Principles of terrestrial ecosystem ecology.pdf

ecosystems experience similar pulses of leaf
and root senescence with the onset of drought.
Senescence and tissue loss are therefore highly
pulsed in most ecosystems and occur just before
the period when conditions are least favorable
for resource acquisition and growth. These seasonal
pulses of senescence cause the greatest
tissue loss in highly seasonal environments.
Leaf longevity varies among plant species
from a few weeks to several years or decades.
In general, plants in high-resource environments
produce short-lived leaves with a high
specific leaf area (SLA) and a high photosynthetic
rate per leaf mass, but they have little
resistance to environmental stresses and are
poorly defended against herbivores. These
disposable leaves are typically shed when conditions
become unfavorable (winter or dry
season) and are replaced the next spring. Both
root and leaf longevity are greater in lowresource
environments (Berendse and Aerts
1987) and lower at high latitudes than in the
tropics (Fig. 6.7).The greater longevity of leaves
from low-resource environments reduces the
nutrient requirement by plants to maintain leaf
area (see Chapter 8).
Senescence enables plants to shed parasites,
pathogens, and herbivores. Because leaves and
fine roots represent relatively large packets
of nutrients and organic matter, they are constantly
under attack from pathogens, parasites,
Turnover (yr -1 )
1.0
0.8
0.6
0.4
0.2
0
Trees: complete system
Trees: fine roots
Grasslands
Shrublands
Wetlands
High latitude Temperate Tropical
Figure 6.7. Synthesis of information on root
turnover in major ecosystem types along a latitudinal
gradient. (Redrawn with permission from New
Phytologist; Gill and Jackson 2000.)
Global Distribution of Biomass and NPP 137
and herbivores. Phyllosphere fungi, for
example, begin colonizing and growing on
leaves shortly after budbreak, initially as parasites
and later as part of the decomposer community,
when the leaf is shed (see Chapter 7).
These fungi account for some of the mottled
appearance of older leaves. Pathogenic root
fungi are a major cause of reduced yields in
agroecosystems and are common in natural
ecosystems. Plants have a variety of mechanisms
for detecting natural enemies and
respond initially through the production of
induced chemical defenses (see Chapter 11)
and, in the case of severe attack, by shedding
tissues.
Large unpredictable biomass losses occur in
most ecosystems. Wind storms, fires, herbivore
outbreaks, and epidemics of pathogens frequently
cause large tissue losses that are unpredictable
and occur before any programmed
senescence of tissues and associated nutrient
resorption can occur. These unpredictable
biomass losses incur approximately twice the
nutrient loss to the plant as that occurring
after senescence (see Chapter 8). They often
increase spatial heterogeneity of light and
nutrient resources in the ecosystem through
creation of gaps, which range in scale from the
loss of individual leaves to the destruction of
biomass over large regions. Most ecosystems
are at some stage in the regrowth after such
biomass losses.
Global Distribution of Biomass
and NPP
Biome Differences in Biomass
The plant biomass of an ecosystem is the
balance between NPP and tissue turnover. NPP
and tissue loss are seldom in perfect balance.
NPP tends to exceed tissue loss shortly after
disturbance; at other times tissue loss exceeds
NPP. As ecosystems or landscapes approach
steady state (see Chapter 14), however, there is
often a consistent relationship between plant
biomass and the climate or biome type that
characterizes that ecosystem. Average plant
biomass varies 50-fold among Earth’s major