CLM20-2 full issue-1 - Flipbook - Page 22
A guide to conservation land management and greenhouse gas emissions
Left Undamaged bogs sequester carbon very slowly, but hold large quantities of carbon in their peat.
The draining of bogs therefore leads to the release of large quantities of CO2 through oxidation of the
peat. Right During the first hundred years following establishment, 1ha of native broadleaved woodland
removes an average of around seven tonnes of CO2 per year. Iain Masterton/Alamy Stock Photo (left); John
Morrison/Alamy Stock Photo (right)
Semi-natural ‘dry’ habitats and farmland
Semi-natural ‘dry’ habitats on soils that are
not derived from drained peat usually exert a
cooling effect on the climate. These habitats
include lowland heathland and sand dunes, and
also dry grassland, upland heath, dry scrub and
dry woodland where they occur on mineral or
organo-mineral soils. The cooling produced by
these habitats is due to their net removal of CO2
from the atmosphere through photosynthesis by
plants, and the subsequent accumulation of a
proportion of the carbon from this in vegetation
and soil. Their net fluxes of methane and nitrous
oxide are generally negligible, unless subject to
artificial fertilisation and/or grazing by ruminants
(i.e. cattle and sheep).
A number of factors can affect the rate that
the vegetation and soil accumulate carbon,
and thereby the effect of a given habitat on the
climate. For instance, the GHG flux of woodland
varies in relation to its age, the species, density
and growth rates of trees and any habitat
management that takes place (above a certain
level, planting higher densities of trees will
have little or no effect on the rate of carbon
sequestration, because it will not increase the
overall rate of tree growth per unit area).
In the case of unmanaged woodland, the net
rate of uptake of CO2 is low when the trees
are small, while soil disturbance caused when
planting trees can even initially result in a net
release of CO2 to the atmosphere. The rate of
uptake of CO2 then increases during the main
growth phase of the trees, only to slow as the
woodland matures – vegetation biomass per unit
area does not continue to increase indefinitely.
Carbon will, though, still tend to continue
accumulating in the soil. Because of this large
variation in GHG flux over time, the GHG flux
of woodland is often estimated over a long period
and then expressed as a mean annual rate. For
example, in Figure 1 we show the mean annual
GHG flux of dry broadleaved woodland over 30
years and 100 years from establishment.
Arable and drained grassland on organic soils
(i.e. those derived from peat) both produce a big
warming effect per unit area (see Figure 1). This
is due mainly to the release of large quantities
of CO2 through oxidation of the dried-out peat,
with this being especially high on arable due
to cultivation repeatedly exposing the peat to
microbial action. In addition, intensive arable
farming on both organic and mineral soils leads
to significant emissions of GHGs from the
manufacture and use of nitrate fertilisers applied
to them, and also from the use of machinery (e.g.
Williams et al. 2010). Intensive grassland on
both organic and mineral soils also results in the
20 Conservation Land Management Summer 2022 | Vol. 20 No. 2
