Invasive palms have more efficient and prolonged CO2 assimilation compared to native sub-Mediterranean vegetation

In sub-Mediterranean ecosystems, shade-tolerant broadleaf evergreens, especially the invasive Trachycarpus fortunei, are spreading uncontrollably in the forest understorey, impeding the regeneration of the native deciduous woody vegetation. Most invasive species benefit from high-light environments, as they often have a resource-acquisitive strategy with high photosynthetic rates. Yet, evergreen neophytes, such as T. fortunei, grow mainly in dark understoreys, which raises the question about the physiological mechanisms allowing it to thrive and, eventually, outcompete the native vegetation. For one year, we compared the photosynthetic light-use efficiency of T. fortunei and seven competitors (one evergreen and six deciduous species) to understand possible physiological drivers of this invasion process. The work was done in invaded sub-Mediterranean forests in Southern Switzerland and Northern Italy. Measurements included the seasonal photosynthetic capacity (Amax), net assimilation (Anet), apparent quantum yield (qLCP), light compensation point of photosynthesis (LCP), chlorophyll content (CC), chlorophyll fluorescence (Fv /Fm), leaf soluble sugar (SSL) and starch concentrations (STL), nitrogen (LNC), and dry matter (LDMC) contents. During the entire year, T. fortunei had similar assimilation rates to the evergreen and deciduous competitors. However, in October, Amax and Anet of T. fortunei were higher than the other evergreen species, while most deciduous plants started losing their foliage. While no photosynthetic disadvantage was found for T. fortunei, we observed less seasonal variability in Anet, qLCP, SSL, STL, and LDMC, and a lower or similar qLCP, Fv/Fm, CC, SSL, STL, and LDMC in this species than in its competitors. We highlight that October was the only month when T. fortunei had a photosynthetic advantage over the native sub-Mediterranean evergreen vegetation, while during the rest of the year, it showed similar assimilation rates as evergreen and deciduous species. Forests with sparse canopies in October might be particularly favourable for T. fortunei, due to the benefits of high light conditions for its assimilation rates compared to native evergreens. Moreover, the low seasonal variability in physiological traits indicates that as winters get milder and growing seasons longer, the invasive palm may take a further competitive advantage over the native vegetation.