The influence of insects on nutrient cycling in eucalypt forests: A beneficial role?

Insect herbivory is common in many Australian eucalypt forests and, some have suggested, may be greater than in comparable forests overseas. It has been hypothesized by several authors that herbivores max nutritionally advantage either individual plants or plant communities by hastening the biogeochemical cycling of nutrients. This coevolutionary development, it is argued, would be a particular advantage in poor soils. These suggestions conflict, however, with evidence that eucalypts commonly conserve nutrients by nutrient withdrawal prior to leaf abscission and recycling a considerable proportion of nutrients through an internal biochemical cycle. The various hypotheses that have been proposed are reviewed. Although there are some situations both at the individual and community level where herbivory may confer nutritional advantages it appears unlikely that high levels of herbivory in eucalypts are generally nutritionally beneficial to the plants being grazed.     

Does rapid utilization of elevated nutrient availability allow eucalypts to dominate in the tropical savannas of Australia?

Northern Australia's savannas are among the most fire‐prone biomes on Earth and are dominated by eucalypts (Eucalyptus and Corymbia spp.). It is not clear what processes allow this group to dominate under such extreme fire frequencies and whether a superior ability to compete for nutrients and water might play a role. There is evidence that eucalypts are adapted to frequent fires; juvenile eucalypts escape the fire trap by growing rapidly in height between fires. However, non‐eucalypts are less able to escape the fire trap and tend to have stand structures strongly skewed toward suppressed juveniles. The mechanisms that drive these contrasting fire responses are not well understood. Here, we describe the results of a controlled glasshouse seedling experiment that evaluated the relative importance of nutrient and water availability in determining height growth and biomass growth of two eucalypt and one noneucalypt tree species, common in northern Australian savannas. We demonstrate that growth of eucalypt seedlings is particularly responsive to nutrient addition. Eucalypt seedlings are able to rapidly utilize soil nutrients and accumulate biomass at a much greater rate than noneucalypt seedlings. We suggest that a seasonal spike in nutrient availability creates a nutrient‐rich microsite that allows eucalypt seedlings to rapidly gain height and biomass, increasing their likelihood of establishing successfully and reaching a fire‐resistant size. Our results extend our understanding of how eucalypts dominate northern Australian savannas under extremely high fire frequencies.     

Is insect defoliation in eucalypt forests greater than that in other temperate forests?

Claims have been made in the past that insect defoliation in eucalypt forests of Australia is both chronically high and greater than defoliation levels in northern temperate forests. Recent published data, however, indicate that some eucalypt forests may sustain low level defoliation. Careful reexamination of the literature on defoliation in eucalypt forests indicates that there are not enough data available to quantify defoliation on an Australian-wide basis. Therefore it cannot be claimed that insect defoliation in eucalypt forests is greater than that in other temperate forests. There are also insufficient data to claim that insects are a major factor responsible for the growth differential between indigenous and overseas plantation-grown eucalypts. The ecological impact of herbivorous insects in eucalypt and northern temperate forests is also discussed.     

Contrasting demographics of tropical savanna and temperate forest eucalypts provide insight into how savannas and forests function. A case study using Corymbia clarksoniana from north‐eastern Australia

Eucalypts (Eucalyptus spp. and Corymbia spp.) dominate many communities across Australia, including frequently burnt tropical savannas and temperate forests, which receive less frequent but more intense fires. Understanding the demographic characteristics that allow related trees to persist in tropical savannas and temperate forest ecosystems can provide insight into how savannas and forests function, including grass–tree coexistence. This study reviews differences in critical stages in the life cycle of savanna and temperate forest eucalypts, especially in relation to fire. It adds to the limited data on tropical eucalypts, by evaluating the effect of fire regimes on the population biology of Corymbia clarksoniana, a tree that dominates some tropical savannas of north‐eastern Australia. Corymbia clarksoniana displays similar demographic characteristics to other tropical savanna species, except that seedling emergence is enhanced when seed falls onto recently burnt ground during a high rainfall period. In contrast to many temperate forest eucalypts, tropical savanna eucalypts lack canopy‐stored seed banks; time annual seed fall to coincide with the onset of predictable wet season rain; have very rare seedling emergence events, including a lack of mass germination after each fire; possess an abundant sapling bank; and every tropical eucalypt species has the ability to maintain canopy structure by epicormically resprouting after all but the most intense fires. The combination of poor seedling recruitment strategies, coupled with characteristics allowing long‐term persistence of established plants, indicate tropical savanna eucalypts function through the persistence niche rather than the regeneration niche. The high rainfall‐promoted seedling emergence of C. clarksoniana and the reduction of seedling survival and sapling growth by fire, support the predictions that grass–tree coexistence in savannas is governed by rainfall limiting tree seedling recruitment and regular fires limiting the growth of juvenile trees to the canopy.     

