Tree species exceeding 70 m in height are rare globally. Giant gymnosperms are concentrated near the Pacific coast of the USA, while the tallest angiosperms are eucalypts (Eucalyptus spp.) in ...southern and eastern Australia. Giant eucalypts co-occur with rain-forest trees in eastern Australia, creating unique vegetation communities comprising fire-dependent trees above fire-intolerant rain-forest. However, giant eucalypts can also tower over shrubby understoreys (e.g. in Western Australia). The local abundance of giant eucalypts is controlled by interactions between fire activity and landscape setting. Giant eucalypts have features that increase flammability (e.g. oil-rich foliage and open crowns) relative to other rain-forest trees but it is debatable if these features are adaptations. Probable drivers of eucalypt gigantism are intense intra-specific competition following severe fires, and inter-specific competition among adult trees. However, we suggest that this was made possible by a general capacity of eucalypts for ‘hyper-emergence’. We argue that, because giant eucalypts occur in rain-forest climates and share traits with rain-forest pioneers, they should be regarded as long-lived rain-forest pioneers, albeit with a particular dependence on fire for regeneration. These unique ecosystems are of high conservation value, following substantial clearing and logging over 150 yr.
Strawberry guava (Psidium cattleianum) is a shade‐tolerant shrub or small tree invader in tropical and subtropical regions and is considered among the world's top 100 worst invasive species. Studies ...from affected regions report deleterious effects of strawberry guava invasion on native vegetation. Here we examine the life history demographics and environmental determinants of strawberry guava invasions to inform effective weed management in affected rainforest regions. We surveyed the vegetation of 8 mature rainforest and 33 successional sites at various stages of regeneration in the Australian Wet Tropics and found that strawberry guava invasion was largely restricted to successional forests. Strawberry guava exhibited high stem and seedling densities, represented approximately 8% of all individual stems recorded and 20% of all seedlings recorded. The species also had the highest basal area among all the non‐native woody species measured. We compared environmental and community level effects between strawberry guava‐invaded and non‐invaded sites, and modelled how the species basal area and recruitment patterns respond to these effects. Invaded sites differed from non‐invaded sites in several environmental features such as aspect, distance from intact forest blocks, as well as supported higher grass and herb stem densities. Our analysis showed that invasion is currently ongoing in secondary forests, and also that strawberry guava is able to establish and persist under closed canopies. If left unchecked, strawberry guava invasion will have deleterious consequences for native regenerating forest in the Australian Wet Tropics.
Climate change could alter the biogeography of many tree species. However, there have been few studies of tree growth across climatic gradients at a continental scale. Callitris columellaris is a ...widespread conifer that spans many climates and landscape positions across Australia. Our aim was to determine how stem radial growth of C. columellaris varies with tree size and with the biogeographic factors of rainfall, temperature, soil fertility and inter-tree competition. We sampled cores from trees at 85 sites in biomes ranging from tropical savanna to arid desert and temperate forest, and measured widths of the 100 outermost growth rings. We analysed ring width in relation to changes in tree age and diameter, and also evaluated the influence of the biogeographic factors on the width of the ten most recently formed rings. The average width of outermost rings varied only slightly with stem diameter, because the decrease in ring width with age and diameter within trees is offset by an increase with diameter among trees. Our analyses thus explain the weak, inconsistent relationships often observed between stem diameter and growth rate amongst trees. The most important biogeographic factors were the climatic ones: across Australia, ring width increased with both mean annual rainfall and mean annual temperature. These relationships were largely driven by continental scale differences between the tropical and the southern (arid plus temperate) sites, while relationships within climate zones were comparatively weak. Ring width decreased with intense inter-tree competition but showed little correlation with available soil nitrogen or phosphorus.
