Tasmania’s giant eucalypts: discovery, documentation, macroecology and conservation status of the world’s largest angiosperms

Aims and objectives

There has been only one published paper on Tasmania’s tallest trees (Hickey et al. 2000), and none on Tasmania’s most massive trees. Comprehensive surveys of giant trees are lacking, particularly those investigating the local environmental controls and landscape settings. Such studies have been stymied by the lack of systematic remote-sensing and field surveys, high-quality climate data and geospatial data, which have become available only this century. Recently, giant trees have been discovered and documented by combining analysis of aerial survey LiDAR data with field exploration, including specialised tree-climbing techniques that enable accurate height and volume measurements (Sillett et al. 2015). The compilation of these decades of surveys in Victoria and Tasmania presents an important opportunity to undertake the first comprehensive analysis of the Tasmanian giant eucalypts, thereby positioning them into a global context. Specifically, we

Historical review

We reviewed historical written and photographic records of giant-tree measurement in Tasmania and Victoria to contextualise our contemporary measurements. Where possible, we found the primary source material in Tasmanian and Victorian state and university libraries.

Ecological survey

Prospective areas where giant trees may occur were identified by applying a range of methods and technologies including forestry-type maps, LiDAR (Supplementary Fig. S1), satellite imagery, and field reconnaissance (Supplementary Appendix S1 in Supplementary Material). In areas where giant and/or tall trees were found, the location of each giant tree was recorded using a Garmin eTrex 22 × GPS and assigned a field code and a name. During the field survey, we assessed understorey vegetation type around each tree and visually assessed the size-class distribution of the eucalypts within a 50-m radius of the tree.

Tree height

Total tree height was measured as the vertical distance between the mid-point of ground and the tallest part of the tree (Van Pelt 2001), irrespective of whether that tallest part was live foliage or a dead twig, branch, or trunk. The mid-point of ground was calculated by measuring the difference between the high point of ground and low point of ground and dividing by two; this is the best way to approximate the point at which the original seedling first emerged from the ground.

We used climber-deployed tape drops and/or laser rangefinder (methods specified for individual trees in Table 3) to determine tree height. Climber-deployed tape drop is generally considered very accurate, especially if the climber can get to within 2–3 m of the highest point of the tree and then use a measuring pole to touch the top leaves. However, it has the following disadvantages: (i) it is very time-consuming (rarely could you measure more than two trees per day with this method); (ii) there are significant physical dangers in climbing large, old eucalypts, especially when you need to get close to the top of dead or decaying crowns; and (iii) rain and wind can make it difficult to both set a climbing line in the tree to access the higher branches and successfully lower the tape to the ground. Therefore, we used climber-deployed tape drop only when initial laser estimates were higher than 89 m.

Trunk volume

Trunk volume is estimated by segmenting the trunk into a series of truncated cones of (mostly) decreasing diameter (Appendix S1). This requires measuring diameter at multiple known heights up the trunk of the tree, calculating the volume of each truncated cone and summing them. To estimate the trunk volume of Tasmania’s most massive trees, we initially measured likely contenders by using a relatively fast approach adapted from Van Pelt (2001) and Flint (2002). If this showed a tree to be close to or exceed 300 m³ in trunk volume, the volume was then estimated in more detail, as per Kramer et al. (2018).

Initially, by using a tape, we measured the diameter at the high point of the ground and at 1.4 m above this. We combined these lower trunk diameters with trunk diameters measured at 5-m intervals up the bole, calculated with a hand-held Relaskop (Macroscope 25/45 8 × 30 ocular, RF Interscience) in conjunction with a laser rangefinder (Nikon Forestry Pro I and II). When trunks were noticeably oval or irregular, measurements were made from various angles and averaged.

For those trees found to be likely to be 300 m³ in trunk volume, more detailed measurements were made, as follows:

Fig. 1.

An example of a massive tree (AR1, trunk volume 328 m³). (a) The base of the tree with a person for scale (photo: B. Mifsud). (b) A base-map model, showing a series of horizontal cross-sections at various heights up the lower trunk, coloured according to height. (c) It is produced using photogrammetry or LiDAR scanning and allows a detailed volume estimate of the complex lower portion of the tree.

Fig. 2.

Climbers in the process of measuring tree-trunk diameters by using the tape-wrapping method. (Photo: C. Hansen.)

Fire-killed trees

On multiple occasions between 2015 and 2022, we visited the areas burned by wildfires between 2010 and 2019, to assess the condition of previously measured giant trees. We documented those trees found to be fire-killed, and monitored the condition of fire-damaged trees. Using information from this and other sources, we report the largest trees killed by fire between 2000 and 2023.

Data analyses

Using the coordinates of each measured tree, we derived elevation, aspect, and slope (confirmed from field notes) from Listmap (Department of Natural Resources and Environment Tasmania; https://maps.thelist.tas.gov.au/listmap/app/list/map). We classified the stand containing each giant tree as single- or multi-aged on the basis of field data and by analysis of Listmap LiDAR data (Tasmanian Government 2023a). The stand was not classed as multi-aged if there was eucalypt regrowth from human-induced changes such as logging and roads nearby. Preliminary inspection of the data showed no clear north–south difference, but showed an apparent difference between east- and west-facing aspects. Therefore we tested post hoc whether there was an effect of aspect on the count of (i) large trees and (ii) tall trees. Aspect was classed as ‘easterly’ (1–179°) or ‘westerly’ (181–359°), and a binomial test was used to test whether the counts were significantly different, by using the base package of the software R (R Core Team 2024). These models were used to detect statistical differences rather than to predict geographic distributions, given biases in the distribution of remaining old-growth forests arising from accessibility, logging etc. For Tasmanian trees with trunk volume of ≥200 m³, we examined allometric relationships between trunk volume and each of height, diameter at breast height and diameter at 10-m height, using scatter plots and correlation analyses. Using generalised linear modelling, we tested whether there was a difference between live and dead giant trees in the volume–diameter relationship.

