Australia: The Land Where Time Began

A biography of the Australian continent 

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.