Tropical forest response to climate change Dr. Julián Granados Castellanos Calle 55 – B Número 179 Entre 40 y 42 Fraccionamiento Francisco de Montejo Mérida, Yucatán México juliangranados2@yahoo.com.mx

November 2006

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Human disruption on tropical forest ecophysiology

Tropical forest can be found in each of the three land areas, which occur

within the tropical zone and it displays three major tropical regions: The

Neotropical, the Paleotropical, which occupies two African regions, and the South

East Asian tropical forest. Of those regions, Neotropical forest is the most

extensive and diverse in life and portrays a magnificent world biodiversity reservoir,

distributed from Central America to Amazonia. Various vegetation types (i.e.

tropical rain forest, semi evergreen forest, savanna, wetlands, etc.), in which

amalgams of growth forms (trees, lianas, epiphytes, stranglers, herbs, etc.) build

plant communities, which account for the worlds highest arboreal species diversity

number, and even excluding trees, shows the highest world floristic diversity.

Earth biodiversity depends on continuous plant growth resource availability

and solar energy reaching tropical regions has through an evolutionary process

produced a plant productivity-biodiversity binomial, which symbolizes the crown of

atmosphere-biosphere coupling. Atmosphere, through the climate system,

influences Biosphere, and Biosphere reaction due to climate change can be with

water and nutrient cycling changes observed. For exa mple, climatic periodicity

disruptions have substantial effects on precipitation patterns on which ecosystem

process depend. World biomes are sensitive to climate periodicity and Equatorial

tropical forests distribution, formation and species composition has changed with

changing global climates during the last 30,000 years. Since the industrial

revolution a global climate change has been generated as a consequence of

atmospheric greenhouse gases enrichment. Furthermore, pristine tropical forest

has been extensively cleared all over the world (ca. 20 - 30 % of Earth surface),

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which also contributed to the greenhouse gases atmospheric enhancement. These

anthropogenic activities have been generating during the past 256 years an

unparalleled impact on the climatic system with unpredictable consequences to the

life on Earth (Figure 1).

Climate change has been affecting water and nutrient cycles through its

effects on plant productivity (fertilizer effect on photosynthesis) and temperature

enhancement (greenhouse effect). Infrared absorbing gases simultaneously with

natural solar activity contribute to Earth surface warm-up (up to 0.6 oC) and

temperature enhancement stimulate autotrophic and heterotrophic respiration with

its consequent decrement on carbon fixation through time, despite atmospheric

Figure 1. Historical anthropogenic activities that contributes to the CO2 atmosphere enhancement. Land use change ( ) and fuel consumption ( ) has been through time changing their contribution value to the Atmosphere CO2 enrichment, which reflex deeply changes on human life style. (Data from NOAA 2004)

1700 1800 1900 2000 2100

Year Ant

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2000

4000

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CO2-enrichment fertilization potential. Otherwise, intact tropical forest stores a very

high amount of carbon, accumulated in wood for more than a Century. However,

intact pristine forests are not common anymore and this is a consequence of

intensive change in tropical forest land use, which similarly to CO2-enriched

atmosphere, has the potential to enhance tropical forest dynamics, affecting

tropical forest carbon residence time. A reduction in carbon fores t residence time

involves the following aspects: Tree life span depends on tree species and forest

type, and tree longevity determines carbon residence time, which has major

implications on tropical carbon pool size. About 50% of biomass carbon in tropical

forests is bound in canopy trees that represent a minor proportion of the total

number of tropical trees. However, tropical forest is not like a passive sink of

carbon, and successional processes (gap dynamics in pristine forests and forest

succession in secondary vegetation) may be biased by a CO2-enriched

atmosphere to changes in arboreal species forest dominance due to habitat

productivity determination of plant community structure.

