Natural Resources Preservation Program Report Format

TITLE PAGE:

TITLE: Can prescribed fire be used to control yellow sweetclover
(*Melilotus officinalis*) in a cool-season mixed-grass prairie?

PRINCIPAL INVESTIGATOR (PI): Diane L. Larson

PI ADDRESS: 100 Ecology Building, 1987 Upper Buford Cir., St. Paul, MN
55108

CO PI: None

CO PI ADDRESS: N/A

ANNUAL REPORT

Natural Resources Preservation Program

PROJECT NUMBER: Basis+ subtask 1.7 for NPWRC or 2.21 for CR

FISCAL YEAR: 2006

DATE: March 2, 2007

BODY OF REPORT:

ANNUAL REPORT

NRPP FUNDING PROGRAM 2005

NATIONAL PARK SERVICE REGION: Central

TITLE: Can prescribed fire be used to control yellow sweetclover
(*Melilotus officinalis*) in a cool-season mixed-grass prairie?

PRINCIPAL INVESTIGATORS: Diane L. Larson

INTRODUCTION: Yellow sweetclover has become a ubiquitous component of
the vegetation community in several National Parks within the northern
Great Plains. Based on a 1996 survey of 847 transects located in the
south unit of the Theodore Roosevelt National Park, yellow sweetclover
was the sixth most commonly encountered species (Larson et al. 2001).
Similarly, researchers compiling a vegetation map of the north unit of
Badlands National Park (BNP) found yellow sweetclover to be so common in
1998 that it would have compromised interpretation of aerial photography
to classify the vegetation types (Van Loh et al., unpublished report).
Given the fecundity of the species (up to 100,000 seeds/plant) and
longevity of the seedbank ( \> 40 years) (Turkington et al. 1978),
yellow sweetclover will remain a major management issue for Parks for
generations to come.

While sheer numbers are alarming, yellow sweetclover also harbors
nitrogen-fixing organisms that have potential to change the fertility of
soils in this nitrogen-limited grassland system. Nitrogen addition
experiments in tallgrass (Tilman 1987, Wedin and Tilman 1993) and
shortgrass (Dodd and Lauenroth 1979) prairie have been shown to decrease
species diversity and change dominance patterns among plant species..
Consequently,disturbances of anthropogenic origin, such as nitrogen
addition, can result in successional trajectories and fundamental
ecosystem changes that differ substantially from those encountered after
"natural" disturbances (Bazzaz 1996).

Prescribed burning is a key component of prairie management. Benefits
include maintenance of fire-dependent species, fuels management, and in
prairies especially, the reduction of woody plant encroachment. With the
increase in invasive exotic plant populations, however, prescribed fire
often has unintended consequences. Chief among them is the potential for
a change in dominance that can occur when an exotic species shows a
strong positive response to a particular fire regime. Such is the case
with yellow sweetclover, and other exotic species found in the Great
Plains (Grace et al. 2001). Typical prairie management that includes
spring burns at infrequent intervals favors sweetclover establishment
and reproduction (Kline 1986). Prescribed fire may provide a tool for
combating spread of exotic species (Galley and Wilson 2001), but
information is needed to determine optimal burning parameters for
individual species. It is also important to document effects on native
species, so that they are not harmed in the attempt to control exotics.

Dormant season burns, followed by a spring burn to kill the flush of
seedlings that follows, is a typical management prescription for control
of yellow sweetclover (Eckardt 1987). However, resource management staff
at B Nl P did not think this would be a practical solution, given the
vast acreages infested and the limited staff and competing needs for
prescribed burns. In addition, it is not clear that a second fire would
carry so soon after the previous fire in this arid setting.

