ALLOCATION, WITHIN AND BETWEEN ORGANS, AND THE DYNAMICS OF ROOT LENGTH CHANGES IN 2 BIRCH SPECIES

Spatial and temporal dynamics of biomass allocation within and between organs were investigated in seedlings of two birch species of contras ting successional status. Seedlings of Betula alleghaniensis Britt (ye llow birch) and B. populifolia Marsh (gray birch) were grown for 6 wee ks at two nutrient levels in rectangular plexiglass containers to allo w non-destructive estimates of root growth, production and loss. Leaf area and production were simultaneously monitored. Yellow birch respon ded more to nutrient level than gray birch in terms of total biomass, shoot biomass, leaf area and root length. Yellow birch also flexibly a ltered within-organ allocation (specific leaf area, specific root leng th and specific soil amount). In contrast, gray birch altered between- organ allocation patterns (root length:leaf area and soil amount:leaf area ratios) more than yellow birch in response to nutrient level. Yel low birch showed greater overall root density changes within a very co mpact root system, while gray birch showed localized root density chan ges as concentric bands of new root production spread through the soil . Species differ critically in their responses of standing root length and root production and loss rates to nutrient supply. Early successi onal species such as gray birch are hypothesized to exhibit higher pla sticity in varied environments than later successional species such as yellow birch. Our results suggest that different patterns of allocati on, within and between plant organs, do not necessarily follow the sam e trajectories. To characterize thoroughly the nature of functional fl exibility through ontogeny, within- and between-organ patterns of allo cation must be accounted for.