In pristine forest, gap dynamics play a major role in natural regenerat ion

processes, and under gap regeneration conditions CO 2-enhanced liana growth and

competitiveness (i.e. Granados and Korner 2002, Global Change Biology 8:1109-

1117); Granados 2002, dissertation Basel University) have negative influence on

tree growth and CO2-resposivenes which could reduce tree life span and

discrimination against of slow growing, long lived tree species, with obvious

consequences for forest carbon stocking. The relative atmospheric CO 2-

enrichment effect should become stronger as one approaches light compensation

point (where light influence on plant assimilation equals plant respiration) under

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severe light limitation in pristine tropical forest understory CO2-effect on plant

growth declines with plant age and in forest understory where light availability is

limiting, seedlings will be more CO2-sensitive. Liana CO2-response allows liana

effects on forest dynamics enhancing the possibility of forest succession stall in an

early successional stage with a negative impact on forest structure. Changes in

forest structure toward shorter tree life span could lead to a reduction of carbon

residence time in pristine forest. Whereas, a most common form of tropical forest

stage is secondary vegetation, which shows a very different ecology compared

with pristine forests, due to higher species interactions than in pristine forest. For

example, density and biodiversity of lianas and other fast-growing species

decrease as forest stand age increases. However, atmospheric CO 2-enrichment

effects on the secondary forest regeneration process remain a conundrum.

Regarding the biosphere carbon sink, a main concern is biodiversity, and it

has been disregarded that tropical forests are not homogenous sinks of carbon.

The structural components (species diversity) play a major role in the forests

capacity to store carbon. However, species structure and dominance widely varies

within forest types and with forest life history. Thus, a predictive understanding

needs to account for biodiversity effects on tropical forest carbon storage capacity

particularly because productivity and species diversity scale dependently (Figure

2). Another important aspect is the temporal character of the elevated CO 2-effect

on the biota. Growth responses to elevated CO2 are non-linear with the degree of

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Vegetation types

non-linearity depending on species and light availability (i.e. Granados and Körner

2002, Global Change Biology 8:1109-1117), with the strongest effect on the CO2

concentration range between pre-industrial CO2 concentration and 420 ppm CO2

concentration.

Because of the small size of the atmospheric carbon pool, this carbon

reservoir is very sensitive to depletions of biosphere carbon pools and the

exploitable fossil pool. If elevated CO2 will enhance forest regeneration dynamics,

the rate of the biosphere as a carbon sink will change, most likely by diminishing.

p < 0.01 r2 = 0.38

LSF

LHF

LRF

LsH

F

LMH

F

LAM

pHF

HAM

pHF

0

100

200

Figure 2. Arboreal species number within vegetation types. Species richness from low to high land Neotropical forests. LSF Lowland swamp forest; LHF Lowland perhumid forest; LRF Lowland rain forest; LsHF Lowland subhumid forest; LMHF Low altitude mountain humid forest; LAMpHF Low altitude perhumid forest; HAMpHF High altitude mountain forest. Modified from Hartshorn (2002) Biogeografía de los bosques neotropicales. In: Guariguata M y GH Kattan (compiladores) Ecología y conservación de Bosques Neotropicales. Libro Universitario Universal 59 – 81.

Spec

ies

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We need forest CO2-enrichment experiments in the tropics to obtain a wide and

predictable understanding of tropical forest response to climate change.

The Neotropics and Global warming

Global warming is an atmospheric greenhouse gas enrichment

consequence. Main greenhouse gases are: Carbon dioxide (CO 2), Methane (CH4),

Nitrogen oxide (NOx), the so called halocarbon compounds (Cfs), water vapor and

other biogenic trace gases known as Volatile Organic Components (VOC). Those

atmospheric components are infrared absorbing gases and since the Industrial

Revolution have increasingly been warming up the Earth surface (up to 0.6 oC).

