Abstract
Manchurian walnut and larch are key timber species of northeast China but information on (fine) root traits of both species is scarce. Plasticity of root traits in mixed plantations has been studied rarely although this could give important insights into mechanisms of root competition. This study examined root traits by branching order in 30-yr-old monocultures and their plasticity in mixed plantations. In monocultures, Manchurian walnut and larch differed in key fine root traits. Larch roots hold more absorptive root orders, larger diameter and lower specific root length/area. Walnut root orders featured greater cortex:stele ratios, N-concentrations and respiration rates. Under interspecific competition, the proportion of walnut root tips increased, the biomass/length of larch root orders 1–3 decreased. Larch possessed a greater morphological and anatomical plasticity of terminal root orders than walnut. Mycorrhizal colonization rates of walnut were reduced. Both species differed fundamentally in their fine root properties. Absorptive fine root orders reacted plastic under interspecific competition while traits of higher root orders remained unchanged. In mixture, larch roots possessed a greater plasticity in traits related to resource uptake (efficiency) than walnut roots whose reaction norm is suggested to be predominantly based on interference competition via juglone exudation.
Highlights
Tree monocultures can provide large quantities of wood per unit area, this has often come at an expense, e.g. in terms of biodiversity, ecosystem system services and resilience in regard to climate change and biotic disturbances[1,2]
The fine root systems of Manchurian walnut and larch grown in monocultures differed significantly in branching patterns and root order-specific morphological, anatomical, chemical and physiological traits
The data on fine root length and biomass of root orders indicates that larch trees, growing under the given edaphoclimatic conditions, have generally more extensive fine root branches than Manchurian walnut trees
Summary
Tree monocultures can provide large quantities of wood per unit area, this has often come at an expense, e.g. in terms of biodiversity, ecosystem system services and resilience in regard to climate change and biotic disturbances[1,2]. The higher productivity of mixed-species plantations is the positive net result of negative (i.e. competition) and positive interactions (i.e. facilitation) among combined species including both above- and belowground interactions[7,8,9] Both Manchurian walnut (Juglans mandshurica) and larch (Larix gmelinii) are key timber species of northeast China and large plantations have been established in the last decades. Resource competition and allelopathic chemicals, such as juglone, have been shown to reduce the growth of neighboring roots and plants[24,25], and to influence traits of competing root systems to different extends[26,27]. Previous studies addressing belowground competition focused on biomass, morphology and/or tissue chemistry of whole root branches[27,35] but largely neglected anatomical and physiological traits and changes among individual root orders
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