Tree growth responses to climate variability: The case of Clethra arborea in the Azores and Madeira islands.

Forests on oceanic islands present unique opportunities for dendroclimatic research, owing to the pronounced climatic gradients over short geographic distances. In the Azores archipelago, with a temperate oceanic climate, with low thermal amplitude, and mild and relatively wet Summers, Clethra arborea Aiton (Clethraceae) is one of the most recent examples in the Azores of the generally unexpected consequences that the introduction of an exotic species, introduced in São Miguel Island (Azores) probably in 1950s. Madeira island climate is slightly warmer and more humid than the Azores, with distinct variations between the north and south of the island due to its topography. Here, Clethra arborea Aiton is an endemic plant. Forest management is essential for maintaining and improving the provision of ecosystem services, while dendrochronological techniques can be instrumental in supporting this effort. There are not many dendrochronological studies that have targeted this species elsewhere and, both archipelagos, are widely unexplored in terms of dendroclimatology, so, this study in the Azores and Madeira is aimed to understand its climate-growth relationships better and allow the identification of possible limiting factors on growth through tree-ring inter-annual pattern variations. For this purpose, we sampled 606 trees from two populations (São Miguel and Madeira islands). Following standard dendrochronological methods, we obtained four site chronologies from São Miguel Island and four site chronologies from Madeira Island. We used a stepwise modelling approach, with Random Forest and Generalized Linear Models. Our results suggest that Clethra arborea benefits from the conditions of year-round precipitation in the Azores while in Madeira, only on site Encumeada (EN) it benefits from these conditions. For this species, and in the future, higher Winter and Spring temperatures could lead to increased water stress and reduced growth rates. This should be reflected when forecasting the future distribution and productivity of Clethra arborea under diverse climate change scenarios. Our findings provide essential insights for developing management strategies for this species.