Sapwood area drives growth in mountain conifer forests

1. It is expected that climate warming will enhance tree growth of mountain conifer forests in cold regions. However, trees have shown unstable, age-related and site-dependent growth responses to climate throughout the past century, but information on the drivers controlling such responsiveness at the site and tree scales is lacking. We evaluated whether such changing growth responses are more influenced by site features, such as altitude, or by tree features, such as size and sapwood area. 2. We quantified the growth trends at the site and tree levels in Iberian Pinus uncinata forests using dendrochronology. Tree-ring width was converted to basal area increment (BAI) to assess the relationships between growth and site and tree variables over three time periods (19011994, 19011947, 19481994) using structural equation models. 3. Trees were older at higher altitudes, and the amount of sapwood decreased as trees aged. BAI trends were lower in the period 19481994 than in the period 19011947, that is, tree growth is decelerating, despite BAI values of both periods showing the reverse pattern. Sapwood area and, to a minor extent, tree age were the main positive and negative drivers, respectively, controlling BAI during the 20th century, whereas altitude played a minor role. 4. Synthesis. Our results highlight the relevance of tree individual characteristics as the main drivers modulating growth responses to climate warming. We conclude that climate warming will have a lower effect on radial growth in slow-growing high-elevation trees than in fast-growing low-elevation trees, which produce a greater sapwood area. Trees may become relatively insensitive to climate as they age and reach a size-related functional threshold linked to reduced sapwood production.