A major activity in any savanna restoration is the recovery of the understory vegetation, principally forbs and grasses. Depending upon the history of a site, its understory may be rich in species, impoverished, or without any native plants at all. Depending on how hard a site has been grazed, it may be a candidate for complete understory restoration.

Like other plants, savanna plants are products of their evolution. If a plant is to survive in nature, it must be able to grow, flower, set seeds, disperse to new locations, and become established at those new locations. The ability of a species to thrive is a function of its genetics and of how its genetic characteristics interact with its environment.

Each species interacts with its environment in unique ways. Environmental factors that affect plant growth and reproduction include sunlight, moisture, nutrients, soil structure, temperature, carbon dioxide, and oxygen.

Of these, light, moisture, and temperature are probably the most important. For most savanna plants, nutrient deficiencies are not common. Also, gases have little direct effect on growth.

One environmental factor may interact with other factors. For instance, temperature and soil structure can both influence available moisture. Also, light has a direct effect through its requirement for photosynthesis, but also an indirect effect through its effect on temperature.

Another critical factor for the survival of a savanna species is its ability to compete with other species. In most well restored savannas, a large number of species (well over 200) may potentially become established but in any given spot only a few will last. Sometimes this is a matter of chance, influenced by what seeds happen to arrive first on the soil surface. Sometimes it is influenced by how fast a plant is able to grow. If several native plants are competing for the same niche, some will be more likely to become established than others. Exotic weeds are some of the most important species affecting native savanna plants. However, native plants also influence each other.

Environmental heterogeneity

The savanna environment is highly heterogeneous. Tiny hills and valleys are common, variations in soil structure across the site are likely, and rocks of various sizes may protrude.

The most important, and at the same time the most variable factor in savanna heterogeneity are the savanna trees. Because of the tree (oak) canopy, available light varies widely across the savanna. Much depends upon how close to the tree an understory plant is growing, and the size and shape of the canopy. The north side of a tree may be in relative shade while the south side of the same tree may be sunny.

Although available sunlight may have a direct effect on plant growth, the most important effect of sunlight is on temperature and available soil moisture.

Plants vary in their temperature requirements. Some are cool-season plants and others warm-season plants.

Plants vary in their drought tolerance. This depends to a great extent on their root systems. Drought-resistant plants produce very deep root systems, which means that even in a drought when the surface moisture is very low, they are able to obtain water from deep within the soil. Since direct sunlight will warm up the surface of the soil and cause significant evaporation of water, a long period of sunny days (not uncommon in Midwest savannas) can bring about serious depletion of moisture. Drought tolerant plants may be able to survive, but drought sensitive plants may senesce or die. In contrast to prairie plants, many savanna plants are relatively drought sensitive, and grow best in moister environments. Quite a few savanna species will also be found in or near wetlands.

The north and the south sides of a savanna tree are two quite different environments. On the south side may be found prairie grasses and forbs whereas on the north side woodland species may thrive. However, the farther the plant is growing from the tree, the less affect the tree will have on its survival.

The extent of canopy closure will also have major effects on temperature and moisture. A very open savanna is essentially a prairie, and would be expected to have prairie plants, whereas a mostly closed savanna resembles a woodland and would be more likely to have woodland species. Savanna specialist species would most likely thrive in conditions intermediate between very open and mostly closed canopy. This why the Bader list of savanna plants has many species that also thrive in wet and wet-mesic environments.

Because of this wide variation in canopy, species numbers in a well restored savanna will usually be much higher than in a well restored prairie, or even in a good prairie remnant.

This bur oak tree illustrates the complicated environmental conditions in a savanna. Underneath the tree it is shady, providing conditions for growth of savanna and even woodland species. Away from the direct influence of the tree, conditions are open, which favors the growth of prairie plants. The two sides of the tree also differ, since the front side faces the sun whereas the back side is shady. Over 20 species were found within the influence of this single tree.

