We investigated whether 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 vigor of trees growing in a light-limited environment. Eleven plots were established in early spring and two grafted tree pairs and two independent non-grafted trees were selected at each plot. One tree in a grafted pair and one non-grafted tree were shaded at each plot, whereas the remaining trees were non-shaded during the experimental period. Shaded trees had significantly lower carbohydrate reserves and smaller crowns than non-shaded trees following one growing season. Grafted shaded trees had significantly higher root total nonstructural carbohydrate concentrations than non-grafted shaded trees, indicating that root grafts partially offset the effects of shading. Also, large root grafts transferred proportionately more carbohydrates to the shaded trees than small root grafts. Carbohydrates transferred through root grafts could allow grafted trees to persist under conditions where non-grafted trees would be removed by competition.