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)
.
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).
Latin
name |
Common
name |
1st
leaves |
Flower
buds |
1st
flowers |
Last
flowers |
1st
seeds |
1st
collect seeds |
Last
collect seeds |
Comments |
Ranunculus
fascicularis |
Early
buttercup |
|
|
24-Mar |
23-May |
|
7-Jun |
|
|
Oxalis
violacea |
Violet
wood-sorrel |
|
|
14-Apr |
2-Jun |
|
7-Jun |
|
|
Viola
pedata |
Bird's
foot violet |
|
|
14-Apr |
20-May |
23-May |
31-May |
15-Jun |
|
Viola
soraria |
Door-yard
violet |
|
|
14-Apr |
27-May |
|
|
|
|
Antennaria
neglecta |
Field
pussytoes |
10-Apr |
12-Apr |
15-Apr |
13-May |
|
24-May |
10-Jun |
|
Anemone
quinquefolia |
Wood
anemone |
|
14-Apr |
17-Apr |
22-May |
|
|
|
|
Aquilegia
canadensis |
Wild
columbine |
31-Mar |
17-Apr |
20-Apr |
17-Jun |
15-Jun |
30-Jun |
15-Jul |
|
Sisyrinchium
campestre |
Blue-eyed
grass |
|
17-Apr |
25-Apr |
27-May |
10-Jun |
20-Jun |
29-Jun |
|
Polemonium
reptans |
Jacob's
ladder |
11-Apr |
27-Apr |
28-Apr |
22-May |
10-Jun |
25-Jun |
7-Jul |
|
Viola
pedatifida |
Prairie
violet |
|
|
29-Apr |
27-May |
|
|
|
|
Zizia
aurea |
Golden
Alexander |
|
8-Apr |
29-Apr |
7-Jun |
5-Aug |
26-Aug |
15-Sep |
|
Dodecatheon
meadia |
Shooting
star |
31-Mar |
30-Apr |
6-May |
3-Jun |
15-Jul |
5-Aug |
15-Aug |
|
Geranium
maculatum |
Wild
geranium |
10-Apr |
29-Apr |
6-May |
10-Jun |
15-Jun |
30-Jun |
|
|
Polygonatum
biflorum |
Smooth
Solomon's seal |
19-Apr |
6-May |
6-May |
31-May |
15-Aug |
29-Aug |
10-Oct |
|
Heuchera
richardsonii |
Prairie
alum-root |
16-Apr |
6-May |
9-May |
|
15-Jun |
30-Jun |
|
|
Podophyllum
peltatum |
May-apple |
12-Apr |
9-May |
11-May |
|
|
|
|
|
Actaea
rubra |
Red
baneberry |
19-Apr |
27-Apr |
12-May |
31-May |
5-Jul |
20-Jul |
7-Aug |
|
Lupinus
perennis |
Wild
lupine |
19-Apr |
9-May |
12-May |
7-Jun |
15-Jun |
25-Jun |
|
|
Senecio
pauperculus |
Balsam
ragwort |
9-May |
|
13-May |
18-Jun |
|
|
|
|
Osmorhiza
longistylis |
Smooth
sweet cicely |
13-Apr |
|
16-May |
13-Jun |
15-Jul |
7-Aug |
28-Aug |
|
Lysimachia
quadrifolia |
Whorled
loosestrife |
3-Apr |
|
18-May |
8-Sep |
10-Sep |
25-Sep |
|
|
Smilacina
racemosa |
False
Solomon's seal |
13-May |
17-May |
19-May |
10-Jun |
15-Aug |
10-Sep |
15-Oct |
|
Taenidia
integerrima |
Yellow
pimpernel |
19-Apr |
15-May |
22-May |
10-Jun |
23-Jul |
6-Aug |
15-Aug |
|
Triosteum
perfoliatum |
Tinker's
weed |
14-Apr |
11-May |
23-May |
1-Aug |
16-Sep |
10-Oct |
26-Oct |
|
Tradescantia
ohiensis |
Common
spiderwort |
26-Apr |
20-May |
27-May |
9-Jul |
2-Jul |
15-Jul |
30-Jul |
|
Asclepias
exaltata |
Poke
milkweed |
|
22-May |
7-Jun |
22-Jun |
29-Jun |
30-Sep |
21-Oct |
Pods
visible long before seeds ready to collect |
Bromus
kalmii |
Prairie
brome |
|
|
9-Jun |
23-Jul |
23-Jul |
15-Aug |
8-Sep |
|
Thalictrum
dasycarpum |
Purple
meadow-rue |
12-Apr |
16-May |
9-Jun |
23-Jun |
5-Aug |
14-Aug |
15-Sep |
|
Echinacea
pallida |
