Summer limb/branch drop

[daltontrees]

I would love to know why SL/BD happens. My amateur guess has been that since it is related to hot spells and significantly still air, the tree needs to sweat or it will cook, it needs to transpire but the lack of air movement makes gas exchange at leaf surfaces within dense canopy impossible. But if there is also a correlation with non-mulching then it would suggest that lack of ready water is a factor. If it weren't for the still air factor, low pressure in the trunk and limbs would be a starting point for theories about cause. Conversely, the still-air factor suggests the opposite.

 

Looking at the list of susceptible species in the article, it destroys another amateur theory that it affects tree species of dense canopy most i.e species where evapotranspiration is inhibited by the humid microclimate within the canopy. Some of the species e.g. Olea and even Fraxinus aren't dense like FAgus or Aesculus.

 

I wonder if the phenomenon is one of isolated trees or if it happens in say dense Beech woodland or is it just not noticed there as much. Is the weight of cantilevered horizontal limbs just less of a probem there because competition minimises horizontal growth? Have some species missed out in evolution on the need to have a strategy to deal with the conditions of summer branch drop? Perhaps the phenomenon is a reflection ultimately of man putting certain species where nature never meant them to be i.e. exposed to light on a few sides and with long heavy horizontal growth.

 

I'd love to see an example, if I spot one locally I'll get a photo and submit it.

[Matthew Arnold]

Nearly a year to the day when a large beech on a site we manage dropped three limbs without warning. Other than the breaks being tidied up nothing else has been done to the tree.

[Paul Cleaver]

has any organisation ever done a summer branch drop information gathering project? - if not I think it would be a great idea - with all the information we would probably have a better understanding

[agg221]

The data set is insufficient to be statistically valid, but it's certainly an observation that in the two beech trees of which images are posted above, the branches which have failed are vertically in line. Any more trees where this can be validated or dismissed?

 

If this is a consistent feature it suggests that there is more at work than simple ground water levels? It is possible that there is a particular orientation of the branches which is causing them to extend more than on other sides (end loading) but suggests that there may be something relating either to insolation (would show up as always being on the unshaded side, or perhaps always the southern aspect?) or some feature lower down the trunk on that side?

 

Alec

[Daniël Bos]

I saw the "stacked" branches and thought it likely the top one failed and took the lower ones off on its way down.

[Drayton tree care]

I have known summer branch drop when it's been dry for days and then a sudden downpour causes water rushing through the cells rehydrating them extremely fast causes them to literally explode as they expand. The limb actually makes a loud bang or crack and falls strait down almost like doing a step cut. As you can see in those photos the limbs land flat because they havnt torn at all

[AA Teccie (Paul)]

Excellent thread here, thank you to all contributors.

 

I often promote the forum for CPD, not to be overlooked, n this applies equally to myself.

 

I cannot add anything more to the theories aired, it seems a combination of very large, often over-extended, limbs, i.e. poss. bio-mechanical failure, and moisture / water movement related as discussed.

 

As an aside, I was previously delivering a PTI (Prof. Tree Inspection) course at Bicton College and whilst surveying some trees on the main driveway, on a very hot dry day, we heard a very loud, sharp crack (akin to a shot-gun sound) and identified a lower limb on one of the Oaks which cracked a further two times before then being shed across the main driveway...of course that gave an opportunity to use the classic Blue Peter phrase of "here's one I prepared earlier" (thankfully, and luckily perhaps as we were directly in situ, no harm nor damage was done.)

 

Keep it coming and thanks again.

Paul

[Jake Andrews]

I too would like too find the source of the SLD phenomenon, Iv only been here for one summer but the Eucs seem pretty notorious e for this behaviour. I mean not many eucs out there that hasn't got an over extended branch with end weight bias. I personally don't think it's anything to do with a heavy down poor though,rain does not seam too be a factor hear.Also what are the theories why it seams to be species specific.i.e oak beech ect? Would it be that they have usually over extended branches or there are more mature trees around. why not pines?

 

Jake:thumbup1:

[daltontrees]

Interesting point about conifer v broadleaf. And eucalyptus I believe counts as the latter. In the Harris article susceptible trees listed included Pinus and Sequaoiadendron gignateum. But mostly broadleaf.

 

An essential difference between conifer and broadleaf is that the bformer supports cantileverd branches by compression wood on the underside and the latter by tension wood on the upper side. That, and substantially different proprtions of lignin and cellulose. All the pictures suggest rapid (instantaneous) tensile failure on the upper side (obviously). Conifers don't have xylem vessels, they have tracheids which are closely packed and box-sectioned.

 

If some conclusion could be reached about the link between tensile strength of broadleaf tension wood and its relationship to quantities of water in it, particularly if the strength relationship was related to cellulose and/or to the density of xylem vessels, there may be the beginnings of a partial explanation.

 

And if limbs prone to SLD are on a hair-trigger in drought conditions, firstly one limb landing on another would be enough to explain the domino effect of multilpe limb failure and secondly is it just the sudden weight on leaves of heavy rain that triggers failure? I mean, in dry conditions it would be quite a while before rain could infiltrate soil under a dense canopy (that would intercept and store much of it initially) and reach roots and rootlets then be passed into the translocation flow, either swelling xylem or making leaves suddenly heavy. I think the weight of rain explanation is the more likely. But I am just speculating.

[Drayton tree care]

There was a bad case here a young girl of 13 was killed by a poplar limb terrible shame. Sat on park bench and then crack. Poor girl never made it into the ambulance