Greenhouse gases radiative force on climate depends on atmospheric gas

enhancement (source variable) and its atmospheric residence time (atmosphere

molecule stability). According to IPCC (Interguvernamental Panel of Climate

Change), atmospheric life time is defined as: "the burden" (Tg = 1 012 g.) divided by

the mean global sink (Tg/yr) for a gas in steady state (i.e. with unchanging burden)

which denote, if atmospheric burden of gas X is 100 Tg, and the mean global sink

is currently 10 Tg/yr, the life time equals 10 years. However, atmosphe ric lifetime is

difficult to define because source variability depends on reservoir exchange with

reservoirs having a wide range of turnover rates. For example, CH 4 is removed

from the atmosphere by a single process, oxidation by the hydroxyl radical (OH).

This is the reason why it is relatively easy to define it’s atmospheric residence time.

However, atmospheric CH4 concentration increase is to reduce OH concentration,

which in turn, reduces destruction of the additional CH4, lengthening its

atmospheric lifetime. Moreover, industrial atmospheric CO2-enrichment causes

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biogenic CH4 production through its fertilizer effect on photosynthesis due to

carbon belowground distribution induction (i.e. root exudates enhancement), which

could result in CO2 and CH4 emissions enhancement depending on soil anoxia

conditions. Such mechanisms influences atmospheric greenhouse residence time

and have been disregarded to explain radiative force of atmospheric greenhouse

gases. Nevertheless, there are opposite feedback mechanisms to what has been

explained. For example, an increase of N2O can induce chemical reactions leading

to an increase in ultraviolet radiation available to photolyze the N2O, thereby

shortening its atmospheric lifetime. Otherwise, neglected emissions of volatile

organic compounds (VOCs) from vegetation and soil can lead to substantial

underestimate of carbon release from tropical rain forests to the atmosphere. VOC

fluxes can be important for the global carbon cycle and for the troposphere

chemistry due to its importance in regulating air quality and to explain atmospheric

greenhouse gases lifetimes. Neotropical rainforest influence on lifetime of

atmospheric greenhouse gases has been disregard as a climate change motor,

and this process may explain a major atmosphere-biosphere link. Current carbon

cycle models do not include exchange fluxes of different anthropogenic release

and past records of carbon-isotope ratios (13C) show strong shifts of carbon

reservoirs associated with climate warming. Any study of feedback mechanisms

between global warming and carbon release has to account for biogenic source of

atmospheric enhanced greenhouse gases to reach a predictive understanding of

tropical forests role in climate change models.

Terrestrial ecosystem productivity integrates living organisms and

atmospheric processes. Climate and land use feedbacks on climate mainly through

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CO2 and H2O cycles and Neotropical forest biodiversity reservoir has been

disregarded on global climate change framework. Particularly, when plant

community theory often invokes competition to explain why species diversity

declines as productivity increases. For example plant-plant and plant-animal

interactions under rising atmospheric temperature has not been seriously explored.

Rising atmospheric temperature due to greenhouse gases enhancement has a

tremendous potential to affect biosphere physiology, because temperature greatly

affects carbon exchange rates and plant growth. Temperature decreases

photosynthesis when leaf temperature rises. Rubisco activase inhibition due to

temperature enhancement strongly interacts with light and water availability due to

changes in CO2 compensation point. The steady state at which net assimilation

rate equals zero (CO2 compensation point) remarkably depends on temperature

and O2 atmospheric concentration. Under enhanced atmospheric CO 2

concentration, water availability changes, with an increasing global temperature

may influence plant respiration (physiologica process associate with plant

maintenance). Atmospheric CO2-enrichment effect on plant respiration and its

temperature interaction have not been yet clarified. The only satisfactory way to

determine plant respiration and atmospheric CO2 interaction is by experiment. This

neglected topic has not been systematically investigated and its role to explain

atmospheric changes in CO2 concentration with its consequent effect on

atmospheric chemistry has also been neglected. For example, elevated CO 2 may

enhance growth, but higher temperatures may diminish it through increased

respiration. How does this tradeoff affect Neotropical forest productivity when we

account for plant diversity productivity links? A lack of information on such

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temperature per atmospheric CO2-enrichment interaction needs to be filled to

construct a predictive understanding on tropical plant responses under climate

change.

