Pugnaire F.I. Valladares F. Functional Plant Ecology

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312 Functional Plant Ecology

Tropical Forests: Diversity

and Function of Dominant

Life-Forms

Ernesto Medina

CONTENTS

Introduction ....................................................................................................................... 313

Climate ........................................................................................................................... 314

Ecoclimatic Classifications ............................................................................................. 315

Soil Fertility.................................................................................................................... 315

Distribution and Extension ............................................................................................ 317

Species Diversity................................................................................................................. 318

Patterns of Species Diversity .......................................................................................... 318

Low-Diversity Tropical Forests...................................................................................... 320

Diversity and Functional Ecology of Life-Forms .............................................................. 321

Dicotyledonous Woody Plants ....................................................................................... 321

Establishment and Development ................................................................................ 322

Plasticity and Acclimation of Photosynthesis ............................................................. 322

Radiation Load and Photoprotection......................................................................... 325

Nutrient Availability and the Role of Mycorrhizal Symbiosis ................................... 326

Efficiency of Nutrient Use in Carbon Uptake and Organic Matter Production ........ 328

Palms ..............................................................................................................................329

Architecture and Growth Patterns.............................................................................. 329

Light and Water Relations.......................................................................................... 330

Climbing Plants and Hemiepiphytes............................................................................... 332

Lianas.......................................................................................................................... 332

Hemiepiphytes............................................................................................................. 333

Vascular Epiphytes ......................................................................................................... 334

Ecophysiology of CAM Epiphytes ............................................................................. 336

Occurrence of CAM in Specific Groups of Epiphytes ................................................336

Forest Dynamics and Climate Change............................................................................... 339

Conclusions ........................................................................................................................ 340

References .......................................................................................................................... 341

INTRODUCTION

Tropical forests are geographically limited to areas located between the tropics of Cancer and

Capricorn. They harbor the largest diversity of species and ecosystem types on Earth. Their

importance for the maintenance of stable trace of gas concentrations in the atmosphere is

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313

increasingly recognized. As a result, large international projects are currently underway

throughout the tropics to understand the global interactions derived from biogeochemical

cycling in tropical forests and the consequences of large-scale deforestation for loss of

biological diversity and stability of the world climate. Numerous fundamental contributions

on the ecology of tropical forests were produced during the last century, describing composi-

tion, structure, and functioning of those communities (Richards 1952, 1996, Odum and

Pigeon 1970, Walter 1973, Unesco 1978, Vareschi 1980, Leigh et al. 1982, Sutton et al.

1983, Whitmore 1984, 1990, Jordan 1985, Jordan et al. 1989, Proctor 1989, Go

mez-Pompa

et al. 1991, Mulkey et al. 1996, Kellman and Tackaberry 1997). These contributions served as

a conceptual basis for this chapter, aimed at highlighting the functional characteristics of

tropical forest plant components. The chapter gives an outline of the distribution and

edaphoclimatic characteristics of tropical forest as a basis for the more detailed discussion

of diversity and function of main life-forms. The relationships between water and nutrient

availability as regulators of the photosynthetic performance of tropical trees are also

discussed to assess the role of tropical forests in the carbon balance of the atmosphere.

CLIMATE

Climatic delimitation of tropical regions has been discussed by numerous investigators

(Holdridge 1967, Walter 1973, Whitmore 1984, Richards 1996, Schultz 1995). Climatic

characterization emphasizes the lack of seasonal variations in temperature and daylight

duration, but these parameters increase linearly from the equator toward the tropics. Using

temperature criteria, Walter (1973) defined the tropics as areas of the world, where diurnal

variations in temperature are more pronounced than the differences in average temperatures

between the hottest and the coldest month. This characterization applies to the whole range of

tropical forests, from the lowlands to the high-altitude formations well represented in all

tropical regions of the world. Other treatments separate tropical from nontropical systems

using the occurrence of freezing temperatures at altitudes below 100 m above sea-level

(Holdridge 1967).

