The　underground　forest　of　the　degraded　karst　tiankeng　is　mainly　distributed　on　the　inverted　stone　slopes　of　different　slope　aspects.　In　this　paper,　we　explore　the　role　of　environmental　filtering,　competitive　exclusion　and　random　processes　in　the　underground　forest　community　assembly　in　karst　tiankeng,　which　is　helpful　to　reveal　the　value　of　species　refuge　of　the　karst　tiankeng,　and　provide　scientific　reference　for　the　protection　of　tiankeng　plant　diversity　and　ecological　restoration　of　surface　outside　the　karst　tiankeng.　This　study　took　the　Shenxiantang　degraded　tiankeng　in　Zhanyi,　Yunnan　as　an　example.　Mainly　combined　with　functional　trait　ecology　and　phylogenetic　methods,　we　investigated　the　soil　environmental　characteristics,　functional　diversity　variation,　phylogenetic　signals　and　phylogenetic　diversity　of　plant　communities　in　different　slope　aspects.　The　results　showed　that:　(1)　the　soil　environment　varied　greatly　from　slope　to　slope,　with　the　greatest　degree　of　variability　on　the　eastern　slope　compared　to　the　southern　and　northern　slopes.　(2)　The　dominant　populations　varied　considerably　across　the　slopes,　among　them,　Fagaceae　plants　had　relatively　large　abundance,　Quercus　pannosa　had　the　highest　population　abundance　on　the　northern　and　eastern　slopes,　and　Cyclobalanopsis　glauca　had　the　highest　population　abundance　on　the　southern　slope.　The　functional　traits　of　dominant　species　were　different　across　the　slope　aspects,　the　degree　of　variation　was　medium　on　the　whole,　but　the　degree　of　variation　was　the　largest　for　the　dominant　species　on　the　southern　slope.　(3)　Leaf　width,　leaf　aspect　ratio,　leaf　thickness　and　specific　leaf　area　had　strong　and　significant　phylogenetic　signals　(K　＞　1,　P　＜　0.　05),　which　showed　that　the　evolution　of　traits　was　strongly　influenced　by　genetic　factors:　While　leaf　length,　leaf　area,　leaf　dry　weight,　leaf　dry　matter　content　and　maximum　tree　height　had　weak　phylogenetic　signals　(K＜1),　which　indicated　that　the　evolution　of　traits　was　greatly　influenced　by　habitat　factors.　(4)　The　assembly　process　of　plant　community　in　each　slope　acpect　was　dominated　by　the　environmental　filtering　and　competitive　exclusion　effects　of　niche　theory.　During　the　transition　from　the　slope　aspects　from　north　to　east　and　south,　the　environmental　filtering　effect　of　plant　community　assembly　gradually　decreased,　and　the　competitive　exclusion　effect　gradually　increased.　The　northern　slope　may　be　dominated　by　xerophytes　as　the　mainly　constructive　species　for　a　long　time,　while　the　southern　slope　may　eventually　succeed　to　a　subtropical　humid　climatic　top　community　in　the　future.　©　2022　Science　Press.　All　rights　reserved.