Frost Crack Incidence in Northern Hardwood
Forests of the Southern Boreal–North Temperate
Transition Zone
ABSTRACT
Julia I. Burton, Eric K. Zenner, and Lee E. Frelich
Frost cracks are common in northern hardwood stands near their northern range limits. Although they have long been attributed to the regional climate,
temperature fluctuations result in surface cracks largely when internal wounds are present. We examined the relationship between the proportion of trees with
frost cracks and both tree-level diameter class and stand structural characteristics in primary stands with a history of minimal logging (n ⫽ 4) and 67- to
97-year-old second-growth stands subjected to past heavy partial cuts and high grading (n ⫽ 8). We hypothesized that frost crack incidence would (1) be greater
in the second-growth stands and (2) be associated with differences in structural attributes between the two stand types. High levels of frost cracking in primary
stands indicated that cracks are not completely avoidable. However, the proportion of trees with frost cracks was significantly higher in second-growth than
primary stands, particularly on small-diameter trees. For example, the odds for frost cracking were 1.66 –3.74 times greater in second-growth than in primary
stands in the 15-cm diameter class, but were not different in the 45⫹-cm diameter class. Frost cracking was positively associated with increasing diameter
in both stand types. Structural characteristics reflecting tree size, stand basal area, and basal area of hardwoods were positively associated with the proportion
of trees with frost cracks in second-growth stands but not in primary stands. Although the basal area of conifers was negatively associated with frost cracking,
the effect was likely due to a reduction in hardwood basal area in the vicinity of conifers. We suggest that greater frost crack incidence in second-growth stands
is likely a consequence of injuries to residual trees during selective logging.
Keywords: forest structure, frost cracks, high grading, northern hardwoods, sawlog quality
F
rost cracks are included as one of the major visible tree defects
that contribute to the renowned poor quality of many secondgrowth northern hardwood stands in the upper Great Lakes
region (Winget 1969, Miller et al. 1978). Frost cracks are radial and
vertical plane separations in the stems and branches of trees and are
the most important places of entry for up to 28 decay-inducing buttrot and trunk-rot fungal species in sugar maple (Nordin 1954, Vasiliauskas 2001). They reduce wood quality and render stems unsuitable
for veneer, thereby lowering sawlog values (Kubler 1983). Frost cracks
expand when temperatures fall and contract as temperatures rise. In the
spring, bark callus tissues form and bridge the crack, but cracking and
healing may recur for several years, and repeated cycles of freezing and
thawing may lead to frost crack extension along the tree surface (Kubler
1983). Therefore, frost cracking has long been associated with sudden
decreases in temperature and long spells of cold temperatures (Caspary
1855, Hartig 1894).
The frequency and severity of frost cracking is highest where
species are near their northern range limits (Lamprecht 1950, Malaisse 1957), leading to the general supposition that frost cracks are
inevitable at northern latitudes (Butin and Shigo 1981). We now
know, however, that frost cracks are also closely associated with
wounds and stubs (Shigo 1972, Phelps et al. 1975, Kubler 1983)
and result from periodic freezing and thawing largely when internal
wound cracks are already present (Butin and Shigo 1981). Many
second-growth northern hardwood stands in the upper Great Lakes
region developed after heavy partial cuts between 1910 and 1940
that left variable levels of residual stocking (Miller et al. 1978,
Stearns 1997) and have subsequently been selectively harvested,
leaving low-value residual trees. Quite possibly, the high incidence
of frost cracks may be associated with harvesting activities that resulted in tree damage (Winget 1969, Butin and Shigo 1981). It is
unclear, however, what the background level of frost cracks is in
northern hardwood forests that exist near the northern edge of their
climatic range. Furthermore, the proportion of trees with frost
cracks (%FC) in selectively cut, second-growth stands has never
been compared with that of primary stands showing limited to no
evidence of past logging (sensu Frelich et al. 2005).
Primary stands on the North Shore of Lake Superior in Minnesota have a continuous heritage of natural disturbances, followed by
regeneration forming beneath resulting canopy openings (Frelich
and Reich 2003). These stands provide a “natural background level”
of frost cracking. We contrasted the incidence of frost cracks in such
primary stands with nearby second-growth stands that were previously high graded. We hypothesized (1) that %FC would be greater
Received February 1, 2007; accepted November 14, 2007.
