Increased water use efficiency does not prevent growth decline of Pinus canariensis in a semi-arid treeline ecotone in Tenerife, Canary Islands (Spain)

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 CO₂ 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 CO₂ fertilization on tree growth of P. canariensis at its upper distribution limit.

Keywords: Canary Island Pine; Climate change; Mediterranean climate; drought; stable carbon isotope; treeline.