Prior to extensive clearance of the landscape from the 1960s, SASDTF ex- tended eastward from west Vietnam into the lowlands of northern and east- ern Cambodia and southern Laos, across northern-eastern Thailand and into central Myanmar, representing one of the largest continuous tracts of dry tropical dry forest globally (Maxwell and Cox 2011) (range shown on Figure 3.1). Aside from a few occurrences as high as 1000 m ASL, these forests typically occur within lowlands (50 to 800 m) (Ruangpanit 1995). As such, mountain ranges commonly bound this ecoregion (see Figure 3.2 map C). SASDTF habitats are important for supporting a broad range of verte- brates within the region, including (amongst others), kouprey and Eld’s deer (critically endangered), gaur, banteng, wild water buffalo, khting-vor, serow, Javan rhinoceros and Asian elephants (Wikramanayaka et al. 2014a, b).
As discussed in Chapter 2, a mosaic of dry forest units in the absence of large tracts of savanna is a defining characteristic of SASDTF, which is especially the case for the Southeastern Indochina Dry Evergreen Forests (SIDEF) sub-units that occur within the northern portion of the study area (location delimited on Figure 3.1). Here, patches of distinctly different for- est can occur within hundreds of meters of each other (Bunyavejchewin et al. 2011), contributing to the high beta-diversity of the area (Apgaua et al. 2014). The distribution of these units can, at a highly simplified level, be predicted by available moisture (MAP and soil water retention) and el- evation, but is also a likely product of disturbance (see Chapter 2 for de- tails). Semi-evergreen dry forest (SEDF) characteristically occurs at higher latitudes (approximately 700 to 1 000 m) with a MAP of 1 200 to 2 000 mm (Bunyavejchewin et al. 2011), or at lower latitudes in soils with better water retention and / or a higher MAP (Maxwell 1999, Ohnuki et al. 2008). Dry Dipterocarp forest (DDF) and mixed deciduous forest (MDF) occupy areas with elevations <700 m ASL, the former persisting in areas with a lower moisture availability (MAP = 1 000 to 1 500 mm) (Rundel and Boonpragob 1995) and the latter in moister regions (MAP = 1 400 to 1 800 mm). However, these generalisations do not always hold true (Ruangpanit 1995).
Chapter 3. Contemporary site conditions 59
The limited work that has been conducted on the vegetation structure of SASDTF in Cambodia (in particular, Theilade et al. (2011) Tani et al. (2007) Rollet (1972) and Rollet (1962) — the latter two of which have been re- viewed in English by Maxwell (1999)) as well as more generalised descrip- tions of SASDTF which is generally focused on Thai dry forests (Williams 1965, Stott 1976, 1984, 1986, 1988b, a, 1990, Stott et al. 1990, Ruangpanit 1995, Gardner et al. 2000, Bunyavejchewin et al. 2011, McShea et al. 2011) are drawn upon to provide an ecological overview of SASDTF. As nomen- clature used to characterise SASDTF forest units varies between authors and the regions in which they work, the following section adopts the gener- alised classification for the ecoregion used by Bunyavejchewin et al. (2011), with additional units represented within north-east Cambodia following Maxwell (1999) (drawing on Rollet (1962)). Vegetation that is characteris- tic of each forest type has been compiled into Appendix B based on pre- existing research and basic, qualitative site walkovers that were conducted throughout July 2012 and April 2013 field seasons. On-site plant identifica- tion was aided with the help of A. Maxwell (independent academic) and S. Channa (Department of Environment, Banlung).
Dry deciduous forest
Dry deciduous forest make up the core forest units within the CIDF ecore- gion (location delimited on Figure 3.1). Leaf shedding by canopy domi- nants throughout the dry season is the defining characteristic of this forest type. This process makes these forests prone to low intensity annual burn- ing by lowering relative humidity and drying the fine fuels in the under- storey (grasses and leaf litter) (Bunyavejchewin et al. 2011). As such, many dominant tree species within this formation have developed fire adapta- tions such as thick corky bark and root crowns that re-sprout post distur- bance (Wikramanayaka et al. 2014a). In Cambodia, the presence of Xylia xylocarpa (Benth./Roxb.) is taken to be an indicator of dry deciduous forest (Tani et al. 2007).
Distinctions in the vegetation composition of dry deciduous forest means that it is typically sub-divided into two types — Deciduous Dipterocarp Forest (DDF) and mixed deciduous forest (MDF). The distinguishing fea- tures of each are detailed below.
