Plant Trait Responses to Grassland Management and Succession
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PLANT TRAIT RESPONSES TO GRASSLAND MANAGEMENT AND SUCCESSION DISSERTATION ZUR ERLANGUNG DES DOKTORGRADES DER NATURWISSENSCHAFTEN (DR. RER. NAT.) DER NATURWISSENSCHAFTLICHEN FAKULTÄT III – BIOLOGIE UND VORKLINISCHE MEDIZIN DER UNIVERSITÄT REGENSBURG VORGELEGT VON STEFANIE KAHMEN REGENSBURG JUNI 2003 Veröffentlichung: Kahmen, S. (2004) Plant trait responses to grassland management and succession. Dissertationes Botanicae 382, pp. 123. Promotionsgesuch eingereicht am 18. Juni 2003 Mündliche Prüfung am 10. Oktober 2003 Prüfungsausschuss: Prof. Günter Hauska Prof. Peter Poschlod Prof. Michael Kleyer Prof. Jürgen Heinze Die Arbeit wurde angeleitet von Prof. Peter Poschlod Contents CHAPTER 1 GENERAL INTRODUCTION...................................................................1 CHAPTER 2 PLANT FUNCTIONAL TRAIT RESPONSES TO GRASSLAND SUCCESSION OVER 25 YEARS ..............................................................7 CHAPTER 3 CONSERVATION MANAGEMENT OF CALCAREOUS GRASSLANDS. CHANGES IN PLANT SPECIES COMPOSITION AND RESPONSE OF PLANT FUNCTIONAL TRAITS DURING 25 YEARS.................................23 CHAPTER 4 EFFECTS OF GRASSLAND MANAGEMENT ON PLANT FUNCTIONAL TRAIT COMPOSITION .........................................................................35 CHAPTER 5 COMPARISON OF UNIVARIATE AND MULTIVARIATE ANALYSIS OF PLANT TRAIT RESPONSES TO MANAGEMENT TREATMENTS................49 CHAPTER 6 DOES GERMINATION SUCCESS DIFFER WITH RESPECT TO SEED MASS AND GERMINATION SEASON? EXPERIMENTAL TESTING OF PLANT FUNCTIONAL TRAIT RESPONSES TO MANAGEMENT.................59 CHAPTER 7 EVALUATION OF PLANT TRAIT RESPONSES TO DIFFERENT GRAZING INTENSITIES USING A MECHANISTIC, SPATIALLY EXPLICIT SIMULATION MODEL ..........................................................73 SUMMARY .........................................................................................................99 ZUSAMMENFASSUNG ..................................................................................................103 REFERENCES .......................................................................................................107 APPENDIX .......................................................................................................116 DANK .......................................................................................................123 Chapter 1 General Introduction Grassland management and succession Semi-natural grasslands with their specific flora and fauna are of high conservation value in Central Europe. They became threatened when intensification of agricultural practice increased in the middle of the last century. From this time on, semi-natural grasslands have been fertilised, afforested or, alternatively, been abandoned owing to their low agricultural productivity (Poschlod & Schumacher 1998, WallisDeVries et al. 2002). A major goal of nature conservation is to ensure the protection and maintenance of these semi-natural grasslands and their unique floristic and faunistic diversity by conservation management. Aiming to find the best management, i.e. optimal maintenance of the species composition at the lowest cost possible, several studies on management treatments have been started in the last decades (e.g. Schreiber 1977, Krüsi 1981, Schmidt 1981, Kapfer 1988, Bakker 1989, Bobbink & Willems 1993, Huhta & Rautio 1998, Huhta et al. 2001, Fischer & Wipf 2002). Most of these studies investigated the impact of management-induced shifts in fertility on changes in species composition. Due to different environmental conditions and species sets, however, the transfer of conservation knowledge between sites or regions remains difficult. When management ceases and a site is abandoned, grassland succession leads to the establishment of shrubs and trees until the site is entirely covered by forest. The course of succession, however, differs greatly among sites. Species composition, environmental conditions, but also site characteristics like the history of a site, determine the succession process to a large extent, thus preventing the generation of one general, unifying theory (Clements 1916, Margalef 1963, Odum 1969, Connell & Slatyer 1977, Miles 1987, Pickett et al. 1987, Glenn-Lewin & van der Maarel 1992). Without such a theory, predictions of the effects of grassland management or abandonment for the purpose of nature conservation require individual studies for each region and grassland type. As this approach is both unaffordable and time consuming, conservationists have been forced to explore other methods of assessment. In recent years, there has been a growing interest in the role of biological characteristics of plants (plant functional traits) for vegetation changes induced by management or succession (Hodgson 1990, Briemle & Schreiber 1994, Olff et al. 1994, Hobbs 1997, Prach et al. 1997, Poschlod et al. 1998, Dupré & Diekmann 2001, Pywell et al. 2003). Focusing on plant functional traits instead of species is promising since more general processes can be exposed. This thesis investigates plant functional trait responses to grassland management and succession in order to identify regulating mechanisms. 