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Australia: The Land Where Time Began |
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Transfer of Carbohydrate through root grafts to support shaded
trees
According to Fraser et al.
they investigated if root grafts between lodgepole Pine (Pinus
contorta var. latifolia Dougl. Ex. Loud) trees can transfer
sufficient carbohydrate reserves from a source tree to a grafted sink
tree to affect the vigour of trees that were growing in an environment
that was light deficient. In early spring 11 plots were established and
2 grafted tree pairs and 2 independent trees that were not grafted were
selected at each plot. At each plot 1 tree of a grafted pair and 1
non-grafted tree were shaded at each plot, while the remaining trees
were not shaded during the period of the experiment. Trees that were
shaded had significantly lower carbohydrate reserves and smaller crowns
than trees that were not shaded following 1 growing season. Grafted
trees that were shaded had significantly higher total root nonstructural
carbohydrate than shaded trees that were not grafted, which indicates
that the effects of shading were partially offset by root grafts. Also,
proportionately more carbohydrates were transferred by large root grafts
to the shaded trees than small root grafts. Grafted trees were allowed
to persist under conditions in which trees that were not grafted would
be removed by competition, by carbohydrates that were transferred
through root grafts.
Following disturbances that replaced stands, some species of tree
establish at high densities and there is little competition among
seedlings (Cannell et al.,
1984; Kenkel et al., 1987).
Individual rates of growth diverge and trees begin to differentiate into
different crown classes after establishment. It is generally assumed
that competition for critical resources (1.e. light water nutrients) has
control over the rate and pattern of tree mortality in forest stands
(e.g., Mohler et al., 1987;
Knox et al., 1989; Nilsson et
al., 2002) and that the
mortality is concentrated among the smallest individuals (Mohler et
al., 1978; Cannell et
al., 1984; Kenkel et
al., 1997). It is usually
considered that competition for light is asymmetric because taller trees
re able to capture a disproportionate amount of the resource (e.g., Knox
et al., 1989; Berntson &
Wayne, 2000). The result is that smaller trees in dense stands generally
perish because of lack of light.
It has been suggested that the normal competitive relationships among
trees may be altered by the presence of root grafts and influence the
dynamics of forest stands (Kuntz & Riker, 1956; Bormann, 1962; Eis,
1972). It was indicated by previous studies that water (Schultz & Woods,
1967; Stone & Stone, 1975) and carbohydrates (Bormann, 1961; Bormann,
1966) can be transferred across root grafts. It has been demonstrated
that photsynthate from intact trees can be transported to keep the root
system of girdled trees (Bormann, 1966; Stone, 1874) and stumps (e.g.,
Bormann, 1961; Schultz & Woods, 1967; Eis, 1972) alive for years
following of the photosynthesising tops,
though the quantity of water or carbohydrates, or both, that can
be moved across root grafts is not known. However, it is not clear
whether these resources can be transferred to trees in inferior light
environments such as might occur under conditions of asymmetric
competition for light (Knox et al.,
1989; Bernston & Wayne, 2000).
Lodgepole pine (Pinus
contorta var.
latifolia Dougl. Ex.
Loud.) was chose for this study of root grafting because it commonly
regenerates at densities that are extremely high following a wildfire
(Lotan & Critchfield, 1990; Blackwell et
al., 1992) and it has the
ability to form extensive root grafts in dense stands more than 15 years
of age (Fraser et al., 2005).
These dense stands often have poor crown differentiation in the juvenile
stages, and, as a result of this, trees that are suppressed that would
normally be removed through competition often persist and so there may
be little change in the density over time (Blackwell et
al., 1992; Reid et
al., 2003). Fraser et
al. hypothesised that in
lodgepole pines the transfer of resources via root grafts helps to
support the continued existence of trees in dense stands that are in
inferior positions. In order to test this hypothesis it was determined
whether carbohydrate reserves can be transferred to a grafted sink tree
growing in an environment that was light-limited. Fraser et
al. also assessed whether
this relationship affected the vigour of grafted trees relative to
non-grafted trees that were growing in a similar environment.
