Plant Adaptations to Rocky Sites (including glades, barrens, rock outcrops, and cliffs)

Rocky natural communities include rock outcrops, cliffs, and  glades and barrens.  These sites are characterized by thin soils, expanses of bare, gravelly, or lichen-covered rock, and patches of vegetation ranging from isolated plants to small woodland patches.  These communities are often picturesque: the textures, colors, and shapes of the rock act as  beautiful foils to the vegetation. 

​Scenic as they are, however, these are harsh environments, often experiencing periods of drought. Indeed, granite outcrops have been called "biological deserts" because of the heat and dryness they experience: temperatures of up to 140 degrees have been measured on some granite outcrops in the Georgia Piedmont, and they are often bone-dry.

The ability to survive these rough conditions makes the plants of outcrops special.  They are superb competitors on difficult sites, but not in more average conditions.  As a result, often these communities will support rare and/or endemic (found nowhere else) species. 

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The waxy leaves of Georgia oak retain moisture in the leaf.

The thin, small leaves of rock outcrop milkwort enable to plant to lose less moisture by J. Pakchar

The white hairs on the inflorescence (and undersides of leaves) of rabbit tobacco will reflect sunlight.

The leaves of wooly ragwort can tilt straight up, reducing the amount of sunlight they receive.

Diamorpha, a signature plant of granite outcrops, is a succulent.

Diamorpha is also a winter annual. Look carefully and you will see the old flower stalks: it has already flowered and set seed by July.

Stone Mountain daisy is a summer annual. Seeds germinate in spring, and the plant flowers then set seeds in late fall.

Rock moss (Grimmia) will turn black when dry, but then quickly greens up after rain. by Hugh and Carol Nourse

Thin, dry soils limit the height vegetation can reach: too tall and plants will succumb to drought.

Warm season grasses use C4 photosynthesis, which enable them to lose less moisture to transpiration.

Cacti, such as this prickly pear, use CAM photosynthesis: they open their stomata at night when it is cool and lose less water to transpiration.

Plant Adaptations

Plants in these habitats have evolved ways to reduce the amount of sunlight their leaves receive and to retain moisture.  Some of those adaptations include:

• Waxy or leathery leaves, which keep moisture in.
• Thin, small, or finely cut leaves, which reduce the surface area exposed to sunlight.
• White hairs that reflect light on leaves and stems. Hairs (of any color) on  leaves and stems also hold moisture close, reducing transpiration.
• Leaves which tilt in different directions to minimize the amount of direct sunlight they receive in the heat of the day.
• Succulence, which involves the storage of water in special leaf and stem cells.
• An annual life cycle, wherein a  plant lives for less than a year.  It germinates from seed, grows and leafs out, and produces flowers and seeds within one growing season, then dies.  Winter annuals (such as Diamorpha) germinate in the fall and flower and sets seed by the following summer.  Summer annuals (such as Stone Mountain Daisy)  germinate in spring and summer and flower and set seed in late fall.  This life cycle enables plants to minimize exposure to heat, and are possible in thin-soiled areas where plant competition is not too great.​
• Poikilohydry:  the ability to dry out and go dormant, and then green up and start photosynthesizing quickly once moistened.  Mosses do this.
• Slow growth, small size, and stunting:  when moisture and nutrients are low, and the growing season has many dry periods, plants grow more slowly and stay small.
• Special photosynthetic processes.  Warm season grasses use C4 photosynthesis, and cacti use CAM photosynthesis, each of which enable plants to save moisture compared to the C2 photosynthesis of most plants.

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