Grain size affects the relationship between species richness and above-ground biomass in semi-arid rangelands

ABSTRACT

Background: Discrepancies in the shape of the productivity–diversity relationship may arise from differences in spatial scale. We hypothesised that there is a grain size effect on the productivity–diversity relationship.

Aims: To determine the effect of three sampling grain sizes on the productivity–diversity relationship.

Methods: We applied generalised linear mixed effect models on community data from 735 vegetation plots in the Taleghan rangelands, Iran, sampled at three grain sizes (0.25, 1 and 2 m²) to ascertain plant productivity-diversity patterns, while accounting for the effects of site, plant community type, disturbance, and life form.

Results: Overall, relationships between biomass and plant species richness were unimodal at grain sizes of 0.25 and 1 m², and asymptotical at 2 m². The spurious occurrence of a single large shrub may overwhelm a small-sized sampling unit, resulting in a high estimate of the sample’s biomass relative to species richness. However, the relationship between biomass and species richness at larger grain sizes is more likely to reach an asymptote.

Conclusions: Shrubs are partly responsible for driving the relationship between plant biomass and species richness. Given that the frequency of shrubs is highly variable between small plots but not so in large plots, their presence may result in unimodal productivity–diversity relationships at small but not at large grain sizes.

KEYWORDS:

• Above-ground plant biomass
• arid rangelands
• disturbance
• grain-size
• scale
• species richness

Acknowledgements

This study was supported by the University of Tehran (Grant No. 3870306). We would like to thank Mr Mohsen Hosseini and Dr Esmaeil Alizadeh for their assistance with fieldwork. We also thank Dr Azad Rastegar for plant identifications.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplementary data can be accessed here.

Additional information

Funding

This work was supported by the University of Tehran [Grant No. 3870306].

Notes on contributors

Anvar Sanaei

Anvar Sanaei has recently obtained his Ph.D. in range management. This study was part of his doctoral thesis.

Íñigo Granzow-de la Cerda

Íñigo Granzow-de La Cerda is a plant ecologist interested in the structure, and diversity patterns of bryophyte communities in the biological soil crust of arid landscapes, and in the dynamics of tropical rain forest tree communities, especially following intense natural and human-caused disturbances.

Luis Cayuela

Luis Cayuela is a plant ecologist. His research interests encompass the ecology of plant communities in tropical forests, the effects of human impacts on biodiversity, the dynamics of pest species in Mediterranean forests and agroecosystems, and the development of new statistical approaches to data analysis.