Effect of Forest Degradation on Leaf Litter Decomposition Along a Drought Gradient in Thailand

Kasesart University, Thailand

FY 2014-2016 Grant-in-Aid for Scientific Research (KAKENHI)

KUROKAWA, Hiroko

In South East Asia, more than half of the remaining forest area has already become secondary forest, due to land use change. It is therefore important to evaluate the impact of land use change on ecosystem functioning. The leaf litter decomposition is one of the most important biological processes in forest ecosystems and is strongly influenced by the functional composition and microbiome of the tree community. A wide-ranging assessment of ecosystem functions in Southeast Asian tropical forests requires a comprehensive examination of the mechanisms through which changes in functional composition and physical environments of the community caused by forest degradation impact leaf litter decomposition, in addition to wet-dry climate gradients.

In this research project, we targeted four types of forest communities along a drought gradient in Thailand, to examine the two following hypotheses: 1) in humid tropical forests, leaf litter decomposition rate increases as species with fast growth rate produce abundant decomposable leaf litter in response to intensifying competition for light acquisition caused by forest degradation; 2) in dry tropical forests, leaf litter decomposition decreases, as drought-tolerant species producing less decomposable leaf litter become more abundant in response to an increase in drought stress associated with forest degradation.

In this research project, from the humid tropical forest with an unclear dry season to the dry tropical forest with a clear dry season along a drought gradient in Southeast Asia, we investigated the followings in four locations: Khao Chong in the Thai peninsula (moist evergreen forest), Trat in eastern Thailand (dry evergreen forest), Sakaerat in central Thailand (dry evergreen forest, dry deciduous forest), and Maeklong in western Thailand (dried mixed forest, dry deciduous forest).

1) Changes in functional composition and microenvironments of forest tree communities accompanying forest degradation
2) The relative importance of functional traits and microenvironments in leaf litter decomposition

By combining the above 1) and 2) with consideration of the wet-dry gradient, we evaluated the climatic dependency of the impact of forest degradation on leaf litter decomposition through changes in functional composition and microenvironments in tree communities.

Microenvironments which strongly regulate litter decomposition, such as soil temperature and soil moisture content, changed significantly with forest degradation, yet these changes were dependent on forest type. On the other hand, the soil inorganic nitrogen concentration that impacts the functional composition of the tree community varied significantly between forest types, but did not change between primary and secondary forests. In addition, when considering the traits related to primary production (LMA, foliar nitrogen concentration), species with lower LMA and higher foliar nitrogen became more abundant in secondary forests in any forest type. However, the community-weighted mean of traits related to water use efficiency or leaf litter decomposition (13C, lignin concentration) differed significantly between forest types, but did not change between primary and secondary forests. Furthermore, the rate of leaf litter decomposition also varied significantly between forest types. Leaf litter decomposition was faster in dry mixed forests and moist evergreen forests, where species with high lignin concentrations that inhibit leaf litter decomposition were abundant, possibly due to the high soil moisture content in these forests. On the other hand, there was no difference in decomposition rate between primary forest and secondary forest in any forest type.

These results are expected to contribute to the improvement of regional assessments and prediction accuracy of the effects of forest degradation on tropical ecosystem functions (production, litter decomposition).

KACHINA Panida, KUROKAWA Hiroko, AIBA Masahiro, ASANOK Lamthai, MAROD Dokrak, NAKASHIZUKA Tohru (2016) Changes in functional composition along forest degradation in different types of tropical forests in Thailand. Abstracts of the Annual Meeting of the Ecological Society of Japan 63: E1-11.

KUROKAWA Hiroko, AIBA Masahiro, ONODA Yusuke, HYODO Fujio, KACHINA Panida, ICHIE Tomoaki, NAKASHIZUKA Tohru (2017) Variations in plant functional traits of Asian woody species. Abstracts of the Annual Meeting of the Ecological Society of Japan 64: P2-K-327.

KACHINA Panida, KUROKAWA Hiroko, AIBA Masahiro, UEDA Miki, ASANOK Lamthai, MAROD Dokrak, THINKAMPHEANG Sathid, NAKASHIZUKA Tohru (2017) Impacts of human-induced changes on litter production and decomposition in tropical forests along a drought gradient. Abstracts of the Annual Meeting of the Ecological Society of Japan 64: L01-10.

UEDA Miki, KACHINA Panida, MAROD Dokrak, NAKASHIZUKA Tohru, KUROKAWA Hiroko (2017) Soil properties and gross nitrogen dynamics in old growth and secondary forest in four types of tropical forests in Thailand. Forest Ecology and Management, 398: 130-139.

KUROKAWA Hiroko, AIBA Masahiro, ONODA Yusuke, OGURO Michio, NAKASHIZUKA Tohru (2018) Variation in functional traits and decomposition processes. Abstracts of the Annual Meeting of the Ecological Society of Japan 65: S08-5.