Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco Comparison of wood density in roots and stems of black spruce before and after commercial thinning Audrey Lemay a, ⁎ , Cornelia Krause a , Alexis Achim b a Département des sciences fondamentales, Université du Québec à Chicoutimi, 555 boulevard de l’Université, Chicoutimi (Québec) G7H 2B1, Canada b Département des sciences du bois et de la forêt, Université Laval, 2405 rue de la Terrasse, Québec (Québec) G1V 0A6, Canada ARTICLE INFO Keywords: Growth Earlywood density Tracheid dimensions Picea mariana Boreal forest ABSTRACT Roots play an important physiological and mechanical role in the survival and growth of a tree, but also in the success of silvicultural treatments. Studies comparing the xylem in roots and stems have shown that conifer tracheids tend to be wider and longer in roots, which renders root wood less dense and more susceptible to cavitation and embolism. The increased radial growth often observed after thinning may induce changes in wood anatomy that could alter wood properties, such as wood density, in the stem and roots. The aim of this study was to compare growth, wood density and tracheid dimensions between the stem and roots of black spruce trees growing in the boreal forest. We also evaluated whether these wood properties were altered by the application of a commercial thinning treatment. Six black spruce trees were harvested in four commercially thinned stands. Samples were collected from the stem and two locations of each root. Radial growth, wood density and tracheid dimensions were measured on each sample. Results show that all wood density components, especially early- wood density, were higher in the roots than in the stem in black spruce. This denser wood in roots might provide increased safety against cavitation in a part of the xylem where hydraulic stresses are higher. After thinning, growth was increased in the stem and particularly in roots, resulting in slight wood density decreases, which should not influence the vulnerability of roots to cavitation or wood quality in the stem. These results lead us to suppose that the hydraulic network of the black spruce root system may not be so vulnerable to cavitation. 1. Introduction The stem and roots of trees are both physiologically and mechani- cally important, but survival and growth are to a great extent de- termined by the root system. Roots provide anchorage and stability (Danjon and Reubens, 2008), as well as the water and nutrients needed for growth and development, and are important for storage of reserves and synthesis of certain growth hormones (Pallardy, 2008). Despite its importance, the root system of mature trees receives less scientific at- tention than the stem because of the difficulty in accessing the roots and the lack of commercial interest in this part of the tree (Fayle, 1968; Marcati et al., 2014). Most of the water taken up and transported by the root system is returned to the atmosphere via transpiration (Jackson et al., 2000). This long-distance water transport in the soil-plant-atmosphere continuum requires an efficient conduit network (Holbrook and Zwieniecki, 2005) that is also resistant to cavitation and embolism. Cavitation, which refers to the formation of water vapor bubbles in columns of water subjected to tensile stresses that exceed the tensile strength of water, results in the formation of an embolism that breaks the continuity of the water column, preventing water transport in this part of the xylem (Tyree and Sperry, 1989; Hacke et al., 2001; Niklas and Spatz, 2012). Vulnerability to cavitation is known to be influenced by the struc- ture of the xylem (Hacke et al., 2001). Strong conduits with a high proportion of wall material per unit volume will be resistant to im- plosion and provide protection against cavitation (Sperry, 2003). The amount of cell wall material is a strong determinant of wood density because the density of the cell wall material itself is rather constant in wood (Panshin and De Zeeuw, 1970; Saranpää, 2003). Denser wood is thus considered to be more resistant to xylem cavitation and confers a greater hydraulic safety (Meinzer et al., 2003; Jacobsen et al., 2005). However, density is also determined by the dimensions of the cells (Butterfield, 2003), and in turn their length, diameter, and wall thick- ness can all influence xylem flow resistance, protection against cavi- tation and risk of wall collapse (Sperry et al., 2006). Studies comparing the vulnerability of root and stem xylem have shown that roots are more vulnerable to cavitation (Alder et al., 1996; Hacke and Sauter, 1996), suggesting that they might be the weakest http://dx.doi.org/10.1016/j.foreco.2017.10.042 Received 20 September 2017; Received in revised form 20 October 2017; Accepted 20 October 2017 ⁎ Corresponding author. E-mail address: audrey.lemay@uqac.ca (A. Lemay). Forest Ecology and Management 408 (2018) 94–102 0378-1127/ © 2017 Elsevier B.V. All rights reserved. MARK