Root turnover is a critical component of ecosystem nutrient dynamics and carbon sequestration and is also an important sink for plant primary productivity. We tested global controls on root turnover across climatic gradients and for plant functional groups by using a database of 190 published studies. Root turnover rates increased exponentially with mean annual temperature for fine roots of grasslands (r2 = 0.48) and forests (r2 = 0.17) and for total root biomass in shrublands (r2 = 0.55). On the basis of the best-fit exponential model, the Q10 for root turnover was 1.4 for forest small diameter roots (5 mm or less), 1.6 for grassland fine roots, and 1.9 for shrublands. Surprisingly, after accounting for temperature, there was no such global relationship between precipitation and root turnover. The slowest average turnover rates were observed for entire tree root systems (10% annually), followed by 34% for shrubland total roots, 53% for grassland fine roots, 55% for wetland fine roots, and 56% for forest fine roots. Root turnover decreased from tropical to high-latitude systems for all plant functional groups. To test whether global relationships can be used to predict interannual variability in root turnover, we evaluated 14 yr of published root turnover data from a shortgrass steppe site in northeastern Colorado, USA. At this site there was no correlation between interannual variability in mean annual temperature and root turnover. Rather, turnover was positively correlated with the ratio of growing season precipitation and maximum monthly temperature (r2 = 0.61). We conclude that there are global patterns in rates of root turnover between plant groups and across climatic gradients but that these patterns cannot always be used for the successful prediction of the relationship of root turnover to climate change at a particular site.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2000.00681.x

Global patterns of root turnover for terrestrial ecosystems

Forest age, which is affected by stand-replacing ecosystem disturbances (such as forest fires, harvesting, or insects), plays a distinguishing role in determining the distribution of carbon (C) pools and fluxes in different forested ecosystems. In this synthesis, net primary productivity (NPP), net ecosystem productivity (NEP), and five pools of C (living biomass, coarse woody debris, organic soil horizons, soil, and total ecosystem) are summarized by age class for tropical, temperate, and boreal forest biomes. Estimates of variability in NPP, NEP, and C pools are provided for each biome-age class combination and the sources of variability are discussed. Aggregated biome-level estimates of NPP and NEP were higher in intermediate-aged forests (e.g., 30–120 years), while older forests (e.g., >120 years) were generally less productive. The mean NEP in the youngest forests (0–10 years) was negative (source to the atmosphere) in both boreal and temperate biomes (−0.1 and –1.9 Mg C ha−1 yr−1, respectively). Forest age is a highly significant source of variability in NEP at the biome scale; for example, mean temperate forest NEP was −1.9, 4.5, 2.4, 1.9 and 1.7 Mg C ha−1 yr−1 across five age classes (0–10, 11–30, 31–70, 71–120, 121–200 years, respectively). In general, median NPP and NEP are strongly correlated (R2=0.83) across all biomes and age classes, with the exception of the youngest temperate forests. Using the information gained from calculating the summary statistics for NPP and NEP, we calculated heterotrophic soil respiration (Rh) for each age class in each biome. The mean Rh was high in the youngest temperate age class (9.7 Mg C ha−1 yr−1) and declined with age, implying that forest ecosystem respiration peaks when forests are young, not old. With notable exceptions, carbon pool sizes increased with age in all biomes, including soil C. Age trends in C cycling and storage are very apparent in all three biomes and it is clear that a better understanding of how forest age and disturbance history interact will greatly improve our fundamental knowledge of the terrestrial C cycle.

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Carbon cycling and storage in world forests: biome patterns related to forest age.

Conceptual frameworks relating plant traits to ecosystem processes such as organic matter dynamics are progressively moving from a leaf-centred to a whole-plant perspective. Through the use of meta-analysis and global literature data, we quantified the relative roles of litters from above- and below-ground plant organs in ecosystem labile organic matter dynamics. We found that decomposition rates of leaves, fine roots and fine stems were coordinated across species worldwide although less strongly within ecosystems. We also show that fine roots and stems had lower decomposition rates relative to leaves, with large differences between woody and herbaceous species. Further, we estimated that on average below-ground litter represents approximately 33 and 48% of annual litter inputs in grasslands and forests, respectively. These results suggest a major role for below-ground litter as a driver of ecosystem organic matter dynamics. We also suggest that, given that fine stem and fine root litters decompose approximately 1.5 and 2.8 times slower, respectively, than leaf litter derived from the same species, cycling of labile organic matter is likely to be much slower than predicted by data from leaf litter decomposition only. Synthesis. Our results provide evidence that within ecosystems, the relative inputs of above- versus below-ground litter strongly control the overall quality of the litter entering the decomposition system. This in turn determines soil labile organic matter dynamics and associated nutrient release in the ecosystem, which potentially feeds back to the mineral nutrition of plants and therefore plant trait values and plant community composition. © 2013 The Authors. Journal of Ecology © 2013 British Ecological Society.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2745.12092

Linking litter decomposition of above‐ and below‐ground organs to plant–soil feedbacks worldwide

Summary 1. Conceptual frameworks relating plant traits to ecosystem processes such as organic matter dynamics are progressively moving from a leaf-centred to a whole-plant perspective. Through the use of meta-analysis and global literature data, we quantified the relative roles of litters from above- and below-ground plant organs in ecosystem labile organic matter dynamics. 2. We found that decomposition rates of leaves, fine roots and fine stems were coordinated across species worldwide although less strongly within ecosystems. We also show that fine roots and stems had lower decomposition rates relative to leaves, with large differences between woody and herbaceous species. Further, we estimated that on average below-ground litter represents approximately 33 and 48% of annual litter inputs in grasslands and forests, respectively. 3. These results suggest a major role for below-ground litter as a driver of ecosystem organic matter dynamics. We also suggest that, given that fine stem and fine root litters decompose approximately 1.5 and 2.8 times slower, respectively, than leaf litter derived from the same species, cycling of labile organic matter is likely to be much slower than predicted by data from leaf litter decomposition only. 4. Synthesis. Our results provide evidence that within ecosystems, the relative inputs of above- versus belowground litter strongly control the overall quality of the litter entering the decomposition system. This in turn determines soil labile organic matter dynamics and associated nutrient release in the ecosystem, which potentially feeds back to the mineral nutrition of plants and therefore plant trait values and plant community composition.