Climate analyses

We extracted data to describe the climate envelope of Tasmanian and Victorian wet forests, and the climate of the specific locations of the tallest and most massive eucalypts. Gridded climatic means for the 1981–2020 period were obtained from CHELSA (Karger et al. 2017) at a resolution of 1-arc-second (approximately 700 m at the study-area latitude), for mean annual precipitation, mean annual temperature, and mean monthly climate moisture index (CMI). Values of these measures were extracted from the grids for the location of each tree. The Australian National Vegetation Inventory System (NVIS 4.1; Department of the Environment 2012) vegetation community polygons were cropped to the area of interest encompassing Tasmania, eastern Victoria, and a portion of south-eastern New South Wales near the Victorian border (MGA Zone 55, eastings 52,665–822,568, northings 5,113,824–6,043,746). Vegetation classes were simplified into temperate rainforest (NVIS Code 1), wet forest (NVIS Code 3), and open forests (NVIS Codes 4, 5, 8, 28, 54 and 60) groups. Cell values for climate variables were extracted for each cell centre within these three aggregated vegetation types, and plots were produced, showing the distribution of identified trees and the broader vegetation polygons across these climate variables, for both tall and massive trees, and Tasmanian and Victorian trees.

We used generalised linear models to test whether there was a difference in climate between where tall and massive trees are located. We used the the following three climatic variables: mean annual temperature (MAT), mean annual precipitation (MAP) and CMI, which was calculated as precipitation (mm) – potential evapotranspiration (mm) (Hogg 1997). State (Victoria and Tasmania) was also included in these models. We also tested whether there was a difference in these climate variables between the locations of the recently dead Tasmanian trees that had previously been measured to have a trunk volume of ≥250 m³ and their 11 counterparts that were still alive in 2022/23, acknowledging that there is potential autocorrelation in these data.

History of giant-tree measurement in Tasmania and Victoria

Most of the tallest and most massive eucalypts are found in south-eastern Australia, in the mainland state of Victoria and the island state of Tasmania (Williams et al. 2023). Since the mid-19th century, Victoria has developed a rich literature on its tallest (Caire 1905; Hardy 1911, 1921, 1923; Simpfendorfer 1982; Mace 1996; Bowman 2001; Mifsud 2003, 2012) and most massive (Hardy 1935; Mifsud and Harris 2016) trees. Unlike Victoria, which has numerous historical reports of very tall trees from a variety of districts (Caire 1905; Cornthwaite 1925a, 1925b; Hardy 1935; Griffiths 1992), there are relatively few historical records of trees over 90 m tall being measured in Tasmania in the period from settlement to the early 1900s. Two blue gums (E. globulus), one at Mount Wellington and another on the Huon River, were recorded as being 101 m tall (Lewin 1906. A 97-m blue gum was recorded from about 1890 without listing a locality and a 94-m blue gum was recorded along the Huon Road in 1911. The recorded maximum heights are substantially greater in Victoria than Tasmania: superlative tree heights listed in Victoria range from a believable 114 m (Guinness World Records 2023b) to 143 m (Mace 1996). Last, all the tallest historical recorded trees in Tasmania were species of E. globulus, whereas all the historical tall-tree heights from Victoria were of E. regnans trees. Whereas there are only a few surviving photographs taken of very large trees in Tasmania from the mid-1800s to the early 1900s, the giant trees in photos from early colonisation in Tasmania appear no larger than trees currently known. This is unlike Victoria, where photographed trees of the mid- to late 1800s dwarf any trees alive now (Fig. 3). Possible explanations for the differences between the Tasmanian and Victorian records include the following: a greater interest in tall-tree measurements in Victoria in the late 1800s, spurred on by the influential botanist, Baron Von Mueller (Mace 1996); exaggeration by early colonists in Victoria; Eucalyptus globulus is a tree species that is more widespread and existed closer to colonial settlements in Tasmania and was the tree early settlers measured, rather than the more dense forest species, E. regnans; overestimations of tree height because of poor surveying techniques (or even recording rough height estimations as carefully measured ‘facts’). Therefore, whereas the reported heights for historical Tasmanian trees are significantly lower than those often quoted from Victoria from the mid- to late 1800s, the accuracy of these earlier measurements cannot be verified. As Tasmania entered the era of industrial forestry, foresters would measure tree heights in designated coupes before they were logged and occasionally reported heights of very tall trees to the then Tasmanian Forest Commission. For example, in the late 1950s, the newsprint manufacturing company Australian Newsprint Mills (ANM) reported significant discoveries of tall trees in the following two locations in the Styx Valley: a 98-m tall E. regnans tree was measured in what is now known as the Styx Big Tree reserve, and a few kilometres to the east, at what is now known as the Andromeda reserve, 11 E. regnans trees were measured being >90 m tall, with the tallest of these being 98.7 m. These measurements were summarised by Mount (1960). In their paper ‘Tasmania’s Tallest Trees’, Hickey et al. (2000) compiled data from an unpublished report by Kostoglou (2000). Before this, apart from Mount (1960), all formal measuring and recording of tall (or massive) tree data was unpublished and held by the Tasmanian Government Forestry department (Forestry Commission 1947–1994, Forestry Tasmania 1994–2016, Sustainable Timber Tasmania 2016–present) (Felton 2006). One such report recorded tree heights measured in the Styx and Florentine valleys by a registered surveyor, D. G. Potter, in October 1987 (Potter 1987). The tallest tree recorded there was a E. regnans 90.1 m tall, with no others recorded being >90 m. The same 90.1-m tree was remeasured and reported in the 2000 paper to be 92 m tall, but remeasurement via climber tape-drop in 2001 found that the tree was 87 m. This is a clear example of how evolving surveying techniques have affected tree-height measurement and, hence, cautions uncritical acceptance of historical measurements. Unlike the numerous measurements and reports of tall trees in Tasmania over the past seven decades, there has been little recognition of massive trees over that time. The only recorded measurements of a massive tree before the 1990s were from a specimen growing on Nicholls Spur near Junee, which was cut down by ANM for its paper mill in 1945. Once felled, measurements taken near the base and at various points along the fallen trunk indicate a trunk volume of approximately 330 m³ (Helms 1945). From the accompanying photographs, the tree looked in excellent health and the cut trunk displayed no evidence of decay. Although not being quite as massive as the top four living trees measured in this study, it ranks in the top-10 most massive trees ever measured in Tasmania.

