Tropical Climates and Ecosystems

Mihai Tomescu

With Special Reference to the Neotropics, andParticular Reference to French Guiana

Neotropical Rainforest Ecology SeminarPBIO 693

Spring 2002

The questions I am trying to answer

What are the parameters that circumscribetropical climates

How does El Nino function and what are its effects

What are the major types of neotropical ecosystems

What do we know about the recent climatic historyof northern South America

1.2.3.4.

The TropicsThe region that straddles the Equator, bounded by the two tropical circles

2327’N

2327’S

47-degree band of latitude

Cancer

Capricorn

Climate: tropical, but also subtropical at the two latitudinal extremes

Tropical climates

Consistently warm,often hot temperatures

High relative humidity

High precipitations

Seasonality

Trade winds

!

Temperature and humidity

31C (88F) day22C (72F) night

Daily temperature fluctuationexceeds average annual seasonal fluctuation.

Relative humidity is around 80%.Varies seasonally: 88% rainy season, 77% dry seasonReaches 90-95% at ground level in tropical rainforests

Seasonal fluctuation of temperature is extremely low:maximum 5C between mean temperatures of warmestand coldest month.

Precipitations and windsTropical areas fall within the trade-wind belts and encompass theInterTropical Convergence (ITC)

Evapo(transpi)ration(very important!)

Humid, warm air rises and iscarried North and South awayfrom the ITC

Air cools down releasingprecipitations and falling

Backward flow toward theEquator

Precipitations: 1,500 – 3,000 mm annually in the Amazon basin

Seasonality

The Tropics are located at 2327’ latitude North and South

Earth’s axial tilt is also 2327’NI wonder why ?

Seasonality

Day/night length varies (even by over 35’)Heat patterns of air masses change, resulting in seasonality of rainfall.

Dry season: <100 mm rainfall/monthmany deciduous trees shed leavesmost trees flower - insect pollinators more active

Animals et al. also respond to seasonality.

Common misconception: there is no seasonality in the Tropics

Wet season Dry season. Sometimes two of each.

Rainy north of the Equator Dry south of the Equator and viceversa.

Wet season: up to 1,000 mm rainfall/month or moreseeds germinate at onset of rainy season, BUT...Most fruiting at peak of rainy season, BUT...

El Nino Southern Oscillation (ENSO)

El Nino conditionsHigh pressure weather system and westward circulation disruptedUpwelling shut downWarm oceanic waters and precipitations move westwardDrought in Australia, Indonesia

Normal conditionsHigh pressure weather system stable over eastern PacificWestward trade-winds and currentsUpwelling and cold water along South American coastWarm waters in western PacificPrecipitations over Australia, Indonesia

ENSO effects

A recent correlation analysis of past ENSO events and dengue epidemics in Indonesia and French Guiana (Gagnon et al., 2001) has demonstrated a significant correlation between the two. The two regions experience warmer temperatures and less rainfall during ENSO years.

Major disruptions to ecosystems, especially marine

Changes in weather patterns, with accompanying effects - forest fires, floods, mudslides etc.

Maybe even species extinctions

Catastrophic for ecosystems sensitive to seasonal variation

The Andes and their effect on climate and vegetationThe Andes are still rising, as we’re talking here!

0°

23°27’

Act as a wall: prevent moisture-laden air in the Amazon basin from reaching the western coastRain shadow effect

Eastern slopes very humid - rainforestsWestern slopes and coast extremely dry - deserts

Precipitation is recycled within the Amazon basin

Ecosystems on eastern and western slopes differ dramatically

Stute et al., 1995 Climatic history of northern South America

A bit of climatic history

Reconstructed past temperatures (last 30 ky) based on concentrationsof noble gases (Ne, Ar, Kr, Xe) in groundwaters (NE Brazil).

Relative concentration of the gases reflect the temperature of the watersthat incorporated them, which in turn reflect the mean annual groundtemperature at the depth of the water table.

Showed the differences in meanannual temperatures between thepresent interglacial and the lastglacial period.

Interpolating, for French Guianathe difference is of about 5-6C.

