树木细根生产与周转研究方法评述

树木细根在森林生态系统能量流动和物质循环中起重要的作用。树木细根研究及方法探讨也成为当今森林生态学的研究热点。在中国,对树木细根生产和周转的研究尚未引起充分重视。在此介绍了目前国外普遍采用的树木细根研究方法及其优缺点、适用性以及不同方法的研究比较,以期对我国开展树木细根方面的研究有所裨益。     

树木细根养分内循环

养分内循环是树木减少养分损失,提高养分利用效率的重要途径。树木细根寿命短、周转快．每年大量凋落死亡,因此,近20多年来树木细根养分内循环的研究逐渐受到人们的重视。关于树木细根养分内循环目前的研究结论比较复杂。本文从细根在树木地下部分养分内循环中的重要地位、细根养分内循环对树木减少养分损失的重要性、细根中各养分元素内循环的研究现状以及细根养分内循环研究方法存在的问题等方面综合论述了国内外的进展情况,并对今后的研究趋势进行了展望。     

鼎湖山南亚热带森林细根分解干物质损失和元素动态

鼎湖山南亚热带森林细根分解干物质损失和元素动态温达志（中国科学院华南植物研究所,广州５１０６５０）魏平张佑昌（中国科学院华南植物研究所鼎湖山树木园,肇庆５２６０７０）孔国辉（中国科学院华南植物研究所,广州５１０６５０）ＤｒｙＭａｓＬｏｓａｎｄＣｈｅｍ...     

长白山阔叶红松林细根周转的研究

系统研究了长白山阔叶红松林细根生物量、生产力、年周转率及其在净生产力分配中的作用,生物量调查结果表明,阔叶红松林活细根的生物量为5049kg.ha~(-1),死细根生物量的平均值为1883kg.ha~(-1),细根的年周转率为0.96,年生产量为4860kg.ha~(-1),约占总净初级生产力的19.40％,年死亡量为2343kg.ha~(-1),相当于阔叶红松林枯枝落叶年凋落量的60％,由此提出了森林凋落物应包括枯枝落叶和根系凋落物的论点。     

树木位置和胸径对人工林细根水平分布的影响

通过研究福建三明莘口林场33年生格氏栲和杉木人工林细根生物量与树木位置和胸径大小的关系,探讨人工林细根水平分布特点。用土芯法(土钻内径6 .8cm,深10 0 cm)测定细根生物量,格氏栲和杉木人工林分别随机取土芯4 1个和4 0个,同时记录离取样点最近的第1棵、第2棵和第3棵树的距离和胸径。格氏栲和杉木人工林细根生物量平均值分别为3.2 6 6 t/ hm2和2 .0 4 8t/ hm2 ,变异系数分别达37.3%和4 2 .8% ,细根生物量均遵从正态分布(p<0 .0 5 )。格氏栲和杉木人工林细根生物量均与离取样点最近第1棵、第2棵树的距离有显著的负相关,且以与最近第1棵树距离的相关系数最大。格氏栲人工林细根生物量与最近第1棵树的胸径呈显著的正相关(p<0 .0 1) ,而与最近第2、第3棵树的胸径无关(p>0 .0 5 ) ;而杉木人工林细根生物量则与最近第1、第2和第3棵树的胸径均无显著相关(p>0 .0 5 )。逐步多元线性回归分析表明,离取样点最近第1棵树距离和胸径可解释格氏栲人工林细根生物量水平变异的4 1.0 % ,而离取样点最近第1、2棵树距离则可解释杉木人工林细根生物量水平变异的4 0 .6 %。由于人工林细根水平分布呈现特定模式,规则取样估计细根生物量将产生系统误差。     

高山森林三种细根分解初期微生物生物量动态

为了解川西高山森林凋落物分解过程的微生物生物量特征,采用凋落物分解袋法,测定了粗枝云杉(Picea asperata)、岷江冷杉(Abies faxoniana)和红桦(Betula albosinensi)细根分解几个关键时期微生物生物量碳(MBC)、氮(MBN)和磷(MBP)的动态特征。3个树种细根分解过程中的MBC均表现为在土壤深冻期下降至全年最低点后缓慢上升,至土壤融冻中期再次下降,到生长季节增长的趋势。然而,粗枝云杉与岷江冷杉细根分解过程中的MBC最大值出现在生长季节末期,红桦细根分解过程中的MBC最大值出现在土壤冻结初期。3个树种细根分解过程中的MBN表现出相似的动态规律:土壤深冻期急剧下降至全年最低,随后在冻融季节无显著变化,生长季节明显增加,到生长季节末期达到全年最大值。另外,粗枝云杉和岷江冷杉细根分解过程中MBP均随着分解的进行呈现增加趋势,而红桦细根分解过程中的MBP在土壤融冻末期出现最大值,在生长季节中期出现另一峰值,生长季节末期明显下降。这些结果表明冬季细根分解过程中仍存在一定的土壤微生物,但受到细根质量、温度及其驱动的环境因子的深刻影响。     

