Principles of terrestrial ecosystem ecology.pdf

194 8. Terrestrial Plant Nutrient Use
impact of herbivory on plants. There has therefore
been strong selection for effective chemical
and morphological defenses that deter
herbivores and pathogens. These defenses are
best developed in tissues that are long lived
and in environments where there is an inadequate
supply of nutrients to readily replace
nutrients lost to herbivores (Coley et al. 1985,
Gulmon and Mooney 1986, Herms and Mattson
1992). Most nutrients transferred from plants to
herbivores are rapidly returned to the soil in
feces and urine, where they quickly become
available to plants. In this way herbivory speeds
up nutrient cycling (see Chapter 11), particularly
in ecosystems that are managed for
grazing. Nutrients are susceptible to loss from
the ecosystem in situations in which overgrazing
reduces plant biomass to the point that
plants cannot absorb the nutrients returned to
the soil by herbivores.
Other Avenues of Nutrient
Loss from Plants
Other avenues of nutrient loss are poorly
known. Although laboratory studies suggest
that root exudates containing amino acids may
be a significant component of the plant carbon
budget (Rovira 1969), the magnitude of nitrogen
loss from plants by this avenue is unknown.
Other avenues of nutrient loss from plants
include plant parasites such as mistletoe and
nutrient transfers by mycorrhizae from one
plant to another.Although these nutrient transfers
may be critical to the nutrient distribution
among species in the community, they do not
greatly alter nutrient retention or loss by vegetation
as a whole.
Disturbances cause occasional large pulses
of nutrient release. Fire, wind, disease epidemics,
and other catastrophic disturbances cause
massive nutrient losses from vegetation when
they occur (see Chapter 13), but these losses
are small (less than 1% of nutrients cycled
through vegetation) when averaged over the
entire disturbance cycle. Even in fire-prone
savannas and grasslands, fires generally burn
during the dry season after senescence has
occurred and burn more litter than live plant
biomass. Most plant nutrients in these ecosys-
tems are stored belowground during times
when fires are likely to occur.
Summary
Nutrient availability is a major constraint on
the productivity of the terrestrial biosphere.
Whereas carbon acquisition by plants is determined
primarily by plant traits (leaf area
and photosynthetic capacity), nutrient uptake is
usually governed more strongly by environment
(the rate of supply by the soil) rather than
by plant traits. In early succession, however,
plant traits can have a significant impact on
nutrient uptake by vegetation at the ecosystem
level. Diffusion is the major process that delivers
nutrients from the bulk soil to the root
surface. Mass flow of nutrients in flowing soil
water augments this nutrient supply for abundant
nutrients or nutrients that are required in
small amounts by plants. Root biomass differs
less among ecosystems than does aboveground
biomass because those ecosystems that are
highly productive and produce a large aboveground
biomass have a relatively low allocation
to roots.
Plants adjust their capacity to acquire nutrients
in several ways. Preferential allocation to
roots under conditions of nutrient limitation
maximizes the root length available to absorb
nutrients. Root growth is concentrated in hot
spots of relatively high nutrient availability,
maximizing the nutrient return for roots that
are produced. Plants further increase their
capacity to acquire nutrients through symbiotic
associations with mycorrhizal fungi. Plants alter
the kinetics of nutrient uptake in response to
their demand for nutrients. Plants that grow
rapidly, due either to a favorable environment
or a high relative growth rate, have a high
capacity to absorb nutrients. Plants adjust the
absorption of specific nutrients by maximizing
the capacity to absorb those elements that most
strongly limit growth. In the case of nitrogen,
which is frequently the most strongly limiting
nutrient, plants typically absorb whatever
forms are available in the soil. When all forms
are equally available, most plants preferentially
absorb ammonium or amino acids rather than