Quantifying the Impacts of 23-years of Warming and Drying Trends on Peatland/Upland Succession in Central Alberta

English Abstract:

In this study we compare peatland ecosystem resilience (stability following climatic perturbations) using an index of evapotranspiration (ET) (EI) (actual ET/ precipitation) vs. dryness index (DI) (potential ET / precipitation)) and water yield within six heterogeneous upland/lowland catchments in the increasingly arid Central Boreal Plains (CBP) region of north-central Alberta from 1986-2009. The impacts of increased atmospheric drying over time within energy limited (DI < 1) and water limited (DI > 1) ecosystems and their resilience (or elasticity) is quantified using time series slope and residual analysis of the Normalize Difference Vegetation Index (NDVI) from Landsat TM combined with SRTM (Shuttle Radar) topographic positioning. This allows us to determine if ecosystems are experiencing long-term browning (declining NDVI trends), greening (increasing NDVI trends), resilience (recovery) or threshold (tipping point) change in boreal regions where NDVI is used as a spatio-temporal proxy indicator of ecosystem trajectory following annual atmospheric drying and catchment runoff. We find that catchments with low (detrended) topographic variability containing mostly peatland land cover types are highly resilient due to abundant water supply. Despite their resiliency, these catchments are characterised by declining water yield over the period of study and increased greening via succession of woody shrub and tree biomass into ~76% of peatlands. This indicates rapid loss and conversion to riparian/forest land cover types as peatlands dry. Meanwhile, catchments containing greater topographic variability have experienced increased water yields and declining NDVI trends during the same period. These catchments are typically water limited, highly sensitive to atmospheric warming and as such, have experienced significant browning (loss of chlorophyll, leaf cover) within 74% of upland treed and shrub peatlands with lagged recovery. A likely explanation for increased catchment yield coincident with reduced green foliage cover is a reduction in ET losses from upland land covers. The results of this study indicate that wide-spread browning trends observed in parts of the central boreal forest are complicated by energy-/water-limited systems, spatio-temporal lags and underlying geology.