DOI: 10.5061/dryad.z34tmpghm
title: Does natural root grafting make trees better competitors?

description: Natural root grafts (anastomoses) are morphological unions formed between
 roots of different trees. Common root systems allow translocation of
 water, nutrients and photosynthesis products between grafted trees,
 affecting their growth and their physiology. As carbohydrates are
 redistributed among grafted trees, the formation of a common root system
 could reduce the negative effect of intraspecific competition for light or
 soil resources within stands. The aim of this study was to investigate the
 role of root grafting on intraspecific competition and growth of balsam
 fir (Abies balsamea (L.) Mill). We studied inter-tree relationships in
 three natural balsam fir stands of the boreal forest of Quebec (Canada)
 that contained an average 36% of grafted trees. At each stand, ring width
 and basal area of trees were measured using dendrochronology techniques.
 We used mixed linear models to test the effect of root grafting and
 intraspecific competition on annual basal area increment of trees. Trees
 before grafting had higher growth rates than trees once grafted. Thus,
 root grafting did not improve tree growth. Growth of grafted trees was
 more negatively affected by intraspecific competition than growth of
 non-grafted trees. Thus, grafted trees cannot be considered as better
 competitors than non-grafted trees. Under high intraspecific competition,
 growth of larger grafted trees was less affected than that of smaller
 trees suggesting that they were able to divert resources at their
 advantage within a union. Our study demonstrated that grafted trees acted
 on each other’s growth and provides support for the idea that grafted
 trees respond to competition for resources more as a community rather than
 as individual trees.
description: Field work was performed in
 summer 2017. At each site,
 distance between trees were measured then all trees were felled with a
 chainsaw and cross-sectional disks were collected at ground level (0 m).
 Cross-sectional disks were air dried and sanded (80-400 grit) to reveal
 growth rings. Tree ring widths series were measured on cross-sectional
 disks from the stem base using a Velmex “TA Unislide” measurement system
 with ACU-Rite linear encoder and QC10V digital readout device (Velmex,
 Inc., Bloomfield, New York) interfaced with Measure J2X (Version 5.0x)
 ring-reading software (Project J2X, Voortech Consulting, Holderness, NH).
 Ring-width series were measured with 0.001 mm precision along two rays per
 wood disk when possible (i.e. when the rings were clearly visible) and
 visually cross-dated. To obtain a single series per tree, chronologies
 were averaged for each tree when two of them were available. A total of
 110 tree ring width series were obtained.
 We converted tree ring width
 series into basal area increment (BAI) to calculate annual tree growth
 (Biondi and Qeadan
 2008) following equation
 (1) : BAI<sub>t</sub> =
 πR<sub>t</sub><sup>2</sup>-
 πR<sup>2</sup><sub>t-1</sub>
 (1)   Where R<sub>t</sub> is the stem
 radius at year t and R<sub>t-1</sub> is the stem radius at
 year t-1. Intraspecific competition between trees was
 estimated using distance-independent (BAL: Basal area of the Larger tree)
 and distance-dependent (HEG: Hegyi competition index) competition indices.
 The first index (BAL) was
 developed by Wykoff et al. (1982) and corresponds to the sum of the of the basal
 area of neighbor trees that are larger than the subject tree:
 The second index (HEG) was developed by Hegyi (1974) and
 corresponds to the sum of the ratios of diameters of a subject tree and
 its competitors, weighted by the distance from the subject tree. We considered that each tree found in a radius
 of 6 m or less around the subject tree was a potential
 competitor.
description: Excel software is required to open
 the data files (.csv).