Leaf litter nutrient dynamics of Koompassia malaccensis and Shorea uliginosa in a tropical peat swamp forest of Peninsular Malaysia

2017-01-24T00:39:22Z (GMT) by Ong, Sue Peng
Indo-Malaysian peatswamp forests are waterlogged, seasonally flooded, nutrient limited habitats that harbour a wide variety of plant species including many endemic and endangered species. Nutrient stress is a major plant growth-limiting factor which becomes more severe for plants growing in such an extreme environment. This study investigated the mechanisms involved in leaf litter nutrient dynamics of two emergent tree species in Malaysia, Koompassia malaccensis (nitrogen fixing legume) and Shorea uliginosa (dipterocarp), which represent species with adaptations to survive in such nutrient limited environments. Some of the questions included in this study are: (1) what are the mechanisms involved in nutrient release and retention of the selected TPSF tree species?, (ii) do they differ between a legume and a dipterocarp? and, (iii) are the selected tree species important for the formation of peat and thus, maintenance of TPSF? The main hypothesis of this study is that the nutrient regulation process will differ between a legume and a dipterocarp with species-specific properties especially litter litter properties playing a bigger role overriding micro-environmental conditions of the TPSF making them important tree species for the maintenance of the forest. To test this hypothesis, the project was divided into three main objectives which examined three processes associated with leaf litter nutrient dynamics: leaf litterfall, decomposition and abiotic leaching. The first part of the study investigated the contribution of litterfall to nutrient supply and regulation of K. malaccensis and S. uliginosa. This was done via litterfall collection using litter traps set up in the TPSF. To determine the nutrient retranslocation efficiency (NRE) of both tree species, mature green leaves were also collected. Both leaf litter and green leaves were then assessed for their physico-chemical properties and compared. Results obtained indicate that K. malaccensis relies on a combination of high litter quality (higher N coupled with lower amount of tannins, fibre and cellulose), low annual litterfall mass (388.17 g m-2) along with an ability to retranslocate N for nutrient regulation while the opposite strategy is used by S. uliginosa, which was found to produce a high annual litterfall mass of 918.14 g m-2. Due to the aseasonal environment, no significant relationships were observed between climatic variables and litterfall. Therefore, litterfall characteristics in aseasonal tropical regions may depend more on species-specific physico-chemical properties than on climatic variables. The second part of the study investigated the events that happen after leaf litter reaches the forest floor. What affects the nutrient release or retention of leaf litter? To investigate this, a field litterbag decomposition experiment was conducted using the selected tree species. The leaf properties tested for both studies included leaf mass, nutrient content and a range of physico-chemical properties over a one year period. Variations in nutrient release by leaf litter during decomposition in different microenvironments, hollows and hummocks, which represent waterlogged and non-waterlogged conditions respectively, were also examined. Results showed that both micro-environmental conditions and leaf litter properties shaped the patterns of decomposition with little or no invertebrate effect. Anaerobic decomposition (in hollows) reduced decomposability and release of nutrients from both litter types as expected but under similar micro-environmental conditions, the effects of litter quality prevailed. S. uliginosa litter decomposed and released nutrients at a slower rate compared to K. malaccensis. This was suggested to occur as an adaptation to protect its nutrients and allow time for the parent tree roots to reach the litter as the first part of this study showed that S. uliginosa did not have the ability to retranslocate nutrients from senescing leaves back to the parent tree. Lastly, early decomposition of both litter types was shown to be regulated by nutrients especially nitrogen with litter lignin or lignin:N coming into play at the later stages. The final part of the study focused on the role of abiotic leaching in nutrient release from leaf litter as the forest floor is waterlogged and seasonally flooded. In order to address this, we conducted a laboratory leaf litter leaching experiment on litter collected from K. malaccensis and S. uliginosa. Our findings suggest that abiotic leaching contributed to the loss of nutrients from the TPSF litter and that the process occurred rapidly following leaf fall. However, the leaching dynamics of the nutrients were variable in the order of P > N > C. The latter two were released mainly through microbial decay at the later stages of decomposition. Interspecific variations were not observed for leachable fractions of P but it appeared to be the driving force of leachable fractions of N whereby the amount N released from the K. malaccensis litter was lower compared to S. uliginosa. Overall, this indicates that tree species are adapted to the TPSF conditions using different mechanisms of litter breakdown to survive in the nutrient poor environment. This study gave an insight on how two emergent tree species survive in the extreme environment of the TPSF. Both tree species are adapted to the harsh environment of the TPSF using different mechanisms for survival in the forest. The overall decomposition rate of both litter types were found to be very slow with S. uliginosa litter still having 50% of its mass remaining by the end of the 18 months incubation, which is expected to contribute to the accumulation of peat that makes up the substrate of the TPSF. Therefore, both tree species should be included in replanting projects for TPSF. It is clear that maintenance of the water table is vital for protection, maintenance and rehabilitation of TPSF but it is not the only factor to consider as the individual tree species also play a major role.