ECR Feature: Qiong Cai on niche breadth and range size in beech trees

Qiong Cai is a postdoc at the College of Urban and Environmental Sciences, Peking University. She is a community ecologist interested in forest ecosystems. Qiong shares her recent work exploring the “niche breadth hypothesis” across the worldwide distribution of Fagus beech tree species.

Qiong Cai.

Personal links. Google Scholar

Institute. College of Urban and Environmental Sciences, Peking University

Academic life stage. Postdoc

Major research themes. Community ecology of forest ecosystems; Phytogeography

Current study system. Currently, my research focuses on plant communities in forest ecosystems, especially beech (Fagus) forests in China and worldwide. Species in the genus Fagus are one of the most dominant and representative tree species in the temperate forests of the Northern Hemisphere, playing vital roles in ecosystem functions and wood production. I’m particularly interested in exploring how the differences and similarities among beech dominated forests relate to the disjoint distribution of Fagus species across the Holartic region.

Qiong in the field.

Recent paper in JBI. Cai, Q., Welk, E., Ji, C., Fang, W., Sabatini, F. M., Zhu, J., … & Bruelheide, H. (2021). The relationship between niche breadth and range size of beech (Fagus) species worldwide. Journal of Biogeography.

Motivation behind this paper. Do species with capacity to grow under a wider set of environmental conditions (wide niche breadths) also have broader geographic distribution ranges? This question has been extensively discussed in the field of macroecology and biogeography and is referred to as the “niche breadth hypothesis”. Despite some empirical confirmations, a positive relationship between niche breadth and geographic range has not been systematically observed. We have no idea whether this depends on the taxonomic groups examined or on the methodological approaches used to estimate niche sizes. During my PhD, one of my research focuses was the community ecology of beech (Fagus) forests in China. Later I spent one year at the Martin-Luther University Halle-Wittenberg (Germany), where I used the global vegetation plot database, sPlot (Bruelheide et al., 2019), as well as the data we sampled in China, to test whether the niche breadth hypothesis holds in this key genus of northern temperate forests.

Key methodologies. We estimated niche breadth using a novel approach, the co-occurrence-based method, that uses community turnover rates across plots (the taxonomic β diversity) as a measure of species’ niche breadth. The fundamental assumption is that generalist species should occur within a broader range of communities and have a higher number of co-occurring species, compared to specialists. Using species co-occurrences to quantify the realized niche of a species allows accounting for species’ response to multi-dimensional environment gradients as well as species interactions. This overcomes issues in older resource-based methods such as depending heavily on the selection of environmental variables, because any selection of variables will only partially represent the whole multidimensional niche as defined by Hutchinson.

Illustrations of two beech species in China. (left) Fagus longipetiolata, and (right) F. lucida. Painted by Qiong Cai.

Major challenges. The major challenge was to combine different data sources. We had to harmonize the species nomenclature of data in China with the taxonomic backbone of sPlot. Given the huge number of species occurring in beech-dominated communities worldwide, this process turned out to be extremely challenging. Yet, this allowed us to establish one of the largest datasets of Fagus plots ever assembled.

Major results. We found no correlation between the size of a species’ niche and its geographic range, and this result was independent from measuring niches using biotic or climatic criteria. Notably, the widespread North American beech (F. grandifolia) had a distinctly smaller biotic niche breadth than the Chinese beech species, whose distributions are restricted to few isolated mountain ranges. In addition, neither biotic nor climatic niche breadths were under phylogenetic control. We interpret the lack of a general positive range size–niche breadth relationship within the genus Fagus as the result of the widespread distribution of these species, the high among-region variation in available niche space, landscape heterogeneity, and Quaternary history. Our study could improve our understanding of the possible roles of niche breadths in shaping species’ geographic distributions for different taxa/clades.

Beech forests in Germany (left) and in China (right), with more diverse understory layer in the latter.

Next steps. The geographic distribution of a species is not just controlled by its niche breadths, but results from complex interactions among a species’ extrinsic and intrinsic ecological requirements, its evolutionary history and the landscape configuration. This emphasizes the need to quantify the role of other factors (e.g., niche position) in shaping range size of the beech species in further studies.

If you could study any organism, what would it be? Actually, plants are my favourite organisms. Well, it might also be interesting to study the interactions among different organisms, including plants, animals and/or microorganisms. Organisms are not independent individuals but a tiny part of the ecosystems.

Anything else? In China, beeches are far less known and studied compared to those in Europe and North America, possibly because the Chinese beeches are usually distributed in remote mountains. To collect the raw plot data, my colleagues and I climbed over 60 mountains during several years (hard yet awe-inspiring experience!). So, you could imagine how surprised and excited I was when I visited Germany and realized beech trees could be commonly found in gardens and roadsides! This research would not have been possible without all the vegetation plot data, which people from all over the world deposited in the global vegetation database, sPlot. Thus, I would like to thank again all the data contributors. Such large databases, and the sharing philosophy that inspires them, are becoming more and more crucial for macroecology and biogeography research, and will play an increasingly important role in shaping our understanding of the patterns and trends of the world’s biodiversity.

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