Compare their reconstructionswith previous results.

A bit of climatic historyMora and Pratt, 2001 Climatic history of northern South America

Reconstructed the climate of thelast glacial stage based on O andH isotopic composition of mineralsin weathering profiles of theColombian Andes.

There is a strong correlationbetween kaolinite isotope data(O and H) and surface temperatureand rainfall.

Weathering profiles from Bogota basin located at altitude of (A) 3150 m and (B) 2980 m, showing soil horizons, radiocarbon dates on

charcoal, and distribution with depth of kaolinite oxygen (18O) and hydrogen (D) isotopic composition

Mora and Pratt (continued)

Isotope data from paleosols developed during the last glacial stage(LGS) suggest approximately 6C cooler temperatures.

The data indicate higherisotope values of rainwaterduring the LGS,interpreted to reflect drierconditions.

The temperatures matchthose cited by Stute et al.for the region, based onpollen spectra and treeline positions.

A bit of climatic history

Values of 18O vs. D for kaolinites in weathering profiles. Diamonds correspond to mean values in modern (Holocene) soils, and circles correspond to mean

values in last glacial stage (LGS) paleosols. Discrete values within modern soils and paleosols are shown as small crosses.

Major neotropical ecosystemsThe Tropical RainforestLush vegetation, many vines and epiphytes.

Essentially nonseasonal, dominated bybroad-leaved, evergreen trees.

The actual rainforest

Annual precipitation >2,000 mm, spreadevenly from month to month.The moist forestMore important participation of deciduous trees.In seasonally dry climates, precipitation >1,000 mm, mean annualtemperature >24C.

Tropical forests are among the most productive and extensive ecosystemson Earth.Debate going on whether their biomass has been increasing in responseto atmospheric changes over the last century.

Latest studies (Phillips et al., 2002) show that it has been increasing,providing a modest negative feedback to atmospheric CO2 accumulation.

“Major neotropical ecosystems”The Jungle – Disturbed Forest Areas

Jungles actually represent early successional stages of the rainforestfollowing disturbance.

Fast-growing, shade-intolerant species.

Thin-boled trees growing very close together. Palms, bamboos, vinesabound.

Even rainforests are mosaics of successional stages triggered bydisturbance represented by tree-falls.

Major neotropical ecosystemsRiverine and Floodplain EcosystemsGallery forests border rivers.

Varzea floodplain forests run along whitewater rivers (sediment-rich)Igapo floodplain forests run along blackwater rivers (rich in humic matter)

Rio Xingu

Major neotropical ecosystemsSavannas

May be relatively wet, or dry and sandy.Seasonal climate, occasional wildfires.

Dry forestsOpen woodlands with many deciduous trees.In areas with a pronounced dry season, often intermixed with savannas.

Low Quite high species diversity.

Grasslands scattered with trees and shrubs.

Major neotropical ecosystemsCoastal ecosystemsMangal – the dominant vegetation along tropical coastlines.Formed by mangroves – salt-tolerant tree species.

Seagrasses

Coral reefs

Mangroves and seagrasses contribute to the health of neighboring coral reefs

Mangroves have an essential role in the ecology of coastal areas: habitat for animals, improve water quality prevent erosion.

French GuianaReliefLocated on the Guiana ShieldOld rocks (Precambrian, 2.1-1.9 Gy), very stableVery eroded – flat relief (highest altitude 800m)Basalt dikes associated with the opening of the SouthAtlantic - form inselbergs

French GuianaClimate

Tropical – hot and humid

Average annual precipitation > 2,500mm> 3,000mm/year on most of the coast< 2,000mm/year in southern French Guiana

Seasonality of climate due to latitude and to the position of theInterTropical Convergence (ITC): Two Seasons

Difference between longest and shortest days of the year 35’August-October: ITC north of French Guiana – Dry SeasonDecember-June: ITC migrates south – Wet Season (May worst)

Average temperatures 23C (73F) - 33C (91F) in CayenneAround 27C in Saul

Daily temperature fluctuations greater than annual fluctuations

French GuianaVegetation

18 kya

12 kya

All of the major ecosystems are present.

