Relative effects of different leaf attributes on sapling growth in tropical dry forest

Soil moisture content (SMC) influences establishment, survival and development of plant species and is considered as the most important limiting factor in tropical dry forest (TDF). In this study we attempt to establish the relationship between leaf attributes and of tree saplings in TDF and address the following questions: (i) how are the functional attributes of dominant tree saplings of TDF affected by seasonal changes in SMC at different habitats?, (ii) what is the relationship of functional attributes with each other?, (iii) how are the functional attributes and their plasticity affected by habitat conditions? and (iv) can the functional attributes in single or in combination predict the growth rate of tree saplings of TDF? The study was conducted on four sites (Hathinala, Gaighat, Harnakachar and Ranitali, listed in order of decreasing SMC) within the tropical dry deciduous forest in northern India. We analysed eight leaf attributes, specific leaf area (SLA); leaf dry matter content (LDMC); leaf nitrogen concentration (leaf N); leaf phosphorus concentration (leaf P); chlorophyll concentration (Chl); mass-based photosynthetic rate (A (mass)); mass-based stomatal conductance (Gs(mass)); intrinsic water use efficiency (WUEi) and three growth attributes, relative diameter increment (RDI); relative height increment (RHI); relative growth rate (RGR) of the 10 dominant tree saplings (viz., Acacia catechu, Anogeissus latifolia, Boswellia serrata, Buchanania lanzan, Diospyros melanoxylon, Hardwickia binata, Lagerstroemia parviflora, Lannea coromandelica, Shorea robusta and Terminalia tomentosa) of a TDF and observed the effects of site, season and species for a period of 2 years. Saplings were selected in gradients of deciduousness. Step-wise multiple regression was performed to predict RDI, RHI and RGR from mean values of SMC and leaf attributes. All the 11 attributes were interrelated and differed significantly among the 10 saplings. Species response varied across sites and seasons. Across the SMC gradient, the attributes showed variable plasticity that differed across species. Among the 10 saplings, the highly deciduous Boswellia serrata showed the maximum plasticity in seven functional attributes. According to the step-wise multiple regressions, 65% variability in RDI and 67% variability in RGR were due to Gs(mass), and for RHI, 61% variability was due to A (mass). SMC and the other attributes, viz., SLA, Chl, WUEi and LDMC in combination could contribute only for similar to 2-6% of the variability in RDI, RHI and RGR, which indicates that other traits/factors, not accounted in this study are also important in modulating the growth of tree saplings in TDFs. In conclusion, growth of the tree saplings in the tropical dry environment is determined by soil moisture, whereas the response of saplings of different tree species is modulated by alterations in key functional attributes such as SLA, Chl, WUEi and LDMC.