Root size and soil environments determine root lifespan: evidence from an alpine meadow on the Tibetan Plateau
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O R I GI N A L A R T IC L E
Yibo Wu • Yongcui Deng • Jing Zhang Jing Wu • Yanhong Tang • Guangmin Cao Fawei Zhang • Xiaoyong Cui
Root size and soil environments determine root lifespan: evidence from an alpine meadow on the Tibetan Plateau Received: 21 May 2012 / Accepted: 21 February 2013 / Published online: 8 March 2013 The Ecological Society of Japan 2013
Abstract We used a minirhizotron system to investigate the influence of three major factors—root morphology, root depth, and season of root emergence—on root survivorship and longevity in a Kobresia humilis meadow on the Tibetan Plateau during the growing season of 2009. Root longevity was assessed by survival analysis, Kaplan–Meier analysis, and Cox proportional hazards regression. Root longevity was correlated positively with root diameter. A 17.5 % decrease in the risk of mortality was associated with a 0.1-mm increase in diameter. Roots distributed in the top 10 cm of the soil had significantly shorter longevities than roots at greater depths, with a 48 % decrease of mortality risk for each 10-cm increase in soil depth from the surface to 40 cm. Of all the factors examined, the season of root emergence had the strongest effect on root lifespan. Roots that emerged in May and June had shorter longevity than roots that emerged later in the year, and roots that emerged in September and October were more likely to survive over winter. Our findings indicated that lifehistory traits of roots in K. humilis meadows are highly heterogeneous, and this heterogeneity should be considered when modeling the contribution of roots to carbon and nitrogen fluxes in this type of meadow ecosystem. Moreover, temporal, spatial, and compositional Y. Wu College of Architectural, Civil Engineering and Environment, Ningbo University, Ningbo 315211, China Y. Wu Æ Y. Deng Æ J. Zhang Æ J. Wu Æ X. Cui (&) College of Life Sciences, University of Chinese Academy of Sciences, Beijing 100049, China E-mail: [email protected] Tel.: +86-10-88256497 Fax: +86-10-88256497 Y. Tang National Institute for Environment Studies, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan G. Cao Æ F. Zhang Northwest Plateau Institute of Biology, Chinese Academy of Sciences, Xining 810001, China
variations in root longevity must be considered. Keywords Minirhizotron Æ Alpine meadow Æ Root lifespan Æ Survival analysis Æ Seasonal dynamics
Introduction Fine roots—the most active fraction of root systems—are an integral component of ecosystem-level processes, especially carbon and nutrient cycling. Assuming that fine roots turn over once per year, the expense of fine root production accounts for nearly 30 % of total global annual net primary production (NPP) (Jackson et al. 1997). Moreover, in forest ecosystems as much as 40–50 % of annual NPP may be allocated to fine roots (Vogt et al. 1996). In addition to the important role of fine roots in the plant’s carbon budget, the amount of nitrogen returned to the soil from fine root death and decomposition equals or exceeds that from leaf litterfall (Vogt et al. 1986). Although the
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