Different species on two sides of the same bur oak
Gray goldenrod (Solidago nemoralis), a dry prairie species, growing on the hot (south side) of a bur oak in an open savanna.

Zig-zag goldenrod (Solidago flexicaulis), a woodland species, growing on the cool (north side) of the same bur oak.

Life cycles of plants

The life cycle of a species plays a major role in plant establishment.

Annual plants are those that grow from seed, flourish, flower, set seed, and die all in the same growing season. Very few annuals are savanna plants.

Biennial plants require two seasons to complete a life cycle. A seed germinates the first year and the plant grows but does not flower. During this first year the plant is establishing a root system; often the above-ground parts of the plant are insubstantial. After a winter hardening period (cold weather is usually required to induce formation of flower buds), the plant grows in the second season, flowers, sets seeds, and dies. The dispersed seeds carry the species on.

Biennial plants that might be found in savannas include tall bellflower (Campanula americana), woodland thistle (Cirsium altissimum), old-field thistle (Cirsium discolor), and stiff gentian (Gentiana quinquefolia)
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Perennial plants live three or more years. The longevity of plants vary, some being relatively short-lived, whereas others have long lives. Trees, of course, are the best known long-lived plants, but even some herbaceous plants may have long lives. Detailed estimates of prairie blazing star (Liatris aspera) found a maximum age of 34 years, with an average of about 25 years. Preliminary estimates of a few other prairie forbs were similar. No specific research has been done on savanna forbs, but research on forbs that live in woodlands showed maximum ages of about 15 years for individual plants.

Another factor in the life cycle of perennial plants is the age at which flowering first takes place. Although some perennials flower in their first growing season, many do not. During the first few years of their life, individual plants may be establishing root systems, and their above-ground plant material may be minimal. Only after the root system is well established is flowering initiated.
Since well established perennials also flower and set seeds, if environmental conditions remain relatively constant an established population of a species may continue to occupy a given site for many years.

Monocarpic plants are plants that flower once and then die. Biennial plants are a subset of monocarpic plants. Monocarpic species may grow vegetatively for two or three years, perhaps more, before flowering. These species probably need more time to establish a substantial root system before initiating flowering. Under certain conditions, some monocarpic plants can act as annuals or biennials, carrying out their whole life cycles in a single or in two years.

An example of a monocarpic savanna species is lion's foot (Prenanthes alba). Often in manuals monocarpic plants are included with perennials, since they can be long-lived.

Tall American bellflower (Campanula americana) is a biennial plant. It is also a prolific seed producer, so even though each individual plant lives only two years, the "population" maintains itself, although often showing better growth one year than the next.
Butterfly milkweed (Asclepias tuberosa) is a long-lived perennial, over 20 years in careful research observations.
Compass plant (Silphium laciniatum) is one of the longer-lived native perennials. It has a very deep root system so that it can survive drought years.
Great blue lobelia (Lobelia siphilitica) is an example of a monocarpic plant. It may grow several years in the vegetative state but once it flowers, it dies. When planted in a new area, it may take several years before it grows big enough to flower, and flowering populations often fluctuate widely from year to year.

Start with a seed

Most native plants of prairies and savannas are perennials. The seed is the dormant structure that brings about the dispersal of the species to a new environment. Seed germination is the process by which the seed changes from a dormant structure to a growing plant.

The seeds of many plants remain dormant until activated by some environmental stimulus. In the Midwest, most of the plants that flower in late summer or early fall produce seeds that need a cold moist period in order for the germination process to be activated. Because of the Midwest climate, this activation happens naturally during the winter. The following spring, as the temperature of the soil warms, the seed germinates and produces a seedling.

The ability to recognize plant species at the seedling stage is very useful when working on a savanna restoration. Each species does have its characteristic structure and once one has made the visual connection between the seedling and the mature plant, it is generally possible to identify seedlings.