Tropical ecosystems are affected by seasonality of rainfall distribution, a climatic trait

associated with the displacement of the intertropical convergence zone. Most ecological

descriptions of tropical ecosystems distinguish humid from dry tropics. The distinction is

based on the annual ratio between precipitation and potential evapotranspiration (P=E ratio).

The actual value of this ratio varies from less than 0.5 in very dry regions to more than five in

wet tropical regions (Holdridge 1967). Practically all tropical climates present varying degree

of seasonality in rainfall, measured as absolute rainfall or as frequency of dry days (daily

rainfall less than evapotranspiration), dry season lasting from few days to several months.

In the last 20 years more attention has been given to climate variability as a driving factor

in tropical latitudes. This approach is emphasized because the emerging understanding of the

impact of global climate change. Generally aseasonal tropical regions may be subjected to

occasional, at times severe, drought that increase tree growth and mortality. These processes

were well documented during the 1997–1998 El Nin

o (Walsh and Newbery 1999, Slik 2004). In

addition to those drought events long-term changes in climate have been observed in tropical

rainforest regions. Malhi and Wright (2004) conducted a worldwide analysis of temperature

and rainfall records in these areas. They showed that since 1970 average temperature increased

0.268C per decade, in probable association with global temperature rise, whereas rainfall

decreased approximately 1% per decade, decline being more pronounced in northern tropical

Africa than in south-east Asia and the Amazon basin. These tendencies of increasing

temperature and declining rainfall may offset the CO

effect on photosynthesis (Clark 2004).

Dendrochronology is producing much needed information on growth rhythms associated

with climatic events.These studies have been made possible by the finding that many tropical

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314 Functional Plant Ecology

trees form annual rings in areas where growth is limited during part of the year by drought or

flooding. The advent of new approaches such as the analysis of

C and stable carbon and

oxygen isotope ratios has generated hard data on growth dynamics in several tropical forests

systems (Worbes and Junk 1989, Verheyden et al. 2006).

Brienen and Zuidema (2006) conducted tree ring analyses on six tree species from

the Bolivian lowland moist forest forming annual rings. They found that age of trees

with the same diameter may differ widely as a consequence of the variation of suppression

and release events during the life time of the tree. They concluded that the species studied

reach the canopy passing through periods of rapid and slow growth, in a pattern probably

determined by light availability.

Scho

ngart et al. (2002) and Fichtler et al. (2003) used a combination of the

C method of

Worbes and Junk (1989) with conventional wood anatomy to study seasonality of growth

species in wet environments with seasonal flooding in the upper Amazon or in rainforests in

Costa Rica. These studies showed that many trees form annual rings, so that tree growth may

be correlated with environmental conditions. Indeed, Scho

ngart et al. (2004) showed that the

growth of Piranhea trifoliata (Euphorbiaceae) is a reliable indicator of El Nin

o events during

the last 200 years.

ECOCLIMATIC CLASSIFICATIONS

Rainfall and temperature account for the large majority of changes in forest structure and

species composition in the tropics. Within a certain temperature–rainfall regime, soil structure

and fertility regulate the differentiation of forest communities (Medina and Cuevas 1994,

Schultz 1995, Swaine 1996).

Temperature and rainfall interact to determine water availability at a given site.

Temperature is directly correlated with evaporative demand of the atmosphere, whereas

rainfall provides water supply through the roots and modify evaporative demand through

changes in atmospheric water content. Humidity indices are frequently calculated using

temperature and rainfall data alone, mainly because of the lack of direct measurement of

evaporation (see discussions on climate classification including tropical areas in Lal 1987,

Cramer and Leemans 1993, Schulz 1995, Olson et al. 2001).

