Growth of tree seedlings in a tropical dry forest in relation to soil moisture and leaf traits

"AimsThe growth of plant species in tropical dry forest (TDF) is expected to be largely governed by the availability of soil moisture. In this study we attempt to identify mechanisms by which seedlings of dry tropical trees cope with water stress by adjusting their leaf characteristics to water availability and micro environments, and address following questions: How are leaf traits and relative growth rate (RGR) of the dominant seedling species of TDF affected by seasonal changes in soil moisture content (SMC)? What is the relationship of functional traits with each other? Can leaf traits singly or in combination predict the growth rate of seedling species of TDF? The study was conducted in situ on four sites (viz., Hathinala, Gaighat, Harnakachar and Ranitali, listed in order of decreasing SMC) within the tropical dry deciduous forest in northern India. Methods Five leaf traits viz., specific leaf area (SLA), leaf dry matter content (LDMC), concentrations of leaf nitrogen (leaf N), phosphorus (leaf P) and chlorophyll (Chl) and two physiological processes, viz., stomatal conductance (Gs net) and photosynthetic rate (A net), and RGR, of four dominant tree seedling species of a TDF (viz., Buchanania lanzan, Diospyros melanoxylon, Shorea robusta and Terminalia tomentosa) on four sites were analysed for species, site and season effects over a 2-year period. Step-wise multiple regression was performed to predict RGR from mean values of SMC, leaf traits and physiological processes. Principal component analysis (PCA) was performed to observe the extent of intra- vs. inter-specific variability in the leaf traits and physiological rates.Important findings All the traits and physiological rates were interrelated and showed significant positive relationship with RGR except for the correlation of LDMC with RGR which was not significant. Further, relationships of SMC with all leaf traits, physiological rates and RGR were significant, except for that between SMC and SLA for B. lanzan and D. melanoxylon. The slope of seedling trait:SMC relationship, a measure of phenotypic plasticity in response to soil moisture gradient, varied among species. Among the four species, T. tomentosa was the most plastic and S. robusta the least. In conclusion, leaf traits and physiological processes were strongly related to soil water availability on the one hand and seedling growth on the other. Gs net is the most important variable which accounted for the greatest amount of variability (62%) in RGR, emphasizing the role of stomatal conductance in shaping growth patterns across spatial and temporal gradients of soil water availability. Gs net and SMC together explained 64% variability in RGR, indicating that other traits/factors, not studied by us are also important in modulating the growth of tropical tree seedlings.     

Effect of soil nitrogen,carbon and moisture on methane uptake by dry tropical forest soils

Methane uptake was measured for two consecutive years for four forest and one savanna sites in a seasonally dry tropical region of India. The soils were nutrient-poor and well drained. These sites differed in vegetational cover and physico-chemical features of the soil. There were significant differences in CH₄ consumption rates during the two years (mean 0.43 and 0.49 mg m^-2 h^-1), and at different sites (mean 0.36 to 0.57 mg m^-2 h^-1). The mean uptake rate was higher (P < 0.05) in dry seasons than in the rainy season at all the sites. There was a significant season and site interaction, indicating that the effect of different seasons differed across the sites. There was a positive relation between soil moisture and CH₄ uptake rates during summer (the driest period) and a negative relation during the rest of the year. The results suggested that seasonally dry tropical forests are a strong sink for CH₄, and C and N status of soils regulates the strength of the sink in the long term.     

Comparative response of seedlings of selected native dry tropical and alien invasive species to CO2 enrichment

Aims Global climate change and ongoing plant invasion are the two prominent ecological issues threatening biodiversity world wide. Among invasive species, Lantana camara and Hyptis suaveolens are the two most important invaders in the dry deciduous forest in India. We monitored the growth of these two invasive species and seedlings of four native dry deciduous species (Acacia catechu, Bauhinia variegata, Dalbergia latifolia and Tectona grandis) under ambient (375–395 μ mol mol^-1) and elevated CO₂ (700–750 μ mol mol^-1) to study the differential growth response of invasive and native seedlings.Methods Seedlings of all the species were exposed to ambient and elevated CO₂. After 60 days of exposure, seedlings were harvested and all the growth-related parameters like plant height; biomass of root, stem and leaves; total seedling biomass; R/S ratio; allocation parameters; net assimilation rate (NAR) and relative growth rate (RGR) were determined.Important findings Biomass, RGR and NAR of all the species increased under elevated CO₂ but the increase was higher in invasive species and they formed larger seedlings than natives. Therefore under the CO₂ -enriched future atmosphere, competitive hierarchies could change and may interfere with the species composition of the invaded area.     

Methane flux from rice/wheat agroecosystem as affected by crop phenology, fertilization and water level

Methane flux was measured for a rice/wheat agroecosystem of Gangetic Plains, with and without application of chemical fertilizer and wheat straw (WS). Three treatments of control, fertilizer application and fertilizer + WS application, were established in a completely randomized block design and measurements were made for two consecutive years (1993 and 1994). CH₄ measurements during growth of the rice crop period showed that there were significant difference in flux rates during the two years. Maximum emission occurred at the time of anthesis and minimum at the seedling stage. The flux rates were significantly higher for fertilizer or fertilizer + WS treatments. The effects of the treatments were similar across phenological stages and years. In the subsequent wheat crop and fallow period, the soils consumed CH₄. There were significant differences in CH₄ uptake rates between the two years. Fertilizer treatments reduced CH₄ uptake in both the years. The results suggested that tropical agroecosystems may consume substantial amounts of CH₄ and that the methane output can be reduced by lowering the submergence level in rice paddies.