BIODIVERSITAS ISSN: 1412-033X Volume 19, Number 6, November 2018 E-ISSN: 2085-4722 Pages: 2219-2231 DOI: 10.13057/biodiv/d190630 Ideal planting space for merbau (Intsia bijuga) forest plantations in Papua based on distance-dependent competition ELIESER VIKTOR SIRAMI 1,2,♥ , DJOKO MARSONO 1 , RONGGO SADONO 1 , MUHAMMAD ALI IMRON 1 1 Faculty of Forestry, Universitas Gadjah Mada. Jl. Agro No. 1, Bulaksumur, Sleman 55281, Yogyakarta, Indonesia 2 Faculty of Forestry, Universitas Papua. Jl. Gunung Salju, Amban, Manokwari 98314, West Papua, Indonesia. Tel.: +62-986-211065, Fax.: +62- 986-211065, ♥ email: siramieli@yahoo.co.id Manuscript received: 30 September 2018. Revision accepted: 9 November 2018. Abstract. Sirami EV, Marsono D, Sadono R, Imron MA. 2018. Ideal planting space for merbau (Intsia bijuga) forest plantations in Papua based on distance-dependent competition. Biodiversitas 19: 2219-2231. Distance-based competition between merbau (Intsia bijuga (Colebr.) Kuntze) and neighboring trees could be applied to determine the planting space between merbau trees and shade trees in plantations. This research was conducted to identify the characteristics of merbau competition with neighboring trees and determine the ideal spacing of merbau trees. The sampling design using was the systematic line technique with hypothetical plot and sample tree as the quadrant center. The competitors were determined using a virtual high approach and header contact. Distance-based competitions were identified using the Hegyi index. Sixteen species had the highest competition index because of their dominance in the tree structure, namely Pometia coriacea, Intsia bijuga, Pimelodendron amboinicum, Horsfieldia laevigata, Palaquium amboinense, Pometia pinnata, Garcinia sp., Spathiostemon javensis, Prunus costata, Sterculia macrophylla, Terminalia complanata, Lepiniopsis ternatensis, Horsfieldia irya, Dysoxylum octandrum, Buchanania arborescens, and Ficus similis. Merbau responded to the high intensity of competition by tilting its canopy in the opposite direction to the position of the competitors’ canopy, making an irregular canopy shape, and growing the first branch at the lowest point on the stem. The ideal planting space for merbau trees in the plantations was 3-7 m. Keywords: Distance-dependent competition, Gunung Meja, merbau plantations, planting space, West Papua INTRODUCTION Currently, merbau (Intsia bijuga (Colebr.) Kuntze) population faces serious threats due to timber production (Newman and Lawson 2005; Marler 2015) and its natural habitat destruction in the lowland rainforest of New Guinea (Vincent et al. 2015; Margono et al. 2014). A plantation forest with native tree species is an alternative to prevent these threats (Barua et al. 2014; Jacovelli 2014; Bremer and Farley 2010). However, the cultivation of native trees requires the availability of relevant ecological information, one of which is the characteristic of merbau competition with surrounding trees. In the Papua forest, merbau trees naturally grow in hight dense vegetation conditions due to the density of plants. The density of plants in tropical forests makes distances between trees get closer. It affects the survival of the trees (Zhu et al. 2015; Fraver et al. 2014), because the close distance between trees increases the effect of competition (Maleki et al. 2015; Contreras et al. 2011; Tome and Burkhart 1989). Competition affects the diameter growth, height, width, and shape of tree canopy (Kunstler et al. 2011; Potvin and Dutilleul 2009; Thorpe et al. 2010). Therefore, competition is an essential factor driving forest dynamics (Sanchez-Salguero et al. 2015; Coomes and Allen 2007; Kunstler et al. 2011). Several facts regarding competition between trees which become important references in silviculture of plantation forests, such as determining planting space, have been explained in previous studies. Competition is one of the plant structuring processes that led to the formation of a variety of stand structures (Craine and Dybzinski 2013; Amiri and Naghdi 2016). Differences of the nature of individuals or species of the tree have a strong influence on the competition (Bennett et al. 2016). If competition is based on niche differentiation, then same-species competition is stronger than different species competition (MacArthur and Levins 1967). However, in natural forests, same-species competition can occur altogether with different species competition. Therefore, each tree species will provide different intensity and effect of competition depending on the ability to compete with others. The ability of competition is a function of growth space, activity, and distribution of space and time of each plant to get resources depending on the combination of plant characteristics (Grime 1979). Furthermore, the intensity of competition depends on the spatial relationship between plants and their neighbors, the impact of the availability of resources on the two competing trees, and the ability of the plant to compensate for the effects of competition through architectural and physiological plasticity (Grace 1995). Spatial relationship between trees is one of the most important factors in competition. Therefore, a distance- based competition study is very important to be carried out as a reference to formulate the ideal spacing of merbau tree for its cultivation in plantations. Distance-based competition, when associated with morphological character of the stand, produces certain distance with the certain intensity of competition. The higher size of the trees and the closer the space between trees, the more intense the