Interspecific competition: A mechanism for rodent succession after fire in wet heathland

Abstract A competitive interaction between two species of sympatric native rodents was demonstrated experimentally in Myall Lakes National Park (NSW, Australia). The previously documented replacement of Pseudomys gracilicaudatus by Rattus lutreolus in areas of wet heath in this region implied that competition may be the mechanism facilitating this succession. We present experimental evidence to support this suggestion. Removal of the larger R. lutreolus caused a significant increase in abundance of P. gracilicaudatus on five experimental sites in comparison to five unmanipulated control sites. The sites used in this experiment were chosen from three ages of wet heath regenerating after fire. These three ages represent key stages in the replacement of P. gracilicaudatus by R. lutreolus and the occurrence of competition in each of these ages indicates that competition plays an important role in this succession. In the initial stages of the experiment when R. lutreolus were removed they were replaced by new R. lutreolus individuals. This has been interpreted as evidence of strong intraspecific competition. After removal of R. lutreolus, P. gracilicaudatus expanded its habitat range into microhabitats that had formerly been occupied by R. lutreolus and reduced its range in microhabitats previously occupied by P. gracilicaudatus. This leads us to believe that P. gracilicaudatus was occupying inferior microhabitat before the removal of R. lutreolus, which had excluded it from more preferable microhabitat.     

A model of the effects of water stress on seed advancement and germination

A model of seed germination is proposed which uses a variable with the units of an osmotic potential (virtual osmotic potential) to integrate the effect of a constant or a varying water potential. This differs from existing models that describe the effects of fixed water potentials on germination, or the effects of fixed priming water potentials on the subsequent germination at a fixed water potential. When a seed is sown, the virtual osmotic potential is assumed to fall at a rate that depends on the ambient water potential, and on the difference between its current and a minimum value. Radicle growth is assumed to initiate when the difference between the ambient water potential and the virtual osmotic potential exceeds a threshold. The germination of carrot and onion seeds at various fixed potentials below 0 MPa was well described by the virtual osmotic potential model. The model was also used to simulate the results of experiments in which seeds were given a single step change in water potential.     

Comparison of regeneration following burning,clearing or mineral sand mining at Tomago,NSW: I. Structure and growth of the vegetation

Abstract We have begun a long-term ecological research project to address questions about the impact of multiple disturbances on the species richness of communities and whether multiple disturbances are additive or interactive. A protected water catchment area was chosen, which is subjected to fires, sand mining and clearing, and for which detailed records are available. The study area, at Tomago (32°52′S, 151°45′E), has forest, woodland, shrubland and swamp on a sand substrate, with the vegetated dunes forming part of a coastal embayment. Forty-four sites were located in forested areas that had undergone disturbance by either fire, sand mining or clearing. Sites of each disturbance type were grouped into four age classes: less than 1 year since disturbance, nominally 1991; 5 years, nominally 1986; 11 years, nominally 1980; and 17 years, nominally 1974. A set of burned sites, with the time of the last fire matched to the times of the other disturbances, was used as the control response. In this paper we describe the study area and sites, then examine the effects of each single disturbance on vegetation structure. Canopy cover increased with time and type of disturbance, with 17 year old cleared or mined sites similar to the cover of 11 year old burned sites. In the first two years after disturbance, burned sites had significantly more understorey vegetation than cleared or mined sites, but by 5 years all three were similar. The data presented here show that regeneration of mined sites at Tomago is substantially slower than regeneration following disturbance by fire, with the regeneration of cleared sites intermediate but closer to mining than fire. After 17 years regeneration, cleared and sand mined sites had not returned to the vegetation structure of the pre-disturbance state. Understorey height and the amount of vegetation on cleared or mined sites have not achieved the levels in the original forest, although canopy cover did seem to have reached pre-disturbance levels. Current rehabilitation techniques are more sophisticated than those used 17 years ago and continued monitoring of sites currently being rehabilitated may show a faster return to pre-disturbance states. Having established the hierarchy and nature of the response to each single disturbance here, we are now in a position to investigate the impact of multiple disturbances.     

