Resin harvesting’s effect on maritime pine growth varies with tree age

Resin is a natural defensive substance produced by trees in response to injury and infection and is a sustainable and valuable non-timber forest product. Resin tapping is a traditional activity that besides increasing the economic profit of forest plantations, it provides ecosystem services and promotes the development of rural areas. However, the effect of resin tapping on tree growth remains elusive. Furthermore, there is evidence connecting resin production with climatic conditions, but tree-growth responses to climatic conditions and stand characteristics in resin-tapped and non-tapped trees remains unclear. Our aim is to understand if resin tapping affects tree growth, and how tree age, edaphoclimatic conditions and stand characteristics interplay. A dendrochronological study was conducted in six Pinus pinaster stands across a latitudinal gradient from North to the Centre of Portugal. The stands that had been resin-tapped for 5 to 7 years, presented different climate conditions and characteristics, i.e. different cambial ages. When tree-growth (tree ring-width; TRW) on tapped and untapped trunk sides was compared during the resin-extraction period, there was a slight enhancement of growth in the resin-tapped side of the youngest populations (<30 years) and no changes in older populations (>40 years). Annual resin-tapping impact (RTI), calculated as the ratio between TRW during resin harvesting years and the 5-year average TRW before tapping, was below (above) 1 in the younger (older) stands. Among other stand characteristics, RTI was negatively correlated with tree competition. Climatic conditions did not have a major influence on tree growth in response to resin tapping. In conclusion, the effect of resin extraction on growth is age dependent, with younger trees being more negatively affected. Our results seem to indicate that the co-production of resin and timber should be performed in pine populations older than 40 years.