Abstract
KEY MESSAGE: Intrinsic water-use efficiency of Pinus canariensis (Sweet ex Spreng.) growing at a semi-arid treeline has increased during the past 37 years. Tree-ring width by contrast has declined, likely caused by reduced stomatal conductance due to increasing aridity. CONTEXT: Rising atmospheric CO(2) concentration (C(a) ) has been related to tree growth enhancement accompanied by increasing intrinsic water-use-efficiency (iWUE). Nevertheless, the extent of rising C(a) on long-term changes in iWUE and growth has remained poorly understood to date in Mediterranean treeline ecosystems. AIMS: This study aimed to examine radial growth and physiological responses of P. canariensis in relation to rising C(a) and increasing aridity at treeline in Tenerife, Canary Islands, Spain. METHODS: We evaluated temporal changes in secondary growth (tree-ring width; TRW) and tree ring stable C isotope signature for assessing iWUE from 1975 through 2011. RESULTS: Precipitation was the main factor controlling secondary growth. Over the last 36 years P. canariensis showed a decline in TRW at enhanced iWUE, likely caused by reduced stomatal conductance due to increasing aridity. CONCLUSION: Our results indicate that increasing aridity has overridden the potential CO(2) fertilization on tree growth of P. canariensis at its upper distribution limit.