Tapping practices to maximize yields over the long-term

By Abby van den Berg, Timothy Perkins, Mark Isselhardt, Joël Boutin, Wade Bosley, and Brendan Haynes, University of Vermont Proctor Maple Research Center, University of Vermont Extension, Club d’Encadrement Technique Acéricole des Appalaches

In addition to location and the timing, frequency and duration of weather conditions for sap flow, there are four main factors which influence the total yields achievable from individual trees: the level of vacuum at the taphole, the sanitation of the spout and dropline, tapping practices such as taphole depth and diameter, and tree characteristics like size and genetics (sap sugar concentration, etc.). The choice of tapping practices like taphole depth and diameter is one of the most important decisions we make each year because it not only strongly influences the yields we might obtain in the current season, but will also impact the yields achievable in future seasons. Because of this, over the past several years experiments at the UVM Proctor Maple Research Center have been aimed at quantifying the impacts of various tapping practices on yields using current collection practices (vacuum, spout and dropline sanitation, etc.) to provide data to help guide choices of tapping practices that will maximize yields not only in the current season, but also over the long-term.

Taphole Depth. We conducted a 3-year study to quantify the impact of taphole depth on total syrup yields under vacuum. On average, 1”-deep tapholes produced about 63% of the 1.5”-deep tapholes, while 2”-deep tapholes produced about 25% more than 1.5”- deep tapholes (Fig. 1). Drilling deeper than 2” actually didn’t result in additional gains over 2” tapholes.

Taphole Diameter. We also conducted several studies to determine the impact of taphole diameter on yields. With all factors equal – the same spout material, level of vacuum, etc. – on
average, yields were lower with smaller taphole diameters (Fig. 2). For example, 1/4”-tapholes produced about 10% less than 5/16”-tapholes. Notably, this is almost identical to the results of a similar study at 28” Hg by Centre Acer.

Number of Taps per Tree. We’re conducting ongoing research to determine the additional yield from adding a second taphole under vacuum conditions. Average total syrup yields from the first two years of the study were used to calculate the estimated gain derived from the second taphole by tree diameter (Fig. 3). With these data, the estimated percentage gain from the
second tap ranged from about 46% for an 18” tree, to about 48% for a 28” tree. This is slightly lower than the gains observed in previous studies, possibly due to the smaller tree diameters
included in the present study. We’re currently conducting this study in trees of larger diameters (average = 24.7”) and will update these results as needed after the 2023 and 2024 seasons.

So we know that the depth, diameter, and number of tapholes influence the yields we can obtain, but of course they also impact the amount of nonconductive wood generated each year. The tree’s response to each taphole wound generates a column of wood above and below the wound that is permanently nonconductive to sap or water flow for the tree. The amount of
nonconductive wood (NCW) generated is generally proportional to the size ofthe wound, so wider, deeper tapholes, and more tapholes per tree result in greater amounts of NCW (https:/youtu.be/LbUqFKVAUI0?t=1870). And this is an important factor in choosing tapping practices to maximize yields over the long-term, because tapholes drilled into NCW produce significantly less sap than those drilled into clean wood – in a study conducted by UVM Extension at PMRC, tapholes drilled into stained wood produced an average of 75% less sap, with greater reductions observed the more NCW that was intercepted by the taphole. (https:/mapleresearch.org/pub/reduced-sap-yields-whentapping-into-non-conductive-wood/). So the greater the amount of NCW that accumulates in the tree’s tapping zone, the greater the chances of hitting it and obtaining reduced sap yields during subsequent tapping. This is how the choice of tapping practices in the current season can affect the yields of the future.

So what are optimal tapping practices to maximize yields not just in the current season, but over the long-term as well? In reality, they’ll vary depending on tree growth rates and health status, tapping history (i.e. how much NCW is already present), site characteristics, tree size, and other factors including the level of vacuum and other operation features and tapping practices (dropline length, etc.). But by far the most important factor is the tree’s radial growth rate – this is what determines how much new, conductive wood is added to the tapping zone each year. The greater the growth rate, the more NCW can be supported because new conductive wood is being “replenished” at a faster rate. So healthy trees with good radial growth rates (and no history of over-tapping) can support less conservative tapping practices – deeper taphole depths, larger taphole diameters, and smaller diameters for the first and second tapholes. This is why practices to encourage vigorous growth and health of crop trees – including appropriate thinning and other forest management best practices, and soil amendments if tests indicate they’re needed – are the foundation for maximizing yields. For trees with lower growth rates, and/ or when other “suboptimal” conditions exist – trees whose crowns are in a suppressed position in the forest canopy, trees that have been recently stressed or are exhibiting signs of stress (branch dieback, fine twig mortality, slow wound healing, etc.), trees that have a history of over-tapping, or if NCW is hit frequently when tapping – more conservative practices like shallower tapping depths, smaller spout diameters, and larger minimum diameters for the first and second tapholes are more appropriate.

Other factors will also impact the choice of tapping practices. For example, any practice which effectively increases the size of the tapping zone – longer dropline lengths, moving the lateral line system vertically, and tapping below the lateral line – increases the amount of conductive wood available for tapping and can help support the sustainability of less conservative tapping practices. Tree size is also an important factor to consider. In general, smaller trees produce lower yields than larger trees (https:/mapleresearch.org/pub/m0218treesize/). The volume of NCW from a single taphole comprises a larger proportion of the tapping zone in smaller trees than it does in larger trees. Because of this, NCW can accumulate more rapidly in smaller trees, particularly those with lower growth rates. For this reason, the tradeoff between yields and NCW accumulation is important to consider when choosing the minimum diameter for a single or second tap in smaller-diameter trees.

Our current recommendations are outlined in Table 6.1 of the 3rd Edition of the North American Maple Syrup Producers Manual (www.mapleresearch.org/manual). The choice of tapping
practices should be an ongoing, continuous process, and be frequently assessed and adjusted in response to conditions as they change over time – as growth rates improve or decline, NCW wood is hit less or more frequently, less or more conservative practices can be implemented as appropriate. In all situations, the basic principle is always to aim for practices that will result in the maximum yields possible in the current season that also support the availability of sufficient future conductive wood, and thus maximum potential future yields, given the conditions of the trees and site.

We’ll continue our studies of tapping practices and yields as equipment and practices change and improve, and update findings as needed. In addition, it’s important to note that these
recommendations could change if we find that sap collection itself impacts the radial growth rates of trees – however to date, after 10 years of an ongoing long-term experiment at PMRC, we’ve found no significant impact on growth rates from either gravity or high-vacuum sap collection (https:/youtu.be/LbUqFKVAUI0?t=744).

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