Seedling of compass plant (Silphium laciniatum) growing in a planted restoration. Root growth is the principal activity of this species for the first few years. This particular seedling is in its third year. Often, flowering does not occur in compass plant until the fifth or sixth year after planting.
Seedling of rattlesnake master (Eryngium yuccifolium) in a planted restoration.
Seedling of pale Indian plantain (Arnoglossum atriplicifolium). This savanna indicator species has a very characteristic leaf structure and is easy to recognize even in young plantings.
Seedling of spiderwort (Tradescantia ohiensis) in a savanna restoration. Although usually considered a "prairie" plant, this species also grows well in open savannas.
Seedling of wood betony (Pedicularis canadensis), growing at the edge of a savanna restoration. This species grows hemiparasitically on other plants, which helps it to become established and spread. However, it never dominates a restoration.

Many species grow only vegetatively their first year. After overwintering, more vigorous above ground growth occurs the second year and may lead to flowering. Most, but not all, native plants will flower the second year. Flowering is followed by seed set and the cycle is completed.

Not all plants require cold moist activation. Most grasses, as well as forbs that produce seeds early in the season, will germinate without special activation. In fact, species such as lupine which flower early have already set seed by the beginning of July. After dispersal, these seeds initiate growth the same year and form small first year plants that are able to overwinter. The following spring the life cycle is completed. Lupine seeds sometimes begin to germinate while still inside the seed pod.

The seeds of many biennial species, including some notorious invasive weeds, germinate the same year they are produced, usually in the early fall. They grow until the ground freezes, then go dormant until spring.

A related phenomenon is fall regrowth. Many species go dormant for a while after setting seed but then initiate a new cycle of vegetative growth in the fall. Many prairie and savanna plants exhibit this phenomenon, remaining green until the ground freezes.

Phenology of plant growth

Phenology is the discipline that studies the development of plants (or animals) in relation to the seasons of the year. Knowledge of plant phenology provides important insights into the experimental establishment of plant species in the savanna. A knowledge of phenology is also useful when monitoring plant growth in newly restored savannas.

Some plants appear early in the growing season, whereas others appear in summer, and still others in the fall. Some plants may appear early in the growing season but do not flower until very late in the year. The restorationist must be familiar with the phenology of the plants that are becoming established.

The table here gives phenological data for some plant species found in savannas. The dates are only approximate and apply to southern Wisconsin (and probably northern Illinois).

Effect of daylength

A key environmental factor affecting plant phenology is the duration of sunlight (daylength). Some plants are “short-day” plants, only flowering when day length is short, whereas others are “long-day” plants, flowering in mid summer. The term “photoperiod” is used to refer to the responses of plants to differing daylengths.

In the Midwest, there are striking differences between the photoperiod at different times of year. The flowering process in many plants is affected by the length of day, generally unrelated to the “amount” of sunlight received. Even though the amount of sunlight a plant receives in the early morning is much less than the amount it receives at midday, this small amount of light can affect the development of flowers. A plant hormone called “phytochrome”, which is sensitive to small amounts of sunlight, affects the initiation of flower buds.

Short-day plants, flowering only when the daylength is shorter, include a number of species that do not flower until fall. The phenology table above can be used to locate some of these species.

Long-day plants are those which flower in mid-summer, when daylength is longest.

The principal affect of daylength on flowering is on the “initiation” of flower bud formation. Once flower buds form, further vegetative growth of the stem containing the flower bud ceases (although elongation of the stem as part of the flowering process may continue). Prairie and savanna plants that are short-day plants bloom mainly in the fall (September/October). These include, among others, asters, goldenrods, and ragweeds. Most prairie grasses are long-day plants, initiating flower bud formation in mid summer.

