Drought and the Impact on Sap Production

[Sugar Bear]

Sap flow is more dependent on soil moisture during the spring itself...so snowmelt and rain during the spring are quite important. However, there seems to be a couple of impacts on sap sugar content and tree growth. Think of leaves as the engine of the tree. The fuel is sunlight. But in order for that engine to run at peak levels, everything else needs to be operating correctly. If soil moisture is lacking, stomates in the leaves close, so CO2 cannot enter the leaves and be "fixed" into sugars. So drought reduces photosynthetic carbon gain (production of sugars), resulting in reduced growth and less storage or sugars in that ring of wood formed during that time. Fortunately, the sugar we collect as maple producers in sap comes from many tree rings (20-30 depending on tree growth rate and taphole depth), but the outermost tree rings tend to be the most productive (more sugar and higher hydraulic conductivity...meaning the younger pipes/vessels work better), so there can be some small reduction in sap sugar content due to drought. This is more apparent when we have several years of drought in a row.

I apologize if this seems too complicated...but that is really the simple explanation. The details would make heads spin.

Interesting stuff.

Do Medullary Rays ever shut down at a certain depth/growth ring and eventually abandon sugar deep within the cellulose of a trunk? I.E. if we have drought in the spring and poor sap flow over consecutive years.

Its seems the answer would be yes or most woods would not have that sweet smell when I cut them as lumber. Even the Pith section almost as well.

And so my ultimate question here would be does drought in any given season increase the rate or amount of abandonment over time?

Or is the abandonment only nominal after several drought seasons in a row?

[DrTimPerkins]

Do Medullary Rays ever shut down at a certain depth/growth ring and eventually abandon sugar deep within the cellulose of a trunk? I.E. if we have drought in the spring and poor sap flow over consecutive years.

Heartwood formation is not programmed that specifically (to certain rings or ages) in maple trees. It is quite variable as you'll have noticed when cutting trees up.
Hydraulic function (ability to move liquid) and carbohydrate levels (starch and sugar) drop off from the outermost to innermost rings. There is certainly almost always some residual non-structural carbohydrate remaining in wood that gets walled off, and is therefore no longer accessible to the tree. Of course, stained wood produced by tapping is also no longer functional for liquid movement or carbohydrate storage. It is essentially structural at that point, but little else.

And so my ultimate question here would be does drought in any given season increase the rate or amount of abandonment over time?

Or is the abandonment only nominal after several drought seasons in a row?

Good questions. No good answers. I doubt that (in sugar maple at least) there is a real strong relationship for any one particular season of dry weather. Drought will clearly reduce basal growth and carbohydrate production/storage. Perhaps there would be a stronger relationship in ring-porous trees like oak where functionality drops off very quickly with depth, but I can't say for sure, partly because maple is a bit of an oddball species and partly because we think mostly in terms of what is going on with sap flow during an odd part of the year for most plants. Most tree physiology deals with sap flow during the growing season. We deal with sap flow during the so-called "dormant" season.

[Sugar Bear]

Yes, what you say makes sense, and we should all be able to deduce this because it seems it would be senseless for a tree to evolve itself to rely so heavily on one summer worth of food production.

To me it seems like a tree that did that would be a serious evolutionary failure.

But I suppose some trees may make a go of it that way.

[Jim Foster]

Here in southwestern Nebraska we have been in a severe drought for years. We went 2 months in the middle of summer with 1/4" of rain. I can tap trees on the creek bottom and get good sap, go up hill 50 feet to a beautiful tree and not get a drop. Drought does effect sap flow very much.

Jim

[DrTimPerkins]

Drought does affect sap flow very much.

No argument there. Drought in either the spring or summer affects sap flow, but in different ways.