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Responses of tropical native and invader C4 grasses to clipping, fire and increased atmospheric CO2 concentration

Baruch, Z, RB Jackson
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The invasion of African grasses into Neotropical savannas has altered savanna composition, structure and function. The projected increase in atmospheric CO2 concentration has the potential to further alter the competitive relationship between native and invader grasses. The objective of this study was to quantify the responses of two populations of a widespread native C4 grass (Trachypogon plumosus) and two African C4 grass invaders (Hyparrhenia rufa and Melinis minutiflora) to high CO2 concentration interacting with two primary savanna stressors: drought and herbivory. Elevated CO2 increased the competitive potential of invader grasses in several ways. Germination and seedling size was promoted in introduced grasses. Under high CO2, the relative growth rate of young introduced grasses was twice that of native grass (0.58 vs 0.25 g g-1 wk-1). This initial growth advantage was maintained throughout the course of the study. Well-watered and unstressed African grasses also responded more to high CO2 than did the native grass (biomass increases of 21 to 47% compared with decreases of 13 to 51%). Observed higher water and nitrogen use efficiency of invader grasses may aid their establishment and competitive strength in unfertile sites, specially if the climate becomes drier. In addition, high CO2 promoted lower leaf N content more in the invader grasses. The more intensive land use, predicted to occur in this region, may interact with high CO2 to favor the African grasses, as they generally recovered faster after simulated herbivory. The superiority of invader grasses under high CO2 suggests further increases in their competitive strength and a potential increased rate of displacement of the native savannas in the future by grasslands dominated by introduced African species.

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