Project Framework and study areas

The project was conducted within the framework of the German-Chilean DFG priority program

‘EarthShape – Erath Surface Shaping by Biota’, which investigates the “influence of plant, animals, and microorganisms on the formation of soils and the shape of topography (earthshape.net)”. The Chilean Coastal Cordillera was chosen as a study region, because sites could be selected here with similar granitoid parent material along a precipitation gradient from (hyper)arid to humid. The whole gradient was glaciation free during the last glacial maximum and well-documented climate change records exist. The four study areas studied in this thesis are located in the Cordillera from 29° to 38° southern latitude and cover a >900 km long precipitation gradient from 80 mm a^-1 mean annual precipitation (MAP) in the north to >1500 mm a^-1 in the south (Fick and Hijmans, 2017).

Figure 1.1-2: Overview of the aspects investigated in objective 1 (nutrient availability), objective 2 (plant nutrient recycling and uplift and plant resource economics), and objective 3 (agents and symbionts for plant nutrient acquisition).

Figure 1.2-1: Study site overview. Showing mean annual precipitation (MAP) and mean annual temperature (MAT) along the gradient (WorldClima data Version 2, Fick and Hijmans et al., 2017). Study site locations indicated by triangles from north to south: arid shrubland, coastal matorral, Humid-temperate forest. Adapted from Stock et al.

(2019).

The northern sites are classified as arid ecosystems (‘arid shrublands’) with an aridity index of 0.06 and 0.05 (Trabucco and Zomer, 2018). The first shrubland site is located in a grazing exclusion area in Quebrada de Talca (30.05 S, 71.09 W), at an altitude of 645 m a.s.l. and in 23 km distance from the Pacific Ocean. The second arid shrubland is located in the Reserva Santa Gracia (29.76 S, 71.14 W) at an altitude of 680 m a.s.l. and in 23 km distance to the Pacific Ocean as well. Both arid shrublands resemble each other in their environmental conditions, with an mean annual precipitation (MAP) and mean annual temperature (MAT) of 80 mm and 13.8 °C (Fick and Hijmans, 2017). Cambisols (pH 5.5-7.0) are 30-40% covered with vegetation, which is dominated by drought-deciduous shrubs and cacti.

The third site along the precipitation gradient is classified as a Mediterranean ecosystem (‘Mediterranean coastal matorral’) with an aridity index of 0.24 (Trabucco and Zomer, 2018), and represents the intermediate level of the three aridity intensities. The site is located in the National Park La Campana (32.96 S, 71.06 W) at approx. 70 km northwest of Santiago and 43 km landwards, at an altitude of 730 m a.s.l. and with a MAP and MAT of approx. 400 mm and 13.1 °C (Fick and Hijmans, 2017). The vegetation covered 100% of the Cambisols (pH 4.5-6.1), dominated by evergreen-sclerophyllous trees, deciduous shrubs, and a dense herb layer (Bernhard et al., 2018).

The southernmost and fourth site is classified as a humid ecosystem (‘humid-temperate forest’) with an aridity index of 1.4 (Trabucco and Zomer, 2018). The site is located in the National Park Nahuelbuta (37.81 S, 73.01 W) at 1240 m a.s.l. elevation and in 55 km distance to the Pacific Ocean, with a MAP and MAT of >1500 mm and 7.4 °C (Fick and Hijmans, 2017). Vegetation here covered 100% of the Umbrisols and Podzols (pH 3.7-5.1), dominated by the evergreen

conifers Araucaria araucana (Mol.) K. Koch and winter deciduous broadleaved Nothofagus spp.

trees, with a rich understory. From north to south, soils developed all on similar granitoide parent material. The extent of the soil profiles (including A, Bw, and BCw horizons) varied markedly between sites. Under the arid shrubland in the north, the transition from BCw horizon to the underlaying weathered rock is located around 50 cm soil depth, while under the Mediterranean coastal matorral, the transition lies at around 80 cm depth (Bernhard et al., 2018). Under humid-temperate forest in the south, the transition from BCw horizon to saprolite occurs around 100 cm depth (Bernhard et al., 2018). Thickness of the A horizon varies as well, with 10-20 cm under shrubland, 10-40 cm under woodland, and 30-50 cm under forest (Oeser et al., 2018). For a detailed site description, please refer to Bernhard et al. (2018) and Oeser et al. (2018).

