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One of the largest problems facing land management and land restoration agencies is the invasion of ecosystems by non-native species (Hobbs and Humphries 1995; Morgan 1994; Westman 1990). For example, invasion of grasslands by diffuse knapweed (Centaurea diffusa) is a large problem in Boulder Open Space. To find a way to protect areas like this against invasion, we must first understand the processes that make an area susceptible to invasion. Two processes that contribute to such susceptibility are disturbance and nutrient additions (Hobbs and Humphries 1995; Cowie and Werner 1993; Lodge 1993; Hobbs and Huenneke 1992; Huenneke et al. 1990; Westman 1990; Hobbs 1989). While each is quite powerful at enabling invasion on its own, the combination of the two processes makes invasion even more likely (Hobbs 1989). Disturbance leading to secondary succession is often characterized by a rapid increase in soil nitrogen followed by a gradual decrease (Vitousek et al. 1989). As some disturbance is inevitable, completely stopping disturbance is not a practical land management objective. Instead, to combat invasion, we may want to consider strategies that lower elevated nutrient levels resulting from nutrient availability responses to disturbance and decomposition of weedy species and speed the process of recovery from these disturbances.
Examining the characteristics of invasive species offers further insight into techniques that may stop invasion. Many of these invasive species are colonizing species that are capable of living in highly disturbed habitats (Hobbs and Huenneke 1992). Many are early seral species with these characteristics: high population growth rates, short generation times, abundant seed production, and very effective seed dispersal strategies (Rejmánek and Richardson 1996; Lodge 1993; Westman 1990). Therefore, the factors that favor native species over these invasive species are the same factors that favor late seral species over early seral species, such as ability to persist at low nutrient levels and capacity to out-compete other species under such conditions (Redente et al. 1992; Wedin and Tilman 1990).
Reducing soil nitrogen generally decreases plant biomass, particularly the biomass of invasive and early seral species (Redente et al. 1992; Huenneke et al. 1990). Studies testing the relationship between nitrogen level and biomass in grasses and forbs have reported different results depending on whether the forbs and grasses are early seral species, invasive species with other growth strategies, or native species. Native grass species adapted to low nitrogen environments generally show less of a biomass response when given additional nitrogen (Seastedt et al. 1991; Morgan 1994). However, exceptions include the C3 grass Western wheat (Agropyron smithii), which has been shown to have a lower biomass in lower nitrogen plots (Hunt et al. 1988). Studies looking at responses to higher nitrogen levels show differences in response depending on plant type and location. Native forb species found in serpentine grassland decrease in biomass in response to higher nitrogen levels because higher nitrogen levels increase competition from non-native grasses (Huenneke et al. 1990). In contrast, native forb species in tallgrass prairie tend to increase in biomass in response to higher nitrogen levels (Seastedt et al. 1991). Redente et al. (1992) studied the effects of fertilization on forbs, grasses, and shrubs, and found the greatest biomass increase in early seral forbs and grasses. In general, the biomass of early seral species and invasive species has a high dependency on nitrogen while the biomass of native and late seral species has a low dependency.
Soil carbon amendments are being studied as a means to lower soil nitrogen levels and favor later seral species. Carbon amendment treatment involves the addition of organic matter -- such as sugar, sawdust, straw or grain hulls -- that is high in carbon and low in nitrogen to the soil of an experimental site (Morgan 1994). The addition of carbon stimulates soil microbe growth, and the soil microbes accumulate soil nitrogen in their biomass which makes it unavailable to plants (Morgan 1994; McLendon and Redente 1992; Hunt et al. 1988; Vitousek 1982). Because native plants and late seral species are more able to out-compete invasive species and early seral species in the resulting low nitrogen environment (Redente et al. 1992; Wedin and Tilman 1990), the nitrogen-lowering effects of carbon amendment treatment can help speed the rate of succession in disturbed areas by countering the nutrient increases due to disturbance and by favoring late seral species.
