David Y. P. Tng

Giant eucalypts – globally unique fire‐adapted rain‐forest trees?

CONTENTS: Summary xa0xa0xa01 I. Introduction xa0xa0xa01 II. Giant eucalypts in a global context xa0xa0xa02 III. Giant eucalypts - taxonomy and distribution xa0xa0xa04 IV. Growth of giant eucalypts xa0xa0xa06 V. Fire and regeneration of giant eucalypts xa0xa0xa08 VI. Are giant eucalypts different from other rain-forest trees? xa0xa0xa09 VII. Conclusions 10 Acknowledgements 11 References 11 SUMMARY: Tree species exceeding 70xa0m 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 150xa0yr.

Humid tropical rain forest has expanded into eucalypt forest and savanna over the last 50 years

Tropical rain forest expansion and savanna woody vegetation thickening appear to be a global trend, but there remains uncertainty about whether there is a common set of global drivers. Using geographic information techniques, we analyzed aerial photography of five areas in the humid tropics of northeastern Queensland, Australia, taken in the 1950s and 2008, to determine if changes in rain forest extent match those reported for the Australian monsoon tropics using similar techniques. Mapping of the 1950s aerial photography showed that of the combined study area (64,430 ha), 63% was classified as eucalypt forests/woodland and 37% as rain forest. Our mapping revealed that although most boundaries remained stable, there was a net increase of 732 ha of the original rain forest area over the study period, and negligible conversion of rain forest to eucalypt forest/woodland. Statistical modeling, controlling for spatial autocorrelation, indicated distance from preexisting rain forest as the strongest determinant of rain forest expansion. Margin extension had a mean rate across the five sites of 0.6 m per decade. Expansion was greater in tall open forest types but also occurred in shorter, more flammable woodland vegetation types. No correlations were detected with other local variables (aspect, elevation, geology, topography, drainage). Using a geographically weighted mean rate of rain forest margin extension across the whole region, we predict that over 25% of tall open forest (a forest type of high conservation significance) would still remain after 2000 years of rain forest expansion. This slow replacement is due to the convoluted nature of the rain forest boundary and the irregular shape of the tall open forest patches. Our analyses point to the increased concentration of atmospheric CO2 as the most likely global driver of indiscriminate rain forest expansion occurring in northeastern Australia, by increasing tree growth and thereby overriding the effects of fire disturbance.

Plant Traits Demonstrate That Temperate and Tropical Giant Eucalypt Forests Are Ecologically Convergent with Rainforest Not Savanna

Ecological theory differentiates rainforest and open vegetation in many regions as functionally divergent alternative stable states with transitional (ecotonal) vegetation between the two forming transient unstable states. This transitional vegetation is of considerable significance, not only as a test case for theories of vegetation dynamics, but also because this type of vegetation is of major economic importance, and is home to a suite of species of conservation significance, including the world’s tallest flowering plants. We therefore created predictions of patterns in plant functional traits that would test the alternative stable states model of these systems. We measured functional traits of 128 trees and shrubs across tropical and temperate rainforest – open vegetation transitions in Australia, with giant eucalypt forests situated between these vegetation types. We analysed a set of functional traits: leaf carbon isotopes, leaf area, leaf mass per area, leaf slenderness, wood density, maximum height and bark thickness, using univariate and multivariate methods. For most traits, giant eucalypt forest was similar to rainforest, while rainforest, particularly tropical rainforest, was significantly different from the open vegetation. In multivariate analyses, tropical and temperate rainforest diverged functionally, and both segregated from open vegetation. Furthermore, the giant eucalypt forests overlapped in function with their respective rainforests. The two types of giant eucalypt forests also exhibited greater overall functional similarity to each other than to any of the open vegetation types. We conclude that tropical and temperate giant eucalypt forests are ecologically and functionally convergent. The lack of clear functional differentiation from rainforest suggests that giant eucalypt forests are unstable states within the basin of attraction of rainforest. Our results have important implications for giant eucalypt forest management.

Beta-diversity in seasonally dry tropical forests (SDTF) in the Caatinga Biogeographic Domain, Brazil, and its implications for conservation

Tropical biomes are species rich, but some biomes such as seasonally dry tropical forests (SDTFs) are still inadequately studied compared to their co-occurring rain forest and savanna. SDTFs occur in areas of high environmental heterogeneity, resulting in high beta (β)-diversity or species turnover, but this has so far only been accessed using a single β-diversity measure, and at a spatial scale that is of limited applicability for reserve planning. The Caatinga Biogeographic Domain in Brazil contains the largest known extent of SDTF which are poorly studied and inadequately reserved. We therefore studied the variation in species richness and species turnover among SDTF between localities and between known floristic communities. From six localities within the Caatinga Biogeographic Domain we recorded all tree species with a circumference at breast height equaling or exceeding 10xa0cm within 106 400xa0m2 survey plots. From the species presence/absence data we calculated three measures of β-diversity between pairs of study localities and between different floristic communities representing: (i) species similarity, (ii) differences between species richness, and (iii) species gain and loss. Our results confirm the high β-diversity of SDTFs and species turnover between localities and also between floristic communities. The three indices were also complementary to each other and can be used to maximize accuracy in β-diversity studies. The implications of our study for conservation and reserve planning of SDTFs are discussed.

Does moisture affect the partitioning of bryophytes between terrestrial and epiphytic substrates within cool temperate rain forests

Abstract Bryophyte communities are highly sensitive to moisture and/or humidity levels. Most studies on the subject focus on bryophytes on either tree or ground habitats and do not consider how bryophytes partition themselves across both ground and epiphytic substrates within the same forest. Sampling mesic temperate forest sites of the same physiognomy from two Tasmanian regions with slightly different moisture levels (a wetter northwest versus a drier northeast region), we examine various aspects of the community structure (overall liverwort and moss cover; species richness; and liverwort to moss ratios) of both tree and ground communities with respects to moisture availability. We then test the hypothesis that a wetter site will exhibit a greater magnitude of bryophytes inhabiting both tree and ground habitats. Results of the analyses show that the ground habitat in the northwest sites exhibited a significantly higher mean species richness, higher overall and mean liverwort to moss ratio, and a higher liverwort cover than the northeast sites. This suggests that the northwest had a more ameliorated ground microclimate than the northeast. In terms of habitat partitioning, a significantly higher percentage of taxa occupied both tree and ground habitats in the northwest, compared to the northeast, which exhibited a higher percentage of taxa restricted to trees. It is proposed that within a single vegetation type, a higher site moisture level may create microclimates conducive to more bryophyte species in both tree and ground habitats, especially the latter, thereby enabling taxa to colonize and coexist on both substrates more freely.

Continental-scale climatic drivers of growth ring variability in an Australian conifer

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 2117xa0mm 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 (r2xa0=xa00.49) as a result of wider and more tracheids per ring. For this species tracheid lumen diameter was correlated with annual rainfall (r2xa0=xa00.61), 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.

Seasonal pollen distribution in the atmosphere of Hobart, Tasmania: preliminary observations and congruence with flowering phenology

The atmospheric pollen loads of Hobart, Tasmania, Australia, were monitored between September 2007 and July 2009. To examine the match of the airborne pollen composition with the flowering duration of their contributing plants, the phenology of native and non-native plants in various habitats near the pollen-trapping site was undertaken between August 2008 and July 2009. The pollen load was found to have a strong seasonal component associated with the start of spring in September. This is incongruent with the peak flowering season of the total taxa in October. In most taxa, atmospheric pollen signatures appeared before flowering was observed in the field. The presence of most pollen types in the atmosphere also exceeded the observed flowering duration of potential pollen-source taxa. Reasons for this may be related to the sampling effort of phenological monitoring, pollen blown in from earlier flowering populations outside of the sampling area, the ability of pollen to be reworked, and the large pollen production of some wind-pollinated taxa. In 2007–2008, 15 pollen types dominated the atmosphere, accounting for 90% of the airborne pollen load. The top six pollen types belonged to Betula, Cupressaceae, Myrtaceae, Salix, Poaceae and Ulmus. Comparatively, the annual pollen load of Hobart is lower than in most other Australian cities; however, the pollen signal of Betula is inordinately high. Native plants play a minor role as pollen contributors, despite the proximity of native habitats to the pollen-sampling location. The implications of the aerobiological observations are discussed in relation to public health.

Letting giants be – rethinking active fire management of old‐growth eucalypt forest in the Australian tropics

Tall old-growth forests are of global social-economic, political nand ecological significance. These forests contribute significantly nto the global carbon budget and are of high nconservation value given sustained logging and clearing nover the past two centuries (Tng et al . 2012a). In Australia, nthese old-growth forests extend from tropical to temperate nregions of Australia in areas where rainfall exceeds n1000 mm per year, being characterized by emergent neucalypt trees attaining statures of 30 m to more than n80 m, with canopy and understorey layers consisting of nmesophytic broad-leaved trees and treelets, sclerophyllous nshrubs and graminoids (Fig. 1). These forests support some nof the tallest flowering plants in the world, are important nhabitats for a unique suite of flora and fauna, and are nimportant forest cover for metropolitan water catchments – nvalues that make giant eucalypt forests a focal point of nscientific study and ecotourism (Tng et al . 2012a). Until recently, these eucalypt forests were extensively nexploited as a timber resource, but now, most remaining nold-growth stands have been set aside for conservation. In nsome regions containing giant eucalypt forest, native nforestry activities either have ceased or are based on shortrotation nharvests of regrowth forests, meaning that the ntrees can never achieve their potential size. Regeneration ntypically occurs after landscape fires, and fire is also used to ninitiate regeneration of temperate eucalypts after logging nand to reduce fuel loads (Attiwill et al ., in press). In temperate nregions, fire management of the remaining stands of nold-growth giant eucalypt forest is largely based upon fire nsuppression and fuel reduction burning in surrounding nopen forests, as fires in giant eucalypt forests are extremely ndifficult to control because these forests are only flammable nunder dangerous fire weather conditions (Bowman et al . n2013). In subtropical and tropical forests, fuel reduction nburning is used to reduce fire hazard within the giant nforests as well as in adjacent open forests and savannas. What constitutes the most appropriate and ecologically nsustainable fire management practices of these giant nforests remains a controversial issue among scientists, nland managers and conservationists. Here, we outline nrecent advances in landscape ecology theory, palaeoecology nand functional biology research on a giant eucalypt nforest type in the Wet Tropics region of northeast Australia nto explore options to achieve sustainable management nof these systems. Given the ecological similarity between nAustralia’s giant eucalypt forests and other old-growth nforests in the Northern Hemisphere (Tng et al . 2012a), nthe insights gleaned herein have implications for a wide nrange of old-growth forests. Further, the expansion of nrain forest into surrounding savannas has implications for nthe management of savannas and grasslands, where there nremains debate as to whether increased woody biomass nshould be managed using fire or allowed to accumulate n(Bond & Parr 2010).

Variation in stem radial growth of the Australian conifer, Callitris columellaris, across the world’s driest and least fertile vegetated continent

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.

From desert to rainforest, sapwood width is similar in the widespread conifer Callitris columellaris

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 168–2,117xa0mm in mean annual rainfall, and 14–28xa0°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.