Fertilization effect on photosynthesis by atmospheric elevated CO 2 concentration

Productivity is the rate of resource conversion to biomass per unit area per

unit time and species richness is affected by productivity. Worldwide, Neotropical

forest has no biodiversity parallel and Global climate change due to enhanced

atmospheric CO2 concentration may exert biodiversity effects due to species-

specific differential growth responses which may engage in unpredictable

consequences on global carbon cycle and consequently on climate change. Global

carbon balance responses to global change are highly dependent on factors

limiting primary productivity and net primary production (NPP) linearly increases as

temperature increase, but declines with high precipitation in tropical forest. At an

intermediate spatial scale (within biomes but across communities) the productivity -

diversity relationship is often uni-modal, with diversity peaking at intermediate

productivity. Natural mechanisms research, linking primary production with

biodiversity is a most predictable understanding value to the climate change

problem. For example, factors that change ecosystem composition, such as novel

organism invasion, nitrogen deposition, disturbance frequency, forest

fragmentation, predator decimation, alternative management practices, and global

change itself, are likely to strongly affect ecosystem processes, which represent

changes in biodiversity reservoir.

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Plants through their photosynthesis depending on CO2, have been growing

during at least 420,000 years under atmospheric CO2 concentrations below 280

ppm. Therefore, atmospheric CO2 anthropogenic enrichment which has been

occurring during 256 years, has been much faster than evolutionary processes in

higher plants, slow growing species in particular. A higher atmospheric CO 2

concentration stimulates plant growth with distinct species growth response due to

inherent lifespan cycles. Within any habitat, an individual plant is more likely to

interact with neighboring plants than within more distant ones. A fast increment on

atmospheric CO2 concentration could be a major determinant of enhanced forest

dynamics due to differential plant growth response within co-occurring competitors.

The existing relationship between primary productivity and species richness

(one aspect of biodiversity concept) can be expressed as follows: productivity

influences species richness. A general agreement of most authors in the literature

express that productivity affects biodiversity. However, no general mechanism has

been described to clarify productivity-biodiversity relationship. Despite that

knowledge gap some major mechanisms, which are susceptible to be by climate

change, has been described. I address three of these mechanisms, pointing out

their capability to affect tropical forest carbon storage capacity due to their

sensitivity to climate change: (1) Neotropical forest vegetation types; (2) Pristine

forest natural regeneration (gap dynamics) and (3) Secondary forest regeneration.

Neotropical forest vegetation types.

Vegetation type is mainly determined by water availability, edaphic

conditions (topography and soil fertility) and altitude. A feature of rainwater

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availability on Neotropical forest vegetation composition-structure depends on

precipitation in the Yucatan Peninsula. From North to South a precipitation

gradient, annually and geographically ranging from 400 to 3000 mm determines

structural and compositional differences among vegetation types. Recently White

and Hoods (2004 Journal of vegetation science 15:151 – 160) Yucatan Peninsula

study demonstrated a floristic variation consequent to an overall West to East

environmental rainfall gradient similar to what the great Mexican Botanist Miranda

previously described in 1959.

Diverse authors have classified Neotropical forest depending on rainfall

precipitation, temperature, and altitude, which have resulted in eight vegetation

type groups, structurally, floristically and physiognomically different at large.

Arboreal species, which are main forest carbon reservoirs, widely varies in floristic

composition within vegetation types. A wide spectrum of forest responses to

climate change can be expected under such arboreal species variety. Far as I

know, no work had taken such differences in account, and it is a critical aspect to

construct a predictive understanding of climate change and Neotropical forest

growth response. I propose to the scientific community, the construction o f an

understanding framework in which precipitation gradients interacting with soil

fertility factorizes CO2-enrichment arboreal species growth responses. This,

because vegetation types productivity are mainly on those growth factors

dependent.

