Change in Piñon-Juniper Woodland Cover Since Euro-American Settlement: Expansion Versus Contraction Associated with Soil Properties

Abstract

Woodland and forest ecosystems across western North America have experienced increased density and expansion since the early 1900s, including in the widely distributed piñonjuniper vegetation type of the U.S. Southwest. Fire suppression and grazing are often cited as the main drivers of these historic changes and have led to extensive tree-reduction treatments across the region. However, much of the scientific literature on piñon-juniper expansion dates back only to the early 1900s, which is generally a half a century after Euro-American settlement. This study uses General Land Office (GLO) surveys to establish piñon-juniper woodland extent in the late 19 th century at the incipient stages of Euro-American settlement in southeastern Colorado and compares this data with 2017 aerial imagery of woodland cover. We found substantial amounts of woodland contraction as well as expansion: approximately 61% of historically dense woodland is now savanna or open (treeless) whereas approximately 57% of historically open areas are now savannas or woodlands, although analyses at finer spatial scales suggest considerably more contraction relative to expansion. We assessed change in woodland cover and extent as a function of soil type, a dominant biophysical control, and found that the highest rates of expansion occurred upon shallow, rocky soil types with low soil available water capacity (AWC). These low soil AWC areas support little herbaceous vegetation and thus had less grazing pressure and were unlikely to carry frequent surface fires historically, suggesting iii that fire suppression and grazing were not the primary drivers of expansion. Meanwhile, the significant contractions in woodland extent occurred on deeper, upland soils with higher soil AWC, which support greater herbaceous cover and were likely where early settlement and treecutting was prevalent. Our results provide mixed support for the often widespread assumption of woodland expansion since Euro-American settlement and suggest that the expansion that has occurred in our study area is unlikely a result of past grazing or fire suppression. This paper uses important, underutilized sources of ecological data in order to more directly assess the earliest effects of Euro-American settlement on one of the U.S. Southwest's most prevalent and important vegetation types.

Mastication Treatments Increase Perennial Herbaceous Cover Across Soil Types in Southeastern Colorado Piñon-Juniper Woodlands

“…4) may have resulted in similar percent cover changes, even if changes in abundance of individual plants differed (e.g., a small change in abundance of a species with a large canopy will show a similar cover increase to a large change in abundance of a species with a small canopy). Tree cover was also relatively sparse across all soil types (8% ± 0.13 SE) representative of a piñon-juniper savanna (Romme et al 2009; Amme et al 2020), so competition between trees and herbaceous plants may not have played a big role in this study.…”

“…(Pinaceae) (two-needle piñon; hereafter "piñon") along canyon rims ( Amme et al 2020 ). A previous study that quantified changes in woodland distribution at Chancellor Ranch found evidence of both woodland expansion and contraction since Euro-American settlement ( Amme et al 2020 ). Woodland expansion occurred on rocky, shallow soils that are commonly associated with canyon edges, whereas contraction occurred primarily in deeper soils, possibly due to woodcutting over the past century.…”

“…(one-seed juniper; hereafter "juniper") savanna in the uplands, with persistent woodlands containing both juniper and Pinus edulis Engelm. (Pinaceae) (two-needle piñon; hereafter "piñon") along canyon rims ( Amme et al 2020 ). A previous study that quantified changes in woodland distribution at Chancellor Ranch found evidence of both woodland expansion and contraction since Euro-American settlement ( Amme et al 2020 ).…”

Managing for ecological resilience of pinyon–juniper ecosystems during an era of woodland contraction

“…Harvesting for railroad and mining operations was extensive in the Great Basin in particular, where hundreds of thousands of acres of pinyon-juniper woodlands are estimated to have been cut down to provide fuelwood, charcoal, and timbers for mining operations (Ko et al, 2011;Young & Budy, 1979;Young & Svejcar, 1999). Homestead-related woodcutting in the late 1800s was likely more widespread across pinyon-juniper ecosystems, and was associated with substantial contractions in tree cover in the limited areas examined (Amme et al, 2020;Bahre & Hutchinson, 1985). Grazing by domestic livestock has significantly impacted western landscapes over the last two centuries, shifting the composition of plant communities and altering the distribution of fuels (Miller & Rose, 1999).…”

Living on the edge: Predicting songbird response to management and environmental changes across an ecotone

“…These ecosystems are experiencing rapid changes to environmental conditions due to anthropogenic development, spread of invasive annual grasses, and a changing climate (Schroeder et al., 2004). In addition, pinyon‐juniper woodlands have been undergoing regional expansion, contraction, and infilling due to changes in land‐use and climate (Amme et al., 2020; Falkowski et al., 2017), representing an additional threat to the sagebrush system. As a result, mechanical removal of pinyon pine ( Pinus edulis and Pinus monophylla ) and juniper ( Juniperus monosperma , J. osteosperma , and J. scopulorum ; hereafter, “pinyon‐juniper removal”) is currently being conducted to reduce the risk and severity of wildfire (Infrastructure Investment and Jobs Act, 2021; Vaillant & Reinhardt, 2017), enhance grazing productivity (Naugle et al., 2019), and restore sagebrush ecosystems (Miller et al., 2017).…”