Presentation on theme: "HOW A RAINFOREST FUNCTIONS Dawn R. Black. Questions 1.What factors influence productivity? 2.How does primary productivity in tropical rainforests compare."— Presentation transcript:
Questions 1.What factors influence productivity? 2.How does primary productivity in tropical rainforests compare to other biomes? 3.Where are most of the rapidly recycling minerals in tropical rainforests found? 4.What are the three general types of soils found in the tropics?
Questions 5.What are the nutrient retention adaptations found in oligotrophic soils? 6.How do rainforest plants receive nitrogen?
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
Factors Influencing Productivity Adequate light (low light intensity limits understory species) Moisture CO 2 levels Soil minerals/nutrients (many soils old and mineral poor)
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
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
Nutrient Cycling Decomposition and subsequent recycling is the process by which materials move between the living and nonliving components of an ecosystem
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
Leaching of Nutrients Leaching – washing of essential minerals and other chemicals from leaves and soils by water
Leaf adaptations Drip tips (speed water runoff) Protective cuticle with lipid-soluble secondary compounds that retard water loss & discourage herbivores and fungi
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
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).
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)
Soil Characteristics Determined by several factors (Jenny 1941): –Climate –Vegetation –Topographic position –Parent material –Soil age
Rainforest Soil Types Three general classifications of soils throughout humid tropics 1.Ultisols 2.Oxisols 3.Alfisols Comprise ~71% of land surface in humid tropics worldwide Only ~15% of moist tropical forests moderately fertile (in young soils of recent origin)
Ultisols Well-weathered Minerals leached from upper parts of soils
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
Alfisols Closer to neutral pH (still acidic) Less overall leaching Common in subhumid & semiarid tropics
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
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
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
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
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
Forest Demographics Forest turnover varies with species & region –La Selva, Costa Rica ~118 years –Cocha Cashu, Peru 63 years –Manaus, Brazil 82-89 years
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
Answers 1.What factors influence productivity? Light levels, moisture, CO 2 levels, soil minerals/nutrients 2. How does primary productivity in tropical rainforests compare to other biomes? Both GPP & NPP are higher than other biomes
Answers (cont.) 3.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
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
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 δ 15 N method to estimate nitrogen input by N 2 -fixing legumes to natural rainforest
Roggy et al. (1999) Results –N 2 -fixing legumes contributed 136 t ha -1 to total above-ground plant biomass –N 2 -fixation estimated to be 7 kg ha -1 y -1 –δ 15 N of non- N 2 -fixing plants could be related to soil nitrogen availability Could be used as indicator of nitrogen-cycling efficiency in rain forests
Chave et al. (2001) Biomass study 2 study sites –Nouragues Research Station (100 km inland) –Piste de Saint-Elie Research Station (coastal rain forest
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
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.
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
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