A

B

C

MonarchPass3448m

Summit3626m

200m

Transect Plot

AD 400 - 800

Undated

300 BC - AD 1

Remnants - inside dates

Monarch Pass samples

Alpine Treeline Changes in the Central Rocky Mountains: A Progress Report

Connie A. Woodhouse, Department of Geography and Regional Development, University of Arizona (conniew1@email.arizona.edu), and Jeffrey J. Lukas, INSTAAR, University of Colorado (lukas@colorado.edu)

GC41A-0111

COTTONWOOD PASS

SITE DESCRIPTION

This site is located about 1 km west of Cottonwood Pass, at the Continental Divide. There is no clear evidence for a previously higher treeline, but there is evidence of fire and a denser forest prior to burning. The forest below the burn is sparse, with some living tree islands above. The burn area is being colonized by a few “krummholz” lodgepole, and by limber and bristlecone pine trees. The burned trees appear to be Englemann spruce. Two standing snags with bark were burned when living will help determine the year of the fire.

SAMPLING STRATEGY

From the lower end of the burn, to the elevation of the highest remnant (3610-3680m), we defined a rectangular area (100m across and 200m up/down slope). Within this area, 34 remnants were sampled, photographed and GPSed. Each living tree > 10 cm in diameter was cored, and the locations and diameters of smaller living trees was recorded.

Cottonwood Pass Site Questions

�What is the date of the fire? From the era of miners or earlier? Concurrent drought and/or the Monarch burn?

�What was the species and age composition of the burned forest?

�Are there trees currently in this burn area that are survivors of the fire or have all of them established since the fire?

University of Colorado Boulder

SHEEP MOUNTAIN SITE

SITE DESCRIPTION

This site is located on the northern side of Sheep Mountain, approximately 10 km west of Leadville, CO. The forests going up to treeline are bands of 5-needle (limber and bristlecone pine), and Engleman spruce, grading into spruce krummholz. The sampling site was in a high steep, southfacing drainage with a treeline of 5-needle pines and many large, full tree remnants above the living trees. Pine and spruce tree islands extended the elevation of the remnants, higher along the side of the drainage.

SAMPLING STRATEGY

24 remnants were sampled, starting at the highest (3545m) to the lowest (3520m) which graded into the current treeline forest. Each remnant was photographed and GPSed. In addition, we cored 22 old living trees (a mix of bristlecone and limber) growing in a stand below the remnants and around to a cliff above Limber Grove (a previously sampled stand of old limber pine).

Sheep Mountain Pass Questions

�What are dates of remnants and we determine the date of trees establishment?

�When did the living trees become established?

�Are the remnants a distinctly different population (time-wise) from the living trees?

� If not, can an extended tree-ring chronology be compiled from both remnant and living tree collections?

� What climate variable limits tree-growth at this site?

� Is the climate signal different for the limber and bristlecone pine trees?

ReferencesLaMarche, V. C., and H. A. Mooney. 1967. Altithermal timberline advance in western United States. Nature 213, 980-982.

Lloyd, A.H. and L.J. Graumlich. 1997. Holocene dynamics of treeline forests in the Sierra Nevada. Ecology 78, 1199-1210.

Millar, C.I. 2007. Treeline Response to Climate: Up, Down, All Around. Presentation for CNPS at White Mountain Research Station, CA.

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

Scuderi, L. A. 1987. Late-Holocene upper timberline variation in the southern Sierra Nevada. Nature 325, 242-244.

AcknowledgementsThis work was made possible through the interest and support of Robert S. Thompson and the Earth Surface Processes - Central Region of the U.S. Geological Survey. We thank Mark Losleben, Kurt Chowanski, and Cody Routson for field assistance.

INTRODUCTION

Mountain environments are expected to be extremely sensitive to human-induced changes in 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 of alpine treeline can reflect climate variability and change, as well as patterns of disturbance which themselves may be mediated by climate variability. Information about past treelines may provide insight into the likely character of future changes.

