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Natural History of Zizania palustris




Life History
Wild rice (Zizania palustris) is an aquatic annual grass which grows from seed each year. This herbaceous plant resembles other grasses and similar shoreline plants like the bulrushes and sedges, but is also unique in a number of ways. The plant has a terminal panicle, and is composed of two spikelets: the female flowers produce the grain and the male flowers produce the pollen. The stem of the plant is stout (from .25 to 2 inches) and erect, hollow, and at the nodes are divided by walls and diaphragms. The leaves are large and flat, developing from the nodes. From the first node, they function at the floating leaf stage and the subsequent leaves are erect, up to 111.5 cm in length and less than 1 cm to 6 cm in width. Tillers are clusters of branches formed at the first and second nodes. Internodes are at the location of tiller and leaf formation. The roots are shallow and plants possess only one at germination. Other roots arise at nodes, the first set are normally short and thick and ones that develop later are higher on the stem and are long and fibrous as they are mainly there for anchoring and not the uptake of nutrients.

The life cycle of wild rice is separated into a number of distinct stages based on morphology: germination, submerged-leaf stage, floating-leaf stage, aerial-leaf and tillering stage, flowering stage, seed formation stage, seed shatter stage, and dormancy.

In the spring, seeds buried in the sediment from previous seasons germinate when increases in water temperature and surrounding sediment signal a lessening of germination inhibitors (Atkins 1986), optimally between 12 and 25o C. Faster germination occurs at the upper end of the range. The earlier the seedlings are established, the better they can compete with other aquatic macrophytes for light and nutrients later in the growing season. When anaerobic conditions are present, germination is very successful. In very flocculent sediments, the less anaerobic lake conditions have been associated with sporadic germination patterns (Atkins 1986).

The submerged-leaf stage is initiated by the formation of a roseate of leaves that are submerged, and lasts up to six weeks. There can be from one to four leaves formed on average, with the greatest numbers found in deep water. Opposite from seed germination, the growth from these undifferentiated leaves is impeded by low oxygen concentrations. Seedlings can survive short periods of anaerobic conditions, but extended periods below 5ppm can kill the plants. The seedlings that form in these anaerobic conditions are poorly rooted in the flocculent sediments. The boom and bust annual patterns are thought to be particularly influenced at this stage in the plant’s life cycle. The straw mat that forms after a very productive year can inhibit seedling development due to the oxygen demand required for the decomposers of the detritus. Water temperatures and light availability are two other specific needs that must be met for optimum growth to occur. Below 150C growth rates are low. In stained or deep water, and in cloudy conditions during this life stage, productivity at maturation is greatly reduced. (Atkins 1986).

In late June, a shallowly rooted cluster of ribbon-like leaves float on the water surface during the floating-leaf stage of the plant’s life cycle. There are usually two to four leaves produced at this juncture in the life cycle. The plant’s greatest photosynthetic activity is at this time because there is a large degree of growth put into the development of the newly formed shoot (Atkin 1986). It is during this stage that the plant is most vulnerable to disturbances, including: rapid changes in water levels, carp and crayfish activities, extended high water periods, and wave or wind action. These types of stresses can result in either less robust mature plants with lower productivity, or high mortality rates (Fannucchi 1986).





The aerial-leaf stage begins when the upright blossom stalks emerge from the water, normally in early July. An increase in lignin causes these stalks and their associated leaves to be more fully extended from the water and to be more resistant to changing environmental conditions (Atkins 1986). Frequently one seed will produce more than one blossom stalk, which is referred to as tillering. Tillering is light induced (Lee 1979, Lee 1986), and increases the yield of the plant and the productivity of the bed. Each panicle formed at the top of each main stalk or tillered stalk is composed of two parts. The female floret is clustered in a spike at the top of the flower head that produces the grain. These remain attached to the plant until the seeds are ripe. The male floret is spread along short branches below and produce the pollen. These fall off the plant once they have shed their pollen. Cross-pollination is guaranteed through this positioning (Atkins 1986, Dore 1969).

