Florian Wittmann

A Classification of Major Naturally-Occurring Amazonian Lowland Wetlands

Our estimates indicate that about 30% of the seven million square kilometers that make up the Amazon basin comply with international criteria for wetland definition. Most countries sharing the Amazon basin have signed the Ramsar Convention on Wetlands of International Importance but still lack complete wetland inventories, classification systems, and management plans. Amazonian wetlands vary considerably with respect to hydrology, water and soil fertility, vegetation cover, diversity of plant and animal species, and primary and secondary productivity. They also play important roles in the hydrology and biogeochemical cycles of the basin. Here, we propose a classification system for large Amazonian wetland types based on climatic, hydrological, hydrochemical, and botanical parameters. The classification scheme divides natural wetlands into one group with rather stable water levels and another with oscillating water levels. These groups are subdivided into 14 major wetland types. The types are characterized and their distributions and extents are mapped.

Tree species distribution and community structure of central Amazonian várzea forests by remote-sensing techniques

In central Amazonian white-water floodplains (vairzea), different forest types become established in relation to the flood-level gradient. The forma- tions are characterized by typical patterns of species composition, and their archi- tecture results in different light reflectance patterns, which can be detected by Landsat TM image data. Ground checking comprised a detailed forest inventory of 4 ha, with Digital Elevation Models (DEM) being generated for all sites. The results indicate that, at the average flood level of 3 m, species diversity and archi- tecture of the forests changes, thus justifying the classification into the categories of low vairzea (vairzea baixa) and high varzea (vairzea alta). In a first step to scale up, the study sites were observed by aerial photography. Tree heights, crown sizes, the projected crown area coverage and the gap frequencies provide information, which confirms a remotely sensed classification into three different forest types. The structure of low vairzea depends on the successional stage, and species divers- ity increases with increasing age of the formations. In high varzea, only one succes- sional stage was found and species diversity is higher than in all low-varzea forma- tions. The more complex architecture of the high-vairzea forest results in a more diffuse behaviour pattern in pixel distribution, when scanned by TM image data.

An estimate of the number of tropical tree species

Significance People are fascinated by the amazing diversity of tropical forests and will be surprised to learn that robust estimates of the number of tropical tree species are lacking. We show that there are at least 40,000, but possibly more than 53,000, tree species in the tropics, in contrast to only 124 across temperate Europe. Almost all tropical tree species are restricted to their respective continents, and the Indo-Pacific region appears to be as species-rich as tropical America, with each of these two regions being almost five times as rich in tree species as African tropical forests. Our study shows that most tree species are extremely rare, meaning that they may be under serious risk of extinction at current deforestation rates. The high species richness of tropical forests has long been recognized, yet there remains substantial uncertainty regarding the actual number of tropical tree species. Using a pantropical tree inventory database from closed canopy forests, consisting of 657,630 trees belonging to 11,371 species, we use a fitted value of Fisher’s alpha and an approximate pantropical stem total to estimate the minimum number of tropical forest tree species to fall between ∼40,000 and ∼53,000, i.e., at the high end of previous estimates. Contrary to common assumption, the Indo-Pacific region was found to be as species-rich as the Neotropics, with both regions having a minimum of ∼19,000–25,000 tree species. Continental Africa is relatively depauperate with a minimum of ∼4,500–6,000 tree species. Very few species are shared among the African, American, and the Indo-Pacific regions. We provide a methodological framework for estimating species richness in trees that may help refine species richness estimates of tree-dependent taxa.

Drought responses of flood-tolerant trees in Amazonian floodplains

BACKGROUND Flood-tolerant tree species of the Amazonian floodplain forests are subjected to an annual dry period of variable severity imposed when low river-water levels coincide with minimal precipitation. Although the responses of these species to flooding have been examined extensively, their responses to drought, in terms of phenology, growth and physiology, have been neglected hitherto, although some information is found in publications that focus on flooding. SCOPE The present review examines the dry phase of the annual flooding cycle. It consolidates existing knowledge regarding responses to drought among adult trees and seedlings of many Amazonian floodplain species. MAIN FINDINGS Flood-tolerant species display variable physiological responses to dry periods and drought that indicate desiccation avoidance, such as reduced photosynthetic activity and reduced root respiration. However, tolerance and avoidance strategies for drought vary markedly among species. Drought can substantially decrease growth, biomass and photosynthetic activity among seedlings in field and laboratory studies. When compared with the responses to flooding, drought can impose higher seedling mortality and slower growth rates, especially among evergreen species. Results indicate that tolerance and avoidance strategies for drought vary markedly between species. Both seedling recruitment and photosynthetic activity are affected by drought, CONCLUSIONS For many species, the effects of drought can be as important as flooding for survival and growth, particularly at the seedling phase of establishment, ultimately influencing species composition. In the context of climate change and predicted decreases in precipitation in the Amazon Basin, the effects of drought on plant physiology and species distribution in tropical floodplain forest ecosystems should not be overlooked.