Julia Burton (jiburton@wisc.edu), University of Wisconsin–Madison, Department of Forest Ecology and Management, 120 Russell Labs, 1630 Linden Drive, Madison, WI 53706.
Eric Zenner, School of Forest Resources, The Pennsylvania State University, University Park, PA 16802. Lee Frelich, Department of Forest Resources, University of Minnesota, 1530
Cleveland Ave. North, St. Paul, MN 55108. Special thanks go to The Nature Conservancy of Minnesota and Lake County Forestry. We are grateful to Meredith Cornett, Chris
Dunham, Dan Spina, Brady Boyce, Craig Ferguson, and Jeri Peck for their contributions to this project. Funding was provided by National Oceanic and Atmospheric Administration
and Minnesota’s Lake Superior Coastal Program. Erik V. Nordheim and Peter M. Crump provided statistical assistance. We also thank Ralph Nyland and several anonymous reviewers
for excellent suggestions on previous drafts.
Copyright © 2008 by the Society of American Foresters.
NORTH. J. APPL. FOR. 25(3) 2008
133
in the second-growth stands. In addition, because primary and second-growth northern hardwood stands often differ in overstory
structure or composition (Goodburn and Lorimer 1999, Hale et al.
1999, McGee et al. 1999, Crow et al. 2002), we hypothesized (2)
that %FC would be associated with differences in structural attributes between the two stand types.
Table 1. Mean (standard deviation) of species composition and
structural characteristics for primary stands and second-growth
stands.
Structural attribute
2
Methods
Study Area
Sugar maple-dominated northern hardwood forests stretch along
the North Shore of Lake Superior in a 7.5-km-wide (4.7-mile-wide)
belt 5 km (3.1 miles) inland from the lake, within the northern
temperate–southern boreal forest transition zone (Pastor and Mladenoff 1992). Elevations range from 200 to 700 m (656 to 2,297 ft),
and the topography is gently rolling to steep. Cliffs and outcroppings of exposed to sparsely vegetated Precambrian rock are common both on the shore and inland. Soils are derived from glacial till
and are sandy loams classified as Udepts (Anderson et al. 2001).
Lake Superior moderates the climate, which is cold-temperate continental, with long snowy winters, a mean growing season length of
128 frost-free days, mean annual temperature of 3.3°C (38°F), and
mean annual precipitation of 70 cm (28 in.), 40% of which is
deposited during the growing season (Albert 1995).
Experimental Design
Twelve sites located within 13 km (approximately 8 miles) of
Lake Superior in Lake County in northeastern Minnesota were selected on the basis of harvest history and lack of recent major natural
disturbance. The four primary stands were located on state lands
and had had some limited-intensity selective logging in the past
(Rusterholz 1996). The eight second-growth stands were located on
nearby county land and were cut heavily in the early 1900s and
selectively harvested for conifers and yellow birch in the 1960s (Dan
Spina and Chris Dunham, Lake County Forestry and The Nature
Conservancy of Minnesota, July 8, 2002). According to structural
threshold definitions based on exposed crown area and diameter
distributions (Lorimer and Frelich 1998), three primary stands and
one second-growth stand were in the old-growth stage of development, whereas the remaining stands were mature (Burton 2004). All
stands were composed predominantly of a sugar maple (Acer saccharum Marsh.) overstory. Less common overstory associates included
yellow birch (Betula allegheniensis Britt.), black ash (Fraxinus nigra
Marsh.), northern white cedar (Thuja occidentalis L.), white spruce
(Picea glauca [Moench] Voss), paper birch (Betula papyrifera
Marsh.), balsam fir (Abies balsamea [L.] P. Mill.), and red maple
(Acer rubrum L.).
