Does natural root grafting make trees better competitors?

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.