Throughfall and Stemflow
219
in net precipitation) is posItIve, since unmeasured dry deposition of Ncontaining gases or aerosols could be adding N to throughfall and masking
canopy uptake.
The origin of the ions removed from the holm oak canopy by throughfall
and stemflow is difficult to determine. In the first Prades study, attempts to
differentiate the contribution of dry deposition from canopy leaching were
made by Bellot (1989) and Lopez (1989) through three approaches: (1) the
use of dry deposition traps, in an experimental setting similar to that described by Miller and Miller (1980), and Lakhani and Miller (1980); (2) the
use of statistical relationships between the net nutrient fluxes, the precipitation amount and the number of rainless days before the event (Lovett and
Lindberg 1984); and (3) a sequential washing of covered and uncovered
branches, as proposed by Johannes et al. (1986) and Kazda (1990). It was
concluded that inputs from atmospheric dry deposition accounted for 90% of
the CI- and Na+ removed from the canopy by precipitation, for 70-80% of
sol- and Ca2+, and for about 50% of Mg2+. The cases of H+ and NOrN were
particular, because their atmospheric inputs in wet and dry deposition were
neutralized or retained by the holm oak canopy. For K+, the results indicated
that more than 90% of its net flux in throughfall and stemflow originated
within the ecosystem through leaching of plant tissues. The alkalinity increase when precipitation washes the holm oak canopy could be produced by
leaching of weak anions or by dissolving dry-deposited calcareous dust.
At Montseny, dry deposition of marine and continental aerosols seems to
be low, as indicated by the moderate enrichment ratios of Na+ and Ca2+
(Table 15.5) and the moderate net throughfall fluxes of these cations (Table
15.6). For instance, using data from the first Montseny study, even if all Na+
in net throughfall were of marine origin, sea-salt aerosol impaction would
only account for 2.5% of Mg2+ and 17% of CI- in net throughfall (Roda et al.
1990). On the other hand, K+ is mostly derived from leaching at this site since
its net throughfall fluxes (1) follow a distinct seasonal pattern, with maxima
in late spring and early summer when flowering and leaf senescence of holm
oak occur, and (2) are highly correlated with the intensity of organic colour
in throughfall. Ions behaving similarly to K+ could be interpreted as also being derived mainly from leaching (Roda 1983). This would be the case of
Mg2+ and P04-P whose net throughfall fluxes showed strong positive correlations with those of K+ at Montseny.
Results from other studies are similar to what is concluded from the
Prades and Montseny sites in that K+ is overwhelmingly and Mg2+ partially
(around 50%) produced by leaching mechanisms, while dry deposition is
thought to be responsible for most Na+, CI- and sol- in net throughfall
(Brinson et al. 1980; Miller and Miller 1980; Parker 1983). Lindberg and Garten (1988) studied the recovery in throughfall of 35S injected in trunks and
concluded that foliar leaching accounted for only 5-7% of total annual sulphate flux in net throughfall. Since most of the S04-S removed from trees by
precipitation enters the ecosystem through dry deposition, the S04-S flux in
219
in net precipitation) is posItIve, since unmeasured dry deposition of Ncontaining gases or aerosols could be adding N to throughfall and masking
canopy uptake.
The origin of the ions removed from the holm oak canopy by throughfall
and stemflow is difficult to determine. In the first Prades study, attempts to
differentiate the contribution of dry deposition from canopy leaching were
made by Bellot (1989) and Lopez (1989) through three approaches: (1) the
use of dry deposition traps, in an experimental setting similar to that described by Miller and Miller (1980), and Lakhani and Miller (1980); (2) the
use of statistical relationships between the net nutrient fluxes, the precipitation amount and the number of rainless days before the event (Lovett and
Lindberg 1984); and (3) a sequential washing of covered and uncovered
branches, as proposed by Johannes et al. (1986) and Kazda (1990). It was
concluded that inputs from atmospheric dry deposition accounted for 90% of
the CI- and Na+ removed from the canopy by precipitation, for 70-80% of
sol- and Ca2+, and for about 50% of Mg2+. The cases of H+ and NOrN were
particular, because their atmospheric inputs in wet and dry deposition were
neutralized or retained by the holm oak canopy. For K+, the results indicated
that more than 90% of its net flux in throughfall and stemflow originated
within the ecosystem through leaching of plant tissues. The alkalinity increase when precipitation washes the holm oak canopy could be produced by
leaching of weak anions or by dissolving dry-deposited calcareous dust.
At Montseny, dry deposition of marine and continental aerosols seems to
be low, as indicated by the moderate enrichment ratios of Na+ and Ca2+
(Table 15.5) and the moderate net throughfall fluxes of these cations (Table
15.6). For instance, using data from the first Montseny study, even if all Na+
in net throughfall were of marine origin, sea-salt aerosol impaction would
only account for 2.5% of Mg2+ and 17% of CI- in net throughfall (Roda et al.
1990). On the other hand, K+ is mostly derived from leaching at this site since
its net throughfall fluxes (1) follow a distinct seasonal pattern, with maxima
in late spring and early summer when flowering and leaf senescence of holm
oak occur, and (2) are highly correlated with the intensity of organic colour
in throughfall. Ions behaving similarly to K+ could be interpreted as also being derived mainly from leaching (Roda 1983). This would be the case of
Mg2+ and P04-P whose net throughfall fluxes showed strong positive correlations with those of K+ at Montseny.
Results from other studies are similar to what is concluded from the
Prades and Montseny sites in that K+ is overwhelmingly and Mg2+ partially
(around 50%) produced by leaching mechanisms, while dry deposition is
thought to be responsible for most Na+, CI- and sol- in net throughfall
(Brinson et al. 1980; Miller and Miller 1980; Parker 1983). Lindberg and Garten (1988) studied the recovery in throughfall of 35S injected in trunks and
concluded that foliar leaching accounted for only 5-7% of total annual sulphate flux in net throughfall. Since most of the S04-S removed from trees by
precipitation enters the ecosystem through dry deposition, the S04-S flux in