Fig. 3.

An example of a past giant tree from Gippsland, Victoria, from the late 19th century. The Bulga Stump was reported as being 10.7 m in diameter (111 feet girth) (State Library of Victoria 1914) and capable of holding eleven horses in its hollow base. Early settlers often lived their lives among the ‘skeletons’ of the burnt forest. Their very livelihoods relied on them being able to clear and farm the land where the former, centuries old forest stood. (Photo: Anon. 1914.)

Tallest trees

The three tallest living trees were all E. regnans and measured 96 m tall; one had a live top and two had tops that were long dead (Table 3, also see figs 2, 10). The 96 m live-topped tree ‘ED1’, commonly known as ‘Centurion’, was discovered by LiDAR in 2008 and initially measured by climber-deployed tape drop to be 99.6 m tall. It was subsequently remeasured to be 99.82 m tall in 2018. Unfortunately, the tree was affected by the 2019 bushfires, and the tallest branches were damaged by ember attack at the junction, with the main trunk about 85 m above ground level, and subsequently fell. Tree ‘AN1’ was the tallest measured tree remaining in the Andromeda stand of tall trees in the Styx Valley. It was first climbed in 2005 and measured by tape-drop to be 97 m and appears to have lost a further metre of height in the intervening years. A further 17 trees were measured to be >90 m tall. All of these were E. regnans, except for a single specimen of E. globulus, which was 90.7 m tall. Both E. obliqua (88.5 m) and E. tasmaniensis (86 m) also had individual trees measured to exceed 85 m in height. The crowns of many of the tallest trees were unhealthy; 6 of the tallest 10 and 13 of the tallest 20 trees had dead or dying tops (Fig. 4). Most of the tallest trees were old; only one tree taller than 90 m was from a younger age class (Tree MR1, approximately 220 years old). A E. viminalis tree, listed as being 88.9 m tall by Hickey et al. (2000), was reported as unhealthy in 2016 (Wondermondo, undated), and dead by 2020 (Steane 2020), and was excluded from the current list.

Fig. 4.

Declining crown health of a giant Eucalyptus regnans tree in the Andromeda reserve Styx Valley. This small stand has 11 trees over 90 m in height, but all have dead or declining crowns. (Photo: Brett Mifsud.)

Massive trees

The most massive tree measured was WH1, a E. regnans with a calculated trunk volume of 463 m³ (Fig. 5, Table 4). That total would be over 480 m³ when the main branches were added and would likely be above 490 m³ if all wood from the smaller branches was included. A further seven trees exceeded 300 m³ in trunk volume. Although five of these were E. regnans, both E. globulus and E. obliqua also produced individuals with a total wood volume greater than 300 m³. Furthermore, we documented 32 trees with estimated trunk volumes of more than 250 m³ and 116 trees exceeding 200 m³. One tree with a girth of 18.5 m and an estimated trunk volume of 330 m³ was logged in 1945.

Fig. 5.

Examples of the lower trunk of giant Tasmanian trees. (a) The lower 10 m of WH1, the largest tree by wood volume in Tasmania. (b) Tree SX1, the second-largest tree by wood volume. (c) Tree BM1, measured to 7.32 m in diameter (23 m girth), had the largest diameter (at 1.4 m height) of any measured tree in Tasmania. (Photos: B. Mifsud.)

Allometry of giant trees

Trunk volume of massive living trees was strongly correlated with diameter at 10-m height (r = 0.83) and weakly correlated with diameter at breast height (r = 0.39), but there was no correlation with height (r = 0.10) (Fig. 6). Diameter at 10 m height is strongly related to trunk volume (Fig. 5b) because it is largely free of the influences of irregularities, such as buttresses and burls, which can distort diameter measurements at breast height. Therefore, this index of stem diameter should be used in subsequent measurements of living and dead massive trees. Dead trees had significantly greater trunk volume (P < 0.001) for a given diameter at breast height than did living massive trees. The cause for this difference is unclear and may reflect geographic differences in tree shape associated with burned and unburnt areas, or variation in measurement of diameter at breast height, which is vulnerable to inaccuracies because of buttressing and sloping ground.

Fig. 6.

Allometry of giant Tasmanian trees. Trunk volume of massive trees (≥200-m³ trunk volume) in relation to (a) diameter at breast height (1.4 m) for dead and live trees, (b) diameter at 10-m height, and (c) tree height. Differences were significant (P < 0.001) between the live and dead trees.

