Influence of Landscape Structure and Climate Variability on a Late Holocene Plant Migration

Abstract

We analyzed and radiocarbon-dated 205 fossil woodrat middens from 14 sites in central and northern Wyoming and adjacent Utah and Montana to document spatiotemporal patterns of Holocene invasion by Utah juniper (Juniperus osteosperma). Holocene migration into central and northern Wyoming and southern Montana from the south proceeded by a series of long-distance dispersal events, which were paced by climate variability and structured by the geographic distribution and connectivity of suitable habitats on the landscape. The migration of Utah juniper into the region involved multiple longdistance dispersal events, ranging from 30 to 135 km. One of the earliest established populations, on East Pryor Mountain in south central Montana, is currently the northernmost population of the species. Establishment by long-distance dispersal of that population and another in the Bighorn Basin occurred during a period of relatively dry climate between 7500 and 5400 years ago. Further expansion of these initial colonizing populations and backfilling to occupy suitable sites to the south was delayed during a wet period from 5400 to 2800 years ago. Development of dry conditions 2800 years ago led to a rapid expansion in which Utah juniper colonized sites throughout its current range. Landscape structure and climate variability play important roles in governing the pattern and pace of natural invasions and deserve close attention in studying and modeling plant invasions, whether exotic or natural.

Ecology and the ratchet of events: Climate variability, niche dimensions, and species distributions

“…Holocene invasion of Utah juniper (Juniperus osteosperma) in north-central Wyoming shows similar contingencies (87). Utah juniper was established on a few sites ca.…”

“…Conversely, had colonization not been interrupted by climate change ca. 5400 years BP, the species might occur across a much larger territory today, filling a number of gaps where suitable habitat occurs but the species is absent (87).…”

Disentangling the dynamics of invasive fireweed (Senecio madagascariensis Poir. species complex) in the Hawaiian Islands

“…Clevenger et al 2002;Lenton et al 2000). The WOE approach compares sets of known locations (response variables) to a prior distribution estimated by evidential themes (layers composed of quantitative and categorical data) theoretically influencing the ecology of the organism of interest (Bonham-Carter 1994;Lenton et al 2000;Lyford et al 2003). The approach has a form similar to regression (Bonham-Carter 1994).…”

Intermountain Presettlement Juniper: Distribution, Abundance, and Influence on Postsettlement Expansion

“…Studies of subfossil pollen deposits and pack rat middens reveal that many lowelevation conifer species, including junipers, piñ ons, and ponderosa pine, have been expanding their ranges throughout the Holocene (the past ,12 000 yr) from glacial refugia in the Southwest and in northern Mexico. In response to increasing temperatures and perhaps aided by moist periods, piñ ons expanded rapidly into the central and northern parts of the western United States at the end of the Pleistocene (Betancourt 1987;Nowak et al 1994;Swetnam et al 1999;Wigand and Rhode 2002), whereas junipers may have expanded with increasing temperatures but during drier periods (Lyford et al 2003).…”

“…J. osteosperma has also been expanding its range in Wyoming and in adjacent sites in Utah and Montana for the past several thousand years, both at a regional scale by moving into new mountain ranges and at local scales by expanding populations where it was already established. In fact, juniper populations in some parts of Wyoming may represent the first generation of trees in these areas (Lyford et al 2003). In addition to latitudinal range expansions following the Pleistocene, piñ ons and junipers have moved to higher or lower elevations in response to the climate changes that have occurred during the Holocene; for example, woodlands in the Great Basin have alternately expanded across large areas of the landscape during favorable climatic periods and retreated to smaller refuge areas during less-favorable periods (Miller and Wigand 1994).…”