Spatial size diversity in natural and planted forest ecosystems: Revisiting and extending the concept of spatial size inequality

Highlights

• Spatial size inequality was successfully extended to better describe local size effects in semi-natural woodlands in China.

• We demonstrated the value of the new size segregation function through cluster analysis.

• Evidence of a gain in information quality was obtained from applying partial spatial reconstruction.

• Spatial species mingling and spatial size inequality are often closely related to each other in complicated ways.

Abstract

Plant size diversity or plant size inequality is a much neglected aspect of plant diversity and biodiversity. And yet size diversity plays an important role in understanding the dynamics of plant diversity and natural mechanisms of maintaining plant diversity, which is of great significance in a time of ongoing global changes. In this study, we introduced and investigated a new measure of spatial size inequality, namely the size segregation function. The new characteristic depends on distance and turned out to be a meaningful and more general extension of the traditional size differentiation index, because it describes local size inequality more extensively. To test the size segregation function we analysed and modelled thirteen large, fully mapped plots from temperate and subtropical forest ecosystems in China. We tested and validated the new function through cluster analysis and a special variant of partial spatial reconstruction. Our results showed that the size segregation function discriminated well between different patterns of spatial size diversity in the complex, mostly natural forests of China. Two random-labelling tests used in the study also highlighted how closely linked spatial species and size diversity are in such species-rich forest ecosystems: Often each species represents a distinctive size range and then spatial size inequality can be interpreted as being a consequence of spatial species mingling. This close relationship between species and size should also be taken into account in analyses of species-rich plant populations. For practical conservation this implies that high size inequality can often be found where there is high species mingling and vice versa.

Introduction

Diversity in forest ecosystems can be considered in various ways including compositional, functional and structural aspects (Peterson, 2019). Plant diversity and its contribution to biodiversity at population and landscape level is in itself important and with ongoing climate change interest in understanding the mechanisms of natural maintenance of biodiversity is growing (Connell, 1971; Fibich et al., 2016; Ford, 1975; Isbell et al., 2018; Janzen, 1970). Plant diversity, however, also has a strong link with population traits, competition-trait relationships and niche complementarity and differentiation. Bourdier et al. (2016) emphasized that plant diversity in particular influences ecosystem productivity and stability.

In the past, structural diversity of plant communities has mainly been studied through the lens of species diversity (Gaston and Spicer, 2004). Particularly in forest ecosystems, the ability of trees to develop considerable sizes and thus to raise their photosynthetically active canopy well above other plants has been an important evolutionary trait innovation. This innovation allowed trees to adapt their sizes according to environmental requirements and thus to elude competition by other plants that did not have this ability. Consequently this trait adaptation of tree populations involving different sizes typically leads to size diversity also referred to as size hierarchy or size inequality (Weiner and Solbrig, 1984) in the ecological literature. Contrary to its importance size diversity has so far not been studied much. In addition, natural disturbances and human management impact on size diversity in forest ecosystems in various ways (Bourdier et al., 2016). For example, traditional forest management terms such as “even-aged” and “uneven-aged” forests or forest management can be interpreted as proxy terms and in reality often refer to forests with “low” and “high size diversity”. They stem from a time when individual-tree age and size were considered to be highly correlated, a relationship that we now know to hold only loosely for certain species and population development stages.

In mixed forest ecosystems, several authors found positive (Hardiman et al., 2011; Lei et al., 2009) and/or neutral (Long and Shaw, 2010) relationships between size diversity or stand structural complexity and productivity. For monospecific stands of Pinus ponderosa Douglas ex C. Lawson, Fagus sylvatica L. and Abies alba Mill. neutral or negative relationships were highlighted (Cordonnier and Kunstler, 2015; Long and Shaw, 2010). In young eucalypt (Eucalyptus spp.) plantations, a loss in wood biomass was also observed due to increased size heterogeneity of stand structure (Ryan et al., 2010). In a coniferous forest, Liang et al., 2005, Liang et al., 2007 found for different species a decrease in stem diameter growth and recruitment or an increase in mortality associated with increased size diversity. Bourdier et al. (2016) concluded from an analysis combining inventory data of ten European species and a light competition model that negative relationships between size inequality and productivity may be the rule in tree populations, i.e. tree size inequality reduces forest productivity.

This is offset by the general hypothesis that forests with greater structural variability should provide for more niche diversity and therefore greater species diversity (Peterson, 2019). Size and species diversity are therefore often related and analyses of both usually complement each other. Pommerening and Uria-Diez (2017) and Wang et al. (2018), for example, found in many very different ecosystems in different parts of the world that trees with high spatial species mingling often tend to be larger sized trees. They referred to this phenomenon as the mingling-size hypothesis. In this context, spatial species mingling is defined as the proportion of heterospecific pairs of plants among the k nearest neighbours of a given plant i (Aguirre et al., 2003; Gadow, 1993; Pommerening, 2002). In an attempt to understand mingling patterns in complex multi-species natural woodlands better, Pommerening et al. (2019) revisited and successfully extended the mingling index, a metric for spatial species diversity. Since size diversity has been much neglected in the past and since there is a relationship between size and species diversity, this paper aims at accomplishing the next logical step by revisiting and extending spatial size inequality.

Most indicators and measures of diversity are non-spatial (Krebs, 1999; Magurran, 2004). Spatially explicit characteristics additionally provide information on spatial scale and can thus better identify localised processes and trends, for example, among the nearest neighbours. In the 1960s and 1970s first indices were proposed providing a single number for a given plant population (Pommerening, 2002). These were later replaced by functions of interplant distance r, particularly in point process statistics (Illian et al., 2008). To date spatial measures of diversity are still rarely used in diversity research.

The objective of this work therefore is to study size inequality in mostly species-rich temperate and subtropical natural and planted forest ecosystems in China. Since both species and size diversity are naturally very high in these ecosystems, they constitute interesting test cases. To achieve this analysis better we introduced and tested an extension of the size differentiation index (Gadow, 1993), which is capable of taking different spatial scales into account in analogy to the species segregation function introduced by Pommerening et al. (2019). We have validated the new size inequality characteristic with the partial reconstruction technique and the mark differentiation function (Hui and Pommerening, 2014; Pommerening et al., 2011).