A few short- and long-day plants

Short-day plant: New England aster (Aster nova-angliae). This is one of the more colorful asters. It begins to flower in late August and is at its peak in September.
Short-day plant: Elm-leaf goldenrod (Solidago ulmifolia). This is a late-flowering goldenrod, usually at its peak in late September.
Short-day plant: Heath aster (Aster ericoides). Heath aster is interesting because it is one of the first plants to appear in the spring, yet it doesn't flower and set seed until late fall. This is a species that forms clones or fairly large patches, and in the spring the leaves are soft and green. As the season wears on, the plants get progressively rougher, until by the time they are flowering in mid October, they are beginning to feel like a Brillo pad. (I suppose that is why it is called "heath" aster, since plants of the heath family have a similar appearance).
Long-day plant: Yellow cone-flower (Ratibida pinnata). This is a classic long-day species. Although it grows vegetgatively throughout the spring, it does not start to make flower buds until late June, and first flowers usually show about the 1st of July.
Long-day plant: Black-eyed Susan (Rudbeckia hirta). Another typical long-day plant, in full flower during mid summer. This species has a longer flowering period than yellow cone-flower. Some specimens begin to flower even in June, whereas others don't start flowering until August, but the main peak of flowering is July.

Warm season (C4) and cool season (C3)plants

Grasses and forbs can be divided into two major groups depending upon the biochemical processes involved in photosynthesis. The biochemical details need not be discussed here. It is sufficient to state that plants can be divided into two groups,called C3 and C4 plants. Most plants are C3 types, but tropical plants and many prairie grasses that thrive in warmer times of the year are C4 plants.

It turns out that C4 plants have a competitive advantage over C3 plants under conditions of drought. Although C4 plants constitute only 1% of the Earth’s species, they account for around 30% of the photosynthetic carbon fixation in terrestrial environments.

In full sunlight, C4 plants are better able to carry out photosynthesis at higher temperatures, and are thus better adapted to warm season conditions. This has important implications for the ecologyof warm-season prairie grasses, especially those living in hot, dry conditions. C4 plants have markedly higher growth rates at their high optimum temperatures than do C3 plants. Also, C4 plants have about twice the water-use efficiency than C3 plants, because of higher photosynthesis rates and lower transpiration. Most of the research has been on crop plants, and C4 plants are especially adapted to hot environments with limited rainfall. Presumably, C4 prairie plants show a similar relationship. C4 grasses outcompete C3 grasses in tropical and subtropical grasslands where mean temperatures are above 25º C and rainfall is mostly in the summer.
The tallgrass prairies of the Midwest are dominated by C4 grasses, especially big and little bluestem, Indian grass, and switchgrass. These are all warm season grasses. All of these grasses are also the important grasses of the sunnier parts of oak savannas. In shadier savannas, several C3 cool-season grasses dominate: silky and riverbank rye, bottlebrush grass, and woodland brome. Follow this link for a list of prairie and savanna grasses and their classification as cool-season and warm-season.

Grasses of the prairie are able to tolerate moisture stress by means of the physiological processes associated with C4 metabolism. On the other hand, prairie forbs, all of which are C3 plants, are able to tolerate moisture stress not because of the biochemistry of photosynthesis but because of the very deep root systems that they maintain.

Fire on the tallgrass prairie is known to enhance the growth of C4 grasses. This is because fire removes the extensive dead plant material remaining from the previous growth season, increasing spring/early summer solar radiation, which warms up the soil surface and thus provides conditions suitable for the growth of warm-season grasses. Frequent fire is very important in allowing C4 grasses to remain dominant.

Cool season grasses begin to develop in the spring and are in flower by mid to late June. These grasses include needle grass, june grass, various savanna ryes (Elymus) and bromes (Bromus). Most of the prairie grasses and many of the prairie forbs develop in mid summer, when conditions are hot and often drier. The C4 process gives the prairie (full sunlight) grasses a competitive advantage over the C3 savanna (partial shade) grasses under these conditions. Warm-season (C4) grasses are big and little bluestem, Indian grass, side oats grama, and prairie dropseed.

There are no C4 forbs in the Midwest. The forbs that develop in the mid summer are the species that are able to tolerate mid summer drought because of their deep root systems.

An extensive summary of savanna species that are found in various light and moisture regimes has been published by Bader and can be accessed via this link.