Storm intensity and old-growth forest disturbances in the Amazon region

We analyzed the pattern of large forest disturbances or blow-downs apparently caused by severe storms in a mostly unmanaged portion of the Brazilian Amazon using 27 Landsat images and daily precipitation estimates from NOAA satellite data. For each Landsat a spectral mixture analysis (SMA) was applied. Based on SMA, we detected and mapped 279 patches (from 5 ha to 2,223 ha) characteristic of blow-downs. A total of 21,931 ha of forest were disturbed. We found a strong correlation between occurrence of blow-downs and frequency of heavy rainfall (Spearman\u27s rank, r2 = 0.84, p \u3c 0.0003). The recurrence intervals of large disturbances were estimated to be 90,000 yr for the eastern Amazon and 27,000 yr for the western Amazon. This suggests that weather patterns affect the frequency of large forest disturbances that may produce different rates of forest turnover in the eastern and western Amazon basin

Size and frequency of natural forest disturbances and Amazon carbon balance

Forest inventory studies in the Amazon indicate a large terrestrial carbon sink. However, field plots may fail to represent forest mortality processes at landscape-scales of tropical forests. Here we characterize the frequency distribution of disturbance events in natural forests from 0.01 ha to 2,651 ha size throughout Amazonia using a novel combination of forest inventory, airborne lidar and satellite remote sensing data. We find that small-scale mortality events are responsible for aboveground biomass losses of B1.28 Pg C y 1 over the entire Amazon region. We also find that intermediate-scale disturbances account for losses of B0.01 Pg C y 1 , and that the largest-scale disturbances as a result of blow-downs only account for losses of B0.003 Pg C y 1 . Simulation of growth and mortality indicates that even when all carbon losses from intermediate and large-scale disturbances are considered, these are outweighed by the net biomass accumulation by tree growth, supporting the inference of an Amazon carbon sink

Severe storms and blow-down disturbances in the Amazon forest

Large natural disturbances (\u3e 1 ha) in old-growth tropical forests are caused by a variety of processes such as landslides, fires, wind, and cyclonic storms. We analyzed the pattern of large forest disturbances apparently caused by severe winds (blow-downs) in a mostly unmanaged portion of the Brazilian Amazon using a longitudinal transect of Landsat images (27 scenes between 6°43\u27W 68°50\u27S and 2°16\u27W 51°51\u27S) and daily precipitation estimates based on NOAA satellite data. We found 170 blow-downs with an average area of 3 km2. Most blow-down disturbances occurred in the Western Amazon between 67°W and 58°W. A map of heavy rainfall (\u3e 20 mm d-1) showed that the maximum frequency of heavy daily rainfall (~80 days y-1) occurred around 63°W in our study region. We found a close relationship between the frequency of heavy storms and the occurrence of blow-down disturbances events. This, in turns, suggests a close connection between severe weather and the rate of forest turnover caused by blow-down disturbances. The forest turnover time calculated for these disturbances within 9 Eastern Landsat scenes studied was almost 9000 years whereas for the 18 scenes in the Western Amazon, turnover time was closer to 1200 year. Large disturbances may have a significant influence on the spatial pattern of forest dynamics and productivity of the Amazon

Forest disturbance spectrum of the Amazon

Tropical forests play an important role in global carbon cycle by storing and potentially sequestering a large amount of carbon and are currently considered for climate mitigation policies associated with REDD (Reduced Emission from Deforestation and Degradation). However, recent studies show that there is a large uncertainty on both the stock and dynamics of these forests associated with patterns of tree mortality from natural disturbances. Based on a forest census network of approximately 150 forests plots, Amazon forests constitute a ~0.6 Pg C y-1 sink. The size of this sink has been questioned because frequent large disturbances would invalidate the interpretation of the sparse plot network. We characterize the spectrum of forest disturbance across the entire Amazon basin based on previously published data and we assess the implications of this spectrum on the tropical old-growth forest carbon sink. We use records of biomass changes from a spatially distributed forest census network in the Amazon supplemented by two large forest plot surveys in the Eastern Amazon (53 ha and 114 ha) and an analysis of multiple satellite images to map, detect and estimate the severity of large wind disturbances (blow-downs). We found two disjoint disturbance regimes – small-scale tree-fall gaps within the range of 0.01-0.1ha and rare larger-scale disturbances caused by severe winds predominantly in the western Amazon. Small disturbances (0.01-0.1 ha) are responsible for above-ground biomass losses of about 2.5 Pg C y-1 over the entire Amazon region, but large-scale disturbances for only 0.01 Pg C y-1 because they are extremely rare. Results of a simple stochastic forest simulator based on growth statistics from the forest census and the basin-wide disturbance spectrum indicate that rare larges disturbances do not change the estimates of the positive growth rate trends of old-growth forests previously reported