Nutrient cycling and nitrogen mineralization in eucalypt forests of south-eastern Australia

Summary Nutrient pools in litter and soil and the major nutrient transfers and additions in rainfall, throughfall and litterfall were measured in eight mature, undisturbed eucalypt forests covering a range of species, climate, productivity and soil type. Litterfall is the major pathway for the return of N, P, Ca and usually Mg, to the soil. The forests covered almost the range of litterfall reported for eucalypt forests and, in conjunction with published data, litterfall was strongly related to climatic variables. Extractable P in the soil and P concentrations in litter and litterfall were significantly higher in two sub-alpine forests (Eucalyptus pauciflora andE. delegatensis) than in all other forests. In general, nutrient turnover, particularly N turnover, was related to the rate of organic matter turnover. Rates of organic matter turnover in these forests and in other studies of eucalypts were correlated with climatic conditions using the simple climatic scalar developed by Vitousek. Nitrogen turnover, especially that proportion cycling via leaf litterfall is primarily a function of organic matter turnover, but litter quality appears also to have an influence.     

Fire frequency effects in a grassy woodland: Trees and grasses

Our study investigated the effects of fire frequency on the tree and grass components of Cumberland Plain Woodland, an endangered grassy eucalypt woodland in eastern Australia. We located three sites within each of three fire frequency classes (high, moderate, low) based on fire history and a similar time since last fire. For trees, we asked whether fire frequency affected density, eucalypt population structure, basal area and spatial patterning. For grasses, we tested for fire frequency effects on total abundance and the abundance of two common species. Density of trees was not significantly affected by fire frequency for juveniles, saplings or adult trees and neither was basal area per hectare. Features of a recruitment bottleneck model were present for eucalypts. There was a large pool of suppressed juveniles making up more than half the population at each fire frequency. Saplings were the smallest group and were susceptible to fire‐induced stem mortality, with particularly low numbers relative to juveniles and small trees where fire was frequent or intense. Despite this difference in sapling mortality, numbers of small trees did not differ with fire frequency. Saplings could transition to small trees in equal numbers if they did so more quickly at high than at lower fire frequencies, and if recruitment of saplings into the tree layer was controlled independently of fire frequency. The size hierarchy of small to medium eucalypt trees was homogeneous over eight of the nine sites, spatial patterning of adult trees was random tending to regular, and mean tree size decreased with density at all sites. These features of eucalypt population structure are indicative of possible resource competition which could regulate tree recruitment. Total grass cover index was high across all fire frequencies, with Themeda triandra dominating at high and moderate fire frequencies and Microlaena stipoides at low fire frequency.     

Factors affecting the growth of Eucalyptus delegatensis seedlings in inhibitory forest and grassland soils

In many highland forests of Eucalyptus delegatensis in Tasmania the establishment and healthy growth of eucalypts is promoted and maintained by fire. In the absence of fire, secondary succession from eucalypt forest to rainforest occurs, during which the eucalypts decline and die prematurely. On sites that are prone to radiation frost severe reduction or removal of a tree canopy allows a sward of tussock grasses to develop, in competition with which seedlings of eucalypts decline in growth and a high proportion dies.Factors of the soil that could contribute to these phenomena were investigated by means of pot experiments that used soils from:o     

Premature Decline of <Emphasis Type="Italic">Eucalyptus</Emphasis> and Altered Ecosystem Processes in the Absence of Fire in Some Australian Forests

We propose a model of ‘premature tree decline’ whereby an absence of fire hastens the mortality of overstorey eucalypts in some forests. This model is relevant to some temperate Australian forests in which fire regimes have shifted from relatively frequent before European settlement to infrequent following settlement. The increased development of midstorey vegetation and litter accumulation has occurred since European settlement in some specific examples of Australian forests and woodlands. Our model proposes that in the long absence of fire: 1. midstorey vegetation reduces the availability of soil water for eucalypts and; 2. Eucalypts have less access to P and/or cations as these elements become locked up in soil, litter and midstorey biomass. We highlight important knowledge gaps and argue that research into ecological burning, for eucalypt health and other values such as biodiversity, is urgently required.     

