Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems

Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.

The global carbon cycle: a test of our knowledge of earth as a system

Motivated by the rapid increase in atmospheric CO2 due to human activities since the Industrial Revolution, several international scientific research programs have analyzed the role of individual components of the Earth system in the global carbon cycle. Our knowledge of the carbon cycle within the oceans, terrestrial ecosystems, and the atmosphere is sufficiently extensive to permit us to conclude that although natural processes can potentially slow the rate of increase in atmospheric CO2, there is no natural "savior" waiting to assimilate all the anthropogenically produced CO2 in the coming century. Our knowledge is insufficient to describe the interactions between the components of the Earth system and the relationship between the carbon cycle and other biogeochemical and climatological processes. Overcoming this limitation requires a systems approach.

Downward flux of water through roots (i.e. inverse hydraulic lift) in dry Kalahari sands

Downward transport of water in roots, in the following termed "inverse hydraulic lift," has previously been shown with heat flux techniques. But water flow into deeper soil layers was demonstrated in this study for the first time when investigating several perennial grass species of the Kalahari Desert under field conditions. Deuterium labelling was used to show that water acquired by roots from moist sand in the upper profile was transported through the root system to roots deeper in the profile and released into the dry sand at these depths. Inverse hydraulic lift may serve as an important mechanism to facilitate root growth through the dry soil layers underlaying the upper profile where precipitation penetrates. This may allow roots to reach deep sources of moisture in water-limited ecosystems such as the Kalahari Desert.

Measurement of bone lengthening forces; an experimental model in the lamb

OBJECTIVE: To obtain the mechanical behaviour pattern of the lengthening process. DESIGN. IN VIVO: measurement of forces during bone lengthening in lambs. BACKGROUND: A series of parameters of a mechanical and biological nature have a bearing on all lengthening processes, and most of them are not fully understood. METHODS: A strain-gauge-monitored unilateral fixator was designed and used to obtain data about the changes which took place in the forces of elongation at a rate of 1 mm/day in four lambs while a 3 cm progressive lengthening of the left tibia was being performed, analysing how these forces behaved from day to day, and how they changed in the course of a single day. RESULTS: The maximum forces in all the animals each day occur after distraction, and the forces reach their greatest magnitude between days 21 and 25 after surgery, attaining values of slightly over 8 kg (40-50% of the animal's weight). The maximum daily force starts to drop 1 h after distraction, and continues to decrease gradually throughout the day until it reaches a value slightly greater than the initial force on the previous day. CONCLUSION: This pattern is due to the distraction of soft tissues which gradually adapt to their new situation, thereby reducing the level of stress. RELEVANCE: In the daily bone lengthening procedure, the greatest forces are produced in a short period of time immediately after lengthening. they could be reduced to decrease pain in the patient and loads on the device by performing lengthening over a greater number of steps or using dynamic equipment able to absorb these forces.

Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia

Above-and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44-45°S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above-and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at-27‰), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80-0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2-3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2-3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.

Correction of angular deformities by physeal distraction

Physeal distraction is an alternative to more conventional treatments for the correction of angular deformities of the long bones. Twenty deformities of the femur and tibia, nine of which also involved associated shortening, were partially or completely corrected. In eight cases, there was physeal bony bridge. Complete correction of the angular deformity was achieved in 17 patients, and in seven patients, more than 80% correction was achieved. There were complications in four patients that hindered complete correction of the deformity, or shortening, or both. The external control of the correction until consolidation occurs is progressive and fairly noninvasive. The method allows external control of the correction until consolidation; it acts at the site of the deformity itself and permits lengthening and angular correction during therapy. In deformities with a physeal bony bridge, correction can be achieved with physeal distraction alone, prior resection of the bridge is not unnecessary. The technique is indicated in cases of angular deformities in patients nearing skeletal maturity and particularly in subjects in whom there is associated shortening.

Experimental physeal distraction in immature sheep

An experimental study on physeal distraction was carried out in the distal femur of 45 two-month-old lambs for the purpose of identifying the basic mechanism of lengthening as well as assessing growth cartilage viability after lengthening. The animals were divided into three groups, each with three subgroups, according to the distraction rate employed (2 mm/day, 1 mm/day, 0.5 mm/day) and the time at which the animals were killed (end of lengthening, 1.5 months postlengthening, and at six months of age). Another group of three animals whose femurs were lengthened at a rate of 0.5 mm/day was killed ten days postoperatively. Roentgenologic, specimen measurements, and histologic studies were performed on all animals. The results obtained showed that the basic mechanism for lengthening is the production of a physeal fracture between degenerative and calcified layers; this finding was consistent. It was also observed that the lower the rate of distraction employed, the greater was the short- and long-term viability of the growth cartilage. More specifically, optimal viability was observed when a distraction rate of 0.5 mm/day was employed.

The Pavlik harness in the treatment of congenital dislocating hip: report on a multicenter study of the European Paediatric Orthopaedic Society

The results of functional treatment using the Pavlik harness in congenital dislocation and congenital dysplasia of the hip in children aged less than 11 months were examined by an EPOS study group. This study was conducted on 3,611 hips in 2,636 patients for a period of 1-9 years after treatment. The reduction rate was 92% in grade Tonnis 2 and 3; the healing rate was 80%. In children with dysplastic hips, the healing rate was 95.35%. Avascular necrosis of the femoral head was observed in 2.38%. The Pavlik harness is designed for outpatient treatment if the parents are compliant.

Bone lengthening by physial distraction. An experimental study

Experimental physial distraction was carried out in the distal part of the femur in 45 two-month old lambs in order to study the basic mechanisms of lengthening as well as the viability of the growth cartilage after using this method. The animals were divided into three groups (A, B and C), and each group into three subgroups (1, 2 and 3) according to the rate of distraction used (2 mm/day, 1 mm/day, 0.5 mm/day) and the time of sacrifice. The results obtained show that the basic lengthening mechanisms consists, firstly, in the production of a fracture between the metaphysis and the epiphysis and, secondly, that the lower the distraction speed employed, the greater is the short-term and long-term viability of the growth cartilage. Optimum viability was observed at a distraction rate of 0.5 mm/day. On this basis we conclude that in clinical practice physical distraction could be indicated for children at an early stage of skeletal growth and repeated later provided that the rate of distraction is kept within reasonable limits.

Breaking bony bridges by physeal distraction. A new approach

Physeal distraction for breaking bony bridges and the late treatment of long bone deformities in children near maturity, has been used in our Department since June, 1983. In all cases (4 bones in 3 patients) a deformity was present at the extremity of a long bone due to a bony bridge caused by a previous injury to the growth cartilage. The results were excellent and in our opinion there are advantages over other methods. The operation itself is not extensive and very little damage is done. There is no need for internal fixation or bone grafts. It is possible to obtain lengthening and to adjust the angular correction during treatment. We conclude that it is possible to correct angular deformities and bone shortening due to bony bridges by physeal distraction without the need for resection of the bony bridge. At present we recommend this method in children near skeletal maturity, especially in those cases with bone shortening.