In a recent Texas Range Webinar, Morgan Treadwell, Texas A&M AgriLife Extension range specialist, revealed a new approach to range management through a better understanding of how native grasses grow. She discussed how an understanding of the reproductive and growth methods of native grasses helps improve management decisions.
Experienced range managers can determine plant composition and forage density by looking across pastures, but plant underground components are often overlooked. Underground plant structures are responsible for what occurs aboveground and provide the science for grassland management.
The functional understanding of plant underground processes and components has not progressed as rapidly as our knowledge of aboveground plant structures. Research has shown that perennial grasses reproduce by vegetative processes through asexual reproduction. The contribution of seed to maintain an established grassland is less than 1 percent. Treadwell noted that this is contradictory to earlier range science research that suggested seed head formation was necessary for perennial grass reproduction.
Bud bank deposits
Plant components within a few inches below the soil surface play a major role in maintaining reproduction and density of every native grass species. Research, beginning in 2000, showed vegetative buds at or beneath the soil surface are responsible for plant reproduction. The amount of buds that exist on a single grass plant is called the bud bank, and in 2004, research documented that more than 99 percent of new tillers are produced from this bank.
Through evolution, bud banks developed in the soil to facilitate regrowth after top growth is destroyed by fire, grazing, or drought. Variances in bud banks occur due to differences in plant photosynthetic pathways (cool- versus warm-season grasses) and growth forms such as bunch grasses versus rhizomatous plants.
Vegetative buds are produced by meristematic tissue that exists in each junction of a leaf and a stem (also called a tiller). In plants, the meristem is an area of tissue from which new growth is formed. Vegetative bud functions are very complex, yet research continues to uncover more information on their function in perennial grass growth processes and how these processes differ among perennial grass species.
Bud banks contain three different types of buds — active, dormant, and dead. Active buds are resources for reproduction, but they require an environmental impulse such as rain, fire, or grazing to initiate tiller growth. Vegetative buds can begin tiller growth in as little as 24 hours after receiving an impulse. These new tillers grow new buds, which replenish the bud bank.
Dormant buds perform like a savings account; they become an active bud depository when stimulated by a disturbance such as fire or grazing. A second impulse mobilizes them to produce tillers and enter the bud replenishment cycle. Dormant buds can live six to 10 years or even longer.
Dead buds do not contain meristem, and as a result, they can never become activated. If a plant has too many dead buds, it becomes meristem limited. When too many plants are meristem limited, that particular species disappears from the plant community. Disappearance may be temporary or permanent depending upon management decisions.
Most growth from buds
Perennial grasses have the ability to produce by both buds and seed even though greater than 99 percent of new tiller growth comes from the bud bank. Buds are long-lived with a life of up to five years or greater, but seeds are short-lived at one year or less. Bud life varies among grass species, and research is currently focused on determining which species have the longer living buds. Vegetative buds respond quickly to environmental change, and examination of the bud bank provides an indication of future plant community composition.
Seeds possess more genetic variability than buds and have
the ability to travel long distances. A disadvantage is the increased mortality of seeds in relation to vegetative buds, which makes perennial grass more difficult to establish by seeding. Even so, establishment of perennial grass from seed has been more heavily researched than establishment by vegetative buds.
Research has revealed differences in bud banks between cool-season and warm-season grasses. Cool-season grasses have a smaller bud bank, and the buds are short-lived at one to two years. The bud banks are almost entirely depleted during a growing season and are sensitive to environmental conditions. Because of their bud bank characteristics, cool-season grasses are prone to disappear from the plant community when environmental conditions are unfavorable for plant growth.
Warm-season grasses have an extensive dormant bud bank, and since the buds are long-lived, they are multi-aged. Due to these characteristics, warm-season grasses can respond quickly and positively to rainfall. In addition, warm-season grasses are better able to sustain themselves through dry periods than are cool-season species.
Seed is still important
Tim Steffens, an extension range specialist at West Texas A&M University, offers the following on the Texas A&M “How Grasses Grow” website (agrilife.org/howgrassesgrow): “Quantifying and describing bud bank densities for dominant grasses will greatly improve the ability to apply appropriate range management strategies. For example, bud banks of
various perennial grasses are affected differently by the season in which natural or prescribed burns occur and the duration and intensity of fire intervals and grazing timing. Employing strategies that maximize bud bank densities is paramount in maintaining healthy native grass populations, plant diversity, and plant community resiliency.”
Steffens continues, “This does not mean, however, that allowing plants to periodically produce seed is unnecessary. What it does mean is that vegetative reproduction is normally the most rapid method of increasing stand percentage of a preferred species. Perennial native grass plants should produce seed often enough to replace dead plants, ensuring a plant population with high vigor and a viable seed bank in the soil.”
Bud banks are assessed by digging a clump of grass and carefully separating the tillers. Buds at the bottom of the tillers are then stained with dye, which causes active buds
to turn red, leaving the dormant buds in their natural colors. The plants are then placed under a microscope and the dormant buds are counted.
Currently available data on bud characteristics and densities of some dominant grasses are shown in the table . An understanding of bud bank influences on aboveground growth habits of the listed grasses is easily seen by studying the data. For instance, KR bluestem — a prolific invader — easily forms monocultures and has from 12 to 22 buds. Research has shown that any type of disturbance will activate this plentiful bud supply to develop new tillers.
This article appeared in the January 2020 issue of Hay & Forage Grower on pages 16 and 17.
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