Climate response of above- and belowground productivity and allocation in European beech Dissertation zur Erlangung des mathematisch-naturwissenschaftlichen Doktorgrades „Doctor rerum naturalium“ der Georg-August-Universität Göttingen im Promotionsprogramm Biologie der Georg-August University School of Science (GAUSS) vorgelegt von Hilmar Müller-Haubold aus Linnich Göttingen, 2014 Betreuungsausschuss Prof. Dr. Christoph Leuschner, Abteilung Pflanzenökologie und Ökosystemforschung, Universität Göttingen Prof. Dr. Markus Hauck, Abteilung Pflanzenökologie und Ökosystemforschung, Universität Göttingen Dr. Dietrich Hertel, Abteilung Pflanzenökologie und Ökosystemforschung, Universität Göttingen (Anleiter) Mitglieder der Prüfungskommission Referent: Prof. Dr. Christoph Leuschner, Abteilung Pflanzenökologie und Ökosystemforschung, Universität Göttingen Korreferent: Prof. Dr. Markus Hauck, Abteilung Pflanzenökologie und Ökosystemforschung, Universität Göttingen Weitere Mitglieder der Prüfungskommission Prof. Dr. Hermann Behling, Abteilung Palynologie und Klimadynamik, Universität Göttingen Prof. Dr. Erwin Bergmeier, Abteilung Vegetationsanalyse und Phytodiversität, Universität Göttingen Prof. Dr. Michael Bredemeier, CBL - Sektion Waldökosystemforschung (SWF), Universität Göttingen Prof. Dr. Dirk Hölscher, Abteilung Waldbau und Waldökologie der Tropen, Universität Göttingen Tag der mündlichen Prüfung: 16.07.2014 Table of contents CHAPTER 1 9 General Introduction CHAPTER 2 29 Material and methods CHAPTER 3 39 Climate responses of aboveground productivity and allocation in Fagus sylvatica: a transect study in mature forests CHAPTER 4 71 Climatic drivers of mast fruiting in European beech and resulting C and N allocation shifts CHAPTER 5 111 Fine root biomass and dynamics in beech forests across a precipitation gradient – is optimal resource partitioning theory applicable to water-limited mature trees? CHAPTER 6 153 Synthesis CHAPTER 7 173 Summary CHAPTER 8 179 Appendix List of abbreviations AWSC Available water storage capacity of the soil C Carbon Ca Area-based carbon concentration Cm Mass-based carbon concentration CEC Cation exchange capacity DBH Diameter at breast height LAI Leaf area index MAP Mean annual precipitation MAT Mean annual temperature N Nitrogen Na Area-based nitrogen concentration Nm Mass-based nitrogen concentration NPP Net primary production NPPa Aboveground net primary production P Precipitation R:S Root per shoot biomass ratio Rad Solar radiation RAI Root area index REW Relative extractable water RSVI Relative stem volume increment RSVIa Relative annual stem volume increment RTD Root tissue density SLA Specific leaf area SRA Specific root area SRL Specific root length SVIa Annual stem volume increment T Temperature VWC Volumetric water content WSC Water storage capacity WUE Water use efficiency ΣfSUT Fraction of fine-grained soil particles < 200 µm ΣUT Fraction of fine-grained soil particles < 63 µm Chapter 1 General Introduction CHAPTER 1 Background Global and regional climate change Human activity over the past 250 years has increased the amount of greenhouse gases in the atmosphere. Since 1750, atmospheric concentrations of CO2 have risen from < 280 ppm to 393 ppm in 2012 (Le Quéré and others 2013). Also the concentrations of several other greenhouse gases, such as methane (CH4) and nitrous oxide (N2O) are increasing as a result of (agro-) industrial activities (IPCC 2013). Elevated concentrations of atmospheric greenhouse gases have changed Earth’s climate, raising the globally averaged combined land and ocean surface temperature by 0.85 ° C between 1880 and 2012 (IPCC 2013). Current models suggest an increase in global temperature by 3.2 – 5.4°C above the mean temperature (1850 – 1900) by the end of the 21st century (IPCC 2013). These changes will very likely cause large impacts on the global hydrological cycle (Huntington 2006; Gerten and others 2007). However, alterations of temperature and, even more, of precipitation will be largely subject to regional and seasonal variations (Klein Tank and others 2002; Brunetti and others 2012). Most climate change scenarios for Central Europe predict a rise in mean annual temperature by 2.5 – 3.5 °C until the end of the 21st century as well as increasing frequency and raised intensity of summer heat waves (Rowell and Jones 2006; Fischer and Schär 2009). Projections of climate change on regional scale for Northern Germany are similar to those referring to Central Europe (Jacob and others 2008; Moseley and others 2012). Concurring shifts of temperature and precipitation will likely result in a substantial aggravation of the climatic water balance during the vegetation period in Germany and many parts of central and southern Europe (Kundzewicz and others 