Discussion
Trees in inferior positions are indicated by the results of this study
to have gained resources from surrounding trees through root grafts. In
course roots the total non-structural carbohydrate (TNC) concentrations
were found to be about 40% greater in that grafted trees that were
shaded (GS) than in shaded (NS) trees that were not grafted. The largest
impact of the grafts was manifest in the maintenance of higher TNC
concentrations in the roots of shaded trees that had grafts compared
with shaded trees that had no root grafts. The effects on crown growth
and concentrations of carbohydrates in the foliage were less clear.
There was a strong trend for less crown recession (P=0.076)
and for greater growth of the stem and greater leader increments in
grafter trees that were shaded compared with shaded trees that had no
grafts. The overall impacts of grafts were, however, on aboveground than
at the root level. As the root system is the first resource sink to be
in contact with the carbohydrates that were obtained from their grafted
neighbours, it is possible that few carbohydrates were passed on to the
organs that are more distal above ground. A similar relationship has
been observed that in herbaceous plants that were grown in soils that
were nutrient poor where the root system sequestered the majority of
nutrients that were available and passed few resources on to the
remainder of the plant (reviewed by Clarkson, 1985).
It is clear that the deep-shading treatment allowed the occurrence of
relative little photosynthesis (see the photosynthetic light response
curves by Landhӓusser & Lieffers, 2001) and it is not likely that the
shaded trees in this study would have survived a second growing season,
because there were declines in root carbohydrates and crown growth in
shaded trees, especially in the shaded trees that were not grafted. As a
consequence, the size of the phloem connection that joined many of the
grafted tree pairs may not have been sufficient to conduct enough
reserves to satisfy the demands of the root system as well as the organs
that were more distal at the top of the shaded trees. The size of
appeared to have been important in determining how much photsynthate was
passed between trees and in
most cases, it appears the grafts were too small to deliver sufficient
carbohydrates to maintain these large and heavily shaded neighbours.
According to Fraser et al. if
the shaded neighbour was proportionately smaller than the tree that was
not shaded, as would be the case in asymmetric competition, maybe the
grafts would have been able to support better the shaded tree.
Fraser et al. predicted that
there may be a parasitic relationship between the grafted shaded (GS)
tree and the grafted tree that was not shaded, with the growth and root
carbohydrate supply of the GN trees relative to non-grated non-shaded
trees being affected negatively by the shaded neighbour. Insignificant
growth reduction in the GN trees over 1 growing season being detected,
which suggested that the GN trees were not affected by the apparent
transfer of TNC to the GS trees. It is possible that the increased
absorptive surface area of the grafted root system improved the supply
of water to the foliage of trees that were not shaded because it is
likely the shaded trees had low stomatal conductance as a result of the
artificial boundary layer and low irradiances. Also, the high sink
strength of the grafted neighbour that was shaded and the grafted root
system may have increased the photosynthetic efficiency of the GN trees
(e.g., Neales & Incoll, 1968; Herold, 1980; Meyers et
al., 1999; Pieters et
al., 2001).
In conclusion, the grafted trees that were growing in environments that
were light-limited were supported partially by the carbohydrates
transferred across root grafts from their more vigorous partners.
However, competitive asymmetry could be reduced by even moderate
transfer of resources to a subordinate tree (Knox et
al., 1989; Kenkel et
al., 1997; Nilsson et
al., 2002), especially when
transfer occurs year after year. Mortality is generally greatest among
the most suppressed individuals; however, suppressed trees that are
grafted to a more vigorous partner may persist longer, especially it is
connected with a large graft. Therefore, the slow thinning of lodgepole
pines stands at high densities may be partially explained by root
grafting.
Fraser, E. C., et al. (2006). "Carbohydrate transfer through root grafts
to support shaded trees." Tree Physiology 26(8):
1019-1023.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |