Extreme climate events such as drought can influence forest dynamics and productivity (Breshears et al. 2005, McDowell et al., 2008). However, it remains unclear under which circumstances trees can maintain functioning and at what stage mortality begins. Therefore, further research is needed to determine species-species tipping points in the face of drought (Allen et al., 2010), for which more evidence is available on dry sites (Etzold et al. 2016).
This analysis will focus on drought effects on European beech (Fagus sylvatica L.) and silver fir (Abies alba), two species that are widely distributed and economically important in Switzerland. In the case of silver fir, there is evidence that it can sustain prolonged summer drought in the Mediterranean area (Tinner et al. 2013). However, our observations on such sites let us hypothesize that this is only the case in deep and moist soils. The research question this analysis will address is: how did beech and silver fir stands respond to the summer drought in 2018 on moderately dry to xeric sites?
The planned work will be a prestudy to a larger project that aims to assess drought effects on adult and recruiting beech and silver fir trees, with a focus on demographic change caused by mortality and physiological responses on site gradients. The project will exploit existing permanent plots.
In this prestudy, the first step will involve finding suitable sites for the analysis. Here we will draw upon permanent plots within the Forest Reserve Network (minimum dbh 4 cm) and the Experimental Forest Management network (minimum dbh 8 cm), and potentially also other long-term plots (LWF) to find a gradient of sites containing beech and fir in the moderately dry to xeric range. The forest reserve database will then be analysed to examine historic trends of the two species in terms of tree stem growth, mortality and recruitment, and relate them to drought periods.
Moreover, appropriate plots will be visited in early 2019 to assess i) individual tree condition (dbh, vitality, needle length) following the summer drought of 2018, and ii) site factors, in particular soil properties to model soil moisture where this information is lacking.