Abstract:
Taken from Boy (2008):
Successful conservation of tropical montane forest, one of the most threatened
ecosystems on earth, requires detailed knowledge of its biogeochemistry. Of particular
interest is the response of the biogeochemical element cycles to external influences such
as element deposition or climate change. Therefore the overall objective of my study
was to contribute to improved understanding of role and functioning of the Andean
tropical montane forest. In detail, my objectives were to determine (1) the role of long-
range transported aerosols and their transport mechanisms, and (2) the role of short-term
extreme climatic events for the element budget of Andean tropical forest.
In a whole-catchment approach including three 8-13 ha microcatchments under
tropical montane forest on the east-exposed slope of the eastern cordillera in the south
Ecuadorian Andes at 1850-2200 m above sea level I monitored at least in weekly
resolution the concentrations and fluxes of Ca, Mg, Na, K, NO3-N, NH4-N, DON, P, S,
TOC, Mn, and Al in bulk deposition, throughfall, litter leachate, soil solution at the 0.15
and 0.3 m depths, and runoff between May 1998 and April 2003. I also used
meteorological data from my study area collected by cooperating researchers and the
Brazilian meteorological service (INPE), as well as remote sensing products of the
North American and European space agencies NASA and ESA.
My results show that (1) there was a strong interannual variation in deposition of Ca
[4.4-29 kg ha-1 a-1], Mg [1.6-12], and K [9.8-30]) between 1998 and 2003. High
deposition changed the Ca and Mg budgets of the catchments from loss to retention,
suggesting that the additionally available Ca and Mg was used by the ecosystem.
Increased base metal deposition was related to dust outbursts of the Sahara and an
Amazonian precipitation pattern with trans-regional dry spells allowing for dust
transport to the Andes. The increased base metal deposition coincided with a strong La
Niña event in 1999/2000. There were also significantly elevated H+, N, and Mn
depositions during the annual biomass burning period in the Amazon basin. Elevated H+
deposition during the biomass burning period caused elevated base metal loss from the
canopy and the organic horizon and deteriorated already low base metal supply of the
vegetation. Nitrogen was only retained during biomass burning but not during non-fire
conditions when deposition was much smaller. Therefore biomass burning-related
aerosol emissions in Amazonia seem large enough to substantially increase element
deposition at the western rim of Amazonia. Particularly the related increase of acid
deposition impoverishes already base-metal scarce ecosystems. As biomass burning is
most intense during El Niño situations, a shortened ENSO cycle because of global
warming likely enhances the acid deposition at my study forest.
(2) Storm events causing near-surface water flow through C- and nutrient-rich
topsoil during rainstorms were the major export pathway for C, N, Al, and Mn
(contributing >50% to the total export of these elements). Near-surface flow also
accounted for one third of total base metal export. This demonstrates that storm-event
related near-surface flow markedly affects the cycling of many nutrients in steep
tropical montane forests. Changes in the rainfall regime possibly associated with global
climate change will therefore also change element export from the study forest.
Element budgets of Andean tropical montane rain forest proved to be markedly
affected by long-range transport of Saharan dust, biomass burning-related aerosols, or
strong rainfalls during storm events. Thus, increased acid and nutrient deposition and
the global climate change probably drive the tropical montane forest to another state
with unknown consequences for its functions and biological diversity.
