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 (K _leaf) 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 K _leaf 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 K _leaf 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.     

Fire controls population structure in four dominant tree species in a tropical savanna

The persistence of mesic savannas has been theorised as being dependent on disturbances that restrict the number of juveniles growing through the sapling size class to become fire-tolerant trees. We analysed the population structures of four dominant tropical savanna tree species from 30 locations in Kakadu National Park (KNP), northern Australia. We found that across KNP as a whole, the population size structures of these species do not exhibit recruitment bottlenecks. However, individual stands had multimodal size-class distributions and mixtures of tree species consistent with episodic and individualistic recruitment of co-occurring tree species. Using information theory and multimodel inference, we examined the relative importance of fire frequency, stand basal area and elevation difference between a site and permanent water in explaining variations in the proportion of sapling to adult stems in four dominant tree species. This showed that the proportion of the tree population made up of saplings was negatively related to both fire frequencies and stand basal area. Overall, fire frequency has density-dependent effects in the regulation of the transition of saplings to trees in this Australian savanna, due to interactions with stem size, regeneration strategies, growth rates and tree–tree competition. Although stable at the regional scale, the spatiotemporal variability of fire can result in structural and floristic diversity of savanna tree populations.     

Juggling carbon: allocation patterns of a dominant tree in a fire-prone savanna

In frequently burnt mesic savannas, trees can get trapped into a cycle of surviving fire-induced stem death (i.e. topkill) by resprouting, only to be topkilled again a year or two later. The ability of savanna saplings to resprout repeatedly after fire is a key component of recent models of tree–grass coexistence in savannas. This study investigated the carbon allocation and biomass partitioning patterns that enable a dominant savanna tree, Acacia karroo, to survive frequent and repeated topkill. Root starch depletion and replenishment, foliage recovery and photosynthesis of burnt and unburnt plants were compared over the first year after a burn. The concentration of starch in the roots of the burnt plants (0.08 ± 0.01 g g⁻¹) was half that of the unburnt plant (0.16 ± 0.01 g g⁻¹) at the end of the first growing season after topkill. However, root starch reserves of the burnt plants were replenished over the dry season and matched that of unburnt plants within 1 year after topkill. The leaf area of resprouting plants recovered to match that of unburnt plants within 4–5 months after topkill. Shoot growth of resprouting plants was restricted to the first few months of the wet season, whereas photosynthetic rates remained high into the dry season, allowing replenishment of root starch reserves. ¹⁴C labeling showed that reserves were initially utilized for shoot growth after topkill. The rapid foliage recovery and the replenishment of reserves within a single year after topkill implies that A. karroo is well adapted to survive recurrent topkill and is poised to take advantage of unusually long fire-free intervals to grow into adults. This paper provides some of the first empirical evidence to explain how savanna trees in frequently burnt savannas are able to withstand frequent burning as juveniles and survive to become adults. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Alternative fire resistance strategies in savanna trees

Bark properties (mainly thickness) are usually presented as the main explanation for tree survival in intense fires. Savanna fires are mild, frequent, and supposed to affect tree recruitment rather than adult survival: trunk profile and growth rate of young trees between two successive fires can also affect survival. These factors and fire severity were measured on a sample of 20 trees near the recruitment stage of two savanna species chosen for their contrasted fire resistance strategies (Crossopteryx febrifuga and Piliostigma thonningii). Crossopteryx has a higher intrinsic resistance to fire (bark properties) than Piliostigma: a 20-mm-diameter stem of Crossopteryx survives exposure to 650°C, while Piliostigma needs a diameter of at least 40 mm to survive. Crossopteryx has a thicker trunk than Piliostigma: for two trees of the same height, the basal diameter of Crossopteryx will be 1.6 times greater. Piliostigma grows 2.26 times faster than Crossopteryx between two successive fires. The two species have different fire resistance strategies: one relies on resistance of aboveground structures to fire, while the other relies on its ability to quickly re-build aboveground structures. Crossopteryx is able to recruit in almost any fire conditions while Piliostigma needs locally or temporarily milder fire conditions. In savannas, fire resistance is a complex property which cannot be assessed simply by measuring only one of its components, such as bark thickness. Bark properties, trunk profile and growth rate define strategies of fire resistance. Fire resistance may interact with competition: we suggest that differences in fire resistance strategies have important effects on the structure and dynamics of savanna ecosystems. Received: 16 August 1996 / Accepted: 4 January 1997     

Carbon balance of a tropical savanna of northern Australia

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO₂ efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204±53 ton C ha^–1, with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0–1 m) was 151±33 ton C ha^–1, accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53±20 ton C ha^–1, 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha^–1, of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha^–1 year^–1, of which 8.0 ton C ha^–1 (73%) was contributed by below-ground net primary production and 3.0 ton C ha^–1 (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha^–1 year^–1. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5–6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha^–1 year^–1.     

