Life history strategies and biomass allocation : the population dynamics of perennial plants in a regional perspective

This study aims to contribute to the knowledge of how plants respond to adverse influences of intensified land use. In particular, attention was paid to the ways in which life history strategies change in order to buffer environmental variation, and which important parts of the life cycle are affected most. Herbaceous plant species are often threatened when the land use in an agricultural landscape is intensified: habitat is destroyed for new fields, infrastructure or urban areas, or habitat becomes less favorable because water levels are lowered for agriculture and because of pollution and nutrient enrichment by the influx of particles through air and water from the neighboring fields. Nutrient enrichment enhances natural succession and causes more competitive species to take over from species that are characteristic for nutrient-poor grasslands. As a result of habitat reduction and deterioration, populations decline in size and are more prone to extinction. The distances between remaining populations also increases, which makes gene flow through pollen or seed dispersal less likely. Decreased gene flow may lead to inbreeding depression and again higher extinction risks.Plant species may counterbalance some of these influences by flexible growth patterns and may adapt for instance to nutrient enrichment. For the conservation of the floral diversity of agricultural landscapes it is therefore important to know to what extent plant species are endangered by the different effects of intensified land use and to what extent these species can buffer the changes in their growing conditions. In this study I concentrate on four co-occurring perennial herbs in nutrient-poor, species-rich meadows. With three species from the Asteraceae ( Cirsium dissectum (L.) Hill, Hypochaeris radicata L. and Centaurea jacea L.) and one from the Dipsacaceae ( Succisa pratensis Moench), the species are phylogenetically related but have contrasting life history strategies as they differ in the life-span of their rosettes and in the rate of clonal propagation.The flexibility of the allocation of biomass to different life history functions (plant growth, sexual reproduction and clonal reproduction) is investigated in chapter 2. In a one-season garden experiment we continuously removed either rosette buds or flower buds to study the responses to damage. All investigated plants compensated for the removed buds. The short-lived species ( H. radicata ) showed stronger compensation for lost flower buds than the longer-lived species ( S. pratensis and C. jacea ) and the species with a high clonal propagation rate ( C. jacea ) compensated more strongly for lost rosette buds than the infrequently ramifying species. However, two species ( H. radicata and S. pratensis ) also switched to increased rosette formation when flower buds were removed. This switch shows, together with increased vegetative plant weight, that sexual reproduction has costs in terms of growth in size and clonal propagation.Costs of sexual reproduction underlie the hypotheses on the response of plants that are gradually outcompeted during succession: plants are expected to either increase their seed production to escape to other sites through seed dispersal (but at the expense of plant growth and future reproduction), or alternatively to persist by investing more in vegetative growth. In chapter 3 we aim to test these long-standing, but largely untested hypotheses in a three-year garden experiment. The target plants were grown together with a dominating, tall grass ( Molinia caerulea (L.) Moench). Costs of sexual reproduction were investigated by continuous flower bud removal and were found in all target species as either reduced plant biomass or reduced number of rosettes. Succession was mimicked by fertilizing of the plots, which caused a threefold increase of the grass biomass. Mortality rates increased under fertilization in two target species ( H. radicata and C. dissectum ) probably because they lacked a means of competing with the grasses. Three target species increased their relative allocation to flowers and seeds when their plots were enriched, while their relative allocation to storage did not decrease but in some species even slightly increased. In the fourth species ( C. jacea ) seed production was only higher in large plants but not in small plants. Allocation to every life history function was strongly and positively dependent on plant size. Consequently, the two hypothesized strategies collapse into a single syndrome: in response to succession plants must increase in size in order to survive and to increase seed production.The sexual reproduction pathway, from flower production to established seedlings, is further investigated in chapter 4. By sampling flowering plants and flower heads with seeds and by performing seed addition experiments we quantified for each species all involved steps: flower head production, flower production, seed set, seed predation and establishment as seedlings. The two infrequently ramifying species ( H. radicata and S. pratensis ) had high numbers of seedlings per flowering rosette: more than one on average. This number was below one in the two more clonally propagating species ( C. dissectum and C. jacea ). Seedling establishment was the largest bottleneck in the pathway, while seed set was important too in one of the more clonal species. The different steps were studied in several sites and over several years. The spatiotemporal coefficients of variation were highest for seedling establishment, but closely followed in magnitude by the coefficients for flower head production and seed set, indicating that selection processes can be expected in different parts of the sexual pathway of the life cycle.The whole life cycle is studied for three species ( S. pratensis , H. radicata and C. jacea ) in chapter 