The physical, chemical and environmental conditions in open cast limestone mines present extreme challenges to the establishment of tree cover, and the eventual return of such sites to original forest ecosystems. Natural events, such as the eruption of Krakatoa in 1883, demonstrate that re-establishment of tropical forests is possible through natural succession, even under the harshest of conditions (Whittaker et al., 1989) but may take a hundred years or more. Forest restoration seeks to enhance, accelerate and direct such natural process, such that such “natural-looking” forests can be restored to limestone mines within reasonable timeframes acceptable to the legal requirement of mine companies to restore natural vegetation, once mining has ceased.
Limestone mines represent “Stage 5 Degradation”, the most extreme form, illustrated overleaf, with extremely poor substrate conditions and no natural sources of forest regeneration (i.e. seed bank, seedlings or live tree stumps) (Elliott et al., 2008). Plants are completely exposed to strong sunlight, causing desiccation and extreme temperature fluctuations between night and day. Since the substrate is largely impermeable, water accumulates on the surface during heavy rainfall events inundated plants. Drainage is impeded and oxygen supply to the roots is prevented. Furthermore, the substrate of limestone mines is highly alkaline, so that only trees evolved to grow under high pH conditions can grow there. This limits the number of tree species which can potential be planted under such conditions and require expensive substrate amelioration operations before tree planting can succeed.
However, most of the limestone mines operated by SCG differ from classic stage-5 degradation in that they are usually surrounded by natural forest, which acts as a reservoir for seed-dispersing animals. This means that once wildlife can be attracted into the restored sites, there is a high probability that they will disperse seeds of natural forest trees into the sites and thus bring about more rapid recovery of vegetation species richness than would usually occur with Stage 5 Degradation sites. Thus, the forest restoration approach on limestone mines can include the framework species method, specifically designed to attract seed-dispersing animals.
The definition of forest restoration used for this booklet is “directing and accelerating ecological succession towards an indigenous forest ecosystem of the maximum biomass, biodiversity, ecological functioning and structural complexity that are self-sustainable within prevailing climatic and soil limitations.”