Principles of terrestrial ecosystem ecology.pdf

potentially ammonium in ecosystems with
low rates of nitrification. Although laboratory
experiments show that plants consistently
exclude mycorrhizae from roots under highnutrient
conditions, the extensive distribution
of mycorrhizae across a wide range of soil fertilities,
including most crop ecosystems, suggests
that mycorrhizae continue to provide a
net benefit to plants even in relatively fertile
soils.
There are a range of mycorrhizal types, but
the most common are arbuscular mycorrhizae
(AM; also termed vesicular arbuscular mycorrhizae,
VAM) and ectomycorrhizae. AM fungi
grow through the cell walls of the root cortex
(i.e., the layers of root cells involved in nutrient
uptake), much as does a root pathogenic
fungus. In contrast to root pathogens, AM
produce arbuscules, which are highly branched
treelike structures produced by the fungus and
surrounded by the plasma membrane of the
root cortical cells. Arbuscules are the structures
through which nutrients and carbohydrates are
exchanged between the fungus and the plant.
AM are most common in herbaceous communities,
such as grasslands, and in phosphoruslimited
tropical forests and early successional
temperate forests. Many AM associations are
relatively nonspecific and can occur even with
ecotmycorrhizal plant species shortly after disturbance.
AM are generally eliminated after
ectomycorrhizae colonize the roots of these
species.
In a given ecosystem type, AM associations
are best developed under conditions of phosphorus
limitation, where they short-circuit the
diffusion limitation of uptake (Allen 1991,
Read 1991). Their effectiveness in overcoming
phosphorus limitation may contribute to the
nitrogen limitation in many temperate ecosystems
(Grogan and Chapin 2000). The AM symbiosis
is a dynamic interaction between plant
and fungus, in which both roots and hyphae
turn over rapidly. Under conditions in which
plant growth is carbon limited, as in young
seedlings or in shaded or highly fertile conditions,
mycorrhizae may act as parasites and
reduce plant growth (Koide 1991). Under these
conditions, the plant reduces the number of
infection points in new roots.As older roots die,
Nutrient Uptake 183
this reduces the proportion of colonized roots,
thus decreasing the carbon drain from the
plant. AM associations might be viewed as a
balanced parasitism between root and fungus
that is carefully regulated by both partners.
Ectomycorrhizae are relatively stable associations
between roots and fungi that occur primarily
in woody plants. The exchange organ
is a mantle or sheath of fungal hyphae that
surround the root plus additional hyphae that
grow through the cell walls of the cortex (the
Hartig net). Roots respond to ectomycorrhizal
colonization by reducing root elongation and
increasing branching, forming short, highly
branched rootlets. Fungal tissue accounts for
about 40% of the volume of these root tips. As
with AM, ectomycorrhizae involve an exchange
of nutrients and carbohydrates between the
fungus and the plant. In contrast to AM, ectomycorrhizae
generally prolong root longevity.
Ectomycorrhizae also differ from AM in that
they have proteases and other enzymes that
attack organic nitrogen compounds. The fungus
then absorbs the resulting amino acids and
transfers them to the plant (Read 1991). Ectomycorrhizae
therefore enhance both nitrogen
and phosphorus uptake by plants.
There are other mycorrhizal associations that
differ functionally from AM and ectomycorrhizae.
Fine-rooted heath plants in the families
Ericaceae and Epacridaceae, for example, form
mycorrhizae in which the fungal tissue accounts
for 80% of the root volume.These mycorrhizae,
like ectomycorrhizae, hydrolyze organic
nitrogen and transfer the resulting amino acids
to their host plants. Many nonphotosynthetic
orchids totally depend on their mycorrhizae for
carbon as well as nutrients. Their mycorrhizal
fungi generally form links between the orchid
and some photosynthetic plant species, especially
conifers. In this case, the plant is clearly
parasitic on the fungus.
As with the orchid–fungal association, ectomycorrhizae
and AM often attach to several
host plants, often of different species. Carbon
and nutrients can be transferred among plants
through this fungal network, although relatively
few studies have shown a net transfer of
carbon among plants (Simard et al. 1997). If
these fungal connections among plants cause