Landscape and vegetation correlates of tall and massive trees

Massive trees were evenly distributed among single-aged and multi-aged stands, whereas 85% of the tallest trees occurred in single-aged stands (Fig. 7). Massive trees were much more likely to have understoreys composed of rainforest species or a mixture of rainforest (fire-intolerant) and ‘wet sclerophyll species’ (fire-adapted) than were the tallest trees, for which the understorey was more likely to be a mix of wet sclerophyll species (Fig. 7) (Bowman 2000). There was no difference in slope between the tall and the massive trees. Aspect was more likely to be easterly than westerly for both massive (P = 0.0003, n = 69) and tall (P = 0.02, n = 37) trees (Fig. 8).

Fig. 7.

Characteristics of giant Tasmanian trees and associated vegetation. Comparison of massive and tall trees in relation to (a) relative frequencies of single-, double- and multi-aged stands, and (b) understorey vegetation types.

Fig. 8.

Distribution of Tasmanian giant trees in relation to slope and aspect. Circular plots of aspect and slope (%) for (a) tallest, and (b) most massive trees. Red indicates the top 10 trees in each category (most massive or tallest).

Many of the tall trees are concentrated in small stands, such as the Andromeda stand, where nine trees are currently recorded to be >90 m, the Manning Stand (1 tree >90 m tall and 4 >85 m) and the Three Huts stand (16 trees >85 m tall). The remote McLeods Creek area also had 30 LiDAR hits of trees between 85 and 88 m tall, of which 10 have been confirmed by ground-based laser. These trees are believed to be in the 220-year-old age class, and most survived the December 2018 Gell River fire, which burnt through parts of the stand.

Climate envelope for giant trees in Tasmania and Victoria

Wet forest occurred in a climate space intermediate between the cooler wetter rainforest (found mostly in Tasmania), and the drier, slightly warmer open forest (Figs 9, 10, S2). There was no difference between tall and massive trees in the three climate variables MAT, MAP or CMI (Figs 10, S3). The Tasmanian giant trees occurred in areas that were, on average, significantly cooler (9.8°C vs 10.6°C MAT) and drier (1096 vs 1266 mm MAP) than those in Victoria, but with similar CMI (Fig. 11). Mean annual precipitation in the wet forest climate space mostly ranged from 800 to 1500 mm, mean annual temperature from 7.5°C to 12.5°C, and CMI from −20 to 100 mm (Fig. 11). Giant trees occurred through most of the wet forest climate space, although they were concentrated towards the cooler, wetter areas (Figs 10, 11). There was one outlier, a massive E. obliqua (‘Mount Cripps’, with 245-m³ trunk volume), found in north-western Tasmania in an area that receives almost 2000 mm MAP.

Fig. 9.

Geographic distribution of Tasmanian and Victorian giant trees. Locations of tall trees (≥85 m tall; yellow circles) and massive trees (≥200-m³ trunk volume for Tasmania, 170 m³ for Vic; black circles) in (a) Tasmania and (b) Victoria (because there were only 14 Victorian trees with a trunk volume of ≥200 m³, we used a lower-volume threshold). Measurements were confirmed by ground-based laser and or climber-deployed tape drop. The distribution of wet eucalypt forest is shown in green, open eucalypt forest in red and cool temperate rainforest in blue.

Fig. 10.

Distribution of Tasmanian and Victorian giant trees in climate space. Climate of locations of tall (yellow dots) and massive (teal dots) trees in relation to the climate envelope of wet eucalypt forest (green pixels), open eucalypt forest (red pixels) and rainforest (blue pixels) in Tasmania and Victoria. The intensity of the colour in each pixel is proportional to the area of that vegetation type represented by that MAT/MAP combination; for example, the most common climate for a wet forest is MAT of 12–12.5°C and 1100–1200 mm MAP. Tall trees are all Tasmanian and Victorian trees ≥85 m tall and massive trees are all live Tasmanian trees with a trunk volume of ≥200 m³ and Victorian trees of ≥170 m³. Because there were only 14 Victorian trees with a trunk volume of ≥200 m³, we used a lower-volume threshold. The outlier is a massive Eucalyptus obliqua tree in north-western Tasmania. Frequency histograms showing the areas of open eucalypt forest, wet eucalypt forest, and rainforest separately for Tasmania and Victoria are presented in Fig. S2.

Fig. 11.

Occurrence of giant Tasmanian and Victorian trees relative to the climate space of mapped wet forest. Black histograms show frequency distribution of (a) climatic moisture index (CMI), (b) mean annual precipitation (MAP), and (c) mean annual temperature (MAT) for wet forest in Tasmania and Victoria. Violin plots show the distribution of giant trees in Tasmania (red) and Victoria (green). There was no difference in climate between exceptionally tall (≥85 m) and massive (≥200 m³ for Tasmania, ≥170 m³ for Victoria) trees; so, they have been combined in this figure.

Fire-killed trees

Fire is a major cause of mortality of giant trees. At least 18 trees with a trunk volume measured to be >250 m³ have been killed by fire this century, including El Grande, which was killed by a forestry regeneration burn in 2004, and at the time was the world’s largest known eucalypt (Table 5). Most giant-tree deaths were caused by wildfires, the most serious of which was ignited by dry lightning strikes on 15 January 2019. One large fire, the Riveaux Road Fire Complex, affected parts of the Southern Forests and was particularly damaging to older and larger trees. It killed 15 of the largest 25 trees known in the state at that time, including Rullah Longatyle, the world’s largest E. globulus (Table 5). By contrast, during this period, logging is known to have killed only one massive tree, in 2012.

More massive trees than tall trees have been killed by fire in Tasmania since 2004. The fire-killed massive trees were from areas that were drier (1024 cf. 1101 mm MAP; P = 0.01), with a lower CMI (9.1 cf. 19.1; P = 0.0015), than areas supporting the live massive trees (Fig. S4). MAT was not significantly different between live and dead massive trees.