Elevated CO2 enhances resprouting of a tropical savanna tree

The savannas (cerrado) of south-central Brazil are currently subjected to frequent anthropogenic burning, causing widespread reduction in tree density. Increasing concentrations of atmospheric CO₂ could reduce the impact of such frequent burning by increasing the availability of nonstructural carbohydrate, which is necessary for resprouting. We tested the hypotheses that elevated CO₂ stimulates resprouting and accelerates replenishment of carbohydrate reserves. Using a factorial experiment, seedlings of a common Brazilian savanna tree, Keilmeyera coriacea, were grown at 350 ppm and 700 ppm CO₂ and at two nutrient levels. To simulate burning, the plants were either clipped at 15 weeks or were left unclipped. Among unclipped plants, CO₂ and nutrients both stimulated growth, with no significant interaction between nutrient and CO₂ effects. Among clipped plants, both CO₂ and nutrients stimulated resprouting. However, there was a strong interaction between CO₂ and nutrient effects, with CO₂ having a significant effect only in the presence of high nutrient availability. Under elevated CO₂, carbohydrate reserves remained at higher levels following clipping. Root total nonstructural carbohydrate remained above 36% in all treatments, so carbohydrate reserves did not limit regrowth. These results indicate that under elevated CO₂ this species may be better able to endure the high frequency of anthropogenic burning in the Brazilian savannas. Received: 19 May 1999 / Accepted: 8 November 1999     

The dynamics of three shrub species in a fire-prone temperate savanna: the interplay between the seed bank, seed rain and fire regime

A model was developed to assess how the seed rain and fire regime affect seed bank dynamics and seedling establishment of three native shrub species (Acanthostyles buniifolius, Baccharis pingraea and Baccharis dracunculifolia) with different regeneration strategies, in temperate South American savanna. Seed bank and seed rain were quantified for each species under different fire regimes, and their relative roles in regeneration were evaluated. All species had short-term persistent seed banks and high annual variability in seed production. A high proportion of seeds deposited in the seed rain produced seedlings after fire; few entered the soil seed bank. Fire killed a high proportion of the seeds in the soil seed bank. Seedlings derived from the seed rain had a higher probability of surviving for 2 years than seedlings emerging from the soil seed bank. In the absence of fire, establishment depended on germination both from the seed rain and the soil seed bank, whereas with annual fire, establishment was primarily dependent on germination of seeds arriving in the annual seed rain, regardless of species’ regeneration strategies. These results help to explain changes in the vegetation of South American temperate savannas as a result of changes in fire regime and grazing management during the last 50 years. By revealing the crucial roles of the soil seed bank and seed rain in regeneration, this study provides vital information for the development of appropriate management practices to control populations of shrub species with different regeneration strategies in South American temperate savannas.     

The quantification of condensed tannins in African savanna tree species

We compared Quebracho with Sorghum tannin as standards for condensed tannin (CT) quantification in selected African savanna tree species in relation to the acid-butanol assay for CTs. Without exception, the use of Quebracho tannin as standard overestimated CTs, ranging from 0.7 to as much as 8.3 times. Sorghum tannin underestimated CTs by 0.26–0.79 times, except in one species where there was no difference in the CT concentration. Condensed tannins in African savanna trees showed qualitative and quantitative differences in chemical composition which may explain the variable reactivity in the acid-butanol assay. We propose the use of condensed tannins purified from the plant under investigation be used as standard since it will closely represent the CT structure and presumably chemical reactivity in the acid-butanol assay.     

Post-fire regeneration strategies and tree bark resistance to heating in frequently burning tropical savanna woodlands and grasslands in Ethiopia

