水曲柳和落叶松细根寿命的估计

ESTIMATING FINE-ROOT LONGEVITY OF FRAXINUS MANDSHURICA AND LARIX GMELINII USING MINI-RHIZOTRONS

树木细根（直径≤2mm）是控制树木与其周围环境进行能量交换和物质分配的主要器官，其寿命的长短决定了每年被分配到土壤中碳和养分的数量。我们使用微根管技术监测了水曲柳（Fraxinus mandshurica）和落叶松（Larix gmelinii）细根生长、衰老、死亡的动态过程，运用Kaplan-Meier方法估计细根存活率及中位值寿命（Median root lifespan，MRL），做存活曲线（Survival curve）。用对数秩检验（Los-rank test）比较不同树种、不同土壤层次、不同季节出生的细根寿命差异程度。研究结果表明，随观测期延长，细根存活率逐渐下降，在观测期内的各个时点上，水曲柳细根存活率显著高于落叶松（P〈0．001），说明水曲柳细根寿命明显长于落叶松，两树种的MRL分别为111±7d和77±4d。无论是水曲柳还是落叶松，土壤下层（20～40cm）的细根存活率始终高于上层（0～20cm），差异程度均达到显著水平（p落=0．001，p水〈0．001），落叶松上下两层的MRL分别为62±11d和95±11d，水曲柳为111±6d和124±20d，这与土壤环境因子的垂直分布有关，下层土壤延长细根寿命。不同同龄根群（Root cohort）的细根寿命不同。落叶松夏季产生的细根存活率显著高于春季（P=0．042），中位值寿命分别是MRL春=47±13d，MRL夏=82±6d。水曲柳不同细根同龄根群与落叶松具有相似的季节性，夏季产生的细根存活率在同一时间点上要显著高于春季（P=0．014）。

Aims Fine-roots （≤2 mm diameter） are important in controlling energy and matter exchange between tree and soil. Fine-root longevity is a critical determinant of fine-root turnover and therefore carbon allocation and nutrients returned to the soil. However, little is known about variation in fine-root longevity in relation to spatial and temporal heterogeneity in availability of soil resources （such as temperature, moisture and nitrogen）. How does availability of soil resources affect fine root longevity？ Do different tree species, soil depths and root cohorts have different fine-root longevity in the same forest site？ To answer these questions it is important to understand the fate of fine roots in soil with heterogeneous resource availability. The objectives of this study were to： 1 ） compare fine-root longevity between ash （ Fraxinus mandshurica ） and larch （ Larix gmelinii ） in the same site, 2） define patterns of fine-root longevity in different soil depths and elucidate the ef- fect of availability of soil resources on fine-root longevity and 3） analyze seasonal changes of free-root longevity for both species. Methods Mini-rhizotrons （root observation tubes） were used to estimate fine-root longevity. We established three 20 m×30 m plots in an ash and larch plantation, and installed six clear PVC （polyvinyl chloride） minirhizotron tubes （90 cm long×5.5 cm inside diameter） in each plot in October 2003. From 16 April to 30 October in 2004, video images were collected from the mini-rhizotron tubes at approximately two-week intervals and analyzed with an image analysis system （RooTracker software）. We compared differences in longevity between tree species, soil depths and seasonal root cohorts using cumulative survival rate and median root longevity （MRL） of fine roots by Kaplan-Meier methods in survival analysis. Important findings Cumulative survival rate of fine roots decreased gradually with time. Survivorship curves showed that cumulative survival rate of fine roots in ash was significantly higher than that in larch （p 〈 0.001） ; MRL was 111 ± 7 d in ash and 77 ± 4 d in larch. Higher fine root mortality was found in 0 - 20 cm soil, as MRL was 62 ± 11 d for larch and 111 ± 6 d for ash, contrasting with 95 ± 11 d for larch and 124 ± 20 d for ash in deep soil. This study also indicated longer lifespans of fine roots produced in summer （82 ± 6 d for larch） than spring （47 ±13 d）. Similar seasonal pattern was found for ash. Fine-root longevity was different between tree species due to different root genetics, physiology and architecture. High soil temperature and high N content can decrease fine root longevity. In summer, photosynthates are allocated to roots and fine roots grow rapidly and have increased longevity. Our results suggested that fine-root longevity is correlated with genetics of the species, physiological status and soil resource availability.