Showing papers by "Martin Zobel published in 2002"

The role of the seed bank in gap regeneration in a calcareous grassland community

Abstract: We conducted a factorial field experiment in order to investigate the role of the soil seed bank in gap recolonization in a calcareous grassland, using 240 experimental gaps (10 X 10 cm). Experimental manipulations included the elimination of the seed bank (by sterilizing the soil), the elimination of short-distance seed rain by removing all flow- erheads in 2 X 2 m plots, and the prevention of lateral clonal spread by surrounding gap soil with 41-pum nylon mesh. The influence of competition on regeneration was also studied, by planting an adult individual of Filipendula vulgaris or Festuca rubra in the center of some gaps. There were 130 species of vascular plants in the established vegetation. Twenty-five species also occurred as seedlings; 51 species were recorded in the seed bank, of which only two were not present in the growing vegetation. There were on average 2362 seeds/ m2 in a 10 cm deep soil layer. Species relative abundances in the established vegetation and in the seed bank were not significantly correlated. Eighty-four vascular plant species emerged from seeds in the experimental gaps. The seed bank contributed on average 5.4 seedlings per gap, short-distance seed rain ( 0.5 m) seed rain 2.7 seedlings. Thirty-one vascular plant species arrived in experimental gaps by lateral clonal spread. Of these, only one was not recorded as a seedling in the experimental gaps. There were on average 2.7 clonally spread shoots per gap. Species with smaller seeds were present in significantly greater numbers in the seed bank than in the vegetation and were also more common colonizers of experimental gaps. There was no evidence of competition or facilitation between seed- lings, or between seedlings and adults. It was concluded that the soil seed bank has an important functional role in a perennial grassland community as a means for population maintenance for many species. Thirty-six percent of the regeneration in small gaps was due to the seed bank, and one may expect that this proportion will increase in larger gaps.

Are invaders disturbance-limited? Conservation of mountain grasslands in Central Argentina

Abstract: Extensive areas in the mountain grasslands of central Argentina are heavily invaded by alien species from Europe. A decrease in biodiversity and a loss of palatable species is also observed. The invasibility of the tall-grass mountain grassland community was investigated in an experiment of factorial design. Six alien species which are widely distributed in the region were sown in plots where soil disturbance, above-ground biomass removal by cutting and burning were used as treatments. Alien species did not establish in undisturbed plots. All three types of disturbances increased the number and cover of alien species; the effects of soil disturbance and biomass removal was cumulative. Cirsium vulgare and Oenothera erythrosepala were the most efficient alien colonizers. In conditions where disturbances did not continue the cover of aliens started to decrease in the second year, by the end of the third season, only a few adults were established. Consequently, disturbances are needed to maintain ali...

Small-scale turnover in a calcareous grassland, its pattern and components

Abstract: We studied small-scale turnover in a calcareous grassland community in 2 cm × 2 cm subplots within 46 10 cm × 10 cm plots from 1996 to 1999. In different years, 73–84% of the subplots were occupied by rooted plant individuals, but the location of empty subplots varied considerably between years. Seedlings were the most mobile components in subplots – during one year (from July to July), seedlings either appeared or disappeared in on average 85% of all subplots where seedlings had occurred during that year. Within a one-year period, vegetative ramets appeared or disappeared in an average of 30% of all subplots that had rooted plants at either the beginning or the end of that year. When only previously empty subplots were taken into account, 80% of appearances were due to clonal growth and 20% due to seedling establishment. Ca. 75% of the seedlings, however, established in subplots that already contained a rooted plant individual. There were significantly less generative appearances when a moist su...

Studying plant competition: from root biomass to general aims

Abstract: Cahill (2002) initiates a discussion on methodological issues of studying plant competition, addressing two main questions: (i) whether we need full separation of aboveand below-ground interactions among plants before we can accurately measure the relative importance of root and shoot competition; and (ii) whether we must measure plant responses both above and below ground. If the first question is not tautological, it is certainly philosophical and it may have no universal answer. It would always be good to know more, but everything depends on the goal of any particular study. Cahill considers that insisting that all competition studies fully separate root and shoot competition ‘greatly increases the complexity of experimental designs’, highlighting the need to ensure that goals are driven by theory rather than methods. However, if the goal of studying the ‘relative importance of root and shoot competition’ has been agreed, then there are several possible ways to proceed. One of these is to rely on indirect evidence, although any effort to be more specific would certainly be welcome. The point that Cahill really wants to address is the question: What data need to be collected from the focal plants? After advocating briefly the use of short-term observations to predict long-term responses in the field, he asks, ‘Must we measure both root and shoot responses of focal plants in studies which separate root and shoot competition?’. The assumption on which this is based is rather optimistic given the field evidence available (e.g. Tilman 1989; Bakker et al. 1996). Can shoot growth alone be a proxy for the long-term effects of the experimental treatments of interest? This question is not theoretical, neither is it connected specifically with the issue of separating shoot and root responses. It is, rather, a practical consideration related to all kinds of plant competition studies. In his final comments, Cahill agrees that information about ‘how plant allocation varies’ may be ‘essential for addressing some very interesting questions’, but argues that it may still be ‘impractical for studies that often have on the order of 500 focal plants’. We would like to continue discussion on the topic of how realistic is the common tendency in competition studies to operate above ground only.