Root ammonium transport efficiency as a determinant in forest colonization patterns: An hypothesis

Ratios of ammonium (NH₄⁺) to nitrate (NO₃^-) in soils are known to increase during forest succession. Using evidence from several previous studies, we hypothesize that a malfunction in NH₄⁺ transport at the membrane level might limit the persistence of early successional tree species in later seral stages. In those studies, ¹³N radiotracing was used to determine unidirectional fluxes and pool sizes of NH₄⁺ and NO₃^- in seedlings of the late-successional species white spruce (Picea glauca) and in the early successional species Douglas-fir (Pseudotsuga menziesii var. glauca) and trembling aspen (Populus tremuloides). At high external NH₄⁺, the two early successional species accumulated excessive NH₄⁺ in the root cytosol, and exhibited high-velocity, low-efficiency (15% to 22%), membrane fluxes of NH₄⁺. In sharp contrast, white spruce had low cytosolic NH₄⁺ accumulation, and lower-velocity but much higher-efficiency (65%), NH₄⁺ fluxes. Because these divergent responses parallel known differences in tolerance and toxicity to NH₄⁺ amongst these species, we propose that they constitute a significant driving force in forest succession, complementing the discrimination against NO₃^- documented in white spruce (Kronzucker et al. 1997).