Alpine Treeline Changes in the Central Rocky Mountains: a Progress Report
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GC41A-0111 Alpine Treeline Changes in the Central Rocky Mountains: A Progress Report Connie A. Woodhouse, Department of Geography and Regional Development, University of Arizona University of Colorado Boulder ([email protected]), and Jeffrey J. Lukas, INSTAAR, University of Colorado ([email protected]) INTRODUCTION MONARCH PASS SITE COTTONWOOD PASS Mountain environments are expected to be extremely sensitive to human-induced changes in SITE DESCRIPTION SITE DESCRIPTION climate, and the impacts of climate change on mountain regions are already well- documented in western North America. Changes in the elevation, composition, and structure This site is directly east and above Monarch Pass on the This site is located about 1 km west of alpine treeline can reflect climate variability and change, as well as patterns of disturbance Continental Divide in Colorado. The current treeline forest of Cottonwood Pass, at the which themselves may be mediated by climate variability. Information about past treelines is composed of primarily Englemann spruce, with a few Continental Divide. There is no clear may provide insight into the likely character of future changes. subalpine fir. The actual treeline is variable in elevation, evidence for a previously higher but generally about 3500 meters. Remnants were scattered treeline, but there is evidence of fire Studies of past treelines are possible in areas where wood remnants above the current in a line roughly 10-50 meters above the current treeline, and a denser forest prior to burning. treeline are well preserved due to cold, dry climate conditions. Alpine treeline changes roughly tracking the topographic undulations in the current The forest below the burn is sparse, during the late Holocene in the Sierra Nevada have been investigated by a number of treeline. Several old burns are embedded in the treeline. In with some living tree islands above. researchers (e.g., LaMarche and Mooney 1972, Scuderi 1987, Lloyd and Graumlich 1997, addition, there are saplings (Englemann spruce, lodgepole, The burn area is being colonized by a Millar 2007), but to date, no similar studies of late Holocene treeline have been undertaken in limber, and bristlecone pine) above the forest line and the few “krummholz” lodgepole, and by SHEEP MOUNTAIN SITE the central Rocky Mountains. spruce tree islands. limber and bristlecone pine trees. The burned trees appear to be SAMPLING STRATEGY Englemann spruce. Two standing SITE DESCRIPTION In this study, we have begun an snags with bark were burned when 41 N investigation of characteristics of the A. REMNANTS living will help determine the year of This site is located on the northern side of Sheep current and past alpine treeline in the Continental COLORADO Mountain, approximately 10 km west of Leadville, CO. Divide 43 remnants of bristlecone pine or limber pine above the the fire. central Rocky Mountains. The goal of current treeline were photographed, GPSed, and cut with a The forests going up to treeline are bands of 5-needle the study is to document the past and (limber and bristlecone pine), and Engleman spruce, Denver chainsaw. SAMPLING STRATEGY present treeline, and treeline grading into spruce krummholz. The sampling site was relationships to climate and fire, using in a high steep, southfacing drainage with a treeline of 5- B. CURRENT TREELINE Monarch Pass Site Questions From the lower end of the burn, to the dendrochronological techniques. In Sheep Mountain 3 transects were run from the highest tree 100 meters elevation of the highest remnant needle pines and many large, full tree remnants above the summer of 2007, we sampled the living trees. Pine and spruce tree islands extended Cottonwood Pass down into the forest. At 20 m intervals, the six closest trees What is the age of the remnant treeline? To what does it correspond, (3610-3680m), we defined a remnants within and above treeline at the elevation of the remnants, higher along the side of Monarch Pass were measured (DBH and height to coring) and cored. All climatically? rectangular area (100m across and three sites, as well as living trees at other trees, remnants, sapling, and seedlings in a 10 m 200m up/down slope). Within this the drainage. the current treeline (locations are radius were noted. What is the age structure/species composition of the current forest to area, 34 remnants were sampled, shown in the map, right). We also SAMPLING STRATEGY treeline? photographed and GPSed. Each recorded the presence of seedlings 0 50 100 150 200 km C. SAPLINGS ABOVE THE CURRENT TREELINE living tree > 10 cm in diameter was and saplings establishing above the 37 N All saplings above the highest tree island were GPSed, cored, and the locations and 24 remnants were sampled, starting at the highest 109 W 102 W What are the dates of the fires in the two burned areas? Are they concurrent current treeline and disturbances at measured and species were noted. Two young trees were (3545m) to the lowest (3520m) which graded into the Locations of 3 sites in this study with each other, and concurrent with the Cottonwood Pass burn? diameters of smaller living trees was two of the sites in the form of burns. cored. recorded. current treeline forest. Each remnant was photographed The preliminary results for this work and GPSed. In addition, we cored 22 old living trees (a What are the ages of the oldest saplings growing above the current treeline? are reported here. D. BURNED AREAS mix of bristlecone and limber) growing in a stand below Will the saplings survive, eventually raising the current treeline or die in harsh the remnants and around to a cliff above Limber Grove Remnants at two burns within the treeline were sampled conditions? (12 and 20 samples) and GPSed. (a previously sampled stand of old limber pine). Cottonwood Pass Site Questions Preliminary Results Sheep Mountain Pass Questions Remnant Treeline What is the date of the fire? From the What are dates of remnants and we determine the date of trees establishment? era of miners or earlier? Concurrent To date, 21 remnants have been dated. The inside and outside dates are shown (below), while drought and/or the Monarch burn? tree abundance (binned by century after Lloyd and Graumlich 1997), are shown (below, right ). No When did the living trees become established? adjustments have been made yet for the loss of sapwood and lack of pith in most samples, but What was the species and age results give a rough first look at approximate dates these trees were alive and growing above the Are the remnants a distinctly different population composition of the burned forest? current treeline. (time-wise) from the living trees? Are there trees currently in this burn area If not, can an extended tree-ring chronology be that are survivors of the fire or have all of compiled from both remnant and living tree collections? them established since the fire? What climate variable limits tree-growth at this site? Monarch Pass samples Transect Plot Is the climate signal different for the limber and Remnants - inside dates bristlecone pine trees? 300 BC - AD 1 A AD 400 - 800 Undated Current treeline References Summit The current treeline forest is composed almost entirely of Engelmann spruce trees, saplings and LaMarche, V. C., and H. A. Mooney. 1967. Altithermal timberline advance in western United States. Nature 213, 980- 3626m C A comparison with seedlings. Estimated pith dates of the trees, grouped into 20-yr age classes, are spread from the 982. 1630s to the 1960s, with a peak in the late 19th century (above). A preliminary chronologies (12 Salzer and Lloyd, A.H. and L.J. Graumlich. 1997. Holocene dynamics of treeline forests in the Sierra Nevada. Ecology 78, 1199- Monarch samples) of standard ring widths shows a rapid shift from high growth to low growth just prior to an Pass Kipfmueller. ’s 1210. 3448m (1997) temperature increase in pith dates, but whether this is related or just a coincidence is unknown (above. top). Millar, C.I. 2007. Treeline Response to Climate: Up, Down, All Around. Presentation for CNPS at White Mountain reconstruction (left) 200m B Research Station, CA. suggests Within the forest, a large number of Englemann spruce trees at this site are seedlings and saplings has established in the last two Salzer, M.W. and K.F. Kipfmueller. 2005. Reconstructed temperature and precipitation on a millennial timescale from tree rings in the southern Colorado Plateau, U.S.A. Climatic Change 70, 465 - 487 sensitive to decades (right). In contrast, above the current treeline, Locations of remnant samples and transects summer small trees, saplings and seedlings are mostly a mix of Scuderi, L. A. 1987. Late-Holocene upper timberline variation in the southern Sierra Nevada. Nature 325, 242-244. (A, B, C) for sampling the current treeline temperatures spruce (11) and 5-needle pines (19, limber and forest are shown above. Undated remnants bristlecone). either do not overlap in age with dated Acknowledgements This work was made possible through the interest and support of Robert S. Thompson and the Earth Surface Processes remnants or have too few rings to accurately - Central Region of the U.S. Geological Survey. We thank Mark Losleben, Kurt Chowanski, and Cody Routson for field crossdate. assistance..