[APWG] Fw: exotic grasses vs. palo verdes

[Jil_Swearingen at nps.gov]

Please see article below.
-Jil

----- Forwarded by Jil Swearingen/NCR/NPS on 07/12/2005 04:48 PM -----

----- Forwarded by Sue Rutman/ORPI/NPS on 07/12/2005 07:48 AM -----
                                                                                                                                      
                      Sherry_Barrett at fw                                                                                               
                      s.gov                    To:       FW2_AZ_ES_Biologists%FWS at fws.gov                                             
                                               cc:       Mike_Coffeen at fws.gov, Sally_Gall at fws.gov, Sue_Rutman at NPS.gov,                
                      07/06/2005 08:45          Susan_Jacobsen at fws.gov                                                                
                      AM MST                   Subject:  exotic grasses vs. palo verdes                                               
                                                                                                                                      




Published: 08.08.2004
Exotic grass vs. palo verde
Two non-native varieties crowding out signature tree
By Mitch Tobin
ARIZONA DAILY STAR

Non-native grasses are killing palo verde trees in Southern Arizona and
resisting land managers' attempts to control the invaders with prescribed
fires, UA researchers are reporting.

Buffel grass, introduced to the Tucson area from Africa around 1940,
hijacks water destined for the roots of palo verdes, one of the Sonoran
Desert's defining plant species.

Lehmann lovegrass, another African species sown in the 1930s to reduce soil
erosion, may even proliferate after intentional fires. That's prompting one
local preserve with an aggressive burning program to consider trying
herbicides or mechanical removal of the grass.

"I think everyone is realizing that Lehmann's is here to stay," said Sally
Gall, acting manager of the Buenos Aires National Wildlife Refuge, south of
Three Points.

The UA biologists' findings were reported in two papers delivered recently
in Portland, Ore., at the annual meeting of the Ecological Society of
America.

Scientists had thought a deep-rooted plant like a palo verde wouldn't
compete with grasses that have shallow roots, said J. Alex Eilts, a
doctoral student in the UA's department of ecology and evolutionary
biology.

But when Eilts visited the century-old Desert Laboratory at Tumamoc Hill,
just west of "A" Mountain, he noticed palo verdes weren't growing where the
buffel grass was. Historic photographs also showed palo verdes used to live
where the grass now dominates.

So Eilts set about testing his hypothesis by studying palo verdes along the
"invasion front" of buffel grass. Using a chamber pressurized with nitrogen
gas, he measured the level of water stress in stems of palo verdes.

Once the buffel grass was removed, the trees' water stress fell
significantly.

"There is the potential for this to harm other native, Sonoran Desert
plants," Eilts said.

To cope with drought, palo verdes will self-prune their branches, turning
them from their namesake green to yellow. Eilts found palo verdes
surrounded by buffel grass lost about 24 percent of their branches, while
those without grass at their base lost only 16 percent.

"Buffel grass takes the drought and accentuates it," said co-author Travis
Huxman, an assistant professor in the department.

Scientists at Tumamoc Hill - known to ecologists around the globe for its
continual study of vegetation plots - also worry that thick stands of
buffel grass will let a wildfire spread, killing saguaros and other plants
that never evolved with fire.

Fire has been a key strategy for combating another pesky invader - Lehmann
lovegrass. But the tactic appears utterly ineffective, researcher Erika
Geiger found.

"It's from South Africa, from a fire-prone system, and it has evolved with
fire," said Geiger, a doctoral candidate in the UA's School of Natural
Resources.

With funding from the Defense Department, Geiger set up 54 plots on Fort
Huachuca, near Sierra Vista, with one-third containing only native grasses,
one third with only invasive species and one third with a mix.

Then firefighters at Fort Huachuca burned some of the 2.5-acre plots as
part of their training to battle wildfires.

Results from the past five years show that no matter what the year's
rainfall or when the grass was burned, the proportion of Lehmann lovegrass
remained constant or increased slightly.

"We've thrown a lot of time, energy and money at Lehmann lovegrass, with no
apparent success," said co-author Guy McPherson, a UA professor of natural
resources. "Perhaps we should dedicate a similar amount of time, energy and
money at the remaining biological invaders."

At 118,000-acre Buenos Aires National Wildlife Refuge, officials have tried
to scorch 15,000 to 20,000 acres a year in an attempt to eradicate Lehmann
lovegrass and restore the high-desert grassland's natural fire cycle.

The lovegrass crowds out native vegetation favored by masked bobwhite
quail, an endangered bird that prompted creation of the refuge in 1985.

"We're realizing that burning may not be having an effect on reducing
Lehmann's," acting manager Gall said.

The prescribed fires are still valuable for controlling an unnatural
proliferation of mes-quites, she said, but the refuge is now considering
herbicides or churning the soil with discs to combat the Lehmann
love-grass.

"We're definitely going to be considering other tools," she said.

● Contact reporter Mitch Tobin at 573-4185 or mtobin at azstarnet.com.