Studies of past treelines are possible in areas where wood remnants above the current treeline are well preserved due to cold, dry climate conditions. Alpine treeline changes during the late Holocene in the Sierra Nevada have been investigated by a number of researchers (e.g., LaMarche and Mooney 1972, Scuderi 1987, Lloyd and Graumlich 1997, Millar 2007), but to date, no similar studies of late Holocene treeline have been undertaken in the central Rocky Mountains.

In this study, we have begun an investigation of characteristics of the current and past alpine treeline in the central Rocky Mountains. The goal of the study is to document the past and present treeline, and treeline relationships to climate and fire, using dendrochronological techniques. In the summer of 2007, we sampled remnants within and above treeline at three sites, as well as living trees at the current treeline (locations are shown in the map, right). We also recorded the presence of seedlings and saplings establishing above the current treeline and disturbances at two of the sites in the form of burns. The preliminary results for this work are reported here.

Locations of 3 sites in this study

COLORADO

Monarch PassCottonwood Pass

Sheep Mountain

Denver

37 N

41 N

102 W109 W

50 1501000 200 km

C nt n nt lo i e a Divide

MONARCH PASS SITE

SITE DESCRIPTION

This site is directly east and above Monarch Pass on the Continental Divide in Colorado. The current treeline forest is composed of primarily Englemann spruce, with a few subalpine fir. The actual treeline is variable in elevation, but generally about 3500 meters. Remnants were scattered in a line roughly 10-50 meters above the current treeline, roughly tracking the topographic undulations in the current treeline. Several old burns are embedded in the treeline. In addition, there are saplings (Englemann spruce, lodgepole, limber, and bristlecone pine) above the forest line and the spruce tree islands.

SAMPLING STRATEGY

A. REMNANTS43 remnants of bristlecone pine or limber pine above the current treeline were photographed, GPSed, and cut with a chainsaw.

B. CURRENT TREELINE 3 transects were run from the highest tree 100 meters down into the forest. At 20 m intervals, the six closest trees were measured (DBH and height to coring) and cored. All other trees, remnants, sapling, and seedlings in a 10 m radius were noted.

C. SAPLINGS ABOVE THE CURRENT TREELINEAll saplings above the highest tree island were GPSed, measured and species were noted. Two young trees were cored.

D. BURNED AREASRemnants at two burns within the treeline were sampled (12 and 20 samples) and GPSed.

Monarch Pass Site Questions

�What is the age of the remnant treeline? To what does it correspond, climatically?

�What is the age structure/species composition of the current forest to treeline?

�What are the dates of the fires in the two burned areas? Are they concurrent with each other, and concurrent with the Cottonwood Pass burn?

�What are the ages of the oldest saplings growing above the current treeline? Will the saplings survive, eventually raising the current treeline or die in harsh conditions?

Preliminary Results

Remnant TreelineTo date, 21 remnants have been dated. The inside and outside dates are shown (below), while tree abundance (binned by century after Lloyd and Graumlich 1997), are shown (below, right ). No adjustments have been made yet for the loss of sapwood and lack of pith in most samples, but results give a rough first look at approximate dates these trees were alive and growing above the current treeline.

.

Current treelineThe current treeline forest is composed almost entirely of Engelmann spruce trees, saplings and seedlings. Estimated pith dates of the trees, grouped into 20-yr age classes, are spread from the 1630s to the 1960s, with a peak in the late 19th century (above). A preliminary chronologies (12 samples) of standard ring widths shows a rapid shift from high growth to low growth just prior to an increase in pith dates, but whether this is related or just a coincidence is unknown (above. top).

Within the forest, a large number of Englemann spruce seedlings and saplings has established in the last two decades (right). In contrast, above the current treeline, small trees, saplings and seedlings are mostly a mix of spruce (11) and 5-needle pines (19, limber and bristlecone).

A comparison with Salzer and Kipfmueller’s (1997) temperature reconstruction (left) suggests trees at this site are sensitive to summer temperatures

Locations of remnant samples and transects (A, B, C) for sampling the current treeline forest are shown above. Undated remnants either do not overlap in age with dated remnants or have too few rings to accurately crossdate.