Spatial patterns of woody plant encroachment in a temperate grassland

Incipient woody plant encroachment signals heightened vulnerability for an intact grassland region

“…Despite being regionally native to the Great Plains and eastern North America, non‐cultivated J. virginiana were rare in the Sandhills and largely restricted to steep riparian slopes during the early 20th century (Pool, 1913). Key life‐history traits allowed this fire‐sensitive, non‐resprouting species to persist within a biome characterized by 5000–8000 years of frequent anthropogenic‐driven fire ignitions (Axelrod, 1985): (1) A broad abiotic affinity, allowing germination and growth in a range of typically inhospitable environments; (2) prolific seed production — mature trees can produce 87,000‐1,592,000 seed‐bearing cones annually (Holthuijzen & Sharik, 1985); (3) potential for long‐distance seed dispersal by birds (Fogarty et al, 2022); (4) a lifespan potential of 500 years (Therrell & Stahle, 1998); (5) drought tolerance (Eggemeyer et al, 2006); (6) volatile oils that make Juniperus spp. unpalatable to most herbivores (Launchbaugh et al, 1997); and (7) dense canopies that displace herbaceous fuels near the canopy (Engle et al, 1987).…”

“…However, these same traits, coupled with reduced fire occurrence, have allowed J. virginiana to encroach previously uninhabitable grasslands. Moreover, tree‐planting programs promoting the establishment of shelterbelts have assisted in the regional expansion of J. virginiana , resulting in increased grassland exposure to seed dispersal (Briggs et al, 2002; Fogarty et al, 2022; Hanberry, 2022).…”

“…We also used proximity to planted J. virginiana stands (J.PROXIMITY) as a predictor variable to examine the role of propagule availability on the density of encroaching adults. J. virginiana has a male:female sex ratio around 1:1 and planted stands are known propagule sources (Stoeckler & Slabaugh, 1965; Vasiliauskas & Aarssen, 1992; Fogarty et al, 2022). In the Sandhills, planted J. virginiana stands (e.g., shelterbelts) are common, often represent the sole propagule source for a site, and have been shown to contribute to encroachment (Donovan et al, 2018; Fogarty et al, 2022).…”

“…J. virginiana has a male:female sex ratio around 1:1 and planted stands are known propagule sources (Stoeckler & Slabaugh, 1965; Vasiliauskas & Aarssen, 1992; Fogarty et al, 2022). In the Sandhills, planted J. virginiana stands (e.g., shelterbelts) are common, often represent the sole propagule source for a site, and have been shown to contribute to encroachment (Donovan et al, 2018; Fogarty et al, 2022). Proximity to planted J. virginiana was calculated in ArcGIS as the mean Euclidian distance of a site's terrestrial area (divided in 30‐m 2 grid cells) to the nearest planted J. virginiana stand, following the methods of Benson (2013).…”

“…Moreover, tree-planting programs promoting the establishment of shelterbelts have assisted in the regional expansion of J. virginiana, resulting in increased grassland exposure to seed dispersal (Briggs et al, 2002;Fogarty et al, 2022;Hanberry, 2022).…”