Fire's Effect on Plant-Plant Interactions : Competition and Facilitation

(The New York Times)

Plants interact with other plants just like animals interact with other animals; it just may be a little more difficult for us as human being to notice. There are numerous ways plants interact with other plants, from out putting chemicals to warn of herbivory to creating shade thus microhabitats for surrounding plants. Regardless of the interaction, there are two major categories that these interactions fall into: Competition and Facilitation. Competition is a reduction in fitness due to shared use of resources that are in limited supply (Gurvitch et al 2006). Simply put, there is not enough sunlight, water, and nutrients for every individual plant to thrive. Reversely, facilitation is one plant creating conditions for another plant to thrive. Just as with previous blog posts, these relationships are much more complex than just a simple definition. Fire further complicates these relationships. How exactly does fire complicate competition and facilitation?

Competition

            After a fire, herbaceous growth rapidly begins sprouting in hopes of growing big and tall, but not every seedling will grow to its full potential or even survive. During this stage, sunlight is not a limiting factor, which is why so many seedlings sprout up in the first place. No matter how much sunlight these seedlings have, water and nutrients are not unlimited. This is where competition begins. To further complicate the situation, not only do the herbaceous seedlings want to survive, but also new shrubs and trees. In the long term, trees and other 'climax' vegetation will out compete the herbaceous 'pioneer' vegetation (which is why we have successional communities), but what about the short-term (Green 1989)?

            Fires essentially reset the succession of a community by allowing the herbaceous vegetation to outcompete woody vegetation in the short term (Curtis & Patch 1948). When an area is burned regularly, annual plants are the first to dominate the new landscape (Curtis & Patch 1948). This is because an annual plant's life cycle is to grow as quickly as possible and create as many seeds as possible to be able to continue on as a species. The following years, perennials and forbs will outcompete the annuals (Curtis & Patch 1948). This is due to the slower growth rates and lower number of seed production. Fire's main role in competition is to simply create it, otherwise without disturbance, the entire planet would be old growth trees, because nothing else could get one of the most sought after resources, sunlight. Even if it is a slow process, one studied showed only a reduction of up to 8% in basal area a year, fire aids in herbaceous growth being able to outcompete trees (Peterson & Reich 2001). 

            Every plant species requires different levels of resources or often-different resources entirely, thus fire effects every species different. In a more connected world than ever, invasive species have become a major issue for many communities and ecosystems. A once widely excepted theory was that areas with high native diversity make it more difficult for invasive species to establish or thrive. According to one study, instead areas with high native plant diversity also have high exotic plant diversity (Huston 2004). Huston believes fire, as well as other disturbances, can allow land managers to better control interaction between species and consequences control invasive species. Many invasive species however, thrive and outcompete native plants easier following a fire. A specific example of this is Cheatgrass (Bromus tectorum). The competitive ability of Cheatgrass following a fire disturbance allows it to greatly enhance its capability to exploit soil nutrients and outcompete native grass species (Melgoza 1990). Often timing of burns can still be used to control invasive species, but due to many invasive species being promoted by fire or being fire-tolerant, prescribed fires should only be used by land managers trying to eradicate invasive species very carefully and thoughtfully (Brooks & Pyke 2002). Fire not only promotes some invasive species, but the higher number of invasive species can increase the number of 'natural' wildfires (Brooks et al. 2004). Once the fire regime is increased due to the increase in invasive species, the invasive species can become incredibly difficult to eliminate. For this reason, there are a limited number of early successional ecosystems, such sagebrush, that fire can negatively impact, and other management options should be explored first.

