The Plant - Agroecology: The Ecology of Sustainable Food Systems - page 41

response. Within a day or two after significant rainfall,
leaves appear on the stems; as long as moisture levels are
sufficient in the soil, the leaves are retained, but immedi-
ately upon reaching the wilting point the leaves are
dropped.

A particular species' range of tolerance limits and
optimum for a factor of the environment is ultimately the
result of how that factor affects each of the physiological
processes of the plant. A species' tolerance of a range of
temperatures, for example, is linked to how temperature
affects photosynthesis, transpiration, and other physiolog-
ical processes of the plant. When all of the abiotic and
biotic factors of the environment are entered into the tol-
erance equation, the full range of a species' adaptability
becomes apparent. An individual's habitat and niche
become fully integrated (Figure 3.3).
A species with a broad set of tolerances of environ-
mental conditions (known as a generalist ) and a broad
ability to interact with other species (often referred to as
a species with a broad niche or the capability of consid-
erable niche overlap) will be more common over a larger
area. In contrast, a species with a narrow set of tolerances
and a very specialized niche (a specialist ), will be less
common over larger areas and only seen as common at a
very localized level. Redwood sorrel ( Oxalis oregana ), an
ecological specialist, can form dense stands in which it
is the locally dominant plant, but it is restricted to the
specific conditions encountered in the partially shaded
understory of a redwood forest. If the shade is too dense,
photosynthetic activity is not great enough to meet the
plant's respiratory needs, and if the sun is too intense,
sorrel is unable to tolerate the desiccating effects of direct
solar radiation. Sorrel's optimum level of light is interme-
diate to these two extremes.
In summary, each individual plant species occurs in
a particular habitat as a result of the development over
time of a particular set of adaptive responses to the envi-
ronment in which it lives. The species' limits of tolerance
restrict individuals of that species to a particular habitat,

Independent Responses

Finally, certain responses in plants occur regardless of
conditions in the immediate environment and are the result
of some internally controlled, physiologically determined
set of factors. For example, a corn plant begins to flower
because a particular stage in growth and development has
been achieved. External conditions may force later or ear-
lier flowering by affecting growth, but the actual shift to
flowering is internally controlled.

L IMITS AND T OLERANCES

The ability of an individual species to occupy its par-
ticular habitat is the result of a set of adaptations that
have evolved over time for that species. These adapta-
tions allow the plant to cope with certain levels of
moisture availability, temperature, light, wind, and other
conditions. For each of the factors that delimit the habitat
for the species, there is a maximum level of tolerance
and a minimum level of tolerance beyond which
that species cannot cope. Between these two extremes
there is an optimum at which the species performs or
functions the best. For example, the tropical plant
banana has a mean monthly temperature optimum of
27°C; above 50°C banana trees suffer sunscorch and
stop growing; below 21°C growth is checked by reduc-
tion in leaf production and delayed shooting of the
bunches.

optimum functioning

Response
to factor

minimum
tolerance level

maximum
tolerance level

Factor intensity

FIGURE 3.3 A plant's range of tolerance for an environmental factor.