A shady business: Effects of pine afforestation on litter decomposition along a precipitation gradient in Patagonia, Argentina

Background/Question/Methods

Water availability is one of the primary regional controls on ecosystem processes in terrestrial ecosystems, and much attention has been paid to the observed linear relationship between mean annual precipitation and net primary production. Along spatial precipitation gradients, however, a lack of correlation with litter decomposition has been observed frequently and alternative controls have been postulated. Changes in plant life form, increased cover, and decreased incident solar radiation as precipitation increases may eclipse the direct effects of water availability on organic matter degradation. As these changes occur simultaneously along natural precipitation gradients, it is difficult to disentangle the relative importance of these controls. We took advantage of an unplanned natural experiment along a broad precipitation gradient (250-2200mm mean annual precipitation) in Patagonia, Argentina, where extensive areas of steppe and native forest have been converted to pine plantation of a single species (Pinus ponderosa). We established five paired sites, a natural vegetation site and a paired pine plantation of similar density and age. This particular design enabled us to explore how simultaneously changes in plant life forms and precipitation affect litter decomposition, but also, by disentangling these two factors, to isolate effects of precipitation and vegetation on C turnover. We conducted a litter bag experiment evaluating common litter substrates and in situ litter during two year period at all sites along the gradient.

Results/Conclusions

Litter decomposition differed markedly among natural and modified ecosystems and demonstrated a positive relationship with precipitation only in pine plantations. The largest differences for common substrates were in the arid portion of the gradient, where litter decomposition was ~50% faster in the natural steppe vegetation than in paired plantations. In natural ecosystems, in situ decomposition was variable along the precipitation gradient as a result of interactive effects of photodegradation (diminishing as vegetative cover increases) and different litter types, and was systematically higher than in their pine plantation counterpart. Decreased solar irradiance (shade) and recalcitrant litter appear to be major controls affecting the carbon balance in these ecosystems, and these effects could be considered as more important than alterations in net primary production for affecting carbon sequestration potential.