Tropical Forest Community Ecology

128 Egbert G. Leigh, Jr

DPdevoted to making leaves and fine roots, and
mfandmrtheir loss rates. Let foliage dry massF
and root dry massR, in tons ha−^1 , b ein st eady
state, sopfDP=mfFandprDP=mrR. Finally,
let wood production dw/dt(including branches,
bark, and coarse roots) use the resources left over
from making leaves and fine roots, so dw/dt=
( 1 −pf−pr)DPtons ha−^1 year−^1.
Th erat eUof th efor est’s N uptak eis th esoil N
used in making new leaves, fine roots, and wood.
Let the N concentration bexin leaves,arxin roots,
andawxin new wood; let trees reabsorb fractions
zfandzrof the N in dying leaves and roots, so that
fractions 1−zfand 1−zrof the N in new leaves
and roots must b edrawn from th esoil, and l et
plants translocat ea proportionzwof th enitrog en
in new wood from older wood. Then

U=xpf( 1 −zf)DP+arxpr( 1 −zr)DP

+awx( 1 −zw)dw/dt

SincepfDP=mfF+prDP=mrR,

U=x( 1 −zf)mfF+arx( 1 −zr)mrR

+awx( 1 −zw)dw/dt

AsDPis a function ofU 0 , th eav erag erat eof sup-
ply of nutrients from litterfall, atmosphere, and
bedrock, we can calculateDPfromU 0 and th efor-
est’s allocation to wood, leaves, and roots. King
(1993) assumed that root biomass was in a com-
petitive equilibrium where no tree could benefit
by increasing its root biomass, and he used these
equations, appropriately parametrized, to calcu-
late numerically what allocation gives highestDP
for a givenU 0. H efound that foliag eallocation
stayed constant, but that asU 0 declined, the pro-
portion ofDPdevoted to fine roots increased at the
expense of wood production.
As predicted, a higher proportion of forest pro-
duction and biomass is below ground on poorer
soil. In Venezuelan Amazonia, the ratio of above-
to below-ground biomass is lower on poorer soil
(Medina and Cuevas 1989). Despite its better
soil, the rainforest at La Selva, Costa Rica, has
slightly lower gross production than central Ama-
zonian rainforest at Cuieiras, but the Cuieiras

Table 8.2 Total annual dry matter production

(DPtot), above-ground production (DPabove), and

below-ground production (DPbelow), in metric tons

ha−^1 , in two nearby 40-year-old Douglas fir stands on

soils of very different quality.

Soil DPtot DPabove DPbelow

Goo dsoil 17.8 13.7 4.1

Poor soil 15.4 7.3 8.1

Source: Keyes an dGrier (1981).

forest devotes a proportion of its gross produc-

tion to below-ground activities 7/4 times that

at La Selva (Table 8.1). In two nearby 40-year-

old stands of Douglas fir on soils of contrasting

quality in western Washington, total dry mat-

ter production was 16% higher on good soil, but

above-ground production was nearly twice as high

on good soil, whil eth er ev ers ewas tru efor b elow-

ground production (Tabl e8.2, from K ey es and

Grier 1981).

Nutrient conservation

What factors influ enc eth eavailability of nutri-

ents in leaf and soil? How can trees affect this

availability? The preceding section’s last equation

can be solved for leaf nitrogen concentrationx:

x=U/{[( 1 −zf)mfF+ar( 1 −zr)mrR

+aw( 1 −zw)]dw/dt}

For fixedU,xcan be increased by decreasing

the turnover ratesmfof leaves andmrof roots,

increasing the proportions of nutrientszfandzr

recovered from dying leaves and roots, increas-

ing th etranslocationzwfrom old to new wood,

or decreasing wood production dw/dt.

To learn what factors influence the rateU 0

of supply of nitrogen per unit area of soil, King

(1993) assumes that a fractionkof th eunus ed

supplyU 0 −Uleachesaway. When a tree falls, its

roots die, and its root biomass takes a few years

to recover: King (1993) assumes that, each year,

th ewhol eofU 0 is lost in a fraction of th efor est’s

area equal to twice the proportionmof trees dying