Sometime leaf of the seedling or sapling stages are found compound and, turns into simple when they adult e.g. Artocarpus chama, Artocarpus heterophyllus etc. Although, the evolution and development of leaf architectures are varies with geographic, climatic and edaphic factors. It is not clear that, whether adaptation for fast growth during favourable, or either during unfavourable conditions brings such modifications. Even, we do not have quantitative data to conclude that simple leaf bearing trees are abundant or compound leaf bearing trees are abundant in Tropical forest.
There are two likely explanations that immediately come to mind with this question. Firstly, seedings and saplings of forest tree species are at a competitive disadvantage in terms of light harvesting in the understorey. While the leaves of some species are simply larger during the early phases of development, other species may increase the surface area of the leaf by producing compound forms to increase photosynthetic efficiency. Secondly, compound leaves can be beneficial to a sapling if the plant is attacked by insect predators. If a simple leaf is being eaten, it is likely the whole leaf will be lost before the plant can stimulate production of secondary compounds in defense. In compound leaves, insect attack on a single leaflet can stimulate defense mechanisms which might allow the remaining leaflets on the leaf blade to be protected. This is important in terms of conserving energy.
Compound leaves are also very common in dry tropical forest trees, possibly to decrease water loss as well as lower C inputs needed to make a leaf for the same leaf surface area ( lower leaf thickness in compound leaflets as opposed to a single leaf of the same photosynthetic area). This could be an artifact of lower water availability drving lower photosynthesis in semi arid zones. I do not have any references for this..
in big trees there is an obvious leaf shape adaptation to frictional resistance by increasing leaf osmolarity and decreasing leaf area, but other genetic regulations are produced by juvenility. Avoiding flowering is a strong strategy for minimizing energy losses in small plants. Shape changes according to friction increase with distance, whereas juvenility loss is a time dependent irreversible adaptation..Transitions and adaptations depend on growth rates, dominance and longevity and are variables in a natural population and climax species often develop long juvenility and shade tolerance as well shape transition. For juvenility a review of Huijser and Schmid can qualify differences http://dev.biologists.org/content/138/19/4117.full
Along the tropics of Capricorn in southern Africa, Namibia, Botswana, South Africa, occurs a mosaic of compound (microphyllous) versus broadleaf savanna and woodland. Compound leafs tend to dominate in drier environments (climate/substrate). At these semi-arid to sub-humid edges of the tropical belt not insects (Alexsandra), but large wild and domestic ungulates are the dominant leaf consumers. The compound leaf may represent a defence mechanism as it seems associated with thorns (thorn bush savanna).
Article Savannas of southern Africa: attributes and use of some type...