Damming the rivers of the Amazon basin

More than a hundred hydropower dams have already been built in the Amazon basin and numerous proposals for further dam constructions are under consideration. The accumulated negative environmental effects of existing dams and proposed dams, if constructed, will trigger massive hydrophysical and biotic disturbances that will affect the Amazon basin’s floodplains, estuary and sediment plume. We introduce a Dam Environmental Vulnerability Index to quantify the current and potential impacts of dams in the basin. The scale of foreseeable environmental degradation indicates the need for collective action among nations and states to avoid cumulative, far-reaching impacts. We suggest institutional innovations to assess and avoid the likely impoverishment of Amazon rivers.

Struggle in the flood: tree responses to flooding stress in four tropical floodplain systems

This review examines species diversity and structural, physiological and biochemical characteristics associated with survival of annual deep flooding of trees in four contrasting tropical floodplain ecosystems.

A classification of major natural habitats of Amazonian white-water river floodplains (varzeas)

Most countries sharing the Amazon basin have signed the Ramsar Convention on Wetlands of International Importance but still lack complete wetland inventories, classification systems, and management plans. Amazonian wetlands vary considerably with respect to hydrology, water and soil fertility, vegetation cover, diversity in plant and animal species and primary and secondary productivity. Here, we propose a classification system of major natural habitats of Amazonian white-water river floodplains (várzeas) based on hydrological, water and soil chemistry and biological parameters. The Amazonian várzea is one of the largest Amazonian wetlands. It is exceptionally rich in plant and animal species and plays important roles in landscape history, evolution, hydrology and biogeochemical cycles of the Amazon basin. Most of Amazonia’s rural population lives in or along the várzea, emphasizing the economic importance of its natural resources. Our classification indicates five major systems, which are subdivided into 10 main habitats and up to 40 functional (vegetation) units of which the most important mesohabitats are described. We understand this classification as a dynamic system, as it is open to the inclusion of future research attempts and habitats without affecting the entire classification system. Our classification may be used for scientific purposes, such as comparative studies on biomass, productivity, biogeochemical cycles and biodiversity. Also, because the classification builds on habitat types and/or vegetation and functional units already distinguished by the local population it may be especially useful in guiding intelligent use of várzea habitat for specific management activities, such as agriculture, animal husbandry, forestry, fisheries, and conservation.

Management criteria for Ficus insipida Willd. (Moraceae) in Amazonian white-water floodplain forests defined by tree-ring analysis

Ficus insipida Willd. (Moraceae) is a fast growing tree species of early successional stages in the Amazonian nutrient-rich white-water floodplains (várzea). The species is one of the most economically important low-density wood species in the community-based forest management project in the Mamirauá Sustainable Development Reserve (MSDR) in Central Amazonia, where timber species are managed using a polycyclic selection system with a minimum logging diameter (MLD) of 50 cm and a cutting cycle of 25 years. In this study we analyze the floristic composition, stand structure and forest regeneration of a natural 20 year-old stand at an early successional stage and we model tree growth of diameter, height and volume of F. inspida based on tree-ring analysis to define management criteria. The volume growth model indicates that the preferred period for logging should be at a tree age of 17 years when the current annual volume increment peaks. This age corresponds to a diameter of 55 cm, which would be an appropriate MLD.RésuméFicus insipida Willd. (Moraceae) est une essence à croissance rapide présente dans les premiers stades de succession dans les forêts inondables sur sols riches d’Amazonie («varzea»). Cette essence est l’une des plus importantes essences productrice de bois de faible densité, dans le cadre du projet de gestion forestière communautaire durable de la réserve de Mamiraua, en Amazonie Centrale. Ces forêts sont gérées sur le principe d’un système polycyclique avec récolte des arbres présentant un diamètre minimal de 50 cm et une révolution de 25 ans entre récoltes. La présente étude analyse la composition floristique, la structure des peuplements et la régénération dans une forêt naturelle âgée de 20 ans et issue d’une phase de régénération. Un modèle de croissance en diamètre, hauteur et volume a été adapté à Ficus insipida sur la base d’une analyse de cernes, afin de définir des critères de gestion. Le modèle de croissance en volume indique que l’âge de récolte optimal est d’environ 17 ans, au moment du pic de production courante annuelle. Á cet âge, les arbres atteignent un diamètre de 55 cm, qui constituerait ainsi un diamètre minimal de récolte (DMR) tout à fait approprié.

