Effect of lopping on water potential, transpiration, regrowth, (14)C-photosynthate distribution and biomass production in Alnus glutinosa.

The effects of light, moderate and heavy branch pruning or lopping treatments (resulting in removal of 28-31, 56-60 and 80-82% of the foliage, respectively, unlopped control = 0%) were studied in 3-year-old black alder (Alnus glutinosa L. Gaertn.) plants. Within 24 h of lopping, transpiration rates decreased and water potentials increased. The effects of lopping continued for 48 days. The improved water status of the lopped plants enhanced water-use efficiency during the first 30 days. Regrowth was related to a combination of enhanced net assimilation rates during the first 30 days after lopping, use of current photoassimilates and stored carbohydrates in the roots (particularly in medium and heavily lopped plants) for the production of new leaf area, and delayed leaf senescence. The lightly and moderately lopped plants developed 62 and 57% more leaf area, respectively, than the unlopped plants, but total leaf area development was slower in the heavily lopped plants. Lightly lopped plants produced total biomass equivalent to that of unlopped plants, but the moderately and heavily lopped plants produced less biomass than the unlopped plants. During the first 30 days after lopping, there was a net loss of nodule dry matter and recovery to control values did not occur during the period of study. Immediately (24 h) after lopping, more (14)C-photosynthate was translocated to the roots of lopped plants than to the roots of unlopped plants. When a branch near the base of the main stem was fed (14)CO(2), (14)C-photosynthate translocation to the shoot increased with increasing intensity of lopping, at 50 days after lopping.