Tallest Australian trees

Tasmania has six species with specimens that are either more than 70 m tall or 11.5 m in girth, namely, E. regnans; E. globulus, E. obliqua, E. tasmaniensis, E. viminalis and E. dalrympleana. Nearly all of the tallest and most massive trees are E. regnans. There are 19 Tasmanian trees taller than 90 m (18 are E. regnans), including the tallest known live-topped tree in Australia (the 96-m ‘Centurion’) (Table 3, Fig. 12a), and 41 taller than 87 m (38 are E. regnans). By comparison, the tallest known tree in Victoria is a 93-m-tall E. regnans originating from the 1926 bushfires, and a further five trees are known to exceed 90 m, (all E. regnans) (Mifsud 2012). Victoria also has three other species known to exceed 80 m tall, namely E. cypellocarpa at 85 m, E. nitens at 84 m, and E. viminalis at 80 m. Karri, E. diversicolor, is Western Australia’s tallest tree, with the tallest known living tree a 78-m-tall specimen from the Shannon National Park (Du Guesclin 2024). There is disagreement as to which tree in New South Wales is currently the state’s tallest tree. ‘The Grandis’, a E. grandis at Buladelah, has been measured to be 71 m tall, as has the Woodford tree, a E. deanei (National Register of Big Trees 2023a). There was also a specimen of E. nobilis located in Cunnawarra New South Wales that was measured to be 79 m tall in 1997 (New South Wales Government 2023). However, remeasurement is required because the time that has elapsed since it was measured and also because the tree was not measured by climber-deployed tape drop, laser or LiDAR. The tallest known tree in Queensland is ‘Big Bob’, a 72.8-m-tall E. grandis growing in the Sunshine Coast hinterland and found by LiDAR (Stumm 2012). No trees exceeding 70 m tall are known from South Australia, the Northern Territory, or the Australian Capital Territory (Supplementary Table S1).

Fig. 12.

Examples of exceptionally tall Tasmanian trees. (a) Tree ED1, the tallest living tree in Tasmania, photographed in 2008 when still intact at 99.6 m. Note that the bole is covered with epicormic growth, usually a response to past crown damage. (b) Tree TH4, an 88-m-tall, 220-year-old tree in Three Huts Reserve, Florentine Valley. (Photo: B. Mifsud.)

Massive Australian trees

The Tasmanian E. regnans tree with a trunk volume of 463 m³ is easily the largest known tree in Australia (Table 4, Fig. 5). In comparison, the largest E. regnans from Victoria measures only 245 m³ (Table S2). There is also a E. nitens measured to be 229 m³ and a E. cypellocarpa measured to be 163 m³ from Victoria. (Mifsud and Harris 2016). In Western Australia, the largest measured trees are Tingle (E. jacksonii, 202 m³) and Karri (E. diversicolor, 202 m³) (Du Guesclin 2023). In New South Wales, the largest eucalypts by trunk volume include the ‘Bird Tree’ (E. pilularis, 155 m³) and ‘The Grandis’ (E. grandis, 137 m³) (R. Du Guesclin, pers. comm.). Whereas these are all species of Eucalyptus, contenders for the title of the largest trees in New South Wales and Queensland are members of the genus Ficus. Impressive individual trees include the ‘Temple Fig’, Ficus virens, near Murwillimbah, ‘The Curtain Fig’ and ‘Cathedral Fig’ from the Atherton tablelands and a Moreton Bay Fig, Ficus macrophylla, growing near Bellingen (National Register of Big Trees 2023a). Although these fig trees have huge girths owing to their extensive plank buttresses and coalesced roots, they do not contain wood volumes comparable to the largest eucalypts. This is due to a combination of factors, including the enormous amount of air contained between the plank buttresses and the basal and aerial roots, the fact that these buttresses extend high up the trunk and thus the trunks never become round, and in some examples of ‘strangler’ figs, there is no actual ‘trunk’, just a vast entanglement of aerial roots. Thus, making a volume estimate on these trees is extremely difficult, but it is likely that the largest specimens contain wood volumes close to 200 m³. There are no individual trees known to exceed 100 m³ in wood volume in South Australia, the Northern Territory, or the Australian Capital Territory.

Global comparisons

On the basis of the height of the tallest extant individuals, E. regnans was, until 2020, in second place on the list of the world’s tallest tree species. However, ‘Centurion’ has since lost height and is now 96 m tall, so E. regnans currently ranks sixth on the list of the world’s tallest living tree species, behind the coast redwood (Sequoia sempervirens, 115.92 m), Himalayan cypress (Cupressus gigantea – a 102.3-m specimen was recently discovered in China) (Fangyu 2023), sitka spruce (Picea sitchensis), Douglas fir (Pseudotsuga menziesii), (all conifers) and a tropical species, Shorea faguetiana, from Danum Reserve, Sabah, Malaysia, which currently measures 98.53 m (Shenkin et al. 2019). The 90.7-m-tall E. globulus ranks eighth and the 88.5-m-tall E. obliqua ranks 11th in list of the world’s tallest living tree species. The most massive known trees on the planet are giant sequoias, Sequoiadendron giganteum, from the western slopes of the Sierra Nevada ranges, California. The largest known giant sequoia, ‘General Sherman’, has a calculated trunk volume of 1395.2 m³ (Sillett et al. 2021). The only other tree species known to exceed 1000 m³ in trunk volume is the coast redwood, Sequoia sempervirens, which occurs in a 750-km strip from central California to southern Oregon, with the largest measured tree at 1083.7 m³ (Sillett et al. 2022). In terms of age, giant sequoia trees can live in excess of 3000 years and coast redwoods to beyond 2000 years (Sillett et al. 2021), whereas it is unlikely that E. regnans can live much beyond 600 years (see below). Another tree from the Pacific Northwest, the western red cedar, Thuja plicata, attains sizes comparable to the largest E. regnans, with the largest living specimen listed to be 449 m³ (Van Pelt 2001). The three Pacific Northwest species listed above are all conifers, whereas E. regnans and the other large eucalypt trees are angiosperms. The largest eucalypts are probably the world’s largest known angiosperms as measured by trunk volume and wood mass. Their nearest rivals are baobab trees in southern Africa, Adansonia digitata, and Madagascar, Adansonia grandidieri, the largest of which has been recorded to have a stem volume of 414 m³ (Guinness World Records 2023a). However, the ‘wood’ of a baobab is fibrous, 70% water and very soft, and of low density (about 130 kg m⁻³, compared with 680 kg m⁻³ for E. regnans; Guinness Book of Records), so that the largest baobabs contain much less biomass than do the largest eucalypts. The largest eucalypts are also likely the to be the largest-trunked trees in the southern hemisphere, their main rival being the New Zealand kauri, Agathis australis. The largest extant kauri tree, named ‘Tane Mahuta’, Māori for ‘Lord of the Forest’, has been measured to have a trunk volume of between 255 m³ and 317 m³, depending on what part of the tree is included as ‘trunk’ (Earle 2023; R. Van Pelt, pers. comm).

