Woodpecker with a Sweet Tooth (or Tongue): Red-breasted Sapsucker

The Island Oak (Quercus tomentella) in front of the glass house is the first tree in the Botanic Garden that I came across to have rows of holes drilled by a Red-breasted Sapsucker. It was Celia Zavatsky who pointed them out to me years ago. Celia, who passed away last year, was the quintessential docent, having a phenomenal knowledge of, and an infectious enthusiasm for California native plants. Since that initiation by her, I have found numerous sap wells on trees in the garden, both on hardwoods and conifers. 

The best display of sap wells on conifers in the Garden belongs to a Western Red Cedar (Thuja plicata) in Pacific Rain Forest section. The holes climb its trunk from 12 feet above ground, and up without stop for as high as you can see. They do not seem to affect the health of the tree. From their dark color and dry circumference, the holes look old, and tree sap no longer flows out of them. In fact, the majority of the sap wells we see in the garden are remnants of years past. 

If you see sapsuckers flying around and follow their moves closely, you will likely be rewarded with the discovery of fresh wells. The new wells are light brown in color, maintained diligently by the birds to keep the sap flowing. That is how the bird surveying docents first saw the sap wells on Leather Ash (Fraxinus velutina) four years ago. Sapsuckers have come back to the ashes at the north end of Southern California section adjacent to the grassy glade every year since then. All three ash trees growing there sport lines of old and new wells interspersing with each other, going up from the base of stems to where the stems branch.

There are four species of sapsuckers in North America, and what we have in the garden is Red-breasted Sapsucker. Lee Friedman summarizes that our sapsucker breeds to our east, and spends fall and winter in the Bay Area. Sapsucker is in the woodpecker family (Picidae), with the physical characteristics of zygodactyl toes (two pointing forward and two pointing backward) for grasping, a fortified skull for protecting its brain from being damaged by their hammering movements, and a sharp drill bit for a beak. It differs from other woodpeckers in its sugar-dense diet. The tip of its tongue is brush-like, specialized for lapping up the sap by capillary action.

Most of the holes we encounter in the Garden are xylem sap wells. Xylem is the tissue in vascular plants that transports water, minerals and other molecules from roots to other parts of the plants. Many deciduous trees have sap run in spring (such as maples, birches, and walnuts) which moves a large quantity of fluid through their xylem. (Check out this post on tapping various trees for syrup-making, drinking, and fermentation. Another well-written blog talks about mechanisms of xylem transportation, and explains in detail the relationship between sap flow and syrup production.) 

The fact is all trees have sugar in their xylem sap throughout the year, but the sugar concentration varies widely, depending on environmental factors as well as inter- and intra-species differences. Figure 1 shows that sucrose percentage of xylem sap in a willow peaks in August, and can be three times higher than in other months.

Figure 1 Sucrose level of xylem sap extracted from Salix fragilisat monthly intervals except December. 

Source of Figure 1: “Sugar content of xylem sap and susceptibility of willow to Chondrostereum purpureum,” by Sandra D. Stanislawek, P. G. Long and L. K. Davis. New Zealand Journal of Botany, 25:2, 263-269. 1987.

The other transportation tissue in vascular plants is phloem, which is closer to the bark and conducts nutrients from leaves to the rest of the plants. In general, phloem sap is a lot sweeter than xylem sap. Birds of North America Online (a paid account is needed for full access) cites that on average xylem sap has 2-3% sucrose, while phloem sap has over 10%. Again, sugar content of phloem sap differs across seasons, species, and individuals. See Table 1 for the great diversity of sucrose levels. One species, White Ash, marked by light blue color, shows a range of 8.5-25.0%.

Table 1 Sucrose in Phloem Sap

Species	Common name	% (wt/wt)
Salix viminalis	Basket Willow	5.0
Fraxinus americana	White Ash	8.5
Fraxinus pennsylvanica	Green Ash	11.1
Pinus sylvestris	Scots Pine	11.5
Acer platanoides	Norway Maple	17.9
Acer pseudoplatanus	Sycamore Maple	20.6
Fraxinus americana	White Ash	22.1
Fraxinus americana	White Ash	25.0
Eucalyptus globulus	Tasmanian Bluegum	25.1
Acer platanoides	Norway Maple	25.2
Prunus persica	Peach	37.9

Table 1 is excerpted from a larger table in “Optimal concentration for sugar transport in plants,” by K.H. Jensen, J.A. Savage and N.M. Holbrook. Journal of the Royal Society Interface. 2013.

Considering the average higher sugar content in phloem than xylem, we would expect that sapsuckers favor the sweeter phloem sap and dig more phloem wells. Yet, so far we have found only one tree in the Garden with extensive phloem sap wells. It is a Catalina Cherry (Prunus ilicifolia ssp. leonii) in Channel Island section. Pressed against the wire fence along Wildcat Canyon Road, this small hardwood tree with shiny green leaves has large rectangular sap wells on its trunk and a large branch.

Red-breasted Sapsucker on Catalina Cherry, March 1, 2017. Taken by Wen Hsu.
The rectangular shape of wells on the left branch is the hallmark of phloem wells.

Why do we see more xylem wells in the Garden? Could it be that the xylem sap in the seasons when Red-breasted Sapsuckers come to the Garden (late fall through early spring) has enough sugar to warrant their effort of drilling? Could the phloem sap at this time of the year be too sticky to flow, and thus less favored by them? There do not seem to be ready answers to these questions. In the section Priorities for Future Research, BNA Online says, “The criteria by which trees are chosen for sap wells are not known. …Most studies have been made during the breeding season...much remains to be learned about behavior and ecology during other times of the year.” That means our sapsucker observations in the Garden can make important contributions to the study of these beautiful birds!