Successional trajectories and conditioning factors of the natural regeneration of cerrado in abandoned pastures

Author(s)

Mário Guilherme De Biagi Cava

• Document type: Doctoral Thesis
• Press: Botucatu. 2019-03-26.
• Institution: Universidade Estadual Paulista (Unesp). Faculdade de Ciências Agronômicas. Botucatu
• Defense date: 2019-03-07
• Advisor: Giselda Durigan

Abstract

Despite the demand for Cerrado large-scale restoration in abandoned pastures, little is known about the recovery dynamics of these systems.Thus, scarce information is available to support the decision-making about which restoration approach should be applied to recovery this tropical savanna. Here, we sampled 31 secondary savannas resulting from the conversion of old-growth savannas into pastures and subsequent abandonment and assessed the natural regeneration of vegetation. Firstly, we constructed a chronosequence with 29 of these pastures (time since abandonment varying between 3 and 25 years) and modeled, using simple linear regressions, the temporal trajectories of vegetation attributes. Based on these models, we estimated the time required for these attributes to match the attributes of two reference ecosystems, the Cerrado stricto sensu (old-growth savanna that represents the pre-disturbance state) and the Cerradão (encroached savanna resulting from fire suppression, which represents an alternative ecological state). We also analyzed the composition of the studied plant communities. Later, we modeled the effect of time since pasture abandonment, soil and landscape attributes on the annual rate of recovery of secondary savannas (n=29), using generalized linear models and simple regressions, in order to investigate the influence of these factors on the rhythm of vegetation recovery. Based on the chronosequence-models, we verified that canopy cover, richness and density of trees rapidly increased with the time since pasture abandonment, easily surpassing the values of Cerrado stricto sensu (28 years old) and reaching the values of Cerradão 49 years after abandonment. The cover and richness of the ground layer increased at a much slower pace. Since the species of this layer are shade intolerant, they will be eliminated by canopy closure over time. Up to 25 years after abandonment, secondary savannas continued to lack many Cerrado stricto sensu species, mostly from the ground layer. We found no influence of time since pasture abandonment or distance from the nearest remnant of native vegetation on the annual rate of recovery of secondary savannas. On the other hand, our models demonstrated that the soil attributes (proportion of fine particles, penetration resistance and base saturation) and the surrounding native-vegetation cover explained a small portion of the variation in the vegetation recovery rhythm. However, we did not find an isolated factor or a single set of factors that explain the variation of the annual recovery rate for all vegetation components. Despite the high natural regeneration potential of the Cerrado, the end state of secondary savannas will be similar to the Cerradão (alternative state). Thus, if the restoration goal is to recover the pre-disturbance state, active restoration (e.g. reintroduction of fire, removal of trees, planting native grasses) will be necessary. Considering that there is a wide variation in the rhythm of vegetation recovery among sites and that our generalized linear models shows low predictive power, constraining its application for large-scale decision making, we suggest that vegetation monitoring should be done case-by-case, to inform the potential of natural regeneration of the Cerrado and the level of effort required to restore the system to the pre-disturbance state, that was a typical savanna. (AU)

• FAPESP's process: 15/23131-9 - Diagnosis and conditioning factors of the Cerrado vegetation resilience in abandoned pastures
• Grantee: Mário Guilherme de Biagi Cava
• Support Opportunities: Scholarships in Brazil - Doctorate