Drivers of forest dynamics: Joint effects of climate and competition Dissertation zur Erlangung des akademischen Grades Dr. rer. nat. vorgelegt der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth von Frau Dipl. Geoökologin Klara Dolos geboren am 03.03.1982 in Stuttgart Die vorliegende Arbeit von Frau Dipl.-Geoökol. Klara Dolos, geb. am 03.03.1982 in Stuttgart, wurde in der Zeit von Mai 2009 bis April 2013 unter der Betreuung von Herrn Prof. Dr. Björn Reineking an der Juniorprofessur Biogeographische Model- lierung der Universität Bayreuth angefertigt. Vollständiger Abdruck der von der Fakultät für Biologie, Chemie und Geowis- senschaften der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (Dr. rer. Nat). Dissertation eingereicht am: 25.4. 2013 Zulassung durch die Prüfungskommission: 22.5.2013 Wissenschaftliches Kolloquium: 31.10.2013 Amtierender Dekan: Prof. Dr. Rhett Kempe Prüfungsausschuss: Prof. Dr. B. Reineking (Erstgutachter) Prof. Dr. B. Engelbrecht (Zweitgutachterin) Prof. Dr. M. Hauhs (Vorsitz) Prof. Dr. B. Huwe PD Dr. G Aas 1 Wer immer nur tut, was er schon kann, bleibt immer nur dass, was er schon ist. Henry Ford Table of contents Abstract........................................................................................................II Zusammenfassung....................................................................................IV Introduction..................................................................................................1 Background.............................................................................................1 Natural forest dynamics..........................................................................3 Demographic processes.....................................................................4 Forest disturbances............................................................................8 Forest succession...............................................................................9 Climatic gradients..................................................................................10 Spatial climatic gradients..................................................................11 Climate change.................................................................................12 Forest modelling....................................................................................13 Statistical models..............................................................................14 Simulation models............................................................................15 Mathematical models........................................................................17 Limits.................................................................................................17 Conclusions...........................................................................................19 References............................................................................................20 Manuscripts...............................................................................................33 Summary of the following manuscripts.................................................33 Manuscript 1: Long-term vegetation dynamics in New Zealand...........35 Manuscript 2: Climatic turning point for beech and oak........................69 Manuscript 3: Symmetric and asymmetric competition......................103 Manuscript 4: Ecotype mixing as climate change adaptation.............133 Manuscript 5: Disturbance interactions...............................................157 Acknowledgements.................................................................................190 Supplement.............................................................................................191 List of manuscripts and specification of own contribution..................191 Presentations on this research...........................................................193 Non-refereed periodicals.....................................................................194 Further publications............................................................................194 Erklärung.............................................................................................195 I Abstract The present dissertation thesis addresses different aspects of forest dynamics and possible changes due to climate change. Various modelling approaches are used to explore joint effects of climate and competition on forest dynamics with a focus on temperate forests. Motivated by scientific interest, this thesis is aimed at contributing to the establishment of fundamental knowledge for proper ecosystem management. Each of the research projects of this thesis explores a facet of forest dynamics. It appears that for projections of forest dynamics under climate change particularly, it is critical to consider competition among trees. In the first study, the joint effect of climate and competition on forest dynamics in a mountain forest of New Zealand was investigated. The landscape simulation model LandClim was calibrated based on empirical data and applied to reproduce a 1700 years forest succession under stationary climate at the slope of Mt. Hauhungatahi, North Island. Although designed for European temperate forests, LandClim was capable of simulating NZ´s forest dynamics. Under non-stationary climate, forests likely remain in disequilibrium with cli- mate for some time due to the longevity of trees and competitive prevention of establishment. This aspect was investigated in a mixed beech-oak forest in Ger- many, using LandClim and the forest gap model SILVA in a cooperation study. Furthermore, a possible ‘climatic turning point’ was investigated, the point at which species dominances change due to changes in competitiveness caused by climate change. Both models projected a potential climatic turning point at a mean annual temperature of 11-12 °C and precipitation sum of 500-530 mm. However, the change of species composition in existing mixed stands was much slower since the turning point also depended on inherited stand structure. Based on these projections the promotion of oak at dry sites seems advisable due to its superior resistance and resilience to drought. The applied simulation models consider joint effects of climate and competi- tion but no changes in species sensitivity to competition along climatic gradients. The Spanish National Forest Inventories provided a solid basis to develop a stat- istical model for the influence of climate and competition on tree growth. The res- ults indicated that in Mediterranean forests the effect of competition increases with aridity potentially resulting in an additional disadvantage for drought sens- itive oaks compared to pines under climate change. II Under the prerequisite that forest dynamics will be affected by climate change, different forest management strategies on adaptation are currently discussed. One option is the promotion of tree species that are better adapted to anticipated future climates (such as oak instead of beech in Germany) and also the introduction of non-local ecotypes of local species. This increase in biodiversity intuitively appears promising because it is in line with the insurance hypothesis and the port- folio effect theory. Within this thesis the effect of ecotype mixing as an increase of within stand diversity under consideration of self-thinning was assessed. It was shown that ecotype mixing in forest stands might lower the risk of yield losses and at the same time might exempt the portfolio effect from its drawback of lower chances for high yields. Climate not only affects demographic rates of tree species but also all other ecosystem components. Disturbances are an important component of forest dynamics because they initiate successions and thereby influence species coexist- ence. Climate will alter disturbance regimes not only directly but also due to inter- actions among disturbances, for example an increased risk of insect outbreaks due to weak tree defence caused by severe drought stress. Most disturbance interac- tions have been observed to be positive, implying that increases of disturbances in quality and quantity due to climate change will be amplified. Furthermore, sys- tems containing positive feedback loops are considered to be mostly unstable, which would result in forest collapse. A theoretical study on disturbance interac- tions showed why positive feedback loops of disturbances do not necessarily lead to a forest collapse. Disturbance interactions might cause only a minor part of dis- turbances, whereas direct changes due to climate change are of much higher importance. The described studies reflect the diversity of the research field forest dynamics and innovative ecological methodology. Nevertheless, the present thesis is not an exhaustive discussion of drivers of forest dynamics under climate change. Forest dynamics and its drivers provide a range of open research questions posing a chal- lenge for fundamental an applied research of high relevance for society. III Zusammenfassung Diese Dissertation befasst sich mit Walddynamik und möglichen Verände- rungen in Folge des Klimawandels. Verschiedene Modellierungsansätze werden verwendet, um den Einfluss von Klima und Konkurrenz auf die Walddynamik zu untersuchen. Motiviert durch wissenschaftliche Neugier soll diese Arbeit auch dazu beitragen, fundiertes Wissen als Basis für nachhaltiges Forstmanagement