Climate change in the Pleistocene strongly affected pluvial precipitation

patterns; similar to what has been presently occurring on climatic periodicity

(substantial effects on precipitation patterns) due to the Industrial Revolution

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influence on atmospheric greenhouse gases concentration. Neotropical forest flora

as we presently know was during Miocene, some 23 to 6 million years ago, more

variegate than contemporary forests. Additionally, the time period during which

Pleistocene climate change occurred was on the order of thousands of years and

generated drastic changes on temperature and precipitation patterns producing a

reduction in tropical forest Earth surface cover. Present Industrial Revolution

greenhouse concentration enhancement was happening during the past 256 years,

and Earth surface temperature warm-up, which has unpredictable consequences

on dynamic, composition and structure of tropical plant communities, is a CO 2

fertilization - greenhouse effect conundrum on Neotropical forest. In figure 3, I

present a hierarchical structure of factors involved in the carbon pool of tropical

forest dynamics. Vegetation type differences and human land use are at most,

determinant of final vegetation recover. A predictable understanding of past,

present and future atmospheric CO2 concentration has to account for such

hierarchical interactions due to atmospheric CO2-enrichment influence on forest

dynamics (i.e. Granados 2002 dissertation Basel University).

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Tropical Forest (1) Geographical situation

(2) Community vegetation (composition and structure)

Tropical rain forest Tropical wet forest Savanna

(3) Human disruption (a) Natural disruption (Gap dynamics) (b)

Gap

Atmospheric CO2 concentration (planetary scale)

Physical disruption

- Deforestation explains ~ 45 % of increased atmospheric CO2 concentration since 1850 (Malhi et al. 2002)

Ecophysiological disruption

- Elevated CO2 enhances liana growth and competition

- Carbon emission from fossil fuels has surpassed those from defor-estation.

- Forest dynamics enhanced which decreases the probability of carbon sequestration

Figure 3. A hierarchical structure of factors involved in the carbon pool of tropical forest dynamics. (1) Planetary scale; (2) Landscape scale; (3) Regional scale: (a) Human induced disruption, (b) Natural processes. From Granados (2002) Ph.D. Dissertation Basel University

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Natural regeneration in pristine forest (gap dynamics)

Competition itself induces a set of non-equilibrium species oscillations that

allow coexistence of more species than would normally be expected on the basis

of the number of limiting resources. Whether or not this natural mechanism driv es

Neotropical biodiversity, actual industrial atmospheric CO2 concentration

enrichment powers species bias in a high biodiverse forest Neotropical like. Which

species will profit more than others from the stimulant CO2-fertilizer effect, and

when, is not clear. Yoda’s Self-thinning rule establishes that as the number of plant

competitors decreases, the biomass of competing plants increases. Therefore,

under photosynthetic fertilization effects due to atmospheric CO 2-enrichment, what

kind of competitive mechanisms are able to be change and under what kind of

vegetal communities processes? A wide spectrum of plant-plant and plant-animal

interaction can be listed. I will focus on the following: Natural regeneration in

pristine forest and Secondary forest regeneration.

In 1994, Phillips and Gentry (Science 263:954-958) suggested that

according to large-scale surveys, tropical forest tree turnover seems to have

increased worldwide. All experts do not share this view. However, it was not

disputed that there is a possibility that accelerated tree turnover could be mediated

by increased vigor of climbing plants or other fast growing arboreal species, which

in itself could result from atmospheric CO2-enrichment. The juvenile growth stage

of trees is a critical step in forest regeneration and species-specific interactions can

strongly influence recruitment success.

Tree turnover is determined by a series of factors from which stem size,

plant age, tree size, mechanical and physical damage stand density and light

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availability are the most important. The forest understory is a very competitive

environment in which light availability varies widely in time and intensity. It is well

known that in general, elevated CO2 improves growth resource use efficiency and

thus may increase plant competitive ability. For example, elevated CO2 reduces

light compensation point of photosynthesis, having a multiplicative effect on plant

biomass production, at severe light limitation. Additionally, the stimulative effect of

CO2 declines with time and thus plant will also decline its CO2 growth response

with age. Consequently, natural forest regeneration at initial conditions due to gap

dynamics are susceptible to bias under atmospheric elevated CO 2 in favor of

plants with a high CO2-growth responsiveness which not necessarily will be fast

grow species because differences in light and CO2 compensation point within plant

life forms will determine differences on CO2-growth responses and

competitiveness.

