Persönlicher Status und Werkzeuge

Sprachwahl

Extreme drought and progressively limiting water reserves do not only pose huge challenges on mankind, but also on forest ecosystems. In which ways do trees meet such challenges? This question is jointly explored by forest scientists and biologists of the Technische Universität München (TUM) and the Helmholtz-Zentrum München by means of the »Kranzberg Forest Roof Experiment« (KROOF).

The “Kranzberg Roof Project” (KROOF) has been initiated in 2013 and received funding by DFG as well as by the Bavarian State Ministry for Nutrition, Agriculture and Forestry and to the Bavarian State Ministry for Environment and Consumer Protection.

Project phase 1 was to explore the growth potential of European beech (Fagus sylvatica) and Norway spruce (Picea abies) in mixture relative to monocultures under exacerbating summer drought, as predicted for upcoming climate change in Central Europe (Leuschner 2009; Allen et al., 2015). Paramount motivation was the widely observed over-yielding phenomenon of mixtures surpassing productivity of adjacent beech and spruce monocultures within gradients towards resource-poor sites (Pretzsch 2005; 2009; Grossiord et al., 2014). Compliance exists with the “Stress Gradient Hypothesis” (SGH) that claims competition between neighboring species to turn to facilitation (i.e. to mutualistic synergies: (Bertness and Callaway, 1994; Callaway, 2013), given spatio-temporal complementarity in resource use (Pretzsch et al., 2010; del Rio et al., 2014). In line also is the “Optimal Partitioning Theory” (Hertel et al., 2013; McCarthy and Enquist, 2007), with favored allocation belowground under water and nutrient limitation versus aboveground under shading. Facilitation under water limitation might be a pathway to over-yielding (Pretzsch et al., 2014a).

Understanding processes towards mutualistic biotic interactions that foster tree performance under severe stress is crucial for mixed-species forest management in anticipating exacerbating drought and developing management practices of stress mitigation. Beech and spruce are species of concern (Metz et al., 2016; Scherer-Lorenzen et al., 2005), doubted to withstand drought under climate change (Geßler et al., 2007; Zimmermann et al., 2015) but being crucial in Central Europe in ecological and economical terms (Ellenberg and Leuschner, 2010). Complementarity in resource is likely, given the contrasting deciduous and evergreen foliages (Richards and Schmidt, 2010) and differential stomatal responsiveness i.e. indicated isohydric stomatal behavior in spruce relative to more anisohydric strategy in beech (Klein, 2014; McDowell et al., 2008). Isohydry would conserve water during drought by stomatal disruption of gas exchange, while evidence for C starvation is scarce if not absent (Hartmann et al. 2013; Körner 2015). Conversely, anisohydry would sustain water use with risk for hydraulic collapse. Such definitions focusing on stomatal regulation underline the need for conceptual widening and understanding isohydry/anisohydry as physiological whole-tree/system syndromes. Such an extended view is pursued by KROOF.

Project phase 1 initiated the integrated KROOF approach based on experimental throughfall exclusion (TEE) at Kranzberg Forest with novel, rain-controlled roof closure and on five study sites of a precipitation gradient (PGR) across Bavaria, with triplets of beech-spruce mixtures and monocultures (Pretzsch et al. 2014a; see appendix). Two overarching hypotheses were tested:

H I     Drought stress limits the water consumption of Norway spruce more severely than that of European beech, so that the latter benefits from water re-partitioning in mixture.

H II    Increasing water limitation along a precipitation gradient drives the competitive strength of European beech relative to Norway spruce.

General achievements of project phase 1: Through the TEE, spruce and beech have been examined for contrasting water use and partitioning, i.e. isohydry versus anisohydry (see experimental design in appendix and Pretzsch et al. 2014a). The impact of root trenching in 2010 on tree growth was shown to be overcome by the beginning of the TEE in 2013 (Pretzsch et al., 2016). Throughfall exclusion (TE) was conducted from mid-March through mid-December in 2014, 2015 and 2016, nearly depleting the plant-available soil water (depletion of TE >> control and in spruce > beech). When adjacent to beech, spruce’s fine-root production and water uptake shifted towards shallow soil horizons prone to drought (Goisser et al., 2016). The severe natural drought in 2015 incited bark beetle attack across entire Kranzberg Forest, including the TEE site, where a total of 8 spruce trees on plots 2 and 10 had to be removed (reducing spruce study trees to 40). Starting in 2015, bark beetle damage was confined through annually spraying (contact insecticide Karate Forst liquid) to spruce crown and stem surfaces, using the canopy crane.

 

Overall, lowered fine root production and ramification resulted in decreased soil exploitation in spruce when growing together with beech, supporting H I (Part C). Ectomycorrhizal (ECM) community composition was changed and number of vital mycorrhizae was similarly reduced by drought in spruce and beech. Species mixture hardly counteracted drought. Enzyme activity (EA) profiles of ECM communities were qualitatively maintained under drought but total EA per soil volume strongly decreased due to the decline of vital fine roots. Corresponding reductions in leaf N concentrations, however, were not yet observed. Spruce distinctly decreased stomatal conductance under drought, along with sap flow and net CO2 uptake rate (Part B). Stomatal control of spruce followed a more isohydric strategy under TE relative to beech supporting H I. Phloem transport velocity in beech was not impeded by drought. However, the retention time of assimilates in beech leaves was enhanced. Notwithstanding, species interaction effects were absent in leaf gas exchange and stem growth during the 6-week summer drought in 2013 (Goisser et al., 2016). Part A revealed the initial three years of TEE to decrease growth of spruce more distinctly under drought than of beech thus further supporting H I, as mixing effects, however, need further investigation. Along PGR, diameter increments of beech were higher during dry 2015 compared to 2014, although spruce distinctly reduced stem diameter growth under drought, particularly on moist sites. Drought-related increment variations between trees in pure and mixed stands were higher in beech than spruce. PGR showed group-wise species mixture to enhance competition by beech along group edges of spruce. Hence, beech competitiveness for water stayed limited and group-wise beech/spruce mixture is concluded as the favorable silvicultural option in anticipating climate change (Goisser et al., 2016). Regarding H II, increase in beech competitiveness resulted in enhanced stand-level productivity, as spruce lost superiority in growth. Hence, H II was rejected.