Colonization of Tree Species Along an Interior-Exterior Gradient Across the Forest Edge in a Tropical Montane Forest, Northwest Thailand
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TROPICS Vol. 21 (3) Issued November 30, 2012 Colonization of tree species along an interior-exterior gradient across the forest edge in a tropical montane forest, northwest Thailand Lamthai Asanok1,2,＊, Dokrak Marod3, Anak Pattanavibool4, Tohru Nakashizuka1 1 Graduate School of Life Sciences, Tohoku University, Sendai 980-8578, Japan. 2 Department of Agroforestry, Maejo University, Phrae Campus, Phrae 54140, Thailand. 3 Department of Forest Biology, Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand. 4 Wildlife Conservation Society (WCS) Thailand Program, Nonthaburi 11120, Thailand. ＊Corresponding author: E-mail: [email protected] ABSTRACT We investigated the environmental factors and tree species characteristics that are important for colonization of an interior-exterior gradient across the forest edge, for application to the restoration of abandoned shifting-cultivation areas in tropical montane forests in the Umphang Wildlife Sanctuary, northwest Thailand. The relative importance of physical environment and recruit limitation was evaluated in relation to the regeneration traits of tree species. Three belt transect plots (150 m x 20 m) were established at the transition from secondary forest (edge interior) to open areas (edge exterior) of different ages (1, 3, and 5 years) after abandonment of shifting cultivation. We also set three belts (20 m x 50 m each) in a primary forest remnant. The species composition of canopy trees, regenerated seedlings, and saplings was studied, together with aspects of the physical environment. We found that it was difficult for primary forest species to effectively colonize the forest edge exterior, mostly due to recruitment limitations rather than the physical environment. Many of secondary forest species and generalists were also affected by recruitment limitation (significant negative correlation with the distance from forest edge), though they were also affected by factors related to the physical environment and forest structure and more abundant in open area. Only a few species, like Choerospondias axillaris (primary forest species), Wendlandia tinctoria (secondary forest species), Colona elobata, and Ficus hispida (generalist species) did not suggest recruitment limitation. These results suggested that natural regeneration of secondary forest and generalist species could be utilized as a first step in restoration expecting their facilitation effects for primary forest species. Key words: forest edge; forest restoration after shifting cultivation; recruitment limitation; tree species traits; tropical montane forest. INTRODUCTION Shifting cultivation is the main cause of tropical forest loss and fragmentation in many countries of Tropical montane forests host high biodiversity, although Southeast Asia and South America (Fukushima et al. human activities are leading to their decrease and 2008, Harttera et al. 2008, Do et al. 2010, Klemick 2011). fragmentation (Tabarelli et al. 1999, Garcia et al. 2005, Both fragmentation and decreases in forest area have Zang et al. 2005, Cayuela et al. 2006, Toledo-Aceves et al. caused serious losses of biological diversity (Sole et al. 2011). In human-dominated agricultural landscapes in 2004, Bailey 2007, Conceicao and Oliveira 2010). The tropical highland regions throughout the world much of forest edge is the line dividing edge interior and edge the original forest cover has been converted into cropland exterior, and the structure and species composition and pastures, including shifting cultivation with cultivated differs between the interior and exterior of the forest temporarily crops and abandoned after harvesting (Mertz edge (Thomas et al. 1979, Oosterhoorn and Kappelle 2009), or semi-permanent land-use systems were crop- 2000, Lopez-Barrera et al. 2006, Marchand and Houle fallow cycle (Manlay et al. 2001), resulting in mosaics of 2006), which increases with increasing fragmentation. agricultural land, secondary forest (SF) with re-growth of Vegetation at the forest edge interior consists mainly of vegetation that covers land (Perz and Skole 2003), and secondar y shrub and tree species; edge effects are primary forest (PF) patches (Mottet et al. 2006, Soliva et sometimes expressed as a reduction in canopy height and al. 2008, Calvo-Iglesias et al. 2009). an increase in subcanopy stature from the forest interior 68 Lamthai Asanok, Dokrak Marod, Anak Pattanavibool, Tohru Nakashizuka towards the edge (Oosterhoorn and Kappelle 2000). tropical forest. However, forest edges may play an important role in Most mountainous areas of northern Thailand restoration of forests (Park 2001, Asbjornsen et al. 2004), covered by lower montane forest (Bunavejchewin et al. and much more information on interior-exterior the edge 2011), long history of shifting cultivation by local and hill- vegetation and tree regeneration is required. tribe people has also caused gradual fragmentation of PFs Edge-related responses of tree species and their (Buergin 2003, Barnaud et al. 2008, Fukushima et al. consequences for plant community composition can be 2008) and increases forest edge especially, in protected ameliorated or exacerbated as a result of differences in area such as wildlife sanctuary and national park (Royal edge type, which is determined by the type of vegetation Forest Department 2010). Local people cultivated rice, adjoining the forest fragment (Lopez-Barrera et al. 2006, maize, cabbages, and fruit crops after slash-and-burn Pauchard and Alaback 2006) and the age of open land clearing of forest areas, though some of them have been after abandonment (Landenberger and Ostergren 2002, abandoned. So, the restoration of abandoned areas Wulder et al. 2009). Forest structure can also affect these especially, the area transition forest is now urgently gradients. Edge contrast (the difference in canopy height required for biodiversity conservation in the vicinity of between cleared and intact forest) and edge closure (the protected areas. The previous study in tropical forest density and vertical distribution of foliage along the edge) found that important factor of tree colonization around can affect light penetration and air movement and thus the edge. There could be many factors af fecting gradients in temperature, humidity, and other colonization around forest edge, the types of forest edge microclimate variables at the area connection of forest (vegetation adjoining the forest fragment and age of open edge (Heithecker and Halpern 2007, Li et al. 2007, Wright land after abandonment) and distance from the edge to et al. 2010). These interior-edge-exterior environment open areas outside the forest (Lopez-Barrera et al. 2006, changes substantially, especially when abrupt transitions Landenberger and Ostergren 2002, Wulder et al. 2009), occur between vegetation communities with distinct forest structure changing on high-low disturbance regime structures and compositions (Asbjornsen et al. 2004, (Kennard et al. 2002), and interior-edge-exterior Heithecker and Halpern 2007). environmental factors such as light intensity and soil In tropical forest, seedling survival and growth can conditions (organic, moisture and bulk density) differ be enhanced at around forest edges or in the understory along the forest edges (Williams-Linera et al. 1998), and due to the ameliorating effects of mature trees and increasing rates of tree mortality (Laurance et al. 2002). shrubs on abiotic microsite conditions (Murcia 1995, However, both environmental factors and recruitment Sizer and Tanner 1999, Piessens et al. 2006). limitations depend on the specific traits of tree species, Environments around the edge may provide critical and studies to separate these components are necessary regeneration sites in fragmented landscapes (Park 2001, to understand mechanisms of tree regeneration at Asbjornsen et al. 2004), while aggregated forests are interior-exterior gradient across the forest edge. Although sufficiently buffered to maintain species that are sensitive regeneration of light-demanding tree species is high to environmental changes (Hewitt and Kellman 2004). around forest edges, the goal of restoration and Soil moisture availability is often the most important regeneration of PFs, rather than disturbance-dependent factor affecting plant establishment and growth following forest (Elliott et al. 2003, Dent and Wright 2009), and the disturbance (Kolka and Smidt 2004, Fay and Schultz 2009, traits related to PF species are of particular concern in Gaduno et al. 2010, Yang et al. 2010). A high light restoration practice. environment after a large canopy disturbance can In this study, we conducted in Umphang Wildlife promote growth of seedlings of some species (Ashton Sanctuary forested fragmentation, which has been caused 1995), while other species grow better in smaller canopy by shifting and permanent cultivation activities by local openings (Brown 1996). The light regime of the forest hill tribes. We investigated the recruitment limitations, understory and gap edges favors seedling growth of the environmental factors and tree species characteristics more shade-tolerant species (Saldana-Acosta et al. 2009, that are important for colonization of interior-exterior Chazdon et al. 2010). Thus, understanding the potential gradient across the forest edge, for application to the responses of tree species to environmental of interior- restoration of abandoned shifting-cultivation areas in edge-exterior gradient is critical to designing systems tropical montane forests. Specifically,