Because of their dependence on physical support, lianas have the capacity

to influence natural tropical forest regeneration and they represent an important

selective force to tree survival and thus diversity. Initial conditions can be

influenced by liana presence, slowing forest regeneration in an early

developmental stage. At a later stage lianas can enhance tree mortality thus,

having a significant effect on the carbon forest storage capacity. Recent forest

inventories in the Neotropics show that liana dominance has indeed increased over

the last 30 years, which supports Phillips and Gentry original hypothesis and that

has been reinforced by Granados and Körner (2002 Global Change Biology

8:1109-1117) and Granados (2002 dissertation Basel University) data.

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Intact pristine forest stores very high amounts of carbon, accumulated in

wood for more than a century. However, pristine forests are not common anymore,

and since the Industrial Revolution ca. 20 - 30 % of the original forest area on

Earth has been lost. Additionally, carbon residence time in the forest depends on

tree life span and this depends on arboreal species and forest type. Few studies

consider tree longevity as a main focus. Studies show an age range between 8 to

1400 years, almost depending on species life strategy (pioneer and non-pioneer

arboreal species). Estimates of tree life span for pioneer species are 8 - 40 years

and for non-pioneer tree species between 200 - 1400 years. Remarkably,

atmospheric CO2-enrichment has been happening almost during 255 years and

worldwide forestation criteria have not addressed this aspect of climate change.

Tree life span determines carbon residence time, which has major implications on

forest carbon pool size, because about 50% of biomass carbon is bound in canopy

trees that normally represent a minor proportion of the total number of tropical

trees. As has been demonstrated by Granados and Körner (2002 Global Change

Biology 8:1109-1117) and Granados (2002 dissertation Basel University), under

gap regeneration conditions CO2-enhanced liana growth and competitiveness have

a negative influence on tree growth and CO2-responsivenness, which over

generations could result in reduced tree life span, which in turn will produce a

decrement on carbon forest storage capacity. Figure 4, synthesizes a general

mechanism for CO2 effects on gap dynamics and carbon forest storage capacity. In

a fully structured forest

Figure 4. A general mechanism to explain a probable impact of CO2-enhanced liana growth on tropical forest dynamics. Part (a) modified from Schnitzer et al. (2000) . From Granados 2002. Ph.D. Dissertation Basel University

CO2 effect C

ano

py

U

nder

stor

y Gap dynamics Liana impact on

forest dynamics

- Natural history of the place where gap formed

- Sapling species

natural abundance

- Density-dependence of saplings survivorship

Liana density (per 20 m2)

Tree

den

sity

(per

20

m2 )

Forest structure under elevated CO2

Pioneer tree dominance

Carbon Tropical forest

storage capacity

Higher

Lower

Lower

Higher

Saplings

Trees

Pio

nee

r N

on

-pio

nee

r

Pioneer tree

Gap

-Long life-span -Slow growing -Not abundant

-Short life-span -Fast growing -Abundant

Non-pioneer trees

Pioneer trees

Survivorship factors - Seed reserves - Physical damage - Herb ivory - Pests - Drought - Shading - Competition - Species growth rate

Non-pioneer tree dominance

(a) Non-pioneer tree

when a tree falls, a canopy gap opens and a successional event starts being

a major determinant of the new canopy tree species composition. Liana presence

in pristine forest can stall forest succession in early successional stages, favoring

pioneer arboreal species during forest structure build-up, while changes in forest

structure toward shorter tree life span could lead to a reduction of carbon residence

time in forest.

