Responses of drought-tolerant Tipuana tipu trees to long-term irrigation: A tree-ring study in Lisbon, Portugal

Tree planting is a key strategy for the adaptation of cities to climate change. However, cities in semi-arid and Mediterranean climates struggle to sustain tree growth, often relying on irrigation systems. Literature suggests that irrigation enhances tree growth and ecosystem service provision at the cost of structural and physiological adjustments that can lead to lower wood density, compromising tree’s mechanical stability and pathogen resistance. Yet, the response of trees to irrigation is species dependent, raising uncertainty about its possible costs and benefits. We investigate how drought-tolerant Tipuana tipu trees – a common species in Lisbon and other cities worldwide – responded to six years of irrigation using a multi-proxy tree-ring approach. We examined even sized and aged trees growing under comparable conditions in the Duque de Saldanha Square, differing only in irrigation regime. The positive relationship between growth and oxygen stable isotopes confirms that trees grow during the dry and hot summer and is limited by stomatal conductance. The irrigation started in 2017 reduced the stomatal conductance limitations of the treatment trees as evidenced by the significant decrease in stable carbon isotope values and a twofold increase in growth rate compared to control trees. Notably, irrigation did not result in any significant wood density reduction. These findings challenge the assumption of wood density reduction under irrigation and further highlight this drought-tolerant species capacity to increase growth rate and maintain wood density. A response likely to enhance ecosystem services delivery without compromising mechanical stability under long-term irrigation. • Irrigation systems are largely used in cities to support tree growth. • Literature points to a trade-off between growth and wood density in irrigated trees. • We studied the responses of Tipuana tipu to six years irrigation in Lisbon, Portugal. • Tipuanas doubled the growth rate without a significant reduction in wood density. • This response arises from a reduction in growth limitation by stomatal conductance.