Principles of terrestrial ecosystem ecology.pdf

calcium and iron are immobile in the phloem
so plants cannot resorb these nutrients from
senescing tissues. Because these nutrients
seldom limit plant growth, their lack of resorption
has little direct nutritional impact on
plants, except where acid rain greatly reduces
their availability (Aber et al. 1998, Driscoll
et al. 2001).
Leaching Loss from Plants
Leaching of nutrients from leaves is an important
secondary avenue of nutrient loss from
plants. Leachates account for about 15% of the
annual nutrient return from aboveground plant
parts to the soil. Rain dissolves nutrients on leaf
and stem surfaces and carries these to the soil as
throughfall (water that drops from the canopy)
or stem flow (water that flows down stems).
Stem flow typically has high concentrations of
nutrients due to leaching of the stem surface;
however, only a small amount of water moves
by this pathway. Throughfall typically accounts
for 90% of the nutrients leached from plants.
Although plants with high nutrient status lose
more nutrients per leaf, the proportion of nutrients
recycled by leaching is surprisingly similar
across a wide range of ecosystems (Table 8.7).
Leaching loss is most pronounced for those
nutrients that are highly soluble or are not
resorbed. As much as 50% of the calcium and
80% of the potassium in an apple leaf, for
example, can be leached within 24h. Leaching
Table 8.7. Nutrients leached from the canopy
(throughfall) as a percentage of the total aboveground
nutrient return from plants to the soil.
Throughfall (% of annual return) a
Nutrient Evergreen forests Deciduous forests
Nitrogen 14 ± 3 15 ± 3
Phosphorus 15 ± 3 15 ± 3
Potassium 59 ± 6 48 ± 4
Calcium 27 ± 6 24 ± 5
Magnesium 33 ± 6 38 ± 5
a
Means ± SE, for 12 deciduous and 12 evergreen forests.
Data from Chapin (1991b).
Nutrient Loss from Plants 193
rate is highest when rain first contacts a leaf and
then declines exponentially with time. Ecosystems
with very different rainfall regimes may
therefore return similar proportions of nutrients
to the soil through leaching vs. senescence.
Although leaching loss is quantitatively important
to plant nutrient budgets, there are no clear
adaptations to minimize leaching loss.The thick
cuticle of evergreen leaves was once thought to
reduce leaching loss and explain the presence of
evergreen leaves in wet, nutrient-poor forests.
There is no evidence, however, that leaching
loss is related to cuticle thickness. Like nutrient
resorption, leaching loss from plants is a quantitatively
important term in plant nutrient
budgets that is not well understood. Biologists
understand the acquisition of carbon and nutrients
by plants much better than the loss of these
resources.
Plant canopies can also absorb soluble nutrients
from precipitation. Canopy uptake from
precipitation is greatest in ecosystems where
there is strong growth limitation by a given
nutrient. In Germany, for example, nitrogen
inputs in precipitation are so high that forest
growth has switched from nitrogen to phosphorus
limitation. These forest canopies absorb
phosphorus directly from precipitation.
Herbivory
Herbivores are sometimes a major avenue
of nutrient loss from plants. Herbivores consume
a relatively small proportion (1 to 10%)
of plant production in many ecosystems. In
ecosystems such as productive grasslands,
however, herbivores regularly eat a large
proportion of plant production; and, during
outbreaks of herbivore population, herbivores
may consume most aboveground production
(see Chapter 11). Herbivory has a much larger
impact on plant nutrient budgets than the
biomass losses suggest, because herbivory precedes
resorption, so vegetation loses approximately
twice as much nitrogen and phosphorus
per unit biomass to herbivores than it does
through senescence. Animals also generally
feed on tissues that are rich in nitrogen and
phosphorus, thus maximizing the nutritional