Trends in growth and soil nutrients following fire and logging

Abstract

Understanding the impact of natural and anthropogenic disturbance on soil fertility and tree growth is critical to the sustainability of forest management, yet there remains much uncertainty regarding how both harvesting and wildfire affect soil nutrient dynamics, espe-cially over several decades and in varying overstory types. Historically, wildfire has been the dominant stand-replacing disturbance and an important mechanism of ecosystem renewal. However, intensive forest harvesting is replacing fire as the primary disturbance in many parts of the world, sparking concerns about nutrient depletion and decreased site productivity associated with biomass removals. I conducted a global meta-analysis of northern forest ecosystems to examine the ef-fects of forest harvesting on total concentrations and stoichiometric ratios of soil carbon ([C]), nitrogen ([N]), and phosphorus ([P]) relative to natural, uncut control stands, and whether these effects differed as a function of harvest intensity, soil depth, overstory type, and time since harvesting. I then used an age chronosequence from 7- to 33-years-old to ex-amine and compare the effects of clearcut harvesting and fire on soil nutrient concentrations and tree growth in three predominant upland boreal stand types (dominated by deciduous or coniferous trees, or a combination of both) during early stand development. The results of the meta-analysis indicated that harvesting negatively affects [C], [N], and [P] in the forest floor soil layer, but has positive or neutral effects on the mineral soils layer, except for when it is coupled with fire disturbance (through post-fire salvage logging or prescribed burn following harvest), which resulted in strong, negative effects on mineral soil [C] and [N]. The negative effect of harvesting on forest floor nutrients increased with harvesting intensity. Time since harvesting had a positive effect on soil [C] and [N], but less so for [P], which likely requires more time to recover given its reliance on inputs from min-eral weathering. Our chronosequence study also highlighted the important role of stand age (i.e. time since disturbance). Although fire resulted in more dramatic effects on soil nutrient concentra-tions and stand basal area than harvesting in the 7-year-old stands, these differences con-verged in the 15- and 33-year-old stands. Similar to the meta-analysis, the effects of disturb-ance were most profound in the forest floor layer, and temporal trends differed between bio-logically controlled nutrients (C and N), which recovered rapidly and linearly, and nutrients that are more geochemically controlled (P, K, and Ca). The results of both studies indicated that conifer stands are more sensitive to nutrient loss following disturbance than deciduous stands, and mixedwood stands are intermediate. Our findings highlight the importance of harvest intensity and rotation length on long-term soil nutrient health when managing northern forest ecosystems, particularly in conifer stands.