Tree vigour influences secondary growth but not responsiveness to climatic variability in Holm oak

Many tree species from Mediterranean regions have started to show increased rates of crown defoliation, reduced groth, and dieback associated with the increase in temperatures and changes in the frequency and intensity of drought events experienced during the last decades. In this regard, Quercus ilex L. subsp. ballota (Desf.) (Holm oak), despite being a drought-tolerant species widely distributed in the Mediterranean basin, it has recently started to show acute signs of decline, extended areas from Spain being affected. However, few studies have assessed the role of climatic variability (i.e., temperature, precipitation, and drought) on the decline and resilience of Holm oak. Here, we measured secondary growth of seventy Holm oaks from a coppice stand located in central Spain. Sampled trees had different stages of decline, so they were classified into four vigour groups considering their crown foliar lost: healthy (0%), low defoliated (25%), highly defoliated (25 70%), and dying (70 100%). Our results showed that during the study period (1980 2009) the highly defoliated and dying Holm oaks grew significantly less than their healthy and low defoliated neighbours, suggesting permanent growth reduction in the less vigorous individuals. Despite these differences, all four vigour groups showed similar responses to climatic variations, especially during winter and late spring early summer seasons, and similar resilience after severe drought events, managing to significantly recover to pre-drought growth rates after only two years. Our findings, hence, illustrate that tree vigour influences secondary growth but not responsiveness to climatic variability in Holm oak. Still, as reduced growth rates are frequently associated with the process of tree mortality, we conclude that the less vigorous Holm oaks might not be able to cope with future water stress conditions, leading to increased mortality rates among this emblematic Mediterranean specie

Morphological and physiological responses of Galapagos endemic tree Croton scouleri to site conditions varying through its altitudinal range

Galapagos is a natural laboratory offering a great opportunity to study plants functional traits. This study characterises morphological and ecophysiological responses of Croton scouleri, an endemic tree that habits from humid and upper elevations to semiarid lowlands, throughout an altitudinal gradient and in a manipulative experiment. Croton scouleri trees were gradually smaller with less total leaf area due to a gradual reduction in mean leaf size, and they folded their leaves at lower elevations. These results were also recorded after cutting every deep root. Two physiological traits that allowed Croton scouleri to avoid damages to the photosynthetic apparatus were detected between 30 and 150 m a.s.l. Lower variable fluorescence (Fv) and basal fluorescence (F0) keeping constant maximum photochemical efficiency of PSII (Fv/Fm) denoted a drop in chlorophyll concentration. Concomitantly, the recorded increase in the Quantum efficiency of PSII (ΦPSII) with similar Fv/Fm means that Croton scouleri could be using cyclic electron transport as photoprotective mechanism. On the other hand, a deep root system to reach the water table allowed Croton scouleri to behave as a drought-avoider, which was reflected in: (1) unvarying water status Leaf Water Content and Relative Water Content were always higher than 69 and 58%, respectively; (2) stable and low photoinhibition levels; and (3) unvarying leaf area index. However, Croton scouleri was not able to avoid drought at altitudes lower 30 m a.s.l. where similar responses to those recorded after root cutting were recorded