(Anthelme et al. 2007) and increases in soil C storage (Montané et al. 2007) have
been reported after shrub proliferation into grasslands. Woody encroachment
increases the risk of fire propagation by incrementing both fuel load and fuel
continuity. In these encroachment-prone communities, managers may have to
decide between reducing shrub proliferation to maintain biodiversity and grazing
potential or allowing the shrub proliferation to increase C sequestration.
In the Pyrenees, shepherds have traditionally used fire as a management tool to
improve grass productivity and transform encroached land into grassland. Today
prescribed burning is usually carried out by fire brigades or foresters in winter when
snowy or wet conditions limit the impact of the fire on soils and herbaceous plants
(Rigolot et al. 2002). Mechanical thinning is also applied to revert encroached
grasslands. How these management options affect organic matter dynamics and soil
C stocks remains an issue.
9.5 Conclusions and Further Research Needs
Agricultural land-use changes in the European mountains show antagonist trends,
intensification at the bottom of valleys and other productive grasslands, whereas the
less productive grasslands located on steep slopes and at higher altitudes are being
increasingly abandoned. A mechanistic understanding of how these changes affect
relevant ecological processes, such as biodiversity or C stocks, is necessary to
predict the effects of global change on ecosystem function and deliver appropriate
management recommendations.
The decline of agropastoral activities is especially pervasive in high mountain
grasslands. As a consequence, pasture abandonment, especially of marginal and less
productive lands, and shrub encroachment into grasslands have become the most
significant trends in land use, which may be observed all around European
mountains, to a greater or lesser extent. Short-term changes in soil C stocks as a
consequence of grazing abandonment are difficult to detect due to the large size of
the organic matter pool as compared to the small changes in the C inputs, and also
to the high spatial variability of soil C stocks (Conant and Paustian 2002; Smith
2004). Smith (2004), using a modelling approach, demonstrated that a change in
SOC may not be detectable until about 7–10 year after the experiment, assuming an
increase in soil C input of 20–25% (Smith 2004). Therefore, well monitored
long-term exclusion experiments would be very useful to measure changes in the C
stocks caused by management changes.
Higher soil organic matter after shrub encroachment into grasslands may be
explained by the high content of recalcitrant compounds such as lignin and
polyphenols in the plant-derived organic matter inputs, which slows the decomposition of soil organic matter and delays its incorporation into the protected pools
in the mineral soil.
Due to high fine root density and rhizosphere exudation rates, grassland soils
show higher priming effect on C mineralisation than in woodland soils (Waldrop
224
J. Garcia-Pausas et al.
been reported after shrub proliferation into grasslands. Woody encroachment
increases the risk of fire propagation by incrementing both fuel load and fuel
continuity. In these encroachment-prone communities, managers may have to
decide between reducing shrub proliferation to maintain biodiversity and grazing
potential or allowing the shrub proliferation to increase C sequestration.
In the Pyrenees, shepherds have traditionally used fire as a management tool to
improve grass productivity and transform encroached land into grassland. Today
prescribed burning is usually carried out by fire brigades or foresters in winter when
snowy or wet conditions limit the impact of the fire on soils and herbaceous plants
(Rigolot et al. 2002). Mechanical thinning is also applied to revert encroached
grasslands. How these management options affect organic matter dynamics and soil
C stocks remains an issue.
9.5 Conclusions and Further Research Needs
Agricultural land-use changes in the European mountains show antagonist trends,
intensification at the bottom of valleys and other productive grasslands, whereas the
less productive grasslands located on steep slopes and at higher altitudes are being
increasingly abandoned. A mechanistic understanding of how these changes affect
relevant ecological processes, such as biodiversity or C stocks, is necessary to
predict the effects of global change on ecosystem function and deliver appropriate
management recommendations.
The decline of agropastoral activities is especially pervasive in high mountain
grasslands. As a consequence, pasture abandonment, especially of marginal and less
productive lands, and shrub encroachment into grasslands have become the most
significant trends in land use, which may be observed all around European
mountains, to a greater or lesser extent. Short-term changes in soil C stocks as a
consequence of grazing abandonment are difficult to detect due to the large size of
the organic matter pool as compared to the small changes in the C inputs, and also
to the high spatial variability of soil C stocks (Conant and Paustian 2002; Smith
2004). Smith (2004), using a modelling approach, demonstrated that a change in
SOC may not be detectable until about 7–10 year after the experiment, assuming an
increase in soil C input of 20–25% (Smith 2004). Therefore, well monitored
long-term exclusion experiments would be very useful to measure changes in the C
stocks caused by management changes.
Higher soil organic matter after shrub encroachment into grasslands may be
explained by the high content of recalcitrant compounds such as lignin and
polyphenols in the plant-derived organic matter inputs, which slows the decomposition of soil organic matter and delays its incorporation into the protected pools
in the mineral soil.
Due to high fine root density and rhizosphere exudation rates, grassland soils
show higher priming effect on C mineralisation than in woodland soils (Waldrop
224
J. Garcia-Pausas et al.