Ecological responses of a Bornean heath forest (kerangas) to experimental lime and nitrogen addition

Sellan, Giacomo (2019) Ecological responses of a Bornean heath forest (kerangas) to experimental lime and nitrogen addition. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Heath forests are a rare but widespread forest type found across the tropics that have low productivity and relatively low plant species diversity but with high endemism. These forests develop on strongly weathered spodosols that are acidic soils with low nutrients, which is generally believed to explain heath forest distribution. In this study, I tested whether tropical heath forest productivity is limited by low soil nitrogen (N) availability, soil acidity or an interaction between the two, with a factorial N and CaCO3 addition experiment in a Bornean heath forest. I also assessed the susceptibility of this forest type to the predicted increases of N deposition in tropical areas. I recorded the highest wet inorganic N deposition rate measured in Malaysia (7.45 kg N ha-1 yr-1) that was dominated by NH4+-N and most likely derived from agro-industrial fertilization. Prior to the experimental treatments of my plots, I correlated edaphic and topographical differences among plots with species diversity, distribution and stand structure. Soil acidity had the greatest influence on species distribution and forest structure but the subsequent experimental liming did not affect tree performance, only soil acidity and the decomposer community. On the other hand, N addition increased tree relative growth rates, and foliar N concentration after only two years of treatment, but no effect of N addition was detectable on litterfall mass during one year of fertilisation. Overall, both the correlational study on habitat filtering and N addition found most effect on trees 5-10 cm DBH but both acquisitive and conservative species reacted equally to N addition. My results show that heath forest productivity is N limited; moreover, increased N deposition is likely to further acidify this forests’ soil – as seen in soil leachate after one year of N fertilization - possibly increasing the amount of undecomposed organic matter and reducing the availability of soil nutrients to forest trees. Our results from litterfall, fresh leaves and soil leachate also present further hypotheses to be tested suggesting that soil phosphorus (P) and iron (Fe) could be limiting in this forest typology. In conclusion, I show that tropical heath forest are sensitive to N availability. To conserve this unique forest type, local governments should develop policies to limit atmospheric N input.