Effects of cork oak stripping on tree carbon and water fluxes

Abstract

Cork is a high value periodical forest product which ensures the economic, social and ecological sustainability of cork oak woodlands. Abiotic and biotic stresses lead to tree decline which is endangering the productivity and sustainability of these ecosystems. It is therefore critical to find and implement management practices that minimize the impact of these stresses. The current study was conducted in a certified evergreen cork oak woodland of central Portugal under Mediterranean climate. The main aims of the study were to assess the effects of cork stripping in tree water and carbon fluxes. Results are based on the monitoring of cork stripped and unstripped (control) trees. The experiment was repeated with different sets of trees during two contrasting summers (2014 and 2015). 2014 was a wet year (924 mm) with a typical summer drought pattern and 2015 a dry year (440 mm) with a 31% reduction in annual average precipitation. In 2015 the experimental site was entirely cork harvested and effects on ecosystem CO2 fluxes were evaluated. Results showed that the amount of carbon in harvested cork represents less than 1.5% of net primary production on a yearly basis. In addition, cork tissue is very low demanding in nutrients: primary macronutrients content in cork represents approximately 2% of the yearly nutrient needs of leaf canopy. Regardless of the climatic year, trunk water losses following cork stripping amounted to only 2% of canopy transpiration not affecting significantly summer tree water balance. However, cork stripping induced a 46% decrease on sap flow in the dry year suggesting that cork stripping triggered an increase in stomatal closure through an interaction between stripping traumatic effects and soil water scarcity. Although the effects of summer drought on carbon sequestration are more prominent than cork stripping effects, this superimposed stress led to a significant reduction of summer net carbon ecosystem exchange (ca. 32%). Our results suggest that cork stripping detrimental effects can be especially critical in more vulnerable trees growing near their vitality breakdown threshold. Therefore, and concerning cork oak woodland management, the cork stripping practice should be avoided in severe dry years and in the more stress-prone trees.