A relatively simple approach to separate large groups of climates in the tropics uses

average values of rainfall (as a measure of water availability) and temperature (as a measure

of evaporative demand of the atmosphere, and also a regulator of plant activity). With mean

annual data series, climate types can be separated with increasing water availability (moisture

provinces in Figure 10.1) and with altitudinal belts related to reductions in temperatures

(altitudinal belts in Figure 10.1). One widely used system to classify climatic units in connec-

tion with forest ecosystems is the Life Zone System of Holdridge (1967). Such a comprehen-

sive summary of climatic data is useful to evaluate patterns of forest distribution on a large

scale. For specific studies, the analysis of seasonal variations of environmental conditions is

essential to understand the ecological process, because in the tropics seasonal variations in

rainfall are far more important than variations in temperature. The seasonality of a certain

climate is better observed in synthetic displays of the climatic variables (climate diagrams; see

Walter 1973 for details). Figure 10.2 shows the climatic characterization of a number of

meteorological stations in northern South America using the criteria of Bailey (1979) for

separating humid and dry months. Temperature decreases with altitude, and so does the

amount of monthly rainfall required to compensate the atmospheric evaporative demand.

SOIL FERTILITY

The separation of humid and dry tropics is frequently paraleled by changes in edaphic

characteristics. Soils of the humid tropics tend to be severely leached by heavy rainfall and

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Tropical Forests: Diversity and Function of Dominant Life-Forms 315

show slight to strong acidity, and a tendency to high Al-mobility. These characteristics

represent a condition in which one or more nutrients may limit plant growth and produc-

tivity. In these areas, lower geomorphological settings lead to water logging, compromising

nutrient availability by low oxygen supply to the roots during periods of variable duration. In

humid tropical forests, water and temperature are not limiting, and nutrients acquire the

leading role in differentiating forests types.

In the dry tropics, the main limiting factor for forest development is low water availability

during periods ranging from 2 to 7 months, with soils generally with neutral pH, and cation

saturations of the soils exchange complex generally above 50%.

Large-scale gradients of rainfall and soil fertility put forward the covariance of these two

factors in the tropics. Fertile soils, measured with a combination of total sum of bases,

percentage of base saturation, phosphorus availability, and pH, are more frequently found

in areas with rainfall below 1500 mm. The large-scale studies conducted in Ghana by Swaine

and others (Hall and Swaine 1976, Swaine 1996) showed a good separation of the influence of

rainfall (water availability) and soil fertility (nutrient availability). In humid areas (less than

1500 mm rainfall), soil fertility appeared as a strong determinant of vegetation, whereas in

drier areas, other factors gained more relevance. Investigators were able to separate the group

of 48 tree species surveyed throughout the forest zone of Ghana in four distributions groups

established by a x

technique: (1) biased to dry-fertile sites; (2) biased to wet-infertile sites;

(3) biased to wet-fertile sites; and (4) nonbiased to any specific site. The majority of the species

studied were within the wet-infertile and the nonbiased groups.

Laurance et al. (1999) analyzed the correlation of forest biomass and soil texture and

nutrient availability using 65 one hectare plots within an area covering 1000 km

in central

Mean annual rainfall (mm)

Tropical

Moisture provinces (Bailey 1979)

Premontane

Lower montane

Montane

Sub-alpine

Alpine

Arid

Semiarid

Dry

Moist

Humid

Perhumid

2000

Mean annual temperature (⬚C)

4000 6000 8000

FIGURE 10.1 Basic categorization of ecoclimates in the tropics based on humidity provinces after

Bailey (1979) and altitudinal belts after Holdridge (1967), Humidity provinces are separated according

to rainfall and evaporation ratios, whereas the altitudinal belts are separated according to annual

temperature averages. The dotted horizontal line indicates the altitudinal belt at which frosts are

expected to occur.

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316 Functional Plant Ecology

Amazonia. The aboveground biomass ranged from 231 to 492 metric tons ha

1

, and it was

significantly correlated with total N, total exchangeable bases, K

,Mg

2þ

, clay, and organic C

in soils, and negatively correlated with sand and Al saturation. The authors concluded that

parameters of soil fertility account for more than a third of the variation in forest biomass. On

a smaller scale in Costa Rica, Clark et al. (1999), showed that soil factors are responsible for

nonrandom distribution of trees in upland soils.

