Oak decline and mortality in the Ozark Highlands of Arkansas and Missouri:
temporal and spatial trends from 1999 to 2006 under periodic droughts
Zhaofei Fan, Department of Forestry, Mississippi State University; Xiuli Fan and Hong He, Department of Forestry, University of Missouri
Martin A. Spetich, U.S. Forest Service Southern Research Station, Hot Spring, AR; Stephen R. Shifley, U. S. Forest Service Northern Research Station, Columbia, MO
W. Keith Moser, US Forest Service Northern Research Station, St. Paul, MN
0.18
• We compared oak decline and mortality trends and
palmer drought severity index (PDSI) curves derived
from NOAA data using the cross-correlation analysis
and regression method to evaluate the impact of
drought on oak decline and mortality.
0.2
0.12
0.15
0.1
0.1
0.08
0.06
0.05
0.04
0.02
2001
2002
2003
2004
2005
2006
Inventory year
White oak group mortality (dead ba/total ba)
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B
B
0.1
0.15
0
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0
0.1
-0.1
0.05
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*
0
*
-0.4
L ag of y ear
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Non-oak species mortality (dead ba/total ba)
C
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C
0
0.15
-0.1 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
-0.2
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* * * * *
-0.4
-0.5
L ag of year
Figure 2. Temporal mortality trends (A) and cross-correlation
between red oak mortality and growing season (May-October) PDSI
of single (B) and accumulative (C) current and previous years (*
statistically significant at p=0.05).
5
Arkansas division 1
4
Jan
Feb
Mar
Apr
May
June
July
Aug
Sept
Oct
Nov
Dec
8
Missouri division 4
Figure 3. Spatial mortality (dead basal
area/total basal area) trends of red oak
group, white oak group and non-oak
species in the Ozark Highlands.
Jan
Feb
Mar
Apr
May
June
July
Aug
Sept
Oct
Nov
Dec
6
3
2
4
0
-11990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
PDSI
1
PDSI
0.2
2000
0.1
• We used Gaussian kernel density (smoothing) to
evaluate the spatial trend of oak mortality, in terms of
proportion of dead basal area, by species group over
the Ozark Highlands (four climate divisions: Arkansas
divisions 1 and 2; Missouri divisions 4 and 5).
0.14
A
0.05
• We calculated the percentage of dead trees in terms
of density and basal area (ba) for different species
(groups) by inventory year based on the 1999-2006
annual FIA plots in the Ozark Highlands of Arkansas
and Missouri.
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1999
C o rrela tio n c o effic ien t
Data and Methods
A
White oak group
Red oak group
Nonoak
0
C o rre la tio n c o e ffic ie n t
Since the late 1970’s oak decline and mortality have
plagued Midwestern-upland oak-hickory forests,
particularly oak species in the red oak group (Quercus
Section Lobatae) across the Ozark Highlands of
Missouri, Arkansas and Okalahoma (Dwyer et al. 1995).
Advanced tree age and periodic drought, as well
as opportunistic organisms such as armillaria root fungi
and red oak borers are believed to contribute to oak
decline and mortality. Declining trees first show foliage
wilt and browning followed by progressive branch
dieback in the middle and/or upper crown. Trees
eventually die if crown dieback continues. We analyzed
oak mortality by species (groups) and inventory year to
illustrate the general spatial and temporal trends of oak
decline and mortality by using the 1999-2006 FIA plots
measured annually in the Ozark Highlands of Arkansas
and Missouri.
Mortality (dead ba/total ba)
Introduction
2
-3
-2
-4
-5
Arkansas division 2
Jan
Feb
Mar
Apr
May
June
6
July
Aug
Sept
Oct
Nov
Dec
5
2
3
1
2
0
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
-1
Jan
Feb
Mar
Apr
May
June
July
Aug
Sept
Oct
Nov
Dec
1
0
-11990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
-2
-2
-3
-3
-4
Missouri division 5
4
PDSI
PDSI
3
-4
-4
-5
Compared with non-oak and white oak group species, red oak
group species showed an escalating mortality trend in terms of
proportion of dead basal area. Red oak group species decline
was the major determinant of this trend, while the white oak
group and non-oak species maintained a relatively stable
mortality rate in the sequence (Figure 1A).
Dead basal area for non-oak and white oak group species
fluctuated around 4% during the sequence, while the red oak
group species as a whole increased jaggedly from around 8% in
1999 up to 16-18 % by 2006. Two peaks in the sequence
corresponded to the escalating mortality in years 2000 and
2003 (one year earlier than the inventory year).
Impact of periodic droughts in the Ozarks on red oak decline
and mortality was evident through the red oak mortality and the
PDSI trends (Figures 1 and 2A). The oscillation cycle of 2 to 5
years of PDSI corresponded to the mortality change in a
systematic way. Cross-correlation analyses indicated that
mortality was significantly correlated with the growing season
PDSI and usually lagged 2 to 3 years behind single drought
events (Figure 2B). Moreover, the cumulative impact of
droughts might last up to 10 years based on the past 17 year’s
PDSI data (Figure 2C). The Ozarks experienced a severe
drought period from 1998 to 2000 and a mild drought period in
2005 and 2006. These triggered the escalation of mortality
starting from year 2000 (Figure 2A). The result agrees with the
dendrochronological findings in the Ozarks.
Spatially, high red oak mortality areas (hot spots with a
cumulative proportion of dead basal area >0.15) mainly
occurred in the central area of the Ozarks (Figure 3A). The
moderate mortality (with the proportion of dead basal area of
0.10~0.15) was widely spread over most areas of the Ozarks
and the low mortality was distributed around the outer area of
the Ozarks (Figure 3A). In contrast, for the white oak group and
non-oak species the high mortality was rare, the moderate
mortality was sporadic, and the low mortality was dominant in
the Ozarks. (Figure 3B and3C).
As shown, drought pattern varied spatially and temporally in
the Ozarks (Figure 1) and was the major driver of large-scale
oak decline and mortality patterns. Besides, tree characteristics
(crown class and dbh) and competition condition were most
important to oak decline and mortality at stand or small scales
(Fan et al. 2006, Shifley et al. 2006). We will use a hierarchical
approach to explore how regional climate patterns like drought
interact with local factors to affect oak decline and mortality in
the future study.
Major references
0
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
-2
4
Results and Discussion
Red oak group mortality (dead ba/total ba)
Figure 1. Palmer drought severity index (PDSI) in the four climate divisions of the Ozark Highlands of Arkansas and Missouri.
Fan, Z., J. M. Kabrick, and S. R. Shifley. 2006. Classification
and regression tree based survival analysis in oakdominated forests of Missouri’s Ozark highlands. Can. J. For.
Res. 36: 1740-1748.
Shifley, S. R., Z. Fan, J. M. Kabrick, and R. G. Jensen. 2006.
Oak mortality risk factors and mortality estimation. For. Eco.
Manage. 229:16-26.