Soil conditions required for reaction wood formation of drunken trees in a continuous permafrost region

Abstract

Black spruce trees lean to form “drunken” forest on degrading permafrost; however, the causes of tree leaning on continuous permafrost remain unclear. Leaning events are recorded by reaction wood formation in tree rings, and it remains unclear what soil conditions are required for reaction wood formation of drunken trees. Tree disk morphology and soil hummock properties were examined for fifty tree–mound combinations in Northwest Territories, Canada. Spruce trees growing on mound edges form reaction wood on the downslope sides of their trunks. Reaction wood formation in mature trees was greatest in stem tissues between ground level and 30 cm aboveground. Reaction wood formation occurred only in trees growing on mound edges. The extent of reaction wood formation was higher in trees growing in clayey soils than in trees on sandy soils. For trees growing on clayey mound edges, the extent of reaction wood formation decreased with increasing permafrost table depth. Black spruce tree rings formed between ground level and 30 cm aboveground could record movement of clayey soil hummocks over shallow, underlying permafrost tables. A combination of clayey soil texture and shallow permafrost table is likely required for development of hummocks and drunken forests on the continuous permafrost region studied.

Keywords

• active layer
• earth hummock
• frost heaving
• gelisols
• thermokarst