Tropical Forest Community Ecology

Tropical Forest Ecology: Sterile or Virgin for Theoreticians? 125

Table 8.1 Gross production, soil respiration, litterfall, and below-ground respiration, in tons C ha−^1 year−^1 ,intwo

lowland tropical rainforests.

Site Gross

production

Soil

respiration

Litterfall Below-ground

respiration

La Selva, Costa Rica 29.5 12.5 4.4 8.1

Cuieiras, central Amazonia, Brazil 30.4 No data No data 13.7

Sources: La Selva: Gross production from table 6 of Loescheretal. (2003); other data are averages of residual and old alluvium

from figure 6.1 of Schwendenmann (2002). Cuieiras: All data from table 6 of Malhietal. (1999).

Tree height, tree shape, and forest

structure

Tree height

To understand forest structure, we must first
learn what limits forest height. Early models
assumed that forests grew until the costs of
maintaining unproductiv ewoody biomass l eft no
resources for further growth (Bossel and Krieger
1991). This view is no longer credible. In the
rainforest at La Selva, Costa Rica, above-ground
tree-trunk respiration is only 7–12% of gross pro-
duction (Ryanet al. 1994). This proportion does
not increase quickly enough with tree height to
limit th eh eight of lodg epol epin efor est (Ryan and
Waring 1992).
The height of coast redwoods appears to be
limited by the difficulty of lifting water to their
crowns (Kochet al. 2004). Why are most trees
shorter than redwoods? Since diameters of most
canopy trees keep growing long after their height
growth stops (King 1990, Ryan and Yoder 1997),
resources are not limiting. King (1990, p. 809)
therefore concluded that “adult tree height reflects
an evolutionary balance between the costs and
benefits of stature.” To learn what limits forest
height, King (1990) considered a tree’s height as
its strategy in a game played against its neighbors.
What is the appropriate growth strategy of a tree
in a forest of identical neighbors, each with height
H 0 and leaf areaLA? Set the tree’s stemwood pro-
duction dw/dt=cLA( 1 −H 0 /A), wherecLAis
th ewood production th ecrown would support if
height imposed no extra costs, andH 0 /Adenotes
the proportion by which height-associated costs
r educ ewood production. Now suppos ethat on e

tr e ehas th esam el eaf ar eaLAand crown width

w 0 as each of its neighbors, but a heightH

=H 0 ;

let the light it receives, and its wood production,

be[ 1 +(H−H 0 )z/w 0 ]times that of each neigh-

bor, when each neighbor has heightH 0. For trees

at 45◦N with conical crowns twic eas tall as th ey

ar ewid e,z=0.76 (King 1990). If crown width

wis proportional to tree heightH,thatistosay,

ifw=bH, then the heightHcthat maximizes

each tree’s wood production when they all have

th esam eh eight is giv en by∂(dw/dt)/∂H=0.

This optimum height isHc=A/( 1 +zb): a tree’s

wood production declines if it grows beyond this

height. Using stand tables to estimate the stand

heightAat which wood production of surviving

trees declines to zero, this prediction approximates

observation for many, but not all, even-aged stands

(King 1990).

King recognized that a tree’s height growth

depends on its neighbors’, and assumes that in

a mature forest, all canopy trees are equally tall.

Trees of the genusTachigali(Leguminosae) are

monocarpic (flowering and fruiting only once

before dying), so they grow much faster than

their iterocarpic neighbors without sacrificing

strength (as measured by wood density), but

they grow no taller than their canopy neigh-

bors (Poorteret al. 2005). On th eoth er hand,

King assumed that height-associated costs reduce

wood production in linear proportion to tree

height, and did not predict the constant of pro-

portionality, that is, the height where canopy

trees’ wood production stops. These costs, how-

ever, may increase non-linearly with tree height.

For example, the probability that an Amazonian

canopy tree carries large lianas, which slow its

growth and tripl eits annual probability of dying,