Better lucky than good: How savanna trees escape the fire trap in a variable world

Abstract

Fire controls tree cover in many savannas by suppressing saplings through repeated topkill and resprouting, causing a demographic bottleneck. Tree cover can increase dramatically if even a small fraction of saplings escape this fire trap, so modeling and management of savanna vegetation should account for occasional individuals that escape the fire trap because they are "better" (i.e., they grow faster than average) or because they are "lucky" (they experience an occasional longer-than-average interval without fire or a below-average fire severity). We quantified variation in growth rates and topkill probability in Quercus laevis (turkey oak) in longleaf pine savanna to estimate the percentage of stems expected to escape the fire trap due to variability in (1) growth rate, (2) fire severity, and (3) fire interval. For trees growing at the mean rate and exposed to the mean fire severity and the mean fire interval, no saplings are expected to become adults under typical fire frequencies. Introducing variability in any of these factors, however, allows some individuals to escape the fire trap. A variable fire interval had the greatest influence, allowing 8% of stems to become adults within a century. In contrast, introducing variation in fire severity and growth rate should allow 2.8% and 0.3% of stems to become adults, respectively. Thus, most trees that escape the fire trap do so because of luck. By chance, they experience long fire-free intervals and/or a low-severity fire when they are not yet large enough to resist an average fire. Fewer stems escape the fire trap by being unusually fast-growing individuals. It is important to quantify these sources of variation and their consequences to improve understanding, prediction, and management of vegetation dynamics of fire-maintained savannas. Here we also present a new approach to quantifying variation in fire severity utilizing a latent-variable model of logistic regression. . Bradstock. 2013. Defining pyromes and global syndromes of FIG. 4. The relationship between the shape of the stem survival function and variance in fire severity. Panels A and B show two survival functions, and C and D show the corresponding probability density functions of fire severity. Thus, A and C correspond to a scenario with little variation in fire severity, while B and D correspond to a scenario with greater variation in fire severity.

Fire Affects Asymbiotic Nitrogen Fixation in Southern Amazon Forests

“…ANF has decreased after a fire in temperate grassland soils as a result of greater soil inorganic N (Hobbs & Schimel, ), the uptake of which is energetically favorable compared to atmospheric N 2 fixation (Norman & Friesen, ). Given that fire events can cause different impacts on different ecosystems (i.e., severity; Hoffmann et al, ), future studies in the tropics might further test this fire frequency hypothesis in controlled experimental settings.…”

“…Journal of Geophysical Research: Biogeosciences impacts on different ecosystems (i.e., severity; Hoffmann et al, 2019), future studies in the tropics might further test this fire frequency hypothesis in controlled experimental settings.…”

Not all trees can make a forest: Tree species composition and competition control forest encroachment in a tropical savanna

“…Aggregated trees are likely to be suppressed and reach fire-resistant sizes more slowly (Trouvé et al, 2020), though this is perhaps balanced by the ability for a patch of trees to suppress grassy fuels more readily through their combined leaf area. This variation in growth is an important factor determining which stems are able to reach maturity and which are susceptible to topkill (Hoffmann et al, 2020;Wakeling et al, 2011). Incorporating this source of growth variation, which depends on the spatial arrangement of trees and their neighbours, will likely have large effects on interactions between demographics, fuel and fire regimes, perhaps propagating to landscape-scale effects on tree cover (Li et al, 2019).…”

Woody encroachment happens via intensification, not extensification, of species ranges in an African savanna

“…This, together with the evidence of increased proportion of the small tree class (Fig. 2b), suggests an opportunity for local efforts to prevent woody encroachment by increasing fire frequency, because woody plants <2.5 m are particularly susceptible to top-kill by fires (Higgins et al 2000, Hoffmann et al 2019. Increasing fire frequency has been frequently proposed as an effective local management tool to prevent or reverse woody encroachment across African savannas and elsewhere around the world (Lunt et al 2012, Case and Staver 2017, Miller et al 2017, Schmidt et al 2018.…”