholds are reached quickly, increasing the probability that savanna switches to forest, whereas low-resource sites are likely to remain as savanna even if fire is infrequent. Species traits influence both thresholds; saplings of savanna trees accumulate bark thickness more quickly than forest trees, and are more likely to become fire resistant during fire-free intervals. Forest trees accumulate leaf area more rapidly than savanna trees, thereby accelerating the transition to forest. Thus, multiple factors interact with fire to determine the distribution of savanna and forest by influencing the time needed to reach these thresholds. Future work should decipher multiple environmental controls over the rates of tree growth and canopy closure in savanna.

FIRE AND POPULATION DYNAMICS OF WOODY PLANTS IN A NEOTROPICAL SAVANNA: MATRIX MODEL PROJECTIONS

Human activity has resulted in high fire frequency in many moist tropical savannas. To simulate the effects of increased fire frequency on woody plants of the cerrado savannas of Brazil, I constructed matrix population models for five species, including a subshrub, two shrubs, and two trees, using four years of demographic data. The models projected that four of the five species will decline under frequent burning but will increase in abundance under infrequent burning. For these four, fire intervals of 2-9 yr are required for long-term persistence, depending on the species. The fifth species, a shrub, was virtually unaffected by burning. Similar data for three herbaceous species were obtained from literature sources. Among the eight species, there was a negative relationship between population growth under annual burning and population growth under unburned conditions. Woody plant species performed best under low fire frequency, and herbaceous species performed best under high fire frequency. Three of the study species propagate vegetatively by producing root suckers. Fire stim- ulates vegetative propagation in all three species but reduces sexual reproduction. Elasticity analysis revealed that vegetative propagation is more important than sexual reproduction for population maintenance of these three species, and that vegetative propagation is rel- atively more important under frequent burning. However, stasis and progression were always much more important than either mode of reproduction, with stasis making the largest contribution to population growth. Analyses of life history response experiments were performed to determine what de- mographic variables contributed most to the population decline under frequent burning. The effect of fire on sexual reproduction contributed little to the overall effect of fire on population growth, particularly for clonal species. Fire-induced mortality also had little

Savanna vegetation-fire-climate relationships differ among continents.

Surveying Savannas Savannas are structurally similar across the three major continents where they occur, leading to the assumption that the factors controlling vegetation structure and function are broadly similar, too. Lehmann et al. (p. 548) report the results of an extensive analysis of ground-based tree abundance in savannas, sampled at more than 2000 sites in Africa, Australia, and South America. All savannas, independent of region, shared a common functional property in the way that moisture and fire regulated tree abundance. However, despite qualitative similarity in the moisture–fire–tree-biomass relationships among continents, key quantitative differences exist among the three regions, presumably as a result of unique evolutionary histories and climatic domains. Evolution cannot be overlooked when aiming to predict the potential global impacts on savanna dynamics in a warming world. Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.

The role of topkill in the differential response of savanna woody species to fire

Understanding the impact of fire on the demography of savanna trees and shrubs is necessary for understanding human impacts in tropical savannas. In a replicated experiment, we studied the impact of fire and vegetation cover on survival and growth of two subshrubs (Periandra mediterranea and Protium ovatum), two shrubs (Miconia albicans and Rourea induta) and three trees (Myrsine guianensis, Piptocarpha rotundifolia and Roupala montana) of the Brazilian cerrado savannas. Burning increased complete mortality (i.e. death of the individual) of five of the seven species, but primarily among individuals with stem diameters 2 m) caused greater mortality and topkill than fires of lower intensity (flame length <2 m). Pre-burn vegetation density had little effect on survival or resprout size, but did affect subsequent growth rates. Four species had greater growth rates in open sites, whereas only one species had greater growth rates in dense sites. For the three tree species and one shrub, resprouting individuals did not reach the minimum reproductive size within 1 year of burning, while the other shrub and the two subshrubs were able to reach reproductive size during this time, indicating that growth form largely determines the population response to frequent burning.

