Tree-ring growth and stable-carbon isotope response data to forest restoration treatments in ponderosa pine forests of the Lick Creek Demonstration-Research Forest, Bitterroot National Forest, Montana, USA

This data publication includes the data used in "Forest restoration treatments in a ponderosa pine forest enhance physiological activity and growth under climatic stress" by Tepley et al. (2020). These data include (1) information on each of the cored trees included in the analysis, (2) tree-ring based growth metrics, and (3) stable-carbon isotope data collected in the summer of 2016 at the Lick Creek Demonstration-Research Forest in the Bitterroot National Forest, Montana. There were restoration treatments implemented as two separate experiments on this site — a commercial thinning (referred to as the moderate thinning in Tepley et al. 2020), and a retention shelterwood (the heavy thinning of Tepley et al. 2020). Each experiment includes 12 management units (1.0 to 6.6 ha each), where three were left as untreated controls, and nine were thinned in the spring of 1992. Three of the treated units had no subsequent burning (thin only treatments). The other six were subjected to one of two prescribed burning treatments. In the commercial thinning (moderate thinning) experiment, three of the thinned units were burned in the fall of 1993 and three were burned in the spring of 1994 (spring and fall burn treatments). In the retention shelterwood (heavy thinning) experiment, all burning was conducted in the spring of 1993, with three units burned when the duff was moist in early May and three burned under drier conditions later in the month (wet and dry burn treatments). Twelve circular plots (0.1 acre or 0.0405 hectares each; radius 11.35 meters) were established within each management unit (144 plots per experiment, 288 total). In summer, 2016, 8 of the 12 plots within each management unit were selected at random. In each of these plots, increment cores were collected from the nearest large (> 25.4 centimeters [cm] diameter at breast height [dbh]) and small (≤ 25.4 cm dbh) ponderosa pine tree to the plot center, thereby sampling 16 trees per unit and 192 trees per experiment. Two increment cores were collected from each tree, where one of the cores was only used for measuring growth metrics, and the other was used for a subset of the growth measurements, followed by stable-carbon isotope analyses. The data files include only data for the increment cores that could be accurately crossdated. Increment cores not included in the data files were either missing, mislabeled, or could not be accurately crossdated over the time period relevant for evaluating treatment responses. The information on each of the cored trees includes the height of core extraction, bole diameter at breast height and coring height, bark thickness at the tree base and at breast height, as well as the date of the innermost tree ring and the estimated distance and number of rings to the pith for both of the sampled increment cores. The tree-ring based growth metrics include ring width (RW), earlywood width (EW), latewood width (LW), basal area increment (BAI), earlywood area increment (EWAI), latewood area increment (LWAI), and the proportion of latewood (PLW) by year for each of the cored trees. The variable reported in the stable-carbon isotope data is the carbon-isotope discrimination (Δ13C) for earlywood and latewood samples in each year from 1969 to 2015. Stable-carbon isotope analyses were conducted on samples pooled from six trees per management unit. There were three units for each of the four experimental treatments (control, thin only, and thinning followed by two prescribed burning treatments), which leads to a total of 72 trees sampled for stable-carbon isotopes.
Treatments designed to restore historical stand structure and enhance resistance to high-severity fire might also alleviate drought stress by reducing competition, but the duration of these effects and the underlying mechanisms remain poorly understood. To elucidate these mechanisms, we collected data to evaluate tree growth, mortality, and tree-ring stable-carbon isotope responses to stand-density reduction treatments with and without prescribed fire in a ponderosa pine forest of western Montana. Our analyses span the 23-year windows before and after treatments, enabling us to compare initial responses and the persistence of those changes under a range of variation in climate and external disturbance pressures (e.g., bark beetle activity). We sought to answer: (1) How did the treatments alter trajectories of tree growth and Δ13C? (2) In what ways did the treatments alter the growth and Δ13C responses to climate, and do those changes differ between EW and LW? (3) To what degree are these alterations of growth, Δ13C, and the relationships of both variables to climate reflected in different tree mortality rates between treated and untreated stands, and specifically, have the treatments reduced mortality rates compared to unthinned units?
These data are part of the Lick Creek Demonstration-Research Forest experiments. Additional vegetation and fuel data, as well as long-term repeat photographs are available in Lutes et al. (2020; https://doi.org/10.2737/RDS-2020-0008). Early treatment responses and site background are described in Smith and Arno (1999). This data publication was originally published on 04/16/2020, with minor updates included on 09/21/2020. On 06/26/2023 a few additional minor metadata updates were made as well as a correction to the legend for: \Supplements\LickCreek_study_design.pdf (descriptions for SB/WB and TO were switched - this has been corrected).