Climate-Growth Analysis for Updated and Existing Tree-ring Network near Columbine Lake, Grand County, Colorado

This data release is tree-ring data near Columbine Lake and surrounding region, Grand County, Colorado (Latitude 40.27˚ N, Longitude -105.83˚ E NAD83). Re-collection of four existing tree-ring sites (Hot Sulphur Springs Psuedotsuga menziessii (HSU), Lexan Creek Picea engelmannii (LCU), Monarch Lake Pinus ponderosa (MLU), and Vasquez Mountain Psuedotsuga menziessii (VMU)) was conducted to update data to the most recent years possible and to maximize data overlap with instrumental records and with historical records of fire occurrence. At the time of collection, initial climate-growth relationships were assessed in a network of previously collected tree-ring sites (collected between 1987 and 2003) to determine which sites expressed the strongest climate signal. Climate-growth relationships were then determined anew on each site chronology to visualize how and when trees in the network are responding to climate inputs. High-resolution environmental information from tree rings overlaps with historic and century-scale drought and fire occurrence (Buechling and Baker, 2004). Tree rings are accurately dated to the year, and they provide synoptic-scale climate information and local-scale fire history when multiple site chronologies are used. The tree-ring archive is capable of producing multiple proxies that reflect climate-growth relationships to better understand precipitation form and dynamics in the context of comprehensive long-term winter-spring precipitation reconstructions (Stahle et al. 2003; Torbenson et al. 2016; Chavardes et al. 2020; Coulthard et al. 2021). Tree-ring proxies also reflect distinct physiological "tree-growth mechanisms related to topographic and climatic site conditions. They provide unique information that may be used in combination to refine our understanding of environmental change at Columbine Lake and assess high-resolution environmental pre-conditioning and past occurrence of wildfire similar in scale and/or severity to wildfire in the historic record. Columbine Lake is located at the western boundary of Rocky Mountain National Park near the town of Grand Lake in Grand County, Colorado. It is within the head waters region of the Colorado River, one of the largest river basins in the western United States and a critical water resource for agriculture, municipalities, and ecosystems in seven states and Mexico. To the west of RMNP, the 2020 East Troublesome fire burned directly over the Columbine Lake area destroying 555 structures, killing two people, and expanding into portions of RMNP. The fire is a stark immediate reminder of the potential scale and impact of severe wildfire in the Colorado River headwaters. Low severity frequent surface fires that are dependent on extreme drought, as well as infrequent stand-replacing fires in less drought prone forests, are believed to be crucial elements influencing the vegetation in the region (Romme and Despain, 1989; Renkin and Despain, 1992; Buechling and Baker, 2004; Kipfmueller and Baker, 2000). However, there is a significant concern that if temperatures continue to increase with climate change, and droughts become more frequent and severe, the probability of large and destructive fires will also increase. Climate-fire nuances revealed in previous studies illustrate that the relationships between drought and wildfire in forests is more complex than a simple connection between aridity and fire ignition and merit investigation at multiple space and time scales (Littell et al. 2016). While drought primarily drives fire frequency and intensity in Colorado forests, seasonal drought may disproportionately precondition forests for fire in the region (Sherriff et al. 2001). Annual-scale measures of drought may mask the seasonal precipitation connection. However, to our knowledge, no systematic evaluation of the interrelationship of climate and fire has been conducted in the headwaters region of the Upper Colorado River basin using long-term paleo records. Work elsewhere provides evidence of a direct relationship between drought severity, fire occurrence, and fire severity in mixed-conifer lower and upper montane forests, and demonstrate that the relationship is modified by location, elevation and species (Swetnam and Baisan, 1996; Touchan et al. 1996; Brown and Sheppard 2001; Swetnam and Baisan 2003; Westerling et al. 2003; Schoennagel et al. 2007; Margolis et al. 2009; Margolis et al. 2017). Wet-year preconditioning present in lower montane forests at lower latitudes in the western U.S. is not apparent in the Columbine Lake region nor in upper montane systems (Brown and Sheppard, 2001; Schoennagel et al. 2005; Sibold et al. 2006). Many of the same factors affecting moisture for growth in vegetation also affect moisture available from cool-season precipitation for streamflow (Meko et al. 2012). Long-term cool-season drought and its coincident timing with snowmelt-driven streamflow in the Colorado River Basin has been evaluated in previous tree-ring based research (Woodhouse et al 2006; Meko et al., 2007; Timilsena and Piechota, 2008; Pederson et al. 2011). These studies demonstrate statistically significant growth relationships with winter precipitation variables for tree-ring chronologies in the region, resulting in robust climate-growth analysis and reconstruction. For example, previous work explains up to 63% of the variance in snowpack in a lower sub-basin of the Colorado River and up to 81% of variance in Colorado river flow (Woodhouse et al. 2003; Woodhouse et al. 2006; Meko et al. 2007). In some instances, tree-ring reconstructions have utilized site chronologies that were also evaluated in the present study (e.g. two chronologies in Timilsena and Piechota, (2008), one chronology in Woodhouse et al. (2003), one chronology in Woodhouse et al. (2006)). The present data shore up temporal gaps in these chronologies, and provide new insights to their research utility. These raw data are now readily accessible to researchers, partners, and educators and create an opportunity for future application should new tree-ring proxy techniques be developed.