Finally getting around to collecting this year’s prunings before the grass gets too high. By my standards I did some severe pruning, and probably raised the lower height of the canopy by an average of at least 30cm!
I also had to cut out two trees which were encroaching on the hydro wires, which allowed me to do something I have wanted to do for a while – relate total subtended branch basal area (their footprint where they join the trunk, if you like) to the tree’s DBH. This is best expressed in the graph, where each point represents a branch plotted against its positional number away from the tree’s tip. There were about 25 branches on each tree.
Why did I want to do this? I have been intrigued for years about the switch from height increase to canopy expansion in open-grown black walnut, as mine are. In their early years the trees ‘concentrate’ on increasing the height of the leading tip. After this period, in an open-grown plantation each branch continues to grow outward and upward, taking some resources from the leader in so doing. With time what I have previously termed the ‘conical growth rule (CGR)’ must break down for the central stem, - DBH continues to jncrease, but as an expression of total canopy subtended rather than vertical height of the leader.
Each tree was just under the 15cm DBH, and I measured both this and the diameter of every subtended branch, working my way up the trunk (though they are plotted numerically in reverse). Height at which they were subtended (where they joined the trunk) was not taken, as there was no apparent relationship here – small branches could be found between larger ones. Hence the dispersion in the graph. Twigs on the trunk only a year old were ignored.
The numbers can be summed up in two ways: either by relating the relative cross-sectional areas (CSA), or by relating the relative perimeters (P; both cumulative in the case of the subtended branches). In both cases both the CSA and P of the subtended portion exceeded that calculated at DBH by between a factor of roughly 2 (CSA) or 6 (P), implying that a switch has indeed occurred. Which measure we use is not important as long as we be consistent. Remember, however, that DBH is measured at a point below the subtended canopy (or almost – the bottom pairs of branches on both trees were still below DBH), so it expresses an integration of accumulation of all biomass above it.
In physiological terms, the tree is growing mostly outwards, somewhere between twice and six times as much as it is growing upwards (a principal leader was still visually detectable on both trees and was not counted). This is in terms of deposition of carbon, the main structural element of the tree. At this DBH or earlier, the CGR is no longer applicable to the central trunk (closed canopy plantations may be the exception) and a ‘subroutine’ needs to be added for the canopy as a whole.
The relationship appears to be quite consistent, though two examples do not make a large sample (for the statisticians among you, the R2 value of each line shown was about 0.50). It does seem that this will be the best way of noting when the competitive effect between trees occurs – if the pooled line (pooled because they are so close) does represent more than just these two trees then a change in slope (downwards, or less slope) should infer this, i.e. the branches will not grow so big (their footprints will be less). Variability in footprint under these conditions may be less.
This variability is curious. In open grown conditions the tree may continually be testing the immediate environment, and some of the variability may be due to competition between branches within the tree. This suggests that the leaves must be sensitive to slight differences in light intensity, because the black walnut does not have what might be termed a dense canopy.
Whatever it is, another relationship emerges, and I am again left marveling at the flexibility in expressed life around us (if the trees were closer the expression would be different).
