TRopical Tree Physiology

LOUIS LOUIS S. SANTIAGO ; GUILLERMO GOLDSTEIN

Springer

Año: 2016 p. 467

978-3-319-27420-1

Our World is changing rapidly, yet, how tropical forests will respond to this changeand in turn dampen or accelerate its ripple effects is essentially a physiologicalquestion. Addressing important questions regarding the impacts of changes in landutilization, such as deforestation, and effects of global climate change will requirespecific information on tropical tree physiology. Earth system modeling scientistsare clamoring for more physiological data from tropical trees. It seems that thescarcity of information on the physiological responses of trees is the greatest sourceof uncertainly in predicting how the tropical rain forests will respond to increasinggreenhouse gases and in particular increasing atmospheric CO2. For example, treespecies can adjust their physiological behavior to increasing global temperatures ordecreases in precipitation, or they can be replaced by other species better adapted tothe new environmental conditions. It is also true that the physiology of tropical treeshas not been as well-studied as the physiology of trees from temperate regions,leading to major gaps in our understanding of how tropical trees interact with theEarth system over a range of scalesIt is known that the physiological behavior of both tropical and temperate trees isregulated by similar mechanisms. The differences, however, are related to theunique selective pressures to which tropical trees have been subjected during theevolutionary process and its adaptive consequences. The idea put forward byTheodosius Dobzhansky in the 1950s that tropics and temperate zones are areaswhere selection operates differently, generated fruitful lines of thinking andresearch. His contention was that in temperate areas mortality was essentially climatically determined, with little or no competition pressure, while in the tropics,where the environment is relatively more constant, at least in terms of seasonalchanges in temperature, mortality is the result of the effects of population size andcompetition. This paradigm of evolutionary pressures has changed substantially butsome aspects of it still remain as a guide for understanding differences in patterns ofadaptation between temperate and tropical plants. Negative density dependenceprevents any single tree species from dominating most tropical forest ecosystems.The reasons for this must be sought not only in ecological and demographicvprocesses but also among the highly diverse physiological characteristics of tropicaltrees. In the tropics, seasonal temperature variations are relatively small comparedto diurnal temperature changes and if soil water is available, growth and metabolicactivities can be maintained throughout the entire year. Heavy herbivore pressure iscontinuous and the adaptive responses of tropical plants to herbivory are impressive. The physiological implications of various types of mutualisms found amongtropical trees are also important. Many trees have a relatively short life span of lessthan 200 years in the wet tropics compared to more than a 1000 years in sometemperate-zone trees.There has been a substantial increase in the number of studies of tropical treephysiology during the last few decades. The reason for this is not only that trees arethe dominant growth form in most tropical ecosystems, but also because ofincreasing availability and refinement of equipment such as portable photosynthesissystems and instruments for studying water relations of plants. Furthermore, asubstantially larger number of tropical biologists are now involved in moremechanistic studies. The use of tower cranes during the last 25 years has allowedscientists to reach the canopy of tropical forests, one of the ultimate frontiers forunveiling not only new organisms but also new processes that were unthinkable justa few years ago.A distinct feature of tropical trees is not only their high species diversity but alsothe large variety of life history traits and growth forms that are mostly unique to thetropics such as hemiepiphytic trees, stem succulent trees such as baobab trees,mangroves, palms and other arborescent monocots, and unusual arborescent plantsnear tree line that are not traditional trees. The wide range of shade tolerance fromrapidly growing pioneer trees during gap-phase regeneration to species that cansurvive by growing slowly in deep shade contribute to this diversity.Tropical trees tend to grow in habitats where soil water availability is high allyear round or at least in habitats were it is seasonally available. They grow in aridenvironments were they access deep soil water such as in the case of phreatophytictrees. They also occur along altitudinal gradients within the tropics up to the uppertree line and in some cases, such as the caulescent giant rosette plants, they cangrow above the continuous forest line. They extend to the subtropics, and in manycases they share close phylogenetic relationships with subtropical tree species, andthe structure and function of subtropical forests are in many cases similar to tropicalforests, in regards to gap-phase regeneration and the high abundance of lianas