Growth-defence trade-offs in tapped pines on anatomical and resin production

Abstract

Resin harvesting is a traditional forestry activity that provides economic revenue along with crucial ecosystem services (i.e., fire protection and employment) thus contributing to the socio-economic development of rural areas. In the Iberian Peninsula, Pinus pinaster Aiton has traditionally been exploited for resin to complement the profitability of timber harvesting. Trade-offs between resin yield and radial growth are not fully understood, however. Resin ducts, the anatomical structures that produce and transport resin within the tree, have been used as a proxy for resin yield or resistance to pathogens as they represent the tree’s investment on resin defences throughout its lifespan. It is not yet clear if these structures can be linked to growth or how well the resin duct network correlates with resin yield in tapped trees. To answer these questions, we investigate annual tree-ring width, resin duct traits (absolute and standardized), and resin yield of two maritime pine stands tapped for resin extraction in the North of Portugal (VPA and PCO). Resin duct features are assessed before and during tapping periods as may show the constitutive investment, as well as the combination of constitutive and induced ones. Before resin tapping, average tree-ring width in PCO and VPA was ~2.25 and ~1.13 mm year−1, respectively. During tapping, standardized growth rates increased by ca. 40% in VPA, the population living under lower water availability and shorter growing seasons, while growth rates were almost unaffected in PCO. Median resin production in 2016 was 2.9 kg (PCO) and 1.8 kg (VPA), but inter-tree variance was high in both sites. Resin duct production increased exponentially with tree-ring width similarly in the different sites and for the same periods, but wider rings and ducts in PCO resulted in greater areas occupied by resin ducts. Within the population, constitutive anatomical resin-based defences did not trade-off with radial growth, although, lower growth rates were related to higher resin duct investments in response to tapping. Despite this, annual resin production did not correlate with resin duct density or relative area. Conversely, resin yield was positively associated with trunk perimeter, average growth, and absolute resin duct traits. These variables will thus be the most valuable when trying to identify the best resin producers in a stand. Differences in growth sensitivity to resin tapping between sites suggested that the impact of resin harvesting on radial growth is site-dependent and may not be as negative as previously thought.