TROPICAL DRY DECIDUOUS FORESTS

Tropical deciduous forests cover extensive areas in climates with highly seasonal distribution of rainfall (Murphy and Lugo, 1986), characterized by the dominant proportion of deciduous woody components

Proportion of deciduous species varies from 40 to100 % depending on the location within the rainfall gradient.

Deciduousness is frequently facultative, duration of the leafless periods depending on soil water availability.

The structural components change more or less monotonously as annual rainfall decreases

- the proportion of deciduous trees and shrubs increases - epiphytes and hemiparasites frequency decreases- lianas (woody climbing plants) reach important proportion - proportion of succulent plants, including CAM performing cacti and

stem succulents, increases

Area and net primary production of organic matter (expressed as g C per unit area) of tropical forests and savannas estimated by direct measurements and using a process-

based ecosystem simulation model

Vegetation Units Area % NPP Total NPP % (x 106 km2) g C m-2 yr- 1 1015 g C yr-1

Whittaker and Likens (1973)World Total 149.0 58.8Tropical Rain forest 17.0 11.4 1100 18.7 31.8Tropical seasonal forest 7.5 5.0 800 6.0 10.2Savanna 15.0 10.1 450 6.8 11.6Total tropical 39.5 26.5 31.5 53.6

Melillo et al. (1993)World Total 127.3 53.2Tropical evergreen forest 17.4 13.7 1098 19.1 35.9Tropical deciduous forest 4.6 3.6 871 4.0 7.5Tropical Savanna 13.7 10.8 393 5.4 10.2Xeromorphic forests 6.8 5.3 461 3.1 5.8Total tropical 42.5 33.0 31.6 59.4

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Characteristic ecosystem processes regulated by seasonality of water availability and variability of growing period (begin to end of the rainy season)

Primary Producers Decomposers Consumers

Rainy Season Begin Leaf flushing and fine Rapid decomposition Activity of detritivores

root production. of above- and below- herbivores, and seed-Carbohydrate and organic matter fruit eaterswater transport Net losses of CO2 andto the shoots N oxides

Middle Full canopy develop- Accumulation of Maximum herbivorement, carbohydrate microbial biomass activitylitter production

End Growth stop Slow decrease of Reproduction andLeaf and fine root microbial activity begin of restingshedding phase. Faunal

migrations

Dry SeasonCarbohydrate Soil biological Pollinatorsexpenditures for activity strongly and nectar consumersflowering and reduced or nil reproductionmaintenance respirationof roots and stems

Drought tolerance and ecological costs associated to contrasting phenological behaviors: deciduous and evergreen trees

Deciduous trees

Total or partial leaf abscision at moderate water potential values

Associated costs

Seasonal regeneration of leaf mass Productive capacity of limited duration Respiration losses of non photosynythetic organs remaining at

high water potential during the dry season High requirement of nitrogen

Evergreen Trees

Protoplasmatic tolerance to low water potential maintaining metabolic activity. Many species are osmoregulators. Partial evasion to recurrent drought through development of deep root systems

Associated costs

Lower photosynthetic rates Requirement of thermotolerant tissues Protection against light damage of the photosynthetic system Development of deept roots

Examples of life and growth-forms in tropical dry forests (includes tropical deciduous forest, thorn forest and thorn scrub). With data from Aristeguieta 1968, Wikander 1984, Bullock 1985, Ponce & Trujillo 1985

Trees and shrubs (woody plants)Evergreen Capparis, Casearia, Castela, Coccoloba, Guaiacum,

Haematoxylon, JacquiniaDeciduous (obligate Acacia, Albizzia, Bumelia, Pisonia, Pithecelobium,or facultative) Prosopis, Swietenia, TabebuiaGreen-stemmed CercidiumSucculent deciduous Bursera, Cochlospermum, Erythrina, Jatropha, Manihot

Pereskia, SpondiasCAM-type (cacti) Lemaireocereus, Opuntia, Pilosocereus, Ritterocereus

Herbs & RosettesDicots C3 Bastardia, Croton, Melochia, Weddelia

C4 Alternanthera, Atriplex, Euphorbia, Portulaca, TrianthemaGrasses C4 Anthephora, Aristida, Cenchrus, Setaria, SporobolusBromeliads CAM Aechmea, Bromelia

Lianes and creeping plantsC3 Arrabidaea, Mansoa, Macfadyena, Cydista, Cissus (one species

CAM inducible)CAM Acanthocereus, Selenicereus

EpiphytesC3 Peperomia, PhilodendronCAM Brassavola, Schomburgkia, Tillandsia

Hemi-parasites Ixocactus, Phoradendron, Phtyrusa

Anatomical characteristics associated with wind dispersion of reproductive units in trees and shrubs and climbers in a tropical dry

forest (from Wikander 1980)

Type of dispersal unit Trees and shrubs Lianes

Feathery seedsCeiba pentandra Marsdenia condensiflora

Cochlospermum vitifolium Matelea maritimaMatelea planifloraMatelea urceolataMetastelma sp

Winged seeds Tabebuia ochracea Cydista equinoctialisMacfadyena unguis-catiMansoa verruciferaPhryganocidia corymbosaPithecoctenium crucigerumPrestonia exerta

Very small seeds Pogonopus speciosus

Syncarpic fruits Centrolobium paraense Heteropteris purpureawith one wing Fissicalix fendlerii Machaerium moritzianum

Lonchocarpus dipteroneurus Nyssolia fruticosaMachaerium latialatum Securidaca diversifoliaMachaerium robiniaefolium Securidaca sp.Myriospermum frustescens

Schizocarpic, Beureria cumanensis Serjania spwinged fruits Bulnesia arborea

Multi-winged fruits Piscidia carthaginensis Combretum fruticosusTerminalia sp. Paullinia sp.

