Summary
- Disturbance is widely believed to facilitate invasions by exotic plants, but
also is important for the persistence of many native species. Here, I report the
results of a series of field experiments designed to investigate the effects of
soil disturbance on natives and aliens in Californian grassland vegetation. I
also compare the effects of different types of soil disturbance to establish
whether some favour aliens to a greater degree than others.
- In two experiments, conducted at two different locations, three types of soil
disturbance (excavation, burial, and simulated gopher mounds) were created, and
their revegetation was compared with changes in undisturbed control plots over
the next three years. A third experiment was used to provide data on the effects
of soil disturbance on soil temperature, moisture, and KCl-extractable nitrogen.
- Disturbance affected both soil temperature and chemistry. Buried plots
contained the most KCl-extractable nitrogen, and also were the warmest. Effects
on soil moisture were relatively small.
- Initially, most disturbances greatly reduced the numerical abundance of both
groups dominated by natives (perennial graminoids and bulbs) and those dominated
by aliens (annual graminoids). Disturbance also reduced maximal (summer) species
richness, but in some cases increased the fraction of richness contributed by
natives.
- In subsequent years, richness rebounded as natives and exotics re-invaded.
Native bulbs and perennial graminoids were slow to recover; instead, most
disturbances increasingly became numerically dominated by exotic annual grasses,
accentuating the effects of a multi-year drought.
- The differing effects of experimental disturbances on aliens and natives can
best be explained by considering relationships between sources of propagules,
life histories, and geographic origins.
- Some types of disturbance were less damaging to native-dominated groups than
others, but most ultimately favoured exotics. Consequently, it may be difficult
to develop management strategies that preserve the diversity of
disturbance-dependent natives while still excluding weedy aliens.
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