High fire frequency in California chaparral reduces post-fire shrub regeneration and native plant diversity

Fire is crucial for maintaining species diversity and resilience in
fire-adapted shrublands of the world’s Mediterranean climate zones (MCZs),
which include the chaparral shrublands of the North American MCZ.
Chaparral is adapted to high intensity burning, with long intervals
between fires (30-100 years) typifying undegraded conditions. In much of
the range of chaparral, modern fire frequencies are much higher, driven
largely by high densities of human ignitions and coincidence between
ignitions and severe weather conditions. This change in the fire regime
has major implications for biodiversity, leading to exotic invasion,
decreased ecosystem services, and potential type conversion of shrubland
to grassland dominated by exotic species. We studied the impact of
increased fire frequencies on the composition and abundance of herbaceous
and woody species in the Interior Coast Range of northern California. Our
study area is one of the most frequently burned areas in California, which
afforded us the opportunity to investigate higher fire frequencies than
heretofore reported in the scientific literature for California. We
surveyed fifty-four 250-m2 plots to assess changes in plant community
composition and postfire regeneration of chaparral shrubs across a wide
range of fire frequencies, including plots that have burned up to six
times in the past 30 years. Our findings reveal that short-interval fires
significantly reduced post-fire native woody regeneration, with obligate
seeding species experiencing a 99% reduction and facultative species
showing an 83% reduction in regeneration in the most frequently burned
plots. Moreover, the overall marginal effect of one additional short
interval fire decreased the proportion of native species cover by 12% and
both richness and Shannon diversity by 4%. Consequently, areas with higher
fire recurrence supported a more structurally and botanically homogeneous
landscape, dominated by a similar group of non-native species.