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Effects of water availability on the competition of Chromolaena odorata and Pan icuin maximum.

Elschot, K. (2006) Effects of water availability on the competition of Chromolaena odorata and Pan icuin maximum. Master's Thesis / Essay, Biology.

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Invasions of species form a major threat to biodiversity. Chromolaena odorata is one of these invasive species, native from Central America, but has invaded most tropical and subtropical area's around the world. In Africa it forms a major problem to ecosystems, lowering biodiversity and blocking the access to water. First it mainly occupies river banks and the edges of woodlands but then slowly invades into the savannas. While invading it's competing with a native grass species, Panicum maximum. Field observations showed establishment of C. odorata if rainfall was high. At low rainfall it stayed restricted to the riverbanks and woodlands. In this study we examine whether the successful spread and invasive capability of C. odorata in savannas depends on the water availability. A competition experiment was set up in the greenhouse in a complete randomized block design. Seeds were used of C. odorata of both Puerto Rico and South Africa to determine any differences between the native and invasive populations and seeds of P. maximum from South Africa. They had to compete in both an additive and a replacement design with low and high water availability. Biomass production was compared between mono and mixed cultures and Relative Crowding Coefficience (RCC) calculated to determine the competitive advantages and aggressiveness of each species. In this experiment P. maximum has an advantage at both high and low water availability, it is more aggressive and out competes C. odorata. It has a better developed roots system, can extract more water and uses it more efficiently compared with C. odorata. C. odorata meanwhile invests more energy in leaves, resulting in a higher Specific Leaf Area (SLA), Leaf Area Ratio (LAR) and Leaf Weigh Ratio (LWR). This would indicate C. odorata is the superior competitor for light in the long term. A possible explanation for the fact that C. dorata is out competed by P. maximum could be the seed mass. Bigger/heavier seeds result in bigger/heavier seedlings. Although P. maximum doesn't ave heavier seeds, it has significantly heavier seedlings. Although C. odorata has a higher LAR and presumably a higher Relative Growth Rate, it needs time to make up for the disadvantage of lower seedling mass. In the experiment P. maximum remains taller and manages to over shade C. odorata during the entire experiment. To have a successful establishment a disturbance, like fire or herbivory, might be necessary during seedling stage, to take away the competition and give C. odorata the opportunity to outgrow P. maximum. In the field they grow under canopy, meaning a light limitation for P. maximum and less nutrients available (during the experiment we added enough nutrients to prevent limitation). Growth of P. maximum is highly dependable on nutrients and will be limited in the field, resulting in a decrease in length. The LAR, LWR and SLA of C. odorata decreases at low water availability, which means to succeed in establishment it needs to have high rainfall next to a disturbance as well.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Biology
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 07:31
Last Modified: 15 Feb 2018 07:31

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