Data from: Nitrogen deposition suppresses ephemeral post-fire plant diversity

Fire is a dominant force shaping patterns of plant diversity in
Mediterranean-type ecosystems. In these biodiversity hotspots, including
California’s endangered coastal scrub, many species remain hidden
belowground as seeds and bulbs, only to emerge and flower when sufficient
rainfall occurs after wildfire. The unique adaptations possessed by these
species enable survival during prolonged periods of unfavorable
conditions, but their continued persistence could be threatened by
nonnative plant invasion and environmental change. Furthermore, their
fleeting presence aboveground makes evaluating these threats in situ a
challenge. For example, nitrogen (N) deposition resulting from air
pollution is a well-recognized threat to plant diversity worldwide, but
impacts on fire-following species are not well understood. We
experimentally evaluated the impact of N deposition on post-fire
vegetation cover and richness for three years in stands of coastal sage
scrub that had recently burned in a large wildfire in southern California.
We installed plots receiving four levels of N addition that corresponded
to the range of N deposition rates in the region. We assessed the impact
of pre-fire invasion status on vegetation dynamics by including plots in
areas that had previously been invaded by nonnative grasses, as well as
adjacent uninvaded areas. We found that N addition reduced native forb
cover in the second year post-fire while increasing the abundance of
nonnative forbs. As is typical in fire-prone ecosystems, species richness
declined over the three years of the study. However, N addition hastened
this process, and native forb richness was severely reduced under high N
availability, especially in previously invaded shrublands. An indicator
species analysis also revealed that six functionally and taxonomically
diverse forb species were especially sensitive to N addition. Our results
highlight a new potential mechanism for the depletion of native species
through the suppression of ephemeral post-fire biodiversity.