Occupying less than 7 % of Earth's land surface, tropical rain forests harbor perhaps half of the species on Earth and are ecologically, economically, and culturally crucial for issues in global food security, climate change, biodiversity, and human health. Geographically located between the latitudes 10°N and 10°S of the equator, lowland tropical rain forest ecosystems share similar physical structure but vary in geology, species composition, and anthropogenic threats across the forests of Southeast Asia, Australia, Africa, and Central and South America. Mature tropical rain forests are stratified by multiple canopy and understory layers, and physiognomic properties include evergreen broadleaf tree species, a preponderance of species with large leaves to aid with sunlight capture in the light-limited understory, and leaf properties such as entire margins and drip tips that channel water efficiently from the leaf surface. Lianas are increasing in abundance and biomass in a number of tropical rain forests. The additive effects of an increase in liana biomass are correlated with a reduction in tropical forest carbon (C) storage, a value that is currently not considered in global vegetation models. Most rain forest tree species do not grow, flower, or fruit year-round. Peaks in leaf flushing, flowering, and fruiting coincide with the high irradiance and low water stress associated with the onset of the wet season. This synchrony is common and largely driven by resource availability, though biotic explanations for synchrony include selection to attract pollinators or seed dispersers and to avoid herbivory and seed predation. With few exceptions, species richness across the tree of life is highest in equatorial tropical regions and decreases towards the poles. Tropical rain forests harbor approximately two thirds of the estimated 350,000-500,000 extant flowering plant species on Earth, with high rates of endemism and large numbers of rare species. Numerous evolutionary and ecological hypotheses to explain the origin and maintenance of high biological diversity in tropical forests have garnered support and include biogeographic history, evolutionary mechanisms of adaptation and speciation, range size and distribution constraints, and ecological mechanisms promoting species coexistence. Continental drift, climate constraints, and long-distance dispersal are responsible for some of the similarities and differences in species across tropical regions. Familial similarity among forests in Amazonia and Southeast Asia can be as high as 50 %, while independent diversification and species radiation mean that much fewer genera (around 10 %) are shared. Gradients in climate, parent material and soil age, topography and landscape stability, and atmospheric deposition result in strong heterogeneity in soil nutrient availability from local to regional scales. Soil order, which is generally correlated with soil fertility as a strong predictor of aboveground net primary productivity in tropical forests. Tropical forests account for approximately 40 % of terrestrial net primary productivity (NPP), store half of Earth's vegetative C stocks but less than 10 % of its soil C stocks. The relationships between rainfall, temperature, soil fertility, and NPP are complex and require more experimental manipulations to tease apart the interactions. Intact tropical forests are net C sinks, but the uptake of C (1.1 ± 0.3 Pg C year-1) in intact tropical forests is counteracted by the emissions from tropical biome conversion - a net C source to the atmosphere of 1.3 ± 0.2 Pg C year-1 that results in a tropical biome net C balance of approximately zero. Stronger El Nino Southern Oscillation (ENSO) effects are increasing the frequency and severity of droughts, fires, hurricanes and cyclones, and flooding events. Recovery of aboveground biomass, species composition, and forest structure all depend on the type and severity of disturbance and its effect on soil fertility. Greater use of remote sensing imagery from satellites, airborne Light Detection an Ranging (LiDAR) data, and unmanned drones will allow accurate tracking of disturbance and C stocks as well as monitoring of phenology, foliar canopy chemistry, individual species identification, and biodiversity estimates from local to regional scales. The tropical biome is undergoing significant change. Understanding the drivers and impacts of these changes will require sustained advances across multiple disciplines. Ultimately as a society, we are left asking what is the capacity of our remaining and regrowing tropical rain forests to adapt to long-term anthropogenic and climate change and what can we do to moderate these effects while nourishing a healthy human population. © Springer Science+Business Media New York 2014.
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