European beech forests on calcareous soil are endangered by climate change

moderately dry areas of the sub-mountainous altitude range in Central Europe (Ellenberg and Leuschner 1996). One third of the potential beech forest area is spread on calcareous soil (Fig. 4.1) that is highly susceptible to water deprivation. For future forestry in Central Europe, it has even been suggested to replace spruce by beech (Tarp et al. 2000;

Moosmayer 2002). However, the apparent drought sensitivity of beech is a current matter of concern and debate (Leuschner et al. 2001; Rennenberg et al. 2006, 2009; Geßler et al.

2007; Kreuzwieser and Gessler 2010), due to observations of increased heat waves and drought periods in wide regions of Central Europe (Coumou et al. 2013). This trend is expected to continue and intensify in the coming decades (Seneviratne et al. 2006; Smiatek et al. 2009). Based on statistical species distribution models driven by climatic predictors (Hanewinkel et al. 2013 a, b), we computed the distribution range of beech forests on calcareous soil in Europe and found drastic reductions by almost 80% under an SRES A2 scenario (IPCC 2000) until the year 2080 (Fig. 4.1). However, the physiological processes behind this dramatic biome shift on calcareous soil are rarely understood.

Two different mechanisms have been proposed to explain the sensitivity of beech to increased temperature and drought (Geßler et al. 2007; Rennenberg et al. 2009;

Kreuzwieser and Gessler 2010): (1) physiological limitations such as xylem embolism, restricted nutrient uptake capacity and reduced growth; and (2) impaired provision of bioavailable N by soil microbes. The latter mechanism may be of particular importance in calcareous soil, because Rendzic Leptosols derived from limestone are poor in bioavailable N (Dannenmann et al. 2006, 2009). Furthermore, these soils are characterized by a shallow profile, high gravel content, heavy texture, and low water and nutrient retention capacity.

Hence, N is frequently limiting growth of this forest type (Dannenmann et al. 2009;

Rennenberg et al. 2009; Simon et al. 2011; Weber et al. 2013).

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Figure 4.1: Modelled potential distribution of beech forests on calcareous soils in Europe (green colour) under current climatic conditions (left panel). These forests would cover an area of 31.4 million ha, i. e. approximately one third of the potential beech forests in Europe (104.0 million ha). For the SRES A2 scenario (IPCC 2000), we computed a potential distribution of 7.2 million ha in the year 2080 (right panel), i. e. a reduction to 22% of the current distribution.

Here we present an isotope-based experimental approach to simultaneously quantify all major N turnover processes in undisturbed beech seedling-soil-microbe systems, thereby maintaining plant-soil microbe interaction and –competition for N. By translocation of intact soil mesocosms containing natural beech regeneration across a narrow valley from the northwest (NW) the southwest (SW) aspect, we combined this approach with a space-for-time climate change experiment (Fig. 4.2). This treatment increased soil temperature on average by 1 °C (Fig. 4.3) and persistently decreased soil water availability over the entire growing season (Figs. 4.4, 4.5). The NW exposure corresponds to a model climate for present day conditions of many beech forests in Central Europe, while the SW exposure is considered a model for climatic conditions expected for the coming decades (Geßler et al.

2004). Additionally, a roof system accelerated drought during a 39 days period at SW aspect (Fig. 4.4). Supporting measurements included microbial N cycle gene abundances,

mycorrhizal colonization and N metabolite levels in fine roots as well as plant biomass and long-term isotope recovery in beech seedlings.

1

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Figure 4.2: Experimental design. The figure illustrates coring of beech-soil-mesocosms by use of stainless steel cylinders (16.4 cm inner diameter, 15 cm height) with subsequent pre-incubation for one year either under cool-moist microclimate at the coring site (NW exposure, control) or warm-dry microclimate (SW-exposure, climate change). After pre-incubation and equilibration, homogenous and reproducible labelling of the intact plant-soil-microbe systems (40 injections per beech-soil-mesocosm) with ¹⁵N/¹³C-enriched glutamine, ¹⁵N-ammonium (NH4

+) or ¹⁵N-nitrate (NO3

-) and subsequent double harvests (6 and 48 hours after labelling) were conducted twice. Analysis of total N and ¹⁵N enrichment in soil, microbial, mycorrhizal and plant N pools facilitated determination of simultaneously occurring gross N turnover rates in the plant-soil-microbe system in June (comparison of ambient NW vs. SW climatic conditions) and August (ambient NW conditions vs. roof-intensified drought at SW) via ¹⁵N tracing and pool dilution calculations, while plant-soil microbe interaction and competition for N persisted throughout the experimental incubation period. A final harvest of mesocosms labelled in June allowed investigating long-term isotope recovery in September. All three harvest dates were accompanied by determination of supporting soil and plant parameters such as microbial N cycle gene abundance, mycorrhization and ¹⁵N signature of mycorrhizal root tips, and plant metabolites.

Generally, the replication was n=8 for every harvesting date, exposure treatment, and labelling

treatment. Parallel harvests of unlabelled beech-soil-mesocosms (n=4 to 8) were conducted in order to quantify ¹⁵N natural abundance and background N concentrations (not shown in the graph).

2010 2011: 3 labelling/harvest cycles in vegetation period

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Figure 4.3: Soil temperature differences (5 cm depth) between beech-soil-mesocosms incubated at SW exposure (warm-dry microclimate, climate change treatment) and at NW exposure (cool-moist microclimate, control treatment). Data represent mean values of five temperature probes per treatment directly installed horizontally in soil of transferred beech-soil-mesocosms. Arrows indicate the three harvest campaigns. The period between the harvests in June and September equals the roof period of 39 days (see Fig. 4.2).

Figure 4.4: Dynamics of volumetric soil moisture in 5 cm depth (mean values of n=5 measurements) in intact beech-soil-mesocosms of the control treatment (NW exposure, cool-moist microclimate) and climate change treatment (SW exposure, warm-dry microclimate) in the growing season 2011, i. e. 1 year after implementation of treatments by transferring beech seedling-soil-mesocosms within NW exposure or to SW exposure in summer 2010. Arrows indicate harvest campaigns (see Fig. 4.2).

Mar 2011 Apr May Jun Jul Aug Sep Oct Nov

-1

Soil temperature SW - NW (°C)

May 2011 Jun Jul Aug Sep Oct Nov 30.11.2011

0.159

Volumetric soil moisture in 5 cm depth (m3 /m3 )

NW

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Figure 4.5: Gravimetric soil moisture related to water holding capacity (WHC) as determined from harvested labelled (n=48) and unlabelled (n=4 to 8) beech-soil-mesocosms during harvests in June (ambient conditions at both exposures), August (intensified drought at SW exposure due to roof) and September (final harvest). Asterisks indicate significant differences (p<0.05) between NW and SW exposure at the respective harvest. Different indices indicate significant differences between different harvesting dates and labelled and unlabelled beech-soil-mesocosms.

4.2.2 Climate change decelerates beech N nutrition as a consequence of impaired