observe great changes in vegetation richness and composition after two decades of
grazing exclusion in subalpine grasslands in the central Pyrenees. In Collada de
Montalto (Central Pyrenees), shrub encroachment occurred mostly in grass patches
inside the shrublands while woody proliferation into open grassland was less evident. Comparing the border between grassland and shrubland, we estimated a mean
shrub expansion of the border into the mesic grassland of 2.0 ± 1.4 m (n = 263) in
the period between 1997 and 2014 (unpublished data).
When woody plant invasion occurs, the shift from grass- to a shrub-dominated
ecosystem entails significant changes in the production and placement of the inputs
of litter (i.e. aboveground or belowground) and on factors that regulate soil organic
matter mineralisation such as microclimate, biochemical quality of organic matter
and the structure of the microbial community.
9.4.4.1 Litter Inputs
After shrub encroachment, the pattern of litter inputs changes from a belowground
predominance in grasslands to an increase of aboveground deposition in shrublands. This shift is due to the differential allocation patterns between grasses and
shrubs (Lett et al. 2004). For instance, in Cytisus balansae shrublands that had
invaded subalpine grasslands of the Pyrenees, aboveground litter input was estimated as high as fourfold the root litter inputs in the top 15 cm of soil (Montané
et al. 2010).
Surface litter is partially decomposed to CO 2 , but a fraction is incorporated into
the mineral horizons as a dissolved or particulate organic matter where it is mineralised or stabilised. Indeed, litter layers under shrubs may be an important source
of dissolved organic C into the mineral soil, with a flux that may represent up to
35% of the annual litterfall C (Kalbitz and Kaiser 2008). Also, free particulate
organic matter plays a significant role in the increase of soil organic C in the
uppermost layers after woody plant encroachment in grasslands (Liao et al. 2006).
Consequently, shrub encroachment into grasslands modifies the amount and
placement of organic matter into the soil, but the effect on C sequestration also
depends on the C loss from decomposing litter and soil organic matter.
9.4.4.2 Microclimate
Changes in the plant cover after pasture abandonment and shrub encroachment
modify incoming solar radiation and precipitation to the soil. After grazing removal,
the accumulation of standing necromass and litter reduces soil temperature and may
increase soil water content (Rosset et al. 2001). In the Pyrenees, cumulative
degree-days above 0 °C from May to November at 5 cm-depth soil were about 20%
lower beneath woody canopies than under grasses (Montané et al. 2010). Lower
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221
grazing exclusion in subalpine grasslands in the central Pyrenees. In Collada de
Montalto (Central Pyrenees), shrub encroachment occurred mostly in grass patches
inside the shrublands while woody proliferation into open grassland was less evident. Comparing the border between grassland and shrubland, we estimated a mean
shrub expansion of the border into the mesic grassland of 2.0 ± 1.4 m (n = 263) in
the period between 1997 and 2014 (unpublished data).
When woody plant invasion occurs, the shift from grass- to a shrub-dominated
ecosystem entails significant changes in the production and placement of the inputs
of litter (i.e. aboveground or belowground) and on factors that regulate soil organic
matter mineralisation such as microclimate, biochemical quality of organic matter
and the structure of the microbial community.
9.4.4.1 Litter Inputs
After shrub encroachment, the pattern of litter inputs changes from a belowground
predominance in grasslands to an increase of aboveground deposition in shrublands. This shift is due to the differential allocation patterns between grasses and
shrubs (Lett et al. 2004). For instance, in Cytisus balansae shrublands that had
invaded subalpine grasslands of the Pyrenees, aboveground litter input was estimated as high as fourfold the root litter inputs in the top 15 cm of soil (Montané
et al. 2010).
Surface litter is partially decomposed to CO 2 , but a fraction is incorporated into
the mineral horizons as a dissolved or particulate organic matter where it is mineralised or stabilised. Indeed, litter layers under shrubs may be an important source
of dissolved organic C into the mineral soil, with a flux that may represent up to
35% of the annual litterfall C (Kalbitz and Kaiser 2008). Also, free particulate
organic matter plays a significant role in the increase of soil organic C in the
uppermost layers after woody plant encroachment in grasslands (Liao et al. 2006).
Consequently, shrub encroachment into grasslands modifies the amount and
placement of organic matter into the soil, but the effect on C sequestration also
depends on the C loss from decomposing litter and soil organic matter.
9.4.4.2 Microclimate
Changes in the plant cover after pasture abandonment and shrub encroachment
modify incoming solar radiation and precipitation to the soil. After grazing removal,
the accumulation of standing necromass and litter reduces soil temperature and may
increase soil water content (Rosset et al. 2001). In the Pyrenees, cumulative
degree-days above 0 °C from May to November at 5 cm-depth soil were about 20%
lower beneath woody canopies than under grasses (Montané et al. 2010). Lower
9 Are Soil Carbon Stocks in Mountain Grasslands …
221