HOW A RAINFOREST FUNCTIONS

1. HOW A RAINFOREST FUNCTIONS Dawn R. Black

2. Questions • What factors influence productivity? • How does primary productivity in tropical rainforests compare to other biomes? • Where are most of the rapidly recycling minerals in tropical rainforests found? • What are the three general types of soils found in the tropics?

3. Questions • What are the nutrient retention adaptations found in oligotrophic soils? • How do rainforest plants receive nitrogen?

4. GPP & NPP/Biomass • Gross Primary Productivity (GPP) • The total amount of photosynthesis accomplished • Net Primary Productivity (NPP) • Amount of carbon added to the plant for growth and reproduction • Biomass + Detritus + Soil Organic Matter • Biomass • Total storage of organic carbon in plant tissues

5. Factors Influencing Productivity • Adequate light (low light intensity limits understory species) • Moisture • CO2 levels • Soil minerals/nutrients (many soils old and mineral poor)

6. Tropical vs. Other Ecosystems • GPP vastly higher in rainforests than in any other ecosystem • High rates of respiration (temperature stress) • 50-60% of GPP spent on maintenance • NPP higher than any terrestrial ecosystem

7. Comparisons of NPP

8. Global Distribution of Carbon in Plant Biomass

9. Productivity by Biome

10. Tropical vs. Temperate PP • Huston (1994) • Productivity per unit time no greater in the tropics than in temperate zone (high PP due to length of growing season) • Kricher (1997) • Maybe plant tissue grows faster in tropics • Tropical species grew by an order of magnitude more than temperate species (red oak, red maple) when length of growing season was corrected for • Suggests that per tree productivity is considerably enhanced in the tropics

11. Nutrient Cycling • Decomposition and subsequent recycling is the process by which materials move between the living and nonliving components of an ecosystem

12. Decomposition • Fungi & bacteria – convert dead organic tissue back into simple inorganic compounds reavailable to plant root systems • Fungi immensely abundant in tropics • Mycelial mesh covers parts of some tropical forest floors

13. Supporting Decomposers Slime molds Actinomycetes Algae Animals (vultures, arthropods, earthworms, invertebrates) Protozoans

14. Leaching of Nutrients • Leaching – washing of essential minerals and other chemicals from leaves and soils by water

15. Leaf adaptations • Drip tips (speed water runoff) • Protective cuticle with lipid-soluble secondary compounds that retard water loss & discourage herbivores and fungi

16. Leaching of Soil • Rainfall increases H+ ions in soil (lowers pH), which bind to (-)-charged humus & clay • (+)-charged minerals (Ca, K) washed to deeper part of soil • Acidity of soil increased

17. Rapid Recycling of Nutrients • Most of rapidly recycling minerals are in the biomass in the tropics • Decomposition & recycling of fallen parts occur with much greater speed in rainforests than in temperate forests (thin litter layer) • ~80% of total leaf matter in Amazon rainforest annually returned to soil (Klinge et al. 1975).

18. Role of Mycorrhizae • Substitute for poorly developed root hairs • Mostly vesicular-arbuscular (VAM) • Aid in uptake of phosphorous • Some ectomycorrhizae, especially in poor soils • Aid in uptake of both minerals and water • VAM status of Dicorynia guianensis seedlings is critical factor controlling regeneration in primary tropical forest of French Guiana (Bereau et al., 1997)

19. Soil Characteristics • Determined by several factors (Jenny 1941): • Climate • Vegetation • Topographic position • Parent material • Soil age

20. Rainforest Soil Types Three general classifications of soils throughout humid tropics • Ultisols • Oxisols • Alfisols • Comprise ~71% of land surface in humid tropics worldwide • Only ~15% of moist tropical forests moderately fertile (in young soils of recent origin)

21. Ultisols • Well-weathered • Minerals leached from upper parts of soils

22. Oxisols • Deeply weathered • Old • Acidic • Found on well-drained soils of humid regions • Also found on Guianan Shield (common throughout global tropics) • Reddish color due to iron & aluminum oxides 1 M

23. Alfisols • Closer to neutral pH (still acidic) • Less overall leaching • Common in subhumid & semiarid tropics

24. Mineral Cycling on Oligotrophic Soils • Up to 26% of roots on the surface • Root mats several cm thick can develop • Root mat & mycorrhizae directly absorb available minerals • 99.9% of Ca & P absorbed into root mat in Amazon • Presence of buttresses may allow roots to spread widely at surface, where they reclaim minerals

25. Nutrient Retention Adaptations • Surface roots/mats • Apogeotropic roots – roots grow upward from soil onto stems of neighboring trees, absorb nutrients leached from trees from throughfall • Arrested litter – epiphytes & understory plants catch litter from canopy • Canopy leaves – algae & lichens on leaves absorb nutrients from rainfall and trap on leaf

26. Nitrogen Fixation • Legumes & Rhizobium – abundant in biomass & biodiversity in tropics, take up gaseous N from atmosphere & convert to nitrate • Certain epiphytic lichens fix nitrogen • Leaf-surface microbes & liverworts may facilitate uptake of gaseous nitrogen • Termites – N-fixation due to activities of microbes in termite guts

27. Rainforest Gaps • Microclimates dependent on gap size • Affects light, moisture, & wind conditions • Treefalls are normal part of rainforest function, peak in rainy season • Creates heterogeneous forest

28. Gap-Dependent Pioneer Species • Produce an abundance of small seeds dispersed by bats or birds • Seeds capable of long dormancy periods • Different growth patterns among pioneers may explain coexistence of so many different species in rainforest ecosystems

