Soil modifications created by ants along a post-fire chronosequence in lichen-spruce woodland

Abstract

Ants appeared to be an important component of the soil fauna in azonal lichen-spruce woodland in southern Québec, but their role in nutrient distribution and vegetation dynamics remained unknown. A study was undertaken to compare physical, chemical, and microbial properties of nest mound soils to those of surrounding soils to test the hypothesis that ants create patches of fertile soil and that these modifications are greater than changes in soil fertility due to forest succession. We also quantified ground cover, sexually regenerated black spruce seedlings, and ant nests along the chronosequence and verified whether nest mounds constituted safe sites for black spruce seedling establishment. Surface nest mounds had a significant effect on most measured soil variables. Compared to surrounding soils, nest soils had lower moisture content and bulk density, higher organic matter content and pH, higher available C for microbial growth, higher plant-available N, and were enriched in several major nutrients (K, Ca, Mg). Extractable-P was, however, lower in nest mounds than in surrounding soils. On the other hand, age-class significantly affected fewer soil variables (moisture, microbial biomass, extractable Na, and total Ca), and these changes could be explained by fire disturbance and the subsequent development of the tree stratum. Exposed mineral soil was the dominant ground cover in the 1-year-old and 9-year-old plots, while lichen cover was dominant in the three older age-classes. Sexually regenerated black spruce seedlings occurred mainly in the sparse moss layer in the 1-year-old and 9-year-old age-classes and almost exclusively in the lichen layer in the two older age-classes. Seedling density was 15 ha-1 the year following fire disturbance, but varied between 52 and 121 ha-1 in the four older age-classes. The average densities of occupied ant nests varied between 137-188 nests ha-1 across age-classes and occurred directly in lichen or moss, under bare mineral soil, in coarse woody debris, or under conspicuous sand mounds. We found no seedlings growing on occupied or abandoned nest mounds, which numbered only 15 ha-1 in the 1-year-old age-class and between 44 and 74 ha-1 in subsequent age-classes. We conclude that ants in this lichen-spruce woodland create nutrient-rich patches that can have a positive effect on tree growth and accelerate canopy closure, but we reject the hypothesis that nest mounds are safe sites for black spruce seedling establishment.