Vegetation and soil

Beech forestEcosystem Function and Productivity

An ecosystem is a functioning and interacting system composed of plants, animals and other living organisms (the biomass) and the surrounding environment to which they are linked physically, chemically and biologically. Ecosystems are open systems, with their continued functioning dependent upon external inputs of solar energy and materials such as water and dissolved nutrients. The annual rate of biomass growth in an ecosystem is termed net primary productivity (NPP).

The inter-relatedness of the component parts of an ecosystem are apparent in the study of (i) the flows of energy that occur along food chains and within food webs and (ii) the nutrient cycles that maintain plant and animal growth over time. Food chains are sustained by sunlight, captured by the leaves of green plants and converted into carbohydrates as part of the process of photosynthesis. Nutrient cycles are fed by inputs of dissolved minerals in rainfall, countering losses from leaching and run-off.

Ecosystems, food chains and nutrient cycles are all said to exist in a state of dynamic equilibrium, where inputs of energy and matter are balanced with outputs in the long term. However this assumes relatively constant inputs of solar radiation and precipitation (and outputs such as evaporation). Climate change means that these inputs are changing.

How will climate change modify the UK's ecosystems?

There is no firm consensus from scientists on exactly how climate will change in the future and the consequent impact that this will have on photosynthesis and nutrient cycling. However, data suggests that:

  • Global atmospheric concentration of CO2 has increased to a current level of 380 parts per million.
  • Growing seasons for plants in central England have already lengthened by about one month since 1900.
  • Heat-waves have become more frequent in summer (e.g. 2006), while there are now fewer frosts and winter cold spells.
  • Wetter winters have arisen relative to summers throughout the UK over the last 200 years but with a larger proportion of precipitation falling on heavy rainfall days (causing surface run-off, meaning that less water soaks into the soil).

This wide range of climatic changes means that impacts on productivity, food webs and nutrient cycles are complex and often contradictory. They are neither uniformly negative nor positive across the whole of the UK. For instance, growth rates may be increasing wherever plants are not limited by a lack of water or nutrient availability - for instance on flood plains with a high water table. This is due to increased photosynthesis due to a longer thermal growing season and increased levels of atmospheric CO2.

However, where water supplies are limited - for instance in well-drained areas with permeable soils or parent material - then trees will become stressed and experience leaf fall during heat-waves. Grasses will die back to their roots. This leads to a decline in biomass and productivity within the autotrophic layer (and in turn will adversely effect other organisms in the food chain).

Student Practice Question:

Outline the factors that influence rates of ecosystem productivity

As well as looking at how rates vary from place-to-place, a good answer might describe climate change as a factor that may have a long-term impact on rates of productivity over time.