As I walk the walnuts - 2

As I walk the walnuts, and given that there are about 2,500 of them it can take quite a while, a discussion ensues: morning wally, g’day wally, looking good wally, etc., etc. My wife will be happy to know that no words are actually spoken, but the occasional murmur after visual inspection is unavoidable. Kahlua occasionally intrudes, with notice that she has uncovered something edible, but generally it is a peaceful intrusion, a thrust of wet nose into the hand. She knows my path, and will meet me at the end of it.

So, to answer your unspoken question, do I prune the trees at all? Well, yes, but probably not in the way you think. Of the 2,500, there are about 1,800 that receive an inspection, generally in March, with a rapid appraisal of growth and form, and perhaps a cut here and a cut there. All inspection is from the top downwards, with most attention paid to serious clefts which could prejudice future strength. I have learned not to worry too much about lack of obvious leaders. The tree has its own means of assigning leadership among its myriad of branches, and it is rare to find instances of consistent competing dominance which could seriously deform a tree. If there is a cleft I don’t like, it is a two-year job to remove it, to avoid a major wound before carbohydrate resources have been reassigned within the tree, increasing relative deposition rate on the full branch remaining. Regular cuts are made above the collar, and the edges of the cut surface trimmed so that as the new cambial layer slowly engulfs the wound it does not have to negotiate right-angled bends in surfaces. A trimmed cut will be covered within half the time, or even less (thank you, Malcolm Olson).

The remaining 700 are still small enough that pruning is a process of early formation, to avoid problems that would make correction more difficult later. But even then a tree is best left alone, with only minor corrections. Splinting or taping to correct obtuse forks is the only exception.

Stay away from bottom-up pruning. It would satisfy the College of Foresters, but it will leave you with fewer future options, reduced growth, and a tendency not to address higher canopy concerns. Let the tree fill all the space it can.

Cooked spinach

The New Scientist has just published an article (A forest is born, Vol 196, no 2631, p38, 24 Nov 2007; www.newscientist.com) which addresses the evolutionary emergence of large-leaved trees, which can be said to be the majority of deciduous trees we see today (my northern hemispheric interpretation). The important point to note is that while large leaves are very efficient at photosynthesis, they are very difficult to keep cool, which they manage by release of water vapour through the stomata. Apparently, the number of stomata is regulated by a genetic switch in response to CO₂ levels in the atmosphere. Unfortunately, it appears that this is an inverse relationship: the number of stomata decrease as the CO₂ level rises. Extrapolating from this article, it may just be that we are about to render our forest canopies just so much cooked spinach.

Equally interesting is that the article notes that it was plants’ rooting ability (and thus underground C₂ sequestration) which brought about the climatic upheaval which allowed large leaves to evolve.

Perhaps disappointingly, from my point of view at least, is that the article does not suggest a temperature at which declining numbers of stomata will cause “spinach-fall”, nor whether the CO₂ levels we are expected to approach this century will cause stomatal numbers to approach this critical point. Heat and aridity certainly cause leaf drop in our young walnuts, but perhaps I have been misinterpreting the cause. The leaflets are typically yellow at the point they fall. Were they still green, I might be able to provide the data required for the “spinach” test.