Structure and Dynamics of a Tropical Dry Forest Plant Community
This thesis is spurred by the overarching question “why is a plant where it is in space and time?”, which, when asked in different global communities over the last century or so, has contributed to the development of general theories of plant community ecology and has provided information relevant to understanding, managing, and predicting the future of those communities. The question is asked in the context of a seasonally dry tropical forest (SDTF) plant community in southern India, based on long-term research conducted in a permanent 50-ha sampling plot. We employ a layered approach to answering this question, wherein we deconstruct the structure and dynamics of the plant community by first establishing the spatial structure of soils, topography and lithology in the plot. Next we assess how this spatial structure, together with temporal variation in precipitation, affects plant abundances in space and time. Next we break up abundance variation into the components of recruitment, mortality and stem radial growth and assess how these respond to variation in environmental factors such as precipitation, temperature, soils, topography and fire, and biotic neighborhoods.

In Chapter 2, we examine the roles of lithology, topography, vegetation and fire in generating local-scale (<1 km2) soil spatial variability in the 50-ha plot. For this, we mapped soil (available nutrients, Al, total C, pH, moisture and texture in the top 10cm), rock outcrops, topography, all native woody plants ≥1 cm diameter at breast height (DBH), and spatial variation in fire frequency (times burnt during the 17 years preceding soil sampling) in a permanent 50-ha plot. Unlike classic catenas, lower elevation soils had lesser moisture, plant-available Ca, Cu, Mn, Mg, Zn, B, clay and total C. The distribution of plant-available Ca, Cu, Mn and Mg appeared to largely be determined by the whole-rock chemical composition differences between amphibolites and hornblende-biotite gneisses. Amphibolites were associated with summit positions, while gneisses dominated lower elevations, an observation that concurs with other studies in the region which suggest that hillslope-scale topography has been shaped by differential weathering of lithologies. This “inverse catena” pattern is possibly reinforced by topography due to nutrient leaching and clay depletion in the drainage area. Neither NO3 – N nor NH4+-N was explained by the basal area of trees belonging to Fabaceae, a family associated with N-fixing species, and no long-term effects of fire on soil parameters were detected. A strong SW-NE trending P pattern remained unexplained by any of the factors considered. Local-scale lithological variation is an important first-order control over soil variability at the hillslope scale in this SDTF, by both direct influence on nutrient stocks and indirect influence via control of local relief.

 
The extent to which interspecific niche differences structure plant communities is highly debated, with extreme viewpoints ranging from fine-scaled niche partitioning,…