Similar analyses conducted in drier areas in the tropics may render much insight into the

ecological factors determining ecophysiological demands for seedling establishment in these

areas (see reviews in Bullock et al. 1995). This differentiation may prove to be useful in

designing experiments to establish nutritional and hydric dependence of seedling establish-

ment and development under natural conditions.

DISTRIBUTION AND EXTENSION

The accurate estimation of distribution and extension of tropical forests is essential to: (1)

determine the potential carbon fixation capacity of tropical areas of the earth; and (2) assess

the potential effects of increasing concentration of atmospheric CO

on global climate

change. The literature before 1975 has been analyzed and used by Lieth and Whittaker (1975)

3.0 3.0

2.5

2.0

1.5

1.0

0.5

0.0

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Tropical semiarid forest (Coro, 16 m)

B.I.= 2.3; No. dry months: 12

Rain: 427 mm; Temp.: 27.7⬚C

Rainfall Gradient

Premontane semiarid forest (Barquisimeto 813 m)

B.I.= 3.3; No. dry months: 12

Rain: 517 mm; Temp.: 23.8⬚C

Premontane dry forest (Caracas 835 m)

B.I.: 6.1; No. dry months: 6

Rain: 885 mm; Temp.: 21.7⬚C

Premontane moist forest (Mérida 1479 m)

B.I. 13.0; No. dry months: 3

Rain: 1677 mm; Temp.: 19.0⬚C

Tropical dry forest (San Fernando, 47 m)

B.I.: 8.1; No. dry months: 6

Rain: 1447 mm; Temp.: 27.1⬚C

Tropical most forest (Pto. Ayacucho, 73 m)

B.I.: 12.2; No. dry months: 4

Rain: 2144 mm; Temp.: 27.1⬚C

2.5

2.0

1.5

1.0

0.5

0.0

3.0 3.0

2.5

2.0

1.5

1.0

0.5

0.0

3.0

2.5

2.0

1.5

1.0

0.5

0.0

2.5

2.0

1.5

1.0

0.5

0.0

FIGURE 10.2 Examples of seasonal variations in water availability along rainfall and temperature

gradients in tropical forest sites in northern South America. The Y-axes give the monthly S index of

Bailey (1979). The horizontal thick line gives the value of the S index separating the humid from the

dry realms.

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Tropical Forests: Diversity and Function of Dominant Life-Forms 317

to calculate the productivity of organic matter by terrestrial ecosystems. For the estimation of

productivity of terrestrial ecosystems as a whole, the point values of productivity at specific

sites were expanded to an area basis using the extension of each ecosystem unit (Lieth 1975).

More recently, estimations using both climatic and geographic criteria based on satellite

estimation of Normalized Vegetation Indices have been published (see summarizing reports

including distribution and areal extension: Cramer and Leemans 1993, Melillo et al. 1993,

Schultz 1995).

Tropical evergreen forests occupy approximately 17 million km

, or nearly 14% of all land

surface of the planet, but account for more than one-third of total net primary production of

terrestrial ecosystems (Mellilo et al. 1993, 1998, Field et al. 1998; Table 10.1). As a whole,

tropical regions of the earth occupy 33% of the total surface, but their total net primary

production accounts for up to 59% of the total primary production of terrestrial ecosystems.

This bigger contribution to total organic matter production is largely the result of longer

growing periods.

SPECIES DIVERSITY

ATTERNS OF SPECIES DIVERSITY

Tropical forests contain the largest set of higher plants species known. Gentry (1988)

reported values as high as 250 tree species 0.1 ha

1

in several wet tropical forests in

South America. Phillips et al. (1994) analyzed tree species surveys in tropical rain forests

throughout the world, and reported species richness of trees greater than or equal to

10 cm diameter at breast height (DBH) surpassing 200 species ha

1

in Malaysia, Sarawak,

Ecuador, and Peru

, while plots in Uganda, Peru

, Venezuela, and Ghana were relatively poor,

with approximately 60–90 species ha

1

. Most sites showed values between 100 and 200

species ha

1

. The highest species richness values reached 235 (Lambir, Sarawak) and 267

(Yanamomo, Peru

) trees greater than or equal to10 cm DBH per 500 stems, while the lowest

values measured in Kibale, Uganda, and Tambopata, Peru

, were approximately 50 species

per 500 stems.

