Forest resilience to fire in eastern Amazon depends on the intensity of pre-fire disturbance

Abstract

Researchers and managers face the challenge of how to determine when frequency, spatial extent and magnitude of disturbances can overcome the resilience of forest ecosystems. Among the disturbances in tropical forests, the mid- and long-term impacts of fire are still poorly known, especially how fire interacts with selective logging. In this study, we approached the two following questions in relation to dense ombrophilous forests with a history of selective logging: How does fire impact forest recovery? What is the relationship between pre-fire forest conditions and post-fire dynamics? We used data for trees with DBH ≥ 5 cm from 36 permanent plots of 0.25-ha (50 m × 50 m) in an area of dense ombrophilous forest monitored over 31 years in the Tapajós National Forest, in Brazil’s eastern Amazon. The area is under forest management that includes logging, the application of post-harvesting silvicultural treatments and an accidental fire. To determine the effects of pre-fire disturbances (logging and thinning) on basal area (G), mortality rate (MR) and recruitment rate (RR), a repeated measures ANOVA was applied. The post-fire forest recovery was also assessed by looking at changes in G, MR and RR. These variables were evaluated through a Linear Mixed Effect Model. In the post-fire period, there was a combined effect of logging commercial species, thinning non-commercial species (F = 9.255; p-value < 0.01) and time (F = 20.210; p-value < 0.01) in G, with no fire effect (F = 0.710; p-value = 0.406). Our study found the dense ombrophilous forest to be resilient enough to recover from logging, thinning, and a superficial fire, and that logging intensity is a determinant factor in forest dynamics. The forest in our control area with no history of previous strong and frequent disturbances was more resistant to fire in terms of lower mortality rates than the logged and thinned areas. In the short-term, the fire affected mainly the dynamics of smaller trees (DBH < 20 cm). In the med-term (15 years after the fire), we observed no fire effects on the reduction of basal area in any of the treatments and the forest maintained a continuous recovery of its lost stocks.