Allium tricoccum Ait. (Alliaceae/Amaryllidaceae), also known as ramps or wild leeks, are an edible wild onion native to forestlands in eastern North America. They are collected during the spring months throughout the eastern United States (U.S.) as an important and popular non-timber forest product (NTFP) (Baumflek and Chamberlain 2019). Ramps have long held cultural importance in Appalachia, including by Native American groups such as the Cherokee, Iroquois, and others (Moerman 2010). Foraging for ramps provides food, medicine, and economic opportunities as well as a connection to the ecological world (Baumflek and Chamberlain 2019; Pugh 2022). However, demand for ramps has grown significantly in recent years, driven by trending culinary interests surrounding foraged seasonal and farm-to-table foods including edible plants and mushrooms (Pugh 2022; Rivers et al. 2014). This increased demand has driven growth in commercial harvesting, which can remove a significant number of plants from wild populations (Pugh 2022). In Canada, ramp harvesting is prohibited for commercial purposes (Bernatchez et al. 2013; Rock et al. 2003), while some edge-of-range states in the U.S., particularly on the southern end of the species range, rank the species as “Critically Imperiled” (S1) or “Imperiled” (S2) (NatureServe, 2024).
Ramps are slow growing perennials that may take at least three years to mature to a sizable bulb (Dion et al. 2016; Rock 2003). Their slow growth can contribute to overharvesting (Dion et al. 2016; Rock 2003), highlighting a need to develop conservation strategies for the species including education and planting initiatives (Pugh 2022). Ramps have been identified as a forest “crop” in the eastern U.S. with potential for commercial production on forestlands (Chamberlain et al. 2014; Bernatchez et al. 2013). One potential production method, forest farming, is a type of agroforestry practice that focuses on the cultivation and management of NTFP’s under a forest canopy (Chamberlain et al. 2009; NAC 2024). Forest farming is of interest to forest landowners in the eastern US (Strong and Jacobson 2006; McLain and Jones 2013) and incorporating ramps into this practice as a NTFP creates an opportunity for both conservation and income generation. Intensive horticultural practices used in forest farming such as bulb division (Dion et al. 2016) and selective harvesting (Dion et al. 2016; Nilson et al. 2023) can increase growth rates and productivity, helping to meet market demand and reduce foraging pressures in the wild (Burkhart 2011; Davis and Persons 2014). This cropping system would also allow landowners to generate income on a more regular basis than through timber harvesting and may be compatible with production of other NTFP’s such as maple syrup, goldenseal, and ginseng.
Understanding ramp habitat is an important component in successful forest farming efforts, especially if minimal site improvements are to be made (Chamberlain and Predny 2003; Davis and Persons 2014). Anecdotal and observational information available in regional or state floras and some technical publications describe ramp habitat as mesic, deciduous forests, along flat stream sides, or on moist slopes (Bernatchez et al. 2013; Rhoads and Block 2007; Rock, et al. 2003; Vasseur and Gagnon 1993). Recent floristic guides note ramp habitat as “cove forests and mesic slope forests” (Weakley 2023). Habitat descriptions often mention that ramps occur within or prefer “moist soils,” a trait shared by many Allium species whose shallow root systems make them vulnerable to drought stress (Bernatchez et al. 2013; Geries et al. 2020; Rock 2003). Previous work on soil nutrients suggests ramps favor soil with a slightly to moderately acidic pH and a high calcium to magnesium ratio (Bernatchez et al. 2013; Davis and Greenfield 2002). Furthermore, studies in North Carolina and Canada have found an increase in ramp survival and growth rates with the application of calcium fertilizer, particularly in highly acidic soils (Bernatchez et al. 2013; Davis and Greenfield 2002; Ritchey and Schumann 2005).
One tool used in forest farming site evaluation is the presence of “indicator species” which are used to identify suitable planting sites (Burkhart 2013; NAC 2024; Davis and Persons 2014). Indicator species which frequently associate with a species of interest are used to characterize habitats and assess habitat suitability (Dufrêne and Legendre 1997; Peck 2016). If an indicator species occurs in an area, then the local habitat is likely suited for the species of interest since forest vegetation is related to climatic, topographic, and edaphic factors (Gilliam 2014). Combining indicator species and other field data with GIS-based habitat suitability models can build an understanding of spatial patterns of suitable ramp habitat and create guidance for forest farming across multiple scales. This guidance will be useful to a large audience of stakeholders, from high level public land managers to owners of small landholdings. Stakeholders could first consult GIS-based habitat suitability models to narrow the breadth of potential sites for surveying or forest farming establishment. Microsite habitat conditions and the presence of indicator species which respond similarly to habitat cues can then inform on the ground observations and decision making (Ren et al. 2010).
GIS-based habitat suitability modeling has often used multiple logistic regression (Le Duc et al. 1992; Guisan and Zimmerman 2000; Hirzel and Guisan 2002). However, traditional presence-absence modeling using logistic regression has limitations for culturally significant plants such as ramps. While ramps can occupy extensive areas, they are underrepresented in statewide systematic botanical surveys. Absence data is also complicated by influences that are difficult to account for, such as sites where ramps may have been extirpated by harvesting or other land use legacies and have failed to recolonize due to lack of dispersal (Bellemare 2002). Alternatively, opportunistic sightings recorded in citizen science tools such as iNaturalist are numerous and distributed throughout the state. Presence-only modeling can take advantage of these opportunistic sightings and avoid some of the limitations of presence-absence modeling (Elith et al. 2011). One commonly used method, Maximum entropy (Maxent) (Phillips et al. 2006), has been found to outperform other presence-only methods, and presence/absence modeling techniques such as GLM (Hirzel and Guisan 2002), GAM (Yee and Mitchell 1991), and BIOCLIM (Busby 1991), based on the area under the Receiver Operating Characteristic curve’s (AUC) ability to differentiate between sites where a species is present and sites where it is absent (Elith et al. 2006).
This study examined ramp occurrence and forest farming suitability on forestlands in the state of Pennsylvania (PA). Previous research efforts documenting ramp habitat and creating forest farming guidance have largely focused on Canada and the southern Appalachian Mountains due to conservation concern in these regions. Despite the cultural importance of ramps in PA, which is situated within the core-range of ramps (BONAP 2014), no efforts have been made to document habitat preferences and distribution within the state. Ramps occur throughout PA, a state which is climatically (The Pennsylvania State Climatologist 2021), geologically (PA DCNR 2001), and floristically (PNHP 2024) diverse. These conditions make the state an ideal location for investigating the range of habitat conditions under which ramps can grow. Results can also be used to locate extant populations and identify site factors important to continued cultivation efforts both in and ex situ.
The following study questions were used to guide this effort:
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What abiotic site factors are associated with ramp occurrences throughout PA?
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How do factors encountered in the field compare with presence-only modeling results?
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What flora is associated with ramp occurrences, and which species might be useful for site selection?