Predicting the abundance of hollow-bearing trees in montane forests of southeastern Australia

Poisson regression analyses were used to relate the number of hollow-bearing trees on 523 sites in the montane ash forests of the Central Highlands of Victoria to a range of environmental variables. Region, logging history, the dominant species of eucalypt, slope angle, topographic position and the age of the stand influenced significantly the number of hollow-bearing trees. A simple predictive model containing these factors has possible application in geographic information systems and may assist in the assessment of wildlife habitat. The number of multi-aged stands and living trees with fire scars indicates that some individuals of ‘fire-sensitive’ montane ash eucalypts appear to survive intense wildfire.     

Decomposition of litter from three major plant species of jarrah (Eucalyptus marginata Donn ex Sm.) forest in relation to site fire history and soil type

Rates of weight loss and release of N, P, K, S, Ca, Mg, Na, and Cl from litter of several species in jarrah (E. marginata Donn ex Sm.) forest were measured in relation to site fire history and soil type. Weight loss from leaf litter decreased in the order jarrah > marri (E. calophylla R. Br. ex Lindl.) > Banksia grandis Willd. After 18 months on the forest floor senesced leaves of jarrah, marri and Banksia had lost 45%, 42% and 19%, respectively, of their original weight. Although greatest rates of decomposition occurred on a site burnt 3 y previously by an intense autumn fire and slowest rates on a site which had not been burnt for 8 y, the differences between burn sites were small in comparison with the total weight loss from decomposing litter. The order of release of nutrients from decomposing eucalypt litter was P<N<Ca<S<Mg<Cl<K<Na. There appears to be only slow release of N and P from the litter layer of these forests in the period between successive control burns. Fresh jarrah leaves, which are similar in chemical composition to leaf litter falling after crowns have been scorched by intense fire, decompose rapidly in comparison with senescent leaf tissue. Release of nutrients, particularly N and P, is also more rapid from fresh leaves than from leaf litter. Rates of decomposition of green leaves differed between soil types in the order reddish gravels > dark sandy duplex soil > yellow gravels. These differences may be related to the higher nutrient status of the reddish gravel soils.     

Resorption of foliar nutrients in a regenerating southern Appalachian forest

Summary Leaves were sampled in a successional, southern Appalachian forest to estimate autumn foliar nutrient dynamics. Resorption of N and P in a successional forest equaled, or exceeded, resorption estimates for a more mature control forest. Foliar nutrient leaching was not sufficient to account for changes in autumn leaf N, P, Ca and Mg concentrations. The resorption process conserves nutrients by reducing nutrient losses from leaching and litter-fall, thereby closing the nutrient cycle in successional forests. We hypothesize that rapid recovery of primary productivity early in forest regeneration is the result of maximum nutrient resorption of limiting nutrients. Implications of these results for successional nutrient cycling theory are discussed.     

Transferring soils from high- to low-elevation forests increases nitrogen cycling rates: climate change implications

It is well recognized that photosynthesis of C3 plants is highly responsive to CO₂ concentration. However, in natural ecosystems, plants are subject to a range of feed-back effects that can interact with increased photosynthetic carbon gain in different ways so that it is not clear to what extent increased photosynthesis will translate into increased growth. To assess the probable growth response of nutrient-limited forests to increasing CO₂ concentration, we use a previously developed modelling framework and apply it under conditions where the supply of nutrients is affected by a range of different factors. Our analysis indicates that forest growth is likely to be highly stimulated by increasing CO₂ concentration in forests with high fertility, in forests with nitrogen fixing plants, in those subject to fire or where nitrogen in wood is effectively removed from the biologically active cycle either through physical removal of stems in harvesting or through continued stem growth over long time periods. Forest growth is likely to be stimulated by CO₂ concentration in both phosphorus- and sulphur-limited forests provided nutrients in heartwood of trees are removed from the active nutrient cycle. Without this removal from the cycling system, however, sulphur-limited forests should show little response to increasing CO₂. In phosphorus-limited forests without phosphorus removal, the response to increasing CO₂ depends further on the equilibration state of the large pool of unavailable secondary phosphorus. Considered over periods of centuries during which the secondary pool has equilibrated, growth of phosphorus-limited forests is likely to be only weakly stimulated by increasing CO₂ concentration. However, over shorter periods, increasing CO₂ concentration should lead to a substantial increase in productivity. In general, it can be concluded that systems that are more open with respect to nutrient gains and losses are likely to be more responsive to increasing CO₂ concentration than systems where the amount of available nutrients is less variable. In more open systems, operation at a lower internal nutrient concentration as a result of increasing atmospheric CO₂ concentration can lead to reduced nutrient losses per unit carbon gain. Our analysis shows that the effect of increasing CO₂ on forest growth can differ substantially between forests due to interactions with a range of factors that affect nutrient supply. The response of a particular forest to increasing CO₂ concentration can only be predicted if the main factors controlling nutrient supply and growth in that forest are understood and incorporated into an assessment.     