桢楠种源幼苗细根形态和生物量研究

为了解桢楠(Phoebe zhennan)不同种源细根形态和生物量分配的差异,采用全根调查的方法,对桢楠自然分布区13个种源2.5年生幼苗的细根形态和生物量进行了研究。结果表明,桢楠种源间各级细根的平均直径、总根长和表面积差异显著,在种源内细根的平均直径随根序的增加而增加,但根序间总根长和表面积差异规律不明显。根序生物量分配随根序增加而增加,1~4级根生物量分配分别为6.33%、14.47%、25.03%和54.17%。通过综合评价,以HT、LF、ES和WC种源的根系最优,具有较高的生长潜力。     

西双版纳不同热带森林群落土壤表层的细根年动态

用钻土蕊法和内生长土蕊法研究了西双版纳 4个不同的热带森林群落一年内细根现存量和两个群落长入细根量的动态变化 ,结果表明 :在原始群落中活细根现存量 6～ 1 2月间相对较大 ,峰值为 1 0月份 ,在 2～ 6月间相对较少 ,死细根现存量高值出现于 4月中后期 ,最小值出现于 8月。人为干扰较大的 1 5年生群落和人工群落活细根现存量在各个月份出现不规律的变化 ,死细根现存量与原始林有类似的变化规律。 3 0年生群落活细根现存量在 6～ 1 0月份相对较大 ,低值出现于 2月 ,死细根现存量高峰值则出现于 6月 ,低峰值出现在 8月。细根长入量在原始群落和人工群落 4～ 6月期间量最大 ,人工群落于上年 1 2～ 2月份出现最低值。     

淡竹蔸根和鞭根细根寿命特征

植物细根寿命是影响细根周转的重要因素,在陆地生态系统碳和养分循环中扮演着重要角色。但是,目前对竹类植物根寿命的直接观测资料和研究仍显不足,尤其缺乏关于竹类植物蔸根和鞭根两种类型根寿命特征的研究。利用根箱对淡竹(Phyllostachys glauca)蔸根和鞭根细根生长动态开展研究,结果表明：(1)淡竹蔸根和鞭根细根寿命间没有显著差异,但是根序对根寿命有显著影响。1级根寿命显著低于2级根寿命,2级根寿命显著低于3级根寿命。(2)蔸根和鞭根的出生和死亡具有不同的季节模式。鞭根新根出生集中于7、8、9月,在7月达到峰值,死亡高峰发生在春季(3—5月)。蔸根新根发生没有单一峰值,在3月、7月、11月各有一次新根发生的小高峰,死亡高峰发生在夏季(6—8月)。(3)淡竹蔸根和鞭根的细根寿命与出生时间的关系不同。蔸根夏季出生的细根中值寿命最长,春季出生的细根中值寿命最短。鞭根春季出生的细根中值寿命最长,冬季(12—2月)出生的细根中值寿命最短。发现淡竹蔸根和鞭根的细根具有不同的生长规律,其寿命与根序级别、出生时期密切相关,研究结果可为竹林生产和管理提供理论依据。     

凋落物输入对中亚热带不同森林细根生物量及分布的影响

在全球变化背景下,植物凋落物输入的改变对森林生态系统地下生态过程具有重要的影响。中亚热带森林中,细根进入凋落物层生长是一种常见现象,然而凋落物量的改变对细根生长影响的研究较少。通过对中国中亚热带针叶林、针阔混交林及常绿阔叶林这3种典型森林进行地上凋落物添加和去除实验,研究不同凋落物处理水平下细根生物量、垂直分布及形态特征的变化。结果表明:与对照(CK)相比,地上凋落物去除(LR)分别导致针叶林和针阔混交林细根总生物量显著降低40.3%和37.5%,而凋落物添加(LA)使常绿阔叶林中的细根总生物量明显提高了19.4%。不同层次的细根生物量对凋落物处理的响应不同,从针叶林到常绿阔叶林,凋落物量的改变对细根的垂直分布的影响加剧。LA处理明显提高常绿阔叶林凋落物层的细根生物量百分比(相比对照提高了10.6%)以及降低7.5—15 cm土层的细根生物量百分比(相比对照降低了10.4%)。凋落物层中生长的细根生物量和凋落物层厚度呈高度线性相关(R~2=0.742,P0.01),并且和凋落物层生物量也呈显著线性相关(R~2=0.521,P0.01)。3种森林类型细根的根长密度(RLD)和比根长(SRL)变化趋势与细根所处的层次紧密相关,而不同凋落物处理对它们的影响均不明显,说明细根对养分的获取策略表现为在养分丰富的凋落物层和表土层投资更多的生物量和更活跃的代谢,而不是改变细根形态的表型可塑性。     

