Mesophication of Oak Landscapes: Evidence, Knowledge Gaps, and Future Research

Abstract

Pyrophytic oak landscapes across the central and eastern United States are losing dominance as shade-tolerant, fire-sensitive, or opportunistic tree species encroach into these ecosystems in the absence of periodic, low-intensity surface fires. Mesophication, a hypothesized process initiated by intentional fire exclusion by which these encroaching species progressively create conditions favorable for their own persistence at the expense of pyrophytic species, is commonly cited as causing this structural and compositional transition. However, many questions remain regarding mesophication and its role in declining oak dominance. In the present article, we review support and key knowledge gaps for the mesophication hypothesis. We then pose avenues for future research that consider which tree species and tree traits create self-perpetuating conditions and under what conditions tree-level processes might affect forest flammability at broader scales. Our goal is to promote research that can better inform restoration and conservation of oak ecosystems experiencing structural and compositional shifts across the region.

Litter Flammability of 50 Southeastern North American Tree Species: Evidence for Mesophication Gradients Across Multiple Ecosystems

“…Over the last two decades, few southeastern species have been evaluated for their flammability, despite strong suggestions that many of ecosystems are highly flammable (Platt et al, 2016) and the widespread acknowledgment that mesophication is a much more common phenomenon than previously thought (Nowacki and Abrams, 2008;Alexander et al, 2021;Kane et al, 2021). This compilation provides data for 50 tree species, which is a fraction of the high diversity of overstory dominants in the eastern United States.…”

“…Understanding which leaf or fuelbed traits slow or hasten moisture loss (e.g., leaf curling or cuticle retention) may enable analyses of past flammability data to elucidate these patterns. The interaction of shading from overstory crowns or continuous canopies highlight these needs (Kreye et al, 2018a;Alexander et al, 2021). More work on including moisture retention as an interacting factor with flammability is clearly warranted (Kreye et al, 2013(Kreye et al, , 2018b.…”

“…For example, determining the mesophication impact of different species may enable managers to prioritize removal of species in a more targeted way. These removals over time will lead to the reduction of residual mesophytic litter and the canopy shade that often facilitates the growth and expansion of mesophytes while also diminishing future fires (Alexander et al, 2021). Research should be prioritized to determine the relative effectiveness of individual mesophytic species across the region.…”

“…The recruitment of firesensitive mesophytes coincided with increasing tree density, the formation of heavily shaded, multi-canopied forests, and development of moist forest floor conditions that resist ignition (Kreye et al, 2018a,b). This positive feedback phenomenon has been widely observed and is commonly referred to as "mesophication" (Nowacki and Abrams, 2008;Hanberry et al, 2020;Alexander et al, 2021).…”

Human‐mediated trophic mismatch between fire, plants and herbivores

“…While the increasing focus on ecosystem restoration with the use of prescribed fire has led anthropogenic fire regimes to match more accurately that of lightning fires in some areas (Knapp et al 2009), our data and the work of others indicates there is still a large-scale phenological mismatch (Miller et al 2019), and policy barriers to shifting phenology across the global terrestrial landscape will prove more difficult. Many systems have already begun a transition to less (Alexander et al 2021) or more (Seidl et al 2017) flammable plant communities because of fire suppression and climate change so expanding the burning window may be a necessary step to circumvent ecosystem shifts to lower quality habitat or more dangerous states. With so many global changes rapidly occurring concurrently, a deeper understanding of the collective net effects changes in fire regimes are needed to best inform conservation policy.…”