The architectural and functional aspects of the rooting system determine the survival of the trees (Christina et al. 2017). Fine root distribution in a particular area resonates with the nutrient availability within that ecosystem (Vitousek & Sanford, 1986). Soil depth, canopy closure, density of the trees in the stand, aboveground biomass, site specific conditions and other management practices profoundly influence the FRB and FRT (Finer et al. 2011; Pie et al. 2018).
Fine root biomass
Across the investigated sites, the mean FRB ranged between 118.48 and 805.95 kg ha-1 (0.11-0.80 Mg ha-1) which was somewhat similar to the value (0.6–22.7 Mg ha-1) reported for tropical broadleaf evergreen forests by Vogt et al. (1996) and Finer et al. (2011) but relatively lower (1.1–10.6 Mg ha-1) than the value reported by Wen et al. (1999) and Yang et al. (2004) in world’s subtropical forests and by Wang et al. (2017) in Chinese forests (2.78 Mg ha-1).Compared with agroforestry trees (78.39 and 528.59 kg ha-1) of the bhabhar region as reported by Karki et al. (2021), the FRB of S. robusta was relatively higher.
In conformity to the earlier published reports (Upadhaya et al. 2005, Yuan et al. 2010) among all the four sites, fine root biomass reached its top notch value in the rainy season. These findings suggested that in subtropical and tropical forests highest FRB reported in the rainy season corresponds to periods of nutrients release. Yuan and Chen (2012) reported the role of soil microclimatic conditions in regulating intrannual fluctuations in FRB. Fine roots because of their morphological and anatomical properties have more prominent role in water absorption rather than nutrient absorption, thus water serves as a crucial factor in controlling FRB and FRP (Eissenstat & Yanai, 2000). The FRB increased in the moist humid climate of the rainy season (July-September) during the study which corresponds to immediate rapid vegetative growth of the trees after summer induced dormancy due to hot and dry climate. Higher precipitation during the rainy season causes the soil to become rich in soil moisture content thus, increases the transport of nutrients from soil to the surface of roots which results in abundance of nutrients at superficial surface which leads to vigorous expansion and proliferation of root systems. Some other reports also supported our observation as according to them, fine root biomass wavered seasonally and fine root growth reached maximal limits during warm and moist months (Tierney et al. 2003; Kitajima et al. 2010). Arunachalam et al. (1996) and Sundarpandian & Swamy (1996) observed the seasonal fluctuations in FRB in sub tropical and tropical forests and higher FRB was documented during moist and nutrient rich rainy season. However, FRB was not only linked to fine root growth but was also associated with fine root lifespan and FRT.
The results obtained for the vertical distribution of FRB were in accordance with findings of Karki et al. (2021a and b) Barbhuiya et al. (2012) in forest ecosystems and land use systems of Himalayan region which specifies the abundance of fine roots in upper layer of soil and decreased toward deeper soil (40-60cm) in all the sites and seasons. Across all the sites and among all the seasons, the uppermost soil depth contributed (19.05-57.64%) of total FRB, sub surface (20-40 cm) layer contributed (21.52-50.68%) while,lowermost soil depth (40-60 cm) contributed (22.10-50.50%)(Fig. 7). Kaushal et al. (2019) and Maycock & Congdon (2000) also elucidated that the topmost layer of soil is nutrient rich due to the release of nutrients from the litter which is responsible for ample fine root growth in uppermost layer of soil. In the present study, study sites varied in FRB in response to varying soil depth. Site S1 demonstrated highest depth related variation, having the highest slope value, whereas site S3 displayed lowest variation with minimum slope value (Fig.8).
The present study elucidates that nutrient availability in association with soil physical and chemical characteristics is one of the crucial factor that influences morphological and physiological aspects of tree fine roots (Yuan and Chen 2012). The horizontal distribution of FRB was similar to the trend reported by Karki et al (2021a). In all the sites, the FRB decreased with increasing distance from tree bole, with the highest FRB in the 1 m distance.
The girth class usually correspond to the tree age i.e. girth size increases with increasing tree age as a result mature trees has higher girth size as compared to young trees. In this study, FRB showed a fluctuating trend; increased first and then decreased with increasing girth class (Fig. 9). Among all the sites, the higher FRB values were contributed by younger trees representing size classes A(43.38- 54.94 %) and C (43.22 - 61.23%) during the rainy season as compared to older trees with size classes D (43.70-46.20% ) and E (35.27-56.54%) (Fig.4). The greater proportion of fine root in small girth classes represents a strategy to produce more fine roots to increase absorption area for acquisition of resources and in turn, return organic matter to the soil to enhance plant growth. The age of the stand has a huge impact on FRB and generally, the younger stands possess more FRB in comparison to older stands (Claus and George (2005). Similar findings were reported by Pei et al (2018), while negative interaction among stand age and FRB was observed by Leuschner and Hertel 2003. Several studies conducted on FRB dynamics (Finer et al. 2011; Di Lorio et al. 2013) also reported that FRB was tremendously affected by tree age. The variation in FRB at different girth sizes in forest may be affected by several climatic and edaphic factors, stand density, canopy closure and management practices (Finer et al. 2011). During the initial years of planting, plants tend to absorb more nutrients and water for ample growth and development of above ground components thus, higher amount of nutrients are assigned to fine roots that brings about higher rate of production of fine roots with the progression of stand age that subsequently declines after reaching a maximum limit (Jagodzinski et al. 2016).
