The Boundary of Biodiversity: What governs forest expansion into the hyperdiverse fynbos biome in the absence of fire?

Abstract

Fynbos is a heathland-like vegetation type of exceptional plant biodiversity, with an astonishing nearly 70% endemism. It is found only in South Africa, where it exists in an inexplicable mosaic of sharp boundaries between fynbos and indigenous forests. Natural fires are considered to be the main factor controlling the expansion of forests, which do not burn or tolerate fire, into the fire-prone fynbos. Fire cannot be the only controlling factor, however. Soil type, nutrients, moisture, seed dispersal, winds, and human disturbance have all been proposed to govern where forests will establish in the absence of fire, but their relative effects are hard to measure and understand due to the frequent fire regime over the last centuries. The Orangekloof of Cape Town's Table Mountain provides a unique opportunity to study the dynamics between the two biomes in the absence of fire, as it has been protected from fire since 1933, and high-quality aerial photographs of the area are available from every decade since 1944. This study used aerial photographs to evaluate the establishment of trees into fynbos with relation to the forest edge. An unequal rate and concentration of forest expansion were found in different parts of the study site, and increased distance from a forest patch related to an approximately exponential decrease in establishment likelihood. Soil and plant leaf samples from fynbos and forest areas showed marked differences between the two. Sampling of transitional areas, which have experienced a large increase in tree cover over the past 70 years, and ones that have not been colonized by forest species to the same extent, did not show any significant differences in soil and plant leaf constitutions, except for N isotope ratio in plant tissue, which might hold one of the keys to understanding what dictates how quickly an area will be overtaken by forest.