[MCN] Drought hits Rocky Mountain forests recovery after fire

[Lance Olsen]

Drought alters recovery of Rocky Mountain forests after fire

UNIVERSITY OF WISCONSIN-MADISON
http://www.eurekalert.org/pub_releases/2016-03/uow-dar032116.php

MADISON, Wis. -- A changing climate is altering 
the ability of Rocky Mountain forests to recover 
from wildfire, according to a new study published 
in the journal Global Ecology and Biogeography.

When warm, dry conditions lead to drought in the 
years following fires, it impedes the growth and 
establishment of vulnerable new post-fire 
seedlings. The study also shows that forest 
recovery has been negatively affected by 
increased distances between burned areas and the 
sources of seeds that typically replace trees 
lost to fire.

"Fires that are followed by warm, dry conditions 
offer us a window into the future," says Brian 
Harvey, lead author of the study and a former 
University of Wisconsin-Madison graduate student 
in the laboratory of Monica Turner, E.P. Odum 
Professor of Ecology and Vilas Research Professor 
of Zoology. Harvey is now a postdoctoral Smith 
Fellow at the University of Colorado Boulder.

"From all the best available data and modeling, 
and expectations about future climate, these are 
the kinds of fires and post-fire climates that 
we're going to see more of in the future," he 
says.

The new data positions researchers to better 
understand how forests could change in coming 
decades and may yield valuable information for 
the development of robust simulation models. 
Turner says the dataset -- which is the first to 
comprehensively demonstrate the impact of drought 
on forest recovery in the context of a changing 
climate -- "provides unambiguous evidence that 
the climate conditions following fires are really 
going to matter."

The forests of the Rocky Mountains are 
well-adapted to fire. For instance, lodgepole 
pines, a dominant species in the Mountain West, 
possess seed cones that are opened by fire, with 
each tree releasing thousands of seeds when 
burned. However, how the forests will adapt to a 
changing climate is still unknown.

"Fires that are followed by drought -- which we 
are very likely going to see more of with climate 
change -- really do set a new context in which 
these forests are not recovering as quickly," 
says Harvey. "It's a double whammy because even 
if seeds can get to a burned patch, they still 
need to survive once they get there. That may be 
much harder to do in a warmer, drier climate."

The Turner lab's interest in how forest recovery 
after fire may change in the future began in the 
summer of 2000, when large wildfires raged across 
the southern portion of Yellowstone National Park 
in Wyoming. Thousands of acres of forest burned.
Turner had previously studied the massive 1988 
Yellowstone Fires and in the summers following 
the 2000 Glade Fire, she and collaborators 
visited the forest to collect as much data as 
possible. They were surprised to see that tree 
density in the years after the 2000 fire was 10 
times lower than comparable forests recovering 
from the fires of 1988.

A plausible explanation? The year following the 
Glade Fire was unusually hot and dry, with just 
30 percent of normal summer precipitation. But 
one fire followed by a dry summer could not 
provide the data to rigorously test the theory.

By 2013, however, multiple well-documented fires 
had burned throughout the Rockies and that 
summer, Harvey conducted field studies in 
Yellowstone and Glacier national parks.

He and the research team visited 184 sites where 
11 wildfires burned between 1994 and 2003. The 
work was exhilarating, but also grueling, he 
says, as most sites were well off trail in the 
backcountry of some of America's wildest places 
and the research team was on its hands and knees 
examining nearly 10,000 individual tree seedlings.

The team collected data on the overall character 
of the forest before fire and the number, 
species, size and age of trees after fire; the 
ground cover on the forest floor (for example, 
the presence of shrubs or grasses) following 
fire; and other qualities of the site, such as 
whether the forest stands were on cooler/moister 
north-facing slopes or warmer/drier south-facing 
slopes.

The researchers also turned to existing climate 
records to assess the drought severity of each 
location in the three years immediately following 
each fire and examined the distance from each 
plot to the nearest source of seeds -- other 
still-living trees typically outside the burn 
patch.

"By going into areas that had burned at least 10 
years ago -- enough time for many post-fire tree 
seedlings to establish -- we were able to 
characterize how these forests will likely look 
for some time in the future," says Harvey.

They found that overall, fewer post-fire tree 
seedlings established in years when fire was 
followed by severe drought and when seed sources 
were farther away, compared with cooler, wetter 
years and when burned areas were closer to seed 
sources.

Subalpine tree species, including Engelmann 
spruce and subalpine fir, were more negatively 
affected by drought after fire than species that 
grow at warmer, drier low elevations -- 
Douglas-fir and quaking aspen, for instance -- 
and those at the upper tree line, such as 
whitebark pine.

"The warm/dry post-fire climate conditions really 
hammered the species that currently dominate 
subalpine forests," Harvey says.

One exception, however, was the lodgepole pine, 
which was less impacted by drought or seed source 
distance. However, these and the other species 
from lower elevations have not moved quickly 
enough into higher-elevation burned areas to 
replace the more sensitive subalpine tree species.

This means that in addition to shifts in the 
composition of post-burned forests in the 
Northern Rockies, forest densities will also 
likely be lower in areas where fires are followed 
by drought, Harvey says -- at least in the near 
and medium term.

Turner hopes the findings are valuable to land 
managers who routinely work with wildfires, which 
often leave behind a mosaic of burned and 
unburned trees. These islands of live, unburned 
trees in a burnt desert may serve as valuable 
seed sources for forest recovery and should 
likely be left intact, she says.

Harvey and Turner's larger goals are to 
understand the conditions under which forests can 
or cannot recover after fire and to anticipate 
what the forest landscape might look like over 
the next few decades or centuries.

"Trees grow slowly, and we can't just wait 100 
years to get enough opportunities to study forest 
recovery from fire," says Turner.

With the study data, Turner and her collaborators 
will try to create models to predict future 
change. She is also involved in experimental 
efforts to understand the moisture thresholds 
various tree species require to germinate and 
become successfully rooted. She hopes to better 
understand forest recovery in the context of 
climate projections through mid-century.

"We want to tease this apart so we can anticipate 
what the ecosystem services (like carbon storage 
and wildlife habitat) will be in the future, what 
the landscape will look like, what it means for 
people's ability to recreate and where their 
communities are," she says. "There are a lot of 
human dimensions that come from the character and 
distribution of these forests."
###
Daniel Donato, a former member of Turner's lab 
now with the Washington Department of Natural 
Resources, is a co-author on the study, which was 
funded by the U.S. Joint Fire Science Program and 
the National Park Service.

-- 
  ********************************************************************************
"Our results indicate that future reductions in 
Arctic sea ice cover could significantly reduce 
available water in the American west...."

Jacob O. Sewall and Lisa Cirbus Sloan. 
Disappearing Arctic sea ice reduces available 
water in the American west
GEOPHYSICAL RESEARCH LETTERS VOL. 31, 2004

******************************************************
"Linkages between northern high-latitude climate 
and precipitation in the Sierra Nevada suggested 
here could indicate that, under conditions of 
continued global warming, this drought-prone 
region may experience a reduction in 
Pacific-sourced moisture."

Jessica L. Oster et al. Late Pleistocene 
California droughts during deglaciation and 
Arctic warming. Earth and Planetary Science 
Letters 2009
*******************************************************
" Š the proportion of the land surface in extreme 
drought is predicted to increase from 1% for the 
present day to 30% by the end of the twenty-first 
century." 

Eleanor J. Burke et al. "Modeling the Recent 
Evolution of Global Drought and Projections for 
the Twenty-First Century with the Hadley Centre 
Climate Model." JOURNAL OF HYDROMETEOROLOGY 
OCTOBER 2006
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