Els boscos ibèrics davant del canvi climàtic

The pre-industrial atmospheric CO2 concentration of 270 ppmv has increased up to the present values of 380 ppmv and, very likely, it will continue to increase. Consequently, the average temperature of Europe has increased 1ºC during the past century. Both atmospheric circulation and rainfall patterns show evident changes. Models predict a decrease of rainfall in South Europe of up to 1% per decade. In this frame of climate change, the crucial question is what role can play forests in sequestering CO2? Water use efficiency in Mediterranean forests is about 5 mmols of C/mol of H2O or, in other words, to fix between 2 and 3 g of C, plants have to transpire 1000 g of water (i.e., between 300 and 500 times the weight of the fixed C). Given the role of water as a limiting factor in Mediterranean ecosystems, it is clear that the amount of carbon fixed in Mediterranean-type ecosystems is linked to water availability. Both autotrophic and heterotrophic respiration return to the atmosphere an important proportion of the carbon fixed in photosynthesis. Usually, more than 50% of fixed carbon is returned to the atmosphere through respiration processes. The annual net ecosystem exchange of Mediterranean forests is around 5 tones of CO2 per hectare, while the human emissions in our country are around 10 tones per habitant and year. In these conditions, the role of forests to mitigate the human emissions is very limited. To compensate man-induced emissions, the forest area in our country must be 10 times the current forest area or around twice the total area of Spain. Our forests are experiencing deep changes. Temperate ecosystems are progressively being replaced by Mediterranean ecosystems as a response to increasing temperatures. The length of the growth period is increasing. A simulation analysis of the length of the growth period in Europe, using the GOTILWA+ forest growth model, shows that the growth period of Mediterranean forests can increase up to 50 days in the present century. Modelling results and field experiments indicate that these new growth conditions in a Mediterranean world with less rainfall will have negative effects on the vegetation survival.