Carbon accumulation and nitrogen pool recovery during transitions from savanna to forest in central Brazil

Abstract

The expansion of tropical forest into savanna may potentially be a large carbon sink, but little is known about the patterns of carbon sequestration during transitional forest formation. Moreover, it is unclear how nutrient limitation, due to extended exposure to fire-driven nutrient losses, may constrain carbon accumulation. Here, we sampled plots that spanned a woody biomass gradient from savanna to transitional forest in response to differential fire protection in central Brazil. These plots were used to investigate how the process of transitional forest formation affects the size and distribution of carbon (C) and nitrogen (N) pools. This was paired with a detailed analysis of the nitrogen cycle to explore possible connections between carbon accumulation and nitrogen limitation. An analysis of carbon pools in the vegetation, upper soil, and litter shows that the transition from savanna to transitional forest can result in a fourfold increase in total carbon (from 43 to 179 Mg C/ha) with a doubling of carbon stocks in the litter and soil layers. Total nitrogen in the litter and soil layers increased with forest development in both the bulk (+68%) and plant-available (+150%) pools, with the most pronounced changes occurring in the upper layers. However, the analyses of nitrate concentrations, nitrate:ammonium ratios, plant stoichiometry of carbon and nitrogen, and soil and foliar nitrogen isotope ratios suggest that a conservative nitrogen cycle persists throughout forest development, indicating that nitrogen remains in low supply relative to demand. Furthermore, the lack of variation in underlying soil type (>20 cm depth) suggests that the biogeochemical trends across the gradient are driven by vegetation. Our results provide evidence for high carbon sequestration potential with forest encroachment on savanna, but nitrogen limitation may play a large and persistent role in governing carbon sequestration in savannas or other equally fire-disturbed tropical landscapes. In turn, the link between forest development and nitrogen pool recovery creates a framework for evaluating potential positive feedbacks on savanna-forest boundaries.

Inserting Tropical Dry Forests Into the Discussion on Biome Transitions in the Tropics

“…While cerradão is initially comprised of fire-adapted tree species from the cerrado, dry forest tree species colonize this environment if propagules are available, fire remains absent and soils are sufficiently fertile. These dry forest tree species may eventually outcompete cerrado tree species, since they do not invest in fire defense (Ratajczak et al, 2017), and in the prolonged absence of fire, cerradão may transition to a dry forest if there are positive feedback cycles between forest vegetation, lack of fire and soil fertility (Silva et al, 2013;Pellegrini et al, 2014Pellegrini et al, , 2018. However, if the underlying soils remain poor and/or if there are high aluminum concentrations in the soil that do not attenuate over time, then cerrado tree species, which are adapted to infertile, aluminum-rich soils may continue to dominate the vegetation.…”

Sensitivity of woody carbon stocks to bark investment strategy in Neotropical savannas and forests

“…From the 1910s until reserve formation in the 1970s, the whole landscape was subject to frequent fire at ∼ 2-year intervals (Pellegrini et al, 2014). Since reserve formation, the initiation of fire management strategies has provided a landscape with diverse disturbance histories: fire return interval ranges from a few years in savannas to half a century in some adjacent forest patches (Pellegrini et al, 2014).…”

“…Total AGB was calculated from diameter inventories using the allometric relations from Xu et al (2016) and accounting for bark carbon investment assuming β = 0.077. We performed a 35-year model spin-up in accordance with the disturbance history of the reserves after 1970 (Pellegrini et al, 2014) and then compared the range in simulated AGB over the next 10-year period in a simulation with bark fully included and a control simulation without bark investment to observed AGB estimates and tree size class distributions. In our simulations, our high fire frequency scenario was forced with a fire return interval that ranged from 2 to 8 years, our intermediate fire frequency scenario was forced with a return interval that ranged from 9 to 15 years, and our low fire frequency scenario was forced with a return interval that ranged from 57 to 63 years, in accordance with the disturbance regimes of Pellegrini et al (2014) plots 3, 4-5, and 6, respectively.…”

“…2b). At low-frequency fire, the simulation without bark (Pellegrini et al, 2014). Simulated AGB was compared over the subsequent 10-year period for the model with bark and the model without bark.…”

Resilience of Amazonian forests : the roles of fire, flooding and climate

“…A third hypothesis, however, is that soil can change due to local interactions with fire and vegetation, contributing to stabilize forests and savannas ( Fig. 1.1), or to accelerate transition between both states (Jackson 1968;Wood and Bowman 2012;Silva et al 2013;Pellegrini et al 2014;Franco et al 2014;Paiva et al 2015).…”