Background/Question/Methods In forest ecosystems, big-sized trees (i.e., large-diameter, tall-stature, and big-crown trees) are the main stand structures, and hence play a key role in climate change mitigation strategies. Although approximately three decades ago the ecological role of big-sized trees (also known as large or large-diameter trees) has been addressed in the scientific literature, the quantitative assessments for exploring the effects of big-sized trees relative to remaining-sized trees and species diversity on forest functions are not widely explored. In recent years, we (I and my several colleagues) addressed the following general research questions in multiple studies across study regions in Asian countries. 1) How do big-sized trees affect forest functions (i.e., aboveground biomass in most cases) directly and indirectly via species diversity and remaining-sized trees? 2) How do topography, climate and soil conditions influence big-sized trees, remaining-sized trees, species diversity and forest functions? 3) What are the relative contributions of abiotic factors, big-sized trees, remaining-sized trees and species diversity to forest functions? We used abiotic factors, forest inventory, and functional traits data across several hundred plots in tropical and temperate forests of China, Sri Lanka, Iran, and Pakistan, and then addressed the questions through structural equation modelling.
Results/Conclusions Here, I provided a brief review of results from our several studies for better understanding the big-sized trees' effect on forest diversity, structure and functions. Overall results showed that top 1-25% big-sized trees strongly increased aboveground biomass (i.e., explained over 50% of the accounted variation) compared to species richness (i.e., contributed less than 20%) and 99-75% remaining-sized trees (i.e., contributed less than 10%). Also, species richness increased aboveground biomass indirectly via increasing big-sized trees but via decreasing remaining-sized trees. As such, aboveground biomass gain and loss increased with increasing big-sized trees as compared to species richness and trait composition. In most cases, big-sized trees were sensitive to atmospheric drought, whereas the effects of soil nutrients and other abiotic factors on big-sized trees, remaining-sized trees, species diversity and forest functions varied across regions and biomes. Overall, results from our multiple studies across forest biomes and regions highlight the key overruling effect of big-sized trees on aboveground biomass stock and dynamics in response to environmental factors. I argue that the effects of big-sized trees on forest diversity, structure and functions should be tested for a better understanding of the ecological mechanisms and services that underpin human wellbeing.