Field Sampling
In the spring of 2003, we established a 120 ⫻ 120 m (1.44 ha
[3.56 ac]) research plot in each stand. Twenty-five circular subplots
(10 m [32.8 ft] radius) were established on a 30 ⫻ 30 m (98.4 ft)
grid. Plots were sampled over the summer. We recorded the species,
condition (i.e., alive or dead), diameter at 1.37 m (4.5 ft; dbh), and
the presence or absence of frost cracks for all trees ⱖ10 cm (⬃4 in.)
dbh. For a subsample of 56 trees that were selected to represent a
replicated sample of all diameter classes and species, we measured
crown radii in the four cardinal directions to estimate exposed
crown area (ECA). The proportion of total ECA of overstory trees
and the ratio of ECA in large to mature trees have been used to
134
NORTH. J. APPL. FOR. 25(3) 2008
⫺1
Basal area (m ha )
(ft2 ac⫺1)
Sugar maple (%)
Yellow birch (%)
Conifers (%)
LTD (stems ha⫺1)
(stems ac⫺1)
Sugar maple (%)
Yellow birch (%)
Conifers (%)
Modal D (cm)
(in)
L:M
%Large
Primary stands
(n ⫽ 4)
Second-growth
stands
(n ⫽ 8)
33.5 (1.7)
146.0 (7.4)
68.8 (3.9)
13.7 (6.2)
13.6 (2.5)
563 (80)
228 (32)
82.7 (6.8)
5.0 (3.4)
9.0 (4.5)
49.7 (8.9)
19.6 (3.5)
1.36 (0.6)
33.34 (11.0)
30.1 (4.2)
130.7 (18.3)
80.5 (17.5)
3.7 (2.7)
6.7 (8.2)
598 (105)
242 (42)
86.7 (1.7)
1.3 (0.1)
4.9 (6.9)
42.8 (6.8)
16.9 (2.7)
1.29 (1.1)
25.16 (11.2)
P value
0.145
0.108
0.044
0.060
0.581
0.665
0.114
0.319
0.162
0.912
0.257
LTD refers to live trees per hectare, Modal D is the modal diameter class, L:M is the ratio of
large to mature trees, and %Large is the percentage of large trees in a stand. Modal D, L:M, and
%Large are based on aggregate exposed crown area. P values are reported for t tests; variances
were pooled when they were not significantly different (P ⬎ 0.05).
distinguish forest developmental stages more accurately than stem
density or quadratic mean diameter (Lorimer and Frelich 1998) and
may be more useful than conventional metrics for describing forest
structure (Goodburn and Lorimer 1999). ECA was computed as the
sum of four quarter ellipses for trees where we measured crown radii
and estimated for other trees on the basis of regressions of ECA on
dbh (Burton 2004).
Statistical Analysis
Statistical analyses were limited to sugar maple (86.1% of all
trees, 90.4% of all frost cracks). The %FC of sugar maples was
evaluated with respect to stand type (primary versus second-growth
stands) and selected attributes of forest structure (Table 1): total
basal area (BA) (m2 ha⫺1), basal area of hardwoods (BAH)
(m2 ha⫺1), basal area of conifers (BAC) (m2 ha⫺1), live tree density
(LTD) (trees ha⫺1), modal diameter (the 5-cm diameter class with
the greatest aggregate ECA) (Modal D), the ratio of large trees (ⱖ45
cm [⬃18 in.] dbh) to mature trees (25– 44.9 cm [⬃10 –18 in.] dbh)
(L:M), and the percentage of large trees (%Large). L:M and %Large
were calculated for each subplot on the basis of the aggregate ECA of
large, mature, and pole-sized 10 –24.9 cm (⬃4 –10 in.) dbh trees
and then averaged to the stand level. Although some of the trees at
our sites exceeded 100 cm dbh (⬃39 in.), all trees ⬎40 cm dbh (16
in.) were included in the 45⫹ cm dbh class to obtain adequate
sample sizes (live tree densities) within all diameter classes and
stands (Table 2).
First, we examined the effects of stand type (primary and secondgrowth) on %FC using a two-sample t-test. We examined the effects
of stand type, diameter class (15, 25, 35, and 45⫹ cm [6, 10, 14, and
18⫹ in.]), and the interaction of stand type and diameter class on
%FC in each stand-diameter class combination (12 stands ⫻ 4
diameter classes, diameter class nested in stand) with a general linearized mixed model (GENMOD procedure), accounting for nonconstant variability in diameter classes and assuming a binomial
distribution. This additional test of stand type allowed us to examine the significance of stand type after accounting for variability
imposed by differences in diameter distributions. The association
between %FC and attributes of forest structure was explored using
Table 2. Total number of sugar maples per 10-cm (4-in.) diameter
class in primary and second-growth stands.
