Tropical Ecosystems: Climate, Diversity, and Function

1. Tropical ecosystems The delimitation of tropical regions: rainfall, evaporation, and temperature Seasonal versus daily temperature changes: -latitudinal gradient- frost occurrence at sea level -altitudinal temperature gradient in the tropics -separation of tropical plant formations according to temperature and rainfall Eco-climatic classifications The climate-diagramm (Gaussen-Walter) The Holdridge Life Zone concept Bailey’s humidity index Rainfall-evaporation comparisons Distribution of humid and seasonal dry forests General physiognomic changes along environmental gradients Rainfall (or water availability) 1) medium to high fertility; 2) low fertility Altitudinal gradients Flooding gradients Forest-poor regions in medium to high rainfall areas: the savannas operating factors- soil fertility, fire, flooding

3. Altitudinal temperature gradient (average year temperature ºC vs altitude m) in Venezuela measured in conventional meteorological stations (•,o,x) or using Boussingaults method (∆). In tropical climates (daily temperature range larger than average monthly temperature range) soil temperature below 30 cm depth, under shade, corresponds to average annual temperature

6. Holdridges model of Eco-Climatic classification of potential vegetation

8. Relationship between forest vegetation and rainfall (y-axis) and durastion of the dry period in months (x-axis) in India. I. Evergreen and II. Semi-evergreen tropical rain forest, III. Monsoon forest (A. wetter, B. drier), IV. Savanna (thorn forest), V. Desert (from Walter Die Vegetation der Erde 1973)

10. Seasonal Forest Formation Series Decreasing Rainfall Dry Evergreen Formation Series

11. Increasing altitude Montane Forest Formation Series SeasonalSwamp Formation Series

13. Critical structural and functional properties of tropical plant formations Diversity Facts and figures (Gentry et al.) Explanations and theories Monodominant tropical forests Fertility,Flooding, Salinity, Symbiosis Mutualistic symbiosis Mycorrhiza: ectomycorrhizas and VAM N2-fixing associations: Rhizobia, Life-forms, biotypes, etc. The classical (and still working) Raunkiaer’s system Diversity and abundance of life -forms in different tropical climates Life-forms and ecosystem function in tropcial forests Productivity of tropical forests and their potential as carbon sinks Extension of forest plant communities and Net Primary Productivity Precision and uncertainties: operational models and underground productivity Carbon balance of the biosphere: Houghton’s and Field et al’s estimations

15. Changes in species diversity according to species intrinsic growth rate and frequency or intensity of disturbance Huston 1994)

16. Hypothesis on determination of species diversity based on intrinsic Growth rates and frequency or intensity of disturbance (Huston 1994)

17. Costa Rica Ghana West Malesia Amazonas Inverse relationship between soil fertility and species richness (Huston 1994: Biological Diversity. Cambridge University Press

19. Central role of micorrhyzal mutualistic symbioses in tropical forest nutrition Ectotrophic mycorrhiza----> highly specific ----> monodominant forests 1) in Africa: Gilbertiodendron dewevrei and Brachystegia laurentii (Zaire), Cynometra alexandri (Uganda) and Tetraberlinia tubmaniana (Liberia). 2) in tropical South America: Mora excelsa, M. gonggrijpi, and Eperua falcata (Trinidad and Guyana); Pentaclethra macroloba (Costa Rica). Vesicular arbuscular micorrhiza ----> promiscuous----> species rich forests

21. Diversity of Life-Forms, according to Ellenberg (1979) in tropical forests: notice the larger diversity in dry forests

22. Common life-forms of tropical forests associated with forest structure and function Ewel and Bigelow (1996) Denslow (1996) CANOPY TREES Dicot, long-lived trees Canopy and emergent trees Legumes Dicot, short-lived tress Palms Emergents Rosette trees (palms) UNDERSTORY TREES Understory trees Treelets Pioneer Understory SHRUBS AND HERBS Shrubs Herbs and shrubs Pioneer Giant-leaved herbs Understory Large-leaved Graminoids Small-leaved CLIMBING PLANTS Vines Lianas and vines Lianas Hemi-epiphytes Vines EPIPHYTES Epiphytes Epiphytes and hemiepiphytes Non-parasitic herbs Parasitic and hemiepiphytic trees and shrubs

