Tree Seedling Root Architecture Alteration by Tap Root Pruning1

Abstract Nursery production of strong taprooted woody plants typically includes pruning to interrupt taproot development. To discern the impact this practice could have on seedling root architecture, we quantified changes to root architecture after taproot pruning and restriction separately in Catalpa (Catalpa speciosa) and Kentucky coffee tree (Gymnocladus dioicus). Taproot pruning resulted in a large and significant increase in the number of new, vertically oriented roots from the cut end of the primary root (regenerated taproots) in both species. Catalpa seedlings, which produced many strong laterals on unpruned taproots, showed greater reduction in lateral root number and size after taproot pruning than Kentucky coffee tree (with fewer and smaller natural lateral roots). The two species responded differently to restriction of the single, unpruned taproot by container depth (15, 30, 60 cm). For catalpa, with more shallow laterals naturally, the number of laterals was not significantly changed by restriction of the taproot by air pruning at any container depth, but lateral diameter was reduced by the 15 cm-deep container and biomass was reduced by the 30 cm-deep container, compared to the 60 cm-deep container. For Kentucky coffee tree with fewer natural laterals, restricting the taproot at 15 cm significantly increased the number and diameter of lateral roots compared to the 30 and 60 cm-deep containers, suggesting that restricting the taproot could increase the number of laterals in species that naturally produce fewer. Restricting multiple taproots on root-pruned plants generally did not affect lateral root development for either species, but this may have been due to the low number of lateral roots on those root systems. Index words: root architecture, nursery production, urban soils Species used in this study: Catalpa [Catalpa speciosa (Warder) Warder ex Engelm. ]; Kentucky coffee tree [Gymnocladus dioicus (L.) K. Koch]