Pioneer Trees in Amazonian Floodplains: Three Key Species form Monospecific Stands in Different Habitats

Three pioneer tree species —Salix humboldtiana, Cecropia latiloba, Senna reticulata — form monospecific stands in the Central Amazonian white-water flood plain. In contrast toterra firma forests where species composition is unpredictable even for pioneer species, in Central Amazonianvárzea the occurrence of the main colonizing species seems to be predictable. This predictability is linked to characteristic habitat conditions and the low number of pioneer species. This preference for different habitats is reflected by different germination and early growth, by the structural and physiological characteristics, as well as by the reproductive and morphological adaptations of the three main species. The germination rate was above 90% in all species, and the duration until germination ranged between one day inSalix and 14 days inCecropia. Stem elongation was more than 10 cm per month inSalix andCecropia, and about 50 cm per month inSenna. Wood specific gravity ranged from 0.33 g cm−3 inCecropia to 0.45 g cm−3 inSenna. The annual wood increment increased by 1.20 (Cecropia), 1.23 (Salix) to 2.14 cm per year (Senna). All species produced adventitious roots, lenticels and/or stem hypertrophy. Leaf photosynthesis was between 17 and 20 µmol m−2s−1, and reached a maximum of 30 µmol m−2s−1 inSenna. Flowering and fruiting inSalix occurred throughout the year, whereas inCecropia andSenna they were concentrated in the flooded period.Salix humboldtiana occurs mainly at low sites subjected to long periods of inundation and high sedimentation rates. OnceSalix has formed dense forest stands, sedimentation and water currents are reduced at these sites andCecropia latiloba may take over. This species grows on low to middle elevations in the flooding gradient at sites with lower current and sedimentation rates.Senna reticulata does not tolerate submergence and colonizes habitats that may have strong currents and high sedimentation on higher levels in the flooding gradient.

Biomass and net primary production of central Amazonian floodplain forests

In this chapter the existing knowledge on biomass in floodplain forests and the compounds that contribute to their net primary production (NPP) are presented and discussed in comparison with data from non-flooded upland (terra firme) forests. Fine litterfall in old-growth floodplain forests are similar to litterfall data from terra firme forests. The few existing estimates of root biomass in nutrient-rich white-water floodplain forests (varzea) indicate lower belowground biomasses in floodplain forests than in terra firme forests due to regular flooding which limits the development of deep roots. Along the chronosequence, C-storage in the aboveground coarse live wood biomass (AGWB) of varzea forests indicates a strong increase during the first 50–80 years of successional development, but afterwards no increase in AGWB can be observed. On the other hand C-sequestration in the AGWB of varzea forests declines more than threefold along the successional gradient. In comparison to terra firme forest, the varzea forests have lower C-stocks, but a higher C-sequestration in the AGWB. The estimated aboveground NPP in young successional stages of the central Amazonian varzea is among the highest NPP known for tropical forests, while the NPP of the late succession in the varzea is in the upper range of the NPP of old-growth forests in the terra firme. The available database for nutrient-poor floodplain forests (igapo) is insufficient to estimate their NPP. Climate-growth relationships of tree-ring chronologies of species from central Amazonian terra firme and floodplain forests indicate opposing signals during El Nino years. During these events large areas of terra firme forests release carbon to the atmosphere due to the warmer and drier climate conditions, while the weakened flood-pulse favours tree growth in the floodplain forests which might therefore sequester parts of the climate-induced carbon emissions of terra firme forests.