Pruning and felling

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).

Whoa there with the chainsaw, Fred

It comes as no small surprise to me that 50% of the visitors to this site are, like wasps to a ripe apple, drawn by a single (November) 2007 post: What happens when a tree is cut down? Now, I don’t suggest that they are all asking this question in the same way. The answers could vary. ‘It landed on Fred’s head’. ‘Not much, it’s still there’ But I can guess that they are more interested in carbon cycle issues, and as loggers don’t generally dig up the roots, it is the decay of these that some are interested in. Others may be interested in how the surrounding tree canopy takes over the environmental services of the missing tree.

I didn’t start the blog with these people in mind. And it tells me that I don’t get many walnut-interested readers. I shan’t be doing much in the way of enlightening these readers with more explicit information (Fred’s not expected to recover), but I will add that I estimate on our farm that about 25% of the total sequestered carbon in our trees is underground. This ignores the carbon accruing in the other vegetation, and that added to the soil annually by herbaceous plants which die back over winter.

Managing for growth

In a previous post (A bit more about cultivars, Feb 3) I promised to come back to the topic of managing for growth. All that I’ve seen in our plantations suggests that rapid seedling growth (increasing height, expanding DBH) leads to more rapid physiological maturity, and that this in its turn is displayed as fruit setting (nutting). At the core of management for early growth will be weed control, and if you plant, as I did, in old pastures or hay fields, this means grass control. Grasses are particularly pernicious, if you’ll forgive the alliteration. A little while ago I gave a talk to a local woodlot association, where I was tackled about my use of glyphosate. “Why” I was asked “didn’t I mulch my trees?” Laziness, I suppose, though economists would probably interpret that as awareness of an infinitely low return to labour were I to do so, because I am not inherently lazy. The difficulty lies in the number of trees we have – we would have to mechanize the mulching process if we wanted not to have to work around the clock. Let me first, though, defend myself to the purists. In the early years, I did work around the clock, using a Honduran sickle (don’t ask right now) to control grass and weed growth manually. The trees didn’t grow. I also used plastic brush mats (inorganic mulch, if you like). The trees didn’t grow. So I bought myself a backpack sprayer, having seen the local CN maintenance crews spot-spraying for brush control along the local railroad track. I even bought the same model. The trees began to grow. By this time I had 1,500 trees, and found that I could spot-spray the lot in one good day’s hard work. Much better than bending over for several days with my old Honduran friend in hand, without result. The trick was to lower a piece of old 15cm stovepipe over each tree, for protection, while whipping the spray-wand around it. That tells you how undeveloped they were, because even a 50cm seedling can develop enough branches that you’d need a 50cm diameter stovepipe if you were to lower it over the tree quickly.

So the trees grew, and outgrew my technology. Then I picked up an old orchard sprayer from my local machinery dealer, and gravitated to band spraying (switching a tractor-mounted sprayer off between trees requires more dexterity, accuracy, and even controls, than I had, and doesn’t do the machinery much good). Though by this time the trees were getting to the point where a continuously clear band was only going to be good for expanding root growth, so I didn’t begrudge them that extra bit of herbicide. They grew wonderfully. So the lesson in this is that herbicides can be very useful and, if the sums were done, probably far more economical (let alone effective) than mulching. And in case you believe ‘herbicide’ and ‘conservation’ shouldn’t be mentioned in the same breath, or the same sentence, just try g**gling them together, e.g. http://www.appliedeco.org/conservation-research/prairie-restoration-research, or http://www.ecologyandsociety.org/vol7/iss1/art6/
So, let me finish this post off with a couple of photos. The first is courtesy of the Ontario Ministry of Natural Resources and dates from the early 1980s when a lot of work on hardwoods, especially black walnut, was being done. The photo shows the contrast in tree growth between herbicide (background) and no herbicide (foreground). When I find the booklet (it’s lost in my files somewhere) I will confirm the herbicide (simazine, I think) and the agency involved (Canadian Forest Service, I think). Fred Van Althen was the principal researcher. I think the photo speaks for itself about grass effects on black walnut.

The second is from our farm, and dates from 2004. Two years before, these trees were very much like the trees in the foreground of the previous photo. Glyphosate sprayed at the recommended level of active ingredient per hectare gave the grass control you can see here. Some of these trees nutted in 2004, which does not speak to an immediate herbicide effect but rather that the previous spot-spraying was itself effective. Now, I only spray seedling trees under establishment (mainly the selection blocks). My current weapon of choice (not used in this photo) is an ATV-mounted Enviromist Sprayer, with a 60cm spraydome. I can now do hectares of plantation per hour.

