Fine Root Longevity And Influencing Facotrs In Temperate Tree Species In Northeastern China

Fine root (diameter<2 mm) production and mortality are defined as turnover. Fine root turnover plays a key role in regulating carbon (C) allocation and nutrients cycles in forest ecosystem. Fine root longevity is the important parameter for the precise estimate of fine root turnover. However, conceptual and methodological constraints may have limited our understanding of fine root longevity. The temperate forest in northeastern China accounts for 35.3% and 34.5% of national total area and stocking volume, respectively, thus play a key role in C budgets at regional level and global scale. However, to date, little information about fine root dynamics and longevity of Chinese temperate tree species has been reported. In this study, the minirhizotron approach was used in five temperate tree species in northeastern China to determine the dynamics of fine root production and mortality, and to estimate the root longevity during two growth seasons from 2008 and 2009. Climate factors including air temperature, soil temperature at 10 cm depth and precipitation, and some root structural and physiological traits were measured concurrently. The five species include two deciduous hardwoods Juglans mandshurica and Phellodendron amurense; and three evergreen conifers Pinus koraiensis, Pinus sylvestris var. mongolica, and Picea koraiensis. The aims of this study were:(1) to quantify the seasonal patterns of fine root production and mortality in five tree species, and determine what climate factors may have strong effects on root dynamics; (2) to estimate and compare root longevity across species, and determine how fine root diameter, branch order, season and soil depth of root birth would influence the longevity; (3) to investigate the potential correlation between longevity of the first order root and root structural or physiological traits, as well as the potential correlation between root structural or physiological traits. Our results show that:(1) Total root length production differed markedly among species. The seasonal patterns of root production were similar across tree species, with the peaks consistently occurring in summer in two years. Picea koraiensis had the largest production on the annual basis (1.41 mm cm-2 a-1), while P. amurense was lowest (0.23 mm cm-2 a-1), leaving Pinus koraiensis, J. mandshurica and P. sylvestris var. mongolica in the mediate. Fine root mortality occurred simultaneously with production, but which peaks differed among species and years. Three diameter classes root population, i.e.0-0.5 mm,0.5-1.0 mm and>1 mm, showed similar seasonal pattern in production with total roots, but their mortalities are different between each other. Regression analysis showed that there were positive correlations between total root length production and air temperature, soil temperature at 10 cm depth and rainfall, in which air temperature had the strongest but precipitation weakest impacts. Roots with 0-0.5 mm diameter size showed the most sensitivity to environmental variables, exhibiting the consistant and close correlation with environmental variables. However, there were weak correlations between length mortality of total roots or differed diameter size roots and climate factors.(2) During two growth seasons from 2008 to 2009, median root longevity differed significantly among species, ranging from 276 d (P. amurense) to>430 d(Pinus koraiensis). However, conifers did not consistently have longer root longevity than hardwoods, and Cox proportional hazards regression analysis showed the similar results when controlling for other covariates (factors). Survival analysis (Log-rank test) showed that the longevity tend to be larger in roots with greater diameter, and higher branch order or born in deeper soil depth across species. In comparison, roots born in summer or autumn, more often had longer longevity than those born in spring. However, when controlling for other covariates, Cox analysis showed that only coniferous root longevity significantly increased with diameter increasing, and just root longevity in Picea koraiensis shifted with branch order. In contrast, influences of season and soil depth of root birth were more general. Three out of 5 species displayed significant longer root longevity in summer or autumn, and root longevities in 4 out of 5 species significantly increased with soil depth increasing. (3) Incorporating previous results from Fraxinus mandshurica and Larix gmelinii, the present study showed that even the first order roots, the evergreen confers did not have longer longevity than deciduous hardwoods or conifers. In comparison with three hardwood species, four coniferous species all had higher tissue density, lower nitrogen (N) content, N:C ratio, and maintaining root respiration rate. There were differed extents of the variation of root traits, with the largest CV (coefficient of variation,0.701) in root length and the lowest in N:C ratio (CV=0.156). We did not find the significant correlation between root longevity and any other root traits. In comparison, there were significant positive correlations between specific root length (SRL) and branching ratio (BR), so did to N, and N:C ratio or maintaining root respiration rate. However, there were significant negative correlation between tissue density and N content, N:C ratio or root respiration rate, and between diameter and SRL or BR.In sum, this study reveals the strong seasonality of fine root production and mortality in temperate tree species. In comparison with root length mortality, production of root length is strongly affected by climate factors, especially by air temperature. The thinnest root population (diameter in 0-0.5 mm) primarily with uptakes function, were more sensitive to environmental change. At species level, tree root longevity is controlled by both endogenous and exogenous factors. The lower distal order roots may incline to die as an intact braches with similar and short longevity, indicating these roots may constitute the mainbody of ephemeral portion within tree root branching systems. Season and soil depth of root birth had general impacts on root survivorship across species, but the relationship between the ontogeny of individual roots and longevity still deserved further study. The longevity of the first order root tended to vary independently among tree species. However, the close correlations between root structural traits demonstrated the general trade-off between resource acquisition and conservation in tree root, as well as the difference in resource acquisition in roots among tree species. These findings may improve our understanding on the mechanism of fine root dynamics at species level, and estimation of the belowground C and nutrients cycles in forest ecosystems in northeastern China.