Robert S. Hill

The Ecology and Biogeography of Nothofagus Forests

Focusing on the tree species Nothofagus, or southern beech, ecologists and biogeographers here provide a comprehensive examination of the distribution, history, and ecology of this species that predominates in forests from highland New Guinea at the equator to the subantarctic latitudes of Tierra del Fuego. The Nothofagus genus offers a fascinating key to understanding historical plant geography and modern vegetation patterns.

CHANGES IN SPECIES ASSEMBLAGES WITHIN THE ADELAIDE METROPOLITAN AREA, AUSTRALIA, 1836–2002

Currently, slightly less than half the worlds population lives in dense urban areas, principally cities. In Australia, nearly 85% of people live in towns with 1000 or more residents. Although individual species of urban flora and fauna have often been well studied, little is known of the long-term temporal patterns associated with changes in both the abiotic and biotic environments as urban systems expand. Using historical and current information, the changes in species richness (defined as the native and introduced vertebrates and vascular plants) in Adelaide, South Australia, are described from its founding in 1836 until 2002. Adelaide is an isolated city of over a million inhabitants, bordered by a range of hills and the South Australian coastline. With a Mediterranean climate, a culture that places high importance on private residential gardens, and the presence of extensive public parklands, the metropolitan area has a significant diversity of both native and introduced flora and fauna. Using only the presence or absence of a species, the changes to plant and vertebrate species richness were quantified by analyzing the observed patterns of change at a functional group level. Powerful correlative evidence is provided to explain the development and establishment of patterns in urban ecology. There has been a dramatic change in species composition, with an increase in total species numbers of ∼30%. At least 132 native species of plants and animals have become locally extinct, and a minimum of 648 introduced species have arrived (mostly plants). The plants increased in species richness by 46%. Fifty percent of the native mammal species were lost, and overall, the birds declined by one species, representing 21 extinctions and 20 successful introductions. Amphibians and reptiles showed no net change. The herbaceous perennial and annual plant species richness showed a substantial increase. This temporal approach to urban ecology demonstrates new ways to identify individual species or groups at risk of extinction and provides some long-term management goals for large urban areas.

Turner Review No. 2 - Southern conifers in time and space

The three southern conifer families, Araucariaceae, Cupressaceae and Podocarpaceae, have a long history and continue to be an important part of the vegetation today. The Araucariaceae have the most extensive fossil record, occurring in both hemispheres, and with Araucaria in particular having an ancient origin. In the Southern Hemisphere Araucaria and Agathis have substantial macrofossil records, especially in Australasia, and Wollemia probably also has an important macrofossil record. At least one extinct genus of Araucariaceae is present as a macrofossil during the Cenozoic. Cupressaceae macrofossils are difficult to identify in older sediments, but the southern genera begin their record in the Cretaceous (Athrotaxis) and become more diverse and extensive during the Cenozoic. Several extinct genera of Cupressaceae also occur in Cretaceous and Cenozoic sediments in Australasia. The Podocarpaceae probably begin their macrofossil record in the Triassic, although the early history is still uncertain. Occasional Podocarpaceae macrofossils have been recorded in the Northern Hemisphere, but they are essentially a southern family. The Cenozoic macrofossil record of the Podocarpaceae is extensive, especially in south-eastern Australia, where the majority of the extant genera have been recorded. Some extinct genera have also been reported from across high southern latitudes, confirming an extremely diverse and widespread suite of Podocarpaceae during the Cenozoic in the region. In the Southern Hemisphere today conifers achieve greatest abundance in wet forests. Those which compete successfully with broad-leaved angiosperms in warmer forests produce broad, flat photosynthetic shoots. In the Araucariaceae this is achieved by the planation of multiveined leaves into large compound shoots. In the other two families leaves are now limited to a single vein (except Nageia), and to overcome this limitation many genera have resorted to re-orientation of leaves and twodimensional flattening of shoots. The Podocarpaceae show greatest development of this strategy with 11 of 19 genera producing shoots analogous to compound leaves. The concentration of conifers in wet forest left them vulnerable to the climate change which occurred in the Cenozoic, and decreases in diversity have occurred since the Paleogene in all regions where fossil records are available. Information about the history of the dry forest conifers is extremely limited because of a lack of fossilisation in such environments. The southern conifers, past and present, demonstrate an ability to compete effectively with angiosperms in many habitats and should not be viewed as remnants which are ineffectual against angiosperm competitors.

