Of tropical forests and humans in the Americas: response trajectories of tree diversity and composition to anthropogenic disturbances

Abstract

Tropical forests face more frequent and intense anthropogenic disturbances, such as selective logging, namely the felling and harvesting of a few commercially valuable trees in old-growth forests, while the remaining stand is left for natural regeneration. Many studies focused on this regeneration, particularly on the recovery of carbon and timber stocks, most likely due to a strong interest in climate change mitigation and logging profitability. However, despite the crucial role of biodiversity for ecosystem maintenance and functioning - and its intrinsic value - there have been few studies on the impact of selective logging on biodiversity. Therefore, this thesis - organised in three studies - aimed at characterising the response of tree diversity and composition to logging in tropical American forests.First, we drew upon the long-term forest inventories (1986-2021, trees with a diameter at breast height ≥ 10 cm) from Paracou experimental station to build a Bayesian modelling framework of tree diversity and composition trajectories after selective logging. Paracou is located in French Guiana and was disturbed by silvicultural treatments of different intensities in 1986-1987. We propagated in our Bayesian framework the uncertainty associated with botanical determination and functional trait measurements, and modelled Paracou trajectories of taxonomic, phylogenetic and functional tree diversity and composition at the species level, relatively to their pre-disturbance levels. Additionally, we assessed the effect of pre-disturbance tree community characteristics, biophysical conditions and disturbance properties on our forest attribute trajectories. Second, we used a simplified version of the aforementioned Bayesian modelling framework on long-term forest inventories from sample plots located in Costa Rica and three Amazonian countries (respectively belonging to the Observatorio de los Ecosistemas Forestales de Costa Rica and the Tropical managed Forest Observatory). We modelled their post-logging trajectories of taxonomic and functional tree diversity and composition at the genus level, from which we extracted indicators solely over the inventory timespan of each site. We then assessed the effect of pre-disturbance tree community structure and disturbance properties on such indicators. While more variable in the second study with a broader geographical scope than in the first one, we observed similar trends in both studies: diversity mostly increased after logging and tree communities mainly shifted from resource-conservative strategies to resource-acquisitive strategies. Such changes appeared to be driven by the abundant and transient recruitment of early-successional species with acquisitive trait values, which provided them with a competitive advantage as disturbance intensity - i.e., light and space availability - increased. Indeed, changes in diversity and composition increased in both studies with disturbance intensity whereas disturbance selectivity, pre-disturbance tree community characteristics and biophysical conditions had no significant effect. Third, building up on the paramount importance of disturbance intensity in the two previous studies, we developed an original Bayesian hierarchical model of recovery trajectories, considering disturbed forests in a common framework, through a disturbance intensity gradient. We tested our modelling approach on data from two long-term experiments in Costa Rica and French Guiana, set up after selective logging, agriculture, and clearcutting and fire.Overall, these results opened various perspectives on the methods used to evaluate forest response to disturbance, the forest response itself and the ecological processes underlying forest succession, and how disturbed forests could be considered in forest management and conservation plans