Total precipitation (mm) and frequency (i.e. the number of days with precipitation) from March 2016 till November 2016 (time of the conducted field experiments) are given in Figure 2.1-1 and were recorded by the EarthShape weather stations (Ehlers et al., 2017) for the arid and Mediterranean sites. As the project weather station in the National Park Nahuelbuta was only installed in November 2016, precipitation data were derived from the Center for Climate and Resilience Research (CR)² (2020) for the station Parque Nahuelbuta (37.8233 S, 72.9606 W).

1.2.1.1 Predicted precipitation shifts

In the past century precipitation patterns in Chile changed notably. In a meta-analysis of pre-published data from 271 stations between 26°00’S and 56°30’S, a trend of precipitation decrease from 1960 to 2000 was detected in the Coquimbo and Valparaiso regions (29°20’S to 33°57’S) (Valdés-Pineda et al., 2014), where the arid shrublands and Mediterranean coastal

Figure 1.2-2:

Temperature and precipitation changes over Central and South America from MMD-A1B simulations (MMD:

multi-model data set archived a the Program for Climate Model Diagnosis and Intercomparison PCMDI; SRES A1B:

emission scenario with 850 ppm CO2 in atmosphere in 2100).

From left to right: annual mean, DJF (austral summer), JJA (austral winter). Top row: Temperature (top row) and precipitation (bottom row) changes between 1980 to 1999 and 2080 to 2099, average over 21 models. Modified from Christensen et al. (2007) Figure 11.15.

matorral are located. In the fourth IPCC Assessment Report (IPCC, 2007) are pronounced temperature increases and precipitation decreases of up to 50% compared to the reference period 1980-1999 predicted for 2080-2100 in the region from ~25°S to 45°S, encompassing also the southernmost study site of the humid-temperate forest (Figure 1.2-2; Christensen et al., 2007).

Regional climate models (Figure 1.2-3) predicted as well that precipitation will decrease markedly (>50% compared to 1960-1990) in the region of BioBío and Araucanía, where the humid-temperate forest studied here is located (Garreaud, 2011). The regional climate models project also a decrease of precipitation in the Mediterranean and arid northern part of Chile up to the Atacama Desert, but it is expected to be less pronounced than in the Chilean South (Garreaud, 2011). By 2100, an aridification of the region where the Mediterranean coastal matorral site is located is expected to be uncertain to likely (Klausmeyer and Shaw, 2009).

Not only is a general decrease of precipitation predicted for Central Chile, but also are precipitation patterns expected to become much more variable (IPCC, 2014, 2007; Young et al., 2010), resulting in a concentration of precipitation in few events with longer dry periods in between (Knapp et al., 2008a). In the last decade, for example, was an unusual and unprecedented long period of precipitation reduction recorded from 2010 to 2015 between 30-38° S, which was framed as the

‘megadrought’ (Garreaud et al., 2017). These variations can, for example, be caused by changes in the El Niño/La Niña-Southern oscillation (ENSO) (IPCC, 2012; Sala et al., 2015).

During El Niño episodes, precipitation increases above average between 30°S and 35°S in winter (JJA), but increases in late spring (ON) between 35°S and 38°S (Montecinos and Aceituno, 2003).

During La Niña, patterns are opposite. Changes in the El Niño/La Niña-Southern oscillation (ENSO) lead to increased climate variability and could increase the frequencies of extreme events.

The occurrence of ENSO events, however, is naturally highly variable and no consistent

Figure 1.2-3: Mean annual precipitation change [%] for the period 2070-2100 with 1960-1990 as reference. The change was normalized to the average of the current precipitation. Modelled with PRECIS-DGF (Providing REgional Climate Impact Studies) under the assumption of severe greenhouse gas emissions in the coming decades (SRES emission scenario A2 with 1250 ppm CO2 in atmosphere in 2100). *The region of the Atacama Desert is shown in gray as the precipitation there is (almost) 0 mm.

Taken from Garreaud, (2011) Figure 9. Black stars indicate the approximate study site locations.

projections of changes could be made so far (IPCC, 2012; Young et al., 2010). While some models predict an increase in ENSO variability and frequency, others predict a reduction (van Oldenborgh et al., 2005). It was reported , however, that the frequency and intensity of ENSO events have increased since the 1970, which in turn leads to a decrease in frequency but an increase in intensity of rain events (Carrasco et al., 2005; Young et al., 2010). Especially such prolonged drought periods can compromise the continuity of ecosystems that rely on deep water pools when topsoils are dehydrated, if those deep pools are not replenished sufficiently.