Resumen de Carbon sequestration in Mediterranean ecosytems: critical aspects related to plant respiration, wildfires and nitrogen budget
Victoriano Ramón Vallejo Calzada, P. Rovira, Joaquín Azcón Bieto, I. Fleck
Net primary production (NPP) in the biosphere is limited by the current low atmospheric CO2 con centration (Ca). Hence, the increase in C a due to anthropogenic activities, which is thought to be the main cause of climate change, is expected to result in an increase in NPP, and therefore in an increased Ca sequestration in the biomass. Increases in NPP and C a sequestration in the biota are already detectable in boreal and tropical forests. Increased C a results in a decrease in plant respiration -on average, about 17%-, and therefore the increase in Ca sequestration could be higher than previously assumed. This has led some authors to publish highly optimistic views about the capacity of terrestrial ecosystems to compensate for the excess of C a. Nevertheless, increased Ca results not only in an increase in NPP (different for every plant species), but also in changes in plant morphology (also different for every plant species); these will result in changes in the patterns of space occupation by plants, which make it difficult to predict how the NPP will change in the long term, at a whole-ecosystem level. It is not clear that such optimistic views can be applied to Mediterranean terrestrial ecosystems, in which drought -the main constraint for NPP- is expected to increase in the future as a result of climate change. Increased drought is expected to lead to a greater risk of wildfires, which can generate a highly unstable situation in the ecosystem if the periodicity and intensity of fire events surpasses its capacity to recover (either by resprouting or seeding). Wildfires cause significant losses of N and P by volatilization. They also result in a strong increase in the availability of nutrients (mainly N), and hence an increase in soil fertility. Nevertheless, even though many Mediterranean plant species (in particular Quercusilex) have a high capacity for the accumulation of N in basal organs, likely to support rapid resprouting, their ability to trap and take up the excess of available N released as a consequence of wildfire is of ten more limited. This results in the loss of these released nutrients and aggravates the impoverishment of ecosystem nutrients at each wildfire event.
