Jennifer Read

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.

Plant biomechanics in an ecological context

Fundamental plant traits such as support, anchorage, and protection against environmental stress depend substantially on biomechanical design. The costs, subsequent trade-offs, and effects on plant performance of mechanical traits are not well understood, but it appears that many of these traits have evolved in response to abiotic and biotic mechanical forces and resource deficits. The relationships between environmental stresses and mechanical traits can be specific and direct, as in responses to strong winds, with structural reinforcement related to plant survival. Some traits such as leaf toughness might provide protection from multiple forms of stress. In both cases, the adaptive value of mechanical traits may vary between habitats, so is best considered in the context of the broader growth environment, not just of the proximate stress. Plants can also show considerable phenotypic plasticity in mechanical traits, allowing adjustment to changing environments across a range of spatial and temporal scales. However, it is not always clear whether a mechanical property is adaptive or a consequence of the physiology associated with stress. Mechanical traits do not only affect plant survival; evidence suggests they have downstream effects on ecosystem organization and functioning (e.g., diversity, trophic relationships, and productivity), but these remain poorly explored.

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.

Characterising sclerophylly: some mechanical properties of leaves from heath and forest

Abstract Although sclerophylly is widespread through the world and is often the dominant leaf-form in mediterranean climates, the mechanical properties of sclerophyllous leaves are poorly understood. The term ”sclerophyllous” means hard-leaved, but biologists also use terms such as tough, stiff and leathery to describe sclerophyllous leaves. The latter term has no precise definition that allows quantification. However, each of the former terms is well-defined in materials engineering, although they may be difficult or sometimes inappropriate to measure in leaves because of their size, shape or composite and anisotropic nature. Two of the most appropriate and practically applicable mechanical properties of sclerophyllous leaves are ”strength” and ”toughness”, which in this study were applied using punching, tearing and shearing tests to 19 species of tree and shrub at Wilson’s Promontory, Australia. The results of these tests were compared with leaf specific mass (LSM) and a sclerophylly index derived from botanists’ ranks. Principal components analysis was used to reduce the set of mechanical properties to major axes of variation. Component 1 correlated strongly with the botanists’ ranks. Overall, leaves ranked as sclerophyllous by botanists were both tough and strong in terms of punching and tearing tests. In addition, tough and strong leaves typically had high toughness and strength per unit leaf thickness. There was also a significant correlation between component 1 and LSM. Although more detailed surveys are required, we argue that sclerophylly should be defined in terms of properties that have precise meanings and are measurable, such as toughness and strength, and that relate directly to mechanical properties as implicit in the term.

Responses of some Southern Hemisphere tree species to a prolonged dark period and their implications for high-latitude Cretaceous and Tertiary floras

Abstract Seedlings of some Southern Hemisphere tree species were grown under experimental conditions to investigate their tolerance of long periods of darkness. The taxa used have either a fossil record at high latitudes during the Cretaceous or early Tertiary, or their biogeography suggests that they occurred at high latitudes during this period. The plants were subjected to an approximation of cool/cold, dark polar winters (10 weeks at 4°C) and warm, dark polar winters (15°C). These were compared with plants grown under natural Hobart winter daylengths at 15°C. Most of the plants were able to tolerate this period of darkness. In general, tissue death was less in the 4°C dark treatment than in the 15°C dark treatment. Fourteen of the sixteen species examined survived the 4°C dark treatment with relatively little damage. No significant deterioration in chlorophyll levels was detected at the end of the dark treatment in the plants tested. A negative correlation was recorded between tissue damage of dark-treatment plants and starch levels of light-treatment plants of the same species. There was a weak positive correlation between tissue damage and the dark respiration rate in evergreen species. These preliminary results have several implications for understanding plant survival at high latitudes during the Cretaceous and Tertiary. Firstly, the tolerance of this experimental dark period suggests that woody plants could have survived natural prolonged dark periods at this time, but with a greater tolerance of cool-cold winters than of mild winters. Secondly, the differential tolerance shown by these species suggests that the long dark winters could have acted as a filter of species movements between the high latitude regions of Gondwana. Thirdly, the lengthening winter days as the Australian Plate moved northwards into lower latitudes during the Tertiary may have influenced the decline ofspecies and species habits, e.g. deciduousness, that were highly suited to survival of polar winters.

