Catastrophic ecosystem shifts in dry tropical forest: evidence, mechanisms and implications for climate change

Abstract:

Tropical dry forests have long been used by humans. Has it been sustainable? Not in the southern Yucatan. Biomass accumulation declines with each cycle of shifting cultivation with implications for both internal recycling of nutrients and external inputs of nutrients. We detail the evidence for a decline in P inputs from biomass burning (aboveground biomass, litter, and coarse woody debris), an increase in leaching losses from deep soils, and a decline in atmospheric inputs of new P from Saharan dust following the transition from mature to secondary forest. Canopy trapping of dust is critical to maintaining P balance in this system. Effective trapping is diminished by changes in the structure of secondary forest--loss of height, leaf area and basal area. Experimental studies show that it is atmospheric transport of dust, not microbial shedding or leaching from live tissues, that explains the difference between throughfall P and P in bulk deposition. Because of net losses in P, uptake of carbon during regrowth is slower with each cycle of shifting cultivation. As much of the tropics has moved beyond a mature forest frontier, the decline in carbon sequestration is likely widespread over both dry and wet forests. The terrestrial carbon sink in the tropics may be declining. The capacity to sequester carbon through afforestation, reforestation and restoration has certainly diminished over time, limiting the effectiveness of such efforts to help mitigate climate change.