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

Det

ren

de

d g

row

th r

ate

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

Det

ren

de

d G

row

th R

ate

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

Det

ren

de

d G

row

th R

ate

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

Det

ren

de

d G

row

th R

ate

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

Lif

etim

e a

vg g

row

th (

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

Rin

g W

idth

(m

m)

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

Pro

bab

ility

of

mo

rtal

ity

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