Ecological correlates of Tasmanian giant-tree height, volume, and age

We found that tall and massive trees are restricted to the same climate envelope and show no differences in aspect. Our allometric analysis showed that variation in trunk volume was driven by variation in the diameter of the stem above the basal buttressing, rather than by tree height. As outlined below, we suspect the differences in height and volume are controlled not by the physical environment, but rather are influenced by stand dynamics and stand age. This hypothesis is supported by the observation that the tallest trees are typically in single-aged stands with a wet sclerophyll understorey, whereas massive trees more often occur in multi-aged stands with rainforest understoreys. Tasmanian giant trees occur in wet eucalypt forest, predominantly in areas receiving between 1000 and 1500 mm MAP, noting that the water balance of sites that support giant trees is affected by slope, aspect and elevation. It is easy to explain the drier limit by the large amount of water required to grow and sustain such large trees. Likewise, moisture stress is likely to explain why an easterly aspect was favoured by the giant trees, which would lessen the amount of incoming solar radiation during the hottest part of the day. However, the reason that wet eucalypt forest tends not to occur in the wettest areas is less obvious. We suspect that this can be explained by the fire ecology of these forests; germination and establishment of the giant eucalypt species requires disturbance, usually fire. Without fire, over a time-scale of centuries, the more shade-tolerant rainforest species eventually dominate the understorey, and there is a transition to mixed forest and eventually rainforest (Gilbert 1959; Jackson 1968; Bowman 2000). Gilbert (1959) and Ashton (1976, 1981) established the concept that major fire events in wet forest dominated by E. regnans and other obligate-seeder eucalypts produce predominantly single-aged stands. However, this paradigm has been challenged by more recent research, indicating that multi-aged stands are reasonably common in tall wet eucalypt forest (McCarthy and Lindenmayer 1998; Turner et al. 2009; Prior et al. 2022). Here, we found that 85% of the tallest trees occurred in single-aged stands, whereas the massive trees were evenly distributed among single-aged and multi-aged stands. This may reflect the following: (i) high intensity, stand-replacing fires often lead to a very high density of eucalypt seedlings that face extreme competition for light, and the tallest-growing trees are more likely to survive to maturity (Givnish et al. 2014; Sillett et al. 2015); (ii) the extreme age of the largest trees increases the likelihood that they have survived at least one fire event in their lifetime, which can stimulate eucalypt seedling germination; this increases the chance of them being in a multi-aged stand; and (iii) fire events can damage the crowns of tall trees, so even if the tree survives the fire, crown dieback means it is often no longer extremely tall. This can either be by flames affecting the crown of the tree, or indirectly, by affecting the base of the tree or via embers lodging and structurally damaging the tree higher up the trunk (for example, with the ‘Centurion’ tree). The fact that the understorey of massive trees was more likely to contain rainforest species, whereas tall trees were more likely to have wet sclerophyll species in their understorey, is best explained by the differences in age classes between the two groups. The reasoning here is that for the giant trees to reach such sizes, they must have avoided fire for a long period, allowing the rainforest species to slowly colonise, become dominant and replace the wet sclerophyll species by shading them out (Gilbert 1959). In Victoria, it has been shown that the largest trees are mostly found near cool temperate rainforest and cool temperate mixed forest on streamlines (Trouvé et al. 2024).

On the basis of dendrochronology of rainforest trees and ¹⁴C dating of giant eucalypt stems, the largest trees are likely to be 450–500 years old (Hickey et al. 1999; Wood et al. 2010), whereas most of the tallest trees come from estimated age classes in the 220–350-year range. It is important to note here that dendrochronological techniques do not work well with eucalypts because of their wood anatomy, especially for older stems that are often hollowed out from rot, fire damage or both (Brookhouse 2006). Sillett et al. (2015) used a different approach to determine age through the measurement of a series of E. regnans trees of various known age classes from 90 to 300 years old. Via the development of allometric equations quantifying increases in annual wood volume growth, they estimated the age of three significant Tasmanian giant trees as follows: FL1 at 430 years (95% CI 360–500), ED1 at 320 years (95% CI 260–380) and the deceased El Grande at 480 years (95% CI 400–560).