Secondary forest regeneration

Compared with pristine forest, ecology of secondary vegetation is very

different, and this kind of vegetation community is a most common form of the

actual tropical vegetation stage. In general, secondary vegetation forest ecology

differs from pristine forest ecology mainly because interacting species are much

more diverse. In a world with elevated CO2 concentration the fertilization effect on

photosynthesis has the potential to enhance competition among species. A wide

variety of mechanisms can lead to diverse floristical successional processes and

rates of forest recovery. Regarding liana effect, density and biodiversity of lianas

decrease as forest stand ages increase and this seems to be related with light

availability. Because species widely differ in light requirements and light gradients

explain successional dynamics, tropical tree species coexistence will be more

difficult to predict under secondary forest stages. Moreover, despite the fact that

liana abundance is highest in young stands, no liana species are restricted to

younger forest. Thus, liana species presence is not restricting ageing of stands and

slow growing tree species are normally excluded from early successional stages.

Furthermore, pristine forest conversion to secondary vegetation has been shown to

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result in losses of soil organic matter. Soil organic matter is a main determinant of

soil fertility and a key uncertainty extends to whether the fe rtilization effect is limited

by the availability of other nutrients such as nitrogen and phosphorus. It has been

shown that forest ecosystem responses to CO2-enriched atmosphere are highly

dependent on soil fertility. Depletions on soil organic matter wil l deplete soil fertility

and lianas are equally represented in tropical forest with poor and fertile soils, and

the biggest trees are usually found in tropical forest with fertile soils. Liana impacts

on secondary vegetation are very strong and it has been suggested that by the

protection of such disturbed areas, tropical forest regrowth could be used as a

carbon forest sink. I propose a reassessment of the usefulness of such Kyoto

protocol target.

Forest regeneration confronts plant-plant interactions (in both gap dynamics

and secondary successional processes) that affect the establishment and juvenile

life phase of trees in different forms. Arboreal species juvenile life phase at most

concern tree turnover rates, which took place in the forest understory a nd depend

on local conditions and vegetation type. However, other aspects like latitude,

seasonality, topography heterogeneity and dry periods are also important, but

planetary-scaled processes determine them. For example, long turnover rates are

a function of primary productivity and primary productivity is also a function of

geographical situation due mainly to rainfall variation. Thus, forests that are

intrinsically of low turnover rates are also different in composition and structure and

forest structure-composition is related to biomass fixed and carbon storage

capacity (Figure 3, 4).

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Human civilization confronts major consequences of its development. Fast

atmospheric CO2-enrichment represents a change in the availability of a growth

factor, which can destabilize associations of plant communities due to its abrupt

character. Current atmospheric CO2-enrichment has been occurring in ecological

time scales, and during past epochs atmospheric CO2 concentration varied

between 200 to 400 ppm, and those changes did not appear to be associated with

collapses in plant communities or species extinction events. On the contrary, those

changes were associated with plant biodiversity increments. However, those CO 2

concentration changes took place over millions of years, which represent

evolutionary time scales, a situation quite different from the current one where CO 2

concentrations change far more rapidly than evolutionary processes in most

species. The link between forest regeneration mechanisms and biodiversity is t he

key process to understand the effect of CO2-enhanced liana growth and

competition on tropical forest dynamics and thus in the global carbon cycle.

Species turnover rates appear to be related with habitat variability, and low

turnover rates of species over great distances in the tropical regions suggest that

population densities of some species are limited by unidentified process that may

be associated with dispersal process. If biodiversity is maintained by a tradeoff

between recruitment ability and competitive ability, a CO2-enhanced arboreal fast

growing species and lianas will have negative effects on tree biodiversity and may

have a major impact on tropical forest composition. Predictions of tropical carbon

fluxes will need to account for the changing composition and dynamics of

supposedly pristine forest. A recently report for undisturbed Amazonian forest

found a non-random change in floristic composition in which many faster-growing

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genera are increasing in the basal area (i.e. Laurance et al. 2004 N ature 428:171-

175).

