Relationship of ion absorption to growth rate in taiga trees

Rates of nutrient absorption were measured on excised roots of taiga tree seedlings grown in the laboratory. Phosphate and to a lesser extent ammonium (relatively immobile ions in the soil) were absorbed most rapidly by poplar and aspen, two species with rapid growth rates and most slowly by alder and/or black spruce, species with slow growth rates. In contrast, potassium (which is more mobile in soil) was absorbed most rapidly by slowly growing species. All species had low rates of nitrate and chloride absorption. Absorption rate of each ion was most temperature sensitive in those species that typically occupy the warmest soils (i.e. poplar and aspen). We suggest that in infertile soils a high capacity for uptake is an important component of root competition only in the case of mobile ions (e.g. potassium, nitrate), because only for these ions do diffuison shells of adjacent roots overlap; in contrast plants compete for immobile ions (e.g. phosphate) only by increasing absorptive surface via root growth or mycorrhizal association.