Twenty four hours after the female has been fertilized by the male pollen, the stigmas wither and the ovaries swell and fill the hull with the embryo that will become the seed. The seeds ripen from the top down, unevenly across the stand. At first, the grain is a soft light green and graduates to a semi-firm olive color into the fully mature hard dark brown or black kernel in the course of four weeks. When the seeds are mature, the hulls with enclosed grains fall into the water over a period of up to two weeks. This does not happen uniformly across the stand. When the seeds fall, they are orientated by a rudder like awn and an imbalance of weight. This ensures that the seed is deposited in the proper growth medium close to the parent plant and that it embeds in the soil in the correct direction.

After falling from the parent plant, the seeds undergo dormancy through the following winter, controlled through a relationship between low temperatures and an imbalance of growth regulation hormones (Atkins 1986). They then experience what is called after-ripening, characterized by a lessening of germination inhibiting hormones and an increase in of growth initiating hormones. This loss of dormancy may not occur after one over-wintering season, but may require up to five winters for germination to occur. This secondary dormancy is believed to be caused by sediment temperatures above or below the optimum. According to Atkins (1986), it is believed that 10% of a season’s seed needs more than one winter to germinate, and as much as 40% may need two seasons. Regional experts (unpublished conversations with Jim Meeker, 1996 and Peter David, 1999) believe that about 70% of each years seed germinates the following year. This depends primarily upon the environmental conditions surrounding the seed. This pattern helps sustain the life of wild rice stands by providing a seed bank to draw upon following a bust crop where there are few viable ripe seeds to layover for the next spring.

Ecology
Zizania palustris is found throughout the eastern half of the United States and southern Canada, and is mostly abundant in Northern Minnesota and Wisconsin. The three most important factors regulating wild rice productivity are water depth, competition, and the nutrients associated with the sediment of the body of water. Also important to the health of the plants are water flow, water clarity, water fluctuations, and the weather patterns of a particular season.

Wild rice does best in areas with continuously flowing waters, and is most successful if there are periodic floods season to season to rid the area of competitive aquatic perennials. Optimum water depth for wild rice is between ½ and 3 feet of water. In shallower waters, tillering is more predominant and reduces the impact that plant density has on the number of seed heads produced in an area. In clear water, tillering can develop at depths up to 3 feet, while in darkly stained waters, it does not occur in depths greater than ½ foot. The time it takes for ripening lengthens with an increase in water depth. Plants that grow in deeper water are generally thin, elongated, single stemmed, widely spaced, and later flowering (Atkins 1986, Dore 1969).

Areas that have little disturbance find that the wild rice is often out competed by perennials. Gradual changes throughout the season are not detrimental to the plants, it is rapid fluctuations from flooding or wave action that cause the most damage. Increases of 6 inches of water during the growing season can reduce productivity by half (Atkins 1986, Dole 1969, Fannuchhi 1986). If the water levels rise during the floating leaf stage, while the roots are small, the plants can easily become uprooted from the soft sediment by the buoyancy of their floating leaves.

Wild rice enjoys clear, non-turbid waters, and will not compete strongly with aquatic perennials that are better equipped to deal with turbidity or staining, like the cattail or pickeral weed. Wild rice absorbs its nutrients from the soils in which it is rooted, and the most productive stands are found in sediments characterized by a layer of between 6 inches to 3 feet deep black mucky sediment overlying a limey inorganic sediment that supplies appropriate levels of nitrogen, phosphorus, and potassium. In addition, because wild rice is mostly wind pollinated, it requires mild summer winds for maximum crop production. Productivity decreases in seasons punctuated with long calm stretches or frequent high winds.

add text referring to the role palustris plays in the landscape ie: wetlands functions such as filtering and nurseries for walleye, habitat for red winged blackbirds etc...