Carbon balance of mediterranean evergreen species

Photosynthesis and respiration are the most fundamental plant physiological processes which affect carbon cycle on a scale ranging from the leaf to the globe (Cavaleri et al.,2008). Respiration and photosynthesis are strongly coupled and interdependent in leaves of higher plants (Atkin et al., 2007). The temperature sensitivity of photosynthesis differs from that of respiration, and hence the ratio between the two processes may be altered following a short-term change in temperature (Loveysetal., 2002). In this context, the main object of this research was to analyze variation in the ratio RL/PN of the species co-occurring in the Mediterranean maquis in response to water availability and air temperature during the year. Variations of the ratio RL/PN might imply change in Mediterranean species structure and productivity in the long term (Saxe et al., 2001) by causing changes in species presence and, in turn in vegetation. Improving knowledge on processes and factors influencing carbon balance is important in making accurate models of CO2 exchange between vegetation and the atmosphere (Armstrong et al., 2006). Moreover, taking into account that respiration of terrestrial ecosystems is a major flux in the global carbon cycle and a potentially important positive feedback mechanism for climate change (Schimel, 1995), data on the response of the ratio RL/PN to environmental changes is an important aspect for predicting future global atmospheric CO2 concentration. The results highlight that the ratio RL/PN of the considered species ranged from 0.16± 0.09 (in winter) to 1.44±0.93 (in summer). Variations of RL and PN during the year were attested by the PCA which was carried out using leaf physiological and morphological traits of the considered species. In particular, Cistus incanus having the highest mean yearly PN and RL rates and low LMA and LTD was furthest from the other species emphasizing its drought semi-deciduous habitus, the highest photosynthetic capability in favorable conditions (spring), but low tolerance to drought. Erica multiflora and Rosmarinus officinalis were characterized by the highest LMA and LTD, low PN rates in drought and the lowest PN ones in spring. Erica arborea, Pistacia lentiscus, Phillyrea latifolia and Quercus ilex had the highest drought tolerance. Arbutus unedo and Smilax aspera were close to this group despite lower RL rates during the year. The xeromorphic leaves of the considered evergreen species (i.e. high LMA and LTD) favor carbon gain profits over transpiration losses during drought, nevertheless, the high construction cost of these leaf type justifies the relatively high RL rates. The ratio RL/PN can be used to evaluate plant carbon gain in response to environmental factor variations. Moreover, our findings of a seasonal variation in the ratio RL/PN are consistent with those of Zaragoza-Castells et al. (2008) showing that the ratio increases in response to the increase of temperature and in drought. The hypothesized extension of the dry season in the Mediterranean area, as forecasted in the next decades by climatic models (IPCC, 2007) might favor P. latifolia, P. lentiscus, Q. ilex, A. unedo and E. arborea by their capability to maintain a lower RL/PN ratio during drought, which is indicative of a more positive carbon balance, compared to C. incanus, E. multiflora, R. officinalis and S. aspera. Thus, we can assume that the species able to keep a positive carbon balance during drought will be more adapted in dealing with stronger environmental stress. The vegetation of the Mediterranean shrublands contributes to a relevant amount of carbon sequestration at a global level (Gratani et al.,2013), and in the long term the different competitive ability could determine change in the community structure and composition, resulting in variations of the CO2 sequestration capacity.