Pale
purple coneflower |
17-May |
7-Jun |
10-Jun |
3-Aug |
24-Aug |
13-Sep |
5-Oct |
|
Heliopsis
helianthoides |
Ox-eye
sunflower |
9-May |
10-Jun |
12-Jun |
21-Sep |
7-Oct |
|
|
|
Asclepias
purpurascens |
Purple
milkweed |
13-May |
19-May |
13-Jun |
9-Jul |
15-Aug |
17-Sep |
16-Oct |
Pods
visible long before seeds ready to collect |
Baptisia
lactea |
White
wild indigo |
2-Jun |
7-Jun |
13-Jun |
2-Jul |
15-Aug |
5-Sep |
10-Sep |
Pods |
Euphorbia
corollata |
Flowering
spurge |
7-Jun |
13-Jun |
14-Jun |
26-Aug |
30-Aug |
13-Sep |
20-Sep |
|
Parthenium
integrifolium |
Wild
quinine |
|
|
14-Jun |
12-Sep |
25-Aug |
15-Sep |
10-Oct |
|
Rudbeckia
hirta |
Black-eyed
Susan |
19-May |
14-Jun |
20-Jun |
8-Sep |
13-Sep |
14-Sep |
23-Oct |
|
Elymus
hystrix |
Bottlebrush
grass |
19-Apr |
20-Jun |
22-Jun |
30-Jul |
5-Aug |
15-Aug |
29-Aug |
|
Ceanothus
americanus |
New
Jersey tea |
29-Apr |
3-Jun |
28-Jun |
14-Aug |
15-Sep |
30-Sep |
5-Oct |
|
Desmodium
canadense |
Showy
tick-trefoil |
15-Jun |
22-Jun |
28-Jun |
30-Jul |
6-Aug |
26-Aug |
14-Sep |
|
Monarda
fistulosa |
Wild
bergamot |
16-Apr |
21-Jun |
28-Jun |
22-Aug |
26-Aug |
15-Sep |
25-Sep |
|
Ratibida
pinnata |
Yellow
coneflower |
19-Apr |
14-Jun |
29-Jun |
20-Aug |
10-Sep |
10-Oct |
27-Oct |
|
Verbena
stricta |
Hoary
vervain |
|
24-Jun |
29-Jun |
21-Aug |
26-Aug |
15-Sep |
10-Oct |
|
Dalea
candida |
White
prairie clover |
|
15-Jun |
2-Jul |
30-Jul |
25-Aug |
15-Sep |
30-Sep |
|
Astragalus
canadensis |
Canadian
milkvetch |
17-Apr |
27-Jun |
3-Jul |
5-Aug |
14-Aug |
15-Sep |
5-Oct |
Pods |
Desmodium
illinoense |
Illinois
tick-trefoil |
8-Jun |
24-Jun |
3-Jul |
28-Jul |
22-Jul |
14-Aug |
22-Sep |
|
Silphium
laciniatum |
Compass
plant |
25-Apr |
16-Jun |
3-Jul |
25-Aug |
8-Sep |
15-Sep |
30-Sep |
|
Verbena
hastata |
Blue
vervain |
|
|
5-Jul |
8-Sep |
25-Aug |
15-Sep |
30-Sep |
|
Campanula
americana |
Tall
bellflower |
19-May |
29-Jun |
6-Jul |
29-Aug |
15-Sep |
5-Oct |
25-Oct |
|
Dalea
purpureum |
Purple
prairie clover |
15-Jun |
29-Jun |
6-Jul |
16-Sep |
25-Aug |
15-Sep |
30-Sep |
|
Silphium
terebinthinaceum |
Prairie
dock |
29-Apr |
24-Jun |
10-Jul |
19-Aug |
7-Sep |
13-Sep |
20-Sep |
|
Veronicastrum
virginicum |
Culver's
root |
17-May |
27-Jun |
12-Jul |
28-Aug |
25-Aug |
15-Sep |
30-Sep |
|
Oenothera
biennis |
Common
evening-primrose |
25-May |
|
13-Jul |
30-Sep |
15-Aug |
15-Sep |
15-Oct |
|
Arnoglossum
atriplicifolia |
Pale
Indian plantain |
14-Apr |
29-Jun |
14-Jul |
10-Sep |
16-Sep |
25-Sep |
4-Oct |
|
Silphium
integrifolium |
Rosinweed |
9-May |
1-Jul |
14-Jul |
26-Aug |
8-Sep |
27-Sep |
14-Oct |
|
Vernonia
fasciculata |
Common
ironweed |
17-May |
21-Jun |
14-Jul |
25-Aug |
25-Sep |
10-Oct |
15-Oct |
|
Eupatorium
purpureum |
Purple
joe-pye weed |
16-May |
15-Jun |
18-Jul |
23-Aug |
5-Sep |
15-Sep |
25-Sep |
|
Liatris
pycnostachya |
Prairie
blazing star |
|
29-Jun |
19-Jul |
15-Aug |
20-Sep |
5-Oct |
15-Oct |