Gentry (1988) found that the number of tree species 0.l ha

1

increased with rainfall, from

approximately 50 species at rainfall levels near 1000 mm to 250 at rainfall levels of greater

TABLE 10.1

Area and Net Primary Production of Organic Matter Estimated by Direct Measurements

and Using a Process-Based Ecosystem Simulation Model

Vegetation Units

Area

(310

Net Primary

Production

(g C m

year

)

Total Net Primary

Production

(10

g C year

World total 127.3 53.2

Tropical evergreen forest 17.4 13.7 1098 19.1 35.9

Tropical deciduous forest 4.6 3.6 871 4.0 7.5

Tropical savanna 13.7 10.8 393 5.4 10.2

Xeromorphic forests 6.8 5.3 461 3.1 5.8

Total tropical 42.5 33.0 31.6 59.4

Source: Adapted from Melillo, J.M., McGuire, A.D., Kicklighter, D.W., Moore III B., Vo

smarty, C.J., and

Schloss, A.L., Nature, 363, 234, 1993.

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318 Functional Plant Ecology

than or equal to 4000 mm. In addition, vascular species diversity decreased strongly with

altitude (from 1500 to 3000 m altitude). No clear relationship was found between soil fertility

and diversity, although species composition generally changed strongly in association with soil

fertility. Forests without relatively dry months (rainfall 100 mm) and good soil conditions,

such as La Selva and Mersing, had tree-species richness of approximately 100 species ha

1

while sites in Lambir, Sarawak, and Mishana, Peru

, with no rainfall limitation but low

fertility, had numbers well above 200 species ha

1

(Phillips et al. 1994; Figure 10.3). Using

data from Holdridge et al. (1971) for Costa Rica, Huston (1980) showed that tree-species

richness appeared to be inversely correlated with soil fertility. There were significant negative

correlations with available P, exchangeable K and Ca, total bases, base saturation, and cation

exchange capacity. These results were interpreted as a support for the hypothesis that higher

tree diversity in tropical forests occurs under nutritional conditions that prevent dominance

of single, highly competitive species (Huston 1994). The assessment of soil fertility using

conventional soil chemical analysis, such as nutrient concentration without considering

rooting depth, bulk density, and nutrient availability, is frequently misleading. Therefore,

100

200

300

100

200

Number of tree species per 500 stem

300

Queensland, Australia (4,2)

Sepilok, Saba (0,4)

Belem, Brazil (2,3)

Sepilok, Saba (0,2)

Pasoh, Malaysia (1,4)

Number of tree species ha

−1

Kibale, Uganda (3,3)

Tambopata, Perú (3,1)

San Carlos, Venezuela (0,2)

Kade, Ghana (3,3)

Khade, Ghana (3,3)

La Selva, Costa Rica (0,4)

Mersing, Sarawak (0,4)

Tambopata, Perú (3,5)

Sungei Menyala, Malaysia (1,4)

Tambopata, Perú (3,4)

Bukit, Lalong, Malaysia (0,2)

Tambopata, Perú (3,3)

Manú, Perú (3,5)

Lambir, Sarawak (0,4)

Lambir, Sarawak (0,1)

Jatun Sacha, Ecuador (0,3)

Mishana, Perú (0,1)

Yanamono, Perú (0,4)

Anangú, Ecuador (0,3)

FIGURE 10.3 Diversity of tree species in tropical forest sites throughout the world. Sites have been

ordered a1ong-increasing diversity as number of species per hectare or number of species per 500 stems.

The first number after the site name indicates the number of dry months (0–4), and the second indicates

site fertility increasing from 1 to 5. (From Phillips, O.L., Hall, P., Gentry, A.H., Sawyer, S.A., and

Vasquez, R., Proc. Natl. Acad. Sci. USA, 91, 2805, 1994. With permission.)

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Tropical Forests: Diversity and Function of Dominant Life-Forms 319