Puig et al., 1981 Vegetation formationsClimatic formations:Tropical ombrophilous lowland forest

Precipitations >3,000 mmPrecipitations >2,000 mm

Edaphic formations:MangroveTall grassland with broad- leaved trees (savanna)Middle-altitude savanna

+ Gallery forest & Jungle

French GuianaCharles-Dominique et al., 1998 Small-scale zonation of vegetation, vegetation and climate history

Les Nouraguesresearch station

Annual rainfall > 3,000mmDistributed over 280 days

Seasonality: two dry seasons of unequal durationMain dry season – September-October“Little summer” – around March

Small-scale zonation of vegetationHyperhumid rainforest: mosaic of successional stages eachinitiated by treefallsRock savanna: open vegetation on rocky outcrops and inselbergscyanobacterial crusts – xerophytic herbs – shrubby thicketsLow forest/Transition forest: between the two – low thicketyvegetation adapted to xeric conditionsNot a mere ecotone: harbors almost 200 characteristic speciesLiana forests & bamboo thickets: stagnant regeneration or relictpatches

French Guiana Charles-Dominique et al. (continued)

Vegetation history

Found several charcoal layers in the soil – indicators of large-scaleforest fires.

Fire occurrences most frequent 10,000-8,000 BP6,000-4,000 BP2,000-0 BP

Fires independent of human occuppation.Sometimes occur below human occuppation levels.Earliest anthropogenic remains in French Guiana around 2,000 BP.Not a local phenomenon.

Conclude that forest fires are the only type of major perturbationoccurring at a moderate rate in the rainforest.Hypothesize that they could be favorable to the maintenance of a highspecies diversity by giving more or less periodical impulses to thedynamics of the ecosystem.

French Guiana Charles-Dominique et al. (continued)

If fires are not anthropogenic, then they are triggered by climatictrends: repetition of abnormally long dry seasons over a number ofyears and consequent lowering of the water table.

Found remains of alluvial terraces that could only be formed duringperiods of intensive erosion.Such erosion could be explained only by extensive deforestation,such as that generated by large-scale forest fires.

In the last decades particularly dry years have been recorded inin correlation with high ENSO incidences.Hypothesize that such episodes could be at the origin of droughtperiods characterized by high forest fire frequency.

Although drier periods were characterized by different compositionof the rainforest (fossil pollen, seeds and charcoal), open landscapeswere not present in the area during the last 3,000 years.

Climate history

French GuianaPujos et al., 1996Paleoceanographyof French Guianashelf and climatehistory

Studied the mineralogy of fine sediments in several cores takenon the continental shelf off the French Guiana coast.

Demonstrated that fluctuations in the relative participation of thedifferent continental sources that supplied the sediments reflectfluctuations in water discharge and erosion related to climatefluctuations.

Suggest that the dry episodes reflect periods during which theclimate mimicked present-day ENSO phenomena.

Evidenced dry climatic phases between 2,200-1,200 BP.

French GuianaLoubry, 1994 Phenology of decidual trees

Studied leaf-fall periodicity on 500 treesover a two-year period.The rainforest in French Guiana is evergreenbut contains approximately 100 deciduoustree species.

Leaf-fall periodicity thought to be unrelated to climate (endogenous)based on: wide spread taxonomic distribution of the character

presence or absence among some taxaancient origin of the character

These finds refine earlier hypotheses and suggest the participation ofboth endogenous and exogenous components in the determination ofleaf-fall periodicity.

Leaf-fall periodicity is annual and seasonalis not correlated to rainfall or the dry seasoncorrelated to photoperiodical variations (exogenous)each tree has its own periodicity for leaf sheddding

French GuianaSarthou et al., 2001 Island effects in French Guiana inselberg vegetation

Studied the genetic structure of Pitcairnia geyskesii(Bromeliaceae) on populations distributed on threeinselbergs by analysis of10 isozyme loci.

Inselbergs – rock savannahabitats isolated by expansesof rainforest, therefore expectedto see island effects in plantsrestricted to inselbergs such asPitcairnia.

