Abstract
BackgroundA number of hypotheses and theories, such as the Janzen-Connell hypothesis, have been proposed to explain the natural maintenance of biodiversity in tropical and temperate forest ecosystems. However, to date the details of the processes behind this natural maintenance are still unclear. Recently two new nearest-neighbour characteristics were proposed and in this paper we demonstrate how they can contribute to a better understanding of the ontogenesis of global forest structure from localised neighbourhoods.MethodsWe applied the new species and size segregation functions together with appropriate test procedures to four example woodland data sets from China at Daqingshan, Jiaohe, Jiulongshan and Xiaolongshan forest regions. In addition we quantified the morphology of the new characteristics and modelled a neighbourhood allometric coefficient linking the two functions.ResultsThe results revealed quite different species segregation patterns with both conspecific and heterospecific attraction. We found these to be generally matched by equivalent size segregation patterns of attraction of similar and different sizes. It was straightforward to model the size segregation function from the knowledge of the species segregation function by estimating a neighbourhood allometric coefficient.ConclusionsThe new characteristics have helped to quantify the extent and rate of decline of neighbourhood interactions in terms of spatial species and size diversity. Through the allometric neighbourhood coefficient the analysis highlighted once more how closely related species and size segregation are, thus supporting the mingling-size hypothesis. Using both a traditional and a restricted random-labelling test has provided a valuable tool for understanding the exact nature of species-mingling and size-inequality relationships.
Highlights
A number of hypotheses and theories, such as the Janzen-Connell hypothesis, have been proposed to explain the natural maintenance of biodiversity in tropical and temperate forest ecosystems
Many theories and hypotheses have been proposed to explain the maintenance of plant and tree diversity in forest ecosystems (Pommerening and Uria-Diez 2017)
EM, as explained in Pommerening et al (2019) and in analogy to Pielou’s segregation index, can be used to normalise species mingling in such a way that the resulting measure Ψ is independent of the number of species and refers to the null hypothesis of independent species marks according to Pommerening and Uria-Diez (2017) referred to the index defined in Eq (2) as species segregation index
Summary
A number of hypotheses and theories, such as the Janzen-Connell hypothesis, have been proposed to explain the natural maintenance of biodiversity in tropical and temperate forest ecosystems. The Janzen-Connell hypothesis (Janzen 1970; Connell 1971), for example, proposes that elevated numbers of specialist natural enemies, such as herbivores and pathogens, maintain diversity in plant communities They reduce the survival rates of conspecific seeds and seedlings located close to reproductive adults or in areas of high conspecific density (Comita et al 2014) leading to elevated conspecific self-thinning (Yao et al 2016), i.e. a progressive decline in density in a population of growing individuals of the same species An important effect of the Janzen-Connell hypothesis is the negative density/distance dependence that occurs when nearby conspecific plants negatively affect performance through mechanisms such as intraspecific competition and pest facilitation (Wills et al 1997; Wright 2002; Piao et al 2013; Yao et al 2016) Another important ecological hypothesis in this context, the herd immunity hypothesis, focuses on the variation in heterospecific neighbour densities and predicts that species diversity confers protection from natural enemies by making it more difficult for specialist natural enemies to locate host plants (Wills et al 1997). Individual plant fitness should be enhanced in stands involving many species but reduced in stands with low species diversity (Wills et al 1997; Murphy et al 2016)
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