HOW A RAINFOREST FUNCTIONS

Dawn R. Black

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• CO2 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

Comparisons of NPPNet Primary Production (NPP) of the Major Biome Types Based on Biomass Harvestsa.

Biome

Aboveground NPP (g m-2 yr-1)

Belowground NPP (g m-2 yr-1)

Belowground NPP (% of total)

Total NPP (g m-2 yr-1)

Tropical forests 1,400 1,100 0.44 2,500 Temperate forests 950 600 0.39 1,550 Boreal forests 230 150 0.39 380 Mediterranean shrublands 500 500 0.50 1,000 Tropical savannas and grasslands 540

540 0.50 1,080

Temperate grasslands 250 500 0.67 750 Deserts 150 100 0.40 250 Arctic tundra 80 100 0.57 180 Crops 530 80 0.13 610 a Data from Saugier et al. (2001). NPP is expressed in units of dry mass. NPP estimated from harvests excludes NPP that is not available to harvest, due to consumption by herbivores, root exudation, transfer to mycorrhizae, and volatile emissions.

Global Distribution of Carbon in Plant Biomass

Global distribution of terrestrial biomes and their total carbon in plant biomassa.

Biome

Area (106 km2) Total C pool (Pg C)

Total NPP (Pg C yr-1)

Tropical forests 17.5 340 21.9 Temperate forests 10.4 139 8.1 Boreal forests 13.7 57 2.6 Mediterranean shrublands 2.8 17 1.4 Tropical savannas and grasslands 27.6 79 14.9 Temperate grasslands 15.0 6 5.6 Deserts 27.7 10 3.5 Arctic tundra 5.6 2 0.5 Crops 13.5 4 4.1 Ice 15.5 Total 149.3 652 62.6 a Data from Saugier et al. (2001). Biomass is expressed in units of carbon, assuming that plant biomass is 50% carbon.

Productivity by BiomeProductivity per day and per unit leaf areaa.

Biome

Season lengthb (days)

Daily NPP per ground area (g m-2 d-1)

Total LAIc (m2 m-2)

Daily NPP per leaf area (g m-

2 d-1) Tropical forests 365 6.8 6.0 1.14 Temperate forests 250 6.2 6.0 1.03 Boreal forests 150 2.5 3.5 0.72 Mediterranean shrublands 200 5.0 2.0 2.50 Tropical savannas and grasslands 200 5.4 5.0 1.08 Temperate grasslands 150 5.0 3.5 1.43 Deserts 100 2.5 1.0 2.50 Arctic tundra 100 1.8 1.0 1.80 Crops 200 3.1 4.0 0.76 a Calculated from Table 5.3. NPP is expressed in units of dry mass. b Estimated c Data from Gower (In press).

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

Supporting Decomposers

Slime molds ActinomycetesAlgaeAnimals (vultures, arthropods, earthworms,

invertebrates)Protozoans

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. Ultisols2. Oxisols3. 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, 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

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 δ15N method to estimate nitrogen input by N2-fixing legumes to natural rainforest

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

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

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. Science 282:439-442.

• 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.