PLANT TRAITS THAT ENHANCE POLLUTANT REMOVAL FROM STORMWATER IN BIOFILTRATION SYSTEMS

Plants species have been shown to improve the performance of stormwater biofiltration systems, particularly in removal of N and P. Recent research has shown that plants vary in their contribution to pollutant removal, but little is known about the type of plant that is best suited to use in biofilters in terms of survival, growth rate, and performance. In this study, growth responses of 20 species to applications of semi-synthetic stormwater were measured, and the roles of key plant traits in removal of N, P, and several metals were investigated. There was no evidence of negative effects of stormwater application on plant growth, and plant traits, particularly root traits, were strongly correlated negatively with N and P concentrations of effluent stormwater. The most common and strong contributors to N and P removal appeared to be the length of the longest root, rooting depth, total root length, and root mass. The plants that made the strongest contribution to pollutant removal, e.g., Carex appressa, combined these traits with high growth rates. Investigation of other plant traits (e.g., physiology), causal mechanisms, and effects of more complex planting environments (e.g., species mixtures) should further guide the selection of plants to enhance performance of biofiltration systems.

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.

Dynamics of Nothofagus-dominated rainforest on mainland Australia and lowland Tasmania*

Cool temperate rainforest in Australia is commonly dominated by Nothofagus species. In Victoria and Tasmania, Nothofagus cunninghamii dominates old rainforest on optimal sites and is able to regenerate continuously. Size structure analysis of the major lowland tree species in this forest suggest that no major changes in species composition or dominance are occurring. This contrasts with the status of Nothofagus in lowland rainforest in New Zealand and rainforest below about 1000 m a.s.l. in south-central Chile. N. cunninghamii is a relatively light-demanding species, and is maintaining its dominance by seedling regeneration in canopy gaps created by the death of old individuals. The dynamics of the cool temperate rainforest in northern New South Wales are more complex. These forests are dominated by Nothofagus moorei, and size structure analysis indicates that persistence of this species within the rainforest is dependent on vegetative regeneration. On some sites this rainforest is being actively invaded by warm temperate and sub-tropical elements from lower altitudes. In these areas N. moorei in unable to regenerate beneath the canopy.

Reproductive traits of tropical rain-forest trees in New Caledonia

Reproductive traits of 123 species of rain-forest tree from ultramafic regions of New Caledonia were assessed, mainly from herbarium specimens. Most species had extremely small, simple, pale-coloured flowers that are probably mainly pollinated by small insects, but not including bees. The seeds of most species were considered to be bird dispersed. However, wind is also important for pollination and seed dispersal. The phenological trend was for an increase in the number of species flowering and fruiting around the end of the warm dry season/start of the hot wet season, followed by a decline at the end of the wet season, and lower proportions during the cooler season. Seed size was significantly correlated with fruit size. Other correlations, between flower size and fruit size, and between seed size and seed number, were significant using species as independent observations, but did not hold following phylogenetic correc- tion. Compared with non-dioecious species, dioecious species had significantly larger seeds, and a greater proportion of species with biotic dispersal, abiotic pollination and solitary (female) flowers. The long-term persistence of at least the larger-seeded tree species in New Caledonia is precarious, since the endemic giant pigeon, Ducula goliath, is probably their principal effective disperser, and this species is in decline.

Effects of heat and smoke on germination of soil-stored seed in a south-eastern Australian sand heathland

Various fire-related agents, including heat, smoke, ash and charred wood, have been shown to break dormancy and promote germination of soil-stored seed in a broad range of species in mediterranean-type systems. However, relatively little work has been conducted in south-eastern Australian heathlands. This study examined the effects of heat and smoked water on germination of the soil seed bank in a mature sand heathland within the Gippsland Lakes Coastal Park, in south-eastern Australia. Heat was clearly the most successful treatment for promoting seed germination, followed by smoked water, then controls, with 55% of species present in the germinable soil seed bank requiring a heat or smoke stimulus to promote seed germination. Mean species richness of the germinable soil seed bank was found to be significantly higher in heat-treated soil than in smoke and control treatments. Seedling density of heat-treated soil was almost 10 times that of controls, while smoke-treated soil was almost five times that of controls. Seedling emergence was fastest in heat-treated soil, followed by smoke and control soils. Of the species found in the soil seed bank, 25% were absent from the extant vegetation, suggesting the existence of post-fire colonisers in the soil seed bank. The results have implications for the design of soil seed bank experiments and the use of fire as a tool in vegetation management.