Vascular Plant and Vertebrate Inventory of Saguaro ... - USGS

Non-native Species and Changes to Vegetation
The spread of non-native species within the
district is an important natural resource issue.
In particular, buffelgrass (Pennisetum ciliare),
Lehmann lovegrass (Eragrostis lehmanniana),
red brome (Bromus rubens) and other nonnative
grasses, have increased in the last ten
years (Funicelli et al. 2001). The spread of
some non-native plants used for landscaping,
such as crimson fountaingrass (Pennisetum
setaceum) from development bordering the
district is also a concern. The invasion of nonnative
grasses has led to structural changes in
vegetation, from areas that supported mostly
sparse bunchgrasses to areas of uniform grass.
This change in species composition and structure
can alter the fire regime of the area by supporting
higher fire frequencies, thereby leading to other
changes in vegetation composition and structure
(Anable et al. 1992). Nowhere are these effects
more evident than in the Sonoran Desertscrub
vegetation community, which rarely burned
historically (Steenbergh and Lowe 19 ). Many
native plant species, especially succulents, are not
adapted to short duration but high-intensity fires
and therefore die (Schwalbe et al. 1999, Dimmitt
2000). Fires such as the Mother’s day fire,
which was fueled largely by non-native grasses,
have caused a high mortality of saguaro cactus
(Carnegiae gigantea Britt. & Rose), which is of
great concern to park managers (Schwalbe et al.
1999; see Chapter 3 for additional information).
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Wildland Fire
Since the park began keeping records in 193 ,
there have been 572 fires in the district, and since
1984, park personnel have burned approximately
1,450 ha through their active fire-management
program. Fires play a crucial role in the middle
and high-elevation semi-desert grasslands and
forests by depleting dense understory vegetation
and downed-woody debris. Even in these
fire-adapted ecosystems, however, fire can
be devastating, particularly after decades of
suppression and subsequent buildup of fuel loads.
A number of large fires in the last few decades,
most notably the Chiva and Box Canyon fires,
caused massive runoffs of sediment and ash.
The Chiva fire apparently eliminated lowland
leopard frog habitat and may have destroyed the
district’s only population of (federally listed)
Gila topminnow (Poeciliopsis occidentalis
occidentalis) at Little Wildhorse Tank, though
their status as a natural or introduced population
was uncertain (Don Swann, pers. comm.). The
Box Canyon fire of 1999 led to the sedimentation
of perennial pools, where lowland leopard frogs
once bred (Don Swann, unpubl. data). Despite
some problems, the NPS is committed to
returning natural fire cycles to the high elevation
areas of the district.