Life Forms and Dynamics in Tropical Forests

Tropical forests are certainly the fastest shrinking ecosystems on earth. At the same time, they are the least studied habitats on continental earth. For most areas, we even don't have complete inventories for the best-known groups of organisms, such as the angiosperms, not even mentioning less well-known groups. 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.