Type and Stand
Primary
Caribou River
Egge Pond
Manitou North
Manitou South
Second-growth
Big Pine
Birch Cut
Brush Hog
Cedar Finger
Egge Lake
Hill Top
Power Line
Schoolhouse Trail
15 cm
(6 in.)
25 cm
(10 in.)
35 cm
(14 in.)
45⫹ cm
(18 ⫹ in.)
110
209
220
219
55
124
145
94
48
50
43
37
46
22
15
38
Table 3. Logistic regression results of associations between the
proportion of frost cracks of sugar maple (%FC) and structural
attributes. Main effects are stand type (primary and second-growth
stands) and structural attributes (modal diameter [Modal D], the
ratio of large to mature trees [L:M], the percentage of large trees
>45 cm [>18 in.] [%Large], live tree density [LTD], total stand basal
area [BA], basal area of hardwoods [BAH], and basal area of
residual conifers [BAC]).
Structural attribute
149
142
75
206
268
306
156
318
198
93
69
116
134
146
118
161
80
48
23
67
34
16
47
34
39
51
4
37
17
18
49
27
Modal D
L:M
%Large
LTD
BA
BAH
BAC
Stand
type
Structural
attribute
Stand type ⫻
structural attribute
⬍0.001
⬍0.001
⬍0.001
⬍0.001
⬍0.001
⬍0.001
⬍0.001
⬍0.001
0.455
0.008
0.038
0.815
0.745
⬍0.001
⬍0.001
0.020
⬍0.001
n.s.
0.012
0.008
n.s.
Stand type ⫻ structural attribute represents the interaction of both main effects on %FC.
Interactions that were not statistically significant (n.s.; P ⬎ 0.05) were dropped from the
model. Values in the table are P values.
Figure 1. Relationship between the proportion of sugar maples
with frost cracks (%FC) and dbh. Solid circles and lines (primary
stands) and open circles and dotted lines (second-growth stands)
represent observed and modeled values, respectively.
multiple logistic regressions (GENMOD procedure). Logistic regressions yield estimates for the change in the odds of observing a
frost crack per unit change of a particular explanatory variable.
When a confidence interval for an odds ratio does not include 1, the
relationship is statistically significant. In this study, we used an ␣
level of 0.05. All statistical analyses were performed in SAS for
Windows (version 9.1, SAS Institute 2006).
Results
%FC in sugar maple ranged from 22 to 27% in primary stands
and from 24 to 43% in second-growth stands. The two-sample t
tests showed that, on average, frost crack incidence was significantly
higher (P ⫽ 0.004) in second-growth (35.4 ⫾ 7.3%, mean ⫾ 1 SD)
than in primary stands (24.8 ⫾ 2.2%).
In both stand types, %FC increased with diameter class (P ⬍
0.001), but even after accounting for this trend, %FC remained
significantly higher (P ⫽ 0.010) and consistently more variable in
second-growth compared with primary stands (Figure 1). A weak
interaction between diameter class and stand type (P ⫽ 0.090)
indicated that the difference in %FC between stand types lessened
somewhat with increasing diameter classes. For example, the odds
for frost cracking were 1.66 –3.74 times greater in second-growth
than in primary stands in the 15-cm (6-in.) diameter class; however,
the odds for frost cracking were not significantly different
(0.92–3.01 times; P ⬎ 0.10) between second growth and primary
stands in the 45⫹-cm (16⫹-in.) diameter class.
