Response Of Plant Productivity Research Articles

Analysing the responses of photosynthetic CO2 assimilation to long-term elevation of atmospheric CO2 concentration

Understanding how photosynthetic capacity acclimatises when plants are grown in an atmosphere of rising CO2 concentrations will be vital to the development of mechanistic models of the response of plant productivity to global environmental change. A limitation to the study of acclimatisation is the small amount of material that may be destructively harvested from long-term studies of the effects of elevation of CO2 concentration. Technological developments in the measurement of gas exchange, fluorescence and absorption spectroscopy, coupled with theoretical developments in the interpretation of measured values now allow detailed analyses of limitations to photosynthesisin vivo. The use of leaf chambers with Ulbricht integrating spheres allows separation of change in the maximum efficiency of energy transduction in the assimilation of CO2 from changes in tissue absorptance. Analysis of the response of CO2 assimilation to intercellular CO2 concentration allows quantitative determination of the limitation imposed by stomata, carboxylation efficiency, and the rate of regeneration of ribulose 1:5 bisphosphate. Chlorophyll fluorescence provides a rapid method for detecting photoinhibition in heterogeneously illuminated leaves within canopies in the field. Modulated fluorescence and absorption spectroscopy allow parallel measurements of the efficiency of light utilisation in electron transport through photosystems I and IIin situ.

Ecological conditions that determine when grazing stimulates grass production

We report the results of a pot experiment that examined the effects of three ecologically important factors controlling plant growth rates in savanna grasslands: defoliation, soil nitrogen and soil water availability. The experiment was conducted in the Amboseli region in east Africa, and was designed to simulate natural conditions as far as possible, using local soils and a grass species that is heavily grazed by abundant large herbivores. Productivity by different plant components was reduced, stimulated or unchanged by defoliation, depending on specific watering and fertilization treatments. Total above-ground production was stimulated by defoliation and was maximized at moderate clipping intensities, but this was statistically significant only when plants were watered infrequently (every 8 days), and most important, periods between clipping events were extended (at least 24 days). Under these conditions, plant growth rates were limited by water availability at the time of clipping, and soil water conserved in clipped, compared to unclipped plants. Within a given fertilization treatment, whole-plant production was never stimulated by defoliation because root growth was unaffected or inhibited by clipping. However, when fertilization was coupled to defoliation, as they are in the field, whole-plant production by fertilized and moderately clipped plants exceeded production by infertilized, unclipped plants. Under this interpretation, maximum whole-plant production coincided with optimum conditions for herbivores (maximum nitrogen concentration in grass leaves) when watering was frequent, and plants were moderately defoliated. However, these conditions were not the same as those that maximized relative above-ground stimulation of growth (infrequent watering and clipping).The results indicate that above-ground grass production can be stimulated by grazing, and when that is likely to occur. However, the results emphasize that plant production responses to defoliation can vary widely, contigent upon a complex interaction of ecological factors.

Carbon flows in the biosphere: present and future

The atmosphere, biosphere and ocean surface layers contain approximately equal amounts of carbon (the deep ocean contains over 50 times as much). The flux of carbon between these pools is dominated by the process of photosynthesis which is itself influenced by the CO 2 concentration of the atmosphere. Over the last 200 years deforestation and now fossil fuel combustion have added CO 2 to the atmosphere to increase the concentration by 27%—half the increase occurred over the last 30 years. The resulting greenhouse effect has been accompanied by an increase in the global average temperature of about 0.5°; other 'greenhouse gases' (N 2 O 1 O 3 , CFCs, CH 4 ) are also contributing to the warming. Present uncertainties with estimating carbon flows result from the disputed contribution of deforestation, the level of increased plant productivity and carbon storage due to CO 2 fertilization, and problems with estimating net primary production. Uncertainties in estimating future carbon balances arise from the extent of fossil fuel use especially by USA, USSR and China, the degree of control of greenhouse gas emissions, the CO 2 fertilization effect on plant productivity and vegetation responses (including moisture availability) and the rate of change of the climatic factors resulting from changing carbon fluxes.

An Evaluation of Grazing Intensity Influences on California Annual Range

Influences of grazing intensity on species composition and herbage production of grass-woodland and improved grassland subtypes of annual range were evaluated over a 5-year period in coastal northern Califomia using 3 grazing treatments (100, 150, and 200o of moderate stocking). Herbage utilization did not differ signiflcantly between the 2 subtypes but averaged 42, 52, and 69%o for the respective treatments. Plant species and production responses differed significantly between woodland and grassland subtypes. On woodland, ripgut brome (Bromusrigidus Roth.)l and wild oats (A vena barbata Brot. and A. fatua L.) were most sensitive to grazing intensity while wild barley (Hordeum leporinum Link. and H. hystrix Roth.) and annual fescue (Festuca dertonensis (all.) Asch. and Graebn. and F. megalura Nutt.) were least sensitive. On improved grassland, subterranean clover (Trifolium subterraneum L.) increased and soft chess (Bromus mollis L.) decreased with increasing grazing intensity. Soft chess remained most plentiful on woodland range under heaviest grazing and it continued to be a major species under heavy grazing of grassland, demonstrating tolerance to grazing intensity. Filaree (Erodium cicutarium (L.) L'Her. and E. botrys (Cay.) Bertol.) declined on woodland but increased on grassland as grazing intensified. Peak standing crop was not significantly affected by grazing intensity on woodland range but was greatest at 150%o of moderate stocking and lowest at 200% of moderate stocking on grassland range. Decline in grassland herbage yield under heaviest grazing was due to reduction of soft chess which was displaced by subterranean clover. Effects of grazing intensity on range composition and productivity were confounded by innate differences in ranges and yearly weather patterns. Herbage production was impacted more by annual growing conditions than by grazing regimens, but there was no correlation between total annual precipitation and peak standing crop.