Reconstruction of the Oligocene vegetation at Pioneer, northeast Tasmania

The Oligocene vegetation at Pioneer was closed temperate rainforest dominated by Nothofagus johnstonii Hill, which probably produced N. menziesii-type pollen. However, other angiosperms (Quintinia, Cupaniae, Ilex, Cunoniaceae, Myrtaceae, Proteaceae and Winteraceae) were also present, as well as several conifers (Athrotaxis, Phyllocladus, Podocarpus, Dacrydium, Dacrycarpus and Araucariaceae). This rainforest was floristically more complex that the modern Tasmanian Nothofagus cunninghamii rainforests but contained many taxonomically related elements. One major difference was that a fern similar to extant Cyathea filled the riparian niche now largely occupied by the tree-fern Dicksonia antarctica. There is indirect evidence that species producing Nothofagus brassii-type pollen may have occurred upstream of the site of deposition, suggesting that the Nothofagus species were altitudinally zoned or edaphically restricted. The current absence of many of these Nothofagus species in Tasmania may be due to their in...

The dynamics of some rainforest associations in Tasmania

(1) Spatial and temporal patterns of regeneration of the major Tasmanian cool temperate rainforest canopy species were investigated. Nothofagus cunninghamii, Eucryphia lucida and Atherosperma moschatum generally exhibit continuous regeneration and tend towards self-replacement. Evidence of autogenic replacement of N. cunninghamii and E. lucida by the more shade-tolerant A. moschatum, as predicted by successional theories, was not observed, but would take many generations due to the slow dispersal rate of A. moschatum through rainforest. (2) The endemic conifers, Phyllocladus aspleniifolius and Athrotaxis selaginoides regenerate continuously in open vegetation, and occasionally in large canopy gaps created by natural tree death in closed forest, where competition with co-occurring species is low. However, both species, particularly P. aspleniifolius which is most frequent in scrub rainforests on poor soils in which canopy gaps are small and quickly occupied by species which reproduce vegetatively, commonly occur in a narrow range of size classes. This size structure is indicative of regeneration following a large scale disturbance and inability of the species to regenerate in the mature closed forest. (3) Canopy composition and dynamics in Tasmanian rainforest is, therefore, influenced by differences among species in the modes of reproduction, and in tolerance of the light environment under the canopy. However, autogenic replacement of canopy species by those with greater shade-tolerance is infrequent because of the slow rate of replacement relative to the frequency of catastrophic disturbance.

Biogeography of Nothofagus supports the sequence of Gondwana break-up

The Austral biota reveals many links between Australasia and South America that have challenged biogeographers for many years. Nothofagus, the Southern Beech, is probably the classical example. With the general acceptanceof continental drift, the break-up of Gondwana is regarded as primarily responsible for many disjunct patterns expressed in the Southern Hemisphere biota. Vicariance biogeography is the principal tool used to investigate biogeographic patterns of extant plant groups, resulting in areagrams or general area cladograms. These are often at odds with current geological knowledge, and on this basis, alternative hypotheses of area relationships and geological history have, therefore, been suggested. One such areagram was recently advocated by Linder & Crisp (1995) in a biogeographic analysis of Nothofagus. Three explanations, often in combination, account for incongruence: long-distance dispersals, extinctions, and erroneous geological models. All of these parameters ought to be considered in the analysis. Here we report the result of a historical biogeographic analysis of Nothofagus where we compare the reconciled trees between a well-supported Nothofagus phylogeny and two geological hypotheses: (1) the current view of Gondwana break-up, and (2) the areagram by Linder & Crisp. Our analysis makes use of extant and extinct taxa, as well as the assumption of long-distance dispersals as defined by maximized vicariance. Our results show that Nothofagus existed prior to the break-up of Gondwana and, most importantly, its present distribution supports, and is dependent upon, the traditional break-up sequence of East Gondwana, compatible with three vicariance events. The areagram, conceived as an alternative geological hypothesis, presents a more parsimonious solution, but fails to explain numerous past distributions in areas such as Antarctica, South America, and Tasmania. We therefore recommend a conservative approach to use (general) areagrams in historical biogeography.

