Relationship between species diversity and tree size in natural forests around the Tropic of Cancer

Although numerous hypotheses have been proposed to explain the patterns of species diversity of forests at local and landscape levels,it is still difficult to predict the alpha diversity of species,especially in tropical and subtropical natural forests where trees of different sizes and shapes are highly mixed.Size might characterize species diversity,and the possible correlation between these variables may contribute to the development of easy-to-use growth indicators to predict diversity and to understand the status of trees within a stand.In this study,diameter classes were divided using the equal diameter class(EDC)and minimum measured diameter(MMD)methods,and five species diversity indices were calculated for each diameter class of 14 permanent plots in five national nature reserves surrounding the Tropic of Cancer(23.5°),southwest China.The results show that species richness,abundance,and spatial diversity indices decreased in a typical inverted J-shape pattern with increasing diameter class and MMD,and could be easily modeled by a negative exponential function.The ShannonWiener index showed a linear decrease while Pielou’s evenness index displayed a linear increase,with a small degree of instability.The results suggest that species diversity in subtropical forests is closely related to tree size,and the relationship is possibly independent of habitat.Measuring DBH of some trees in a stand could be informative regarding species diversity and contribute to the investigation and assessment of biodiversity.

Designing near-natural planting patterns for plantation forests in China

Background: China has a long tradition of managing planted forests. Different species of Populus, Eucalyptus, Larix, Cunninghamia and Pinus are planted to satisfy the local demand for wood products and provide ecological services at the same time. Evidence of the greater resilience of natural forests provides the motivation to develop asymmetric planting patterns, which is the focus of this study. We present a new method for designing plantation patterns that follow those observed in natural ecosystems and to maintain some regularity for operational convenience. Methods: Based on the uniform angle index, we analyzed the spatial structure of six natural forests in different regions of China. The uniform angle index describes the degree of spatial uniformity of the n nearest neighbors of a given reference tree. Accordingly, we identified all possible patterns of a neighborhood group within a regular planting pattern and developed a method to optimize planting point arrangements that contain some randomness as well as a minimum degree of regularity. Results:(1) There are 13 types of structural units in a regular planting, including seven random units, five even units and one cluster unit;(2) Five near-natural arrangements are presented with a minimum proportion of 50% of random units. These five arrangements represent a combination of regularity for operational convenience and asymmetry. Conclusions: The new planting patterns developed in this study are expected to increase the asymmetric competition and resilience of these important ecosystems. Some experimental plantings, based on our findings, have already been established, e.g., in Pinus tabulaeformis plantations in Tianshui, Gansu Province, and in a Populus deltoides plantation in Fangshan near Beijing.

Diversity and production in an Afromontane Forest

Background：This contribution evaluates the effect of forest structure and tree species diversity on plot productivity and individual tree growth in the unique Knysna forests in Southern Africa using mapped tree data from an observational study that has been re-measured over a period of 40 years.Methods：The effects of tree species diversity and forest structure on tree growth and forest production are evaluated on three levels of resolution：a） the forest community（canopy,sub-canopy species）,b） the subplots（number of trees per ha,skewness of the diameter distribution,diameter coefficient of variation） and c） the immediate neighborhood of selected reference trees（＂Mingling＂,＂Dominance＂,Aggregation＂ and ＂Size Variation＂）.Results：An analysis of the community level identified two distinct clusters,one including dominant/canopy species with the highest growth rates and a greater variation of growth,and another cluster which includes the remaining subcanopy species which have a smaller maximum size and lower rates of growth.The area-based structure variables on plot level have a highly significant effect on total basal area growth.However,the effects of forest density and species richness on productivity were not straight forward.Maximum basal area production of about 0.75 m^2/ha/year is achieved at medium levels of richness（around 20 species per ha） and medium levels of density（around 30 m^2/ha basal area） using percentile regression estimates.The relative ＂Dominance＂ of a selected reference tree had a highly significant effect on individual tree growth on all investigated species.Other neighbourhood structure variables were only occasionally significant or not significant at all.Conclusion：This contribution presents a new theoretical framework for analysing natural forests that includes community,plot and neighborhood variables of forest structure and diversity,and a first specific analysis of the structure and dynamics of the Knysna Afromontane Forest,based on a unique set of longterm observations.The species-area（SAR） model developed in this study,represents a new general approach that can be used to derive a common standard of tree species diversity for different plot sizes,the species richness per hectare.