Fine root production (FRP) and Fine root turnover (FRT)
The current study estimated annual FRP within a range of 488 -2483 kg ha-1 yr-1. Sites and soil depth influenced the annual FRP as annual FRP varied among the sites and decreased with the rise in the soil depth. Ford and Deans (1977) also reported similar trend of fine root allocation depthwise. Forest litter releases nutrients that govern and alter the production of fine roots (Cuevas and Medina 1988). Higher proportion of litter accumulated in the surface of the soil regulate the microclimatic conditions of the soil thus, provides an appropriate strata for growth and development of fine roots (Jordan and Escalante 1980). The FRP decreased with increasing distance from the tree stem. Among girth classes, highest FRP (2483.60 kg ha-1 yr-1) was reported for girth class C. Pei et al. (2018) and Yuan and Chen (2010) also reported that FRP first increase with increasing stand age to a peak and then declined towards old aged stands.The relationship of FRP with basal area (Helmisaari et al. 2007) is well established. However, tree age may negatively (Messier and Puttonen 1993), positively (Helmissari et al. 2002) affect the FRP or have no effect (Finer et al 2011). DBH also affects FRP positively (Li et al. 2003). Site characteristics and variability among species also serve as a key factor that shapes the FRP (Aerts et al 1992). The higher FRP in site S4 corresponds to the prominent tree density, elevated moisture content of the soil and nutrient –rich condition (high soil C and N) prevailing in this forest site (Table 1 and 2).
Fine root turnover tends to fluctuate with tree species composition, stand age as well as soil characteristics and plays a crucial role in determining the global C budget, nutrient flux forest ecosystems (Fonseca et al. 2012). The turnover rates reported in the experimental sites (2.03-3.52 yr–1) was relatively greater than the values (0.4–2.8 year−1) reported by Fukuzawa et al. (2013), McCormack et al. (2014) in fruit trees. Root turnover also varied across the sites and soil depth and the values reported surpassed the values reported in Himalayan Banj Oak and Chir Pine forests (Usman et al. 1999) and forests of Sweden (Persson 1983). Higher turnover rate might correspond to higher soil temperature as with the increase temperature upto a certain extent the microbial activities also increase which results in higher nutrient availability. In boreal forest ecosystems broad leaved species had higher FRT than needle leaved species (Yuan et al. 2010).
Effects of soil characteristics on FRB, FRP and FRT
Pearson’s correlation test (Linear) was performed to determine the relationships between different soil parameters and fine root characteristics (Fig. 10).The result of the correlation matrix revealed that few soil parameters were highly correlated with each other expressing both negative and positive correlation while there was no correlation between certain studied variables. Soil depth showed strong negative correlation with FRB FRP, TN C: N ratio and porosity as these variables decreased with increasing soil depth while bulk density showed strong positive correlation with soil depth. Bulk density and porosity were negatively correlated with each other. FRB and FRP showed positive correlation with C:N ratio while FRP and FRT showed significant positive correlation with TN. Plants have a tendency of imparting more biomass to roots during the scarcity of resources in the environment as per the concept of optimal partitioning theory (Bloom et al. 1985). FRB is negatively while FRT is positively influenced by nitrogen content of the soil.
Soil pH showed negative correlation with FRB and FRP and positive correlation with FRT indicating that FRP is immensely affected by pH of the soil. Zhou et al. (2017) reported that acidic soils inhibit the microbial growth and activity thus, root growth can be stimulates in soils with a higher pH. FRB and FRP showed strong positive correlation with porosity. Climatic, edaphic and geographical factors, determine the variations in FRB, FRP and FRT (Hendrick and Pregitzer, 1993; Pregitzer et al. 2000; Finer et al. 2011) as these are the major determinants regulating lifespan, survival and death of fine roots. The variations in soil characteristics among sites are related to space and time because of variations in topography, climate, weathering activities (Bargali et al. 1992 and 1993; Baumler and Zech 1994; Bargali et al. 2018). Baumler (2015) also stated that the soil characteristics frequently vary within the short distances in the region. FRT showed negative correlation with FRB and FRP (Fig 10).