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

Anna T Trugman,Adam F A Pellegrini,William A Hoffmann,David Medvigy

doi:10.5194/bg-15-233-2018

Abstract

Abstract. Fire frequencies are changing in Neotropical savannas and forests as a result of forest fragmentation and increasing drought. Such changes in fire regime and climate are hypothesized to destabilize tropical carbon storage, but there has been little consideration of the widespread variability in tree fire tolerance strategies. To test how aboveground carbon stocks change with fire frequency and composition of plants with different fire tolerance strategies, we update the Ecosystem Demography model 2 (ED2) with (i) a fire survivorship module based on tree bark thickness (a key fire-tolerance trait across woody plants in savannas and forests), and (ii) plant functional types representative of trees in the region. With these updates, the model is better able to predict how fire frequency affects population demography and aboveground woody carbon. Simulations illustrate that the high survival rate of thick-barked, large trees reduces carbon losses with increasing fire frequency, with high investment in bark being particularly important in reducing losses in the wettest sites. Additionally, in landscapes that frequently burn, bark investment can broaden the range of climate and fire conditions under which savannas occur by reducing the range of conditions leading to either complete tree loss or complete grass loss. These results highlight that tropical vegetation dynamics depend not only on rainfall and changing fire frequencies but also on tree fire survival strategy. Further, our results indicate that fire survival strategy is fundamentally important in regulating tree size demography in ecosystems exposed to fire, which increases the preservation of aboveground carbon stocks and the coexistence of different plant functional groups.

Highlights

Tropical savannas and forests are important components of the land carbon sink (Pan et al, 2011; Liu et al, 2015; Ahlström et al, 2015)
We found substantial differences in the fraction of aboveground woody carbon (AGB) present in different tree size classes between the original model without bark and the updated model with a bark investment strategy, but these differences depended on fire frequency and precipitation
We found that tree size distributions were largely unaffected by mean annual precipitation (MAP) in the presence of frequent fire in simulations with a bark investment strategy (Figs. 3a, c, e, S4); only minor impacts were found at intermediate fire frequency and high MAP (Fig. S3c)

Summary

Introduction

Tropical savannas and forests are important components of the land carbon sink (Pan et al, 2011; Liu et al, 2015; Ahlström et al, 2015). Their ability to continue sequestering carbon is uncertain (Malhi et al, 2008), due in part to the impact of projected increases in drought frequency and changes in fire regime on woody carbon stocks (Brando et al, 2014). Fire is critical in defining the vegetation structure and distribution of tropical savannas and forests (Bond et al, 2005; Hoffmann et al, 2012a; Staver et al, 2011b). Despite high fire frequencies in many sa-

Methods

Results

Discussion

Conclusion