Variation in understorey irradiance is both a cause and consequence of the structure and dynamics of closed-canopy forests, which are also influenced by soil nutrients and water availability. We tested the hypothesis that understorey light regimes differ among four mixed dipterocarp forest types that share the same rainfall, but grow on different soils along an edaphic gradient at one site in Borneo. Based on data from photosynthetically active radiation sensors deployed at 1-m height at 36 locations for 351 sensor-days, we found significant soil-related variation in irradiance. The more productive forest types on clay and fine loam had lower daily photosynthetic photon flux density (PPFD) than those on the more nutrient-depleted and better-drained sandy loam and loam. They also had fewer moderate to high-intensity sunflecks, and a greater proportion of their daily PPFD came from low-intensity light. Understorey irradiance did not, however, monotonically decline with increasing soil resources. Forests on intermediate soils had greater irradiance than those with more and less soil resources, due to steeper slopes. Plant communities arrayed on resource gradients are commonly used to test hypotheses of environmental factors driving their assembly. Our results indicate that consideration of multiple resource dimensions in such tests is critical.

Understanding exotic–native plant relationships within the context of landscape-scale environmental factors such as rainfall, topography, disturbance and forest structure, is important for distinguishing the role of invasions in native plant community change. In this study, the relationship between Lantana camara invasion and native forest understorey vegetation was described after accounting for environmental influences. Rainfall, terrain slope, altitude and fire frequency were measured from GIS layers, and tree density, dry above-ground L. camara biomass, and native plant abundance and species richness were measured in nested plots (four 1 × 1-m plots for grass, tree seedlings and L. camara within one 5-m-radius plot for tree saplings and herbs and shrubs within one 10-m-radius plot for trees) at 80 locations distributed across Bandipur Tiger Reserve, a tropical deciduous forest in southern India. Relationships between environmental factors and L. camara abundance were described using a multiple regression. Further, the role of L. camara abundance in explaining residual variation in native vegetation, after accounting for environmental influences, was described using linear models. Lantana camara abundance decreased with increasing tree density, supporting the notion that it thrives in disturbed forest with fewer trees. Whereas native tree seedling and herb and shrub density showed no relationship to L. camara abundance, both tree sapling density and grass volume decreased under L. camara biomass exceeding 2 kg m−2. These data suggest that, by association with grass decline and decreased recruitment of tree saplings, L. camara may be linked to the gradual transition of these tropical deciduous forests into exotic-dominated shrubland, and overall native biodiversity loss.

As forest fragmentation and intentional burning of grasslands increase, the frequency of fires penetrating the dry and subhumid tropical forests of Bolivia is also likely to increase. To expand our understanding of the role of fire in tropical dry forest, the physical and thermal properties of barks of tree species were studied to determine their relative resistances to cambial damage by fire. For 16 tree species found in the dry forest of the Lomerío region of eastern Bolivia, bark thickness, moisture content, and specific gravity were measured. Insulating capabilities of bark were measured by obtaining cambial and surface temperatures during experimental wick fires. Bark thickness on trees 5-100 cm dbh (diameter at 1.4 m) ranged from 2–51 mm and both thick- and thin-barked species were represented. For all species, bark thickness increased as stem diameter increased. Bark thickness explained more (63%) of the variation in peak cambial temperatures during fires than did bark moisture content (4%) or specific gravity (1%). A threshold bark thickness of 18 mm was associated with the ability to withstand lethal cambial temperatures during the experimental, low intensity fires. For 13 of the 16 species included in this study, trees ≤20 cm dbh have bark thickness below the threshold 18 mm and, therefore, are likely to experience cambial injury from low intensity fires. Our results suggest that the forest presently characteristic of the Lomerío region did not develop with frequent fires and that species composition is likely to be substantially affected by an increase in fire frequency.

The germination of 43 tree species native to the lowland forests of Malaysia was monitored on forest soil in trays placed in closed-canopy forest, an artificial forest gap, and a large clearing. Germination varied significantly among habitats, with only seven species germinating well in all three sites. Seed germination of most species demonstrated clear patterns of shade tolerance or intolerance identical to those long recognized for tree seedlings. Favoured locations for germination were similar to known distributional patterns of adult individuals in mature and secondary forests. Most forest canopy species germinated in the gap, but germination in the large clearing was dramatically reduced or nil; the regeneration of these species in large clearings will be severely retarded even when seeds are present. Canopy-induced inhibition of germination was apparent in several pioneer species; seeds moved from the forest into the gap or clearing germinated rapidly. Results indicate that natural treefall gaps do not inhibit the germination of most species, but do allow pioneer species to germinate, and therefore regenerate. Although germination occurred in all three locations, the level of germination for particular species varied greatly among habitats. We conclude that germination requirements play a very important role in controlling the species composition of regeneration in forest understorey, gap, and large clearing habitats.