Scientifica Acta 2, No. 2, 65 – 69 (2008)

Experimental Ecology and Geobotany

Multi-scale investigations of alpine vascular plant species in theSan Juan Mountains of Colorado, USA GLORIA target region

Julie CrawfordDipartimento di Ecologia del Territorio, Università di Pavia, Via S. Epifanio, 14, 27100 Pavia, Italycolumbine_julie@yahoo.com

The Global Observation Research Initiative in Alpine Environments (GLORIA) is an internationally co-ordinated long-term observation project that assesses and predicts vascular plant diversity change due toaccelerating climate change. There are currently 40 GLORIA target regions worldwide; the San Juan Moun-tain target region (SJM) of southwestern Colorado, USA is one of these. This study uses the baseline datacollected at SJM for immediate interpretation of current vegetation composition, abundance, richness, anddiversity at different spatial and taxonomic scales. It employs a wide range of multivariate techniques usinga continuum of distance coefficients to isolate the relative importance of species composition and speciesabundance on the SJM summits.

1 Introduction

Climate affects vegetation composition and distribution both directly (e.g. temperature) and indirectly (e.g.soil moisture). Human-induced global climate change is a well-documented fact. In recent decades, thewestern mountains of the United States have experienced some of the largest temperature increases in thenation, resulting in reduction in stream and spring flow, increase in evaporation and evapotranspiration,increase in wildfire severity, and outbreaks of tree pathogens [16, 17, 24]. It is estimated that with continu-ation of current emissions releases, this pattern will continue and the west may have increases of over 8˚Fby 2100 [17]. This increase would significantly impact snow pack, stream flow, and land surface drying.The documented 2-3 degree increase in temperature in the western mountains over the last 30 years hasalready led to species moving to higher elevations [5, 7, 14]. This movement of vegetation upslope hasbeen seen worldwide [6, 8, 9, 18, 19, 20].

The Global Observation Research Initiative in Alpine Environments (GLORIA) is an internationallycoordinated long-term observation project that uses alpine plant and temperature data to assess and pre-dict biodiversity change due to accelerating climate change. Data collection follows a rigorous protocoldeveloped by the international GLORIA program (www.gloria.ac.at). Currently, worldwide, there are 40active GLORIA target regions, 12 in set-up, and 17 planned. In the summer of 2006, The Mountain StudiesInstitute of Silverton, Colorado set up the fifth GLORIA target region in the United States in the San JuanMountains of southwestern Colorado (SJM) (Figure 1). Four summits ranging in elevation from 3,717.0meter (12,195 feet) to 4,234.9 meter (13,894 feet) in the Lake City Caldera were chosen based on GLO-RIA summit selection criteria [21]. In this study, the SJM data are used as a contemporary investigationinto alpine plant species in relation to scale. As scale is recognized to be important in ecology [25], andmulti-scale studies are better able to investigate the many abiotic and biotic factors that influence and helpexplain vegetation richness, association, pattern, and distribution [10, 27, 28], this study investigates fourlevels of spatial scale and two levels of taxonomic scale.

1.1 Methods

Field data collection and data input strictly followed the GLORIA Field Manual – Multi-Summit Approach[21]. Low impact monitoring plots were installed on each of the four summits using a specific observation

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Fig. 1: GLORIA target region set up in the San Juan Mountains of southwestern Colorado, USA.

protocol. Sampling design included four upper and four lower summit area section plots and a series of 1x 1 meter plots per each aspect of each summit. Plots of varying size allowed for a comparison of spatialscale. In total, plot areas follow: sixteen 10-meter summit area sections = ∼ 6,000 square meters, sixteen5-meter summit area sections = ∼2,500 square meters, and of both the sixteen 2 x 2 meter plots and sixty-four 1 x 1 meter plots = 64 square meters each. Two taxonomic scales, species and family, were alsoinvestigated. Multivariate analyses follow the work of M. J. Anderson [1, 2, 3, 4].

1.1.1 Results

There were a total of 97 vascular plant species from 28 families found within the plots of SJM. Thisrichness value is ranked 16th among the current target regions worldwide. Thirty-five of the 97 totalspecies are considered alpine obligates [11, 26] and are most threatened by global climate change. SJMspecies with a more arctic distribution are also greatly threatened, and those species at their southern limitof distribution are at risk of local extinction [14]. Two alpine obligates that are endemic to Colorado, Drabastreptobrachia and Penstemon hallii, are at risk of total extinction. Twenty-two of the 97 total plant speciesobserved on the four alpine summits of SJM are considered subalpine or lower elevation species [11, 26].This suggests the effects of climate change over recent decades are already in progress, that is, species aremoving up in elevation. The lowest elevation summits of SJM are at greatest risk of loosing their alpinecharacter due to encroachment of lower elevation species. Plant diversity may actually increase over time,however, species composition is expected to change in the future.

In general, species richness and diversity were greater with increased plot area (Figure 2). This followsa well-documented rule in ecology that more area equals more habitats (and microhabitats) available forcolonization and greater species richness (e.g. species-area curves; [15, 23]). Alpine environments containa great number of microhabitats created by such factors as contrasting exposure and variable snow coverand substrates. In SJM, the measured abiotic factors of percent cover of scree and litter, as well as overallplot area were the most important factors influencing vegetation composition and abundance. Other studieshave shown that alpine plant species richness is tied with slope aspect; south and east aspects having highestrichness [6, 13, 22]. With a sample size of just four summits within SJM, there was only a trend that southand east slopes were more floristically rich. Overall, the highest summit was least floristically rich andhad the highest cover of scree, the second highest summit had the highest species richness and a variety of

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Fig. 2: Species richness and diversity plotted with symbols proportionate to the plot area, San Juan Mountain GLORIAtarget region, Colorado, USA.

Fig. 3: Species richness in the 2 x 2 meter, 5- and 10- meter summit area section plots by summit and aspect, San JuanMountain GLORIA target region, Colorado, USA. Plots from left to right: lowest to highest in elevation.

microhabitats, and the two lowest summits had intermediate richness levels and more gentle slopes withless scree (Figure 3). This illustrates that habitat, not elevation, has greatest influence on the alpine plantsof SJM.

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The aggregation of finer taxonomic scales into coarser scales distorts data by underestimating speciesrichness [12]. Here, the highest richness on any one peak was 47 species. To further dilute this to thenumber of families represented by these 47 species, 18, made the results less strong. Richness by specieswas much greater and more irregular than richness by family, though similar trends occurred for bothtaxonomic scales. Beta diversity, the measure of differences in species across the four summits, variedby spatial scale, but was similar for both taxonomic scales. At larger spatial scales, when looking atcomposition only, beta diversity was greatest in the highest summits; when looking at abundance only,beta diversity was greatest on the lowest summits. In both instances, the opposite was true of the smallestspatial scale.

Because the choice of dissimilarity measure has important implications for interpreting test results andobserved patterns, a continuum of distance coefficients were used in most multivariate analyses. Theintermediate spatial scale, the smallest taxonomic scale, and the distribution measures of Jaccard and Man-hattan proved to be the best at finding differences in the dataset of SJM. Due to the variety of habitats andplant species found at SJM, multiple spatial scales were necessary to capture the full extent of the alpinevegetation of SJM.

Acknowledgements Special thanks go to my family and friends, Dr. John Wood, Dr. Graziano Rossi, Dr. KorenNydick and the Mountain Studies Institute, the American Alpine Club, the Colorado Mountain Club Foundation, andthe Conservation Biology Wilderness Research Class of Truman State University. In memory of GN and Shan.

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