Longevity of mycorrhizal roots depends on branching order and nutrient availability

The effects are reported of branching order and nitrogen fertilization on t he longevity of mycorrhizal short roots at a Norway spruce (Picea abies) st and in southwest Sweden. A 4-yr qualitative analysis was made, using minirhizotrons, of the emergenc e frequency and life span of individual mycorrhizal roots of three root ord ers (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 myc orrhizal root orders (a fivefold reduction in order 3 compared with control s), 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 ro ots and nitrogen addition decreases branching density while increasing long evity. The survival of mycorrhizal roots, which remain vital and unsuberize d over winter, enhances water and nutrient efficiency of Norway spruce.