Understanding the role of sapwood loss and reaction zone formation on radial growth of Norway spruce (Picea abies) trees decayed by Heterobasidion annosum s.l.

Allocation of photosynthates to defence responses at the expense of biomass increase is a common strategy amongst plants to cope with stress factors. Trees reduce the spread of decay by creating a secondary metabolite-rich reaction zone as fungal ingresses the sapwood. Reaction zone formation implies a sacrificial conversion of sapwood, thus, as decay progresses, the sapwood area of the tree is reduced. The relative contribution that reaction-zone formation and sapwood loss make to radial growth decrease is unclear. To answer this question we reconstructed radial-growth patterns in 100 Norway spruce (Picea abies) trees with a range of reaction zone and sapwood disruption. Basal area increment (BAI) between 1960 and 2007 and its relationship with sapwood reduction and reaction zone formation was assessed using structural equation models (SEM). BAI data showed that over 10 years, trees with small or no decay columns (<40%) and a reaction zone shifted from a growth rate that was similar to trees without a reaction zone towards low growth rate similar to trees with large decay columns. The fitted SEM indicated that: (i) the effects of decay on growth would begin with the formation of the reaction zone, and (ii) the smaller sapwood in decayed trees as compared with healthy trees would not reduce radial growth, but would be in part the result of previous periods of low growth due to reaction-zone formation.

► Growth losses started earlier than extensive decay development. ► Reaction zone contributed to growth reduction besides decay size. ► Small sapwood in decayed trees did not explain low growth. ► Small sapwood in decayed trees may result from previous period of low growth.