Let me mention some of the other consequences of vegetation, mainly grass, control with herbicide. As I generally spray only once, in the spring, there has been an interesting proliferation of broad-leaved species, with an accompanying increase in insect and bird diversity. Note that I say broad-leaved ‘species’, rather than ‘weeds’. The definition of a weed is ‘any plant growing where you do not want it to grow’. While I was quite surprised by the influx of milkweed, and of some other species, it is only ragweed that I truly dislike. Ragweed, apart from its noxious asthmatic-inducing pollen, is a plant with no redeeming features, and can be as oppressive to young black walnut as grass can be. So a second application may be called for. The remaining vegetational diversity I value, as it co-exists with the trees without direct competition. As the trees grow they quickly shade the other species, and I am now mowing beneath them as some grass has returned. There seems to be no juglone intolerance in these broad-leaved species, as I see no major species-difference where they grow between fields with trees or without.

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.

Of tree equivalents and methane

I am not the only one using tree equivalents to account for carbon offsetting. However, the term clearly means different things to different people. If you enter it as a Google search term, you’ll raise all sorts of esoteric stuff, mainly to do with analytical methods in economics, but here’s an interesting analysis from TransCanada Pipelines (http://www.methanetomarkets.org/events/2005/all/docs/cormack.pdf)

TransCanada Pipelines
Emissions saved in no of tree equivalents
Year	2002	2003
tonnes methane	191,000	223,000
tree equivalents	232,000	270,630
tonnes per tree (CO2E)	1.2	1.2

TransCanada is trying to show what impacts the prevention of emissions (basically leaks) from its pipelines will have. It assigns a factor of 1.2 t of carbon dioxide equivalent (CO₂E, different from the straight carbon, C_2, that I have been using – the factor would be 0.32 t C₂) to a tree, and claims that it has prevented emissions equivalent to hundreds of thousands of trees. I am assuming that the analysis attempts to quantify the offset tree planting program it would have to launch if it were to address emissions by this means.

If I have tried to do anything so far, it is to indicate that emissions offsetting must be viewed from the point of view of annual increments in sequestration per tree. A tree does not go from zero to 1 t of sequestration of CO₂E in a single year. Perhaps TransCanada wouldn’t actually plant, and has identified a stand of actively growing larger trees somewhere in the tropics of Costa Rica which have been assessed as capable of sequestering this much annually, and it is these which it has selected for its sequestration factor. This analysis is no better than that of my fuel-dump story yesterday. What is essential is to understand that the process of carbon sequestration in trees builds upon that which has already occurred (there is something called a relative growth rate), and that conditions remain such that the trees can grow actively for the period during which sequestration is being sought. In the early years, sequestration is in the order of kg, not t, per tree.

Methane has a much higher (21 times) global warming potential (GWP) than CO_2, so it is not entirely clear to me that the factor of 1.2 t is actually in CO₂E. This is actually 1.2 t of methane, equivalent to 25.2 t of CO₂ in global warming terms, so if I’ve done my calculations correctly, TransCanada’s tree equivalents should more honestly be numbered in the millions. These data were shown in a Powerpoint presentation, so I’m assuming that if no-one questioned the concepts espoused, they all went home feeling warm and fuzzy.

Of tree planting and offset programmes

These posts should tell you one thing - that planting trees is not the immediate solution to offsetting carbon emissions. I remember seeing an article datelined Australia, which indicated that planners of a recent major celebration in Sydney were going to offset the consequences of a spectacular fuel dump by a military jet (and the fuel's ensuing conflagration) by planting 300 trees somewhere. Given the droughts and fires in Australia, my suspicion is that if the trees were ever planted they are probably already dead. But I'd also be intrigued to know who is auditing or will audit this offset, otherwise it is just as much hot air as was left by the jet in its wake. In the case of our example, it would take close to 15 years to reach the point where the number of trees required to offset that fuel dump did not have a lot of zeros attached. In short, it takes an already-planted well-developed tree growing rapidly to offer a short-term outcome. This is why avoiding emissions has to be the first step, and tree planting an associated initiative to get as much of the free atmospheric carbon dioxide as possible fixed back into some longer-term natural form that enhances biological capital, and, hopefully, our appreciation for the aesthetic values of landscapes rather than subdivisions. I think those Aussie planners were more concerned for simple and immediate pleasures ('aaah'), and felt no personal or professional responsibility for the display's effects. I do not actually know whether the display went ahead. I do know that they should be planning to plant trees now for the probability of any such display (or any other flyby, even in Iraq) 15 years from now. Otherwise, don't do it.

So, how many trees to a tank of gas?

Black Walnut Carbon Sequestration, Field 3 Lostwithiel Farm
	2003	2004	2005	2006
Average C per tree (kg)	2.37	3.54	5.56	6.98
Annual C increment per tree (kg)		1.16	2.02	1.42
Annual C increment per tree (%)		49.0%	57.1%	25.6%
Litres of gasoline equivalent in increment (l)		1.88	3.26	2.29
Tree equivalents per 50 litre tank of gas		26.64	15.35	21.82

What I really mean by this is, how many trees of the average age and size of the trees we have in Field 3 will it take to sequester the carbon emitted by combustion of 50 litres of gas? The answer is, currently, about 20. You can see that the trees are growing quite rapidly, increasing in sequestering capacity by about 50% per year (except 2006, which was very wet and cool, and apparently not favourable for growth; a substantially thinner cone was laid down in 2006 than 2005. We won't know about 2007 until we take measurements while the trees are still dormant in April 2008). Annual estimates are made on the increment in sequestration capacity, and not on the basis of total carbon fixed.

