Transcript how a rainforest functions

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 NPP
Net Primary Production (NPP) of the Major Biome Types Based on Biomass Harvestsa.
Aboveground Belowground NPP Belowground Total NPP
NPP
(g m-2 yr-1)
NPP
(g m-2 yr-1)
Biome
(g m-2 yr-1)
(% of total)
1,100
Tropical forests
1,400
0.44
2,500
600
Temperate forests
950
0.39
1,550
150
Boreal forests
230
0.39
380
500
Mediterranean shrublands
500
0.50
1,000
Tropical savannas and
540
grasslands
540
0.50
1,080
500
Temperate grasslands
250
0.67
750
100
Deserts
150
0.40
250
100
Arctic tundra
80
0.57
180
80
Crops
530
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.
Area (106 km2)
Total C pool
(Pg C)
17.5
10.4
13.7
2.8
27.6
15.0
27.7
5.6
13.5
15.5
149.3
340
139
57
17
79
6
10
2
4
21.9
8.1
2.6
1.4
14.9
5.6
3.5
0.5
4.1
652
62.6
Total NPP
(Pg C yr-1)
Biome
Tropical forests
Temperate forests
Boreal forests
Mediterranean shrublands
Tropical savannas and grasslands
Temperate grasslands
Deserts
Arctic tundra
Crops
Ice
Total
a
Data from Saugier et al. (2001). Biomass is expressed in units of carbon, assuming that
plant biomass is 50% carbon.
Productivity by Biome
Productivity per day and per unit leaf areaa.
Biome
Tropical forests
Temperate forests
Boreal forests
Mediterranean shrublands
Tropical savannas and grasslands
Temperate grasslands
Deserts
Arctic tundra
Crops
a
Season
lengthb
(days)
Daily NPP per
Daily NPP per
c
ground area
Total LAI leaf area (g m2 -1
(g m-2 d-1)
(m2 m-2)
d )
365
6.8
6.0
1.14
250
6.2
6.0
1.03
150
2.5
3.5
0.72
200
5.0
2.0
2.50
200
5.4
5.0
1.08
150
5.0
3.5
1.43
100
2.5
1.0
2.50
100
1.8
1.0
1.80
200
3.1
4.0
0.76
Calculated from Table 5.3. NPP is expressed in units of dry mass.
Estimated
c
Data from Gower (In press).
b
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
Actinomycetes
Algae
Animals (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):
–
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–
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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
•
•
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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
1M
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
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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 longterm 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.