Principles of terrestrial ecosystem ecology.pdf

nitrate. Nitrate absorption is often important,
however, because of its high mobility.
There is an inevitable trade-off between the
maximum rate of nutrient investment in new
growth and the efficiency with which nutrients
are used to produce biomass. Plants produce
biomass most efficiently per unit of nutrient
under nutrient-limiting conditions. Nutrient use
efficiency is maximized by prolonging tissue
longevity—that is, by reducing the rate at which
nutrients are lost. Senescence is the major
avenue by which nutrients are lost from plants.
Plants minimize loss of growth-limiting nutrients
by resorbing about half of the nitrogen,
phosphorus, and potassium from a leaf before it
is shed. About 15% of the annual nutrient
return from aboveground plant parts to the soil
comes as leachates, primarily as through-fall
that drips from the canopy. Herbivores can also
be important avenues of nutrient loss because
they preferentially feed on nutrient-rich tissues
and consume these tissues before resorption
can occur. For these reasons plants lose more
than twice as much nutrients per unit of biomass
to herbivores than through senescence. Other
factors that cause occasional large nutrient
losses from vegetation include disturbances
(e.g., fire and wind) and diseases that kill plants.
Review Questions
1. Mass flow, diffusion, and root interception
are three processes that deliver nutrients to
the root surface. Describe the mechanism
by which each process works. What is the
relative importance of these three
processes in providing nutrients to plants?
How does soil fertility influence the relative
importance of these processes?
2. How do soil and plant properties influence
rates of diffusion and mass flow? How
can the plant maximize these transport
processes?
3. What is the major mechanism by which
plants get nutrients into the plant, once
they have arrived at the root surface?
4. How do plants compensate for (a) low
availability of all nutrients, (b) spatial variability
in nutrients within the soil (localized
Additional Reading 195
hot spots), and (c) imbalance between
nutrients required by plants (e.g., nitrogen
vs. phosphorus availability)?
5. What is the rhizosphere? How do plants
influence the rhizosphere?
6. How does plant growth rate affect nutrient
uptake?
7. What are the major mechanisms by which
mycorrhizae increase nutrient uptake by
plants? Under what circumstances are mycorrhizae
most strongly developed?
8. What are the major processes involved in
converting nitrogen from nitrate to a form
that is biochemically useful to the plant?
Why is nitrogen acquisition by plants so
energetically expensive?
9. Why are nitrogen and carbon flows in
plants so tightly linked? What happens to
nutrient uptake when carbon gain is
restricted? What happens to carbon gain
when nutrient uptake is restricted? What
are the mechanisms by which these adjustments
occur?
10. What is nitrogen use efficiency? What
are the physiological causes of differences
in NUE, and what are the ecosystem
consequences?
11. Why do all aquatic and terrestrial plants
tend to show a similar balance of nutrients
(the Redfield ratio)? How can you use this
information to estimate which nutrient is
most strongly limiting to plants in a particular
site?
12. What are the major differences in types of
species that occur in fertile vs. infertile
soils? What are the advantages and disadvantages
of each plant strategy in each soil
type?
13. What are the major avenues of nutrient loss
from plants? How do all plants minimize
this nutrient loss? What are the major adaptations
that minimize nutrient loss from
plants that are adapted to infertile soils?
Additional Reading
Aerts, R. 1995. Nutrient resorption from senescing
leaves of perennials: Are there general patterns?
Journal of Ecology 84:597–608.