2006; Fischer and others 2012). Responses of plants, populations and species to climate change These, on evolutionary time scale, abrupt changes in growing conditions pose a major threat to present plant populations (Walther and others 2002; Parmesan 2006). The capacity of plants to cope with such radical changes basically rest upon three reaction types with differences regarding the spatial and temporal scale: phenotypic plasticity (acclimation), genotypic evolution (adaptation) and changes in distribution (migration) (Anderson and others 2012). As acclimative responses to environmental changes, plants may alter their physiological, phenological, growth and allocation behaviour by variations of gen expression and 10 General Introduction metabolism within species-specific limits. Adaptation alters the potential of plants to acclimate to environmental variations via micro-evolutionary processes on population level, and is therefore regarded as a key factor for a successful adaption of plants to climate change (Bradshaw and others 2006). However, adaptation processes involve genetic changes and therefore typically require several generations to be put into effect. Comparisons of the historical and current distributions of many species suggested their relationships with climate to be largely constant (Bradshaw 1991; Huntley 1991). Therefore, among response processes of plants to current global climate change, only a minor importance is assumed for adaptation (Jump and Peñuelas 2005). With changes in environmental conditions formerly limiting the species´ distribution range, migration is expected as the most immediate reaction of plants at population level (Thuiller and others 2008). A directional shift of species´ ranges toward higher latitudes and altitudes in response to global warming has been found in paleoecological studies (Prentice and Jolly 2001; Parmesan 2006) as well as in numerous observations of current species´ range shift (Parmesan and Yohe 2003; Peñuelas and Boada 2003; Hickling and others 2006; Chen and others 2011). For the Holocene warming period, several authors estimate the post-glacial migration of Fagus and other temperate tree species to have occurred at rates of 60-170 m y-1 (McLachlan and others 2005; Bialozyt and others 2012; Feurdean and others 2013). In contrast, simulated future migration rates for such tree species are much slower (Meier and others 2012), likely due to a greater influence of competition and habitat fragmentation during the present warming phase. As the current increase in concentrations of atmospheric CO2 is several orders of magnitude greater than in any previous period of rapid change in atmospheric CO2 during the last 500 million years (Peñuelas and others 2013), present global change may likely exceed the capacities of many plant species – at the individual, population and community level – to assimilate them (Leemans and Eickhout 2004). Beyond global warming, plants are additionally threatened by further impacts like N eutrophication, habitat fragmentation and species invasion. Compared to other biological resources, forest ecosystems and silviculture are especially vulnerable to rapid environmental changes because of extensive life spans and long cultivation periods of temperate forest trees (Spellmann and others 2007). 11 CHAPTER 1 Plant responses to shifting growing conditions Global climate change is likely to simultaneously alter many aspects of local growing conditions, regarding climate (e.g. precipitation, temperature, solar radiation) and atmospheric input of elements to forests and other ecosystems. These alterations will directly affect the availability of resources for plants, such as water, light and nutrients (Lindner and others 2010). Besides abiotic conditions, the capture of requisite resources is further influenced by the ability of plants to react to changing conditions with above- and belowground allocation and active incorporation processes. It is widely assumed that elevated CO2 concentrations will enhance photosynthesis and reduce stomatal conductance, which in theory enables plants to conserve water and to enhance their water use efficiency (WUE) (Schäfer and others 2002; Battipaglia and others 2013). This CO2-induced increase in primary productivity and WUE is commonly known as the “fertilization effect” of CO2 (Farquhar 1997; Hättenschwiler and others 1997). Yet, multiple studies demonstrated that enhanced CO2 concentrations will not necessarily lead to an increased drought resistance of temperate forests, because stomatal control of many tree species is widely unresponsive to elevated CO2 (Medlyn and others 2001). In addition, the water conserving effects arising from decreased stomatal conductance can at least partially be compensated by