What
are the criteria to identify a savanna?
Immediately we found differences of opinion, although
not disagreement. Species composition, soil characteristics, topography (related
to fire behavior), and canopy coverage were all aspects of the stand which
individuals in the group (including DNR, ISU, USFWS, and others) identify
savanna. Like prairie, savanna exhibits considerable variability based on
moisture and climate. Experts in the group agreed that savanna is a continuum
community, a transitional stage, never a stationary fixture in the landscape. It
should be considered an ecosystem on the brink of extinction in Iowa.
What
are the tools for managing Savanna?
Burning the undergrowth in a savanna creates an
immediate and dramatic change on some sites, particularly with drier soils.
Burning frequency and timing may be different for savanna systems than for
prairie, and it seems that there are no current standards, only trial and error.
Persistence is definitely required, as transforming the plant community from
non-fire-tolerant species to appropriate fire-tolerant species takes time.
Members of the discussion group see a need for comparison of management
strategies and results for the various savanna types across the state. Some
members of the group felt that unmanaged savanna becomes successional deciduous
forest. Others felt that it would always be simply degraded savanna. The
restoration of a savanna must respect the individuality of each community
system, and management should be a tool to restore a savanna to its own natural
state, rather than a set of calculated specifications.
Cutting and/or spraying invasive shrubs is another
widely used, labor intensive means of exposing the herbaceous layer. In many
cases, attempted burns will not carry through a shrubby degraded savanna. There
is anecdotal evidence that opening the canopy by girdling or removing larger
trees in various spots within the degraded savanna enables patchy restoration
which may be more effective than management across a site (referred to as
“punching holes” in the stand, rather than sweeping through from one end to
the other). Fire will burn more readily across these open spots where ground
cover is heavier after thinning the canopy.
What
are some effects of fire on savanna?
Although many species of oak and other deciduous
trees are appropriate in various savanna types across Iowa, bur oak seems very
common and well-adapted to fire. Oak recruitment is greater in burned areas.
Individual survivors grow more quickly and more stout than unburned bur oak
saplings. Neither cutting nor burning are effective methods for removing small
bur oaks. Their deep root systems are strengthened by injury above ground.
Another noted feature of bur oak is its adaptability to different sites. It is
most common in uplands, but has also been found in sand bar savannas in a flood
plain.
What
role does savanna serve to make it attractive to the public and policy makers?
There is ample evidence of the role of savanna and
prairie in landscape hydrology, which has implications for water quality and
climate. The dense prairie sod that occurs below the trees in savanna holds more
surface runoff than a heavily shaded, shrubby understory, while paradoxically
using less water than an area thick with trees and shrubs. This tendency was
illustrated by the restoration of a north-facing slope at Neil Smith where trees
and shrubs were removed to encourage a degraded prairie plant community,
including a patch of sedges, on the hillside. The result was that the
bottomland, which had been a dry crop field, became a wet meadow. The
restoration of natural hydrology to the soils provides needed habitat for
invertebrates, creating more diverse wildlife habitat, and improves
rainwater/snowmelt infiltration rates, which protects surface and ground water
from contamination. The native vegetation also increases the available surface
area for condensation, which decreases humidity in the immediate area during hot
days.
Carbon sequestration: Savanna has the potential to
store more carbon than a forest, since the trees store carbon above ground,
where it is eventually released with the death of the tree, while perennial
native grasses and forbs have most of their biomass underground, where it is
trapped (banked) permanently.
Should all else fail, savanna is beautiful. It seems
likely that early settlers built their houses within savannas, which were
attractive, full of fuel wood and familiar because of the trees. Even now,
savanna makes a nice backdrop for newer homes, although restoration and/or
protection of the savanna may suffer from the proximity of a residence.
What
is the fate of the savanna in Iowa?
Iowa DNR, The Nature Conservancy, Indian Creek Nature Center, US Fish & Wildlife and others have used the maintenance techniques described previously to restore degraded savanna, and all agreed that with persistence their efforts were successful, although almost no consistent monitoring has been done. Mesic savanna seems to be the most common and the least likely to be restored, since it is easily grazed or allowed to fill up with invasive species that give it the appearance of a forest. Wet savannas, the most difficult to restore, are probably most rare. There is rarely any financial or political incentive for landowners to protect, restore or reconstruct savanna. The state has specified that for a wooded acre to be eligible for tax forgiveness, it must contain 200 or more trees, which is more than a “healthy” savanna supports. On the other hand, Iowa’s Slough Bill, intended to provide tax relief for property allowed to remain in “prairie”, is optional for each county government. When it is implemented, a county assessor may increase the taxes on the remainder of the property, which denies the landowner the intended credit.
Members of the discussion group suggested promoting a
system of water credits or carbon credits for landowners who maintain healthy
savanna or prairie remnants and buffers. The EPAs Phase II Stormwater
regulations may spur more greenscaping in urban environments. Eventually our
whole society must realize the value of protecting water, air, habitat and
biodiversity.