Vegetation–Climate Feedbacks in the Conversion of Tropical Savanna to Grassland

Tropical savannas have been heavily impacted by human activity, with large expanses transformed from a mixture of trees and grasses to open grassland and agriculture. The National Center for Atmospheric Research (NCAR) CCM3 general circulation model, coupled with the NCAR Land Surface Model, was used to simulate the effects of this conversion on regional climate. Conversion of savanna to grassland reduced precipitation by approximately 10% in four of the five savanna regions under study; only the northern African savannas showed no significant decline. Associated with this decline was an increase in the frequency of dry periods within the wet season, a change that could be particularly damaging to shallow-rooted crops. The overall decline in precipitation is almost equally attributable to changes in albedo and roughness length. Conversion to grassland increased mean surface air temperature of all the regions by 0.58C, primarily because of reductions in surface roughness length. Rooting depth, which decreases dramatically with the conversion of savanna to grassland, contributed little to the overall effect of savanna conversion, but deeper rooting had a small positive effect on latent heat flux with a corresponding reduction in sensible heat flux. The authors propose that the interdependence of climate and vegetation in these regions is manifested as a positive feedback loop in which anthropogenic impacts on savanna vegetation are exacerbated by declines in precipitation.

THE EFFECTS OF FIRE AND COVER ON SEEDLING ESTABLISHMENT IN A NEOTROPICAL SAVANNA

1 It was hypothesized that if facilitation is important for seedling establishment in savanna, then fire should reduce seedling establishment. 2 This was tested in the cerrado savanna of Brazil with a factorial experiment designed to evaluate the effects of cover and prescribed burning on seedling establishment. 3 Seeds of 12 species of trees and shrubs were sown in plots located in sites providing three densities of woody cover and four times since last burning. 4 Seedling establishment generally was greater under the crowns of trees than in open grassland, but individual species responded differently to cover. Eight of the 12 species responded favourably to cover, but a single species showed reduced establishment with increasing cover. 5 Burning had an overall negative effect on seedling establishment in the first year following burning. By the second year following burning, establishment returned to control levels. 6 An irrigation experiment with two species demonstrated that rain-free periods within the wet season had negative effects on seedling establishment. 7 A litter-removal experiment with Miconia albicans revealed that, at least for this species, litter has a negative effect upon seedling establishment in densely wooded sites and positive effects in open grassland.

Constraints to seedling success of savanna and forest trees across the savanna-forest boundary

Tropical savannas and closed forests are characterized by distinct tree communities, with most species occurring almost exclusively in only one of the two environments. The ecology of these two groups of species will largely determine the structure and dynamics of the savanna-forest boundary, but little is known about the ecological and physiological differences that might control their distributions. We performed field and nursery experiments to compare seedling establishment success, predawn leaf water potential, biomass allocation, and root carbohydrate concentration of congeneric species, each composed of one savanna species and one forest species. Seedling establishment of savanna and forest species responded differently to vegetation cover, with forest species having lowest establishment success in the open savanna and savanna species having lowest success in forest. Subsequent survival followed similar patterns, resulting in even greater differences in cumulative success. The low survival of forest species in the savanna appears related to drought stress, as seedlings of forest species had lower predawn leaf water potential than savanna species. Seedlings of savanna species had greater root: shoot ratios and root total nonstructural carbohydrate (TNC) concentration, particularly among evergreen genera. Among evergreen genera, root TNC per shoot mass, which may largely determine resprout capacity, was seven times higher in savanna species than forest species. Although water availability and microclimate may reduce the success of forest species, these factors appear unable to completely exclude forest seedling establishment in savanna. Fire, on the other hand, appears to be a much more absolute constraint to success of forest species in savanna.

