Understanding the factors controlling productivity is crucial for modelling current and predicting future forest growth and carbon sequestration potential. Although abiotic conditions exert a strong influence on productivity, it is becoming increasingly evident that plant community composition can dramatically influence ecosystem processes. However, much of our understanding of these processes in forests comes from correlative studies or field experiments in short-statured, short-lived vegetation. Here, we present the background, design and implementation success of the Australian Forest Evenness Experiment (AFEX), which was designed to investigate the influence of community composition on the processes that contribute to forest productivity. Eighty 25 × 25-m plots, covering 5 ha in a logged, burnt forest coupe in south-eastern Tasmania were sown with four tree species, namely Eucalyptus delegatensis R.T.Baker, E. regnans F.Muell., Acacia dealbata Link and Pomaderris apetala Labill., in varying combinations to provide a range of evenness levels with each of the four species as target dominant. Despite some differences between sown composition and realised composition 1year after sowing, a substantial range of community evenness and local neighbourhood densities and compositions existed in the experiment. Thus, this site provides a unique opportunity to determine the influence of local neighbourhood composition on a range of ecological processes.
Aims: Climate change will impact plant communities and populations but also individual plant performance. Most predictive models of community responses to climate change ignore individual-level biotic interactions despite their known importance for community diversity and functioning. Here, we consider plant fitness and diversity responses to climate change associated factors at three organisational levels: communities, populations and individual plants, to increase our understanding of how plant communities respond to climate change.
Plant morphology and architecture are essential characteristics for all plants, but perhaps most importantly for agricultural species because economic traits are linked to simple features such as blade length and plant height. Key morphological traits likely respond to CO2 concentration ([CO2]), and the degree of this response could be influenced by water availability; however, this has received comparatively little research attention. This study aimed to determine the impacts of [CO2] on gross morphology of perennial ryegrass (Lolium perenne L.), the most widespread temperate pasture species, and whether these impacts are influenced by water availability. Perennial ryegrass cv. Base AR37 was grown in a well-fertilised FACE (free-air carbon dioxide enrichment) experiment in southern Tasmania. Plants were exposed to three CO2 concentrations (~400 (ambient), 475 and 550 µmol mol–1) at three watering-treatment levels (adequate, limited and excess). Shoot dry weight, height, total leaf area, leaf-blade separation, leaf size, relative water content and specific leaf area were determined, as well as shoot density per unit area as a measure of tillering. Plant morphology responded dramatically to elevated [CO2], plants being smaller with shorter leaf-blade separation lengths and smaller leaves than in ambient (control) plots. Elevated [CO2] increased tillering but did not substantially affect relative water content or specific leaf area. Water supply did not affect any measured trait or the response to elevated [CO2]. Observed impacts of elevated [CO2] on the morphology of a globally important forage crop could have profound implications for pasture productivity. The reductions in plant and leaf size were consistent across a range of soil-water availability, indicating that they are likely to be uniform. Elucidating the mechanisms driving these responses will be essential to improving predictability of these changes and may assist in breeding varieties suited to future conditions.
Leaf area index (LAI) is an important driver of primary productivity, and affects water and nutrient cycling. Extra leaves have both a cost and a benefit to a plant in terms of carbon and water balance and nutrient economics. Greater leaf area increases photosynthetic area, but also incurs a respiratory cost to the plant in terms of leaf construction and maintenance. Optimal leaf area is therefore influenced by the trade-off between carbon gains through photosynthesis and carbon loss through respiration, but is also influenced by transpirational demands. Furthermore, optimal leaf area responds to environmental factors such as nutrition, temperature and water supply. Using three field experiments across a rainfall and temperature gradient in Tasmania, I investigated the way in which nutrient supply influences the optimal leaf area of the globally-important plantation tree, Eucalyptus nitens. Results show that the costs and benefits of extra leaf area depend on nutrient supply as well as site characteristics. Specifically, LAI was highest at intermediate nitrogen levels over the first growing season, with associated changes to maximum net photosynthetic rate, dark respiration and stomatal conductance. Thus, leaf area response to nutrition is decidedly non-linear in this system with corresponding influences on plant water use and physiology. These results will contribute to the development of efficient nutrition management of production forests through an improved ability to predict and model the impact of fertiliser on productivity.
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