Influences of Root Diameter, Tree Age, Soil Depth and Season on Fine Root Survivorship in Prunus avium

The rapid turnover of the fine root system is a major pathway of carbon and nutrient flow from plant to soil in forest ecosystems. In order to quantify these fluxes there is a need to understand how fine root demography is influenced by edaphic, environmental and plant ontogenetic factors. We studied the influence of four major factors (season, depth, root diameter and tree age) on the survivorship and longevity of fine roots of Prunus avium L. (wild cherry) over two years in North East Scotland. Survival analysis of data derived from minirhizotron observations showed that, for the range of root diameters studied, an increase in root diameter of 0.1 mm was associated with a 16% decrease in the risk of death. Depth was also an important factor; roots present at a depth of 10 cm had significantly lower survivorship than did roots at all lower depths studied. The effects of tree age and season on root production were more complex. Roots of old trees were more likely to die in the spring and roots of young trees were more likely to die in the autumn. Our data illustrate the complex factors that must be taken into account when scaling up information from individual observations of root longevity to model the contribution of fine roots to C and nutrient fluxes in forest ecosystems.     

Fine Root Dynamics and Forest Production Across a Calcium Gradient in Northern Hardwood and Conifer Ecosystems

Losses of soil base cations due to acid rain have been implicated in declines of red spruce and sugar maple in the northeastern USA. We studied fine root and aboveground biomass and production in five northern hardwood and three conifer stands differing in soil Ca status at Sleepers River, VT; Hubbard Brook, NH; and Cone Pond, NH. Neither aboveground biomass and production nor belowground biomass were related to soil Ca or Ca:Al ratios across this gradient. Hardwood stands had 37% higher aboveground biomass (P = 0.03) and 44% higher leaf litter production (P < 0.01) than the conifer stands, on average. Fine root biomass (<2 mm in diameter) in the upper 35 cm of the soil, including the forest floor, was very similar in hardwoods and conifers (5.92 and 5.93 Mg ha⁻¹). The turnover coefficient (TC) of fine roots smaller than 1 mm ranged from 0.62 to 1.86 y⁻¹ and increased significantly with soil exchangeable Ca (P = 0.03). As a result, calculated fine root production was clearly higher in sites with higher soil Ca (P = 0.02). Fine root production (biomass times turnover) ranged from 1.2 to 3.7 Mg ha⁻¹ y⁻¹ for hardwood stands and from 0.9 to 2.3 Mg ha⁻¹ y⁻¹ for conifer stands. The relationship we observed between soil Ca availability and root production suggests that cation depletion might lead to reduced carbon allocation to roots in these ecosystems.     

杉木(Cunninghamia lanceolata)、火力楠(Michelia macclurei)纯林及其混交林细根分布、分解与养分归还

用土钻法研究了杉木（Ｃｕｎｎｉｎｇｈａｍｉａｌａｎｃｅｏｌａｔａ）、火力楠（Ｍｉｃｈｅｌｉａｍａｃｌｕｒｅｉ）纯林和混交林的细根分布,用分解袋法研究了杉木和火力楠细根的分解,计算了３个林分中细根分解的Ｎ,Ｐ,Ｋ,Ｃａ,Ｍｇ的归还量。活细根的垂直分布以火力楠纯林层次性最强,混交林次之,杉木纯林最差。火力楠细根的养分含量比杉木细根高,而Ｃ／Ｎ比低。火力楠细根年分解率比杉木快,火力楠为５７．７％,而杉木为３２．７８％。细根分解的养分归还量多少顺序依次为：火力楠纯林、杉木火力楠混交林和杉木纯林。混交林中,细根分解的Ｎ,Ｐ,Ｋ,Ｃａ和Ｍｇ归还量分别为枯枝落叶的３３．３８％,５．８２％,２６９．３３％,３４．１２％和３７６．０８％。细根在３个林分的物质循环和周转中起着不可忽视的作用。     

Measuring Fine Root Turnover in Forest Ecosystems

Development of direct and indirect methods for measuring root turnover and the status of knowledge on fine root turnover in forest ecosystems are discussed. While soil and ingrowth cores give estimates of standing root biomass and relative growth, respectively, minirhizotrons provide estimates of median root longevity (turnover time) i.e., the time by which 50% of the roots are dead. Advanced minirhizotron and carbon tracer studies combined with demographic statistical methods and new models hold the promise of improving our fundamental understanding of the factors controlling root turnover. Using minirhizotron data, fine root turnover (y⁻¹) can be estimated in two ways: as the ratio of annual root length production to average live root length observed and as the inverse of median root longevity. Fine root production and mortality can be estimated by combining data from minirhizotrons and soil cores, provided that these data are based on roots of the same diameter class (e.g., < 1 mm in diameter) and changes in the same time steps. Fluxes of carbon and nutrients via fine root mortality can then be estimated by multiplying the amount of carbon and nutrients in fine root biomass by fine root turnover. It is suggested that the minirhizotron method is suitable for estimating median fine root longevity. In comparison to the minirhizotron method, the radio carbon technique favor larger fine roots that are less dynamics. We need to reconcile and improve both methods to develop a more complete understanding of root turnover.     

