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2006 SOUTHEASTERN NATURALIST 5(1):127–134
Characteristics of Sap Trees Used by Overwintering
Sphyrapicus varius (Yellow-bellied Sapsuckers) in an
Old-growth Pine Forest
J. MORGAN VARNER III1,2, JOHN S. KUSH1,3,*, AND RALPH S. MELDAHL1
Abstract - Sphyrapicus varius (Yellow-bellied Sapsuckers) are winter residents of
mature Pinus palustris (longleaf pine) forests in the southeastern US. Sapsuckers
pierce the bark of mature pines to create wells on living trees and consume the sugarrich
exudate and insects attracted to this food source. To determine sapsucker
preferences for individual trees, for locations along boles, and for edge vs. interior
habitat, we surveyed an old-growth Pinus palustris stand in lower Alabama with
recent sapsucker activity. Individual tree characteristics and stand conditions were
evaluated to assess their influence on the number and location of sap wells. Of 596
pines sampled, 74 (12.4%) contained wells. Sapsuckers selected trees with greater
diameter at breast height (mean dbh of welled trees = 40.4 cm; non-welled trees =
24.6 cm; P < 0.01). Among pines with wells, sapsuckers fed differentially on
different aspects and stem heights. Sap wells were concentrated on the north aspect
of the bole (P < 0.05), where wells were 40% more likely to occur than any other
aspect. No stand characteristics (plot distance to stand edge, plot basal area, plot tree
density, 5-year radial increment growth) significantly influenced sapsucker treeselection
Sphyrapicus varius L. (Yellow-bellied Sapsuckers) are migratory woodpeckers
that create horizontal rows of wells in the bark of more than 200
native woody plants (McAtee 1911). Sapsucker wells are small rectangular
holes pierced through the bark primarily into the sap-filled phloem, but also
into the cambium and xylem (Eberhardt 2000). Sapsucker wells fill with sap
that comprises between 20 and 100% of the sapsucker’s diet (Beal 1911,
Emlen 1978, Tate 1973, Walters et al. 2002). The sugar-rich sap also attracts
at least 14 arthropod families (Rissler et al. 1995) and 35 avian and six
mammalian species (Foster and Tate 1966).
A major priority in understanding the behavior and ecology of Yellowbellied
Sapsuckers is their selection of individual trees and the location of
wells along the boles of selected trees (Eberhardt 2000, Walters et al. 2002).
In the Yellow-bellied Sapsucker’s breeding range, Erdmann and Oberg
(1974) found that they select more vigorous Betula alleghaniensis Britton
(yellow birch) over less vigorous trees. Work by several investigators
1School of Forestry and Wildlife Sciences, Auburn University, AL 36849. 2Current
address - Department of Forestry and Watershed Management, One Harpst Street,
Humboldt State University, Arcata, CA 95521. 3Corresponding author -
128 Southeastern Naturalist Vol. 5, No. 1
(Eberhardt 2000, Ohman and Kessler 1964) suggests that sapsuckers choose
older, diseased, lower-vigor trees for a combination of both sucrose and
amino acids. The diet of sapsuckers consists of a mix of insect prey and sap
(Eberhardt 2000, Foster and Tate 1966, Kilham 1983, Rissler et al. 1995,
Tate 1973), though investigators disagree over proportions and preferences.
Better understanding of individual tree selection patterns could help clarify
this debate by demonstrating preferences among trees of contrasting vigor
(i.e., if insects are the primary prey item then damaged, lower vigor trees
would be favored, whereas if sap was the primary prey, then higher vigor
trees would be favored by sapsuckers) or other characteristics (e.g., age,
dimensions, species, and location).
Pinus palustris Miller (longleaf pine), native to the southeastern US, is a
species often used by wintering Yellow-bellied Sapsuckers (Repenning and
Labisky 1985). Sapsuckers, like Picoides borealis Vieillot (Red-cockaded
Woodpeckers), show significant roosting and foraging preferences for mature
longleaf pine forests (Repenning and Labisky 1985). There has been a
focus of recent scientific and conservation interest in Pinus palustris ecosystems
due to the precipitous decline in area (97% lost since European settlement),
interruption of frequent fire regimes, loss of old-growth forests, and
the imperiled status of much of their flora and fauna (Varner and Kush
2004). In an assessment of native ecosystems, Noss et al. (1995) listed the
Pinus palustris ecosystem as the third most endangered ecosystem in the US.
Many natural history elements are poorly understood within longleaf pine
ecosystems, including information on overwintering Yellow-bellied Sapsuckers.