Tropical grassy biomes: misunderstood, neglected, and under threat

Tropical grassy biomes (TGBs) are globally extensive, provide critical ecosystem services, and influence the earth-atmosphere system. Yet, globally applied biome definitions ignore vegetation characteristics that are critical to their functioning and evolutionary history. Hence, TGB identification is inconsistent and misinterprets the ecological processes governing vegetation structure, with cascading negative consequences for biodiversity. Here, we discuss threats linked to the definition of TGB, the Clean Development Mechanism (CDM) and Reducing Emissions from Deforestation and Forest Degradation schemes (REDD+), and enhanced atmospheric CO2, which may facilitate future state shifts. TGB degradation is insidious and less visible than in forested biomes. With human reliance on TGBs and their propensity for woody change, ecology and evolutionary history are fundamental to not only the identification of TGBs, but also their management for future persistence.

Stem and leaf hydraulics of congeneric tree species from adjacent tropical savanna and forest ecosystems.

Leaf and stem functional traits related to plant water relations were studied for six congeneric species pairs, each composed of one tree species typical of savanna habitats and another typical of adjacent forest habitats, to determine whether there were intrinsic differences in plant hydraulics between these two functional types. Only individuals growing in savanna habitats were studied. Most stem traits, including wood density, the xylem water potential at 50% loss of hydraulic conductivity, sapwood area specific conductivity, and leaf area specific conductivity did not differ significantly between savanna and forest species. However, maximum leaf hydraulic conductance (Kleaf) and leaf capacitance tended to be higher in savanna species. Predawn leaf water potential and leaf mass per area were also higher in savanna species in all congeneric pairs. Hydraulic vulnerability curves of stems and leaves indicated that leaves were more vulnerable to drought-induced cavitation than terminal branches regardless of genus. The midday Kleaf values estimated from leaf vulnerability curves were very low implying that daily embolism repair may occur in leaves. An electric circuit analog model predicted that, compared to forest species, savanna species took longer for their leaf water potentials to drop from predawn values to values corresponding to 50% loss of Kleaf or to the turgor loss points, suggesting that savanna species were more buffered from changes in leaf water potential. The results of this study suggest that the relative success of savanna over forest species in savanna is related in part to their ability to cope with drought, which is determined more by leaf than by stem hydraulic traits. Variation among genera accounted for a large proportion of the total variance in most traits, which indicates that, despite different selective pressures in savanna and forest habitats, phylogeny has a stronger effect than habitat in determining most hydraulic traits.

Nitrogen Controls on Climate Model Evapotranspiration

Most evapotranspiration over land occurs through vegetation. The fraction of net radiation balanced by evapotranspiration depends on stomatal controls. Stomates transpire water for the leaf to assimilate carbon, depending on the canopy carbon demand, and on root uptake, if it is limiting. Canopy carbon demand in turn depends on the balancing between visible photon-driven and enzyme-driven steps in the leaf carbon physiology. The enzymedriven component is here represented by a Rubisco-related nitrogen reservoir that interacts with plant‐soil nitrogen cycling and other components of a climate model. Previous canopy carbon models included in GCMs have assumed either fixed leaf nitrogen, that is, prescribed photosynthetic capacities, or an optimization between leaf nitrogen and light levels so that in either case stomatal conductance varied only with light levels and temperature. A nitrogen model is coupled to a previously derived but here modified carbon model and includes, besides the enzyme reservoir, additional plant stores for leaf structure and roots. It also includes organic and mineral reservoirs in the soil; the latter are generated, exchanged, and lost by biological fixation, deposition and fertilization, mineralization, nitrification, root uptake, denitrification, and leaching. The root nutrient uptake model is a novel and simple, but rigorous, treatment of soil transport and root physiological uptake. The other soil components are largely derived from previously published parameterizations and global budget constraints. The feasibility of applying the derived biogeochemical cycling model to climate model calculations of evapotranspiration is demonstrated through its incorporation in the Biosphere‐Atmosphere Transfer Scheme land model and a 17-yr Atmospheric Model Inter comparison Project II integration with the NCAR CCM3 GCM. The derived global budgets show land net primary production (NPP), fine root carbon, and various aspects of the nitrogen cycling are reasonably consistent with past studies. Time series for monthly statistics averaged over model grid points for the Amazon evergreen forest and lower Colorado basin demonstrate the coupled interannual variability of modeled precipitation, evapotranspiration, NPP, and canopy Rubisco enzymes.