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Submitted to: The Progressive Rancher
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 8/27/2015 Publication Date: 9/7/2015 Citation: Harmon, D.N., Clements, D.D. 2015. The importance of seed germination in rangeland research. The Progressive Rancher. 15(7):24-25. Interpretive Summary: The USDA, Agricultural Research Service, Great Basin Rangelands Research Unit, Wildland Seed Laboratory, located in Reno, NV, has been studying seed germination for the past 40 years. The wildland seed laboratory has collected, processed, and quantified germination characteristics of hundreds of native and introduced rangeland plant populations. The research laboratory uses multiple large incubators to test seed germination at multiple temperatures representative of Great Basin seedbed temperatures. By mimicking the day and night fluctuations of seasonal temperature regimes we are able to determine optimal germination temperatures and seasons for a particular species. Generally, successful plants often have a wider range of germination temperatures. There has probably been no species studied more for germination in the Great Basin than cheatgrass. This exotic invasive annual weed has contributed to the loss of millions of acres of habitat, by providing fine-textured early maturing fuels that increase the chance, rate, season and spread of wildfires. This fuel is coupled with its strong competitive inherent ability to limit the recruitment of native plants. Like many annual plants it has a “bet hedging” seed strategy where seeds can be non-dormant and germinate after the first rain or they can be dormant, where even with moisture, the seed will not germinate. Seed dormancy insures that there will be seed carried over in the soil to the next year or longer, and in the case of cheatgrass, can lead to 100s of seeds per square foot in the soil seed bank. Cheatgrass has developed increased “bet hedging” (seed dormancy) in populations from “riskier” habitats. Populations from drier hotter environments, salt deserts, show greater seed dormancy, quicker flowering, and greater seed production, than populations from less risky environments. Some annual plants like Russian thistle and tumble mustard take advantage of highly variable weather events. These plants do not express seed dormancy and will germinate rapidly after any summer rain. Unlike annuals, most native perennial grasses do not rely on seedling establishment every year nor do they build seed persistent seed banks. However, like most things in nature there are exceptions to the rule. Indian ricegrass, a native perennial bunchgrass, has the inherent ability to produce a variety of phenotypic seeds from big and brown, to small and naked with no covering. The naked seeds are not dormant, but the dark brown seeds are strongly dormant in order to maintain a seed bank. In favorable years the dormant seeds can break dormancy, germinate and provide a flush of new seedlings. Many physical properties of the seed can affect germination. Hard seed coats inhibit moisture uptake and require scarification, where the seed coat is partially broken to allow the seed to imbibe moisture. Some seeds, often mustards, can produce a mucilage layer that promotes water uptake. The sticky mucilage can also aid in dispersal by sticking to animals or other objects. Seed hairs and root radicle modifications can also improve moisture retention, surface soil attachment and wind dispersal. Some seed dispersal appendages, such as the hairs on winterfat seed, make rehabilitation seeding efforts very difficult because the seed cannot readily pass through seed drills. Applying the knowledge of the physical seed and germination properties can improve management efforts in order to preserve a sustainable rangeland resource. Technical Abstract: The USDA, Agricultural Research Service (ARS)/ Great Basin Rangelands Research Unit, Wildland Seed Laboratory, located in Reno, NV, has been studying seed germination for the past 40 years. The wildland seed laboratory has collected, processed, and quantified germination characteristics of hundreds of native and introduced rangeland plant populations. The research laboratory uses multiple large incubators to test seed germination at multiple temperatures representative of Great Basin seedbed temperatures. By mimicking the day and night fluctuations of seasonal temperature regimes we are able to determine optimal germination temperatures and seasons for a particular species. For each standardized test 5,500 seeds are used. Moving seeds in germination petri dishes from warm to colder incubators during the day and night allows us to test 55 separate constant and fluctuating 24 hour temperature regimes. For example, one set of seeds may be in 8 hours at 68F during the day and then 16 hours at 41F for the night. By testing all 55 different temperature combinations from 32F to 104F (9F increments) we can provide a standardized germination temperature profile. Generally, successful plants often have a wider range of germination temperatures. However, to germinate is not always in a seeds best interest. Seeds can maintain a state of dormancy where germination only occurs after environmental cues initiate it. Cheatgrass is a widely studied species in the west. Cheatgrass has a “bet hedging” seed strategy where seeds can be non-dormant and germinate after the first rain or they can be dormant, where even with moisture, the seed will not germinate. Seed dormancy insures that there will be seed carried over in the soil to the next year or longer which can lead to 100s of seeds per square foot in the soil seed bank. The ability of cheatgrass to build persistent seed banks allows this species to “mine the site” until the right opportunity comes along to dominate the site. Seed dormancy also mitigates the risks of seedling death in highly variable environments that can go from warm to cold and wet to dry quickly. Some annual plants like Russian thistle and tumble mustard take advantage of highly variable weather events. These plants do not express seed dormancy and will germinate rapidly after any summer rain. Even though summer months are hot and dry it is an advantage for poor competitors like Russian thistle and tumble mustard to germinate after summer rains, because most strong competing winter annuals like cheatgrass have completed their life cycle by this time. Unlike annuals, most native perennial grasses do not rely on seedling establishment every year nor do they build seed banks that persist for multiple years. However, like most things in nature there are exceptions to the rule. Indian ricegrass, a native perennial bunchgrass, has the inherent ability to produce a variety of phenotypic seeds from big and brown, to small and naked with no covering. The naked seeds are not dormant, but the dark brown seeds are strongly dormant in order to maintain a seed bank. In favorable years the dormant seeds can break dormancy, germinate and provide a flush of new seedlings. Big sagebrush seed does not aquire dormancy which makes sagebrush seeding efforts critical to restore plant populations after a fire. Sagebrush seedlings have a modified root with a barbed tip to anchor the small seedling to the soil. It is important to avoid seeding big sagebrush on top of snow, because it often does not allow the radicle to anchor to the soil. There are mechanisms that can decrease seed dormancy such as stratification, where seeds germinate after being exposed to cool moist conditions. From a plants standpoint, this insures seeds will germinate during cool wetter months. For example, antelope bitterbrush, an important shrub species, |
