Aim of this research project was to identify plant nutrient acquisition strategies under three levels of aridity: arid, Mediterranean, and humid. With the predicted precipitation changes in Chile, it is likely that abiotic conditions and biotic interactions in one ecosystem along the precipitation gradient shift towards the current conditions of another. Based on the results about plant nutrient acquisition strategies, an assessment is made in the following on potential shifts and responses of the arid and humid ecosystems.
1.4.1 Humid-temperate forest
The focus on nutrient reutilization from topsoil in the humid-temperate forest ecosystem (Study 1, Study 5) is an efficient strategy to recycle and reutilize nutrients and to reduce losses by leaching but might be a disadvantage under decreasing water availability. Decreasing precipitation and prolonged dry periods (as predicted by climate change scenarios for that region) will result in higher evaporation and more intensive dehydration of topsoil. Reduced water availability will not only impose water stress on forest plants but also restrictions on their nutrient supply. Low microbial activity and reduced nutrient mobility in dry soils will be a challenge for plants that are adapted to a fast recycling of nutrients in moist soils. Under these conditions deep rooting strategies will be favored over shallow rooting strategies (Knapp et al., 2008b; Wang et al., 2020).
Deep roots of the woody plants in the humid-temperate forest have the potential to access deep water sources, allowing them to avoid dehydrated topsoil, to recapture and uplift leached nutrients, and to maintain nutrient acquisition even under extensive dry periods (Nardini et al., 2016; Zhang et al., 2019). This is only possible if deep water pools are available and replenished during wet seasons or years. Previous-years precipitation was shown to strongly determine aboveground primary production of the subsequent year (Sala et al., 2012a). If multiple dry years occur consecutively, deep water pools might be eventually exhausted and deep rooting species will no longer survive the prolonged drought conditions (Kannenberg et al., 2019; Sala et al., 2012a).
The decrease of water availability will reduce the abiotic mineral dissolution (Belnap, 2011), and thus increase the need for plants to invest into biological weathering agents such as organic acids, to cover their demand of rock-derived nutrients. Thus, plants in the humid-temperate forest would have to intensify their investment into biological weathering agents under water shortage.
Microbial SOM decomposition and nutrient mineralization in dehydrated topsoils, however, are likely more strongly hampered by drought than mineral weathering in potentially moist depths.
The focus of woody plants in the humid-temperate forest on nutrient recycling from topsoil, therefore, would be a disadvantage. Additionally, under prolonged drought periods, where plant activity is suppressed stronger than microbial activity (Dijkstra et al., 2015; Knapp et al., 2008a;
Yahdjian et al., 2006), the decoupling between nutrient mineralization and plant nutrient uptake can lead to an accumulation of inorganic N in topsoils (McCulley et al., 2009; Reichmann et al., 2013), as observed at the site under Mediterranean conditions (Study 1). In subsequent wet periods, plant species that can (1) recover fast from drought (Knapp et al., 2008a) and (2) can exploit available resources fast in competition against other plants but also retain mobile inorganic N against leaching (Austin et al., 2004; Reich, 2014) are in an advantage. Conservative root traits, as expressed by A. araucana in humid-temperate forest ecosystem (Study 2), might be
advantageous or disadvantageous under these conditions, depending on the dominating factor.
Roots with conservative traits can sustain dehydration longer than acquisitive roots, reducing tissue damage and root dieback, and allowing a faster recovery from dry conditions than for plants with acquisitive root traits. The slow acquisition that is possible with ‘conservative’ roots, however, reduces the potential to retain a large pool of highly mobile N against leaching in topsoil, especially with increasing frequencies of concentrated rainfall events (Knapp et al., 2006;
Yahdjian and Sala, 2010). Increased leaching of inorganic N from topsoil even though total precipitation decreases would, therefore, favors deep rooting species that are able to recapture leached nutrients (Yahdjian and Sala, 2010).
AMF support plant nutrient and water acquisition and can, thereby, alleviate environmental stress for plants. The current AMF community, however, is not adapted to prolonged drought or to largely retain mobile nutrients against leaching losses as observed under Mediterranean conditions (Study 2). Therefore, they might not be able to provide these functions for their host plants (Hawkes et al., 2011; Millar and Bennett, 2016).
1.4.2 Arid shrubland ecosystem
The effects of precipitation variability on fine root biomass were shown to be higher in dry ecosystems and decreased with increasing mean annual precipitation (Wang et al., 2020). Heavy rainfall events were proposed to compensate for a reduction of total precipitation in arid regions (Knapp et al., 2015, 2008a; Kulmatiski and Beard, 2013). Greater water availability despite a decrease of total precipitation allows plants with a deeper root system to increase their photosynthetic activity (Hsu et al., 2012; Huxman et al., 2004) and to invest more C in belowground processes for nutrient acquisition. This could accelerate SOM decomposition and nutrient mineralization under greater water availability (Dijkstra et al., 2015), and might increase OM-derived nutrient availability for plants with deeper root systems as G. resinosa in the arid shrubland, which was shown to acquire nutrients uniformly from the whole soil profile (Study 1).
Yahdjian and Sala (2010), however, described that larger water pulses increase N losses by nitrate leaching and gaseous losses by denitrification. With greater mineralization and thus mobility of inorganic N in soil, N leaching could increase depending on the magnitudes of rainfall events (Knapp et al., 2015). Plants in the arid shrubland are not adapted to exploit available resources fast and are likely not able to retain nutrients against leaching (contrary to plants in the Mediterranean coastal matorral), because they express neither acquisitive root traits nor have an extensive (fine) root system (Study 2).
AMF in the arid shrubland did not show a connection to plant N nutrition, whereas in the Mediterranean coastal matorral, characterized by high denudation rates, AMF seemed to support
plants in retaining N against losses from soil (Study 2). Whether AMF in the arid shrubland are also able to provide this ecosystem function is unclear, as AMF in arid regions are not adapted to a high nutrient mobility in soil and to retain nutrients against losses.
If the magnitude of large precipitation events does not lead to run-off and nutrient loss from surfaces, shrub plants have good prerequisites to cope with changing precipitation variability (Knapp et al., 2008b; Kulmatiski and Beard, 2013), since they have deep roots and are adapted to acquire nutrients not only from topsoil, but also from subsoil and saprolite (Study 1). Increased run-off and erosion, and thus nutrient losses from topsoil, however, would likely favor fast growing, acquisitive plant species that concentrate their nutrient acquisition in the upper, nutrient-richer soil and are more efficient in retaining nutrients within the soil. The overall precipitation decrease, however, will eventually favor slow growing, conservative plant species.
The required shifts of nutrient acquisition traits in both ecosystems evaluated here will have to go along with at least a partial shift of species compositions and functional groups (regarding not only plants but also their symbionts such as AMF). Many of the traits beneficial under the predicted climate change scenarios exist in the Mediterranean ecosystem and, thus, are in principle available along the Chilean Coastal Cordillera. The magnitude and temporal dynamic of the climate change and further factors such as plant-plant interactions and plant community stability (Hallett et al., 2014; Lloret et al., 2012; Ploughe et al., 2019), however, will be decisive for whether and to which extent species with the required traits can migrate from the Mediterranean area and immigrate into the adjacent regions. This study showed that the investigation of ecosystems along a climate sequence with similar parent material allows to evaluate the "toolbox" of available nutrient acquisition properties under different climatic conditions in a region. The knowledge of available traits allows to estimate possible and necessary shifts to maintain the functionality of ecosystems.
This knowledge can improve and refine the predictions of ecosystem responses to climatic change.