Synopsis top ↑
Without identification keys and knowledge of the life cycles of the participating organisms, studying interaction in as complex an ecosystem as a tropical forest is a very difficult undertaking.
The twelve contributions in this volume give an idea how diverse the
aspects to be studied can be - and these aspects cover only a small
scope of the possibilities. Some of the contributions were presented
during the XI.th International Botanical Congress, St. Louis,
Missouri in the symposium "Life forms and strategies in tropical
forests". Geographically, most of these studies have been carried out
in the Americas, two in Africa, and none in Asia and
Oceania. This shows that the current hotspot of tropical forest
research lies in South America, which also has the highest diversity
of plants yet undescribed.
The general structure of forests, their diversity and structure is
subject of two papers. Porembski describes a gallery forest in the
northeastern Ivory Coast acting as extrazonal corridor for
Guineo-Congolian rainforest species, allowing migration of these
species far beyond their distributional area under zonal climatic
conditions. Siqueira, Rodal, Linse-Silva & Melo describe a secondary
lowland forest remnant of the much-reduced Brazilian Atlantic
forest. As should be expected in a secondary forest, the canopy is
formed by large pioneer trees, but Siqueira et al. also note an
unusually high percentage of dead trees indicating ongoing
succession. Carvalho & Oliveira-Filho present the third paper on
forest structure with an emphasis on gap dynamics in a Brazilian cloud
forest. Their estimated canopy turnover time of 149 years is in line
with the time found in the Americas in other studies; however, they
cite a much longer turnover time for Africa (Ivory Coast) and a much
shorter one for India, a result asking for confirmation by further
studies and for a study of possible causes.
The bulk of papers refers to particular guilds in a tropical forest.
Lücking studied tropical rainforest leaves as habitat for foliicolous
lichens. Cordova & Del Castillo observed epiphytes of various
systematic affiliation and found that abundances of mosses, liverworts
and vascular plants were positively correlated, while microlichen
cover decreased as the cover of mosses and vascular plants
increased. Only lichen cover was not affected by the age of the stand,
while cover by other epiphytes increased with age. Hemp studied
epiphytic and non-epiphytic ferns on Mt. Kilimanjaro in Africa, where
he could distinguish six fern groups with special strategies and three
types of growth and dormancy correlating well with the altitudinal
zonation on the mountain. Kazda & Mehltreter compared lianas and their
supporting trees and shrubs in terms of mineral content and leaf mass
per unit area. Independent of taxonomic differences, lianas showed a
lower leaf mass per unit area than their self-supporting relatives.
Differences in mineral content, though, seem to be dependent on the
soil conditions at the site. Climatic gradients and their influence on
life form frequency in Cyclanthaceae were studied by Freiberg &
Gottsberger with the result that soil water content was more important
than air microclimate for terrestrial and root climbing species, while
epiphytes would show a reversed pattern.
Life cycles and reproductive biology were in the focus of study of the remaining four papers. Charles-Dominique, Chave, Vezzoli, Dubois & Riéra extended on the life cycle of Astrocaryum sciophilum, an understory palm, with an extremely long juvenile stage which can possibly be used as indicator of forest age. Silberbauer-Gottsberger, Webber, Küchmeister & Gottsberger have studied cantharophily and the associated adaptive traits in four different plant families in the Central Amazonian rainforest. Benítez-Malvido, Martínez-Ramos & Ceccon assessed the relative contribution of the seed rain and soil seed bank on the density and recruitment of tree seedlings in contrasting successional habitats (primary forest, secondary forest, and pasture) in southeastern Mexico. They also investigated effects of seed predators, dispersers, proximity to forest remnants, abiotic conditions and species life history on the recruitment of tree seedlings in different successional forest stages. Finally, Stevens could show that in 19 Bignoniaceae from the cerrados in Central Brazil, investment in flowers, fruits and seeds relative to leaves is independent of growth form and species, while it depends on size distribution of reproductive individuals within populations. Flower production increases exponentially with size of individuals, but fruit production does not. Therefore, old, large individuals sire much more flowers than they develop fruits.
These examples show how many facets of forest biology must be investigated - and how few of them have been studied to date. Even fewer have been explored in a comparative manner in different regions, so that we normally still don't know whether an effect has general or more or less local character. However, these studies also provide a roadmap for future investigations in forest ecosystems.