29. Forest Demographics • Forest turnover varies with species & region • La Selva, Costa Rica ~118 years • Cocha Cashu, Peru 63 years • Manaus, Brazil 82-89 years

30. Disturbance & Ecological Succession • Jungle = early succession in tropics • High species richness • Highly variable from site to site • Early succession – Colonizers • Small in stature, grow fast, produce many-seeded fruits • Late succession – Equilibrium species • Larger, grow more slowly, fewer seeds per fruit, persist in closed canopy • Can take >500 years to reach equilibrium

31. Answers • What factors influence productivity? Light levels, moisture, CO2 levels, soil minerals/nutrients 2. How does primary productivity in tropical rainforests compare to other biomes? Both GPP & NPP are higher than other biomes

32. Answers (cont.) • Where are most of the rapidly recycling minerals in tropical rainforests found? In the plant biomass 4. What are the three general types of soils found in the tropics? Ultisols, Oxisols, Alfisols

33. Answers (cont.) 5. What are the nutrient retention adaptations found in oligotrophic soils? Surface roots/mats, apogeotropic roots, arrested litter, algae/lichens on leaves 6. How do rainforest plants receive nitrogen? Legumes & Rhizobium, epiphytic lichens, leaf-surface microbes/liverworts, termites

34. Roggy et al. (1999) • Study of plant N nutrition in legumes & pioneer species at Piste de St Elie in the ECEREX research area, French Guiana • Used δ15N method to estimate nitrogen input by N2-fixing legumes to natural rainforest

35. Roggy et al. (1999) • Results • N2-fixing legumes contributed 136 t ha-1 to total above-ground plant biomass • N2-fixation estimated to be 7 kg ha-1 y -1 • δ15N of non- N2-fixing plants could be related to soil nitrogen availability • Could be used as indicator of nitrogen-cycling efficiency in rain forests

36. Chave et al. (2001) • Biomass study • 2 study sites • Nouragues Research Station (100 km inland) • Piste de Saint-Elie Research Station (coastal rain forest

37. Chave et al. (2001) • Results • Significant spatial variability of biomass at fine-scale resolution • Illustration of disturbance-driven, mosaic-like pattern in old-growth forest • Biomass accumulation of 3.2 Mg ha-1 y -1 & 2.8 Mg ha-1 y -1, which agrees with literature NPP of 2-4 Mg ha-1 y –1 (Phillips et al., 1998) -Variability of biomass correlated with canopy gap openings

38. Granier et al. (1996) • Transpiration of natural rainforest & its dependence on climatic factors • Objectives: • Analyze transpiration at tree level through sap flow measurements performed on several major species growing in their natural environment • At stand level, analyze dependence of transpiration to climatic factors, by scaling up allowing calculation of stand.

39. Granier et al. (1996) • Dependent Factors • Late stage species (high flow rates) • Pioneer species (low flow rates) • Crown Status • Codominant trees exhibited lower flow rates than dominant trees of same species • Sap flow showed remarkable concordance with variations of air vapor pressure deficit

40. Literature Cited • Bazzaz, F.A. 1984. Dynamics of wet tropical forests and their species strategies. In E. Medina, H.A. Mooney, and C. Vazquez-Yanes (eds). Physiological ecology of plants of the wet tropics. Junk, Dordrecht, pp. 233-243. • Bereau, M., E. Louisanna, and J. Garbaye. 1997. Effect of endomycorrhizas and nematodes on the growth of seedlings of Dicoryniaguianensis Amshoff, a tree species of the tropical rain forest in French Guiana. Annales des Sciences Forestieres 54: 271-277. • Chave, J., B. Riéra, M-A. Dubois. 2001. Estimation of biomass in a neotropical forest of French Guiana: spatial and temporal variability. Journal of Tropical Ecology 17: 79-96. • Granier, A., R. Huc, S.T. Barigah. 1996. Transpiration of natural rain forest and its dependence on climatic factors. Agricultural and Forest Meteorology 78:19-29.  • Huston, M.A. 1994. Biological diversity: the coexistence of species on changing landscapes. Cambridge, England: Cambridge University Press.  • Kricher, J. 1997. A Neotropical Companion: An Introduction to the Animals, Plants, & Ecosystems of the New World Tropics. 2nd ed. Princeton, New Jersey: Princeton University Press.   • Lescure, J-P. and R. Boulet. 1985. Relationship between soil and vegetation in a tropical rain forest in French Guiana. Biotropica 17: 155-164. • Phillips, O. L., Y. Malhi, N. Higuchi, W.F. Laurance, P.V. Núñez, R.M. Vásquez, S.G. Laurence, L.V. Ferreira, M. Stern, S. Brown, & J. Grace. 1998. Changes in the carbon balance of tropical forests: evidence from long- term plots. Science282:439-442. • Roggy, J.C., M.F. Prévost, F. Gourbiere, H. Casabianca, J. Garbaye, and A.M. Domenach. 1999. Leaf natural 15N abundance and total N concentration as potential indicators of plant N nutrition in legumes and pioneer species in a rain forest of French Guiana. Oecologia 120: 171-182.  • Turnbull, M.H., S. Schmidt, P.D. Erskine, S. Richards, G.R. Stewart, M.A. Topa, P.T. Rygiewicz, and J.R. Cumming. 1996. Root adaptation and nitrogen source acquisition in ecosystems. Tree Physiology 16: 11-12, 941-948.