Effects of Topography and Land use on Woody Plant Species Composition and Beta Diversity in an Arid Trans-Himalayan Landscape, Nepal

Distribution patterns of plant species are believed to be impacted by small-scale habitat heterogeneity. However, there have been few comparative studies examining how woody vegetation composition and diversity varies with aspects of different orientations in the Trans-Himalayan region at a local scale. Here, we examined the effects of incoming solar radiation on variation in woody species composition and compared the diversity between the northeast- and southwest-facing slopes in a Trans-Himalayan valley of Nepal. We also examined the implicit interactions between slope orientation and land use in determining the compositional variations between the slopes. We selected two pairs of northeast- and southwest-facing slopes where the first pair has a similar land use and differs in exposure only(Pisang site) while the other pair has clear differences in land use in addition to slope exposure(Braka site). In each site, we sampled 72 plots(36 on each slope) in which the presence and absence of woody species, environmental variables, and disturbance were recorded. Correspondence Analysis(CA) results suggested that the woody species composition significantly varied between northeast- and southwest-facing slopes at both sites, and was significantly correlated with measured environmental variables such as radiation index, altitude, and canopy openness. In the Braka site,mean alpha diversity was significantly higher on southwest-facing slopes. In contrast, beta diversity and gamma diversity were greater on northeast-facing slopes at both sites. Our results suggest that topographic variables(e.g., radiation index) affect species composition between the slopes, likely due to their influence on small scale abiotic environmental variables. However, the effects of land use, such as livestock browsing/grazing may interact with the effects of slope exposure, effectively reducing differences in species composition within slopes but enhancing the differences in beta diversity between contrasting slopes in the Braka. We conclude that slope orientation and land use are important factors in structuring the woody species composition and diversity in the arid Trans-Himalayan region. We suggest that both environmental and land use variables should be taken into consideration in future studies on plant community structure along the cultural landscapes.

The role of light, soil and human factors on the probability of occurrence of an invasive and three native plant species in coastal transitions of coastal mississippi, USA

Aims understanding relationships between the distributions of species and their surrounding environment provides a basis for forecasting how species will respond to future environmental changes.In this study,we examined the effects of environmental factors and human devel-opmental features associated with disturbances on probability of occurrence of juveniles of invasive Triadica sebifera and three native plant species,Baccharis halimifolia,Ilex vomitoria and Morella cer-ifera within a typical coastal transition in coastal mississippi,USA.Methods We recorded presence of juveniles of focal species and measured environmental factors(soil salinity,canopy openness,soil texture and soil carbon to nitrogen ratio)along an 11.3 km transect located at grand bay National Estuarine research reserve.Further,we doc-umented anthropogenic features and associated activities as a proxy for human disturbance.Important Findings With the exception of I.vomitoria,all other species occurrences were significantly influenced by an interaction between soil salinity and canopy openness.The occurrence of I.vomitoria sharply decreased with salinity.B.halimifolia occurrence peaked at moderate levels of salinity and low to moderate levels of canopy openness.occurrences of M.cerifera and the invasive T.sebifera were highest at low levels of salinity and canopy openness.These results indicate that salinity is a strong driver that limits distribution of juvenile native and invasive species in coastal transitions.logistic regression confirmed the positive effects of anthropogenic disturbances on T.sebifera and I.vomitoria occurrence.It appears that while soil salinity is likely to drive retreat of the seaward boundaries of woody plant species distributions,increased human developments along the coast likely enhance the inland spread of species,in particular the invasive T.sebifera by increasing distur-bances and facilitating dispersal.results from this study can offer insight for the development of T.sebifera management and preventive meas-ures for further spread along coastal areas of the southeastern USA.