The process that transforms conductive sapwood to non-conductive heartwood in trees is poorly understood. Here, we use natural variation in climate to examine the environmental control of sapwood ...width in a widespread conifer species. We hypothesised that if sapwood width is linked to transpirational load, there would be a positive association between sapwood width, and continental gradients in mean annual rainfall, whereas age-related conversion to heartwood would be revealed from estimates of the age of the inner-most sapwood ring. Using the widespread Australian conifer Callitris columellaris we took cores from trees at 85 sites spanning a range of 168–2,117 mm in mean annual rainfall, and 14–28 °C in mean annual temperature. We found that sapwood width was remarkably similar throughout the species range, being only slightly lower in the tropics than the arid or temperate zone. There was a weak negative relationship between sapwood width and mean annual rainfall, which is in the opposite direction expected from transpirational control of sapwood width. Sapwood growth rings were wider, but there were fewer of them in the tropics than elsewhere, indicating conversion to heartwood occurred earlier here. Together with an earlier finding that tracheid diameter was largest in the tropics, our results show that differences amongst climate zones more strongly influence the hydraulic properties of sapwood than its amount.
Deadwood is a large global carbon store with its store size partially determined by biotic decay. Microbial wood decay rates are known to respond to changing temperature and precipitation. Termites ...are also important decomposers in the tropics but are less well studied. An understanding of their climate sensitivities is needed to estimate climate change effects on wood carbon pools. Using data from 133 sites spanning six continents, we found that termite wood discovery and consumption were highly sensitive to temperature (with decay increasing >6.8 times per 10°C increase in temperature)-even more so than microbes. Termite decay effects were greatest in tropical seasonal forests, tropical savannas, and subtropical deserts. With tropicalization (i.e., warming shifts to tropical climates), termite wood decay will likely increase as termites access more of Earth's surface.
Callitris is Australia’s most successful and drought tolerant conifer genus. Callitris species are distributed across a huge geographical range from rainforest to arid zones, and hence they provide a ...rare opportunity to view plant growth trends across the continent. Here, we make a continental-scale examination of how climate influences basal diameter growth in Callitris. We sampled a total of five species but focused effort (23 of 28 samples) on the most widespread species, C. columellaris. Cores from a total of 23 trees were sampled from 15 sites that spanned a gradient in mean annual rainfall from 225 to 2117 mm and mean annual temperature from 11.5 to 28.2°C. Ring production is not annual across much of the distribution of the genus, so 14C-AMS dating was used to establish the frequency of ring production for each core. Ring width, tracheid lumen diameter and number of tracheids per ring were also measured on each core. Ring production was close to annual at mesic sites with reliable alternation of rainfall or temperature regimes but was more erratic elsewhere. For C. columellaris, ring width significantly increased with mean annual rainfall (r 2 = 0.49) as a result of wider and more tracheids per ring. For this species tracheid lumen diameter was correlated with annual rainfall (r 2 = 0.61), with a threefold increase from the driest to the wettest sites, lending support to the hypothesis that conifers growing at drier sites will have narrow lumen diameters to maximise mechanical strength of the xylem.
Seasonally Dry Tropical Forests (SDTF) are gaining recognition as a significant biome and poorly conserved tropical biodiversity refuge. Understanding floristic relationships within SDTF is essential ...for their effective conservation. This study examines the floristic variation within SDTF of the Caatinga Biogeographic Domain, in north-eastern Brazil. SDTF trees, shrubs and arborescent cacti were sampled in six localities in the Brazilian states of Minas Gerais and Bahia. A number of soil chemical attributes were also measured to analyse vegetation-soil relationships across the study sites. Five floristically distinct SDTF communities were delineated. Differences in these communities are attributable to influences from adjacent savanna and rain forest surrounding the study sites, and also to soil properties such as Al3+, base saturation and gravel content. The high species richness of, and species dissimilarity between communities reflect the need for judicious conservation planning for SDTF that account for biodiversity values and forest structural integrity.
•Fires are becoming more common with global warming.•Savanna species are more flammable than forest species.•Leaf traits drive these differences in flammability to different extents.•Using less ...flammable species as green firebreaks can help protect biodiversity.
