Plant Taxonomy (BIOL308)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Plants of Central Minnesota Rocky Outcrops

    The granite outcrops that occur in central Minnesota have an interesting flora. Plants that grow in this environment must be able to tolerate harsh conditions.

    Rocky outcrops are xeric, or dry, habitats. Little moisture is available for plants growing in rocky outcrops because there is little soil to hold moisture. The soil depth is rarely more than a few inches deep and it is often restricted to cracks in the rock.  As a result, the plants of these areas are typically herbaceous; there is not enough soil to support trees.

    Any precipitation quickly runs off or evaporates from the surface so these plants must be able to survive on intermittent moisture. Drought tolerance is probably the most important feature of plants that live on these outcrops. Several strategies for water conservation and surviving desiccation including: (1) a thick waxy cuticle; (2) stomata on the lower surface of leaves or sunken in chambers to minimize water loss; (3) smaller leaves or no leaves, and (4) hairs on the surface to minimize water loss.

    The Brittle prickly-pear cactus (Opuntia fragilis) is a good example of a plant that is adapted to dry conditions. It forms a small, but dense mat of plants on the surface of the rock. This plant has a very shallow root system and are ideally adapted to the harsh conditions. Succulent stems, no leaves, thorns, thick waxy outer cuticle, and a small size are additional features that help to conserve water. Fameflower (Talinum parviflorum) is a small perennial with round tubular leaves and a taproot. It grows in the thin soils in the cracks of exposed granite bedrock. Both of these drought-tolerant species are more common in the Minnesota River Valley and the southwest section of the state, though we find them in central Minnesota, for example in Quarry Park.

    Another interesting adaptation for survival in xeric conditions is Crassulacean acid metabolism, abbreviated CAM. The process is a specialized version of photosynthesis in which plants do things "backwards". Most plants open the stomata in their leaves during the daytime to absorb carbon dioxide for photosynthesis. One unavoidable consequence of opening the stomata is that the plant loses water through the pores. For most plants this isn’t a serious problem. However, plants that grow on rocky outcrops can’t afford to lose so much water. Thus, they have evolved a modification that allows them to open up the stomata at night and then store the carbon dioxide until the daytime, when sunlight is available to complete the photosynthetic process. This is a tremendous advantage because it is usually much cooler at night which results in significantly lower water losses. This adaptation is shown by several rocky outcrop species including fameflower, brittle prickly-pear cactus, and a common weedy plant, purslane (Portulaca oleracea).

    Compounding the problem of drought, is temperature. Rocky outcrops are hot in the summer and cold in the winter. The rock absorbs solar radiation increasing the temperature and rate of water loss. On a sunny day the temperature of the rock surface can be several degrees higher than the air temperature or that of adjacent soil. In winter, perennial plants growing in this area must be able to tolerate cold and freezing since the roots have little soil for protection. Many of the adaptations for tolerating drought also do "double duty" to help survive fluctuating temperatures. For example, like many alpine species, plants of the rock surface are often small and have hairs for insulation (and to reduce water loss).

    Rocky outcrops provide a good substrate for lichens. These desiccation-tolerant organisms are formed from a symbiotic association of fungus and algae. Essentially, a lichen is an algal sandwich surrounded by fungal tissue. They are capable of remaining inactive – or dormant - in a dry state for a long time. When moisture is available, lichens absorb water (hydrate) and begin to photosythesize as long as conditions permit. Lichens are typically the first pioneer species of rock. In fact, in the classic bare rock succession, lichens lead the way.

    There are three major growth forms of lichens - crustose, thallose, and folliose. Crustose, lichens form a thin film adhering to the rock. They are nearly impossible to separate from the rock and are early colonizers. Thallose lichens are flat, leaf-like species and foliose are larger, three-dimensional species. These latter types are usually later colonizers. One of the interesting foliose lichens in central Minnesota outcrops includes the highly branched, grayish-green reindeer moss (which is really a lichen). This species is uncommon in central Minnesota – it is more common in the arrowhead region of the state.

    Many mosses and ferns, like the lichens, also exhibit desiccation tolerance. The Rusty woodsia (Woodsia ilvensis), a clump forming species, has numerous hairs to help minimize desiccation and the fronds curl up when they become dry to further reduce water loss.

    Bearberry (Arctostaphylos uvi-ursi) is an evergreen, creeping plant that can be found growing on central Minnesota outcrops.  This plant usually has a more "circumboreal" distribution; that is, you are more likely to find it in a northern Minnesota conifer forest. The Native American name for this plant is kinnickinic. Bearberry leaves, which are rather leathery to resist water loss, have been used to make a tea to treat urinary disorders, but this is not advisable because it contains the toxic chemical, arbutin. Pale corydalis (Corydalis sempervirens), like bearberry, is another plant that grows on the outcrops but is more common in northern Minnesota.

    The vegetation on the surfaces of the rocky outcrops is sparse and occurs primarily in the shallow soils that have accumulated in cracks in the rock. Adjacent to the exposed bedrock is a mixture of grasses and forbs that are growing in the thin soils over buried bedrock. Prairie species such as Big blue stem (Andropogon gerardii) are often found adjacent to the rocks since that is the nature of the surrounding vegetation.

    Fire is an important factor to help maintain the composition of the rocky outcrop communities. In its absence, surrounding areas are invaded by trees, especially red oak (Quercus rubra), northern pin oak (Quercus ellipsoidalis), bur oak (Quercus macrocarpa) and Eastern red cedar (Juniperus virginiana)

    A listing of some of the plants likely to be found on a rocky outcrop in central Minnesota is given in Table 1. In this list there are three major types of species: (1) species characteristic of rocky outcrops (O. fragilis, T. parviflorum); (2) prairie species that grow in the area around the outcrop and in the cracks and depressions with deeper soil; and (3) weeds.

Lab Activity
    During today's lab we will walk around Quarry Park (Waite Park, MN) to find the plants in the Table 1 below.  Quarry Park is a relatively new park in Stearns County.  The park was once the site of major granite-mining operations because the abundance of easily accessible rock near the soil surface.  We will observe several old quarries and the piles of discarded stone.  We will go out rain or shine so dress appropriately.  Out in the field I will describe the key characteristics of these species and provide other information (edibility, uses, etc.) as appropriate.   You will may want to bring a notebook and pen to take notes about each species that we find. 

References:

• Anderson, RJ and J Baskin, eds.  1999.  Savannas, Barrens, and Rock Outcrop Communities of North America.  Cambridge University Press.
• Quarry Park Website including maps and lots of other great information

Last updated:  09/11/2007 / � Copyright by SG Saupe