Tree mortality in the states of the Acre and Amazonas grew from 2% to 5%, on average. The finding is due to longer periods of drought caused by the passage of El Niño and La Niña.. The increase occurred after over a ten-year period of timber exploration. Studies indicate that climate events can cause more impact in the forest than wood extration. Large trees are more affected.. New forest management practices are necessary. Studies on forest dynamics conducted by Embrapa in the states of Acre and Amazon have proven that atypical climate events like El Niño and La Niña elevate tree mortality rates and affect the development of forests in tropical areas. Results of 20 years of research have revealed that with the occurrence of such phenomena, average tree mortality went up from 2% to 5% due to the water deficit caused by the phenomena. According to the Embrapa Acre researcher Marcus Vinício d’Oliveira, in unmanaged Amazonian forests tree mortality varies between 1% and 2% a year. Even in well-managed areas, an increase in tree mortality is expected after forest logging, which ceases over time. In the forests that were studied, rates above 5% were identified, with higher incidence in bigger trees. “In contrast with a gradual reduction, we have observed peaks of mortality, despite the use of adequate management techniques. In several locations, this increase occurred in a period of time of more than ten years after the wood extraction. Therefore, we believe it is not associated with forest management acitivities, but rather with the longer periods of drought resulting from the passage of El Niño and La Niña in this interval of time”, he reports. Large trees are more vulnerable The studies have also shown that forest species with more than 50cm in diameter are susceptible to the effect of climate events. “Water stress situations change the trees' hydraulical mechanisms. Within the plant trunk, the pressure that enables the transportation of the sap from the roots to the leaves decreases. In large-sized trees, this reduction more is accentuated, anc can result in the vaporization of liquids through the formation of bubbles, a phenomenon known as cavitation, which causes overtopping and can lead to plant death”, explains the researcher Luís Cláudio de Oliveira. The research also revealed that the rise in the tree mortality indicator influences the forest's half life, that is, the time estimated for half of its original population to be replaced. According to Marcus d' Oliveira, in the managed areas, the focus of the study, this time was of around 20 years, while in unmanaged forests, with annual mortality of up to 2%, it is estimated that the regeneration process takes between 35 and 70 years. “On the other hand, the high tree mortality opens glades in the forest that increase the availability of light and reduce the competition for nutrients. This allowed the ingress and establishment of new plants, a factor that benefited forest regeneration and biomass gain in smaller trees with diameter between 20 and 50 centimeters”, he states. Climate effects are greater than expected Studies on growth dynamics in Brazilian tropical forests performed by the TMFO (Tropical Managed Forests Observatory) Network also indicate the influence of atypical climate events on such environments. “In the Southwestern Amazon, however, the effects are more visible due to the existence of a more pronounced dry season than in other stretches of this region”, says Marcus d'Oliveira. For the specialist, the evidence that climate events can cause more impacts in the forest than logging was a surprise and shows the need to incorporate new management practices to increase tropical forest resilience to such occurrences. Adequate management can mitigate impacts Understanding forest growth dynamics, resistance mechanisms and vulnerabilities to natural events has been a challenge for research. Such knowledge is the basis for the definition of more sustainable models for forest management, as they allow the establishment of suitable parameters for the volume of timer to be removed and the resting time needed for the forest to recover and restitute its supply, among other aspects that are essential to reduce impacts in forestry. Moreover, they help to indicate efficient ways to mitigate the impacts of climate events on the forest. One of the strategies the study recommends to reduce the mortality rate and increase productivity in managed forests is the application of forestry practices, either individually or in groups of forest species with similar behavior. “Interventions like the removal of large, ill-formed or non-commercially valuable trees or cutting lianas favor the growth of commercial species and increase their density in the forest. These procedures allow the trees to reach the recommended minimum diameter for the management (50 centimeters), with better use of the economic potential of the species and ensuring its life cycle in the forest”, Luis Oliveira underscores. Field work Since 1992, Embrapa has conducted research on the monitoring of managed tropical forests in nine areas that are commercially explored with the use of low-impact management techniques, located in the Amazon Southwest. Performed in partnership with the Acre state government and with companies from the forest sector, the studies aim at determining forest behavior after selective logging. Over a hundred of forest parcels, most of which one-hectare large, have been studied. In Acre, the monitored areas are located in the State Forests of Antimary (Sena Madureira); of the Gregorio, Liberdade, and Mogno rivers; of the Kaxinawá de Nova Olinda Indigenous Land (Tarauacá); in the Filipinas rubber tree forest, located in the Chico Mendes Extractivist Reserve (Xapuri), and at Embrapa Acre (Rio Branco). In Amazonas, the study areas were concentrated in the Iracema II Farm, in the town of Lábrea. The field work involves the measurement of the trees before and after the first logging and remeasurement every three to five years, by forest engineers, botanical identifiers, tree climbers and other professionals. With the aid of geotechnologies like drones and the Lidar (Light detection and ranging) system, the teams collect information on tree height, canopy size and diameter, variables that work as indicators of species growth, and data related to trunk shape and position of the trees with regard to the canopy. According to Luís Oliveira, in addition to allowing the calculation of tree mortality, ingression and growth rates, such variables enable the estimation of the time needed to recompose timber supplies, information that is essential to ensure the sustainability of management activities. With the results, one can evaluate forest behavior and production potential, and project gains in wood volume for future logging. “Moreover, monitoring areas based on continuous forest inventories (CFI) facilitates assessments of aspects of the composition of commercial species and of the tree population in general, with emphasis on the definition of forestry practices to minimize damages and optimize the development of managed forests”, the researcher observes. Photo: Priscila Viudes Long-term research Extreme climate phenomena are events with global impacts on the environment. Research on climate change has shown that El Niño occurs in intervals of two to seven years, in a weak, moderate or strong intensity. In the last 40 years, more intense effects were recorded in 1987, 1997, 2005, 2010, 2011 and 2015, data that has been confirmed by research at Embrapa. According to Marcus d' Oliveira, these results indicate a trend of this climate event becoming more and more frequent, with changes in the composition and structure of the forests. “However, as research on forest dynamics ia a long-term process, two decades is a relatively short time for definitive inferences. In order to more fully understand how forest behavior relates to such phenomena, we need to advance in analyses that consider more comprehensive time series”, the specialist asserts. Photo: Daniel Papa Translation: Mariana Medeiros