Comparative seasonal gas exchange and chlorophyll fluorescence of two dominant woody species in a Holm Oak Forest

Diurnal courses of net CO2 uptake rate, stomatal conductance (gs), maximum photochemical efficiency of PSII (Fv/Fm) and apparent photosynthetic electron transport rate (ETR) were measured in Quercus ilex and Phillyrea latifolia in a holm oak forest throughout the seasons of the year. These measurements were complemented with response curves of photosynthetic rates to PPFD and CO2 concentrations. R latifolia was better adapted to drought and warm conditions and showed higher net CO2 uptake, gs and Fv/Fm values than Q. ilex in summer. But in autumn and specially in winter P. latifolia was more sensitive to low temperatures and experienced lower net CO2 uptake, gs, ETR and Fv/Fm values than Q. ilex. The maximum net CO2 uptake values for P. latifolia occurred under summer high temperatures whereas maximum net CO2 uptake values for Q. ilex occurred under winter low temperatures. However, in summer during midday, both species presented null or slightly negative net CO2 uptake rates. Since in the summer season both species experienced similar ETR values, the lower net CO2 uptake values of Q. ilex suggest that Q. ilex presented greater photorespiration rates. During winter, very low Fv/Fm values were found especially for P. latifolia, indicating that maximal photochemical efficiency of PSII is very sensitive to low temperatures. However, they were not accompanied by low net CO2 uptake rates showing that cold photoinhibition determined a potential but not an actual decrease in photosynthetic performance. Under well watered conditions and with high CO2 concentration and saturated PPFD, Q. ilex was able to increase its photosynthetic rates whereas P. latifolia had lower plasticity to make a profit of optimal environmental conditions. These results show different strategies between these two dominant co-occurring species. They also indicate that the warmer and drier conditions expected for the Mediterranean region in the near decades as a result of climate change will favour drought resistant species with lower photosynthetic rates such as P. latifolia in detriment of more mesic species such as Q. ilex.