Determining sapsucker tree selection patterns can be based on several
tree characteristics. Trees with greater radial growth or percentage of height
occupied by foliage are more vigorous than trees with lower values of these
parameters. In Pinus palustris ecosystems, Hooper (1996) found that less
vigorous trees are preferentially visited by the arthropods that sapsuckers are
known to consume, particularly Hymenopterans (principally Camponotus
spp.), Coleopterans, and Lepidopterans (Kilham 1983). Conversely, more
vigorous trees produce more photosynthate (Zimmerman 1961) and are
visited less by arthropods (Hooper 1996). Location of wells along stems is
also important and may help clarify tree selection and food source
The primary objective of this study was to document sapsucker selection
patterns within the sapsucker’s overwintering range. We also wanted to
determine patterns of individual tree selection among pines in a monospecific
stand. Within the population of trees with sap wells, we sought to better
understand the spatial patterns of well creation along the bole. Finally, we
sought to add to the natural history of Pinus palustris ecosystems and
2006 J.M. Varner III, J.S. Kush, and R.S. Meldah 129
Study Area and Methods
The Flomaton Natural Area (FNA) is a 27-ha old-growth Pinus palustris
stand in Escambia County, AL (31o01'N, 87o15'W). Fire was excluded from
the stand for 45 years until 1994 when an aggressive ecological restoration
program began with complete hardwood (primarily Quercus spp.) harvest
and three prescribed fires (Varner et al. 2000). In May 1996, all hardwood
trees were harvested from the stand. During the following two growing
seasons (1996 and 1997), remnant Pinus palustris increased their growth
(Varner et al. 2000) and stand conditions improved, unintentionally restoring
habitat for Yellow-bellied Sapsuckers. Fieldwork had been conducted in
the stand since 1993, and there was little or no Yellow-bellied Sapsucker
activity before the removal of the hardwoods (J. Kush, Auburn University,
AL, pers. observ.).
As part of a restoration monitoring project at the FNA (Kush et al. 2004),
thirty-two 0.08-ha circular plots were established on a 60-m x 80-m grid.
Within these plots, all trees > 2.5-cm diameter at breast height (DBH) were
examined by at least two observers for presence of sapsucker wells. Sap well
patterns were recorded for each tree with wells present. Patterns included
abundance, heights of sap wells, and aspect of wells on bole (north, south,
east, or west). Heights of sap wells on the stem below the canopy were
classified into thirds of the bole and designated lower, middle, and upper,
regardless of tree height. Individual tree data including DBH, height to base
of living crown (crown height), total tree height, age (number of annual rings
at 1.3 m) and 5-year radial increment growth were obtained from the ongoing
monitoring of the ecosystem restoration project (Kush et al. 2004).
Values for pine density (trees ha-1), pine basal area (m2 ha-1), hardwood basal
area harvested (m2 ha-1), hardwood density harvested (trees ha-1), and distance
to stand edge were also available for each plot.
Differences between pines with sapsucker wells and those without wells
were determined using a chi-square analysis (PROC LOGISTIC; SAS Institute
1999). Differences between proportion of pines with wells per plot
along the stand edge and stand interior were analyzed using a chi-square
analysis (PROC LOGISTIC; SAS Institute 1999). Analyses of well locations
(aspect and stem height) were performed using chi-square analysis (PROC
FREQ; SAS Institute 1999). For all analyses, significance was determined
using α = 0.05.
Of the 597 pines surveyed, 74 (12.4%) contained wells (Fig. 1). Sapsuckers
selected significantly larger diameter trees (P < 0.003) for excavating sap
wells (Table 1). While bole height and total height of pines with wells was
higher than those without, they did not differ significantly (Table 1). Pines
130 Southeastern Naturalist Vol. 5, No. 1
with wells tended to be older (P = 0.06) than non-welled trees (79.4 y for
welled and 51.4 y for non-welled pines, respectively; Table 1). There were
no differences in 5-year radial increment growth between trees with wells
and those that did not have wells (Table 1).
Among plot characteristics (pine density, pine basal area, and hardwood
basal area removed during restoration), there were no significant relationships
between these attributes and the presence or absence of sap wells
(Table 2). In addition, the distance to stand edge was explored, but plots near
an opening had as many trees with sap wells as plots located in the interior of
Figure 1. Pinus palustris forests
are habitat for overwintering
Yellow-bellied Sapsuckers in
the southeastern United States.
Sapsuckers pierce the bark of
longleaf pine to access the sapfilled
phloem and the arthropods
attracted to the sugar-rich food
Table 1. Characteristics of Pinus palustris with and without Yellow-bellied Sapsucker sap
wells. Column headings refer to presence (Wells) or absence (No wells) of sap wells.
No wells Wells
Mean n S.D. Mean n S.D. PA
DBH (cm) 24.7 522 14.21 39.9 74 13.48 0.003
Crown height (m) 14.4 522 3.24 16.6 74 3.04 0.345
Total height (m) 21.7 522 17.86 26.5 74 12.10 0.509
Age (y) 51.4 471 35.50 79.4 57 56.51 0.060
5-year radial increment (cm) 0.30 522 0.387 0.33 74 0.345 0.874
AAll statistical analyses were performed using chi-square analyses with α = 0.05.
