Decoupled leaf-wood phenology in two pine species from contrasting climates: Longer growing seasons do not mean more radial growth

Abstract

Climate warming is expected to lengthen the growing season of tree species and enhance radial growth rates. Alternatively, a longer growing season could not lead to improved radial growth if wood production depends more on growth rate than on growing season length. We test these ideas by comparing leaf phenology data and the estimated start and end dates of wood formation predicted by the VS-Lite growth model. We analyzed long-term series of leaf unfolding and fall dates and reconstructed radial growth of two pine species under contrasting climatic conditions: Scots pine (Pinus sylvestris) in a Russian boreal site and Aleppo pine (Pinus halepensis) in a Spanish Mediterranean site. On average, leaf onset occurred in days 99 and 163 in P. halepensis and P. sylvestris, respectively, about 40 days earlier than the estimated start date of wood formation. The onset of leaf unfolding advanced 2.1 days per decade in P. sylvestris in response to warmer May temperatures. Radial growth was enhanced by warm-wet spring-summer conditions in P. sylvestris and by wet soil conditions from prior winter up to current summer in P. halepensis. In this species the growing season length and the radial growth rate were not coupled because the growing season length shortened during cool-wet periods whereas growth rates increased. In P. sylvestris leaf onset was delayed during years with low growth rates suggesting a potential coupling between warmer spring conditions, earlier leaf onset and enhanced growth whenever soil water content is high enough. Overall, we show that longer growing seasons do not necessarily imply higher radial growth rates.