Regeneration mechanisms of vegetation and the role of tree bark resistance to frequent fire were studied in savanna woodlands and grasslands in Gambella, Western Ethiopia. Data were collected from four sites, each with three replicate plots. The variation between sites in species composition and biomass correlated with the differences in fire intensity. Foliar cover was recorded for individual plant species regenerating by sprouting from older parts of plants that had survived fire or by seedlings; records were made during the dry season and at the beginning of the wet season. Data on bark thickness and tree diameters of 12 dominant tree species were also recorded. Both facultative and obligate sprouters significantly contributed to post‐fire recovery, comprising 98.5 % of total vegetation cover. The contribution of seedlings to cover and abundance immediately following fire was negligible, but seedling density increased in the beginning of the rainy season, 4 to 5 months after fire. The importance of the sprouting and seeding strategies varied between the different plant growth forms. The highest contribution to cover and frequency was made by the most abundant grass species, which reproduced in both ways. Facultative sprouters made up 67.3 % of the vegetation cover, out of which 54 % consisted of grasses. Broad‐leaved herbs and trees/shrubs regenerating mainly by sprouting made up 31.3 % of the vegetation cover. Adaptations to fire in tree species seemed to include the development of a thick bark, once the tree has passed seedling stages. Tree bark thickness and tree diameter at breast height were strongly correlated with the time taken for cambium to reach an assumed lethal temperature of 60°C when exposed to fire, which indicated that mature trees with thick barks might resist stronger fire better than, e.g., small or young trees and trees with thin bark. However, for a given bark thickness the cambium resistance to heat varied three‐fold among species. Hence, site differences in fire intensity seemed to influence the distribution of trees depending on their bark characteristics and resistance to fire.     

Pollen movement to flowering canopies of pistillate individuals of three rain forest tree species in tropical Australia

This study measured the quantities of effective pollen vectors and their pollen loads arriving at the canopies of dioecious tropical rain forest trees in north-east Queensland. Population flowering synchrony, effective pollinator populations and pollen loads transferred between staminate and pistillate trees were compared among three insect-pollinated tree species. All three were visited by a wide range of insects, 75% of which (mostly 3–6 mm long) carried conspecific pollen. Fewer than 8% of individual insects were found to be carrying single-species pollen exclusively and none could be described as specialist pollen foragers. The introduced honeybee carried greater quantities of pollen than any native species but was not necessarily a reliable pollinator. The brief flowering periods in Neolitsea dealbata (3–4 weeks) and Litsea leefeana (4–5 weeks) populations were synchronized among individuals. Flowering in the Diospyros pentamera population extended over 15 weeks and most individuals were in flower for most of this period. Staminate trees began flowering earlier, produced more flowers and attracted relatively more insects than did pistillate trees, suggesting a density-dependent response of pollinators to flowering performance. Pollen was trapped in greater quantities on insects at staminate trees than at pistillate trees. Insect numbers increased at peak flowering periods and Diptera were the most abundant flower visitors. Anthophilous Coleoptera were more numerous at staminate than at pistillate trees in all three tree species populations. Larger quantities of pollen were mobilized during peak flowering times although the greatest quantities were transferred to pistillate canopies towards the end of the population flowering periods. Diptera carried pollen more often to pistillate N. dealbata and L. leefeana trees than did other groups whereas Coleoptera carried pollen more often to pistillate D. pentamera trees. The two contrasting flowering performances in the three tree species are discussed with reference to mechanisms that facilitate pollen transfer between staminate and pistillate trees.     

Wood anatomy and uses of 7 tropical tree species from Mexico

The wood anatomy of Coccoloba cozumelensis Hemsl., Coccoloba spicata Lundell, Gymnanthes lucida Sw., Blomia cupanioides Miranda, Canella winterana (L.) Gaertn., Aspidosperma megalocarpon Müell Arg. and Ehretia tinifolia L., is described. One tree per species was collected in the tropical rain forest of Quintana Roo, Mexico. Their wood has important traditional uses in furniture, tools, rural buildings, posts, fences, railroads and firewood. Macroscopic and microscopic characteristics were described and measured in wood samples, permanent slides and macerated material. These species have diffuse porosity, alternate vessel pits, simple perforation plates, numerous and small rays; libriform fibres are common, as well as ergastic material in gum forms, calcium carbonate and silica crystals. These characteristics explain aesthetical, weight, hardness and resistance (to mechanical and biological damage) characters that fit traditional use by the Maya.     

