DOI: 10.5061/dryad.1g1jwstvx
title: Tree phenology - observer intercalibration and individual tree phenological scoring

description: We report data documenting (1) the dynamics of budburst and leaf
 senescence in three European natural forest tree populations over the
 period od 2012-2015 and (2) the variability among phenological observers
 as documented from seven observer inter-calibration experiments conducted
 in France over 2007-2017 for both the budburst and leaf senescence period.
description: We report three sets of ground phenological observations,
 conducted in different forests: (1) observations of two species (Quercus
 petraea, Carpinus betulus) in the Fontainebleau-Barbeau forest (France)
 from 2013-2017; (2) observations of three species (Quercus petraea,
 Carpinus betulus, Castanea sativa) in the Orsay forest (France) from
 2012-2015; (3) observations of six species (Quercus robur, Fraxinus
 excelsior, Fagus sylvatica, Betula pendula, Corylus avellana, and Acer
 pseudoplatanus) in Wytham Woods (Oxfordshire, UK) in 2013 and 2014. The
 three datasets share the particularity of documenting the whole sequence
 of spring budburst or leaf senescence at the individual tree scale. This
 means that each tree was monitored from winter state (all buds closed) to
 100% budburst and from mature green summer leaves to all leaves colored or
 fallen. In Fontainebleau-Barbeau and Orsay, the same tree individuals were
 monitored during spring and autumn. The three sets of observations were
 each collected consistently by a small group (less than five) of observers
 who have been trained and inter-calibrated before data collection, such
 that the observer variability in this dataset is minimal.<br> At the
 Fontainebleau-Barbeau and Orsay forests, the percentage (on a 10%-step
 scale) of open leaf buds (in the upper-third of the tree crown), leaf
 coloration and leaf fall (over the whole tree crown) for individual trees
 were monitored with binoculars 2-3 times a week from March to June and
 once a week from September to December.<br> At Wytham Woods, the
 development of buds was scored over whole tree crowns using a key of
 phenological stages different from BBCH, at a 3-day interval from March to
 May. A seven-stage key (stage 1 = small dormant buds, 2 = larger, slightly
 elongated buds, 3 = larger, loosened greenish brown buds, 4 = further
 elongated buds with leaves starting to erupt, 5 = leaves emerging but
 still tight, 6 = leaves loosening and extending outwards, 7 = leaves fully
 emerged and unfurled) was used for Quercus robur and a five-stage key
 (stage 1 = dormant bud, 2 = elongated/swollen buds, 3 = budburst, 4 =
 leaves emerging and extending outwards, 5 = leaves fully emerged and
 unfurled) used for the other species.<br> The scale used for the
 observation of budburst in the Fontainebleau-Barbeau and Orsay forests
 differed with that used in Wytham Woods. Indeed, bud observations in the
 Fontainebleau-Barbeau or Orsay forests started from the first signs of
 buds opening (0% buds opened), and ended when 100% buds were open,
 corresponding to the interval from stage 4 to stage 7 using the
 seven-stage key and stage 3 to stage 5 using the five-stage key in Wytham
 Woods. In order to homogenize protocols, we scaled the Wytham Woods
 observations of budburst to a 0-100% scale similar to the one used in
 Fontainebleau-Barbeau and Orsay, setting stage 3 (or 4 depending on the
 species) of the Wytham Woods scale to 0% and setting stage 5 (or 7,
 depending on the species) to 100%.<br> We ensured that the tree
 individual phenological sequences collected at Fontainebleau-Barbeau,
 Orsay and Wytham woods were monotonically increasing with time. If a
 phenological observation record was less than the previous record, we
 corrected it to be equal to the previous record (affects 0.25% / 7.8% of
 all budburst/leaf senescence data). The observer
 inter-calibration dataset was obtained during seven inter-calibration
 sessions of observers conducted in France during 2007-2017, three of which
 concerned budburst observations, and four of which concerned observation
 of leaf senescence. The sessions took place in different locations each
 year and concerned different tree species. An average of 30 individual
 trees (from 12 to 49 trees) were observed by 10 to 37 observers each time
 following the same protocol and scoring scale (BBCH scale). The observers
 were scientists, technicians and doctoral students in biological sciences,
 and most of them participated all the sessions, which lasted one and a
 half day each. The inter-calibration sessions were organized as follows.
 At the first session, all participants were trained to the BBCH scoring
 scale and protocol. At all sessions, all participants first trained
 altogether on a set of tree individuals. Participants subsequently also
 scored another set of tree individuals independently. They used binoculars
 for observing tree crowns. Observers were instructed not to exchange
 information or discuss during this scoring in order to guarantee the
 independence of the scoring. The data obtained with this independent
 scoring were used for this study.