23. Examples of linkages between plant life-forms and processes in tropical forests (Ewel and Bigelow Ecological Studies 122, 101-126.1996) I Life-form Role Dicotyledonous trees, 1. Provide skeletal structure of entire forest long-lived 2. Dominate primary productivity and material flows 3. Influence off-site climate and hydrology 4. Provide shelter and roosts in hollow trunks Dicotyledonous trees, 1.Reduce nutrient loss in early succession Short-lived 2. Reduce likelihood of site takeover by vines and shrubs Rosette trees 1. Channel rainwater toward stem (e.g. palms) 2. Capture and aggregate litter 3. Concentrate Calcium 4. Roots bore through soil pans, creating channels that can be exploited by other plants 5. Root foraging emphasizes scale Understory trees 1. Scavenge sparse radiation in the understory (and have low nitrogen demand) 2. Provide platforms (in humid microenvironment) for nitrogen-fixing epiphylls Shrubs 1. Drive productivity of scansorial rodents and birds that feed on fleshy fruits 2. Retard nutrient loss in early succession

24. Examples of linkages between plant life-forms and processes in tropical forests (Ewel and Bigelow Ecological Studies 122, 101-126.1996) II Life-form Role Giant-leaved herbs 1. Constitute large, homogeneous patches in otherwise heterogeneous understory 2. Foster secondary productivity thropugh nectar and fruit production 3. Provide roosting sites for bats and building sites for carton nests of social insectes Vines 1. Provide trellises for movement of arboreal animals 2. Act as weebing that ties trees together 3. Buffer microclimatic changes by seasing forest edges Graminoids 1. Constitute readily combustible dry-season fuel 2. Provide forage for grazers and food for seed-eating birds, rodentes, ants and fungi Hemiepiphytes 1. Increase mortality rates 2. Provide slender vine trellises (aerial roots) in understory of closed canopy forest Epiphytes 1. Augment leaf area (by colonizing opaque surfaces) 2. Slow nitrogen through-flow 3. Divert water from soil to the atmosphere 4. Redistribute trhrough-fall and stem flow 5. Provide unique habitats essential for reproduction of other species (e.g. insects)

25. Area and net primary production of organic matter (expressed as g C per unit area) of tropical forests and savannas estimated by direct measurements and using a process-based ecosystem simulation model Vegetation Units Area % NPP Total NPP % (x 106 km2) g C m-2 yr- 1 1015 g C yr-1 Whittaker and Likens (1973) World Total 149.0 58.8 Tropical Rain forest 17.0 11.4 1100 18.7 31.8 Tropical seasonal forest 7.5 5.0 800 6.0 10.2 Savanna 15.0 10.1 450 6.8 11.6 Total tropical 39.5 26.5 31.5 53.6 Melillo et al. (1993) World Total 127.3 53.2 Tropical evergreen forest 17.4 13.7 1098 19.1 35.9 Tropical deciduous forest 4.6 3.6 871 4.0 7.5 Tropical Savanna 13.7 10.8 393 5.4 10.2 Xeromorphic forests 6.8 5.3 461 3.1 5.8 Total tropical 42.5 33.0 31.6 59.4

26. Estimated Aboveground Net Primary Production Mg C/ha.yr

27. Estimated Total Net Primary Production Mg C/ha . yr

28. Global Carbon Balance for 1990 (Houghton 1999) Net emissions due to Land-use Change Atmospheric Increase Fossil Fuel Oceans Uptake Residual Terrestrial Sink - - + = 3.3 (±0.2) = 5.5 (±0.5) + 2.0 (±0.8) - 2.0 (±0.8) - 2.2 (±1.3) Units: Pg yr-1 (1) The residual terrestrial sink is attributed to a combination of - CO2 fertilization - Nitrogen deposition - Interannual climatic variation (2) Northern temperate forest show a residual terrestrial uptake of 0.6 ± 0.5 Pg yr-1 (3) Mechanism leading to terrestrial accumulation are not precisely known (4) Several of the potential mechanisms will be less effective in the future leading to a sink reduction or to an aditional terrestrial source

29. Possible mechanisms for the maintenance of a carbon sink in the biosphere (Field et al. 1992) N deposition ≈ 25 Tg yr-1 Increase atmospheric CO2 ≈ 2 ppm yr-1 Increased global temperature 0.5 ± 0.2 ºC in last 100 yr Nutrient Use Efficiency Water Use Efficiency Decomposition Photosynthesis Nutrient Availability Nutrient Availability Decomposition Tissue Nutrients Growth Growth Wood Production Nutrient Availability Decomposition Decomposition Increased Carbon Storage 1.6 ± 1.4 Pg yr-1

32. Summary • Tropical climate regimes vary widely in water availability, from arid to perhumid rainfall regimes, and temperature, from lowland with averages above 25ºC, to high mountains with seasonal and daily frosts • Plant formations vary accordingly in structural development and ecophysiological tolerances • Soil fertility modulates vegetation structure and productivity • Widespread occurrence of Savannas can be related to the interactions between climate, soil fertility, and disturbance regimes represented by herbivory and fire • Climate change represented by atmospheric increase in CO2 concentrations and slowly increases in average temperature, may lead to increased productivity, organic matter turnover and carbon storage in symbiosis-dependent tropical forests