1,720,993 research outputs found
The suitability of the degradation gradient method in arid Namibia
No full textThe Degradation Gradient Method (DGM) is a sophisticated technique for the assessment of range condition. It applies multivariate analyses of herbaceous species data to detect subtle degrees of overgrazing. The suitability of this multivariate method was tested in the central Highland Savanna of Namibia by comparing its results against a univariate analysis of herbaceous data in a simple but robust Range-Unit Model. Despite aridity and topographical heterogeneity, the DGM performed unexpectedly well under these conditions. The relative instability of this dry savanna system favoured the applicability of the DGM by promoting a clear grazing gradient. Using species density data only resulted in an incorrect outcome of the multivariate analysis. The sensitivity of the DGM could be improved by combining density and cover data
Desiccation of undamaged grasses in the topsoil causes Namibia’s fairy circles – Response to Jürgens & Gröngröft (2023)
No full textAsymmetric tree growth at the stand level: Random crown patterns and the response to slope
No full textAsymmetric tree growth is an adaptation to maximise photosynthesis by growing in response to gaps and neighbours, topographical site conditions or incoming solar radiation. Whereas spatial statistics have been widely used to study the distribution of trunk locations, less research has been undertaken to analyse the distribution of crown centres and asymmetric growth at the stand level. It is generally assumed that trees optimise light harvesting via more regular crown patterns. In this study, we primarily ask whether random crown patterns can be found in deciduous and coniferous forests located in continental Europe. Here, we analysed the spatial patterns of trunks, crowns and crowns of overstory trees in different deciduous and coniferous stands, using the scale-dependent g-function and Monte Carlo simulations. We also tested whether the extent of asymmetric growth, that is the crown vector length between the stem-base position and the centroid of the projected crown area, would be greater in angiosperms than in gymnosperms. Finally, we applied circular statistics to test whether trees preferentially bend in slope direction or towards incoming solar radiation. In the deciduous stands, patterns of crowns and upper crowns were random. Response to large-scale heterogeneity in light was strong, because trees bent significantly in downward direction of the slopes. The extent of asymmetric growth was significantly greater in angiosperms than in gymnosperms. The patterns of crowns and upper crowns were regular in a mixed coniferous stand but random in a dense stand with regularly planted Douglas-fir. Mechanical instability caused mutual crown support and attraction between the crowns in this dense stand. The even-aged, slender Douglas-fir clustered significantly in downward direction of the slope. In none of the four stands, trees clustered in southerly direction towards incoming solar radiation. {\textcopyright} 2007 Elsevier B.V. All rights reserved
Spatial patterns and demographics of Streblus macrophyllus trees in a tropical evergreen forest, Vietnam.
No full textStreblus macrophyllus is a shade-tolerant and subcanopy tree species common to tropical evergreen forests in northern Vietnam. However, its ecology is poorly known. We used spatial point pattern analysis to describe the spatial arrangement of tree individuals within a forest community dominated by S. macrophyllus. All individual trees with diameter at breast height larger than 2.5 cm in a 1-ha plot were mapped and measured. The overall pattern of this species was a regular distribution at scales up to 2 m. Its juveniles and subadults were strongly aggregated, but adult trees were regular at scales of up to 3 m, implying evidence of density dependent thinning. The spatial pattern of S. macrophyllus strongly affected the patterning of the whole plot. In S. macrophyllus, juveniles and subadults were similarly distributed relative to adults and showed additional clumping independent of the adults. The overall interspecific association between adults of other species and S. macrophyllus at different life-history stages also showed independence. We conclude that S. macrophyllus is a predominant competitor within the community and it follows a gap-phase regeneration mode
Spatial patterns and demographics of Streblus macrophyllus trees in a tropical evergreen forest, Vietnam.
No full textStreblus macrophyllus is a shade-tolerant and subcanopy tree species common to tropical evergreen forests in northern Vietnam. However, its ecology is poorly known. We used spatial point pattern analysis to describe the spatial arrangement of tree individuals within a forest community dominated by S. macrophyllus. All individual trees with diameter at breast height larger than 2.5 cm in a 1-ha plot were mapped and measured. The overall pattern of this species was a regular distribution at scales up to 2 m. Its juveniles and subadults were strongly aggregated, but adult trees were regular at scales of up to 3 m, implying evidence of density dependent thinning. The spatial pattern of S. macrophyllus strongly affected the patterning of the whole plot. In S. macrophyllus, juveniles and subadults were similarly distributed relative to adults and showed additional clumping independent of the adults. The overall interspecific association between adults of other species and S. macrophyllus at different life-history stages also showed independence. We conclude that S. macrophyllus is a predominant competitor within the community and it follows a gap-phase regeneration mode
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