%FC was strongly associated with several structural attributes
(Table 3). Significant interactions between stand type and structural
attributes indicate that these associations were different in primary
and second-growth stands, however (Figure 2). It is estimated that
the odds of frost cracking in second-growth stands increase by 2.9 to
5.3% (95% CI) for every 1-cm (0.4-in.) increase in Modal D (P ⬍
0.001), 10.5 to 28.1% for every unit increase in L:M (P ⫽ 0.019),
6.2 to 53% for every 10% increase in %Large (P ⬍ 0.001), 3.0 to
6.9% for a 1-m2 ha⫺1 (4.4-ft2 ac⫺1) increase in BA (P ⫽ 0.012),
and increase by 3.4 to 6.8% for every 1-m2 ha⫺1 (4.4-ft2 ac⫺1) in
BAH (P ⫽ 0.008). None of these attributes were statistically significantly associated with %FC in primary stands (P ⫽ 0.356, 0.399,
0.451, 0.161, and 0.127, respectively), however. The lack of significant relationships between stand structure and %FC in primary
stands may be a result of the relatively smaller sample size of primary
stands (n ⫽ 4 stands) and the smaller ranges of the structural attributes encountered in those stands. Nonetheless, results indicate
that differences in the incidence and thus in the odds of frost cracking between second-growth and primary stands increased with increasing values of Modal D, L:M, %Large, stand BA, and BAH
(Figure 2).
In contrast to these structural attributes, %FC was negatively
associated with increasing values of LTD and BAC in both stand
types. It is estimated that the odds of frost cracking decrease between
1.1 to 12.1% for each additional 100 trees ha⫺1 (40 tree ac⫺1) (P ⫽
0.038) and by 3.5 to 10.8% for each additional 1 m2 ha⫺1 (4.4
ft2 ac⫺1) of BAC (P ⬍ 0.001). For any level of tree density and
BAC, the odds of frost cracking in second-growth stands are 1.52 to
2.01 and 1.16 to 1.64 times the odds in primary stands, respectively.
After accounting for BAH in these models, neither BAC (P ⫽
0.235) nor LTD (P ⫽ 0.3496) was significantly associated with
%FC, however.
Discussion
High %FC of sugar maple is typical for regions characterized by
low winter temperatures, sudden large changes in temperature, and
long-lasting low temperatures (Hartig 1894, Ishida 1963). These
NORTH. J. APPL. FOR. 25(3) 2008
135
Figure 2. The proportion of sugar maples with frost cracks (%FC) plotted against structural characteristics (all species included). Solid
circles represent primary stands; open circles represent second-growth stands. Lines represent predicted values of %FC for primary and
second-growth stands based on simple logistic regressions.
events cause expansion of internal frost cracks to the tree surface due
to recurring freeze-drying cycles, intercellular expansion of freezing
water into ice, and differences between tangential and radial shrink136
NORTH. J. APPL. FOR. 25(3) 2008
age, which induce critical tensile stresses at low stem temperatures
(Mayer Wegelin et al. 1962, Schirp 1968, Kubler 1983, 1988).
Perhaps because of these climatic conditions, we found that frost
cracks were common at our study sites on Minnesota’s North Shore,
occurring on about one in four trees in our primary stands. Although no figures on frost crack incidence in other primary stands
are available for comparison, our data suggest that managers should
expect frost cracks to develop in a substantial proportion of trees in
northern hardwood forests along the North Shore of Lake Superior.
However, although our study did not identify exact causes for the
difference in frost crack incidence between stand types, our results
indicate that the regional climate alone does not explain why %FC
in the second-growth stands (about one in three trees) was significantly (43%) higher than in primary stands.
We hypothesize that logging physically damaged appreciable
numbers of residual trees in the second-growth stands, predisposing
them to frost cracking. The underlying cause of frost cracks is the
presence of radial shakes, which are often associated with wounds
and branch stubs that interrupt the continuous circumferential
trunk surface (i.e., frost; Shigo 1963, Shigo et al. 1979, Butin and
Shigo 1981). Injuries that lead to frost cracks in primary stands have
likely come from windthrow, damage to crowns during windstorms,
snow and ice loading, and past light surface fires (Minnesota Department of Natural Resources 2003). Levels of frost crack incidence in our second-growth stands are similar to those among other
second-growth northern hardwoods with comparable histories (e.g.,
30 – 40% in Quebec [Winget 1969]) and likely reflect an added
impact of past logging damage that caused more widespread wounding to residual trees and advance regeneration. Although we have no
quantitative data on the intensity of past logging for these stands,
existing stumps indicate that the cutting ranged from heavy partial
cuts and subsequent high grading to selective cutting of single trees
(Burton 2004). These different histories might have contributed to
the greater variability in frost crack incidence in the second-growth
stands relative to the primary stands. However, even our primary
stands showed some evidence of selective logging (i.e., a few cut
stumps) (Burton 2004). As a consequence, our data may actually
underestimate the true effects of logging in second-growth forests of
the region, given that frost cracking in primary stands may not only
reflect natural injuries, or a natural background level, but also
wounds caused during the selective removal of some single trees.
