Longevity of mycorrhizal roots depends on branching order and nutrient availability

The effects are reported of branching order and nitrogen fertilization on the longevity of mycorrhizal short roots at a Norway spruce (Picea abies) stand in southwest Sweden. A 4-yr qualitative analysis was made, using minirhizotrons, of the emergence frequency and life span of individual mycorrhizal roots of three root orders (unbranched, main axes of branched and side branches of branched short roots, orders 1,2 and 3, respectively). Longevity of mycorrhizal roots varied with branching order and soil depth, being greatest in unbranched (order 1) roots and those at the greatest soil depth (40-85 cm). Nitrogen addition decreased the proportion of higher mycorrhizal root orders (a fivefold reduction in order 3 compared with controls), while increasing the median longevity. Seasonal effects were recorded; mycorrhizal roots produced in spring and summer remained unsuberized during the winter. Mycorrhizal root longevity depends on the branching order of mycorrhizal roots and nitrogen addition decreases branching density while increasing longevity. The survival of mycorrhizal roots, which remain vital and unsuberized over winter, enhances water and nutrient efficiency of Norway spruce.