Linking litter decomposition of above- and below-ground organs to plant-soil feedbacks

Fine root production and turnover in forest ecosystems in relation to stand and environmental characteristics

Litter production and leaf-litter decomposition of selected tree species in tropical forests at Kodayar in the Western Ghats, India

Vegetation structure and species composition of tropical ecosystems were studied through nine transects at Veerapuli and Kalamalai reserve forests in the Western Ghats of Tamil Nadu, India. Species diversity, dominance, species richness and evenness indices of plant communities and also population structure of woody plants were enumerated. A total of 244 species (183 genera and 76 families) were recorded. Species richness (number of species) were 82,142 and 96 species per 0.3 ha respectively for the study areas of low-elevation forest (LEF), mid-elevation forest (MEF) and high elevation forest (HEF). Species diversity indices were greater in MEF compared to the other two forests except juveniles. In contrast, greater dominance value indices were recorded in LEF than other forests. Density and basal area of the MEF were twice greater than the LEF, while HEF showed greater tree density and low basal area when compared to LEF. The stem density and species richness (number of species) decreased with increased size classes of trees observed in the present study indicated good regeneration status. Population structure of juveniles and seedlings also reflects good regeneration status. Terminalia paniculata (IVI of 99.9) and Hopea parviflora (IVI of 103.8) were dominant tree species respectively in LEF and MEF whereas in HEF Agrostistachys meeboldii (63.65), Cullenia excelsa (63.67) and Drypetes oblongifolia (39.67) share the dominance. Past damage (anthropogenic perturbation) may be one of the reasons for single species dominance in LEF and MEF. Occurrence of alien species such as Eupatorium odoratum and Ageratum conyzoides also indicated the past disturbance in LEF. The variations in plant diversity and population structure are largely due to anthropogenic perturbation and other abiotic factors.

Plant species diversity and tree population structure of a humid tropical forest in Tamil Nadu, India

the diversity, dominance, similarity, evenness and species richness indices of plant communities and also to assess the regeneration status of tree species. A total of 58, 77,125 and 105 plant species belonging to 30, 28, 52 and 45 families were recorded in moist deciduous forests (MDF, sites I & II), an evergreen forest (EF, site III) and a forest at higher elevation (HEF, site IV) respectively. Species diversity indices (Shannon index) of tree community were 2.20, 2.37, 2.65 and 2.48, ranking low compared to other studies available in the Western Ghats. Terminalia paniculata, Pterocarpus marsupium and Aporosa lindleyana were the dominant species in the moist deciduous forests, whereas Hopea parvijlora, Valeria indica and Xanthophyllum flavescens dominated in the evergreen forest. The forest at higher elevation was dominated by Agrostistachys meeboldii, Cullenia excelsa and Drypetes oblongifolia. Stem density and basal area of the evergreen forest (748 trees ha"1, 81.38 m2 ha1) were twice those of the moist deciduous forests (352-450 trees ha1,28.05-33.77 m2 ha1), while the forest at higher elevation showed higher density and lower basal area (1173 trees ha"1,72.72 m2 ha'1). The "L" shaped curve of different DBH classes of trees and saplings indicated good regeneration in these forests. Eupatorium and Ageratum are two dominant exotic weeds that form part of the ecosystem in open and disturbed sites here. The changes in species composition are largely due to transition in vegetation types influenced by anthropogenic perturbations and other abiotic factors.

Forest ecosystem structure and composition along an altitudinal gradient in the Western Ghats, South India.

Fine root biomass distribution and productivity patterns under open and closed canopies of tropical forest ecosystems at Kodayar in Western Ghats, South India

Vegetative and reproductive phenology of 42 tree species of tropical forests at Kodayar in the Western Ghats, Tamil Nadu, India was monitored through fortnightly visits for two years. A considerable variation was found in leaf flushing, leaf fall, flowering and fruiting behaviour that could be partly attributed to abiotic factors. Peak activity of leaf fall and leaf emergence that occurred in the early dry period, could be to take full advantage of the first rainy season for vegetative growth and reproduction. Peak flowering activity coincided with leaf fall/leaf flushing, possibly to attract pollinators. The activity of fruit ripening and fruit fall was at its peak in the first rainy season in order to utilize the available soil moisture for seed germination and seedling establishment. Phenological behaviour displayed by the trees is an adaptation to the surrounding abiotic and biotic environment.

P. S. Swamy

Papers

Litter production and leaf-litter decomposition of selected tree species in tropical forests at Kodayar in the Western Ghats, India

Plant species diversity and tree population structure of a humid tropical forest in Tamil Nadu, India

Forest ecosystem structure and composition along an altitudinal gradient in the Western Ghats, South India.

Fine root biomass distribution and productivity patterns under open and closed canopies of tropical forest ecosystems at Kodayar in Western Ghats, South India

Phenological behaviour of selected tree species in tropical forests at Kodayar in the Western Ghats, Tamil Nadu, India