Comparative physiology of canopy tree leaves in evergreen and deciduous forests in lowland Thailand

Three major forest types in lowland Thailand and its adjacent parts in Southeast Asia are mixed deciduous forest (MDF), dry dipterocarp forest (DDF) and dry evergreen forest (DEF). We report the leaf physiology of canopy trees in these forests. The leaf mass-based photosynthetic rates (Amax), stomatal conductance (Gmax) and photosynthetic nitrogen use efficiency were significantly different between the deciduous forests (MDF and DDF) and the evergreen forest (DEF). The canopy trees of MDF with thick, eutrophic soils had the highest intrinsic water use efficiency (Amax/Gmax) among the forest types. Forest-to-forest variations in leaf mass area were related to different nutrient use strategies (less vs. more conservative) associated with different soil nutrients rather than with leaf phenology/longevity. In the interspecific variations within each forest, Amax in MDF and DEF was limited by foliar phosphate, whereas that in DDF was limited by foliar nitrogen. The close association between leaf physiology and soil properties suggests that climate change and increasing human impacts will disrupt this association, leading to forest degradation and dysfunction.

Methods
We examined leaf gas exchange in the canoy leaves of the mixed deciduous forest (MDF), dry dipterocarp forest (DDF) and dry evergreen forest (DEF) in Thailand, using LI-6400. After the mesurements in the fields, we collected leaf disks from the leaves and then examined leaf mass per area, nutrients (nitrogen, phosphate, calcium, magnesium, potassium) and delta 13C in our labopratory.