Sap Flow Changes and Timing Pruning

Below is not a picture of a maple but that's becuase changes in soil temperatuers in the late winter/early spring are impacting sap flow in all deciduous trees, well gymnosperms (deciduous or not) as well. According to a recent study published in Ecosphere (a journal published by the Ecological Society of American; citation below) warmer temperatures are increasing the rates of sap flow in the northern red oak in response to the 0.25 increase (per decade = nearly a 1C increase) in temperatures in the northeastern US since 1970. Couple the two and the result is, trees transpiring more for longer which is the same as increasing the rate of sap flow for longer periods of time in a year. That's going to do multiple things to a tree one of which is change the costs of surviving (if my metabolism increases my food intake will increase and that's my cost of surviving) another is to change the timing of pruning.

Pruning timing is determined by lots of things such as flowering time, sap flow being at its lowest, sufficient metabolic activity to enable wound healing, and avoidance of pathogens (see Bauerle and Bowden 2011). This timing will be dictated not only by tree species but also by the climatic regime of the area and the changes in that regime due to a warming planet. For example, a study by Meinzer et al. (2004) in Oregonshows the importance of vapor pressure (think humidity for simlification) in sap flow. Yi et al. (2016) writing for Tree Physiology points out that changes in drought in the eastern US are going to be a challenge to trees in terms of sap flow. Complicate this further by reading a paper by Herbst et al. (2008) whose work shows that in areas with high soil moisture and humidity the sap flow of identical species will be significantly different from those growing in comparatively drier conditions. We all adapt to our local environment (with serious limitations of course). When I lived in the Sonoran desert my body produced a lot more heat-shock proteins to protect my cells from death due to heat stress. I've since moved to a much cooler climate and no longer produce the same amount of heat shock proteins but that physiological change I underwent didn't happen overnight.

Okay, back to when to prune. In Calgary - midsummer pruning is recommended but the average July temperature in midsummer is 65F/46F with 0% humidity. Eastern TN the average temp is 88F with a humidity of 82%. Safe to assume sap flow will be very different in these two very different climate regimes. This is one reason why I recommend pruning maple, oak, elm, and birch trees in Nov/Dec because temps have dropped to the point that fungal spores are either losing viability or entering 'resting' stages that don't attack plants AND because sap flow is slowing but wounding still functions. Really, the few freezes we see in the winter limited primarily to the latter part of January (typical) and the high rainfall maintaining warm soil temps - those lead me to prune from Nov - Jan for all deciduous trees. I may be wrong; it will not be the first or last time. However, I am basing my decision on a lot of reading and observation.

I'll leave you with this - you can test sap flow by breaking off a twig now and in Dec. That won't tell you about wound healing but it might give you and idea about how much weeping you'll get from pruning at a particular time of year.

As always - reach out to Extensions specialists and other arborists. As with any doctor get multiple opinions. When it's a big (or potentially big) problem, ask a minimum of three specialists and if you get three different answers reach out to another three. Ask them what is leading them to a particular opinion/decision. They should be able to give you experience, education, and research.

( #pruning #landscaping #climatechange #southernplantings #aboriculture #landscapecare #maples

Citations = Bauerle and Bowden 2011 Hort Science 46, Herbst et al. 2008 Tree Physiology 28:959, Yi et al. (2016) TREE PHYSIOLOGY 00:1