Previous studies conducted using carbon addition have met with varied success in preventing invasion or decreasing populations of invasive and early seral species. Two studies used a combination of sawdust and sugar and examined the effectiveness of carbon amendments as a land restoration tool to control invasive species. In the first study, Morgan (1994) mixed carbon amendments into the soil of three sites and planted native grasses and forbs. He found a significant decrease in weed production in one site and no effect on two other sites, which had higher nitrogen soils. Negative results may have been due to insufficient carbon addition to significantly lower soil nitrogen levels, or the treatment may not have lowered soil nitrogen levels quickly enough in high nitrogen soils to affect that season's weed production. In the second study, Seastedt et al. (1996) applied carbon amendments to the surface of disturbed soil and studied the effects of carbon amendments on densities of native and invasive plants. They observed a significant decrease in the density of one of the invasive species but no significant effect on another invasive species or on a native species. Similarly to Morgan's study, they may not have added enough carbon to counter the original high soil nitrogen levels. Both results imply that carbon amendments can help fight against invasive species, but that land managers must carefully assess the amount of carbon needed for control.
The remaining two studies focused on the effects of nitrogen-lowering treatments on succession and decomposition. McLendon and Redente (1992) treated plots with nitrogen addition, no treatment, or sucrose addition to study the effects of a nitrogen gradient on succession. They observed a faster rate of succession in the lower nitrogen plots and concluded that this occurred because the lower nitrogen levels were insufficient to support the biomass of early succession plants. In the second study, Hunt et al. (1988) also treated plots with nitrogen addition, no treatment, or sucrose addition to study the effects of a nutrient gradient on decomposition. They found a decrease in decomposition in lower nitrogen soils and noted that prairie plots with added nitrogen were invaded by Canadian thistle (Cirsium canadensis) while the control plots and the plots with added sugar were not invaded. Both of these studies show the effectiveness of lowering soil nitrogen at lowering populations of invasive and early seral species, but neither proposes addition of carbon as a land management technique.
The nitrogen-lowering effects of carbon amendments also can decrease invasion by increasing species diversity. Nutrient addition decreased species richness in Californian serpentine grassland (Huenneke et al. 1990). A study by Mountford et al. (1996) observed a decrease in species diversity in conjunction with increased nitrogen levels due to fertilization. After three years without fertilization, N-intolerant species increased and N-demanding species declined, but the diversity of these plots was still lower than the diversity found in plots that had never been fertilized. A 25-year study by Willems and van Nieuwstadt (1996) showed that when fertilization ceased, biomass decreased and species number and diversity increased. Species number increased for 16 years and then declined slightly while species diversity increased for 13 years and then began to decrease. Willems and van Nieuwstadt concluded that these decreases occurred because, even 25 years after fertilization, vegetation development still had not stabilized. We learn from these studies that any nitrogen-reduction treatment will increase diversity, but responses to this treatment are gradual.
Scientists looking at the effects of species diversity on invasion by weedy species have largely reported decreasing invasion in more diverse communities. Studies show that plant communities with fewer species are more prone to invasion and are more likely to be changed by invasion (Lodge 1993). Disturbance which removes species lowers competition and allows species invasion (Hobbs and Huenneke 1992). Cowie and Werner (1992) also found that Australian wetland and margin areas, which had high nutrient levels and low species diversity, tended to be more prone to invasion. In contrast, a plant community with high plant density tends to fend off invasion. D'Antonio (1993) found that interference by existing species suppressed invasion of an exotic species, Carprobrotus edulis, by restricting the growth of its seedlings. These studies suggest that a process that increases plant species diversity and number will decrease the likelihood of invasion.
Published research consistently shows that addition of nitrogen to a wide range of habitat types is able to lower species diversity. However, studies like those I have mentioned above, that test whether nitrogen removal or nitrogen reduction can speed succession and/or maintain dominants, have not been conducted in a variety of habitats. Here, I tested the hypothesis that additions of carbon will reduce the amount of inorganic soil nitrogen available to plants. This lower nitrogen availability will have a greater effect on annual or semelparous perennial species, such as alyssum (Alyssum minus) and diffuse knapweed (Centaurea diffusa), than on the native perennial grasses, such as Western wheatgrass (Agropyron smithii). This should increase the populations of native grasses over those of invasive weeds, in particularly the semelparous perennial diffuse knapweed.
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