Changes in Growth Rates of Invasive and Native Plant Species in Response to Elevated CO2 Levels

Rapidly increasing global CO2 levels will have profound effects on the growth of many plant species by directly impacting photosynthetic processes.  Purnima Raizada and her colleagues investigated whether invasive and native dry deciduous species of India respond differently to elevated CO2 levels.  They grew seedlings of two invasive and four native plant species under ambient and elevated CO2 levels and compared the growths of each species.  They found that growth response to elevated CO2 levels varied among species but biomass, relative growth rate, and net assimilation rate of invasive species were higher than those of native species. .— Sanami Nakayama 
Raizada, P., Singh, A., Raghubanshi, A.S., 2009. Comparative Responses of Seedlings of Selected Native Dry Tropical and Alien Invasive Species to CO2 Enrichment. Journal of Plant Ecology 2, 69–75.
The researchers used the invasive species Lantana camara and Hyptis suaveolens, the two most important invaders in dry deciduous forests of India.  They used the four native species Acacia catechu, Bauhinia variegate, Dalbergia latifolia, and Tectona grandis.  They planted seedlings of each species under ambient (375–395 µmol/mol) or elevated CO2 levels (700–750 µmol/mol).  To expose plants to elevated CO2, the researchers used decomposed manure as a source of CO2 and grew the seedling in trenches that were covered with polythene frames.  Seedlings grown under ambient conditions were planted in trenches that were left uncovered, without organic matter.  The researchers used three plants of each species under each treatment.
Initially, before being grown under a treatment, randomly chosen plants of each species were harvested to measure initial growth data.  After 60 days of exposure to ambient or elevated CO2, the researchers measured biomass partition parameters (root shoot ratio, root mass fraction, stem mass fraction, leaf mass fraction, and leaf area ratio) and growth parameters (specific leaf area, relative growth rate, and net assimilation rate) of each plant.  These parameters were then used to compare biomass accumulation and growth of each species. 
Growth performance of seedlings in response to elevated CO2 levels differed across species.  Elevated CO2 significantly promoted growth of seedlings of all six species, but increase in height was greater for the invasive species than the natives.  The invasives also accumulated more biomass under elevated CO2 compared to the natives.  Net assimilation rates and relative growth rates were also higher in the invasives.  The researchers suggest that higher relative growth rates in the invasive species will give them relative advantage over the natives and may help them become competitively dominant under different environmental stresses. 
The results of this study show that as global CO2 levels increase, we can expect the invasive species L. camara and H. suaveolens to perform better than many native species in dry deciduous forests of India.  The invasive plants may become competitively dominant, changing competitive hierarchies and the structures and functions of these forests.  Management of the invasives may become more difficult as CO2 levels increase because of their rapid growth and adaptability under these conditions.  

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