210
Juan Bellot, Anna Avila and Anselm Rodrigo
than in rainwater. The total flux of nutrients to the forest floor in net precipitation is the sum of nutrient fluxes in throughfall and stemflow. When
deposition of any nutrient in incident rainwater is subtracted from that in
throughfall or stemflow, the remainder is called net throughfall or net stemflow. The relative effect of the canopy on these depositions can be estimated
by obtaining the enrichment factor for each element. The main processes of
enrichment are washing of dry deposition and leaching of intercellular solutes from leaves. However, some nutrients do not show such enrichments because they are absorbed by foliar surfaces. This can be important for nutrients in low supply and has been widely reported for nitrogen (Schlesinger et
al. 1982; Parker 1983; Lindberg et al. 1986; Bynterowicz and Fenn 1996).
Chemical changes as rainwater crosses the canopy by throughfall can be expressed in terms of element fluxes (units in kg ha- I year-I), and summarized
in the following expression (Bache 1977):
net throughfall = washout of dry deposition + leaching - canopy uptake.
Much effort has been devoted in the last 20 years to discriminating the
contributions to net throughfall nutrients of atmospheric sources (dry deposition) versus that of plant leaching. The distinction is important because dry
deposition represents (mostly) an external input of nutrients to the ecosystem while leaching represents (mostly) a within-system recirculation of nutrients previously taken up by trees from the soil (Lovett and Lindberg 1984;
Lovett 1994).
In this chapter, we present the results of studies on throughfall, stemflow
and interception in the holm oak forests of Prades and Montseny in NE
Spain with the aim of comparing the hydrological and chemical fluxes at two
sites of different stand structure and climate characteristics. We first deal
with the hydrological pathways through the forest canopy in relation to the
contrasted forest structure of the sites (Chap. 3). We then discuss the biogeochemical behaviour of the main elements, with emphasis on major nutrients
(N and P) and atmospheric pollutants (S).
15.2 The Database
Four throughfall studies have been conducted in the experimental holm oak
forests of Prades and Montseny (Chap. 2), two at each location. The first
study at each site was intended to assess the role of throughfall in the water
and nutrient cycles of holm oak forests. The second Prades study addressed
the effects of N fertilization (Chap. 13) in throughfall chemistry and element
fluxes. As no effect of fertilization was found on throughfall amounts or
chemistry, we have pooled data from treated and untreated plots. The second
Montseny study compared the atmospheric deposition onto holm oak canopies (using throughfall as a proxy) in two sites differently exposed to air
Juan Bellot, Anna Avila and Anselm Rodrigo
than in rainwater. The total flux of nutrients to the forest floor in net precipitation is the sum of nutrient fluxes in throughfall and stemflow. When
deposition of any nutrient in incident rainwater is subtracted from that in
throughfall or stemflow, the remainder is called net throughfall or net stemflow. The relative effect of the canopy on these depositions can be estimated
by obtaining the enrichment factor for each element. The main processes of
enrichment are washing of dry deposition and leaching of intercellular solutes from leaves. However, some nutrients do not show such enrichments because they are absorbed by foliar surfaces. This can be important for nutrients in low supply and has been widely reported for nitrogen (Schlesinger et
al. 1982; Parker 1983; Lindberg et al. 1986; Bynterowicz and Fenn 1996).
Chemical changes as rainwater crosses the canopy by throughfall can be expressed in terms of element fluxes (units in kg ha- I year-I), and summarized
in the following expression (Bache 1977):
net throughfall = washout of dry deposition + leaching - canopy uptake.
Much effort has been devoted in the last 20 years to discriminating the
contributions to net throughfall nutrients of atmospheric sources (dry deposition) versus that of plant leaching. The distinction is important because dry
deposition represents (mostly) an external input of nutrients to the ecosystem while leaching represents (mostly) a within-system recirculation of nutrients previously taken up by trees from the soil (Lovett and Lindberg 1984;
Lovett 1994).
In this chapter, we present the results of studies on throughfall, stemflow
and interception in the holm oak forests of Prades and Montseny in NE
Spain with the aim of comparing the hydrological and chemical fluxes at two
sites of different stand structure and climate characteristics. We first deal
with the hydrological pathways through the forest canopy in relation to the
contrasted forest structure of the sites (Chap. 3). We then discuss the biogeochemical behaviour of the main elements, with emphasis on major nutrients
(N and P) and atmospheric pollutants (S).
15.2 The Database
Four throughfall studies have been conducted in the experimental holm oak
forests of Prades and Montseny (Chap. 2), two at each location. The first
study at each site was intended to assess the role of throughfall in the water
and nutrient cycles of holm oak forests. The second Prades study addressed
the effects of N fertilization (Chap. 13) in throughfall chemistry and element
fluxes. As no effect of fertilization was found on throughfall amounts or
chemistry, we have pooled data from treated and untreated plots. The second
Montseny study compared the atmospheric deposition onto holm oak canopies (using throughfall as a proxy) in two sites differently exposed to air