Most of the 25 tallest trees in Tasmania are estimated to be between 320 and 500+ years old. Only 1 of the 19 trees taller than 90 m as measured was from the 220-year-old age class that was once common throughout much of the Florentine Valley. This tree is from the very small Manning Reserve (alternatively known as Hunns Creek) in the northern part of the valley. A further four trees from the Three Huts Reserve (also a stand of 220-year-old trees located in the Florentine Valley) were between 88 and 89.5 m tall, with a further 12 exceeding 85 m in height (Fig. 12b). On the basis of previous studies on 300-year-old E. regnans trees in Victoria (Sillett et al. 2015), these 220-year-old trees could continue to slowly grow in height as long as their upper trunks remain intact. Thus, there are a few very tall trees in the 200–220-year age-bracket that could potentially produce trees up to 100 m tall in the next 50–100 years, to replace the current tallest but older and declining trees with dead or broken tops (Fig. 4). However, the situation for giant trees is very different. This is because the 220-year-old trees mentioned above, although very tall, have trunk volumes of only 40–80 m³. Modelling from Sillett et al. (2015) indicates that although volume increments can increase with age in a healthy E. regnans tree, for a 220-year-old tree to grow from 80 m³ to 300 m³ in volume will take from 200–260 years, and getting to 400 m³ will take 260–340 years. In summary, there is evidence that E. regnans, E. obliqua, E. globulus and E. tasmaniensis can live up to 500 years or more. However, it is unclear whether the very largest living trees in Tasmania are survivors from a distant fire event more than 600 years ago or are super-sized outliers from a fire event approximately the year 1500 (Fedrigo et al. 2019). Further study using radiocarbon dating of charcoal deposits in the soil around some of the largest living trees may further extend the known life expectancy of wet forest eucalypts well beyond 20th century estimates.

The age a tree species can attain is limited by the investment a tree makes into bark protection and heartwood defence. In an extreme example, the giant sequoia (Sequoiadendron giganteum) from the Sierra Nevada, California, reaches ages in excess of 3000 years because once a tree reaches a certain size, the combination of its extremely thick bark and insect- and rot-resistant heartwood effectively eliminate most potential threats such as fire, insect attack and fungal decay (Sillett et al. 2021). In comparison, E. regnans has very thin, smooth bark above its basal stocking and thus is easily killed by fire, as well as heartwood that is extremely susceptible to fungal attack, which leads to catastrophic, rot-induced structural failure in its later years as well as creating entry points for fire (Mifsud and Harris 2016). Consequently, E. regnans (and most other eucalypts) cannot attain the extremely old ages listed for some conifer species.

Wildfire mortality

Very old and large eucalypts are especially vulnerable to fire because typically they contain pre-existing fire scars, areas of rot and multiple hollows that burn very readily (Mifsud and Harris 2016). The main cause of death of the largest Tasmanian trees in recent decades has been fire (Bowman et al. 2022). Here, we recorded high mortality of giant trees following the 2019 fires. Although these fires occurred during a pronounced drought that coincided with above-average summer temperatures (Wardlaw 2022), the forest fire-danger index was only low to moderate (Prior et al. 2022). We note that survival rates of the wet forest eucalypts in the 50–120-cm-diameter classes were between 80% and 95% following the fire, reinforcing the point that individuals in the 40–150-year-old age class typically survive lower-intensity fires. In addition to fire intensity and tree size, species is a crucial determinant of survival; E. regnans is the most vulnerable, being an obligate seeder, with very limited resprouting capacity (Waters et al. 2010), whereas E. obliqua can resprout both epicormically and basally (Trouvé et al. 2021; Prior et al. 2022). E. tasmaniensis (formerly E. delegatensis subsp. tasmaniensis) is a resprouter (unlike its mainland close relative, E. delegatensis, an obligate seeder), and has intermediate fire tolerance (Rodriguez-Cubillo et al. 2020). Recognising that fire is the main danger to the existing largest trees, we advocate developing a triage plan to protect the tallest and most massive trees. This would involve fire-behaviour modelling to identify landscape settings that are least fire prone, combined with field surveys to identify individuals most likely to survive fire. This would inform practical measures such as such as removing ground-surface fuels, wrapping trees in aluminium foil and irrigation (Bowman et al. 2022).

Climate change and fire

Our climatic analysis showed that the fire-killed giant trees were in areas that were, on average, significantly drier than those where the surviving giant trees occur. This accords with Victorian research showing that surviving giant trees are strongly associated with mesic vegetation such as rainforest (Trouvé et al. 2024). Although acknowledging these data are spatially autocorrelated, one would expect the warmer, drier parts of the wet eucalypt forest range to be most at risk from increased fire frequency and severity. Given enough time, if fire frequency increases, the wet forests could potentially expand into areas of what is currently rainforest. However, the rapidity of climate change combined with the long lifespans of these species, their slow dispersal rate and the fragmentation of the remaining wet forest mean that this is unlikely to occur. In addition to exacerbating the severity and frequency of wildfire, climate change poses a direct threat to giant trees by increasing temperatures and therefore evapotranspiration and drought stress (Lindenmayer and Laurance 2017). Maximum tree height represents a balance between hydraulic limitation and allocational allometry (Givnish et al. 2014), and globally, most forest species operate within narrow hydraulic safety margins (Choat et al. 2012). Therefore, large, old trees are likely to be particularly vulnerable to rising temperatures, because they established under, and are adapted to, a milder climate than that projected for the future (Fox-Hughes et al. 2014; McDowell and Allen 2015). Indeed, large trees were the most likely to die during an experimentally imposed 10-year drought in a tropical rainforest (Rowland et al. 2015). Tall trees experience greater hydraulic stress than do shorter ones, and stomatal limitation of photosynthesis was found to increase with height of E. regnans trees between 61 and 92 m tall in Victoria and Tasmania (Koch et al. 2015). Catastrophic failure of the hydraulic system is a major cause of tree mortality during drought (Rowland et al. 2015; Choat et al. 2018). Prolonged drought stress has apparently already led to the deaths of the tallest known E. viminalis trees (including one of ~89 m tall) in eastern Tasmania in the late 2010s (Steane 2020). The adverse effects of drought on trees can persist for years, because trees need to repair their water-transport system by regrowing damaged xylem, and the number of years required for recovery increases with tree size (Trugman et al. 2018; Arend et al. 2022). Of concern is the poor crown health of many of Tasmania’s giant trees, such as those in the Styx Valley (Fig. 4). Effects of future droughts will be worsened by increasing temperatures (Choat et al. 2018). In addition, hotter conditions could limit the size attained by wet forest eucalypts in the future. A prolonged, unusually warm period was shown to switch a tall E. obliqua forest from a carbon sink to a carbon source (Wardlaw 2022), and warmer temperatures disproportionately reduce the growth rate of large eucalypts relative to smaller ones (Prior and Bowman 2014).

