Growth and mortality of adult invasive Rhamnus cathartica (European buckthorn) in west central Minnesota

1. Growth and mortality of adult invasive Rhamnus cathartica
(European buckthorn) in west central Minnesota
Heidi Swanson, Peter Wyckoff, Drew Larson, Caitlyn Horsch, Ellen Titus, and Abby Mallek
Division of Science and Mathematics, University of Minnesota, Morris
Literature cited
Kurtz, C.M. 2010. Effects of site and climate characteristics on forest invasibility by non-native
plants in the Midwest. Masters Thesis, University of Minnesota.
Potter, R, S. Smidt, H. Lindquist and P. Wyckoff. 2012. Impact of climate on growth of Acer
saccharum (sugar maple) at the prairie-forest border in western Minnesota. ESA annual
meeting, Portland, OR.
Wyckoff PH, Bowers R. 2010. Response of the prairie-forest border to climate change: impacts of
increasing drought may be mitigated by increasing CO2. Journal of Ecology 98: 197-208.
Wyckoff, P. H. and J. S. Clark. 2000. Predicting tree mortality from diameter growth: a
comparison of approaches. Can. J. of Forest Research 30(1): 156-167
Introduction
Projections show Minnesota climate becoming an
analog to contemporary Kansas in the summer and
Illinois in the winter by the end of the 21st century.
This predicted change leads to questions regarding
how forest extent and composition in western
Minnesota might respond.
Previous research reveals a paucity of recruitment
for native tree species at several forest sites along
the prairie-forest ecotone in western Minnesota.
Recruitment of invasive Rhamnus cathartica
(European buckthorn), on the other hand, is quite
robust. Recently we reported preliminary results
for an ongoing seedling transplant experiment
which suggest that while R. cathartica is less
sensitive to deer browsing compared to native
species in our ecotonal forests, growth of R.
cathartica seedlings and saplings is highly sensitive
to summer drought.
Here we extend that work into larger size classes
via tree ring analysis of naturally established R.
cathartica at three sites along a climate gradient.
We place our results in context through comparison
with other recent work we have done on two native
canopy species from the same region: Quercus
macrocarpa (bur oak; Wyckoff and Bowers 2010) and Acer
saccharum (sugar maple; Potter et al. 2012).
Acknowledgements
Funding for the project was provided by NSF/DEB Grant # 1019451 and the. Morris-HHMI Summer
Undergraduate Research program, which is supported by a grant to the University of Minnesota, Morris from
the Howard Hughes Medical Institute through the Precollege an Undergraduate Science Education Program.
We would like to thank J. Aday and S. Schuldt for help collecting and measuring tree ring samples from our
NLP site.
Methods
We used increment cores obtained from adult R.
cathartica at three sites (Fig. 2)
Cores were measured and crossdated using
COFECHA. Detrending using the hugershoff equation
was implemented in ARSTAN. We obtained regional
Palmer Drought Severity Index (PDSI) climate data
from NOAA. Statistical analyses were conducted in R.
Growth-mortality equations were calculated using a
second set of tree rings from our Niemackl Lake Park
(NLP) (methods from Wyckoff and Clark (2000)). Growth rate
distributions were scaled to underlying mortality rates
based on a permanent plot data and a survey of the
buckthorn population at NLP.
Results/Discussion
R. cathartica growth is strongly negatively correlated
with summer drought (Fig. 2), despite little canopy
exposure.
Understory trees are usually avoided in tree-ring
climate studies because of dampened climate
response, but adult R. cathartica showed more
drought sensitivity than either canopy Q. macrocarpa
or A. saccharum for our study region (not shown).
Dead R. cathartica individuals >5 cm DBH exhibited a
strong growth decline prior to death lasting 7-8 years
and corresponding to > 60% reduced ring width
compared to surviving R. cathartica (Fig. 3).
Dead individuals <5 cm DBH exhibited much less
growth decline prior to death and actually grew >50%
faster in their first 10 years of growth than survivors.
Thus, fast growing small R. cathartica are seemingly
at a higher risk of mortality, but the pattern reverses
for those individuals reaching larger sizes (Fig. 4).
Previous work shows that mortality risk for Q.
macrocarpa remains negligible even at very low
growth rates, which should provide resilience in the
face of warming-induced droughts. R. cathartica,
however, is more threatened by growth reductions
expected in a warmer, drier future (Fig. 5).
)
Old da ta and recent data
Fig. 1. R. cathartica occurrence
in Phase 2 FIA plots
(Figure: Kurtz 2010)
Old da ta and recent data
0
0.5
1
1.5
2
2.5
-10 -5 0 5
Detrendedgrowthrate
PDSI
Sibley State Park
Old da ta and recent data
Fig. 2. (A) Study site locations near the pre-
settlement prairie-forest border. (B-D) slope of
regression relating growth to drought increases from
north to south, indicating increasing drought
sensitivity (ANCOVA shows significant differences
between Ginseng Road Farm and Niemackl Lake
Park, p=0.013)
2A.
2B. R. cathartica growth declines with
increasing drought ….
2C. … Slope of response increases….
2D. … from north to south.
Fig. 3. Growth decline precedes death for R.
cathartica
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-10 -5 0 5 10
DetrendedGrowthRate
PDSI
Ginseng Road Farm
Gr = 0.023 * July PDSI + 0.90
r = 0.36
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-10 -5 0 5 10
DetrendedGrowthRate
PDSI
Niemackl Lake Park
Gr = 0.030 * July PDSI + 0.97
r = 0.53
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-10 -5 0 5 10
DetrendedGrowthRate
PDSI
Monson Lake State Park
Gr = 0.045 * July PDSI + 0.92
r = 0.46
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 5 10 15
Lifetimeavggrowth(mm)
DBH (cm)
Dead
Live
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1995 2000 2005 2010 2015
RingWidth(mm)
Live
Dead
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Probabilityofmortality
Average annual growth (most recent five years)
Fig. 5. Non-parametric fitted growth
mortality-functions for R. cathartica and
Q. macrocarpa (Bur Oak )
RHCA
QUMA
Fig.4. Small R. cathartica that live fast,
die young
DRY WET
DRY WET
DRY WET