Mortalidade de cacaueiros (Theobroma cacao L.) em várzeas após cheia extrema no baixo Rio Madeira

Abstract

The cocoa has been cultivated for centuries in Amazonia floodplains. This was the product most exported of this region and nowadays it has been an important non-timber forest product for riverine populations of white water rivers. In 2014, the rivers water level in lower Madeira river reached the highest level ever recorded, resulting in the mortality of many cocoa trees on the floodplains. Despite some reports of cocoa tree mortality after extreme floods, these are events rarely studied in the amazon region. Then, the goals of this paper were: understand the floods dynamic in the lower Madeira river; to estimate the cocoa tree mortality rate after an extreme flood; and to identify the influence of environmental and dendrometric variables over this mortality rate. It was found a growth trend of the higher water levels reached over the last three decades, highlighting the flood of 2014, which exceeded 3.71 meters the mean of the maximum levels of this historical series. It was observed that since the beginning of the 21st century it has happened an increase in the cocoa plantations in the region, since the majority (85%) of the cocoa plantations were set down over the last 15 years. This fact potentialized the mortality caused by the extreme flood, considering that small cocoa trees experience higher mortality rates after floods. The extreme flood occurred in 2014 caused the mortality of 100% of young cocoa tress and 46% of adult cocoa trees. Taking into account only adult cocoa trees, those bigger in diameter experienced lower mortality because they were more resistant to the action of the flow and the injuries caused in the trunk. The increase in canopy cover contributed to lower mortality probably by reducing the impact of logs brought by the flow and reducing the stress caused by excess radiation in cocoa trees, which is a shade tolerant species. Mortality was not influenced by the depth of the sediment layer deposited during the flood; this result can be attributed to the species capacity to emit adventitious root from the trunk. The variation in the flooding duration, between 70 and 105 days, also did not influence mortality. It is possible that this result is due to the extreme magnitude of this flood, which did not allow to sample cocoa plantations that went through less than 70 days of flood. The pruning and the distance of the cocoa plantation at the riverbank also did not influence the mortality. Thus, in view of the trend of increasing flood peaks in the lower Madeira river and of intensification of new cocoa plantations in the region, the results of this work are important steps for understanding these mortality events and for adaptation of the cultivation of cocoa tree in the floodplains.