Deciduous dipterocarp forest (DDF) is structurally the most open of the SASDTF forest units (Bunyavejchewin 1983), but does support a range of canopy coverage, from from semi-dense to a more “open savanna” forest (Stott 1984, 1988b, 1990). This variability is commonly linked to micro- climatic, edaphic, topographic and fire-legacy conditions (Maxwell 1999). Canopy height is usually limited to 20 m, but can be as low at 10 m in xeric conditions and up to 30 to 35 m in especially favourable conditions (Bun- yavejchewin et al. 2011). The distribution of DDF units is often associated with thin, acidic, shallow, sandy (freely draining), often lateritic and nu- trient deficient soils (Stott 1990, Bunyavejchewin et al. 2011). In Thailand (where this forest type has been most described), these are associated with outcropping sandstones from the Mesozoic red-bed terranes (e.g. Khorat
Plateau associations), old alluvium and steep granitic slopes (Stott 1976). The typical MAP for DDF is 1 000 to 1 500 mm, occurring alongside a rela- tively prolonged (five to six month) dry season.
Indicator species within DDF plots surveyed in both Cambodian and north- ern Thailand include four Dipterocarpaceae species (Dipterocarpus obtusi- folius (Teijsm. ex Miq.) (which can occur as near-pure stands), D. tubercu- latus, Shorea obtusa and S. siamesis (Tani et al. 2007, Bunyavejchewin et al. 2011)), and Terminalia tomentosa ((Roxb.) Wight & Arn) (Maxwell 1999). The dominance of dry Dipterocarpaceae species (up to 90% of basal area (Max- well 1999)) in the absence of conspicuous clumps of bamboo is a defining characteristic of DDF (Tani et al. 2007, Bunyavejchewin et al. 2011). Other important canopy and mid-storey species within this unit include Pterocar- pus macrocarpus (Kurz), Xylia xylocarpa (Benth./Roxb.), Gluta usitata ((Wall.) Ding Hou), Aporusa villosa (Lindl.), and Strychnos nux-blanda (A.W. Hill). Dwarf bamboo (Arundinaria), Dillenia sp., Cycas siamensis (Miq.), Corypha lecomtei (Becc. Ex Lecomte) and Phoenix acaulis (Roxb.) growing alongside grasses (wet season) are widespread ground species in DDF (Maxwell 1999, Bunyavejchewin et al. 2011). A more comprehensive list of DDF species is outlined in Appendix B.
Periodic, low-to-moderate intensity ground fires appear important for main- taining DDF by facilitating removal of leaf litter and reinforcing the low organic content of soil (Ogawa et al. 1961, Bunyavejchewin et al. 2011). These burns are, in some cases, considered to be stabilising by preventing the establishment of more-dense forest type (Stott 1984, 1988b, a). The de- ciduous Dipterocarpaceae indicator species within this unit are especially well adapted, physiognomically, physiologically and phenologically, to fire (Stott 1984). The peak period for fires in DDF typically corresponds with the first half of the dry season, between late December and early March, correlating with peak accumulation of leaf litter and sufficient drying of understorey grasses (Stott et al. 1990). Hottest burns typically occur mid- season (Stott et al. 1990). Other edaphic features, particularly the soil water retention characteristics, appear important in internal structural variations within DDF forest-types, but this is not well-studied (Bunyavejchewin et al. 2011).
Mixed deciduous forest (MDF) is typically taller (25 to 30 m) and denser than DDF, and grows in deeper, less acidic (pH 5 to 6), slightly more fertile (clay loam) soil types (Ruangpanit 1995, Bunyavejchewin et al. 2011). Com- mon canopy species in MDF include Lagerstroemia spp., Pterocarpus macro- carpus (Kurz), Xylia xylocarpa (Benth./Roxb.), Bombax insigne (Wall.), Afzelia xylocarpa ((Kurz.) Craib), Anogeissus acuminata (Wall.), Dalbergia spp., Hal- dina cordifolia ((Roxb.) Ridsdale) and Terminalia spp. (Ruangpanit 1995, Tani et al. 2007, Bunyavejchewin et al. 2011). Irvingia malayana (Oliv. ex A.W. Benn.), Syzygium cumini ((L.) Skeels.) and Phyllanthus emblica (L.) are common sub-canopy and understorey species (Bunyavejchewin et al. 2011). MDF ground cover is characterised by the presence of herbs and small shrubs, and where there is more light penetration, grasses growing alongside gingers (Ruangpanit 1995, Bunyavejchewin et al. 2011). Bamboos (especially deciduous bamboo) are an indicator species of this forest-type.
Chapter 3. Contemporary site conditions 61
Dipterocarpus and Shorea species form a relatively minor component of these forests, though their relative proportion increases along a moisture gradi- ent (Rundel and Boonpragob 1995). A more comprehensive list of common MDF species in outlined in Appendix B.