Found high levels of geneticvariation within each population – probably as a result of associationof genet longevity (clonal replication) and recruitment of new genetsby efficient sexual reproduction.Substantial genetic differentiation and low gene flow among inselbergs.

The answers to the questions

What are the parameters that circumscribetropical climates ?1.Hot, often humid and seasonal

Daily temperature fluctuations exceed average annual seasonal fluctuations

Seasonality expressed principally in precipitation: dry season vs. rainy season

The answers to the questions

How does El Nino function and what are its effects2.Disruption of westward oceanic and air circulation west of South America; upwelling along western coast of South America stops; hot surface waters move eastward

Abnormal weather patternsForest fires, floodsDisruption of ecosystemsEpidemics

The answers to the questions

What do we know about the recent climatic historyof northern South America3.Last glacial stage:Dryer, Mean annual temperatures 5-8C lowerMean sea surface temperatures 2C lower

After last glacial:Warming of climateAlternation of wetter and dryer episodes; dry episodes related to high incidence of ENSO

The answers to the questions

What are the major types of neotropical ecosystems4.Rainforest (jungle)

Savannas and dry forests

Riverine and floodplain

Coastal

Adams, J., 2002. South America during the last 150,000 years. http://www.esd.ornl.gov/projects/qen/ nercSOUTHAMERICA.htmlAlden, A., 2001. Geologic map of French Guiana. http://geology.about.com/library/bl/maps/ blfrenchguianamap.htmCharles-Dominique, P., Blanc, P., Larpin, D., Ledru, M.-P., Riéra, B., Sarthou, C., Servant, M., Tardy, C., 1998. Forest perturbations and biodiversity during the last ten thousand years in French Guiana. Acta Oecologica 19(3), 295-302.Gagnon, A,.S., Bush, A.B.G., Smoyer-Tomic, K.E., 1996. Dengue epidemics and the El Nino Southern Oscillation. Climate Research 19(1), 35-43.Kricher, J., 1997. A neotropical companion. Second edition. Princeton University Press.Loubry, D., 1994. Phenology of deciduous trees in a French-Guianan forest (5 degrees latitude North) - case of a determinism with endogenous and exogenous components. Canadian Journal of Botany 72(12), 1843-1857.Mora, G., Pratt, L.M., 2001. Isotopic evidence for cooler and drier conditions in the tropical Andes during the last glacial stage. Geology 29(6), 519-522.Phillips, O.L., Malhi, Y., Vinceti, B., Baker, T., Lewis, S.L., Higuchi, N., Laurance, W.F., Vargas, P.N., Martinez, R.V., Laurance, S., Ferreira, L.V., Stern, M., Brown, S., Grace, J., 2002. Changes in growth of tropical forests: evaluating potential biases. Ecological Applications 12(2), 576-587.Puig, H., Blasco, F., Bellam, M.P., 1981. Vegetation map of South America. Explanatory notes. UNESCO, Paris.Pujos, M., Latouche, C., Maillet, N., 1996. Late Quaternary paleoceanography of the French Guiana continental shelf: clay-mineral evidence. Oceanologica Acta 19(5), 477-487.Rudant, J.-P., 1994. French Guiana through the clouds: first complete satellite coverage. European Space Agency EOQ 44. http://esapub.esrin.esa.it/eoq/eoq44/rudant.htmSarthou, C., Samadi, S., Boisselier-Dubayle, M.C., 2001. Genetic structure of the saxicole Pitcairnia geyskesii (Bromeliaceae) on inselsbergs in French Guiana. American Journal of Botany 88(5), 861-868.Shirah, G.W. et al., 1998. El Nino visualisations. 1997-98 Temperatures Beneath Sea. http://nsipp.gsfc. nasa.gov/enso/visualizations/index.html

Stute, M., Forster, M., Frischkorn, H., Serejo, A., Clark, J.F., Schlosser, P., Broecker, W.S., Bonani, G., 1995. Cooling of tropical Brazil (5C) during the last glacial maximum. Science 269, 379-383.

Bibliography