Cebu City's ecological balance has been destroyed decades or maybe even centuries ago the moment we embraced the planting of exotic and invasive trees in an effort to restore our widely denuded forest in Cebu. The mono-cropping of Gmelina and Mahogany trees in Campo Syete and the hectars of Teak trees in Cebu's watershed area has brought nothing but destruction to our Ecosystem.
These trees reportedly release substances that are toxic to the microbiota, accounting for very little organic matter in the soil. For lack of an absorbent upper soil layer, heavy rains from Typhoon flows downstream thus resulting to an overflowing of large amount of water in the low-level areas.
The following statement is from an article/researcher paper I read a few months ago describing Mahogany and other plants as an invasive tree species -
Mahogany is successful at invading natural forests due to the following attributes of the species. The fruit of mahogany is a capsule and contains an average of 62 winged seeds. The number of seeds a mahogany mother tree can disperse is considerable. Assuming 50 capsules, 3000 seeds can be blown away from the mother tree. The seeds can be blown some 20 to 40 meters from the mother tree. The seeds, being recalcitrant, germinate in less than a month. Mahogany seeds contain food reserves and germinate hypogeal. This means that even if the initial light is relatively poor, the young mahogany plant develops even without initial photosynthesis. The first young leaves of mahogany are scale leaves and not green. True photosynthetic leaves come later and are adapted to sun-flecked shade and partial shade. Hardened mahogany seedlings can tolerate open fields as long as soil moisture is not limiting. The leaves of mahogany are rarely attacked by herbivores. Thus, a mahogany plantation is like a "green desert" to wildlife. Dipterocarps fruit and seed irregularly in intervals of four to five years and therefore stand no chance competing with mahogany.
When mother trees shed their leaves during the months of February, they form a thick litter mat. Dry mahogany leaves are red and can be very rich in tannin. The leaves are intact during the whole length of the dry season. This litter mat could be one reason why very few seedlings are recruited under the mahogany plantation, including their own seedlings. Dispersed recalcitrant seeds rest on top of the litter mat instead of reaching the moist soil and hence die due to desiccation.
They may also be allelopathic. Extracts from the leaves of mahogany were shown to retard the growth of narra (Pterocarpus indicus) test seedlings. Recruits increase away from the mahogany plantation and this increase is proportional to the competition offered by mahogany wildlings . The importance of mahogany seedlings is negatively correlated with the Shannon-Weiner Diversity Indices of quadrats positioned from the mahogany plantation and away from it. In other words, diversity of the quadrats decreases as the importance of mahogany increases.
While mahogany invades regenerating dipterocarp forests and may give the dipterocarps a hard time in competition, the paper mulberry (*Broussonetia papyrifera) also gives indigenous gap and pioneer tree species very keen competition. Ocular observation shows that where paper mulberry forms pure stand thickets, the usual indigenous pioneer tree species. - end of statement
Apart from Mahogany being an invasive species, most of our native wild animals also do not recognize these trees nor the kinds of fruits or flowers that these trees produce. Not many birds prefer Mahogany as their nesting place and most insects do not consume them. Without insects, there will be no birds and rodents.. without small animals there will be no predatory animals. Long story short, the flora and fauna now becomes incompatible with each other. These man-made forest are nothing but a silent and dead green forest. Its not just the poachers and hunters that put the numbers of wild species of animals in a total decline, we too are somehow responsible by depriving them of their natural habitat.
Reforestation is not about quantity but its the quality of trees. Clearly we should plant indigenous trees for reforestation but which kind? Native flora has co-evolved with local organisms for thousands of years, including the bacteria and fungi of decay, hence such forests have a healthy layer of decomposing litter. In all forests there are the pioneer trees that grow first and are mostly rapid growers. They then provide a canopy of shade and shelter for the slower and hardier trees to grow under. Examples of the usual indigenous pioneer tree species are the following: Anabiong (Trema orientalis), binunga (Macaranga tanarius), alim (Melanolepis multiglandulosus), banato (Mallotus philippinensis), tibig (Ficus nota), hauili (F. septica), is-is (F. ulmifolia), sablot (Litsea sebifera), paguringon (Cratoxylon sumatranum), and malapapaya (Polyscias nodosa).
By the time your slow growers are strong and increased in size the pioneer trees are more or less at the end of their cycle and fall down to create space and sunlight for the new generation of large hardwood native trees like Narra (Pterocarpus indicus), Molave (Vitex parviflora), Yakal (Shorea astylosa), Toog (Petersianthus quadrialatus), Apitong (Dipterocarpus grandiflorus), Almaciga (Agathis philippinensis), Lawaan (Shorea), Ipil (Intsia bijuga), and many other large species of trees.
By planting exotic and invasive trees we are basically working against nature. Working with natural patterns will speed up the process. Wild animals must also be taken into consideration when talking about reforestation projects, ergo we must be able to provide for their food source and natural habitat before planting any other tree species. For example, the wild birds... they are very useful in reforestation, birds help to spread seeds through their manure. The manure will add nutrients to the soil and some of the seeds will grow into new trees, same goes for the rest of the other native wild animals.
Reforestation areas are areas where the natural forest is restored. It is a less intensive system, and will provide less produce than agriculture. However, this system is very important for preserving the environment and stopping erosion, and it will provide many essential products in the long run such as bamboo, oils, fiber, timber, honey, medicines, rain and most importantly the oxygen we breath.
Areas that have forests need to be protected and carefully managed. These forests are the seed banks of the future. Reforestation and tree crops are a long term solution for protecting the soil and stopping erosion. And if reforestation is done properly it may also helps to repair damaged land, while providing food, wood, oils, medicines, fiber and many other products for income. These are all sustainable incomes.
I am no expert here but most of the details written are backed with ideas I learned from reading books, thorough research and from my personal experiences in the wilderness. Nature is always working towards a healthier environment lets try and keep it that way.