|
Helianthus
divaricatus |
Woodland
sunflower |
20-May |
2-Jul |
23-Jul |
2-Sep |
|
|
|
|
Agastache
nepetoides |
Yellow
giant hyssop |
20-Apr |
6-Jul |
25-Jul |
30-Aug |
25-Sep |
5-Oct |
25-Oct |
|
Andropogon
gerardii |
Big
bluestem |
1-Jul |
15-Jul |
25-Jul |
2-Sep |
20-Sep |
5-Oct |
20-Oct |
|
Aureolaria
grandiflora |
Yellow
false foxglove |
16-Apr |
11-Jul |
26-Jul |
21-Aug |
25-Sep |
15-Oct |
30-Oct |
Pods |
Cirsium
discolor |
Pasture
thistle |
20-May |
19-Jul |
26-Jul |
2-Sep |
12-Sep |
20-Sep |
30-Sep |
|
Allium
cernuum |
Nodding
wild onion |
|
|
27-Jul |
22-Aug |
2-Sep |
15-Sep |
|
|
Eupatorium
sessilifolium |
Upland
boneset |
25-Apr |
20-Jun |
29-Jul |
2-Sep |
25-Sep |
7-Oct |
20-Oct |
|
Sorghastrum
nutans |
Indian
grass |
1-May |
|
30-Jul |
16-Sep |
21-Sep |
7-Oct |
30-Oct |
|
Lobelia
siphilitica |
Great
blue lobelia |
|
27-Jul |
3-Aug |
26-Sep |
3-Jul |
5-Oct |
15-Oct |
|
Aster
sagittifolius |
Arrow-leaved
aster |
14-Apr |
|
5-Aug |
5-Oct |
|
15-Oct |
30-Oct |
|
Solidago
ulmifolia |
Elm-leaved
goldenrod |
|
7-Jul |
5-Aug |
15-Sep |
5-Oct |
20-Oct |
5-Nov |
|
Eupatorium
altissimum |
Tall
boneset |
19-May |
1-Jul |
6-Aug |
15-Sep |
26-Sep |
11-Oct |
26-Oct |
|
Aster
novae-angliae |
New
England aster |
|
|
14-Aug |
5-Oct |
|
15-Oct |
30-Oct |
|
Aster
laevis |
Smooth
blue aster |
1-Jun |
26-Jul |
15-Aug |
5-Oct |
6-Oct |
25-Oct |
5-Nov |
|
Solidago
rigida |
Stiff
goldenrod |
19-Apr |
3-Jul |
15-Aug |
15-Sep |
25-Sep |
5-Oct |
24-Oct |
|
Solidago
speciosa |
Showy
goldenrod |
17-May |
5-Aug |
20-Aug |
26-Sep |
6-Oct |
21-Oct |
7-Nov |
|
Solidago
nemoralis |
Gray
goldenrod |
|
3-Aug |
21-Aug |
21-Oct |
15-Oct |
24-Oct |
5-Nov |
|
Gentiana
alba |
Cream
gentian |
4-May |
14-Aug |
23-Aug |
13-Sep |
|
25-Oct |
5-Nov |
|
Solidago
flexicaulis |
Zig-zag
goldenrod |
21-Apr |
15-Aug |
23-Aug |
26-Sep |
24-Oct |
30-Oct |
5-Nov |
|
Gentianella
quinquefolia |
Stiff
gentian |
3-Jun |
20-Aug |
26-Aug |
21-Sep |
5-Oct |
15-Oct |
30-Oct |
|
Helenium
autumnale |
Sneezeweed |
22-Jun |
21-Aug |
26-Aug |
2-Oct |
25-Sep |
10-Oct |
21-Oct |
|
Aster
lateriflorus |
Calico
aster |
1-Jul |
23-Aug |
30-Aug |
5-Oct |
|
25-Oct |
5-Nov |
|
Prenanthes
alba |
Lion's
foot |
21-Apr |
19-Aug |
8-Sep |
17-Sep |
30-Sep |
10-Oct |
21-Oct |
|
Aster
ericoides |
Heath
aster |
6-Jul |
|
11-Sep |
26-Oct |
|
30-Oct |
5-Nov |
|
Aster
oolentangiensis |
Sky-blue
aster |
17-May |
27-Aug |
20-Sep |
10-Oct |
|
15-Oct |
30-Oct |
|
Helianthus
pauciflorus |
Prairie
sunflower |
27-Jul |
9-Aug |
27-Sep |
|
20-Sep |
4-Oct |
|
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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. |
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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. |
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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.
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