Examining the peatland shrubification‐evapotranspiration feedback following multi‐decadal water table manipulation

Abstract Northern peatlands are globally important long‐term sinks of carbon due to their predominantly saturated conditions. However, these ecosystems are expected to become drier with climate change, potentially leading to shrubification. As such, the response of the shrubification–evapotranspiration (ET) feedback may be of critical importance to future peatland energy, water and carbon dynamics. We examined the effect of multi‐decadal peatland water table (WT) alteration at three adjacent sites with increasing depth to WT (WET, INTermediate, and DRY). In order to better understand the WT–shrubification–ET feedback, we measured peatland vegetation composition, microtopography and ET partitioning, where ET was measured at the ecosystem, microform, and leaf level using eddy covariance (EC), chambers and porometry, respectively. Averaged across microforms and WT treatments, there was a difference in the median measured leaf resistance ( r leaf ) between plant functional types ranging from 213 s m −1 for erect dwarf shrubs, 325 s m −1 for graminoids/sedges, and 520 s m −1 for prostrate dwarf shrubs. Scaled based on LAI, the low r leaf of erect dwarf shrubs dominated hummocks, where sites with a higher proportion of hummocks had lower median canopy resistance ( r v ) of 141, 133 and 130 s m −1 at the WET, INT and DRY sites respectively. Nevertheless, ET was highest at the WET site and similar between the INT and DRY sites, with greater evaporation from the moss surface at the WET site. Porometry and EC data along with a three‐source model were used to independently assess the evaporative contribution from the moss surface, which ranged from 17% to 40%. For moderate and persistent changes in WT from land‐use or climate change, our results suggest vegetation succession is minimal, but the microtopographic development and the concomitant differences in LAI for the various plant functional types is key to understanding changes in total ET and partitioning.