Several aspects of a prescribed fire may interact with sweetclover
biology to either promote or retard its growth and reproduction. Seeds
of yellow sweetclover require scarification to germinate (Smith and Gorz
1965); fire is known to provide the needed scarification (Eckardt 1987),
but the intensity of fire required for scarification has not been
quantified. Fire temperatures in the northern mixed-grass prairie have
been quantified (Archibold et al. 1998); highest temperatures (500
degrees C) were recorded 10 cm above the soil surface during spring
burns, but quickly reverted to ambient conditions once the fire had
passed. Fire in this grassland environment did not elevate soil
temperature below 5 cm depth. A summer fire might be expected to be
somewhat more intense and have less spatial heterogeneity if additional
fuels have accumulated and if existing vegetation holds less moisture.
Because prescriptions can be written in such a way that intensity of the
burn is predictable, knowledge of sweetclover seed response to different
intensities would give managers an avenue by which sweetclover densities
and age structure could be manipulated.

Timing of a prescribed fire is also under control of the manager and can
influence subsequent survival and reproduction of yellow sweetclover
(Kline 1986, Turkington et al. 1978). Yellow sweetclover does not
survive fire, so burning at any time kills the existing plants. However,
seed germination is promoted, so that a flush of seedlings is likely
following burning. If seedlings emerge in the late summer or fall, their
chances of surviving the winter are small (Turkington et al. 1978). A
carefully timed burn may therefore both kill existing plants before they
set seed and potentially also start to deplete the substantial seedbank.
However, damage to late-season native forbs and grasses is a potential
undesirable side-effect of such a burn. Howe (1994, 1995) demonstrated
that, in Wisconsin, prescribed burns conducted in July favored
early-flowering prairie plants, while dormant-season burns favored
late-season species. Because the prairies invaded by yellow sweetclover
at Badlands National Park are dominated by early-season species, I
anticipate that later burns would produce only minor changes in their
species composition, compared with earlier spring burns. Late season
burns would more closely mimic the natural, lightening-caused fires
typical of the Great Plains (Howe 1994, 1995). However, the outcome of
prescribed burns in the arid Badlands region is also dependent on
subsequent weather that influences moisture availability (Whisenant and
Uresk 1989).

**Objectives**

The goal of the research is to estimate the effect of prescribed fires
that differ in intensity and season on subsequent population dynamics of
yellow sweetclover and on native prairie vegetation within areas invaded
by yellow sweetclover. Specific objectives include:

-   Estimate effects of season and type of burn on yellow sweetclover
    abundance in subsequent years.

-   Estimate effects of season and type of burn on native species
    abundance in subsequent years.

-   Estimate effects of season and type of burn on yellow sweetclover
    seedbanks.

-   Estimate effects of season and type of burn on exotic species
    abundance in subsequent years.

Our expectation is that higher intensity backing fires will stimulate
greater germination of yellow sweetclover and that later season burns
will promote germination yet not allow the plants to mature sufficiently
to survive the winter, thus depleting the seedbank.

METHODS: The study is being conducted at BNP, SD and takes advantage of
a recently-completed study of yellow sweetclover. Twenty random sites,
each with 2 or 3 permanent 2 x 6m plots, were established and
georeferenced in the north unit of the park in relatively accessible
terrain in 2001. Species composition and cover were recorded on most
plots in 2001 and on all in 2002 and 2003. Because yellow sweetclover is
known to vary greatly in abundance from year to year, the preexisting
data, as well as data collected on the unburned plots during the course
of the fire research, will aid in interpretation of fire-related
results.

At each of the 20 sites, the existing plot located at the random point
now serves as one of the five experimental plots we established for this
project. Four additional permanently marked plots were established in
May 2005 along a line running either due north or south. Plots were
spaced 25 meters apart, except in situations where more distance was
necessary to avoid landing in a different vegetation type. Each plot was
randomly assigned one of five treatments:

1\. spring burn, backing fire

2\. spring burn, head fire

3\. late summer/fall burn, backing fire

4\. late summer/fall burn, head fire

5\. control (not burned)

We sampled the seedbank in each plot to determine presence and relative
abundance of yellow sweetclover seed in 2005. The seedbank will be
resampled at the conclusion of the study in 2008. Five 5 cm diameter by
10 cm deep cores were collected systematically across the diagonal of
each plot and composited. Number of yellow sweetclover seeds/plot were
estimated via sieving and seed identification (Forcella et al. 2003).

