302
Anna Avila
Three scenarios of atmospheric deposition were envisaged. In scenario a,
deposition was kept at background (pre-industrial) levels throughout the
simulation. In scenario b, deposition matched the evolution of S02 emissions
in Europe over the past 140 years until 1985 followed by a 60% reduction
over the next 20 years. In scenario c, deposition was similar to b until the present day, but constant at the 1985 levels thereafter. All ions in atmospheric
deposition were considered to vary in parallel to S02, except for Na+, which
remained at the pre-industrial level. Dry deposition was considered constant
in all forest management scenarios, increasing the wet deposition by a factor
of 1.8 for chloride, 1.9 for sulphate and 1.5 for the rest of the ions (Table 21.1).
The evolution of two variables (soil base saturation and streamwater alkalinity) which represent the general response of soil and streamwater chemistry to the different scenarios is shown in Figs. 21.2 and 21.3. The effect of atmospheric deposition was similar in all forestry simulations: there was a decrease in soil base saturation and streamwater alkalinity in the simulations of
the historical buildup of sulphur (scenarios b and c) with respect to the preindustrial scenario (scenario a), being most marked in the case where deposition remained at 1985 levels (scenario c). These downward trends resulting
from atmospheric pollution were similar for the forestry scenarios of continuous harvest and periodic harvest (Figs. 21.2a, band 21.3a, b). However,
the simulation of periodical thinning/harvest showed a superimposed sinusoidal pattern. The pronounced dip in the streamwater alkalinity (Fig. 21.3b,
c) reflected the dilution of streamwater due to reduced evapotranspiration
after tree felling. The sinusoidal waves indicated the cycles of replenishment
and depletion of soil base cations during phases of low nutrient uptake and
higher uptake according to the phase in the management cycle. This pattern
also occurred in all cation concentrations, but will not be shown here. Net
uptake was set to zero at the beginning of regrowth after forest felling because the reduced biomass could only account for small nutrient uptake rates
which were supplied by slash decomposition. The phase of highest uptake
was between 20 and 32 years after felling, when the soil base saturation decreased because the base cation inputs in atmospheric deposition and weathering were not high enough to compensate for the output in vegetation uptake and runoff. From 32 to 41 years after felling, net uptake decreased linearly because of lower requirements in a phase of forest maturity: soil base
saturation increased. At year 41, the forest was harvested again. If the thinning!
harvest practice stopped, base cations accumulated in the soil as a result of
-+
Fig. 21.2. Soil base cation saturation over time for the Montseny catchment, under different forestry
practices (a-c) and the following atmospheric deposition scenarios: a continued preindustrial
deposition; band c increasing atmospheric deposition over the last 140 years in line with trends in
European sulphur emissions up to 1985. For b 1985 levels are reduced by 60% over the next 20 years
and kept constant thereafter; for c, there is no reduction: 1985 levels are maintained
Anna Avila
Three scenarios of atmospheric deposition were envisaged. In scenario a,
deposition was kept at background (pre-industrial) levels throughout the
simulation. In scenario b, deposition matched the evolution of S02 emissions
in Europe over the past 140 years until 1985 followed by a 60% reduction
over the next 20 years. In scenario c, deposition was similar to b until the present day, but constant at the 1985 levels thereafter. All ions in atmospheric
deposition were considered to vary in parallel to S02, except for Na+, which
remained at the pre-industrial level. Dry deposition was considered constant
in all forest management scenarios, increasing the wet deposition by a factor
of 1.8 for chloride, 1.9 for sulphate and 1.5 for the rest of the ions (Table 21.1).
The evolution of two variables (soil base saturation and streamwater alkalinity) which represent the general response of soil and streamwater chemistry to the different scenarios is shown in Figs. 21.2 and 21.3. The effect of atmospheric deposition was similar in all forestry simulations: there was a decrease in soil base saturation and streamwater alkalinity in the simulations of
the historical buildup of sulphur (scenarios b and c) with respect to the preindustrial scenario (scenario a), being most marked in the case where deposition remained at 1985 levels (scenario c). These downward trends resulting
from atmospheric pollution were similar for the forestry scenarios of continuous harvest and periodic harvest (Figs. 21.2a, band 21.3a, b). However,
the simulation of periodical thinning/harvest showed a superimposed sinusoidal pattern. The pronounced dip in the streamwater alkalinity (Fig. 21.3b,
c) reflected the dilution of streamwater due to reduced evapotranspiration
after tree felling. The sinusoidal waves indicated the cycles of replenishment
and depletion of soil base cations during phases of low nutrient uptake and
higher uptake according to the phase in the management cycle. This pattern
also occurred in all cation concentrations, but will not be shown here. Net
uptake was set to zero at the beginning of regrowth after forest felling because the reduced biomass could only account for small nutrient uptake rates
which were supplied by slash decomposition. The phase of highest uptake
was between 20 and 32 years after felling, when the soil base saturation decreased because the base cation inputs in atmospheric deposition and weathering were not high enough to compensate for the output in vegetation uptake and runoff. From 32 to 41 years after felling, net uptake decreased linearly because of lower requirements in a phase of forest maturity: soil base
saturation increased. At year 41, the forest was harvested again. If the thinning!
harvest practice stopped, base cations accumulated in the soil as a result of
-+
Fig. 21.2. Soil base cation saturation over time for the Montseny catchment, under different forestry
practices (a-c) and the following atmospheric deposition scenarios: a continued preindustrial
deposition; band c increasing atmospheric deposition over the last 140 years in line with trends in
European sulphur emissions up to 1985. For b 1985 levels are reduced by 60% over the next 20 years
and kept constant thereafter; for c, there is no reduction: 1985 levels are maintained