Abstract
We reviewed approaches for combining genetic and cultural tools to solve challenges associated with the improvement of tree seedling root systems for enhanced survival and growth. Literature regarding sources of variation in root traits, relationships between root traits and seedling growth and survival, and root trait heritability estimates from published studies were used to illustrate the utility of combining myriad approaches to enhance the efficiency of tree seedling improvement programs. We found that: (1) evidence exists for genetic variation in numerous root traits, (2) root traits tend to have intermediate heritability on average, and (3) evidence supports the possibility of harnessing maternal effects to modify root traits at least temporarily. Across eight root traits from 11 studies, median family narrow-sense heritability estimates (h(2)) were intermediate (0.25 <= h(2) <= 0.50) for root diameter, root count, root biomass, the ratio of root-to-stem dry biomass, and projected root area, and low (h(2) < 0.25) for root length, specific root length, and the ratio of total belowground-to-aboveground dry biomass. Findings from this review suggest that plant improvement pipelines can be optimized to harness root trait variation due to genetics, maternal effects, and nursery cultural regimes. However, the following related gaps in the literature emerged as fundamental obstacles to progress: What is the duration of root system responses to stress conditioning treatments? Which root traits correlate best with seedling survival and thus should be prioritized for improvement? Is it possible to reliably harness variation in root traits resulting from maternal effects?