In spring (late May-June) and summer (August) 2005 we measured percent
cover of each species on each experimental and control plot. In addition
to percent cover, we recorded number of individuals of sweetclover
because change in numbers of seedlings is an anticipated response to
burning and may not be adequately reflected in cover estimates. These
data will allow us to compare our treatment plots and control plots, as
well as allowing for pre- and post- treatment comparisons.

RESULTS:

*Seedbank:* The sweetclover seedbank was quite patchy (Table 1). Eleven
of the sites had no sweetclover seed in any of the plots.

*Prescribed burns:* It has become clear that this study plan, as
originally conceived, stands little chance of achieving its objectives.
During the 2006 field season, in which we had planned to conduct spring
and summer burns on 80 plots at 20 sites, limitations of weather and
manpower resulted in the completion of only 19 plots (at 10 sites)
between May 18 and June 28; no late summer burns were done. If the
situation improves in 2007, we will attempt to burn the remaining sites
as weather and manpower permit. However, to achieve the objective of
understanding sweetclover response to seasonality of burning, the
following changes in methodology will be implemented in 2007 -- 2008.

*Experimental planting of sweetclover seeds*

Seeds of yellow sweetclover plants near existing plots were collected
during July and August 2006. These seeds will be scarified and planted
at a site (fenced to exclude rabbits) near the fire cache at B N P every
two weeks from May 15 through October 15, 2007, approximate average date
of first killing frost. Approximately 50 seeds will be planted at each
date (actual number of seeds planted/plot will depend on results of
germination trials in winter 2006-7); half will be watered twice weekly
so that they receive [\>]{.underline} twice the average precipitation
each month, while the other half will receive only ambient rainfall.
Locations of planted seeds will be marked with cocktail toothpicks;
plants will be individually tagged with colored thread once they are
large enough, to distinguish the planted seedlings from volunteers;
volunteers will be pulled so that experimental plants can be
distinguished in 2008. We will use a randomized block design for the
planting (11 planting events x 2 treatments x 6 reps). We will record
germination (% of seeds planted that produce cotyledons) and
establishment (% of germinated seeds that survive to produce true
leaves) in the year of planting and survival (% of established plants
still extant) and flower production (number of flowers/plant) in June -
July 2008. Plants will be destroyed prior to seed-set.

*Native vegetation response to seasonality of fire*

The study will continue as described in the original study plan. Cover
will be estimated on all plots that we hope to burn in 2007 plus
controls prior to burning and post-burn in 2007 and at peak biomass in
2008. Cover will be estimated at all plots plus controls that were
burned in 2006 at peak biomass in 2007 and 2008. It will be critical
that burns occur in late summer in 2007 to estimate effects of burning
season on native plant species composition and productivity, however.

DISCUSSION: No discussion is currently possible, given the lack of late
season burns.

SUMMARY AND RECOMMENDATIONS: Given the difficulty we had assembling a
fire crew and burning within prescription this year, it seems unlikely
that the park will be able to implement a prescribed fire program that
would affect sweetclover, even if we find that seasonality of burn can
be used to deplete sweetclover seedbanks. However, the revised study
will give the park some understanding of the dependence of sweetclover
survival on timing of burns, should conditions arise that allow more
flexibility in burning.

LITERATURE CITED:

Archibold, O., L. Nelson, E. Ripley, and L. Delanoy. 1998. Fire
temperatures in plant communities of the northern mixed prairie.
Canadian Field-Naturalist **112**:234-240.

Bazzaz, F. A. 1996. Plants in changing environments: linking
physiological, population, and community ecology. Cambridge University
Press, Cambridge, U.K.

Dodd, J. L. and W. K. Lauenroth. 1979. Analysis of the response of a
grassland ecosystem to stress. Pages 43-58 [in]{.underline} French, N.
R., French, N. R. Perspectives in Grassland Ecology. Springer-Verlag,
New York.

