# Inter-Annual Variability of Peatland Vegetation Captured Using Phenocam- and UAV Imagery
## 3 Results  
### Overview of the Study Period (_Hydrometeorological Conditions and CO\({}_{2}\) Balance_)  
The four years examined were characterized by a number of weather events, which affected both vegetation phenology and physiology, as well as ecosystem functioning. The year 2018 was the most unusual year with respect to the long-term record. Cold conditions and late snowfall during spring led to a delayed start to the growing season, and were followed by a European-wide summer heat and drought event [89; 90]. Monthly air temperatures in June and July were consistently above the long-term average, and the site experienced a dry spell (defined as a period longer than 7 days with WTD more than 5 cm below the surface) of over two months. During this period WTD remained below 10 cm for 77 days (28 June to 12 September 2018), and reached a minimum of 47 cm (Figure 3). \begin{table}
\begin{tabular}{l} \hline \multicolumn{2}{l}{**(2) Raw data spikes**} \\ \hline Data excluded if spikes (i.e., values \textgreater{}\pm\) 2 standard deviations from the mean) in vertical wind velocity, CO\({}_{2}\) mole fraction, or water-vapour (H\({}_{2}\)O) mole fraction \textgreater{} 1\% total number of data points per 30-min flux averaging period (i.e., \textgreater{}\geq\)18 for 10 Hz data) \\ \hline \multicolumn{2}{l}{**(3) Sufficient turbulence**} \\ \hline Discard measurements with insufficient turbulence, defined by a friction velocity (u\({}^{\text{a}}\)) threshold [83] \\ \hline \multicolumn{2}{l}{**(4) Stationarity**} \\ \hline Check that the time-series is in steady state [84] \\ \hline \multicolumn{2}{l}{**(5) Flux within sensible range for site**} \\ \hline Discard half-hourly CO\({}_{2}\) fluxes which fall outside the range \(\pm\) 30 \(\mu\) mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._From visual field observation, this dry spell resulted in the desiccation of vegetation and early senescence. This year was also unique in terms of its CO\({}_{2}\) budget. Coinciding with the warm, dry conditions, the year 2018 had a positive NEE (7.92 \(\pm\) 0.5 mol CO\({}_{2}\) m\({}^{-2}\) y\({}^{-1}\)), which is in contrast to the long-term sink function observed at the site [79; 91]. Unusually, daily cumulative NEE did not cross the zero line at any point during this year, and instead remained consistently positive. The year 2019 was characterised by more normal conditions with respect to the long-term (2002-2017) average. This year experienced a warm start, with monthly Tair in February 2019 being almost twice the average, and daily mean Tair was consistently above zero from early February (day of year (DOY) 35). It follows that the growing season also commenced early during this year, with green-up in late April (see Table 5). All months were wetter than average from April onwards, although there were a few dry spells, the most notable of which occurred on DOY 105-128 (23 days, mid-April to early May; Figure 3). Summer WTD was closer to the surface than in 2018, with mean WTD in July for example being close to average (WTD anomaly for July 2019 = + 3.7 cm, vs. + 30.7 cm for July 2018; both with respect to the 2007-2017 average), and Tair for the rest of the year was within average range for the site. Zero-crossing of cumulative NEE (i.e., when cumulative NEE first became negative), occurred on DOY 166, and over this year the peatland functioned as a net sink of atmospheric CO\({}_{2}\) (annual budget = \(-\)2.73 \(\pm\) 0.6 mol CO\({}_{2}\) m\({}^{-2}\) y\({}^{-1}\)). The year 2020 had a dry start to the growing season and was affected by the longest dry spell in the study period (84 days: 7 April-29 June 2020; Figure 3). This year also exhibited the shortest growing season, characterised by a prolonged green-up period (>20 days longer than estimated for the other years) and a short peak (ca. 60% shorter than for other  
Figure 3: Overview of hydrometeorological conditions over the study period: 2018, 2019, 2020 and 2021 (from top to bottom).