The Biosphere

Casey Ryan

The changing carbon dynamics in miombo woodlands

War, colonial trading patterns, tsetse fly and a lack of infrastructure have limited land use change in Southern African forests. This is now changing, and rapid destruction and degradation is underway, particularly in miombo woodlands, the most widespread forest in Africa below the Equator.

This change is globally significant: it threatens the provision of food, grazing, fuel, timber and medicine to 45 million of the world's poorest people, and will alter the role of miombo in the functioning of the Earth System. This is for many, interconnected reasons, but only 3 will be described here.

1. The aboveground biomass and soil carbon in miombo is a vulnerable stock of carbon ~10Pg in size. Well managed, this stock can be increased whilst still preserving the ecosystem services that are critical to the woodland's inhabitants. Currently, because population growth far outstrips economic growth, subsistence slash and burn agriculture is expanding and carbon is being lost to the atmosphere.

2. The near-annual fires in miombo incompletely combust dead grass, leaf litter and woody debris, and result in large fluxes of gases and particles that are radiatively active (CO₂, CH₄, N₂O, soot), affect the oxidation capacity of the atmosphere (isoprene, terpenes, dimethyl sulphide) and alter cloud formation processes.

3. These fires are considered a small net sink of atmospheric CO₂ from the biosphere. The combusted grass and shrubs soon regrow but the fires convert ~1% of the carbon burnt to a super-refractory form that persists on geological time scales. The processes which return this 'black carbon' to the biological cycle are unknown, but without them all atmospheric CO₂ would be converted to black carbon in 100 000 years.

To investigate these processes, 15 x 1ha sample plots have been established in central Mozambique. Long term measurements of leaf area, litter fall, grass and shrub biomass, and tree growth and mortality will be combined with soil surveys, in situ and remote sensing of fire intensity, and satellite measurements of atmospheric chemistry to try to quantify the role of biomass burning and land use change in miombo carbon dynamics.

Casey's presentation

Andrew Cross

Using stable isotopes to study carbon flows in forest ecosystems with relation to climate change

Forests capture some 120 billion tonnes of carbon each year through photosynthesis and release 60 billion tonnes back into the atmosphere as plant respiration. The remaining 60 billion tonnes enters two large pools, the biomass and the soil organic carbon. The balance between photosynthesis and respiration determines whether the landscape acts as a sink or source of CO₂. New approaches are being developed to study these dynamic changes and to see how rapidly they respond to climate change. This project will measure plant and microbial respiration of forests and soils, using a novel technique which can distinguish between the isotopes of carbon. There is a large amount of uncertainty surrounding the use and allocation of carbon in forest ecosystems. Temporal variability of isotopic ecosystem respiration is often greater than spatial variability, and changes in variables such as temperature and moisture affect the isotopic ratio (¹³C/¹²C) of respired CO₂. Differences in the isotope ratio of photosynthesis and respiration can be used to separate net ecosystem exchange into the fluxes of photosynthetic uptake and respiratory loss, and isotopic measurements of CO₂ can also be used as tracers to determine how long it takes for the ecosystem to cycle carbon. Traditional techniques used for measuring carbon isotopes are laborious and have poor temporal resolution. In this project however we will use a tunable diode laser, which is capable of measuring CO₂, ¹²C and ¹³C at high frequencies (10Hz) and accuracy. Measurements will initially be lab-based focusing on soil respiration as a function of temperature and moisture, using soils from different ecotones. Measurements will then be made in various forests with experimental treatments such as droughting. The results will help to identify pathways taken by carbon in forest ecosystems, and will also be integrated into carbon cycle models.

Andy's presentation

Sigrid Dengel

Biological response of a Sitka Sruce forest to changes in sky condition, by using a eddy flux system

This project is part of the CARBOEUROPE-IP programme, which aims to better understand the current and future carbon balance of Europe at a local as well as continental scale. The work will include the measurement of CO₂ and H₂O fluxes, but also micrometeorological and physiological measurements at two sites: a mature Sitka Spruce (Picea sitchensis) forest and a recently clear felled plot. Both are situated in Harwood Forest in Northumberland, England. The aim of the project is to develop relationships between the carbon assimilation (photosynthesis), the disturbance factor (soil respiration/fluxes from roots, debris left behind in the clear fell), and the climatological variables (diffuse light/sky conditions/humidity).

Both sites will be equipped with eddy covariance systems, the forest site with a closed path Li-COR6262 Infra-Red gas analyser and the clear fell with an open path Li-COR7500. Ultrasonic anemometers will provide the 3D component of the wind direction. Both sites will contain meteorological stations. WebCams will monitor sky conditions and canopy phenology. Furthermore, aircraft data will be included in this work.

It is well known that diffuse light enhances photosynthesis, but no distinction has been made between the phenomena causing diffuse irradiance. These include: haze (aerosols) clouds (dry canopy, shading), fog (humidity stress - long term, higher chemical concentration of intercepting H2O, higher radiation than under just cloudy conditions, no shading effect) or even rain (humidity stress - short term, drip off effect, nutrients input via ground, no shading effect, lower radiation values, less chemical concentrated H₂O).

It is also planned to use data from the nearest MetOffice station in order to detect changes in sky properties over the past 30 years. This should give us an overview on annual sky condition in this area and whether or not these variables have changed in the last years, as part of ongoing climate change/global dimming.

In its natural range on the west coast of North America, Sitka spruce is known to grow in predominantly humid environments. Concurrently we also know that diffuse light is enhancing carbon uptake but no work exists on simultaneous measurements of these two aspects. This project will investigate the main attributes of diffuse light and humidity, in order to determine which variable is stimulating carbon uptake.

Sigrid's Presentation

Witness Mojermane

The impact of site preparation for afforestation on soil properties & trace gas emissions

Soil plays an important role in the formation, maintenance and turnover of soil organic matter influencing flow storage and the sink-source and balance of carbon. Soil carbon plays a crucial role in the biological, chemical and physical health of soil. Soil contains three times as much carbon as in plants and approximately twice as much as in the atmosphere. Site preparation has been an integral part of stand establishment for many years with approximately 15 million ha of peatlands and wetlands drained for forestry in Europe. Site preparation has been used to improve conditions for tree growth by changing soil site features. It has been important in ensuring successful and cost effective establishment of plantations

In Britain cultivation has been the most frequently used method of preparing planting sites. Soil and its use contribute substantially to the greenhouse effect; i.e. the enhanced warming of the earth surface and atmosphere as a result of increased emissions of CO₂ and trace gases into the atmosphere. The vital role of soils as a sink or source for C at the global scale and in offsetting atmospheric CO₂ concentration makes it important to evaluate accurately the impact of site preparation on soil properties and greenhouse gas emissions to the atmosphere. Peatlands are natural carbon accumulating ecosystems which affect the global climate by decreasing carbon dioxide and increasing methane. This study is designed to determine the impact of site preparation for afforestation on soil properties and trace gas emissions. Site preparation (drainage ditches, mounding and fertilization) will be carried out in unplanted experimental plots to be established at Harwood Forest. Closed static chambers will be used to measure trace gases (CO2, N₂O and CH₄) and soil properties will be determined in the laboratory by analyzing soil samples collected from experimental plots. The study is important in evaluating the impact of site preparation on soil properties, and emissions of the most important greenhouse gases (CO₂, N₂O and CH₄).

Witness's Presentation