Impact of eucalyptus and pine growing on rural livelihood: the lesson from Bukoba area, north western Tanzania

Sustainable resource management intends to allocate resources in such a way that unnecessary deterioration of biophysical and socio-economical systems is avoided. In Bukoba Area where rainfalls are plenty, evergreen grasslands were expansive and forests were limited, eucalypts ( eucalyptus spp.) and pines ( pinus spp.) were grown on grasslands for preventing land degradation through deforestation and for providing additional source of income for rural poor. This study shows that in addition to detrimental consequence of eucalyptus and pine forests on soil resources, conversion of Bukoban grasslands to forests has negatively impacted livelihood of the rural poor. Growing eucalypts and pines on grasslands prevented a farming system that enabled integration of grasslands, cattle keeping and crop production. Consequently, the grasslands role of nutrients cycling was disrupted, food crop production reduced, home-gardens productivity declined, majority deprived important livelihood asset and foreign income flow into the area reduced.     

Cattle grazing does not reduce fire severity in eucalypt forests and woodlands of the Australian Alps

Large grazing herbivores can change fire regimes by altering fuel types and abundance, particularly in savanna biomes where the dominant fuel is grass. The use of herbivores as a fire management tool is receiving increasing consideration globally, but this intervention has a limited evidence‐base and is controversial because of potential deleterious ecological effects. These issues are well illustrated by the political and scientific debate about the capacity of cattle grazing to reduce fire hazard in the Victorian Alps of Australia; there have been remarkably few scientific studies to illuminate this issue. Here we use remote sensing and geographic information system analysis to determine the effect of active grazing licences on fire severity (crown scorch) in eucalypt forests and woodlands following large fires in the Alps during the summers of 2002/2003 and 2006/2007. Our statistical analyses, which controlled for spatial autocorrelation, found crown scorch was strongly related to vegetation type but there was no evidence that cattle grazing reduced fire severity. There was some evidence that grazing could increase fire severity by possibly changing fuel arrays. Such landscape analyses are a critical approach given that large‐scale grazing × fire trials are prohibitively expensive and impractical to conduct.     

Biomass and nutrient distribution in two eucalypt forest ecosystems

The distribution of biomass and nutrients (N, P, K, Na, Mg, and Ca) among components of a Eucalyptus regnans forest and a mixed Eucalyptus obliqua-Eucalyptus dives forest near Melbourne in southern Victoria have been determined and are discussed. Both forests were found to have relatively low root biomass. Trees and soil accounted for more than 80% of each nutrient present in both ecosystems. The results of nutrient distribution studies in Australian eucalypt forests and in temperate forests elsewhere in the world are compared. These comparisons suggest that the above-ground living biomass in eucalypt forests contains no abnormally high or low quantities of macronutrients in relation to coniferous or deciduous hardwood forests. Detailed comparison among ecosystems is rendered difficult by the variable treatment given to roots and soils.     

No detectable impacts of frequent burning on foliar C and N or insect herbivory in an Australian eucalypt forest

Question: What is the effect of frequent low intensity prescribed fire on foliar nutrients and insect herbivory in an Australian eucalypt forest? Location: Lorne State Forest (Bulls Ground Frequent Burning Study), mid‐north coast, New South Wales, Australia. Methods: Eighteen independent sites were studied representing three experimental fire regimes: fire exclusion (at least 45 years), frequently burnt (every 3 years for 35 years) and fire exclusion followed by the recent introduction of frequent burning (two fires in 6 years). Mature leaves were collected from the canopy of Eucalyptus pilularis trees at each site and analysed for nutrients and damage by invertebrate herbivores. Results: Almost 75% of all leaves showed some signs of leaf damage. The frequency of past fires had no effect on carbon and nitrogen content of canopy leaves. These results were consistent with assessments of herbivore damage where no significant differences were found in the amount of invertebrate herbivory damage to leaves across fire treatments. Conclusions: This eucalypt forest displayed a high degree of resilience to both frequent burning and fire exclusion as determined by foliar nutrients and damage by insect herbivores. Fire frequency had no detectable ecological impact on this aspect of forest health.     

