Under the relay floristic model of forest succession, tree plantations may enhance tropical rainforest recovery through shade and soil effects on plant regeneration. The effects of different types of tree plantations – such as commercial monocultures (e.g. Eucalyptus spp.) and mixed native species restoration plantings – on seed germination, seedling survival and growth, and mediating influences of shade and soil, remain poorly understood. We examined tree plantation effects on rainforest tree regeneration by conducting a shade-house experiment and a sample plot-based study of tree regeneration patterns in the Western Ghats biodiversity hotspot of southern India. In the experimental study which employed a replicated factorial design, we monitored seed germination and seedling survival and growth of four rainforest tree species (240 seeds per species) over six months across three soil treatments, namely monoculture Eucalyptus plantations (MP), ecologically-restored diverse tree plantations (ER) and heavily degraded rainforests not subject to any planting treatment (UP), and two shade levels (low: 50% and high: 75%). A plot-based field survey is currently underway (Feb – Apr 2018) to characterize rainforest tree regeneration patterns within the three habitats.
Results/Conclusions
In the shade-house experiment, seed germination and survival were 14% higher (preliminary result – figures may change) under high than under low shade, while seedling growth rates were 9% higher in the low-shade treatment. Seed germination and survival were unrelated to soil treatments while growth rates were highest in the UP soil treatment. Species-level responses to soil and shade are currently being examined. These preliminary results suggest that plantation monocultures might play similar roles as diverse restored forests within the relay floristic framework, at least in terms of enhancing rainforest tree seed germination and growth. Differences in rainforest recovery under these contrasting plantation strategies might still arise due to differences in incoming seed dispersal and other longer-term influences on seedling and sapling mortality. The results of our ongoing plot-based study will likely provide insights connecting our experimental findings to other factors shaping rainforest regeneration and improve understanding of tropical forest relay floristic dynamics under monoculture tree plantations and ecologically restored forests.