Unraveling the Unfathomable Domain of Fine Root Dynamics in the Forest Ecosystems

Introduction

The fine roots are the key determinants of belowground ecological processes that govern sustenance of the forest ecosystems. Fine roots have a major role in water and nutrient acquisition and regulation of carbon, nitrogen dynamics through belowground carbon and nitrogen fluxes and decomposition.^1-4 Fine root production rates vary within a growing season with peak values during months with ample amount of resource availability and suitable microclimatic conditions.^5,6 Fine roots are functionally important fraction of net primary production which act as a source and sink for carbon in the terrestrial ecosystems. Due to their high turnover rates and association with soil biota, they have a major role in soil stabilization and nutrient restoration.^7,8

The ecologically significant physiological and functional traits of fine roots have drawn the attention of ecologists in the recent years resulting in researches focused on fine root ecology. Further researches are required to explore the complex relationships between root traits and strengthen the foundation of their functional importance at ecosystem level. Multidisciplinary assessment including detailed investigation of fine root morphology, physiology, chemical attributes, and their interaction with the soil environment is prerequisite to link belowground traits to ecosystem functioning.^9,10 The forest land use changes due to rapid urbanization and population growth and its deleterious impact on forest structure and belowground systems need intricate detailing to interpret the overall ecological impact.^11-13

Multifaceted influencers of fine root dynamics

Fine root dynamics encompasses complex network of biotic and abiotic factors which are entangled to balance the ecological proceedings at ecosystem level. Fine roots adapt to different strategies for growth and survival under varying environmental gradients.^14,15 The abiotic factors such as temperature, moisture, soil texture, soil pH significantly affect the microbial activities, fine root growth and decomposition¹⁶. The relationship between fine root productivity and temperature, precipitation in forest ecosystems has been well investigated and documented.^17,18 The organic constituents of the fine roots such as lignin, carbon and nitrogen content also have a role in fine root dynamics, as higher lignin contents make the fine roots recalcitrant and less susceptible to degradation. The microclimatic conditions of the soil such as nutrient, water availability, aeration, microbial communities along with the ecological associations of fine roots with other species also influences the physiological and functional ability of fine roots and affects fine root production.^19,20

The floristic composition, forest structure, diversity and density of tree species also affect the fine root dynamics, as fine root system attributes vary tremendously among different tree species.²¹ The multifariousness in soil nutrient distribution also affects the spatial distribution of fine roots. High species richness has a positive impact on fine root dynamics, as it results in competition among the species for resources, ultimately leading to niche differentiation and better use of soil resources for higher fine root productivity.²² 

Soil and fine root Interface

Fine roots play a crucial role in altering physical, chemical and biological activity of soil and thus ability to transform and stabilize soil and in turn morphological and physiological changes in fine root with changing edaphic environment.²³ Fine root proliferation results in increased porosity of soil that improves air circulation and enhances microbial activities. The soil bulk density also effects the fine root dynamics, as higher bulk density of soil results in mechanical resistance that obstruct the proliferation of fine roots.²⁴ The compaction of soil is reduced which facilitates water infiltration.^25,26 The nitrogen and carbon concentrations of the soil regulate the metabolic activities in the fine roots and soil microbes.^27-29, thus affect the physiological functions such as nutrient uptake and transport and resource allocation.^30,31 Fine roots exudates various substances such sugars, amino acids, proteins, phenolic compounds that act as nutrient and energy supplier for the microbial communities residing the soil, thus are involved in rhizosphere priming and maintaining soil health.^32,33 

Ecological aspects of fine roots

Fine roots account for a major fraction of net primary productivity in forest ecosystems and the turnover and decomposition of fine roots adds 30–80% of organic carbon to the soil.^34,35 Fine root litter accounts for 33% of annual forest litter inputs, thus have a crucial role in returning nutrients back to the ecosystem.^36-37 Fine roots also play a crucial role in belowground carbon sequestration and nutrient cycling.³⁸ Fine roots are highly sensitive to changing environments and adapt to different resource use strategies under varying environmental limitations.³⁹

Conclusions

The current strengths, challenges, and gaps in the field of fine root needs to be identified and evaluated to formulate future research in the field of root ecology based on systematic framework of hypothesis to establish well-fortified links between root traits and their benefaction to the effectiveness of a forest ecosystem. The impact of fine root functional traits on rhizosphere environment needs to be assessed to determine overall community structure and resource utilization in forest ecosystems. Coupling vegetational attributes, soil characteristics and factors involved with fine roots at wider scale might have implications for sustainable management of forest lands under constantly changing landscapes. 

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