Many interesting plant to plant relationships exist, covering the spectrum from mutually
beneficial to wholly parasitic.An example of a
beneficial, plant-plant relationship familiar to many gardeners is the "Three Sisters
Garden." Native Americans planted three crops together—corn, beans, and
squash—knowing that each had something to offer the others. The corn plants grew
straight and tall, giving the pole beans something to climb on. The beans, since they are
legumes, contributed nitrogen to the soil. And the pumpkins shaded out competing weeds.
And even something as simple as the relationship of a tree
to the groundcover beneath it can be considered a beneficial, plant-plant relationship.
The tree casts shade, providing habitat for a shade-loving groundcover, and the
groundcover in turn keeps more deep-rooted and competitive grasses at bay.
Epiphytes. One interesting group of plants are the
epiphytes. Relatively rare in temperate regions, epiphytes are quite common in tropical
rainforests. An epiphyte is a plant that grows on another plant, neither harming nor
helping it. For example, mosses can be epiphytic, growing harmlessly on tree trunks.
More exclusively epiphytic plants are the bromeliads and
some orchids.
Bromeliads are plants that commonly grow high in the
branches of tropical rainforest trees. They are often found in the joint where a branch
meets the trunk; there, fallen plant debris collects, providing a source of nutrients to
the bromeliad. Some species of bromeliad have cup-shaped leaf rosettes. The cup fills with
water during the frequent rains, and the plant is able to use this supply to fill its
water needs. Though bromeliads perch in the branches, they do no harm to the tree. They
photosynthesize their own food—their roots never penetrate the tree’s bark, so
don’t draw any of their nutrients from the host tree. They simply perch there, high
in the canopy, where light is more plentiful than on the forest floor.
Epiphytic orchids can also be found perched in trees in
the rainforest; like bromeliads, they collect nutrients from organic debris. Orchids are
able to fill at least some of their water requirements by absorbing water vapor through
their long, aerial roots.
Now let’s look at some less benign relationships
between plants.