Climate change has led to increased fire risk in many tropical regions, with concerning implications for fire-sensitive forests. Understanding the comparative fire ecology of co-occurring fire-prone and fire sensitive habitats and the drivers of canopy fires is paramount for recognizing fire risk and for selecting plants that may be planted in green firebreaks that could mitigate wildfire damage to these habitats. However, baseline plant flammability data is needed from both habitats. To accomplish this, we compared three flammability measures (maximum temperature, total burn time and burnt biomass) by burning branches of 35 species in a fire-protected forest and fire-prone savanna in tropical northeast Australia. We also measured a set of leaf functional traits (leaf area, leaf mass per unit area and leaf dry matter content) on these species and examined their relationship with shoot flammability. We found that maximum temperature, burnt biomass, and burn time were significantly higher in savanna compared to forest species. Leaf area and leaf mass per unit area did not influence flammability measures, but species with higher leaf dry matter content had a higher percentage of biomass burnt, burnt for longer and hotter. Underpinning these observations, savanna species had significantly higher leaf dry matter content than forest species. Our results enable us to recommend species with low flammability that could be used in the green firebreaks in the study area to mitigate fire risk in sensitive forest habitats. Future studies should look experimentally at the effectiveness of green firebreaks using low flammability species, and examine their post fire recovery.
•Post-fire responses in forests, savannas and their ecotones is proposed to differ.•Two years post-fire, forest and ecotone had higher species diversity than savanna.•Some different patterns of ...change were found, but mostly ecotones resembled forests.•Forests and savannas are alternative stable states with different dynamics.•In the short term, ecotones may behave more like forests in their fire responses.
Fire is a major environmental factor influencing vegetation heterogeneity, with closed forest and savanna ecosystems having different management needs due to their different responses to fire disturbance. However, the differences in post-fire vegetation dynamics between these ecosystems have seldom been compared using a uniform set of parameters. Additionally, post-fire dynamics of forest-savanna ecotones is poorly characterized. With the hypothesis that closed forest, savannas and ecotones will exhibit different post-fire responses, we studied the vegetation diversity, structure and dynamics in an upland forest-savanna vegetation mosaic in Minas Gerais, Brazil following a fire that occurred in September 2011. In January 2012, we identified, tagged, and measured the basal diameter of all regenerating juvenile tree stems within forty-six 4m2 plots in closed forest, savanna and ecotone vegetation, and conducted recensuses in 2013 and 2014. We modelled the relationship between short-term dynamics parameters (recruitment, mortality, basal area loss and gain, and the turnover and net changes in the number of individual stems and basal areas) and vegetation type. Species diversity was higher in closed forests and ecotones than in savanna. Across all vegetation types, stem density decreased and basal area increased. Parameters such as recruitment, net changes in the number of individuals, and the gain, loss and turnover in basal area did not differ across vegetation types. However, stem mortality was higher in closed forest and ecotones combined than in savannas, and the net change in the number of individuals was the lowest in the savanna. Overall, our results support that within a climatically-similar vegetation mosaic, closed forests exhibit different post-fire regeneration dynamics from savanna as expected. Ecotones exhibited post-fire responses and dynamics more similar to closed forests than to savanna, but more studies will be needed to establish if this pattern is applicable to other areas. Understanding the longer-term vegetation dynamics and plant regeneration patterns is a potential next step that will help inform fire management strategies for forest-savanna mosaics.
Seasonally Dry Tropical Forests (SDTF) are gaining recognition as a habitat of global conservation significance. However, these forests often occur on rich soils, and are therefore prone to ...disturbance by agricultural activities. Across their range, many SDTFs are in various stages of regeneration, although little is known about their regeneration dynamics and temporal change in general. To fill this knowledge gap, we studied the changes in species diversity and community structure of an SDTF tree community with a history of past selective logging located in northern Minas Gerais, Brazil. We hypothesized an increase in species diversity and biomass-related parameters, along with a reduction in stem density for the community after five years of regeneration. In 2005, we measured all trees (diameter ≥ 3.18 cm at 1.3 m) within twenty 20 x 20 m (400 m²) plots in the patch, and we remeasured the trees in 2010. Between the two censuses, we found no significant changes in species richness and diversity, suggesting that species accumulation had leveled out. However, we found significant gains in tree basal area and recruitment, suggesting continual regeneration. Diameter class distributions of the five most abundant tree species showed similar patterns. We conclude that past disturbances can have ongoing effects on SDTF vegetation, although inter-annual variations may also play a regulatory role on community dynamics.
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