by Leta Ames
It is well known that fire can play a crucial role in the reproduction and development of plant populations. The availability of water and CO2 also impact plant growth, especially of larger species. It is believed that the interactions of climate, fire, and CO2 greatly influence the shift between savanna and tropical forest ecosystems and their permanence thereafter. Previous research has relied on data collected from intact tropical forests, but although useful, these data only provide a snapshot of the impact of CO2, fire, and climate on these ecosystems. To gain a better understanding of what factors influence tropical ecosystems Shanahan et al. (2016) used the concentrations of carbon and hydrogen stable isotopes from sedimentary leaf wax n-alkanes (δ13Cwax and δDwax) and the frequency of charcoal layers from sediment obtained from Lake Bosumtwi in Ghana to construct a history of changes in vegetation and hydrology, as well as to estimate the annual fire frequency.
Lake Bosumtwi is in lowland tropical forest, far south of the current savanna-forest edge. The sediment offered a 28,000-year record of hydroclimate and fire frequency of the forest that surrounds the lake. Based on the sediment record, climate, fire frequency, and the dominant ecosystem have shifted significantly in the last 28,000 years. At the beginning of the investigated timeframe (15,000 ̶ 28,000 year BP) the δDwax values were positive and δ13Cwax values were higher than current levels— indicating a more arid landscape and that drought-tolerant grasses dominated over woody species. The frequency of charcoal layers indicated that fires were more frequent. Nearly annual burns keep woody species from advancing, because stems burned before they were large enough to withstand fires. After 15,001 year BP, more humid conditions with higher precipitation developed, evidenced by decreasing δDwax values. During this time the frequency of fires decreased as well. These declines were gradual, but the δ13Cwax values indicate that the proportion of woody plants changes by ~25% over <150 years— a drastic change. In the remainder of time before present day the proportion of grasses and woody species was variable and mixed. Throughout most of the 28,000 years it appears that precipitation controlled the proportions of grasses and woody species. However, in more recent periods drastic changes in δDwax values were accompanied by minor changes in δ13Cwax values, suggesting that once established, the tropical forest ecosystems could persist in more arid climates. As suggested by other recent studies, the changes in vegetation dominance are the result of interactions between CO2 concentration, climate, and fire.
Shanahan et al. also propose a threshold to explain abrupt vegetation change.
The threshold would explain the time lag between changes in hydroclimate and a shift from highly precipitation-dependent vegetation to a system resistant to changes in precipitation. However, not all the changes in vegetation throughout the timeframe support the threshold explanation. Once woody plants dominated, decreased fire frequency, rising atmospheric carbon, and vegetation only responding to precipitation on the centennial to millennial timescale characterized periods without abrupt changes. This is likely because higher CO2 concentrations promote woody species growth rather than grass growth. Also, woody forests can resist colonization by grass species due to their shaded canopy.
Overall, the impacts of climate, CO2, and fire shift depending on the role of the other factors. Increased levels of CO2 were shown to function as a buffer— stabilizing woody forest vegetation— however as the levels rise the stabilizing effects decrease. As the world’s climate changes, West Africa could become more arid. If the changes in hydroclimate and fire overcome the stabilizing effects of CO2, the region could quickly transform from a mosaic of savanna grassland and tropical forest to completely savanna.
Shanahan, T. M., Hughen, K. A., McKay, N. P., et al. 2016. CO2 and fire influence tropical ecosystem stability in response to climate change. Scientific reports, 6.