Partial cutting and high grading across entire stands should result
in more injuries to the residual trees than occurs with intermittent
patchy natural disturbances. However, only the former has been
documented. For example, among second-growth northern hardwood stands in northern Wisconsin, Arbogast (1950) found that for
each large cull tree felled (64 cm [25 in.] dbh on average), 1.5 trees
ranging from 2.5 to 25⫹ cm dbh (1–10⫹ in. dbh) were damaged
beyond recovery, and 3.5 trees were damaged less severely. Where
the crown of a felled tree landed and was dragged across the forest
floor, about half the trees in the pathway of the crown were damaged
to some extent, and half of those were less than 10 cm dbh (4 in.).
Most trees injured during felling operations (35–70% of damage
during harvest operations in northern hardwood stands) sustain
“major” damage (60 – 80%; Nyland and Gabriel 1971, Irwin 1972),
and more than half of trees injured from felling or skidding are either
saplings or poles (Deitschman and Herrick 1957, Nyland and Gabriel 1971, Davis and Nyland 1991). Small trees that survive the
initial wounds and shakes and subsequently grow into sawtimber
may have an increased probability of developing frost cracks. To
reduce this possibility, Butin and Shigo (1981) have argued that
foresters need to reduce the incidence of even seemingly minor
wounds, particularly to the lower bole of trees.
Our results show that proportionally fewer frost cracks are found
on small sugar maples and that frost crack incidence increases with
increasing dbh (Busse 1910, Ishida 1963). This is also reflected in
the observed positive association of %FC with many structural attributes that reflect tree sizes (e.g., Modal D, L:M, and %Large).
Although frost cracks can develop on trees of any size (Schirp 1968),
owing to the difficulties of retrospectively dating the origin of frost
cracks, we could not determine when larger trees first developed
cracks. Consequently, we do not know whether frost cracks in larger
trees gradually accumulated over time in response to disturbance
events that injured them either as juvenile or mature trees or whether
they developed across all size classes during particularly harsh or
variable winters.
Although the incidence of frost cracks on sugar maple was lower
in stands that had a higher conifer component, the presence of
conifers does not appear to have indirectly affected frost crack frequencies (e.g., through a moderating effect on the microclimate).
The lack of an association of conifer basal area on frost crack incidence after including hardwood basal area in the model indicates
that the effect of conifer basal area on frost crack incidence was most
likely due to a reduction of the hardwood basal area in the area where
residual conifers were present (r ⫽ ⫺0.459).
Conclusions and Silvicultural Implications
In the southern boreal–northern temperate forest transition
zone, the development of some frost cracks on sugar maple trees
seems inevitable. However, because frost cracks often form after a
discrete tree injury, we infer that heavy partial cutting followed by
high grading in these second-growth stands may be responsible for
higher levels of frost cracking compared with background levels
found in primary stands. Because of the limited cases of active forest
management for high-quality sawtimber in this region, we do not
know what proportion of trees would exhibit frost cracks in carefully
managed stands. However, frost crack frequencies in excess of
30 – 40% greatly limit opportunities to produce quality sawtimber
sugar maple. To reduce frost crack incidence in second-growth
stands, we suggest that managers increase efforts to reduce mechanical (logging) damage to both residual trees and advance regeneration using techniques such as directional felling, a well-designed
permanent skid trail layout, and cut-to-length logging. Furthermore, although managers may remove smaller wounded trees (⬍25
cm or 10 in. dbh) during cleaning, improvement cuts, or thinning,
we recommend girdling mature and large trees (ⱖ25 cm or 10 in.
dbh) to avoid inflicting additional damage to trees in the understory
(Arbogast 1950).
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