Eucalyptus obliqua seedling growth in organic vs. mineral soil horizons

Abstract

Eucalyptus obliqua, the most widespread timber tree in Tasmania, is a pioneer after fire which can eliminate the organic layer of forest soil, exposing the underlying mineral soil. We compared seedling growth, mycorrhiza formation, and mineral nutrient limitation in organic layer vs. mineral soil. We grew E. obliqua seedlings separately in pots of organic layer and mineral soil in a glasshouse. Additional treatments of organic soil only, involved fully crossed methyl-bromide fumigation and fertilization. Fertilization comprised chelated iron for 121 days after transplant (DAT) followed by soluble phosphorus. At 357 DAT, whole plant dry weight was three times greater in ambient organic than in mineral soil. In organic soil, fumigation halved ectomycorrhiza abundance and reduced seedling growth at 149 DAT, but by 357 DAT when negative effects of fumigation on seedling growth had disappeared, neither fumigation nor fertilization affected mycorrhiza abundance. Iron fertilization diminished seedling growth, but subsequent phosphorus fertilization improved it. E. obliqua seedlings grow much better in organic layer soil than in mineral soil, although phosphorus remains limiting. The prevalent forestry practice of burning to mineral soil after timber harvest exposes a poor growth medium likely only partially compensated by fire-induced mineral soil alterations.

Can Air Quality Management Drive Sustainable Fuels Management at the Temperate Wildland–Urban Interface?

“…Another source of smoke pollution in the autumn months is slash burning in the woody debris created by logging Eucalyptus forests. Even though the biological basis of this silvicultural practice is poorly understood, foresters assert that slash burning is necessary for effective regeneration of fire-dependent Eucalyptus forests, and that smoke is a necessary side effect [50][51][52].…”

A commentary on “Eucalyptus obliqua seedling growth in organic vs. mineral soil horizons”

“…In a recent paper in this journal, Barry et al ( 2015 ) reported on a pot-based trial comparing the growth of transplanted Eucalyptus obliqua seedlings in mineral vs. organic soils. Unfortunately in “Forestry Management Implications” (p. 11) they make claims regarding the appropriateness of fire as a regeneration tool in temperate Australian native wet eucalypt forests following harvesting.…”

“…Barry et al ( 2015 ) then cite the studies of Neyland et al ( 2009 ) and Scott et al ( 2012 ) to attempt to raise doubts as to the necessity of post-harvest regeneration burning for satisfactory regeneration. They state that the dispersed retention and aggregated retention silvicultural treatments studied by these authors demonstrated satisfactory regeneration despite poor burns.…”

“…They state that the dispersed retention and aggregated retention silvicultural treatments studied by these authors demonstrated satisfactory regeneration despite poor burns. In doing so, Barry et al ( 2015 ) have confounded stocking (the percentage of plots stocked with seedlings) with seedling density (the number of seedlings established per hectare). In both examples cited, the stocking was indeed in the acceptable range, but the overall seedling density was well below the levels considered necessary for future development of a productive regrowth eucalypt forest, as discussed at length in the papers cited.…”

“…Barry et al ( 2015 ) then suggest that perhaps mechanically removing the CWD and the competing vegetation regrowth by “several bouts of repeated re-clearing prior to eucalypt re-sowing would be an appropriate site preparation technique as an alternative to burning.” This suggestion, if implemented, would not only be a severe economic impost to forest managers, it would also profoundly alter the forest's ecology, setting it on a trajectory far removed from the natural disturbance regimes to which most species in the forest are presumably adapted (Whelan, 1995 ; Gill et al, 2002 ; Bowman and Murphy, 2010 ; Tng et al, 2012 ; New, 2014 ). Their thinking may be based on a fallacious assumption that regeneration burns largely eliminate CWD.…”