Assessment of the Severity and Impact of Alder Dieback and Mortality in Alaska
Lori Trummer, USFS R10 FHP, Anchorage
Roger Ruess, Jack McFarland, University of Alaska, Fairbanks
ABSTRACT: Substantial dieback and mortality of thin-leafed alder (Alnus tenuifolia) is occurring in riparian areas across south-central and interior Alaska. The dieback has raised concern
since alders are considered “keystone species” due to their importance in riparian ecosystems and capacity to fix atmospheric nitrogen. Decline and mortality of alder is expected to alter
ecosystem nitrogen accumulation, forest stand development, and likely forage quality for harvestable subsistence species, such as moose. In 2005, two projects were undertaken to assess
the severity and impact of alder dieback and mortality: 1. Installation and monitoring of transects, and 2. Assessment of changes to nitrogen fixation rates.
IMPACTS OF ALDER DIEBACK AND MORTALITY ON ECOSYSTEM NITROGEN BALANCE IN ALASKA
TRANSECT INSTALLATION AND MONITORING
INTRODUCTION
Dieback and mortality of A. tenuifolia in riparian areas in Alaska has been reported since 2003. The damage
appears recent in most locations, occurring within the last decade, and continuing. We installed transects to:
‰ Monitor the spread and intensification of dieback and mortality
‰ Assess site conditions, associated fungi, insect defoliation, and climate factors
The objectives of the study were to 1) establish replicated long-term plots along riparian forests within 3 Alaskan regions
(Tanana Valley, Anchorage Basin - Eagle River, and Kenai Peninsula - Quartz Creek), 2) permanently tag and assess
stem canker infection and the degree of basal area & canopy losses from stem canker within these plots, 3) measure
nitrogen fixation rates and associated characteristics of both soils (climate and chemistry) and plants (nodule biomass,
leaf morphology and leaf chemistry, Frankia genetic structure) across a range of infected plants within each plot.
METHODS
100-ft permanent transects were installed across south-central Alaska in the Matanuska/Susitna Valley and on
the Kenai Peninsula. For each A. tenuifolia genet, we assessed stem condition and sprout abundance.
Dominant overstory and understory vegetation and various site characteristics were recorded. For 3 genets per
transect, every stem was marked and additional data were collected.
The population of stems (N=512) from plants randomly selected for N fixation measurements constituted approximately
14% of the greater than 3500 stems surveyed in all plots across the 3 landscape regions. Preliminary results indicate:
38%
Live with
dieback
31%
Fig 1: Within the 10 transects established in 2005, stems were
nearly evenly divided among three categories: live & healthy,
live with dieback, and dead (n=755).
2005
Y=
1.3897 + 0.7187*X
r = 0.67, P<0.0001
Y = 1.3897 + 0.7187*X
2
r2= 0.067, P<0.0001
40
20
0
20
40
60
80
100
Y= Y-4.1117
+ 1.2336*X
= -4.1117 + 1.2336*x
r2= r0.15,
= 0.15, P<0.0001
P<0.001
2
30
20
10
0
4
6
8
Genet mortality
Fig 1: Across the 90 plants selected for N fixation measurements,
the proportion of total basal area dead from canker infection scaled
linearly with the associated percentage of leaf area lost from these
plants.
10
12
14
16
Soil Temperature (°C)
Proportion
of BasalArea
Area Dead
Canker
(%) (%)
Proportion
of Basal
Deadfrom
from
Canker
healthy
live with
dieback
dead
Fig 2: In one year, the proportion of healthy stems decreased
14% and dead stems increased 16% (n=440 stems). This
indicates that alder dieback and mortality are continuing at a
rapid rate.
a
13
a
11
9
b
7
5
Eagle River
Eagle River
Alder woolly sawfly (Eriocampa ovata)
-1
60
0
40
-1
80
Soil Temperature (oC)
Fig 2: Across plants from all sites, N fixation rate was
positively correlated with soil temperature. Soil moisture had
no effect on N fixation rates.
(%)
Healthy
2004
Eagle
River
Eagle River
Quartz
Creek
Quartz Creek
Tanana
Tanana
Severe
defoliation
Dieback symptoms
Drought stress
Fig 3: A complex of host stressors is apparent at many affected sites, including severe defoliation by the introduced alder woolly
sawfly and continued record-breaking hot, dry summers since 2003. The role of these and other stressors in causing or
exacerbating alder dieback and mortality is poorly understood and under investigation.
Tanana
Tanana
Fig 3: There were significant differences among regions in N fixation
rates (F2,80= 4.98, P=< 0.01), with rates being highest in plants growing
along the Tanana River, lowest at the Quartz Creek drainage, and
intermediate along the Eagle River.
a
25
20
%
Basal Area Dead
% Basal Area Dead
% Leaf Area Dead
%
Leaf Area Dead
A
a
A
15
10
b
B
5
0
Eagle River
Eagle River
Quartz Creek
Quartz Crk
Tanana
Tanana
Fig 4: However, using an analysis of covariance, significant regional
differences in N fixation rates could be completely explained by the
combined independent effects of soil temperature (F1,80= 2.74, P =<
0.10) and differences in both percentage of total basal area dead from
canker (F1,80= 3.82, P= < 0.05) and percentage of leaf area killed by
canker (F1,80= 3.70, P =< 0.06) among regions.
DISCUSSION
These data suggest that greater levels of canker infection are leading to declines in N fixation rates at a regional scale.
Although we haven’t completed calculations or analysis of nodule biomass data among plots, our observations during
field sampling suggest that substantial amounts of nodule biomass were dying in plots most heavily infected by canker.
Thus, we believe that combining N fixation rates with nodule biomass to yield total N inputs will result in even greater
effects of canker on N balance at the ecosystem level.
FUTURE MONITORING AND ASSESSMENTS
‰ Continued monitoring transects for dieback and mortality;
‰ Establish transects in Interior Alaska in 2006;
‰ Continue assessing relationships between incidence of dieback /mortality and host stressors including
associated canker fungi, insect defoliation, climate change, and other factors.
For more information: Lori Trummer, USFS R10 FHP, ltrummer@fs.fed.us, (907) 743-9460
Quartz Creek
Quartz Crk
Proportion
Basal
Area oror
Leaf
Area
Dead
Fromfrom
Canker
(%)
Proportion
of Basal
Leaf
Area
Dead
Canker
Dead
50
45
40
35
30
25
20
15
10
5
0
-1 -1-1hr -1)
N Fixation
Rate
nodule
N Fixation
Rate(µmol
(μmol NNggnodule
hr )
DWTDWT
31%
Proportion of Stems (%)
RESULTS
Ten transects were established in south-central Alaska in 2005, where a total of 755 stems were examined
within 140 genets. Additionally, within six transects established in 2004, 440 stems within 93 genets were reassessed in 2005. All transects had some level of dieback occurring. A stem canker, putatively caused by Valsa
melanodiscus, appears to be the main cause of dieback (see top picture at right). V. melanodiscus has been
isolated from cankered tissue and pathogenicity tests are underway. Preliminary results indicate:
100
N Fixation Rate (μmol N g noduleDWT -1hr )-1
N Fixation
Rate (µmol N g noduleDWT hr )
Proportionof
ofLeaf
Leaf Area
Area Dead
(%)(%)
Proportion
Deadfrom
fromCanker
Canker
Valsa melanodiscus canker
Healthy Frankia root nodules
For more information: Roger Ruess, University of Alaska – Fairbanks, ffrwr@uaf.edu, (907) 474-7153