2 Plant functional traits Plant functional trait responses to ecosystem processes are of increasing interest in plant ecology (Gitay & Noble 1997, Lavorel et al. 1997, Grime 2001). Plant traits are biological characteristics like morphological or regenerative features. These features may determine whether a plant species is able to establish, survive or increase in abundance under specific environmental conditions or processes like disturbance, land use, climate change, fire etc. A plant trait that is sensitive to an environmental factor is defined as functional. As a consequence, plant functional traits are biological characteristics of plant species that respond to environmental conditions or processes in an ecosystem (Keddy 1992b, Kelly 1996, Gitay & Noble 1997, Lavorel et al. 1997). The benefit of knowing about the relationship between plant functional traits and specific processes is twofold. Firstly, by interpreting trait responses ecologists gain a better understanding of the mechanisms underlying the processes. Secondly, examining traits instead of species is useful for predictive purposes. Predictions based on species responses to a specific process are of a limited validity because most sites differ with respect to the species set and these differences increase with increasing spatial scale. In contrast to that, trait responses are more general, and sites with different environmental conditions or different species sets are thus rendered comparable. In a valuable study, Díaz et al. (2001) found similar trait responses to grazing for two geographic regions with completely different floras (Argentina and Israel). Functional groups versus functional traits There is a great variety of studies on functional trait responses (e.g. Boutin & Keddy 1993, Kelly 1996, Noble & Gitay 1996, Skarpe 1996, Thompson et al. 1996, Díaz & Marcelo 1997, Kleyer 1999, Lavorel et al. 1999b, Leishman 1999, Díaz et al. 2001, Thompson et al. 2001, Pywell et al. 2003). This has led to an equally great variety of methods used in plant functional trait analyses. According to their methodology, studies may, however, be broadly grouped into two categories, as they either focus on single plant traits or plant groups. Lavorel et al. (1997) as well as Gitay & Noble (1997) gave an overview of the approaches to identifying plant groups, functional types sensu Lavorel et al. (1997) and response groups sensu Gitay & Noble (1997). Based on a set of common plant traits, plant functional types are defined as non-phylogenetic groups of species that exhibit similar responses to ecosystem processes (Kelly 1996, Gitay & Noble 1997, Lavorel et al. 1997). Reducing the high amount of species to some plant functional types is appealing for comparative as well as for predictive reasons. Unfortunately, plant functional types are highly context-specific so that a set of specific types has to be defined for each process. Although a lot of studies identified similar plant functional types that respond to grazing (Díaz et al. 1992, Lavorel et al. 1998, Hadar et al. 1999, Landsberg et al. 1999, Lavorel et al. 1999a, Sternberg et al. 2000), until now each study has used its own specific types. Gitay & Noble (1997) devised four criteria for plant functional types, namely uniqueness, repeatability, congruency and convergence but these criteria are seldom met. In the temperate zone, widely applied functional concepts comprise the life form groups of Raunkiaer (1934) and the CSR-strategy scheme proposed by Grime (1974). GENERAL INTRODUCTION 3 ‘Life form’ classifies plants with respect to the location of the regeneration buds as strategy to survive cold winters (Raunkiaer 1934). It encapsulates sets of correlated traits relating to plant persistence and architecture. The CSR strategy scheme uses resource availability and disturbance as two orthogonal dimensions for plant classification. Both concepts help to identify important ecological processes and to predict vegetation responses under various conditions. However, considering processes that exhibit a low variability (e.g. grassland management) rather than steep environmental gradients, ‘life form’ alone is too broad a classification by which to describe and predict vegetation changes (Kelly 1996, Semenova & van der Maarel 2000). Even though the CSR strategies are better able to capture more subtle forms of variation, in most studies