Dataset purpose This dataset is the budburst date monitoring of the OPTMix device (https://www.optmix.efno.fr). The spring phenology of close to 130 oak trees (Quercus petraea) and 130 pine trees (Pinus sylvestris) is monitored since 2013 on 21 experimental plots. The dataset contains the budburst dates for each tree, plot and year.

Site description Data were collected in the south-eastern part of the Orléans National Forest (France, 47°49’ N, 2°29’ E) on 0.5-ha plots in the OPTMix experimental area (https://www.optmix.efno.fr). The Orleans National Forest is about 35 000 ha and is mainly composed of sessile oak and Scots pine. Elevation ranges from 107 m to 174 m above sea level. The area has a temperate continental climate with an oceanic influence: mean annual temperature is 10.8 °C and mean annual rainfall is 729 mm. The OPTMix experimental plots are several kilometers apart from each other with a maximum distance of 26 km. All the plots were set up in a same soil type characterized by a layer of sandy soil on surface and a clay layer appearing between 40 and 70cm depth. This type of soil is relatively poor and acidic with a sandy clay-loam texture, and is classified as a planosol. The stands are between 70 and 80 years old with an average dominant tree height of 20 m for oak and 22 m for pine. In the OPTMix experimental area, all understorey above two meters in height but shorter than the base of the main tree canopy, has been removed so that, the tree canopy is made up of a single layer. Each plot has an area of 0.5ha and is surrounded by a 20-meter-wide buffer zone with the same stand characteristics as the measurement plot. Among the 33 plots at the OPTMix site, we selected 18 plots: 6 monospecific sessile oak plots, 6 monospecific Scots pine plots and 6 plots of mixed stands of sessile oak and Scots pine. At the end of 2017, basal area was 16.9 m²/ha on average for oak monospecific plots, 23.9 m²/ha on average for pine monospecific plots and 20.9 m²/ha on average for oak-pine mixed plots. Phenology observations and determining budburst dates Phenological monitoring on the OPTMix experimental plots started in 2013. For each year, visual observations on budburst were made on a sampling of trees per plot. The objective of these observations was to obtain an estimate of the budburst date in order to define the beginning of the growing season for the different stands and tree species in the OPTMix experiment. On each plot, budburst was observed using binoculars on nine trees per species with one observation every weeks or every two weeks from April 1st to July 15th. The nine trees were randomly selected in three size classes of the stand and were all in the upper stratum of the stand. The sample trees were monitored over the years of this study. To limit operator effects, the observations were mainly made by one and the same person for the whole budburst period of one year and, when possible, the same person conducted the observations for several consecutive years. The phenological stage chosen for the observations was aligned with that of the RENECOFOR network to monitor sessile oak and Scots pine budburst over France: the BBCH09 stage of the BBCH scale. For sessile oak, stage BBCH09 corresponds to open buds visibly green in color and with possibly one unfolded leaf. This is the stage just before the complete development of the leaves. For Scots pine, stage BBCH09 corresponds to the moment when the needles begin to come out their sheath. At each observation date, on each tree, the percentage of buds having reached the target stage was recorded in five classes: 0%, 1 à 20%, 21 à 80%, 81 à 99% and 100%. For further analyses, we used the mean value of each class. For each selected tree, we therefore recorded the change over time in the proportion of buds having reached the target stage. We used the date when 50% of the buds reached the target stage to characterize budburst date. For each species, to obtain the value of the budburst date for one plot and one year, we fitted a the logistic regression (for more details see Perot, T., P. Balandier, C. Couteau, N. Delpierre, F. Jean, S. Perret and N. Korboulewsky (2021). "Budburst date of Quercus petraea is delayed in mixed stands with Pinus sylvestris." Agricultural and Forest Meteorology 300: 108326 DOI: https://doi.org/10.1016/j.agrformet.2021.108326).