Theory of disturbances at the level of plant communities offers plant
ecologists a lot of interesting questions concerning vegetation
dynamics, succession and biodiversity. The importance of disturbance
and regeneration in vegetation has been generalized into the
``mosaic-cycle'' concept of ecosystems (REMMERT 1991). In this view,
vegetation is a mosaic of patches at different stages of a temporal
cycle of aging, destruction and rejuvenation.
Even though thesis presented is embraced on a large scale the
individual chapters are hierarchically and logically structurized. The
first part of the thesis focuses on the theoretical concept of
disturbance ecology including definitions, scaling and
methodology. The author gives a review of disturbances and emphasizes
the influence of traditional land use on habitat fragmentation. The
second part describes vegetation ecology of dry acidic grasslands in
general, and using several experiments verifies proposed research
hypotheses. As VAN DER MAAREL (2005) argues, grasslands are especially
suited for the study of patch dynamics because the gaps are small, can
be easily manipulated and the appearance and disappearance of plant
units (seedlings), can be accurately followed.
Theoretical survey of disturbance types and regimes is supplemented
with a series of experiments: the influence of individual natural (by
ants and rabbits) and artificial disturbances (topsoil removal,
topsoil turnover and compaction) on species composition, species
turnover, nutrient availability and soil seed bank composition were
studied in dry acidic grasslands during 1998-2000. Jentsch compares
spatio-temporal patterns of ant and rabbit disturbances and discuss
their influence on vegetation heterogeneity and patchiness.
Natural disturbances by ants and rabbits were very abundant in the
studied area. Disturbed patches were frequent, of small spatial scale,
small magnitude and short duration (in terms of months). They covered
1-15% of the grassland area dominated by Corynephorus canescens. Both
ants and rabbits disturbances have large effects on plant assemblages
and ants can be also considered as key-stone species for vegetation
Moreover, the influence of mechanical disturbances (topsoil removal,
soil turnover and compaction) on nutrient availability, namely on
nitrogen increase was studied. In summary, experimental soil
disturbances caused a short discrete pulse in nitrate and ammonia, but
no significant effect on soil water content or pH was detected.
The soil seed bank plays the essential role in recolonization dynamics
after disturbance. Analysis of seed bank composition, seed density and
a series of burial experiments were carried out. Bare substrate is, in
general, occupied very quickly, especially by cryptogams. Mosses,
however, such as Polytrichum piliferum have shown to inhibit
germination of forbs and grasses, which is generally attributed to
allelopathy. It is shown that the regeneration potential of dry acid
grasslands is very limited. Similarity in composition of soil seed
bank and established vegetation was poor in all successional stages in
the study area. None of the successional stages exhibited high
proportion of species with a long-term persistent seed bank. Neither
pioneer species, nor endangered plants were present in reasonable
amounts in the soil seed bank.
Special attention was dedicated to variation in species richness among
different successional stages. It is a bit surprising that species
establishing initially after disturbance do not determine the
long-term successional processes of the patch. The results show, that
disturbance has only short-term effects, largest within the first two
years of the experiment (in agreement with management experiments in
dry acidic heathlands, SEDLÁKOVÁ 2003).
The author truly admits that all disturbance intensities tested were
probably below the threshold for changing species diversity at the
most productive site. As the most important factor for driving
vegetation dynamics in dry acidic grasslands seems to be soil nitrogen
together with disturbance regime. Eventually, Jentsch tries to find
generality in studies of disturbance and ecosystem dynamics,
implications for nature conservation and sketches further research
perspectives of disturbance ecology.
The publication is appended with 55 figures (photos, graphs) - some of
them could be of better quality and 20 tables in the text.
I can recommend this book to plant ecologists dealing with the
spatio-temporal heterogeneity of vegetation, as much as to nature
conservationists and to all, who work in applied vegetation science.
Folia Geobotanica 41/3 2006