Water Potential-Water Content Relationships In Apple Leaves

Three methods for determining the relationship between xylempressure potential as measured in a pressure chamber (an estimateof leaf water potential) and leaf relative water content werecompared for apple leaves. A range of leaf water contents wasobtained either by sampling leaves in the field at differenttimes of day and on days with differing evaporative demand,or by allowing evaporation from excised leaves in the laboratory,or by expressing sap by overpressurization in a pressure chamber.The first two methods gave very similar results, but the lasttended to give rather lower water potentials at any given watercontent. A possible explanation for these results and theirimplications for the estimation of osmotic potentials usingpressure-volume curves are discussed. Some osmotic adjustmentwas observed in trees droughted for 3 months, with estimatedosmotic potentials, both at full turgor and zero turgor, beingnearly 0.3 MPa lower than in irrigated controls.     

Growth and Water Relations of Wilty Mutants of Tomato (Lycopersicon esculentum Mill.)

In contrast to some previous reports on the growth of the ABA-deficientwilty mutants of tomato, growth was at least as rapid in themutants as in the wild type, as long as an adequate plant waterstatus was maintained by growing the plants under mist. Moreover,shoot extension was greater and the rate of leaf productionmore rapid in the mutants. Stomatal changes in response to environmentand to time in the light-dark cycle were generally similar inboth wilty mutants and the wild type, though the wild-type weregenerally more closed. Grafting experiments confirmed that thegenotype of the shoot was dominant in determining stomatal aperture,though wild-type rootstocks could cause a slight reduction inthe stomatal conductance of mutant leaves. The effect on plantwater relations of draughting only part of the root system wasinvestigated in a ‘split-root’ experiment. Withholdingwater from only part of the root system was found to lower significantlythe mean leaf water potential, even though the potential evaporationrate was kept very small. Key words: Abscisic acid, stomata, tomato     

A Microcomputer-Based System for Continuous Measurement and Recording Fruit Diameter in Relation to Environmental Factors

A robust yet sensitive field system for continuously measuringand logging diameters of up to eight fruits, together with relevantmeteorological variables such as radiation, temperature andhumidity, is described. The system uses linear variable differentialtransformers to monitor fruit diameter changes and data-loggingis controlled by a microcomputer. A detailed analysis of errorsis presented. Application of the system to the study of environmentalcontrol of diurnal fluctuations in diameter of apple fruitsis described. Key words: Apple, Data logger, LVDTs (linear variable differential transformers)     

Saliva activated transmission (SAT) of Thogoto virus: relationship with vector potential of different haematophagous arthropods

Tick saliva (or salivary gland extract) potentiates the transmission of Thogoto (THO) virus to uninfected ticks feeding on a non-viraemic guinea-pig. This phenomenon has been named saliva activated transmission (SAT). To investigate the potential of different haematophagous arthropods to mediate SAT, guinea-pigs were infested with uninfected R.appendiculatus Neumann nymphs and inoculated with THO virus and salivary gland extract (SGE) derived from a range of ixodid (metastriate and prostriate) or argasid ticks, or mosquitoes; control guinea-pigs were inoculated with virus alone. Enhancement of THO virus transmission was observed only when SGE was derived from metastriate ticks. Comparison with the vector potential of these various arthropod species revealed that enhancement of THO virus transmission was specific for ticks which were competent vectors of the virus. The data indicate a correlation between vector competence and the ability of haematophagous arthropods to mediate SAT of THO virus.     

Surface Conductance and Water Balance of Developing Apple (Malus pumila Mill.) Fruits

Studies of the relative magnitude of the various componentsof the water balance of developing apple fruits are described.Water entering the fruit can be used in growth, evaporationor, subsequently, a reverse flow to other tissues. Estimatesof evaporation were obtained from the weight loss by detachedfruit hanging in their natural positions in the orchard. Evaporationrates could be estimated with reasonable precision using themaximum daily vapour pressure deficit x fruit surface conductanceto water vapour. A method is described for the measurement offruit surface conductance in the laboratory or the field. Althoughthe conductance declined markedly during the season from ca.1.5 x 10^–3 m s^–1 in early May to less than 1.0 x10^–4 m s^–1 in September, the rate of water lossper fruit only declined slightly over this period. There weresignificant varietal differences in surface conductance, withBramley apples having the lowest conductances of those studied,and Egremont Russet the highest. Fruit diameter gauges wereused to provide continuous records of diurnal fluctuations insize of attached fruit. These records were used to estimatediurnal growth and shrinkage. The calculated volume shrinkageeach day averaged 31% of net growth in early August and waslargely accounted for by evaporation, with relatively littlewater flowing out of the fruit to other tissues.