Fire is known to facilitate the invasion of many non-native plant species, but how
invasion into burnt areas varies along environmental gradients is not well-understood.
We used two pre-existing data sets to analyse patterns of invasion by non-native
plant species into burnt areas along gradients of topography, soil and vegetation
structure in Yosemite National Park, California, USA. A total of 46 non-native species
(all herbaceous) were recorded in the two data sets. They occurred in all seven of the
major plant formations in the park, but were least common in subalpine and upper
montane conifer forests. There was no significant difference in species richness or cover
of non-natives between burnt and unburnt areas for either data set, and environmental
gradients had a stronger effect on patterns of non-native species distribution,
abundance and species composition than burning. Cover and species richness of nonnatives
had significant positive correlations with slope (steepness) and herbaceous
cover, while species richness had significant negative correlations with elevation, the
number of years post-burn, and cover of woody vegetation. Non-native species
comprised a relatively minor component of the vegetation in both burnt and unburnt
areas in Yosemite (percentage species = 4%, mean cover < 6.0%), and those species
that did occur in burnt areas tended not to persist over time. The results indicate that in
many western montane ecosystems, fire alone will not necessarily result in increased
rates of invasion into burnt areas. However, it would be premature to conclude that
non-native species could not affect post-fire succession patterns in these systems.
Short fire-return intervals and high fire severity coupled with increased propagule
pressure from areas used heavily by humans could still lead to high rates of invasion,
establishment and spread even in highly protected areas such as Yosemite.
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