The temporal characteristics of atmospheric elevated CO2-fertilization effect

It is completely unknown if and how elevated CO2 will influence the growth

and development of adult tropical trees. Results of the limited number of

experimental studies outside the tropics suggest a reduced sensitivity, as trees get

older. The only test with tall tropical trees was a leaf CO2-enrichment experiment

(Würth et al. 1998 Functional Ecology 12:886-895), which revealed an immediate

accumulation of non-structural carbohydrates (NSC), despite the fact that leaves

where attached to large trees, which could be considered representing a nearly

infinite carbon sink for such leaves. Another aspect to be considered in the adult

trees as a carbon sink is the capacity that each living organism has to change in

size. The growth curve of any living organism presents an inflexion point on which

its growth capacity depletes. Thus, there is a spatial -temporal limit for the trees to

accumulate carbon in its biomass, and for the vegetation community as well due to

density dependence of plant size. Otherwise, the first evidence that elevated CO 2

induces a response which declines with time (where) were presented by Bazzaz et

al. (1995 Proceedings of the nacional academy of sciences of the U.S.A.

98(18):8161-8165). Nevertheless, that work used seedlings; the results obtained in

those experiments are in line with Hattenschwiller et al. (1997 Global Change

Biology 3:436-471) findings, which describe that elevated CO2 effect declines with

tree age. In which period of plant life-span process atmospheric CO2-enrichment

will be critical for final forest regeneration? And how does that critical influence

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determine plant life form competitiveness? This is a major Science unknown, and a

high research priority due to the worldwide environmental crisis.

Forest growth responses to elevated CO2 are non-linear with the degree of

non-linearity depending on species and light availability. Strongest CO2 effects

occur in the CO2 concentration ranges between pre-industrial CO2 concentration

and 420-ppm CO2 concentration, which is only 60 ppm less than current CO2

concentration. Beyond 420 ppm the growth response to CO 2 declines until 560

ppm CO2 concentration. Therefore, CO2 effects are likely to be underway right

now. A predictive understanding of Neotropical forest responses to climate change

has to account for arboreal species, linear and non-linear growth responses to

atmospheric CO2 enrichment.

The exponential increase of Industrial Revolution atmospheric CO2

concentration has no parallel in the planet’s history, and its effects have not been

clarified because existing work contains few considerations of Biosphere

compensatory features. The experimental approach proposed by Granados and

Körner (2002 Global Change Biology 8:) allows the construction of a global change

framework with high predictive value. For example, Figure 5 (a) shows the

Granados and Körner (2002) experiment. The two of three species mixtures show

a maximum response of plant biomass gain at 560 ppm CO2 concentration, but

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H L

L L

b

Gonolobus + ThinouiaGonolobus + CeratophytumCeratophytum + Thinouia

cPlan

t bi

omas

s pe

r co

ntai

ner

(g)

280 0

10

5

30

20

10

0

420 560 700

30

20

10

0

CO2 concentration (ppm)

Figure 5. Non-linear plant biomass responses to a linear atmospheric CO2-enrichment. (a) Plant biomass per container across all species combinations and light regimens; (b) across light regimes but separated by species mix. From Granados and Körner (2002) Global Change Biology 8:1109-1117

a

+ 48 %+ 23 % - 3 %

25

then leveled-off (Figure 5 c). However, graphical linear apperception of third plant

growth mixture (Ceratophytum and Thinouia, the middle and small sized grown

species) is also non-linearly responding, because at 700 ppm CO2 concentration,

no significant differences were found within species mixtures. At the same time this

graphical and statistical particularity shows that although species mixtures fill -up on

growth containers no "pot-bounding" effects were present. I consider that the use

of the Granados and Körner (2002) experimental linearity test can be used in

Neotropical forest response research, due to its capacity to link plant CO 2-growth

responses to atmospheric exponential CO2 enhancement. A predictable

understanding of biosphere climate change effect can be reached using that

experimental-statistical method.