Seasonal Dynamics Of Absorptive Fine Root Biomass And Length Density In Six Temperate Tree Species

Seasonal dynamics of fine root biomass and root length density are important indicators for estimating underground carbon allocation and understanding resource absorptive strategies in forest ecosystems.However,for a long time,the heterogeneity of the structure and function within fine roots has been neglected,which affects the accurate estimation of biomass turnover and nutrient uptake of fine roots.Based on functional classification,fine root group can be divided into two pools:absorptive fine roots and transportive fine roots.The former mainly consists of lower order roots with intact cortex(e.g.1-3 order roots,with root tips defined as first order root),and the latter mainly consists of higher order roots with cortex disintegration or disappear.The composition(the proportion of absorptive root in total fine root number)of absorptive roots varies in different species and sites,even different root orders within one tree species,which would affect the estimation of biomass and root length of absorptive roots.However,until now,the seasonal dynamics of root biomass and root length density and their environmental influence factors are poorly understood.In this paper,six tree species were selected for the experiments in the northeastern of China,including Fraxinus mandshurica,Phellodendron amurense,Juglans mandshurica,Larix gmelinii,Pinus koraiensis and Picea koraiensis.All these species were from the monoculture plantations established in 1986.The anatomy,morphology,biomass and length density of first five roots at 0-10 cm soil layer were measured in May,July and September 2017,respectively.Soil physical and chemical properties were also determined at each sampling time.The main purposes were:(1)to reveal the variation of absorptive roots proportion and its seasonal dynamics across different root orders in six tree species;(2)to identify the seasonal dynamics of the absorptive root biomass and length density in six tree species;(3)to clarify the main factors of soil conditions and root traits affecting the seasonal dynamics of biomass and root length density of absorptive root.The results showed that:(1)The first five orders roots showed a relatively consistent seasonal dynamics in morphology and anatomy across six species.From spring to autumn,root diameter(RD),tissue density(RTD)and the branch ratio(RBR)(the number ratio of 1 st root to 2nd root)showed first increased and then decreased dentency all the root orders,the trend of specific root length(SRL)is opposite,It decreases first and then increases with the season.Seasonal dynamics of root anatomy variations were also found across different root orders.Form spring to autumn,cortical thickness and stele diameter of first four order were greatest in summer across 6 species.However,the ratio of cortical thickness to the stele diameter showed species-specific seasonal dynamics.With the increase of root order,RD and RTD increased across six tree species,cortex thickness and SRL decreased.In first five root orders,RD was thickest in P.amurens(0.56±0.01-1.59±0.04 mm),SRL was lowest in P.koraiensi(25.31±0.58-8.78±0.56 g m-1),while RTD was lowest in p.amurense(0.17±0.01-0.29±0.01 g cm-3).Besides,the SRL of three broad-leaved tree species(F.mandshurica,J.mandshurica and.amurense)was longer than that of three coniferous tree species(L.gmelinii,P.koraiensis and.koraiensis),and the RTD of broad-leaved trees was lower than that of coniferous tree,while the RBR was larger than that of coniferous trees.(2)The proportion of absorptive roots showed a relatively stable seasonal dynamics in six tree species.Because all the 1st and 5th order roots were absorptive and transportive roots in all 6 tree species,respectively,so there were no seasonal variations in the proportion of absorptive roots for both order roots.The proportion of absorptive roots was the highest in July for all the 2nd,3rd and 4th order roots,respectively(38%-75%).At the same time,the proportion was generally higher in September(33%-65%)than that in May(29%-64%).Averagely,the proportion of absorptive roots decreased with the increasing root orders in the growth seasons,100%in 1st order root,(all tree species),68%(P.amurense)-58%(P.koraiensis)in 2nd order root,58%(P.amurense)-45%(P.koraiensis)in 3rd order root,33%(P.amurense)-23%(P.koraiensis)in 4th order root,0 in 5th order root(all species).(3)The biomass and root length density of absorptive root showed a species-and root order-specific seasonal dynamics in six tree species.Both the seasonal dynamics of absorptive root biomass and root length density showed four patterns:increasing and then decreasing,decreasing and then increasing,increasing,decreasing.However,the specific pattern was related to the species and root order,which reflected the dual effects of the proportion of absorptive root number and root biomass.Similarly,the proportion of absorptive roots in total fine root(1-5 order root)biomass and root length density showed species-specific seasonal dynamics.Regardless of the season,the total root biomass and root length density in first four absorptive order roots were quite different across all the species,both were highest in F.mandshurica(66.7±4.6g m-2 and 5544±530m m-2,respectively),lowest in P.koraiensis(13.7±6.2g m-2 and 192±93m m-2,respectively).(4)The seasonal dynamics of root uptake biomass and root length density in different root orders were related with root functional traits and soil conditions,but the influence highly depend on the root order.Root morphology had significant effects on the biomass and root length density of the absorptive roots(except for the biomass of the absorptive roots of the 3rd order roots).While the effects of soil total nutrients(total nitrogen,total carbon and carbon-nitrogen ratio)was only significant on the root biomass and root length density of the first-order root.The influencing factors of the seasonal dynamics of the proportion of absorptive roots varied across root orders.For example,the proportion of absorptive roots was significantly affected by soil nitrate nitrogen and root diameter in 2nd order roots,while it was the TRD for 4th order roots.Our results showed the obvious and consistent seasonal dynamics of absorptive root uptake components in root orders of six tree species.The seasonal dynamics of the absorptive root biomass and root length density varied in different root orders.Additionally,the seasonal variations of absorptive root composition,biomass(and proportion)and root length density(and proportion)were related to root traits and soil conditions,but the potential influencing factors were different among root orders.