The interplay of edaphic and topographic conditions under specific climatic constraints appears important for unit delineation between DDF and MDF. Within MDF, floristics and structure can be quite diverse depending on soil quality (including water retention), elevation and topography. In north-east Cambodia, these forests appear to closely associate with the semi-dense dry forests described in Maxwell (1999) (drawing on Rollet (1962)), which have a preference for rocky basaltic brown earth slopes (the summits of which nearly always transition to denser SEDF). Topographic patterns of expo- sure and soil depth commonly cause sharp delineation of MDF and DDF at higher (>700m ASL) elevations (Rundel and Boonpragob 1995). Where ele- vation is less than 600 m (ASL), however, the sharp transitions that do occur are generally related to edaphic controls (Rundel and Boonpragob 1995). Low frequency ground fire also appears to be an important ecological com- ponent of MDF (Bunyavejchewin et al. 2011). However, different fuels in MDF (vs. DDF) means that fires have the potential to be hotter and more destructive, particularly where tall bamboos facilitate spreading of the fire to the canopy (Stott et al. 1990).
Semi-evergreen dry forest
Semi-evergreen Dry Forest (SEDF) is the densest (250 to 400 stem/ha) of the forest units classified for SASDTF (McKenny et al. 2004) (though, as de- tailed below, some regional sub-classifications have been posited for north- east Cambodia (Rollet 1962)). Structurally, these forest have a relatively closed canopy, 30 to 40 m high, with emergents growing up to 50 to 60 m (Neal 1967). Most canopy species, and nearly all understory species are ev- ergreen. Dominant tree species within this unit include Dipterocarpus alatus (Roxb.), D. dyeri, D. turbinatus (C.F.Gaertn.), D. costatus (C.F.Gaertn.), Ho- pea odorata (Roxb.), H. ferrea (Laness.), Anisoptera spp. that grow alongside Irvingia malayana (Oliv. ex Benn.), Lagerstroemia spp.), Litsea vang (Lecomte), Dehaasia cuneata (Blume), Mesua ferrea (L.), Mangifera indica (L.) and Sindora cochinchinensis (H. Baill) (Maxwell 1999, Bunyavejchewin et al. 2011). In general, year-round canopy cover and lower annual accumulation of leaf litter means that SEDF are less susceptible to annual burning than the de- ciduous dry forest types. As such, it is often held that burning of SEDF can destroy the evergreen tree species, and promote a regime shift to dry deciduous forest (particularly DDF) that is thereafter reinforced by the reg- ular presence of fire (Stott 1984, Stott et al. 1990, Johnson and Dearden 2009). However, as outlined in Chapter 2, this relationship is poorly stud- ied (Maxwell and Cox 2011) and contested (Baker et al. 2008, Baker and Bunyavejchewin 2009).
SEDF sub-classifications — in north-east Cambodia, Maxwell (1999) (draw- ing on Rollet (1962)) classifies an intermediate forest type, transitionary be- tween semi-dense dry forest (MDF) and SEDF. This subunit contains many species of the semi-dense-type MDF, but is closed with a leafy understorey without bamboo. Several SEDF species occur within the emergent layer, with the dominant SEDF Dipterocarpaceae trees gradually being replaced with Lagerstroemia spp. and Fabaceae taxa as the environment gets drier. Common species within this forest include Lagerstroemia angustifolia (Pierre ex Laness.), Xylia xylocarpa (Benth./Roxb.), Pterocarpus macrocarpus (Kurz), Sindora cochinchinensis H. Baill)., Afzelia xylocarpa ((Kurz.) Craib), and Dal- bergia bariensis (Pierre). Dipterocarpus intricatus (Dyer) becomes dominant in these forests as they transition to a dry deciduous forest type. A degree of regular burning (low intensity ground fire) is apparent in this unit (Maxwell 1999).
At the other end of the spectrum, regionally-specific Basaltic red-earth for- est types (BRE) are classified. These represent a variant of SEDF that grows in deep, permeable basaltic soils on uplands and plateau-tops with high water holding capacities. As such, this forest type is weakly tropophilous (Maxwell 1999). Dominant species are similar to that of SEDF, with the ad- dition of Aglaia gigantea (Pierre Pellegr.), Toona sureni (Blume) Merr.), Termi- nalia spp., Pterocarpus macrocarpus (Kurz), Afzelia xylocarpa ((Kurz.) Craib), Peltophorum dasyrachis ((DC.) K. Heyne), Adenanthera pavonina (L.), Lager- stroemia spp. and Tetrameles nudiflora (R. Br.) as common canopy species. This forest type was once likely to have been distributed across the Ratanakiri Volcanic Province basaltic plateau in the vicinity of Yeak Loam. However, owing to the fertility of these soils, much of this forest type has been dis- turbed by swidden agriculture and, more recently, cleared for plantations (this is elaborated upon in Section 3.8).
