## 4 Discussion
### Implications for Peatland Ecosystem Carbon and Water Functions  
It is widely debated whether _Sphagnum_-dominated peatlands will transition to carbon sources as _Sphagnum_ undergoes water stress as a result of climate change, or if _Sphagnum_ may become more productive with longer growing seasons [19; 28; 45; 46; 47; 48]. Despite the possibility of longer growing seasons as the climate changes, several studies have found that water is the constraining factor in _Sphagnum_ productivity [13; 45; 49]. In agreement, our study found that _Sphagnum_ productivity will be limited when water is limited. Our study supports the prediction that _Sphagnum_ dominated peatlands are likely to transition to carbon sources as the climate continues to change and droughts become more frequent or intense. However, our findings also provide hope that peatlands--and specifically hummock forming species of _Sphagnum_ moss--are resilient to some extent.  
This study draws direct connections between _Sphagnum_ traits and peatland hydrology and carbon cycling. Although it is no groundbreaking discovery that hummock masses are able to better tolerate water stress than hollows [11; 19; 50], in this study we identified differences in _Sphagnum_ landform types, observing a new undocumented form of \"mirebreathing\", and identified which specific moss traits may enhance the resistance of carbon cycles under water stress. Taken together, this information improves our understanding of how peatlands will respond to altered hydrology.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/wl4203239/s1](https://www.mdpi.com/article/10.3390/wl4203239/s1)]([https://www.mdpi.com/article/10.3390/wl4203239/s1](https://www.mdpi.com/article/10.3390/wl4203239/s1)). Figure S1. Time series of change in peat height and CO\({}_{2}\) fluxes for the 8 different treatment levels. Please note that the time series for the change in peat height begins 5 days before the time series of CO\({}_{2}\) fluxes because the watering and water table treatments were initiated shortly before the CO\({}_{2}\) flux measurements. The lines represent the mean for each treatment, data points and error bars are not shown because the plot would not be readable. Significant differences are discussed in the main text. Figure S2. Spectral water content (Wetness Index) and photosynthetic capacity (NDVI) determined at the end of the study using a handheld spectroradiometer. The wetness index was significantly reduced water tables in Hollows, and by both reduced water tables and the interaction of reduced water tables and drought in hummocks, while the chlorophyll index was lowered by reduced water tables in hollows, but only by the combination of reduced water tables and drought in hummocks (see Table S1). Table S1. Linear mixed effects models evaluating water table (WT), rainfall, landform (hummock or hollow) and their interactive effects on two hyperspectral indexes of peat surface moisture and plant/Sphagnum moisture stress. See [36] for details.  
**Author Contributions:** Conceptualization, A.O., C.T., E.S.K.; methodology, A.O., C.T., E.S.K.; formal analysis, A.O., C.T.; writing--original draft preparation, A.O., C.T.; writing--review and editing, A.O., C.T., E.S.K.; funding acquisition, A.O., E.S.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the USDA Forest Service Northern Research Station Climate Change Program, the National Science Foundation (DEB-1146149), and the Michigan Technological University College of Forest Resources and Environmental Sciences.  
Data Availability Statement: Data will be made available using Github prior to publication of this manuscript.  
We would additionally like to thank the NRS Peatland Reading Group for providing helpful comments and suggestions on an early draft of this manuscript.  
The authors declare no conflict of interest.