216
Juan Bellot,Anna Avila and Anselm Rodrigo
and stemflow, and between Prades and Montseny, particularly if the large
sampling variabilities and the differences in rainfall, precipitation chemistry,
and sampling layout of each study are borne in mind. Also, the three major
tree species at Prades show an overall similar chemistry of throughfall and
stemflow (Bellot and Escarre 1991; Table 15.4). In contrast with the other
ions, N03 - concentrations in stemflow are lower than in throughfall for the
three species, probably because of biological uptake by cryptogamic epiphytes (mostly crustose lichens) and microbes growing on bark surfaces.
On an equivalent basis, the mean throughfall chemistry at Prades is dominated by Ca 2 + and S04 2 - (Table 15.4). Calcium was also the dominant cation
at Montseny, but the major anion contribution corresponded to alkalinity, at
least in the second study. Throughfall and stemflow present average positive
alkalinities at both sites, indicating the non-acidic nature of these solutions
as opposed to the usual acidic character of throughfall in northern and central Europe and eastern North America (Bredemeier 1988; Lovett 1994; Dambrine et al. 1995). Foliar and bark surfaces of holm oak buffer the incoming
precipitation as indicated by the general alkalinity increase in throughfall.
Enhancement of the alkalinity by foliar contact can be due to the washout of
alkaline dust deposited on the surfaces or to exchange reactions with the intercellular solutes. The relative contribution of each process is unknown. At
both sites, S04-S concentrations in throughfall are higher than in bulk deposition, but most of this enrichment is balanced by base cations.
The annual nutrient inputs to the forest floor in throughfall and stemflow
are shown in Table 15.5. Throughfall fluxes are larger than those in stemflow
mostly because throughfall conveys much higher water amounts. On a mass
basis, K+, Ca 2 +, S04-S and CI- show the highest gross throughfall fluxes at
both sites. Stemflow inputs represent at most 170/0 of the total inputs
(throughfall + stemflow) of each nutrient at Prades. Despite its low proportional contribution, stemflow can play an important role in the forest as it
infIltrates near the tree trunks, presumably providing a localized supply of
water and nutrients to the trees (Young et al. 1984). At Montseny, stemflow
inputs are much smaller due to the lower amounts of water delivered by this
pathway. In this case, stemflow nutrient inputs represent only around 2-30/0
of total dissolved inputs to the soil for all elements.
In Table 15.5, the ratio between gross throughfall and bulk deposition
fluxes (T/BD) is also given. Prades seems to receive a higher marine influence
as the Na+ T/BD ratios are higher than at Montseny. The base cation ratios at
Montseny are similar for the two periods of study, even though the throughfall quantities were very different. At Prades, net throughfall fluxes of Ca 2 +
and Mg2+ and their T/BD ratios were higher in the second period than in the
first one (Table 15.5). Since these ions are present in African dusts and the
incidence of red-rain events delivered by African air masses (Chap. 14) apparently increased in the early 1990s (second period) compared to the early
1980s (first period), it is tempting to speculate whether this increased removal of Ca 2 + and Mg2+ from holm oak canopies was due to dry-deposited
Juan Bellot,Anna Avila and Anselm Rodrigo
and stemflow, and between Prades and Montseny, particularly if the large
sampling variabilities and the differences in rainfall, precipitation chemistry,
and sampling layout of each study are borne in mind. Also, the three major
tree species at Prades show an overall similar chemistry of throughfall and
stemflow (Bellot and Escarre 1991; Table 15.4). In contrast with the other
ions, N03 - concentrations in stemflow are lower than in throughfall for the
three species, probably because of biological uptake by cryptogamic epiphytes (mostly crustose lichens) and microbes growing on bark surfaces.
On an equivalent basis, the mean throughfall chemistry at Prades is dominated by Ca 2 + and S04 2 - (Table 15.4). Calcium was also the dominant cation
at Montseny, but the major anion contribution corresponded to alkalinity, at
least in the second study. Throughfall and stemflow present average positive
alkalinities at both sites, indicating the non-acidic nature of these solutions
as opposed to the usual acidic character of throughfall in northern and central Europe and eastern North America (Bredemeier 1988; Lovett 1994; Dambrine et al. 1995). Foliar and bark surfaces of holm oak buffer the incoming
precipitation as indicated by the general alkalinity increase in throughfall.
Enhancement of the alkalinity by foliar contact can be due to the washout of
alkaline dust deposited on the surfaces or to exchange reactions with the intercellular solutes. The relative contribution of each process is unknown. At
both sites, S04-S concentrations in throughfall are higher than in bulk deposition, but most of this enrichment is balanced by base cations.
The annual nutrient inputs to the forest floor in throughfall and stemflow
are shown in Table 15.5. Throughfall fluxes are larger than those in stemflow
mostly because throughfall conveys much higher water amounts. On a mass
basis, K+, Ca 2 +, S04-S and CI- show the highest gross throughfall fluxes at
both sites. Stemflow inputs represent at most 170/0 of the total inputs
(throughfall + stemflow) of each nutrient at Prades. Despite its low proportional contribution, stemflow can play an important role in the forest as it
infIltrates near the tree trunks, presumably providing a localized supply of
water and nutrients to the trees (Young et al. 1984). At Montseny, stemflow
inputs are much smaller due to the lower amounts of water delivered by this
pathway. In this case, stemflow nutrient inputs represent only around 2-30/0
of total dissolved inputs to the soil for all elements.
In Table 15.5, the ratio between gross throughfall and bulk deposition
fluxes (T/BD) is also given. Prades seems to receive a higher marine influence
as the Na+ T/BD ratios are higher than at Montseny. The base cation ratios at
Montseny are similar for the two periods of study, even though the throughfall quantities were very different. At Prades, net throughfall fluxes of Ca 2 +
and Mg2+ and their T/BD ratios were higher in the second period than in the
first one (Table 15.5). Since these ions are present in African dusts and the
incidence of red-rain events delivered by African air masses (Chap. 14) apparently increased in the early 1990s (second period) compared to the early
1980s (first period), it is tempting to speculate whether this increased removal of Ca 2 + and Mg2+ from holm oak canopies was due to dry-deposited