Fossil plants from the Pliocene Sirius Group, Transantarctic Mountains; evidence for climate from growth rings and fossil leaves

Fossil wood and leaves of Nothofagus beardmorensis Hill, Harwood and Webb occur within the Sirius Group in Antarctica, a sequence of Pliocene glacial sediments that crop out in the Transantarctic Mountains. Growth forms and tree rings in the wood and the morphology of the leaves indicate that these plants were deciduous dwarf trees that developed a prostrate habit with branches which spread across the ground surface. Despite their small size the narrow growth rings indicate that the trees were mature plants which grew very slowly under harsh conditions. Comparison with growing conditions and habits of prostrate shrubs at similar high latitudes in the arctic today suggests that mean annual temperatures in the Transantarctic Mountains were well below freezing, probably about -12°C, with short summer growing seasons with temperatures of around 5°C. This estimate for the Late Pliocene climate of Antarctica is considerably cooler than previous estimates based on these fossil plants.

The angiosperm-dominated woody vegetation of Antarctica: a review

Antarctic vegetation is today mostly restricted to non-vascular plants, with a few small angiosperms clinging to the Antarctic Peninsula. However, probably as recently as the mid-Late Pliocene woody angiosperms were present in inland Antarctica, suggesting an overall presence of complex and diverse vegetation. Angiosperms were introduced into Antarctica during the Cretaceous from South America and possibly also Southeast Asia via Australia. These angiosperms speciated rapidly at the prevailing high latitudes and were an important source for the developing angiosperm-dominated vegetation of the Southern Hemisphere. The migration and evolution of early angiosperms in Gondwana was probably facilitated by a high level of disturbance caused primarily by the rifting of the supercontinent. This high-latitude region was an important source of evolutionary novelty during the Late Cretaceous-Paleogene. As the climate deteriorated during the Cenozoic, the angiosperm flora was reduced in biomass and diversity, finally being restricted to the current remnants. The timing and nature of this major regional extinction is still poorly understood.

Leaf evolution in Southern Hemisphere conifers tracks the angiosperm ecological radiation.

The angiosperm radiation has been linked to sharp declines in gymnosperm diversity and the virtual elimination of conifers from the tropics. The conifer family Podocarpaceae stands as an exception with highest species diversity in wet equatorial forests. It has been hypothesized that efficient light harvesting by the highly flattened leaves of several podocarp genera facilitates persistence with canopy-forming angiosperms, and the angiosperm ecological radiation may have preferentially favoured the diversification of these lineages. To test these ideas, we develop a molecular phylogeny for Podocarpaceae using Bayesian-relaxed clock methods incorporating fossil time constraints. We find several independent origins of flattened foliage types, and that these lineages have diversified predominantly through the Cenozoic and therefore among canopy-forming angiosperms. The onset of sustained foliage flattening podocarp diversification is coincident with a declining diversification rate of scale/needle-leaved lineages and also with ecological and climatic transformations linked to angiosperm foliar evolution. We demonstrate that climatic range evolution is contingent on the underlying state for leaf morphology. Taken together, our findings imply that as angiosperms came to dominate most terrestrial ecosystems, competitive interactions at the foliar level have profoundly shaped podocarp geography and as a consequence, rates of lineage diversification.

The temperature-dependence of photosynthesis of some Australian temperate rainforest trees and its biogeographical significance

The effect of temperature on photosynthesis was examined in several species of Australian temperate rainforest trees to test three hypotheses regarding the distribution and evolution of the species. The results suggest that changing temperatures in south-eastern and eastern Australia during the Tertiary may have affected species distribution and influenced evolution. For Nothofagus, N. moorei is closest to the ancestral species and has a higher optimum temperature for photosynthesis than the derived species N. cunninghamii. It is hypothesized that this difference is the result of the evolution occurring with declining temperatures in south- eastern Australia during the Tertiary, when the ancestral species evolved into N. cunninghamii. The three Eucryphia species tested do not exhibit such a marked trend, but vary considerably in their relative rates of photosynthesis above and below their optimum temperature for photo- synthesis. The species restricted to northern temperate rainforests which have no close relatives in the southern temperate forests (Ceratopetalum apetalum and Doryphora sassafras) have a substantially higher optimum temperature for photosynthesis than the southern species. These results are consistent with the computer-predicted distributions of the species based on the climate profiles of their known distributions.