Fire in Grasslands

It is generally recognized that climate, fire, and grazing are three primary factors that are responsible for the origin, maintenance, and structure of the most extensive natural grasslands. These factors are not always independent (i.e., grazing reduces standing crop biomass which can be viewed simply as a fuel for fire, and biomass is also highly dependent upon the amount of precipitation). Historically, fires were a frequent occurrence in most large grasslands. Most grasslands are not harmed by fire, many benefit from fire, and some depend on fire for their existence. When grasses are dormant, the moisture content of the senesced foliage is low and this fine-textured fuel ignites easily and burns rapidly. The characteristic high wind speeds and lack of natural fire breaks in grasslands allow fire to cover large areas quickly. Because fire moves rapidly and much of the fuel is above the ground, temperatures peak rapidly and soil heating into the range that is biological damaging (>60 °C) occurs for only a short period of time and only at the surface or maybe a few centimeters into the soil. Thus, the important parts of the grasses (roots and buds) have excellent protection against even the most intense grass fires. Fires have been documented to be started by lightning and set intentionally by humans in both tropical and temperate grasslands. Fires are most common in grasslands with high levels of plant productivity, such as tallgrass prairies, and in these grasslands fire is important for keeping trees and adjacent

Raceme

Panicle

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Raceme

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Figure 3 Structure and architecture of the grass plant. From Ohlenbusch et al. (1983).

forests from encroaching into grasslands. Many tree species are killed by fire, or if they are not killed, they are damaged severely because their active growing points are aboveground. Grassland plants survive and even thrive after fire because their buds are belowground where they are protected from lethal temperatures (Figure 4).

The response of grassland species to fire mostly depends upon the production potential of the grassland. In the more highly productive grasslands (e.g., tallgrass prairie), fire in the dormant season (usually right before the growing season) results in an increase in growth of the grasses and thus greater plant production or total biomass. This occurs because the buildup of dead biomass (detritus) from previous years inhibits growth; fire removes this layer. However, in drier grasslands, or even in years in productive grasslands when the precipitation is low, the burning of this dead plant material may cause the soil to become excessively dry due to high evaporation losses. As a result, plants become water-stressed and growth is

Figure 4 Photograph of a spring fire at the Konza Prairie Biological Field Station. The fire in the background is occurring ~2 weeks after the area in the foreground was burned. Photograph by Alan K. Knapp.

1975 1980 1985 1990 1995 2000 Year

Figure 5 Long-term record (26 years) of aboveground net primary production (ANPP) at Konza Prairie Biological Field Station from unburned sites (clear triangles) and annually burned sites (solid circles). The growing season precipitation (AprilSeptember; solid bars) and annual precipitation (clear bars) is also shown.

1975 1980 1985 1990 1995 2000 Year

Figure 5 Long-term record (26 years) of aboveground net primary production (ANPP) at Konza Prairie Biological Field Station from unburned sites (clear triangles) and annually burned sites (solid circles). The growing season precipitation (AprilSeptember; solid bars) and annual precipitation (clear bars) is also shown.

reduced after fire, thus resulting in lower productivity. It is only with long-term data that the true impact of fires on grasslands can be determined (Figure 5).

So what are the mechanism(s) behind the increase in production in mesic grasslands after a fire? One of the most common misconceptions is that fire in grasslands increases productivity by increasing (releasing) the amount of nitrogen (N), a key limiting nutrient in terrestrial ecosystems. Actually, soil N decreases with burning. However, as mentioned above, the primary mechanism by which fire increases production in tallgrass prairie is through the removal of the accumulation of detritus produced in previous years. Standing dead biomass has been reported to accumulate to levels of up to 1000 gm"2 in tallgrass prairie and a steady state is achieved c. 3-5 years after a fire. The specific effects of this blanket of dead biomass on production are numerous and manifest on individual through the ecosystem levels. This detritus may accumulate to >30 cm deep, and this nonphotosyn-thetic biomass shades the soil surface and emerging shoots. This reduction in light available to shoots in sites without fire occurs for up to 2 months and because soil moisture is usually high in the spring, loss of energy at this time is especially critical for primary production. In concert with reductions in light available to the grasses, the early spring temperature environment is much different between burned and unburned sites, with burned sites having a higher temperature favoring the dominant C4 grasses. All of these factors result in less production in unburned tallgrass compared to annually burned prairie (Figure 5). Other evidence that fire does not increase N availability in mesic grasslands comes from N fertilization experiments. Within tallgrass prairie, in annually burned sites, N fertilizer had a strong impact on production, but in sites that have not been burned for several years, additional N did not enhance production and sites with intermediate fire histories had intermediate responses to N fertilization. The results of many studies suggest that one generality regarding grasses and fire is that grasses tolerate fire extremely well and in most cases reach their maximum production in the immediate post-fire years. One qualification to this statement is that the beneficial effect of fire is not uniform across all precipitation gradients. In addition, the growth form type of the dominant grass is also very important. Highly productive grasslands on the high end of precipitation gradients show moderate to high positive response to burning whereas more arid grasslands and some bunchgrass grasslands show reduced productivity in the first few years after fire.

Most grasslands have an active growing season as well as a dormant season. Although fire can occur year-round in many grasslands, fire is most likely to occur during the dormant season and it is most rare in the middle of the growing season during normal (non drought) years. Given the fact that so many aspects of a grassland change during the yearly cycle, it seems fair to expect that a fire in different seasons would have dramatically different impacts. However, in spite of the many studies that have examined the impact of fires at different times of the year, there does not seem to be a general consensus on fire seasonality. Rather, it is probably best to say that grasslands seem somewhat sensitive to 'season of burn'. In one long-term study, it was found that the dominant grass in the tallgrass prairie (Andropogon gerardii) increased with burning in autumn, winter, or spring (dormant season), whereas burning in summer (growing season) resulted in an increase in many of the subdominant grasses with a reduction in A. gerardii.

Research indicates that community structure and ecosystem functioning in grasslands are impacted strongly by fire frequency. Plant species composition, in particular, differs dramatically between annually burned and less frequently burned sites in mesic grasslands. In tallgrass prairie, annually burned sites are dominated strongly by C4 perennial grasses. Although C4 grasses retain dominance at infrequently burned sites, C3 grasses, forbs, and woody species are considerably more abundant resulting in greater diversity and heterogeneity in unburned prairie. In fact, the flora on annually burned sites is a nested subset of that found on less frequently burned areas. Thus, the differences reflect shifts in dominance between frequently and infrequently burned sites, rather than difference in composition per se. Again as with response of production to fire, there appears to be a gradient of response in community structure to grassland fires. In more northern prairies of North America, burning has not been shown to strongly affect community structure. However, these northern grasslands are dominated by C3 grasses, which tend to decrease with burning, unlike the C4 grasses that dominate prairies in warmer climates. Thus, the role of competition and fire in structuring grassland plant communities may increase along a latitudinal gradient throughout the Great Plains.

At a mesic grassland (Konza Prairie Biological Station), a clear picture of fire effects on plant community structure has emerged from the long-term (>20 years) empirical and experimental research done at the site. In the absence of large herbivores, the system is strongly driven by bottom-up forces associated with light, soil resource availability, and differential ability to compete under low-resource conditions. Although light availability increases with burning, the abundance of other critical limiting resources, N and water, declines as fire frequency increases. This is especially true in upland areas (with shallow soils) where production is likely limited by water. These changes in resource availability favor the growth and dominance of a small number of perennial C4 grasses and forbs. As dominance by these competitive species increases, general declines in plant species diversity and community heterogeneity occur.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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