Intra- and interspecific variation of functional traits, growth performance and belowground competition in <i>Populus</i> species

Abstract

Short-rotation forestry (SRF) systems provide sustaining resource supply that is needed to meet the rapidly increasing demand in wood products and renewable energy. Poplar species (<i>Populus</i> spp.) are frequently used for such plantations as they maintain high yield along wide environmental ranges. Specifically aspen (<i>Populus tremula</i> and <i>P. tremuloides</i>) reach considerable productivity even on poor soils and exhibit relatively high drought tolerance when compared to other taxa of the genus. However, the use of aspen in SRF systems is not well established. The aim of this study was to investigate the intra- and interspecific variation of above- and belowground functional traits which determine differences in environmental adaptation, drought tolerance, competitive strength and hence overall species performance in different aspen demes and two further poplar species (<i>P. tremula</i> and <i>P. trichocarpa</i>). This information may contribute to optimize yield and reducing the risk of failure in plantings under current and future climates. Major differences in functional above- and belowground traits of aspen demes of particular environmental adaptation became evident in the course of this study (Chapter 2). However, variance within the aspen fine root system properties is considerably high and above- and belowground trait correspondence remains inconsistent. Fine root properties are to a lesser extent related to genetic distance among demes than leaf-related properties. The high degree of plasticity in belowground (fine root) traits compared to aboveground traits implies a heterogeneous response to high spatial and temporal variability of belowground resources what may be of relevance for species growth performance. This may also explain why the variation in fine root morphological traits was not directly linked to differences in growth among the demes. Instead, much of the within-deme variation of the investigated root morphological traits may be explained by differentiating single root sections according to the hierarchical branching structure (root orders).The intraspecific variation in wood anatomical and hydraulic properties of branches and coarse roots in five genetically distinct demes were related to variation in aboveground productivity and drought resistance (Chapter 3). Genotypic differences in the dependence of growth on branch xylem hydraulic efficiency and on hydraulic safety (cavitation vulnerability) were found. By contrast, a large variation in coarse root anatomical and hydraulic traits did not determine growth performance or drought resistance among the demes but manifest in the observation of ‘high-conductivity roots’ with extraordinarily large vessels which state the functional heterogeneity within the poplar root system. Belowground competition effects on physiologically important fine root morphological and chemical properties of <i>P. tremula</i> and <i>P. trichocarpa</i> saplings along roots of different order and age were identified (Chapter 4). First, the strong control exerted by the root order position or the age of a root segment on different root traits became evident and enabled species differentiation according to the investigated traits. The minor influence of different competition treatments on fine root morphology and chemistry suggests that morphological adaptation in response to competition is not a mandatory phenomenon. Moreover, the comparison of harvest data (fine roots of the box interior) and direct root growth observations (at a transparent front plate) revealed a mismatch between the two types of data suggesting that root polymorphism may strongly impact the interpretation of rhizoscope data. This study is the first to describe the inherent complexity and heterogeneity of the poplar root system in its anatomy, morphology and function on an intraspecific as well as interspecific scale. Further experiments and detailed root analysis within long-term field studies are needed for a better understanding of belowground root-related processes in poplar species.