# Inter-Annual Variability of Peatland Vegetation Captured Using Phenocam- and UAV Imagery
## 4 Discussion
### Impact of Dry Spells  
The present study highlighted the strong impact of weather anomalies on peatland vegetation and ecosystem functioning. We observed that years with prolonged or frequent dry spells functioned as a net source of CO\({}_{2}\) to the atmosphere, which is in contrast with the long-term net uptake measured at the site [79; 91]. Numerous studies have linked drought conditions to a short-term reduction in peatland C uptake [17; 33; 79]. The associated lowering of WTD, which is a key control on GPP [91; 111], can induce vegetation stress and limit CO\({}_{2}\) uptake by canopy photosynthesis (e.g., due to stomatal closure). In particular, our results have highlighted the importance of the timing and duration of dry spells. When dry conditions occurred during green-up (as in the year 2020), we observed a slower development of the vegetation canopy, which causes a substantial delay in cumulative NEE zero-crossing date. Consequently, the peak growing season begins later and is shorter than normal, meaning we observe a reduced period of optimum canopy development and maximum rates of photosynthesis. Changes in the timing and duration of plant canopy development likely made a substantial contribution to the positive annual NEE budget recorded during our study in the year 2020 (i.e., net loss of CO\({}_{2}\)). In contrast, the favourable hydrometeorological conditions during green-up 2021 allowed the canopy to green strongly and rapidly, meaning that the vegetation canopy was fully developed earlier, with a high photosynthetic capacity. Here, we hypothesise that the frequent dry spells experienced during the summer and autumn of this year induced early senescence. The importance of timing on the impact of drought events has been reported previously by [PERSON] et al. [17] for a temperate Swedish peatland. The authors found that a short, severe drought during the peak growing season, when the vegetation canopy was fully developed, had no significant impact on GPP. However, a longer drought period during spring green-up (when the canopy was developing), resulted in a reduction in GPP rates. Studies suggest that during this stage of canopy development, GPP shows the strongest sensitivity and is the least resilient to drought [17; 32]. Similar results have been found in response to heatwaves, with delayed greening observed in an alpine grassland and a 40% reduction in phenocam canopy greenness [42]; which is of similar magnitude to what the present study observed with the UAV data for Dryspell-2.  
Finally, the community-level response to drought in peatland ecosystems is influenced by species composition (e.g., refs. [112; 113]). Vascular plants, with their developed root systems, are generally more tolerant to the effects of periodic drought than _Sphagnum_ spp., for example, which have no root systems (e.g., ref. [114]). In the present study, we employed the spatially referenced UAV data to highlight the differing response of the mapped species to dry conditions (Figures 7 and 8). We found that _M. caerulea_ and _E. vagnatum_ with their deep root systems [115; 116] appeared less sensitive (i.e., exhibited a smaller or no reduction in greenness) to the dry conditions compared to species with shallow (e.g., _J. effusus_[117]) or no roots (e.g., _Sphagnum_ spp., _P. commune_). Although not possible with our employed setup, pixel-level analysis can also be conducted on phenocam imagery, and used to examine variability across plant functional types (e.g., Ref. [42]). These two monitoring approaches hence allow fine-scale heterogeneity in species response to drought to be quantified, and can be used to detect changes in ecosystem functioning not possible with the coarser resolution of satellite data. Such research is key for monitoring the response of peatland ecosystems to future change.