Research: Trees and sap Examining the impact of seed production on sap sugar content J.M. Rapp University of California, Davis and Harvard Forest, Harvard University nnual reports on the sugaring sugar maples grow slowly, and most season are filled with references sugarbush owners do not plant trees Ato the weather. Cold snaps and to tap. For the most part, you are stuck warm-ups alike are blamed for poor with what you have in the sugarbush seasons, and a long season benefits from already. Thinning the sugar bush to “good sugaring weather.” Good weath- give trees more light, or fertilizing3 may er for sap flow is well known to those help to increase sap sugar content of in- who tap maple trees. Cold clear nights dividual trees, but is unlikely to influ- followed by warm sunny days cause the ence the perhaps most enigmatic type sap to flow, so those who tap trees are of variation in sap sugar content: varia- well advised to watch the weather dur- tion in the same trees from year-to-year. ing the tapping season. But sap flow is only part of what determines the total In the October 2014 issue of Maple amount of syrup made (and how much Syrup Digest, M. Isselhardt and col- money ends up in a syrup producer’s leagues described nonstructural car- bohydrates (NSC), the energy stores of pocket). New research suggests sugar 4 makers may be advised to look to their trees. As they noted, some portion of trees’ canopies as well as the weather the total NSC reserve of sugar maples forecast if they want to predict the tap- makes it into a trees’ sap, and can then ping season. be collected to make maple syrup. The balance in this “savings account” may Along with the volume of sap, how vary between years and therefore ac- much sugar is in that sap is a major count for variation in sap sugar con- determining factor of how good a sug- tent. But why does it vary, and why do aring season is. With vacuum lines in- trees have a savings account anyway? creasing sap volume and reverse osmo- As the authors noted, trees use some of sis machines taking out a large portion this reserve in the spring to fuel growth of the water with relatively low energy before leaves are on the trees, and may input, sap sugar content has received call upon their savings in times of need perhaps less emphasis than in the past, – defoliation by insects or frost, damage but it still matters. Sap with a sugar from wind or ice storm, or any number content of 3% will produce 50% more of other stressors. If a tree has recently syrup than sap with 2% sugar, given been damaged, it may use this reserve an equivalent amount of sap. Sap sugar to recover, and this is why many people content can easily vary this much be- will not tap damaged trees. Trees have tween trees, and even in the same tree another use for these NSC reserves that in different years. Some of this variation is often overlooked: making seeds. between trees is due to genetics, and ef- forts to breed maples with sweeter sap Plants can put a tremendous amount have led to some success.1,2 However, of energy into reproduction. Agricul- 14 Maple Syrup Digest ture is largely in the business of maxi- duction. Trees likely don’t have the re- mizing this investment – think of a sources to invest in making seeds year champion pumpkin at the state fair. after year without some trade-off with Forest trees invest less than agricultural other plant functions, such as woody crops and indeed most annual plants. growth or defending against pests and They need to save energy for the next pathogens. Theoretical ecologists have year and invest in woody growth. But seized upon the observation that many maximizing reproduction over the life- plant species alternate high and low time of a tree, what biologists call “fit- years of seed production to develop ness,” is a primary goal for a tree. Like the resource budget model of masting.5 all organisms, trees pass on their genes The idea is that ‘mast years’ – big seed through reproduction, and the tree that years – deplete stored resources (NSC), makes the most seeds is most likely to which plants then replenish during low pass on genes to the next generation. flowering years. If this is true for sugar We can therefore expect trees to invest maples, it provides one reason why considerable resources in reproduction. NSC, and hence the amount of sugar in sap, varies from year to year. Sugar maples are a masting species. This means they don’t produce large In addition to individual trees hav- seed crops every year. Instead, big seed ing a pattern of high and low seed years are followed by years with little years, groups of trees tend to all flower to no flowers and seeds. This in itself and produce seeds at the same time. suggests that there is a cost to repro- Sap: continued on page 17 Sugar Bush Supplies Co. 2611 Okemos Rd, Mason, MI 48854 517-349-5185 fax 517-349-3088 [email protected] We ship daily. Open all year. LEADER Evaporators Tanaka tappers Marcland draw-offs Wes Fab filter presses Tanaka and Honda pumps Testing and filtering supplies Bacon plastic jugs, glass bottles & labels Candy molds, bags and boxes, sugar shakers February 2015 15 Sap: continued from page 15 ing Cooperative. Every year, research- Indeed, my own observations, and ers visit each stand to measure tree data from across the northeastern U.S. growth, canopy condition, and other and adjacent Canada6-10 suggest a high tree health metrics. They also observe degree of regional synchrony in seed whether or not the trees have seeds. production. Not only could masting The syrup data were from the National provide an answer for variation in sap Agricultural Statistics Service, which sugar for individual trees, but masting sends out surveys to maple produc- could influence syrup production re- ers in the U.S. each year. While these gionally. data under-estimate syrup production since not every sugarmaker receives or How does this ecological theory returns a survey, the data do provide hold up in practice? In a recent study a good measure of the year-to-year published in the peer-reviewed jour- variation in syrup production for entire nal, Forest Ecology and Management, states. Syrup production has increased my co-author Elizabeth Crone of Tufts over the past decade due to an increase University and I analyzed data on seed in the number of taps, as well as the and syrup production over 17 years in amount of sap collected per tap as more Vermont. The seed data were from 30 and more taps are put on vacuum. We sugar maple stands throughout Ver- wanted to remove this overall increase mont tracked by the Vermont Monitor- and only look at the year-to-year varia- tion in syrup production. We 300 therefore fit a trend line to the syrup production data, and 200 then subtracted the yearly val- ues from the overall trend. We 100 used these values as the mea- sure of year-to-year change 0 in syrup production. We then compared these data to seed −100 production in the previous year, since we expected seed production to deplete stored (Thousands of gallons) of (Thousands −200 resources, leading to lower −300 syrup production in the fol- lowing year. Relative Syrup Production in current year Syrup current in Production Relative 0 10 20 30 40 50 We found a surprisingly strong relationship between Percent of trees with seeds in last year seed production and syrup Figure 1. The relationship between maple syrup production (Figure 1). Syrup production and the percentage of trees making production for Vermont was seeds in the previous year in Vermont. The y-axis on average more than 200,000 gallons lower after a mast year shows the difference in annual syrup production Figure 1. The relationship between maple syrup production and tfrom the overall (increasing) trend. Following he percentage of mast that in other years, and 43% of trees making seeds in the previouyears, when about 40%s year in Vermont. The y‐axis of sugar maple treesshows the difference in the annual variation in syrup in annual syrup production from the overall (increasing) trend. Following mast years, when about 40% of sugar maplots monitored by the Vermontple trees in plots monitored b Monitoring Coopy the Vermont - Monitoring Cooperative make seeds, syrup production is lower. erative make seeds, syrup production is lower. Sap: continued on page 18 February 2015 17 Sap: continued from page 17 the annual variation in syrup produc- production was explained by seed pro- tion was the model that didn’t include duction alone. any temperature variables at all. Hard- ly resounding support for temperature What about the weather during being important. the tapping season? We also tested if the monthly minimum and maximum Does this mean climate is not im- temperatures from January through portant? No. When we included seed April could explain annual variation in production along with the temperature syrup production, as had been seen in variables in a single analysis, we found Quebec.11 While average monthly tem- that maximum and minimum March, peratures are only coarsely related to and maximum April temperatures the daily temperature fluctuations that helped explain variation in syrup pro- cause sap to flow, a warm or cold sea- duction.