These methods, instrumentation and/or protocols apply to all data in this dataset:| Methods and protocols used in the collection of this data package |
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| Description: | Site Selection:
The Rhone River is an 812-kilometer long alpine river that flows from the Furka Glacier in Switzerland to the Mediterranean Sea (Olivier et al, 2022). In determining from where along the Rhône we would collect increment cores, we sampled sites that (a) had both P. alba and P. nigra represented in the community, (b) represented varying levels of connectedness to the water table, and (c) were close enough geographically to reduce climate variation between sites. Site connectiveness to the water table was determined by a visual observation of the height from the water table to the field site. We chose an 8-kilometer stretch of the
containing four individual sites that met the necessary conditions. A dam upstream of the sites split the river in two, lowering the water level around two of the chosen sites. The split rivers connect further downstream, raising the water level to its original depth where the last two sites are located. They are also far enough downstream where P. alba is present, as P. alba does not grow along the upper portion of the Rhône (Olivier et al, 2022). The close proximity of the four sites to one another indicates that climate variables, such as precipitation and temperature, would be consistent across each site.
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| | Description: | Field Sampling:
Individual trees were selected at random within the site boundaries. The circumference and GPS location of each tree was then recorded. Trees were only selected if they were over 10 meters tall. Eight P. nigra were selected at each site, and eight P. alba were selected each at sites 1 and 2. Due to low availability, six P. alba were selected each at sites 3 and 4. Two core samples were taken from each tree at breast height, at 90 degrees from each other. One was taken with a 4.5-millimeter diameter increment borer, while the other was taken with a 10-millimeter diameter increment borer. The larger diameter cores were stored for future stable isotope analysis of the wood.
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| | Description: | Sample Preparation and Reading:
The cores were stored in paper straws to dry for a week after collection. Once dry, they were mounted and prepped for sanding. The cores were sanded using a belt sander, gradually increasing the fineness from 120 to 400 grit size. They were then further polished by hand using 30-micron sandpaper. After sanding, the cores were read using a sliding scale micrometer linked to Tellervo dendrochronological analysis package (Brewer, 2014). The raw width index was transformed to the basal area increment (BAI) using the dplR package in R (Bunn et al, 2019).
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| | Description: | Statistical Analysis:
Median basal area increment (BAI) chronology was correlated to the species and site connectiveness to the water table. This was done with a mixed model using the nlme package in R (Pinheiro, 2009), with species and site as fixed effects and year as a random effect to account for autocorrelation. We compared five models that included single fixed effects (species and site connectivity), their additive effects (species + site), an interactive model (species x site), and a null (intercept-only) model. An AIC analysis using a likelihood inference approach was applied to determine the best fitting model.
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