Osmotic pressure of leaf sap and xylem tension measured in the field during the middle of the dry and rainy seasons in a dry tropical

forest of the Paraguaná Penísula in Falcón State Venezuela

Species Osmotic Pressure (MPa) Max. Xylem Tension (MPa) Dry Rain Dry Rain

EvergreensCapparis odoratissima 4.31 2.96 5.49 3.57Capparis linearis 3.66 2.95 5.99 4.50

Jacquinia aristata 4.09 1.51 4.96 2.76

DeciduousCasearia tremula 4.55 1.81 6.55 0.78Pithecellobium dulcis 4.26 1.82 5.76 3.19Prosopis juliflora 3.13 2.10 4.17 3.11

Succulent woody plants (deciduous)Jatropha gossypifolia 1.45 1.19 1.38 0.17Pereskia guamacho 1.27 1.09 1.33 0.97

Malacophyllous shrubCroton cf. flavens 4.82 1.28 7.41 0.40

Leaf size of the species studied in the tropical dry forest of Guánica. Leaf sizes classified according to Taylor (1975): Small, Medium and Big

Species Average range Classification Morphological type (cm2)

Exostemma caribaeum (Jacq.) R. & S. 6.5 - 13.8 Microphyll M-B Malacophyll*

Bucida buceras L. 7.8 - 17.4 Microphyll M-B Sclerophyll

Bursera simaruba (L.) Sarg. 13.9 - 23.7 Microphyll B - Mesophyll S Mesophyll, succulent stem

Bourreria succulenta Jacq. 18.4 - 25.4 Microphyll B - Mesophyll S Sclerophyll

Pisonia albida (Heimerl.) Britton 9.5 - 29.1 Microphyll B - Mesophyl S Mesophyll, succulent stem

Thouinia portoricensis Radlk. 23.0 - 33.8 Mesophyll S Sclerophyll

*Malacophylls: plants producing hygromorph, hairy leaves, resistant to low water potentials. They do not reduce tranpsiration at during the dry season, and osmotic pressure increase rapidly. By prolongued drought older leaves are shed, until only the buds remain active.Sclerophylls: plants producing hard, though, fiber rich leaves, capable of efficient transpiration control during drought Mesophylls: plants producing drought intolerant leaves, stomatal closure reduces transpiration at the onset of drought, and in so doing curtail photosynthesis too. Leaves are shed with high water content, and are yellow in color.

Osmolality of the leaf vacuolar sap of woody species from a tropical dry forest in Guánica, Puerto Rico (Medina and Cuevas

1990)

Species mmoles kg-1 Rainy season Dry season

Bourreria succulenta 344 1343

Bucida buceras 475 967

Thouinia portoricensis 510 1232

Pisonia albida 578 634

Bursera simaruba 640 471

Exostemma caribaeum 654 1491

Photosynthesis (A), conductance (g), and intrinsic water use efficiency (IWUE) of woody species in a Tropical Dry forest, Guánica, Puerto Rico, during the rainy and dry seasons

(Medina and Cuevas 1990)

A max g max IWUE µmol m-2 s-1 mmol m-2 s-1 µmol/mmol x 10-2

Rain Dry Rain Dry Rain Dry

Exostemma caribaeum 9.5 6.9 217 180 4.4 3.8

Bucida buceras 8.4 5.7 233 144 3.6 4.0

Bursera simaruba 8.6 6.2 190 142 4.5 4.4

Bourreria succulenta 9.0 6.9 224 177 4.0 3.9

Pisonia albida 7.5 5.6 205 142 3.7 3.9

Thouinia portoricensis 11.6 7.3 339 169 3.4 4.3

Species

Osmotic behavior during leaf development among deciduous forests tree species(from Olivares and Medina 1992)

Leaf composition of Evergreen Xerophytic and deciduos woody plants(Data from Pfundner 1993)

Organic Quaternary

SPECIES Water K Ca ∑ Cations Cl SO4 Acids NH4 comp. ∑cyclitols g/kg frwt. ------------------------------------- mmol kg-1 fresh weight--------------------------------------

Capparis linearis 498 177 141 540 114 283 125 82 6

Capparis odoratissima 444 142 166 418 68 533 63 123 61

Morisonia americana 396 222 94 362 18 329 53 428

8

Belencita nemorosa 416 363 246 801 1 1426 75 50 9

Jacquinia aristata 466 152 121 504 107 226 83 0 62

Jatropha gossypifolia 825 117 1 224 69 13 109 0 33

Pereskia guamacho 915 19 2 178 133 1 74 0 22

Prosopis juliflora 510 146 71 314 162 58 92 4 143

Open fields of research related to the functioning of dry deciduous forests ecosystems

1. Factors regulating leaf shedding and leaf flushingDrought, photoperiodism, temperature

2. Synchronization of flowering during the rainy season- the role of pollinators (Janzen)- water relations (Borchert)

3. The contrast of evergreen and (facultative) deciduous trees- selecting factors:

nutrients tolerance to low water potentialduration of the dry season and carbohydrate balancecontrol of respiration

4. Water and nutrient transport to aboveground growing meristems at the beginning of the rainy season

5. The physiological significance of high levels of soluble Ca in some families (Capparaceae) and the production of oxalate in others (Theophrastaceae)