The Earth system and the cycling of carbon and nutrients - Fundamentals of the Physical Environment - page 515

C-2 3

C-1 350,000

H 700,000

P 6 million

D *UDVVILHOG²S\UDPLGRIQXPEHUV

C-2 21

C-1 383

C 11

H 132 H 3368

g m -2 k cal m -2 yr -1

P 20,800

P 703

E &RUDOUHHI²S\UDPLGRIELRPDVV F 6SULQJ²S\UDPLGRIHQHUJ\

Figure 21.2 Eltonian pyramids of (a) numbers, (b) biomass and (c) energy in different ecosystems. Ccarnivores, Hherbivores,
Pplants.

an ecosystem. Biomass can be measured by harvesting the
above-ground plant parts (i.e. the shoots) in a sampling
plot by clipping at ground level. Large shrubs and trees
are difficult to harvest in this way and usually some
parameters of the trees (e.g. diameter of the trunk at
breast height, DBH) are measured and biomass calculated
using yield tables or appropriate formulae. Underground
biomass (i.e. root biomass) is difficult to measure, but
estimates can be made by washing the plant material from
a volume of soil taken beneath the sampling plot.
Harvested material is dried in the laboratory at 80

Net ecosystem production = B

Net primary productivity

Gross primary Respiration of

= -

production plants

NPP = GPP - Rp

= B + G + D

where G = grazing and D = decomposition. In controlled
experimental conditions, say in a laboratory or green-
house, the flows D and G can usually be made very small
so that:
NPP =

C
until it reaches a constant dry weight, which it usually does
in about twenty-four hours. Dry weight provides the best
estimate of biomass because fresh weight or wet weight
includes the water content, which varies widely among
plant species and even between fresh samples from the
same species.
The energy content of different plant species or plant
parts also varies. Dry weight can be converted to energy
content if the calorific value of the material is known. The
units are calories per gram (cal g -1 ) or joules per gram
(J g -1 ). This value can be determined in the laboratory,
using a calorimeter. The increase in biomass ( B) with
time (

B/ t

GROSS PRIMARY
PRODUCTIVITY
(GPP)

RESPIRATION
LOSSES
(R)

FLOW TO
DETRITUS AND
DECOMPOSERS
(D)

Live
biomass
(B)

t ) is a measure of net ecosystem production
(NEP). In order to use harvest data for calculating net
primary productivity (NPP), the losses of biomass to
herbivores (grazers) and detritus must be taken into
account ( Figure 21.3 ). In equation form:

FLOW TO GRAZERS
(G)

Figure 21.3 Flows and stores needed to calculate net primary
productivity (NPP) and net ecosystem production (NEP).