Historic loss of giant trees: the case of E. regnans

Our study has shown that Tasmanian E. regnans old-growth forests contain the Earth’s largest flowering trees. This is despite E. regnans forests occupying a relatively small area of Tasmania, with an estimated current extent of 76,050 ha (Tasmanian Government 2023b), compared with approximately 100,000 ha pre-1750 (Resource Planning and Development Commission 2002). A question arises whether there are fewer and smaller trees in Victoria as a consequence of historical logging and land clearance. Historical reconstruction suggests that before European colonisation, there were ~250,000 ha of E. regnans old-growth forest in Victoria, but land clearing, logging and extreme bushfires since European settlement have drastically reduced the extent of old-growth E. regnans to less than 1.16% of its pre-1750 extent (Lindenmayer et al. 2021). Although this estimate hinges on definitions of old growth. A more inclusive definition, such as including stands of trees >120 years old (e.g. Trouvé et al. 2024), would increase this amount. However, expanding the definition has no effect on the estimated area where the oldest and largest trees occur (i.e. >400 years old, >200 m³) (Table 4). The Tasmanian forests have suffered considerably less from land-clearing for farming, although some E. regnans wet forests have been converted to plantations of Pinus radiata and E. nitens (Tasmanian Government 2022). The remaining areas of very old E. regnans forest indicate that fires have been less destructive than those in Victoria (Hickey et al. 1999, 2001), possibly reflecting a generally cooler, moister climate in Tasmania (Fig. 11).

The largest living Tasmanian E. regnans trees appear to be close to, if not equal, in size to the very large E. regnans trees depicted in photos from Victorian forests of the Otway and Strzelecki Ranges from the late 1800s to early 1900s (Peirce and Cunningham 1888; Holmes 1949; Griffiths 1992). Although diameter measurements from that time significantly exceed that of any eucalypt alive today (see Fig. 3), the photographs that survive mostly depict the lower 5 m or so of the base of the tree, and consequently the size of the bole above that is not known. Because the correlation between diameter at breast height and wood volume is poor, the trunk volume of these giants of the past is uncertain. However, from the size of the bases alone, some of these trees would have surpassed 350 m³ in trunk volume.

Conservation of Tasmanian giant trees

Recognition of giant trees for conservation

The way tall and massive trees have been recognised, and, in turn, been protected from forestry operations has varied substantially over time. In 1945, ANM noted a tree of outstanding size (Helms 1945), but logged it anyway. However, in the 1950s and 1960s, they created several small informal reserves in their large wood concessions in the Florentine and Styx valleys. These included the Andromeda (6.5 ha) and Styx Big Tree (15 ha) reserves in the Styx Valley, which were specifically created to conserve some very tall E. regnans trees. Other small reserves in the Florentine Valley, such as Lawrence Creek (15 ha), Lady Binney (50 ha), Manning Road/Hunns Creek (17 ha), Pagoda (7 ha) and Three Huts (20 ha), were primarily set aside to showcase representative samples of the original forest prior to logging, and not specifically to conserve trees of outstanding height or size (Kostoglou 2000). Nevertheless, the Three Huts and Manning reserves, despite their small size, contain the best examples of very tall trees from the 220-year age class (Fig. 13). However, it is important to note that none of the ANM reserves listed above contains any of the 50 largest-volume trees in the state. Another example where a giant tree was identified, conserved and showcased concerns the Arve Giant. First measured by district surveyors when planning a logging road in the 1980s (Kostoglou 2000), it was spared from future logging and had a walkway constructed to it, as well as having the road to it sealed for safer public vehicle access. Unfortunately, the Arve Giant was burnt and collapsed following the 2019 fires (Ogilvie 2019; Table 5).

Fig. 13.

Example of a maturing stand of Eucalyptus regnans in the Manning Reserve, Florentine Valley, Tasmania. This single-aged cohort of 220-year-old trees includes individuals up to 91.5 m tall, even though their maximum volume is small (80 m³) relative to massive trees in significantly older giant forests (>500 years). (Photo: B. Mifsud.)

In 1994, the Tasmanian Forestry Commission was rebranded Forestry Tasmania, with a statutory duty to optimise both economic returns and benefits from non-wood values (Dargavel 1995; Felton 2006). Belatedly, in 1999, a formal policy of protecting tall trees on state forest land was introduced where all trees measured 85 m and above would be excluded from logging and given a buffer protection zone from any forestry operations (Whitely 2018). However, the accidental killing of the then largest ever known tree in Tasmania ‘El Grande’, in a forestry regeneration burn in 2003 (The Age 2003), an event which made news overseas (British Broadcasting Corporation 2003), led Forestry Tasmania to introduce a policy that also protected trees deemed to be giants, i.e. determined by a modelled formula, or by detailed measurements, to be 280 m³ or more in trunk volume.