Secondary dry forest
Soil conditions, microclimate and the timing and nature of past distur- bances mean that secondary forests (SeF) are a common component of SASDTF. These widely-varied formations are an ecologically important, but often overlooked formation in Cambodia (Heinimann et al. 2007). The north- east Cambodian variants of SEDF secondary forest types are discussed in Maxwell (1999) (after Rollet (1962)). Here, the most extensive cause for the development of secondary forest is via swidden farming. In Ratanakiri, this practice is generally concentrated in SEDF-supporting landscapes that have more favourable soil characteristics for cropping.
Following low to moderate level disturbance, succession of SEDF usually includes a pioneer assemblage of herbs (if clearance is very frequent) and herbs and shrubs if clearance is less regular. Common pioneer trees are Trema spp., species from the Euphorbiaceae family, including Macaranga spp. and Mallotus spp., Grewia tomentosa (Juss.), Peltophorum dasyrachis ((DC.) K. Heyne), Zizyphus cambodiana (Pierre), Colona sp., Memecylon edule (Roxb.) and Combretum quadrangulare (Kurz). In later stages Lagerstroemia spp., Cra- toxylum sp., Vatica astrotricha (Hance); Dipterocarpus intricatus (Dyer), Shorea roxburghii (G.Don), Hopea sp., Eugenia sp., Sindora sp. and Albizzia sp. form
Chapter 3. Contemporary site conditions 63
secondary forest dominants, which may eventually recover to SEDF de- pending on disturbance events and alternation of original edaphic controls (Rollet 1962) in (Maxwell 1999).
Succession of mainland south-east Asian MDF following particularly in- tense fires or clearance typically results in the establishment of Croton ob- longifolius (Roxb.), Mallotus macrostachys ((Miq.) Müll.Arg.), Trema angusti- folia ((Planch.) Blume) alongside bamboos (Gigantochloa albociliata ((Munro) Kurz) or Dendrocalamus strictus ((Roxb.) Nees). A comprehensive list of species associated with secondary MDF (as well as secondary SEDF, and Basaltic Red Earth forests) is provided in Appendix B.
Secondary grasslands
In some cases, intense, or too frequent disturbance (associated with land clearance or fires) within MDF or SEDF can result in savanna as opposed to secondary forest (Rollet 1962, Maxwell 1999). This can occur following a single destructive event (Goldammer 2002), or as part of a long-term pro- cess whereby annual fire precludes regeneration of canopy species, thus gradually opening up the forest to a grassland community as mature canopy species die out (Ruangpanit 1995). The resultant grassland communities are typically dominated by Oxytenanthera spp. and Imperata cylindrica ((L.) P.Beauv.) (Goldammer 2002). These communities are fire promoting, and tend to persist over forest as long as there is annual fire preventing the establishment of shrubs (and later canopy) species (Ruangpanit 1995, Max- well 1999).
Other SASDTF types
Other forest types that are present in the SASDTF, but not common within the immediate vicinity of the lake study sites, are pine forest, lower mon- tane forest riparian forest, and swamp forest. A summary of the features of each is presented below, as outlined in (Ruangpanit 1995) and Bunyave- jchewin et al. (2011).
Pine forest (PF) can occur across mainland south-east Asia at elevations extending from 400 to 1 500 m ASL. Lower versions of this forest are com- positionally and structurally similar to DDF aside from the dominance of pine (particularly Pinus merkusii (Jungh. & de Vriese)). At higher elevations, these forests are structurally comparable to lower montane types (below), where Pinus kesiya (Royle ex Gordon) and members of the Fagaeae and Lau- raceae families become compositionally important Rollet (1962) in Maxwell (1999).
Lower montane forest (LMF) occurs within south-east Asia at elevations in excess of 800 to 1 000 m ASL (Bunyavejchewin et al. 2011). While no one particular species dominates the canopy, members from the Lauraceae,
Theaceae, Magnoliaceae and Fagaceae family are important in this setting (Bunyavejchewin et al. 2011).
Riparian forest (RF) grows along stream or river lines particularly in basaltic soils, and can form a sharp contact between other forest types where the ground water table rises. These forests generally have a canopy at 20 to 30 m, and show some compositional similarities to SEDF with the addi- tion of Hydnocarpus anthelminticus (Pierre ex Laness.), Barringtonia acutan- gula ((L.) Gaertn.), Nauclea orientalis ((L.) L.) and Sterculia foetida (L.) canopy species (Bunyavejchewin et al. 2011).
Swamp forest (SwF) grows in peat that has accumulated under perma- nently wet ground conditions. These forests can be relatively dense with a high canopy. Common canopy species include some SEDF Dipterocarpaceae alongside Eugenia spp., Sterculia foetida, Myristica spp., and Ficus spp. (Max- well 1999). The understorey is characterised by the presence of palms (Livis- tona cochinchinensis (Merr. ex A.Chev. Syn. L.), Licuala spp. and Calamus sp.) Epiphytes, including Stenochlaena palustris ((Burm. f.) Bedd.) are com- mon in this forest unit (Maxwell 1999).