Assessing various drought indicators in representing summer drought in boreal forests in Finland

Abstract

Droughts can have an impact on forest functioning and production, and even lead to tree mortality. However, drought is an elusive phenomenon that is difficult to quantify and define universally. In this study, we assessed the performance of a set of indicators that have been used to describe drought conditions in the summer months (June, July, August) over a 30-year period (1981-2010) in Finland. Those indicators include the Standardized Precipitation Index (SPI), the Standardized Precipitation-Evapotranspiration Index (SPEI), the Soil Moisture Index (SMI), and the Soil Moisture Anomaly (SMA). Herein, regional soil moisture was produced by the land surface model JSBACH of the Max Planck Institute for Meteorology Earth System Model (MPI-ESM). Results show that the buffering effect of soil moisture and the associated soil moisture memory can impact on the onset and duration of drought as indicated by the SMI and SMA, while the SPI and SPEI are directly controlled by meteorological conditions. In particular, we investigated whether the SMI, SMA and SPEI are able to indicate the Extreme Drought affecting Forest health (EDF), which we defined according to the extreme drought that caused severe forest damages in Finland in 2006. The EDF thresholds for the aforementioned indicators are suggested, based on the reported statistics of forest damages in Finland in 2006. SMI was found to be the best indicator in capturing the spatial extent of forest damage induced by the extreme drought in 2006. In addition, through the application of the EDF thresholds over the summer months of the 30-year study period, the SPEI and SMA tended to show more frequent EDF events and a higher fraction of influenced area than SMI. This is because the SPEI and SMA are standardized indicators that show the degree of anomalies from statistical means over the aggregation period of climate conditions and soil moisture, respectively. However, in boreal forests in Finland, the high initial soil moisture or existence of peat often prevent the EDFs indicated by the SPEI and SMA to produce very low soil moisture that could be indicated as EDFs by the SMI. Therefore, we consider SMI is more appropriate for indicating EDFs in boreal forests. The selected EDF thresholds for those indicators could be calibrated when there are more forest health observation data available. Furthermore, in the context of future climate scenarios, assessments of EDF risks in northern areas should, in addition to climate data, rely on a land surface model capable of reliable prediction of soil moisture. © 2016 Author(s)

Drought identification in the eastern Baltic region using NDVI

“…The Northern Hemisphere boreal forests usually grow in areas of excessive moisture and tree roots are deeper, so they react much slower to the precipitation deficit (such deficit may even lead to higher VCI values). Also, due to the high initial soil moisture, the drought impact on forested areas can be minimal (Gao et al, 2016). Only during the prolonged extreme droughts (e.g.…”

Response of water use efficiency to summer drought in a boreal Scots pine forest in Finland

“…A five-layer soil hydrology scheme was implemented in JS-BACH by Hagemann and Stacke (2015). Gao et al (2016) has demonstrated that JSBACH with its five-layer soil hydrology scheme is able to capture the soil moisture dynamics at sites and on the regional scale of Finland.…”

“…In this study, the soil moisture dynamics are represented by SMI (also referred to as Relative Extractable Water -REW), which has been demonstrated to represent summer drought in boreal forests in Finland (Gao et al, 2016). The SMI describes the ratio of plant available soil moisture to the maximum volume of water available to plants in the soil (Betts, 2004;Seneviratne et al, 2010):…”

“…The V max was set to be 37.5 and the Jmax was 71.3. The soil depth and root depth at the site were derived from maps for the regional JSBACH simulation presented in Gao et al (2016) (see also Hagemann and Stacke, 2015). Those parameter settings in the JSBACH site level simulation for the site are listed in Table 1.…”

“…Even though the occurrence of drought is low in northern Europe, the summer of 2006 in Finland was extremely dry and 24.4 % of the 603 forest health observation sites over entire Finland showed drought damage symptoms by visual examination, in comparison to 2-4 % damaged sites in a normal year (Muukkonen et al, 2015). According to the simulated regional soil moisture, the summer drought in 2006 in southern Finland was the most severe one over the past 30 years , and the spatial distribution of the drought damage has been found to be closely related to the plant available soil moisture (Gao et al, 2016).…”