5. Demographic measurements on rosette survival, clonal propagation and flower head production were made in permanent plots in three or five populations over four years. The sexual pathway of the life cycle was completed with data of the previous chapter. We constructed 42 stage-based projection matrices to study the population dynamics of the three species in different sites over time. The calculated population growth rates were lowest in the most short-lived species ( H. radicata ) and most stable in the most clonal species ( C. jacea ). Using life table response experiment analyses we found that the species differed in their response to the same spatiotemporal variation in environmental conditions. Furthermore, species responded differently in sites in which they had low population growth rates than to years in which they had low population growth rates. In two species ( S. pratensis and H. radicata ) below-average values of some life cycle components in bad sites were partly compensated for by the above-average values of other components, but similar compensation did not occur in bad years. By contrast, this compensation did appear in bad years for the third, more clonal species ( C. jacea ). The implication of this difference between spatial and temporal variation in population dynamics is that temporal variation can not readily be substituted with spatial variation as has previously been done in some time-limited studies.In chapter 6 we investigate the demographic implications of inbreeding depression in order to evaluate if reported effects of inbreeding on several life history components significantly influence the population dynamics and population survival of S. pratensis . For that purpose we constructed a projection matrix model with an outcrossing and an inbreeding cycle. The growth of seedlings and small rosettes were reduced in the latter cycle, as was seed set and germination, using empirically based data. The proportion of seeds entering either cycle was a function of the size of the population: more seeds were inbred in smaller populations. We compared a declining and an increasing population in the simulations. Although only a part of the life cycle was affected, inbreeding reduced the time to extinction in the decreasing population. Inbreeding affected the increasing population only when the initial population was small. Density dependency by limiting habitat size only affected the increasing population.The demography of the most clonal (rhizomatous) and also most endangered herb of the model species ( C. dissectum ) was studied in chapter 7. The results of the allocation experiment of the third chapter are further analyzed: whereas the total biomass of the plants was unaffected by the fertilization treatment, the surviving rosettes were further away from the original planting location and more remnants of dead rosettes were found in the fertilized plots. Field observations on excavated plants and an additional experiment confirmed that both flowering and rhizome formation are size-dependent. This suggests that the high percentage of flowering rosettes in the allocation experiment was the result of a high growth rate of the fertilized rosettes. The total biomass did not differ from the control plots because the mortality rate of the non-flowering rosettes was also higher in the fertilized plots. This pattern of increased flowering and of increased turn-over of biomass and rosettes is consistent with the results of a demographic field study. In the most productive site we found high flowering probabilities and above-average clonal propagation but lower rosette survival. This trend towards higher turn-over rates makes this bad competitor even more prone to extinction when the nutrient enrichment of a meadow continues. The increased seed production has a negligible contribution to the local population growth rate as seedling recruitment seems confined to very open, early-successional habitats.The last chapter was aimed to integrate the population dynamics models and the results of the fertilization experiments to investigate which effects on individual plants have the strongest impact on the dynamics of the population. As fertilization had both positive effects on plant size and seed production and negative effects on survival and seedling establishment, we were also interested in the overall outcome when all effects were incorporated into a model describing the natural population dynamics. In order to model effects on life history components and plant traits we built a hierarchical matrix in which each element was a function of life history components and many of the components in turn were functions of plant traits. The negative effects of fertilization on the population growth rate by increased mortality rates in the two weak competitors ( C. dissectum and H. radicata ) overruled other effects and caused strong population declines. In the other two species ( S. pratensis and C. jacea ), in which mortality was still unaffected, negative impacts of reduced seedling establishment were small or compensated by increased seed production and plant size. However, the model results also suggest that also in these better competitors populations will decline once the mortality rates of individuals increase. Positive effects on plant size and seed number can not compensate for high death rates. We therefore conclude that although the life history responses of these perennials are rather flexible, their populations will go extinct in grasslands that have become more productive through succession and nutrient enrichment. The decline of small population can be expected to be even accelerated by inbreeding depressions. However, when increased allocation to sexual reproduction leads to increased seed production for the total population, this flexible life history response may have positive effects on the dynamics of a species in the landscape through increased colonization probabilities.