Slide 17: What controls tree mortality?
So, what are the main factors governing tree mortality? For the savanna ecosystem, the first is exceedingly well known, since savannas are one of the globe's prime examples of a fire-driven landscape. The second factor governing mortality is elephants - elephants are capable of knocking trees over in order to get at the foliage or fruits at the top, and typically do so, particularly in marginal habitats where other (lower) food sources are being consumed rapidly by other herbivores. These smaller trees are then susceptible to burning. An emerging result from work by Brian van Wilgen and others is that the fraction of the landscape that burns is almost independent of fire management policy! In general, up to about 140 days of growth in a year, the amount of burning increases in response to the large amount of burnable foliage. However, over 170 growth days per year, this proportion decreases because of the large amount of rainfall and concomitant dampness of the area. In other words, climate plays a much larger role in the extent of burning than does management policies.