Latitudinal trends in foliar oils of eucalypts: Environmental correlates and diversity of chrysomelid leaf‐beetles

Eucalypts are characterized by their oleaginous foliage, yet no one has considered the universality of oil expression or its ecological associations and implications for biodiversity. Published literature on the oils of 66 eucalypts was combined with geographic distribution information contained in the Australian National Herbarium (ANHSIR) database to investigate continent‐scale changes in oil yield and composition. The exposure to fire and rainfall of each eucalypt was considered in reference to Walker's data on fire frequency and Australian Bureau of Meteorology 97‐year records of rainfall variability. Host collection records for 69 species of chrysomelid leaf‐beetle were collated from entomologists to consider patterns of association with a subset of 16 eucalypts. Eucalypts endemic to the seasonally arid, sub‐tropical to tropical climates of northern Australia have less oleaginous and aromatic leaves than species endemic to the mesic, temperate climates of the southern parts of the continent. Maximum oil yield and the concentrations of cineole and pinene were positively correlated with minimum fire interval but not with rainfall variability. Low oil contents in more northerly distributed species may facilitate persistence in highly fire‐prone habitats. There were no patterns in the diversity of chrysomelid leaf‐beetles with either the oil yield or the concentrations of 1,8‐cineole or α‐pinene in their hosts. When taken in consideration with the apparent strategy of eucalypts to tolerate insect herbivory, current evidence augurs against high concentrations of cineole or pinene acting alone as antibiotic plant secondary metabolites.     

Litter fall and nutrient cycling in karri (Eucalyptus diversicolor F. Muell.) forest in relation to stand age

Annual litter fall, nutrient concentrations in litter components and annual weight of nutrients in litter fall have been estimated for karri forest stands aged 2, 6, 9 and 40 years and in mature forest. The weight of litter falling annually increases with stand age, ranging from 1.13 t/ha in 2-year-otd regeneration to 9.45 t/ha in mature forest. This increase is due mainly to greater amounts of twigs, bark and fruit falling in older stands. Leaf fait is relatively independent of stand age once the canopy of regenerating stands closes and the understorey has developed. Concentrations of N, P, K, S and Mn in karri leaf litter differ significantly between sites and the differences appear to be related to stand age. Highest levels of these elements are found in karri leaf litter from the youngest stand and the concentrations decrease with increasing stand age. The amounts of annual litter fall and of nutrients cycling in litter are among the largest reported for Australian forests. In particular cycling of Ca, K and Mg in mature karri forest is greater than has been reported for any other eucalypt forest. Karri forest understorey plays a key rote in nutrient cycling in these ecosystems, contributing 30–70% of the weight of many of the nutrients in the leaf component of titter. Understorey leaf material is particularly important in the cycling of N, S and the micro-nutrients Cu and Zn.     

Forest biogeochemistry in response to drought

Trees alter their use and allocation of nutrients in response to drought, and changes in soil nutrient cycling and trace gas flux (N₂O and CH₄) are observed when experimental drought is imposed on forests. In extreme droughts, trees are increasingly susceptible to attack by pests and pathogens, which can lead to major changes in nutrient flux to the soil. Extreme droughts often lead to more common and more intense forest fires, causing dramatic changes in the nutrient storage and loss from forest ecosystems. Changes in the future manifestation of drought will affect carbon uptake and storage in forests, leading to feedbacks to the Earth's climate system. We must improve the recognition of drought in nature, our ability to manage our forests in the face of drought, and the parameterization of drought in earth system models for improved predictions of carbon uptake and storage in the world's forests.     

Effect of Herbivory and Plant Species Replacement on Primary Production

Grazing optimization occurs when herbivory increases primary production at low grazing intensities. In the case of simple plant-herbivore interactions, such an effect can result from recycling of a limiting nutrient. However, in more complex cases, herbivory can also lead to species replacement in plant communities, which in turn alters how primary production is affected by herbivory. Here we explore this issue using a model of a limiting nutrient cycle in an ecosystem with two plant species. We show that two major plant traits determine primary production at equilibrium: plant recycling efficiency (i.e., the fraction of the plant nutrient stock that stays within the ecosystem until it is returned to the nutrient pool in mineral form) and plant ability to deplete the soil mineral nutrient pool through consumption of this resource. In cases where sufficient time has occurred, grazing optimization requires that herbivory improve nutrient conservation in the system sufficiently. This condition sets a minimum threshold for herbivore nutrient recycling efficiency, the fraction of nutrient consumed by herbivores that is recycled within the ecosystem to the mineral nutrient pool. This threshold changes with plant community composition and herbivore preference and is, therefore, strongly affected by plant species replacement. The quantitative effects of these processes on grazing optimization are determined by both the recycling efficiencies and depletion abilities of the plant species. However, grazing optimization remains qualitatively possible even with plant species replacement.