I wrote:
Many "generic" evergreens shed needles (or scales) that are
preemergence herbicides and prevent weeds from coming up from seed. One
good examples is juniper Virginianis. That is why some forests have few
weeds on the forest floor.

"Pam - gardengal" replied:

This is not exactly true - allelopathy is not exactly the same as a
pre-emergent herbicide, although the effects can be somewhat similar.

Allelopathy is a variety of chemical processes that plants use to keep
other plants from growing too close. These compounds include alkaloids,
cyanohydrins, sulphides, flavaniods, terpenoids, steroids, phenolic
acids, aliphatic acids, glycosides, lactones, tannins, organic acids,
purines, nucleotides, cinnamic acid and sugars. In one kind, the plant
that is protecting its space releases growth-compounds from its roots
into the ground. New plants trying to grow near the allelopathic plant
absorb those chemicals from the soil and are unable to live. A second
type of allelopathy releases chemicals that slows or stops the process
of photosynthesis . An allelopathic plant may also release chemicals
that change the amount of chlorophyll a plant has in it. When a plant's
chlorophyll levels are changed, it cannot make the food it needs, and
the plant dies.

Some plants that use allelopathy are black walnut trees, sunflowers,
wormwoods, sagebrushes, and trees of heaven.

There are several ways in which an allelopathic plant can release its
protective chemicals:

Volatilization: Allelopathic trees release a chemical in the form of a
gas through small openings in their leaves. Other plants absorb the
toxic chemical and die.

Leaching: All plants lose leaves. Some plants store protective chemicals
in the leaves they drop. When the leaves fall to the ground, they
decompose. As this happens, the leaves give off chemicals that protect
the plant.

Exudation: Some plants release defensive chemicals into the soil
through their roots. Those chemicals are absorbed by the roots of other
trees near the allelopathic one. As a result, the non-allelopathic
tree is damaged.

Some pine trees are allelopathic. When their needles fall onto the
ground, they begin to decompose . The soil absorbs acid from the
decomposing needles. This acid in the soil keeps unwanted plants from
growing near the pine tree. It has been theorized that phenolic acids
present in pine leaves may play a vital role in the allelopathic
inhibition of understory species. Hence, many pine forests have very
little on the forest floor other than pine needles.

Many junipers produce a phenolic compound found to inhibit seed
germination. Such a compound is called a preemergent herbicide.

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