2006 J.M. Varner III, J.S. Kush, and R.S. Meldah 131
Sap wells were differentially distributed along the boles of Pinus
palustris. Sapsuckers preferred feeding on the north aspect of the bole (P less than
0.05), more than twice as much as either the S or E aspect and nearly 40%
more common than the W aspect (Table 3). We found no differences in
sapsucker preference for bole section (lower, mid, upper).
Given the characteristics of sap trees in our stand, sapsuckers selected
pines with larger diameters (Table 1); this data confirms results found in the
sapsucker’s breeding range (Erdmann and Oberg 1974). However, other
investigators have found that sapsuckers selected weakened trees for sap
wells (Eberhardt 2000, Ohman and Kessler 1964, Walters et al. 2002). This
may be due to the fact that sapsuckers tend to have fidelity for sap trees,
perhaps working the trees until they are weakened and susceptible to disease
(Eberhardt 2000).Sapsuckers may select vigorous pines initially for sugars
and use wells in other species or in other stands for necessary amino acids.
Additional experimental work on tree selection over longer time periods is
needed in both their breeding and over-wintering range (Eberhardt 2000,
Walters et al. 2002).
Trees used by sapsuckers in our study area were older and had larger
diameters than unused trees. The larger trees sapsuckers selected provided
larger areas for feeding and likely for sap production. Photosynthates form
pockets in the phloem tissue of conifers (Zimmerman 1961), and these
Table 2. Plot data used in the analyses of Yellow-bellied Sapsucker activity for the Flomaton
Natural Area, AL.
Mean Min. Max. S.D. PA
Longleaf pine density (stems/ha) 284 62 655 151.77 0.354
Longleaf pine basal area (m2/ha) 18.18 3.57 39.23 8.12 0.467
Hardwood basal area removed (m2/ha) 10.09 4.12 22.86 4.12 0.567
AAll statistical analyses were performed using chi-square analyses with α = 0.05
Table 3.-Yellow-bellied Sapsucker sap well locations along boles of Pinus palustris at
Flomaton Natural Area, AL. Location along bole refers to height of wells along defined thirds
of each longleaf pine bole. Aspect refers to the location of wells within defined quarters of a
stem section (i.e., N denotes presence of sap wells within the 315º to 45º face of a longleaf pine
stem). Units for data in table = number of trees.
Bole location N S E W Total
Lower 33A 24 21 29 107
Middle 36A 15 17 21 89
Upper 42A 12 14 18 86
Total 111 51 52 68 282
Adenotes significant differences among aspect (columns), determined using chi-square analysis,
with α = 0.05.
132 Southeastern Naturalist Vol. 5, No. 1
pockets and the limited sap that moves during winter are a rich food source
for the Yellow-bellied Sapsucker (sensu Zimmerman 1961). Therefore,
larger diameter trees have greater bark surface and should have more photosynthate
pockets for sapsucker exploitation.
Sapsuckers aggregated their sap wells on the boles of pines in our stand,
which may have been a response to insect abundance. Hanula and Franzreb
(1998) found abundance and biomass of insects varied vertically along the
stems of Pinus palustris, however, they did not examine aspect differences.
Although not significant, our results show a consistent trend of greatest
abundance on the lower tree bole where more insects occur (Hanula and
Franzreb 1998). Depending on the preferred food of the Yellow-bellied
Sapsucker during winter, different conclusions can be reached with respect to
well placement on Pinus palustris. Hooper (1996) found that winter arthropod
biomass on Pinus palustris declined as pines aged and as vigor declined
(radial growth in the preceding 6–10 y). Causes for the observed differences in
aspect and stem height locations of sap wells in this study are unknown, but
may be related to photosynthate segregation along the stem (Conner et al.
1975), bark moisture on different aspects, differential bark thickness, or
varying insect prey habitat (Hanula and Franzreb 1998), among others.
Future efforts should focus on experimentation to determine feeding
preferences and foraging behavior of Yellow-bellied Sapsuckers (Walters et
al. 2002). Additionally, stem location preferences deserve further study from
both avian feeding behavior and from tree physiology approaches. This
study also raises questions about biotic response to ecosystem restoration.
At this early juncture, restoration ecology has only scratched the surface of
responses and interactions of associated plants (Varner et al. 2000) and
animals (Ford et al. 1999, Masters et al. 1998) to manipulations for desired
R. Sampson and W. Hauffe assisted with field work. G. Hill, A. Keyser, M.
Mitchell, R. Seman-Varner, and two anonymous reviewers provided constructive
comments on drafts of the manuscript. We thank Champion International Corporation
(now International Paper) and F. Dickard for allowing us to conduct this study.
Financial support was provided by the USDA Forest Service Southern Research
Station, the Alabama Department of Transportation, and the Auburn University
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