Ecological Indicators 48 (2015) 189–197 Contents lists available at ScienceDirect Ecological Indicators j o ur na l ho me page: www.elsevier.com/locate/ecolind Phenology and climate relationships in aspen (Populus tremuloides Michx.) forest and woodland communities of southwestern Colorado Gretchen A. Meier a,∗ , Jesslyn F. Brown b , Ross J. Evelsizer b , James E. Vogelmann b a Inuteq, Contractor to the U. S. Geological Survey (USGS), Earth Resources Observation and Science Center, Sioux Falls, SD 57198, United States b USGS, Earth Resources Observation and Science Center, Sioux Falls, SD 57198, United States a r t i c l e i n f o Article history: Received 23 July 2013 Received in revised form 23 May 2014 Accepted 28 May 2014 Keywords: Phenology, Trembling aspen Populus tremuloides Start of season End of season Remote sensing a b s t r a c t Trembling aspen (Populus tremuloides Michx.) occurs over wide geographical, latitudinal, elevational, and environmental gradients, making it a favorable candidate for a study of phenology and climate relationships. Aspen forests and woodlands provide numerous ecosystem services, such as high pri- mary productivity and biodiversity, retention and storage of environmental variables (precipitation, temperature, snow–water equivalent) that affect the spring and fall phenology of the aspen woodland communities of southwestern Colorado. We assessed the land surface phenology of aspen woodlands using two phenology indices, start of season time (SOST) and end of season time (EOST), from the U.S. Geological Survey (USGS) database of conterminous U.S. phenological indicators over an 11-year time period (2001–2011). These indicators were developed with 250 m resolution remotely sensed data from the Moderate Resolution Imaging Spectroradiometer processed to highlight vegetation response. We compiled data on SOST, EOST, elevation, precipitation, air temperature, and snow water equivalent (SWE) for selected sites having more than 80% cover by aspen woodland communities. In the 11-year time frame of our study, EOST had significant positive correlation with minimum fall temperature and significant negative correlation with fall precipitation. SOST had a significant positive correlation with spring SWE and spring maximum temperature. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Trembling aspen (Populus tremuloides Michx. hereafter referred to as aspen) dominated forest and woodland communities are a prominent, unique and vital landscape feature throughout the Rocky Mountains. These stands contain nearly twice the total nitro- gen (N, NO 3 , and NH 4 ) on a per unit basis, as adjacent conifer stands (Buck and St. Clair, 2012), have greater structural diversity (Debyle and Winokur, 1985) and contain the second highest species diversity (after riparian communities) of any western U.S. vegeta- tion type (McCullough et al., 2013). Aspen stands accumulate and retain more snow and yield larger quantities of water down slope for agriculture and municipal uses than do adjacent mixed-conifer and conifer stands (LaMalfa and Ryle, 2008). The relatively higher soil moisture makes aspen stands act as fire-breaks, and aspen are less likely than other vegetation types to ignite from lightening fire (Morelli and Carr, 2011). ∗ Corresponding author. Tel.: +01 6055942917; fax: +01 6055946529. E-mail address: gmeier@usgs.gov (G.A. Meier). Large patches of dead and declining aspen began appearing in the southern Rocky Mountains in the late 1990’s, and consequently, aspen population dynamics and mortality trends have become a major concern (Worrall et al., 2008). This condition, called Sud- den Aspen Decline (SAD), is characterized as a drought and disease cycle, where terrain and edaphic features predispose stands to decline, severe drought initiates decline, and finally, opportunistic insects and pathogens cause extensive damage and frequent mor- tality (Anderegg et al., 2012; Rehfeldt et al., 2009; Worrall et al., 2008). Researchers have suggested that aspen is undergoing a range contraction in the warmest, driest, and most southerly regions of its distribution (Kuhn et al., 2011; Worrall et al., 2013; Zegler et al., 2012). Aspen decline and mortality is largely attributed to drought events and to the gradual warming and drying conditions brought about by global climate change (Worrall et al., 2013). The typical growing season for aspen occurring in the south- ern Rocky Mountain begins with flower bud burst in April. Leaves sprout in May and June and continue to appear through mid- September, and leaf drop and senescence conclude the growing season in mid-November. During the summer, stems lengthen and new leaves appear until growth ceases in August or September. Bud set is an important final phase, as it signals the end of the growing http://dx.doi.org/10.1016/j.ecolind.2014.05.033 1470-160X/© 2014 Elsevier Ltd. All rights reserved.