[APWG] Burned pinyon-juniper habitat being converted by non-native vegetation - Colorado

[Patricia_DeAngelis at fws.gov]

Conversion of pinyon-juniper woodland to shrubland or herb-dominated
vegetation following wildfires in Mesa Verde National Park, Colorado, USA.

Romme, William*,1, Floyd-Hanna, Lisa2, Hanna, David2, 1 Colorado State
University, Fort Collins, CO2 Prescott College, Prescott, AZ

ABSTRACT- Seven major wildfires during the last 70 years have burned
extensive tracts of pinyon-juniper woodland in Mesa Verde National Park,
located in southwestern Colorado, USA. These were predominantly
stand-replacing fires that killed all above-ground vegetation and consumed
all or nearly all organic litter. Pinyon (Pinus edulis) and juniper
(Juniperus osteosperma) both have failed to reestablish within these burned
areas, even where the fire occurred as long ago as 1934. Instead, post-fire
vegetation recovery has followed two different trajectories. Where
re-sprouting shrubs dominated the pre-fire understory (generally on more
mesic sites), a persistent shrubland has developed, dominated by Quercus
gambelii and Amelanchier utahensis. Where re-sprouting shrubs were lacking
(generally on more xeric sites), a mix of herbs and seed-established shrubs
is now dominant (e.g., Poa fendleriana, Eriogonum racemosa, Purshia
tridentata). Prior to the 1980s, post-fire vegetation was composed mostly
of native species, except locally where non-native species were planted for
rehabilitation purposes (e.g., Bromopsis inermis). Since 1989, however,
most of the pinyon-juniper woodland that lacks re-sprouting native shrubs
has become dominated after fire by invasive non-native herbaceous species
(e.g., Anisantha tectorum and Carduus nutans). The upshot is that a series
of large severe fires during the past 70 years has progressively reduced
the extent of pinyon-juniper woodland within the park, replacing the
woodland either with persistent shrublands of native species or
herb-dominated assemblages of largely non-native species. Pinyon-juniper
woodland, which covered approximately half of Mesa Verde National Park in
1906, is now restricted to less than 25 % of the park area. The biological
reasons for failure of pinyon and juniper to reestablish after severe fires
of the past century are unknown. Nevertheless, it is clear that post-fire
succession in this system is not a deterministic process leading by a
single pathway to recovery of pre-burn vegetation structure. Rather, life
history traits (sprouter vs. obligate seeder), propagule dispersal
mechanisms (animal vs. wind dispersal), and local changes in microclimate
and soils, all interact to predict post-fire vegetation trajectories.

Key words: fire, pinyon-juniper, non-native, succession


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