The water relations of two evergreen tree species in a karst savanna

The ecohydrology of karst has not received much attention, despite the disproportionally large contribution of karst aquifers to freshwater supplies. Karst savannas, like many savannas elsewhere, are encroached by woody plants, with possibly negative consequences on aquifer recharge. However, the role of savanna tree species in hydrological processes remains unclear, not least because the location and water absorption zones of tree roots in the spatially complex subsurface strata are unknown. This study examined the water sources and water relations of two savanna trees, Quercus fusiformis (Small) and Juniperus ashei (Buchholz) in the karst region of the eastern Edwards Plateau, Texas (USA). Stable isotope analysis of stem water revealed that both species took up evaporatively enriched water during the warm season, suggesting a relatively shallow water source in the epikarst, the transition zone between soil and bedrock. Q. fusiformis had consistently higher predawn water potentials than J. ashei during drought, and thus was probably deeper-rooted and less capable of maintaining gas exchange at low water potentials. Although the water potential of both species recovered after drought-breaking spring and summer rain events, associated shifts in stem water isotope ratios did not indicate significant uptake of rainwater from the shallow soil. A hypothesis is developed to explain this phenomenon invoking a piston-flow mechanism that pushes water stored in macropores into the active root zones of the trees. Epikarst structure varied greatly with parent material and topography, and had strong effects on seasonal fluctuations in plant water status. The study suggests that tree species of the Edwards Plateau do not commonly reduce aquifer recharge by tapping directly into perched water tables, but more likely by reducing water storage in the epikarst. A more general conclusion is that models of savanna water relations based on Walter's two-layer model may not apply unequivocally to karst savannas.     

Larval anisakid infections of some tropical fish species from north-west Australia

Despite the commercial and zoonotic importance of larval anisakid infestations of teleosts, their distribution among Australia's diverse marine fish fauna is poorly understood. A preliminary survey of Australia's tropical north-west revealed a generally high prevalence of larval anisakids representing four genera (Anisakis, Terranova, Thynnascaris and Raphidascaris) among only seven fish species. The potential impact of high larval anisakid infections on both the health of recreational fishermen and aquaculture environments is discussed.     

Frequency of cyanogenesis in tropical rainforests of far north Queensland, Australia

BACKGROUND AND AIMS: Plant cyanogenesis is the release of toxic cyanide from endogenous cyanide-containing compounds, typically cyanogenic glycosides. Despite a large body of phytochemical, taxonomic and ecological work on cyanogenic species, little is known of their frequency in natural plant communities. This study aimed to investigate the frequency of cyanogenesis in Australian tropical rainforests. Secondary aims were to quantify the cyanogenic glycoside content of tissues, to investigate intra-plant and intra-population variation in cyanogenic glycoside concentration and to appraise the potential chemotaxonomic significance of any findings in relation to the distribution of cyanogenesis in related taxa. METHODS: All species in six 200 m(2) plots at each of five sites across lowland, upland and highland tropical rainforest were screened for cyanogenesis using Feigl-Anger indicator papers. The concentrations of cyanogenic glycosides were accurately determined for all cyanogenic individuals. KEY RESULTS: Over 400 species from 87 plant families were screened. Overall, 18 species (4.5 %) were cyanogenic, accounting for 7.3 % of total stem basal area. Cyanogenesis has not previously been reported for 17 of the 18 species, 13 of which are endemic to Australia. Several species belong to plant families or orders in which cyanogenesis has been little reported, if at all (e.g. Elaeocarpaceae, Myrsinaceae, Araliaceae and Lamiaceae). A number of species contained concentrations of cyanogenic glycosides among the highest ever reported for mature leaves-up to 5.2 mg CN g(-1) d. wt, for example, in leaves of Elaeocarpus sericopetalus. There was significant variation in cyanogenic glycoside concentration within individuals; young leaves and reproductive tissues typically had higher cyanogen content. In addition, there was substantial variation in cyanogenic glycoside content within populations of single species. CONCLUSIONS: This study expands the limited knowledge of the frequency of cyanogenesis in natural plant communities, includes novel reports of cyanogenesis among a range of taxa and characterizes patterns in intra-plant and intra-population variation of cyanogensis.     

The influence of cattle grazing on tree mortality after drought in savanna woodland in north Queensland

Abstract Live and dead trees were measured in macropod-grazed and cattle-grazed Eucalyptus woodland in north Queensland following a severe drought between 1992 and 1994. ANCOVA revealed no effect of grazing treatment on the proportion of drought death. Twenty-seven per cent of all tree species were killed by the drought and the value ranged from about 4% for Corymbia dallachiana (Benth.), K. D. Hlll & L. A. S. Johnson to 29% for the dominant species Eucalyptus xanthoclada Brooker and A.R.Bean, although differences were non-significant. There was also no significant difference in mortality between poles (< 10 cm d.b.h.) and trees (> 10 cm d.b.h.). The study highlights a natural (as in pre-European), catastrophic structural collapse of tropical eucalypt woodland. Presumably the dramatic declines in basal area as a result of drought are recovered during successive relatively wet years by thickening events, although clearly this severe drought and its after-effects warrant further study.