Chapter 8

Tropical Forest Ecology:

Sterile or Virgin for

Theoreticians?

EgbertG.Leigh,Jr

OVERVIEW

Trees interact primarily with near neighbors, so mathematical theory of forest ecology should be an interactive
dynamics of spatial arrangements of trees of different species. No such dynamics yet exists. Nonetheless, crude theory
of forest structure suggests:
1 Trees’ competition for light causes wildly unequal distribution of light among leaves, greatly reducing forest
productivity.
2 Competition for nutrients favors fine-root investment far beyond that which maximizes forest production.
3 Over a wide range of soil quality, total productivity of lowland tropical forest changes far less than above- versus
below-ground allocation.
4 Trees adapted to poorer soils live longer, have denser wood, and have longer-lived leaves.
5 Long-lasting leaves avoid being eaten by being tough, and avoid drying out by limiting stomatal conductance, thus
reducing photosynthetic capacity.
Testing Hubbell’s neutral theory prediction of how fast initially rare species can spread shows that two tree clades
invading South America 20 million years ago spread non-randomly quickly.They did not replace other clades, implying
that differences between tropical tree species allow them to coexist.
Finally, to understand tropical forest one must consider animals and pathogens. Lotka–Volterra theory predicts:
1 Trees can reduce herbivory by more effective defense (which reduces growth) or by being rare.
2 Many tree species coexist if specialist pests keep each rare enough, as appears true in most tropical forests.
3 In more productive forests, predators limit herbivory.
Therefore, employing animals as pollinators and seed dispersers allowed diverse, productive flowering forest to
replace slower-growing, better-defended gymnosperm forest.

THE QUESTIONS: WHAT MUST WE

UNDERSTAND ABOUT TROPICAL

FORESTS?

Understanding tropical forest ecology requires
answers to several questions.
1 What controls forest productivity?First, why is
th eannual gross production of lowland moist
or wet tropical forests near 3 kg C m−^2 (Leigh

1999, Loescher et al. 2003)? Second, what

governs allocation to above- versus below-ground

activities, such as stem- versus root-making, and

to earlier reproduction versus longer life? Third,

why is average annual mortality among a low-

land tropical forest’s tree’s nearly the same for all

sizes between 10 and 70 cm diameter at breast

height (dbh) (Leigh 1999, p. 122)? Fourth, how

do soil quality and herbivore pressure influence