We’ve been monitoring two sugar maple test plantations in Canterbury for over two years— this post from PhD student Matt Rennie provides an update on his observations to date.
In order to study the effect of climate and coppicing regimes on potential syrup production, the research team partnered with landowners to establish 100-tree test plantations in Canterbury. Sugar maples (Acer saccharum) were planted near Hanmer Springs in July 2020 and near Geraldine in June 2022. The saplings were planted in 10m by 10m plots with 1m spacing. This dense planting system relies on frequent coppicing, so trees have a small stature.
Site Selection & Conditions
Both sites were planted on relatively flat areas, which was more of a coincidence than strategic. In North America, sugar maples will grow in most places, given access to water and sunlight. However, depending on how you intend to collect the maple sap, flat ground or a slight decline would benefit tubing systems such as the proposed vacuum harvesting.
Each tree planted (at both sites) received a handful of blood and bone mix when planted, but the soil wasn’t touched otherwise. Sugar maples will grow in most soil types as long as it’s free-draining and water doesn’t start collecting, or the opposite, where the soil is shallow or dries out often.
I have noticed on both sites that the leaves will start to burn and drop with extended periods of hot, consistent sun during the summer months. This hasn’t detracted from the growth of those trees in the long run. But especially early on, when their roots are shallow and they haven’t developed full canopies yet, this should be avoided.
Both sites had reasonably good access to water, with streams nearby, so the soil never dried out during the hot Canterbury summers. If future sites are prone to hot summer months, it would be advised to consider how to provide water to the trees until they develop enough of a root system to support themselves.
I was surprised by the number of animals and insects that hung around the trees. Almost every time I’ve visited both sites, there have been bumblebees flying around. This may be because the sites are amongst fields but I like to think they sense the sweet sap as well. This is also the case with the more pesky rabbits who, after I accidentally left some of the tree guards off for a week decided they wanted maple syrup too!
Plant Growth
At both sites, the tree growth was quite similar. During the first two years or so, the trees grew relatively slowly, increasing roughly 20cm/yr in height. At the Hanmer Springs site this exploded to about 50 cm/yr in the following two years with the trees now averaging about 200 cm in height.
While not 100% sure of the exact reasoning, it's assumed to be a combination of the trees hitting a critical size where they grow exponentially and the availability of plentiful resources. This may slow down into the future, with the trees fighting for sunlight or promoting them to grow larger and more vertically, but time will tell.
Weather & Climate
Generally, the interior areas across Canterbury, have fairly similar conditions: hot summers where daily maximum temperatures often exceed 30°C and winters with abundant precipitation, the occasional winter snow, and interrupted at times by unseasonably warm days. This isn’t what occurs in Canada or the Northeast of the USA, but smaller-diameter trees reduce the need for consistent below-freezing weather.
The test sites each has a weather station, which I’ve been remotely monitoring for cold spells during which to visit to gain further at-site information and collect long-term weather trends. The stations come with a range of data sensors that collect wind, temperature, pressure, rainfall, and soil moisture, to name a few.
Real time data is available on the Tree Syrup Aotearoa homepage.
Sugar maples are quite resilient when it comes to most conditions. For Canterbury in particular, an eye needs to be kept on the hot dry winds during summer which can cause wilting or burning. Otherwise the number of sunshine hours along with ample water promotes plenty of growth during this period.
While not growth-related, the need for below-zero overnight temperatures during late winter, coupled with above-zero daytime temperatures, promotes the movement of sap within the trees pertinent to sap collection, which is, above all, the main desire for any maple sap operation.
Pruning & Harvest
Around the two-year mark at the Hanmer Springs site, I gave around half of the 100 trees a heading cut. This was done at a height of 40 cm to the closest nodal point, and any branches that had formed were removed. This was done to promote the growth of multiple stems from the top nodal section inline with the proposed multi-stem tree architecture of the vacuum harvesting method.
While optimistic this pruning regime would work, no studies had been previously done on how a sugar maple would react to such a harsh prune – there are varying theories from US maple connections on whether it will take. However, the trees flourished over the following two years, either equal or better in terms of growth compared to the trees not pruned. This provided new knowledge on how many stems can be achieved through this method of pruning.
The first tests of the vacuum harvest technique were achieved this year, with some more tests planned. It is important to note that the proposed vacuum harvest method requires wet soil around the roots during winter, as the stem “acts as a straw”, with the vacuum pulling water up from the roots while starch is converted into sugars in the sap. A blog post on the harvest will be published on this website once the data has been analysed.