 Figure 1: Comparison of Sagebrush Ecosystem to Cheatgrass (google images)

Facilitation

            Not only do fires promote competition, but they also highlight many examples of facilitation. One of the major examples of facilitation is a nurse plant creating a microhabitat. A microhabitat is simply a differing condition or habitat than the surrounding general habitat. Many post-fire shrublands are overwhelmingly vegetative; facilitation by nurse shrubs is vital and differentially effective for different species of nurse shrubs (Raffaele & Veblen 1998). One study in Florida highlighted fire's positive impact on microhabitats for a specific plant specie, Eryngium cuneifolium. The shrub had a significantly lower mortality rate with more frequent fires (Menges & Kimmich 1996). This was due to plants having a higher survival rate the greater distance the plant was from the nearest shrub. (In this specific ecosystem, shrubs dominated the landscape, so having a cleared patch was actually the microhabitat.) With more frequent burns there were more microhabitats, and the less likely the shrub was to die during a fire. Many species like the Eryngium spp. survival actually rely on fire to create the facilitative environment.

(Menges & Kimmich 1996)

Conclusion

            Plants constantly have to interact with other plants in either competitive or facilitative ways. Fire creates more competition by allowing more sunlight to reach the ground, thus allowing herbaceous vegetation to dominate a landscape. However, often fires can also increase the efficiency of invasive plants to compete for resources, so should be avoided. As well as competition, facilitation can also be greater following a fire. As with everything in nature, most situations are species specific and should be observed and evaluated as such.

           

Literature Cited

Brewer, J. Stephen. (1 February 1999). Short-term effects of fire and competition on growth and 

plasticity of the yellow pitcher plant, Sarracenia alata (Sarraceniaceae). American Journal of Botany. http://www.amjbot.org/content/86/9/1264.short

Brooks, Matthew, Pyke, David. (24 Janurary 2002). Invasive Plants and Fire in the Deserts of North America. http://www.firescience.gov/projects/00-1-2-04/project/00-1-2-04_00_1_2_04_Deliverable_04.pdf

Brooks, Matthew L, et al. (2004). Effects of Invasive Alien Plants on Fire Regimes. Bioscience. http://www.bioone.org/doi/abs/10.1641/0006-3568(2004)054%5B0677%3AEOIAPO%5D2.0.CO;2

Curtis, J., & Partch, M. (1948). Effect of Fire on the Competition Between Blue Grass and Certain Prairie Plants. The American Midland Naturalist.

http://www.jstor.org/stable/2421594?seq=1#page_scan_tab_contents

Green, D.G. Vegetatio (10 April 1989). Simulated effects of fire, dispersal and spatial pattern on competition within forest mosaics. http://link.springer.com/article/10.1007%2FBF00045027?LI=true

Gurevitch, J., Scheiner, S. M., & Fox, G. A. (2006). The Ecology of Plants. Sunderland, MA: Sinauer Associates.

Huston, M. A. (2004). Management strategies for plant invasions: manipulating productivity, disturbance, and competition. Diversity and Distributions .http://onlinelibrary.wiley.com/doi/10.1111/j.1366-9516.2004.00083.x/full

Melgoza, G., Nowak, R.S. & Tausch, R.J. Oecologia (21 July 1990). Soil water exploitation after fire: competition between Bromus tectorum (cheatgrass) and two native species. Oecologia. http://link.springer.com/article/10.1007%2FBF00324626?LI=true

Menges, E., Kimmich, J. (February 1996). Microhabitat and Time-Since-Fire: Effects on Demography of Eryngium cuneifolium (Apiaceae), A Florida Scrub Endemic Plant. American Journal of Botany. http://www.jstor.org/stable/2445937?seq=1#page_scan_tab_contents

Peterson, D., Reich, P. (1 June 2001). PRESCRIBED FIRE IN OAK SAVANNA: FIRE FREQUENCY EFFECTS ON STAND STRUCTURE AND DYNAMICS. Ecological Applications: Ecological Society of America. http://onlinelibrary.wiley.com/doi/10.1890/1051-0761(2001)011%5B0914:PFIOSF%5D2.0.CO;2/full

Raffaele, E., Veblen, T. (October 1998). Facilitation by nurse shrubs of resprouting behavior in a post-fire shrubland in northern Patagonia, Argentina. Journal of Vegetation Science. http://onlinelibrary.wiley.com/doi/10.2307/3237287/full