High severity wildfires are a disturbance of growing concern in peatlands from the artic to the tropics (Page et al., 2008; Page & Baird, 2016; Poulter et al., 2006; Turetsky et al., 2014; Usup et al., 2004). The effects of drainage, climate change, and increased burning in adjacent uplands will all likely increase the frequency of peatland fires in the coming decades (Flannigan et al., 2009; Poulter et al., 2006; Turetsky et al., 2014; Watts & Kobziar, 2013). While disturbances, including fires, are natural in peatlands (Loveless, 1959; Watts & Kobziar, 2013), changes to fire frequency and severity alter their influence on the environment (Rein, 2015). For example, peat fires that smolder deeply through the soil profile alter topography and thus water level regimes, affecting ecosystem structure and function (Watts et al., 2015). In drained peatlands, smoldering fires can be expected to burn up to the water table (Link et al., 2023) and have regularly been observed to exceed one meter in depth (Reddy et al., 2015), thus greatly increasing a peatland’s degree of inundation post-fire. The projected changes to fire regimes, and thus hydrology, may weaken peatland resiliency to future perturbations and could result in dramatic vegetation shifts (Kettridge et al., 2015).
Peatland ecosystems are particularly prone to non-native plant invasions given their position on the landscape (Zedler & Kercher, 2004). That high level of vulnerability is exacerbated following disturbance events, particularly changes in hydrologic regime (e.g., via drainage) (Kercher et al., 2004). Critically, as drained systems are more susceptible to deep peat consuming burns, there can be a synergistic effect where the combination of drainage and fire accelerates ecosystem degradation, further reducing resilience to invasions (Page et al., 2008). Severe peatland fires alter hydrology while simultaneously destroying regenerative material both above and below ground (Matlaga et al., 2010), which can greatly influence the species that recolonize post-fire (Gorham & Rochefort, 2003). While it is generally true that invasive species are able to take advantage of degraded ecosystems (MacDougall & Turkington, 2005), species-specific information is required for effective management and is therefore a primary research objective of land managers (Dix et al., 2010).
One invasive species that does exceedingly well in a post-disturbance environment and is of major management interest is the grass Phragmites australis (hereafter referred to as common reed) (Chimner et al., 2016; Ji et al., 2009; Wilcox et al., 2003). Common reed is an aggressive, emergent wetland grass with annual, cane-like stems that can reach up to 6 m tall (Mal & Narine, 2003). Growing in low lying areas, common reed is most commonly found in intermittently or permanently flooded sites with still, shallow water (Haslam, 1972). Through a combination of competitive advantages, including rapid vegetative reproduction and growth (Marks et al., 1994) and abundant windblown seeds (Kettenring & Mock, 2012), common reed can quickly colonize wetlands with dense, near-monospecific stands at the detriment of native species (Farnsworth & Meyerson, 1999). Patches of common reed are comprised of the current year’s growth and dead shoots from prior seasons, which hinder the establishment of other plants (Mal & Narine, 2003) and is poor habitat for many species of fish and wildlife (Able & Hagan, 2003; Robichaud & Rooney, 2017).
Given the aggressive nature of common reed, the impact it has on habitat quality, and how difficult it can be to eradicate (Farnsworth & Meyerson, 1999), it is important to identify the controls on its abundance, including the effects of fire whether direct or indirect (i.e., altered hydrology). Further, common reed patches represent a sizeable fine-fuel load on the landscape and exhibit quick recovery post-burn (Thompson & Shay, 1985), suggesting there may be a self-reinforcing grass-fire feedback cycle present (D'Antonio & Vitousek, 1992). Multiple observations have noted that regular burns facilitate common reed invasions and also maintain common reed stands by controlling less adapted competitors (Norris et al., 2002; Ward, 1968). However, others have shown that prescribed fire can be an effective management tool for common reed in some ecosystems (Kimura & Tsuyuzaki, 2011; Marks et al., 1994; Páramo Pérez et al., 2018). The incongruent role of direct fire effects on common reed coverage aside, no studies to our knowledge have investigated the effect that fire-altered hydrology may have on its abundance. It is well understood that wetland community zonation is driven by hydrology (Hutchinson, 1967), and that common reed can be effectively managed by increasing water levels (Bart & Hartman, 2003; Hellings & Gallagher, 1992; Hudon et al., 2005; Rea, 1996; Rohal et al., 2019; Rolletschek et al., 1999; Weisner et al., 1993) but we do not know how an altered hydrologic regime, caused by drainage and subsequent deep smoldering peat fires, impacts its distribution.
The objective of this study was to investigate the controls on common reed occurrence and coverage, with special attention to disturbance-altered hydrology. Recent fires in a historically drained peatland in the eastern USA coastal plain present an opportunity to observe vegetation recovery following high severity peat-consuming fires. By leveraging remotely sensed data–both satellite imagery and digital elevation models (DEM)–we conducted analyses over large areas at a fine spatial resolution. By pairing water level and remotely sensed data, we sought to uncover a potential relationship between common reed coverage and post-fire hydrologic regimes. In total, our analysis assessed the impacts that drainage and deep-smoldering peat fires have had on system resiliency. We hypothesized that: H1) common reed will be more prevalent in locations where water levels are regularly shallow above the ground surface; and H2) the specific hydrologic conditions conducive to common reed occurrence and dominance will have been generated by a history of drainage and deep peat-consuming fires.