This proceeding is a compilation of findings and progress activities of research collaboration between the Forest Department Sarawak (FDS) and the Japan Research Consortium for Tropical Forests in Sarawak (JRCTS). To highlight the research findings, An International Symposium entitled "Frontier in Tropical Forest Research: Progress in Joint Projects between the Forest Department Sarawak and the Japan Research Consortium for Tropical Forests in Sarawak" was ii held in Kuching, Sarawak on 21-22 September 2015.Nitrogen is one of most limiting nutrients of plants' growth. Lowland tropical rainforests in Southeast Asia are characterized by high species diversity despite limited soil nitrogen conditions. We analyzed δ(15)N values and nitrogen content (N %) in leaves and roots of 108 woody species with different types of symbiotic microorganisms, of different life forms (emergent, canopy, subcanopy, understory, and canopy gap species), and from different families in a Bornean lowland dipterocarp forest to gain more insight into the diversity of nitrogen uptake strategies in the rhizosphere. The plant nitrogen isotope ratio (δ(15)N) changes depending on soil nitrogen source or root symbiotic fungi. Leaf δ(15)N values in the species studied varied widely, from -7.2 to 5.0 ‰, which is comparable to the values of known Asian trees, including those from temperate, subtropical, and tropical mountain forests. Leaf δ(15)N also varied significantly among both life forms and families, though the phylogenetically independent contrast (PIC) relationships were not statistically significant among life form, family, and symbiotic types. Some families showed specific leaf δ(15)N values; Dipterocarpaceae, the dominant family in the canopy layer with symbiotic ectomycorrhiza in Southeast Asia, had small intraspecific variation and higher leaf δ(15)N values (0.03 ‰) compared with species exhibiting arbuscular mycorrhiza, whereas several families such as Burseraceae, Euphorbiaceae, and Phyllanthaceae showed large interspecific variation in leaf δ(15)N (e.g., from -7.2 to 5.0 ‰ in Euphorbiaceae). These variations suggest that tropical species may have family- or species-specific strategies, such as root symbiotic microorganisms, for nitrogen uptake under low-nutrient conditions in tropical rainforests in Southeast Asia.