Of cones and hemispheres

Here is a visualization of the annual superimposition of cones which represents tree growth (lowest/earliest cones are not shown). We can extend this a little further. At the apex of each cone are laid down the buds which will grow into our new twigs next year. These grow in the same manner, expanding into the branches which subtend our expanding canopy (shown here as successive approximately-hemispherical helmets). The relationships which govern our tree’s physical development (which make a black walnut look like a black walnut, or a Norfolk Island Pine like a Norfolk Island Pine) are under close genetic control, and it is basically the annual variability in the environment which results in our tree in any given year growing a little bit more or a little bit less.

Of straight lines and carbon

At the heart of our understanding of the tree’s storage of carbon (beyond the physiological processes) is the discovery that the black walnut trunk from ground to tip grows as a very slender cone. How did we discover this? We take measurements annually of tree height (H) and diameter at breast height (DBH, a forester’s standard; for us 1.35m from the ground) of the 575 trees in Field 3. All trees grow at different rates, so what we were measuring was diameter (D) at 575 different distances from the tree’s tip (DFT) on our ‘average’ tree. When we plotted D against DFT on graph paper, we saw a straight line. Successive annual measurements of H and DBH, converted to D and DFT, of the same 575 trees gave us the same straight line. ‘Same’ in this regard means parallel to the lines for previous years, but with very small differences in lateral distance between lines. Using trigonometry, we can calculate the tip angle of our cone, which turns out to be very close to 1.5^o. In other words, the tree, every year, lays a new cone down on top of the previous one, of uniform thickness at all points along the length (actually height) of that cone.

Farm Carbon Budget - credit

Lostwithiel Farm, Annual carbon budget (t)
Year	2006
Credit
Item	unit	amount	C/unit (t)	total C (t)
Black walnut, field 1	ha	3.02	0.33	1.00
Black walnut, field 2	ha
Black walnut, field 3	ha	1.92	0.39	0.75
Black walnut, F1 blocks	ha
Black walnut, barn field	ha
Hybrid poplar, clone NM6	ha
Hybrid poplar, clone D34	ha
Hybrid polar, clone D182	ha
Native bush	ha
Total				1.75

The credit side of the ledger shows sequestration of 1.75 tonnes in two of our young plantations in 2006, for a negative balance of about 2.23 tonnes. As I noted earlier, we will quantify the other items on the credit side and deliver a truer picture of overall sequestration and benefit. The principal difference between Fields 1 and 3 is plantation density. Trees in Field 1 are at 6m x 12m (20' x 40') spacing and in Field 3 at 6m x 6m (20' x 20'). Field 1 trees are slightly older on average than those in Field 3. We have less data on Field 1, and estimates are based on similar relative expansion of biomass as in Field 3 (these data are slightly different from those posted this morning – mea culpa).

Farm Carbon Budget - debit

Lostwithiel Farm, Annual carbon budget (t)
Year	2006
Debit
Item	unit	amount	C/unit (kg)	total C (t)
Vehicle fuel (gas)	litres	2500	0.62	1.55
Tractor fuel (diesel)	litres	200	0.73	0.15
House, propane	litres	500	0.62	0.31
All buildings, electricity	kwh	17000	0.12	1.97
Total				3.98

My attempts to make available the spreadsheet linked to an earlier post were unsuccessful. Instead, I found that I could cut and paste from the original Excel file. Here is the debit side of the ledger. We are emitting close to 4 tonnes of carbon (not carbon dioxide) in all energy expenditures linked to the farm.

Carbon budget

At the core of our Carbon Club will be the annual carbon budget, our calculation of the net annual sequestration over and above our own emissions. The preliminary estimate for 2006 can be seen at http://spreadsheets.google.com/ccc?key=pu_-I9PdU2MfJcOELwYmOOQ . There are still extensive areas of the farm unaccounted for, but which are contributing fully to an ongoing credit balance. What is important to note is that two of our very young (10-15 years) black walnut plantations have already put us in a credit situation (4.5t at the end of 2006). In these two, sequestration is expanding exponentially. As we perfect our quantitative methods, the other fields will be included. Our methods and assumptions will be verified independently before we launch the Club.

Carbon Club

In these days of emerging offset schemes, where emitters are enabled to buy credits and thus keep emitting, too much is written about national and international schemes and not enough (nothing?) about schemes to which conscientious individuals or small organizations could apply. Through our work at the farm, we have developed an understanding of our own carbon emission and sequestration, and it is our plan to launch a farm-scale carbon club, where our net carbon credit is offered for sale annually. This would probably be as a component of club membership, which would have other associated benefits (e.g. free visits to an open house, nut products). The concept is still under development, as is the carbon price.