Impacts of Water Input Manipulations on Fine Root Production and Mortality in a Mature Hardwood Forest

In order to examine the below ground response of a mature upland hardwood forest in the southeastern U.S., to increases and decreases in water inputs, the gross production, mortality, and net production of fine roots were examined over the first and third years of a long-term water manipulation experiment (Throughfall Displacement Experiment). Treatments involved a 33% decrease (DRY), 33% increase (WET), and ambient (AMB) levels of throughfall to the forest floor, begun in July, 1993. Video images of roots appearing on minirhizotron faces installed on both upper and lower slopes were recorded biweekly to a depth of 90 cm from April through October of 1994 and of 1996. Comparisons were made between treatments in amounts of new root elongation, root mortality, and calculated net root production. Minirhizotron observations during 1994 growing season, immediately following winter 1994 installation, revealed a strong effect of installation disturbance and were therefore not considered valid reflections of the response of the stand to the treatments. The 1996 data, on the other hand, exhibited absence of installation biases inherent in 1994 data because of a longer period since treatment initiation (2 2/3 yr vs. 8 mths), and favorable root growth conditions in all treatments during a greater portion of the year. The 1996 data were, therefore, considered realistic measures of below ground treatment responses. During 1996, net root production at 0-30 cm depth, at the upper slope positions, was significantly greater in DRY than in WET and AMB. Net root production was also greater at the lower slope position, but not significantly so. Treatment differences were the result of gross root production, as patterns of mortality did not differ across treatments. Nor were there significant treatment differences at depths below 30 cm. Whether trees in DRY produced more roots to replace root biomass lost during a previous drought year, or whether a new root:shoot ratio was beginning to develop in response to treatments, will require observations from the response of the stand in future years to be determined.     

Fine root demography in alfalfa (Medicago sativa L.)

In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N₂ fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N₂-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha^–1 and 60 kg N ha^–1, with no differences due to N₂ fixation capacity or root system architecture.     

三峡库区马尾松不同直径细根分解动态及其影响因素

在三峡库区秭归县九岭头林场马尾松人工林进行一年的细根分解试验,研究马尾松直径<0.5、0.5～1和1～2 mm细根的分解动态及其影响因素.结果表明: 细根分解速率随直径增大而减小,直径<0.5、0.5～1和1～2 mm细根年分解率分别为34.0%、28.0% 和25.7%.直径<1 mm细根分解速率随时间增加而逐渐减小,直径1～2 mm细根分解速率随时间增加先逐渐增加再减小.在细根分解过程中,N、P和Ca浓度随时间增加而增加,K浓度呈先降低后上升再下降的趋势.细根分解速率与细根初始N、P、K和Ca浓度,以及C/N、C/P均显著相关,细根Ca浓度和土壤温度是影响细根分解的主导因子.     

Intrinsic and Extrinsic Controls of Fine Root Life Span

Although fine roots play an integral role in biogeochemical cycling and supporting plant function, fundamental understanding of the mechanisms that control fine root life span is limited. Based on literature, we examined how intrinsic plant characteristics including root diameter, root branching order, rooting depth, and mycorrhizal symbiosis affect fine root life span, and how fine root life span differs with plant life form and foliar habit and between early versus late seral species. We also examined how soil nitrogen and water availability, temperature, and atmospheric carbon dioxide concentration influence fine root life span. We focused on evidence from rhizotron and minirhizotron observations which allow for individual roots to be directly monitored in situ. Fine root life span increased with increasing root diameter, was shorter for more distal than proximal roots, and increased with increasing rooting depth, but was not influenced by mycorrhizal symbiosis. Trees had the longest fine root life spans of all the plant life forms, followed by grasses, lianas, shrubs, and forbs. Among trees, deciduous species had shorter fine root life spans than evergreen species. Fine root life span appears to decrease with increasing temperature and increase with soil water availability, whereas the effects of soil nitrogen availability and atmospheric carbon dioxide concentration on fine root life span were highly inconsistent among studies. Our findings indicate that root morphological characteristics and plant traits are useful predictors of fine root life span. However, environmental influences on fine root life span remain poorly understood due to the limited number of respective studies. Future studies of root demographic processes are needed to better understand environmental controls of fine root life span. It is also critical that research continues into developing more direct and less invasive techniques for studying root demographics.