A 40,000‐year woodrat‐midden record of vegetational and biogeographical dynamics in north‐eastern Utah, USA
Authors: Jackson Stephen T. , Betancourt Julio L. , Lyford Mark E. , Gray Stephen T. , Rylander Kate Aasen 
Published: May 2005

Aim A conspicuous climatic and biogeographical transition occurs at 40–45° N in western North America. This pivot point marks a north–south opposition of wet and dry conditions at interannual and decadal time‐scales, as well as the northern and southern limits of many dominant western plant species. Palaeoecologists have yet to focus on past climatic and biotic shifts along this transition, in part because it requires comparisons across dissimilar records [i.e. pollen from lacustrine sediments to the north and plant macrofossils from woodrat (Neotoma) middens to the south]. To overcome these limitations, we are extending the woodrat‐midden record northward into the lowlands of the central Rocky Mountains.

Location Woodrat middens were collected from crevices and rock shelters on south‐facing slopes of Dutch John Mountain (2000–2200 m, 40°57′ N, 109°25′ W), situated on the eastern flanks of the Uinta Mountains in north‐eastern Utah. The site is near the regional limits forPinus ponderosa,P. edulis,P. contorta,Cercocarpus ledifoliusvar.intricatus,Abies concolor,Ephedra viridisand other important western species.

Methods We analysed pollen and plant macrofossils from the 40,000‐year midden sequence. The middens represent brief, depositional episodes (mostly years to decades). Four middens represent the early to full‐glacial period (40,000–18,000 cal‐yrbp), eight middens are from the late‐glacial/early Holocene transition (13,500–9000 cal yrbp), and 33 middens span the mid‐to‐late Holocene (last 7500 years). Temporal density of our Holocene middens (one everyc.210 years) is comparable with typical Holocene pollen sequences from lake sediments.

Results Early to full‐glacial assemblages are characterized by low diversity and occurrence of montane conifers (Picea pungens,Pseudotsuga menziesii,P. flexilis,Juniperus communis) absent from the site today. Diversity increases in the late‐glacial samples with the addition ofJ. scopulorum,J. horizontalis,C. montanus,C. ledifoliusvar.intricatusand mesic understory species. The coniferous trees andJ. communisdeclined andJ. osteospermaappeared during the late‐glacial/Holocene transition.Juniperus osteospermapopulations have occupied the site throughout the Holocene.Pinus ponderosawas established by 7500 cal‐yrbp, and has occurred at least locally ever since. Montane conifers andJ. horizontalispersisted untilc.5500 cal‐yrbp. The signature events of the late Holocene were the invasions ofP. edulisandEphedra viridisand establishment of pinyon–juniper woodland in the last 800 years.

Main conclusions The Dutch John Mountain midden record adds to an emerging picture in which mid‐elevation conifers (P. flexilis,Pseudotsuga menziesii,Picea pungens,J. scopulorum,J. communis) dominated vegetation over a wide area of the Colorado Plateau and adjacent Rocky Mountains. Rather than being fragmented, as often assumed in phylogeographical studies, these species had broader and more‐connected distributions than they do in the region today. Paradoxically, subalpine conifers (Picea engelmannii,A. lasiocarpa) occurred at higher elevations to the south, possibly representing declining precipitation from south to north owing to southward displacement of the polar jet stream. The Dutch John Mountain record displays a series of extinction and invasion events. Most of the extinctions were local in scale; nearly all constituents of fossil midden assemblages occur within a few kilometres of Dutch John Mountain, and some occur at least locally on its slopes. The sole exception isJ. horizontalis, which is regionally extinct. In contrast to extinctions, Holocene invasions were regional in scale;J. osteosperma,P. ponderosa,P. edulisandEphedra viridisimmigrated from glacial‐age source populations far to the south.