Principles of terrestrial ecosystem ecology.pdf

190 8. Terrestrial Plant Nutrient Use
less increase in tissue nutrient concentration.
Tissue nutrient concentrations increase substantially
only when other factors begin to limit
plant growth. The sorting of species by habitat
also contributes to the responsiveness of nutrient
uptake and NPP to variations in nutrient
supply observed across habitats. Species such
as trees that use large quantities of nutrients
dominate sites with high nutrient supply
rates, whereas infertile habitats are dominated
by species with lower capacities for nutrient
absorption and growth. Despite these physiological
and species adjustments, tissue nutrient
concentrations in the field generally increase
with an increase in nutrient supply.
Nutrient use efficiency is greatest where production
is nutrient limited. Differences among
plants in tissue nutrient concentration provide
insight into the quantity of biomass that an
ecosystem can produce per unit of nutrient.
Nutrient use efficiency (NUE) is the ratio of
nutrients to biomass lost in litterfall (i.e., the
inverse of nutrient concentration in plant litter)
(Vitousek 1982). This ratio is highest in unproductive
sites (Fig. 8.8), suggesting that plants
are more efficient in producing biomass per
unit of nutrient acquired and lost when nutrients
are in short supply. Several factors
contribute to this pattern (Chapin 1980). First,
tissue nutrient concentration tends to decline
as soil fertility declines, as described earlier.
Biomass:N ratio of litterfall
280
T
240 C
C
C
C
200 C
C
160
C
120
80
40
C C
C
C
T
C
C C
C
C T
C
C T T T
T T T
T
T T
C
CC
T
C
T
T T
T
T
T T
M C
D T
C
D
C
C D
C
M D
D C
D D
D DT
D
D D D M
M D D
M M
D DD
D
T
D
D
D T
D
N
N
N
T
N
N
N N N N
0
0
2 4 6 8 10 12 14 16 18 20
N in litterfall (g m -2 yr -1 )
Individual plants that are nutrient limited also
produce tissues more slowly and retain
these tissues for a longer period of time, resulting
in an increase in average tissue age.
Older tissues have low nutrient concentrations,
causing a further decline in concentration (i.e.,
increased NUE). Finally, the dominance of
infertile soils by species with long-lived leaves
that have low nutrient concentrations further
contributes to the high NUE of ecosystems on
infertile soils.
Plants maximize NUE in infertile soils by
reducing nutrient loss more than by increasing
nutrient productivity. There are at least two
ways in which a plant might maximize biomass
gained per unit of nutrient (Berendse and
Aerts 1987): through a high nutrient productivity
(an)—that is, a high instantaneous rate
of carbon uptake per unit nutrient, and
through a long residence time (tr)—that is, the
average time that the nutrient remains in the
plant.
NUE = an ¥ tr
g biomass ( gN)
-1 -1
= g biomass ( gN) yr * yr]
(8.1)
Species characteristic of infertile soils have
a long residence time of nutrients but a low
nutrient productivity (Chapin 1980, Lambers
and Poorter 1992), suggesting that the high
NUE in unproductive sites results primarily
T T
T
T
[
-1
Figure 8.8. Relationship between
the amount of nitrogen in litterfall
and nitrogen use efficiency (ratio
of dry mass to nitrogen in that
litterfall). C, conifer forests; D,
temperate deciduous forests; M,
mediterranean-type ecosystems; N,
temperate sites dominated by symbiotic,
nitrogen fixers; T, evergreen
tropical forests. (Redrawn with permission
from American Naturalist;
Vol. 119 © 1982 University of
Chicago Press; Vitousek 1982.)