Eckardt, N. 1987. The Nature Conservancy Element Stewardship Abstract
for *Melilotus alba, Melilotus officinalis*. The Nature Conservancy.
http://tncweeds.ucdavis.edu/esadocs/melioffi.html

Forcella, F., T. Webster, and J. Cardina. 2003. Protocols for weed seed
bank determination in agro-ecosystems. In: Weed management for
developing countries, Addendum I. R. Labrada (Ed.). FAO-Rome Plant
Production and Protection Paper 120, pp. 3-18.

Galley, K. E. M. and T. P. Wilson. 2001. Proceedings of the invasive
species workshop: the role of fire in the control and spread of invasive
species. Miscellaneous Publication. Tallahassee, FL, Tall Timbers
Research Station.

Grace, J. B., M. D. Smith, S. L. Grace, S. L. Collins and T. J.
Stohlgren. 2001. Interactions between fire and invasive plants in
temperate grasslands of North America. Pages 40-65 [in]{.underline}
Galley, K. E. M. and T. P. Wilson, Galley, K. E. M. and T. P. Wilson.
Proceedings of the invasive species workshop: the role of fire in the
control and spread of invasive species. Tall Timbers Research Station,
Tallahassee, FL.

Howe, H. F. 1994. Response of early- and late-flowering plants to fire
season in experimental prairies. Ecological Applications **4**:121-133.

Howe, H. F. 1995. Succession and fire season in experimental prairie
plantings. Ecology **76**:1917-1925.

Kline, V. M. 1986. Response of sweet clover (*Melilotus alba* Desr.) and
associated prairie vegetation to seven experimental burning and mowing
treatments. Proceedings of the North American Prairie Conference
**9**:149-152.

Larson, D. L., P. J. Anderson and W. Newton. 2001. Alien plant invasion
in mixed-grass prairie: effects of vegetation type and anthropogenic
disturbance. Ecological Applications **11**:128-141.

Smith, W. K. and H. J. Gorz. 1965. Sweetclover improvement. Advances in
Agronomy **17**:163-231.

Tilman, D. 1987. Secondary succession and the pattern of plant dominance
along experimental nitrogen gradients. Ecological Monographs
**57**:189-214.

Turkington, R. A., P. B. Cavers and E. Empel. 1978. The biology of
Canadian weeds. 29. *Melilotus alba* Desr. and *M. officinalis* (L.)
Lam. Canadian Journal of Plant Science **58**:523-537.

Wedin, D. and D. Tilman. 1993. Competition among grasses along a
nitrogen gradient: initial conditions and mechanisms of competition.
Ecological Monographs **63**:199-229.

Whisenant, S. G., and D. W. Uresk. 1989. Burning Upland Mixed Prairie in
Badlands National Park South Dakota USA. Prairie Naturalist
**21**:221-227.

Table 1. Estimated number of yellow sweetclover seeds/plot at Badlands
National Park.

  -------------- ------------------ ------------------ ------------------
  Plot \#        Plot letter        Number of seeds     
                                    per sample         

                                    Alive              Dead

  101            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  103            A                  18                 0

                 B                  112                0

                 C                  106                0

                 D                  0                  0

                 E                  38                 0

  104            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  105            A                  0                  0

                 B                  0                  0

                 C                  19                 0

                 D                  0                  0

                 E                  0                  0

  106            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  18

                 E                  0                  0

  107            A                  0                  0

                 B                  0                  0

                 C                  42                 0

                 D                  0                  0

                 E                  0                  0

  108            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  109            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  110            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  111            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  112            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  113            A                  20                 0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  35                 0

  116            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  117            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  24

  118            A                  0                  0

                 B                  52                 0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  119            A                  0                  0

                 B                  0                  0

                 C                  164                0

                 D                  36                 0

                 E                  59                 0

  120            A                  17                 17

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  121            A                  0                  0

                 B                  0                  0

                 C                  0                  0

                 D                  0                  0

                 E                  161                20

  123            A                  0                  0

                 B                  0                  19

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0

  124            A                  0                  0

                 B                  21                 0

                 C                  0                  0

                 D                  0                  0

                 E                  0                  0
  -------------- ------------------ ------------------ ------------------