Scales of Synchrony of Subalpine Ecosystem Changes over the Past Six Millennia

Abstract:

Paleoecological records exhibit numerous shifts in Rocky Mountain forest ecosystem structure and composition through the Holocene and Common Era. Understanding the spatial and temporal distribution of these transitions can help diagnose the drivers and implications of past ecological change, while also providing insight into past forest structure. We apply a systematic approach to assessing the synchrony of subalpine forest transitions in the Southern Rocky Mountains over the last six millennia, with emphasis on the Common Era. We present a new macrofossil record from a small pond, LF2, on the Libby Flats plateau in the Snowy Range, Wyoming. We use Bayesian change point analysis and Bayesian age models to compare this new record to radiocarbon dated subfossil logs and the macrofossil and pollen record from a second small pond, LF15, 145 m away on Libby Flats, and then to four pollen records from the Mount Zirkel Wilderness in the Park Range, Colorado, an adjacent mountain range. The methods enable evaluations of the timing of changes at scales ranging from trees to landscapes. All eight of the ecological timeseries showed patterns of decreasing forest cover through time. At Libby Flats, records suggest a gradual, patchwork opening of the landscape, which first produced alternate forest-meadow ‘ribbon forests’ indicated by differences in the LF2 and LF15 macrofossil records from ca. 4.4-0.8 ka, and then widespread sub-alpine meadows. The patterns of change at the near-by Colorado sites suggest asynchrony across landscapes as a function of elevation with wildfires synchronizing some acceleration of the changes across elevations at ca. 1.1 ka. Regionally, the ecological changes are characterized by scale-invariant asynchrony that, when considered on progressively coarser scales, coalesces to produce a measurable signal of region-wide forest opening consistent with both climate and local disturbances progressively driving many asynchronous changes in the same direction.