Root Ammonium Transport Efficiency as a Determinant in Forest Colonization Patterns: an Hypothesis

Ratios of ammonium (NH 4 ‡) to nitrate (NO 3 ±) in soils are known to increase during forest succession. Using evidence from several previous studies, we hypothesize that a malfunction in NH 4 ‡ transport at the membrane level might limit the persistence of early successional tree species in later seral stages. In those studies, 13 N radiotracing was used to determine unidirectional fluxes and pool sizes of NH 4 ‡ and NO 3 ± 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 NH 4 ‡ , the two early successional species accumulated excessive NH 4 ‡ in the root cytosol, and exhibited high-velocity, low-efficiency (15% to 22%), membrane fluxes of NH 4 ‡. In sharp contrast, white spruce had low cytosolic NH 4 ‡ accumulation, and lower-velocity but much higher-efficiency (65%), NH 4 ‡ fluxes. Because these divergent responses parallel known differences in tolerance and toxicity to NH 4 ‡ amongst these species, we propose that they constitute a significant driving force in forest succession, complementing the discrimination against NO 3 ± documented in white spruce (Kronzucker et al. 1997).