Those spring frosts

The bane of any tender fruit producer’s existence is a spring frost, occurring at bud emergence, and which, depending on severity, can play havoc with the rest of the year. I have noticed that at Lostwithiel Farm the critical period is the first week of May, and that if we can get through it with nothing less than 2^oC our course should set fair. What is the likelihood of this happening?

Environment Canada has just made it easy to search historic weather data for just such trends. I have deliberately chosen a dataset for a location slightly colder than ours (100km to the NE) as being more representative than data collected near Lake Ontario. I have graphed both date and temperature of the latest event in May of equal to or less than 2^oC over the nine-year period for which data is available at this station:

What we immediately see is a high probability of a damaging event occurring at least mid-month or later, and that we should not be surprised by any late spring frost. The 2002 event, a negative reading on the May 25^th, would have tested anyone’s resolve.

However, another factor to consider here is tree size. I collected nuts widely throughout our region in 2002, in fact, it seemed to be a bumper crop, for I have not seen the like since. Frosts generally occur as a result of a temperature inversion, and my hypothesis is that a tall tree has most of its canopy above the freezing zone. What I hope eventually to see, as our trees grow taller, is a declining effect of spring frosts on our own nut production. It is possible that many of the trees out in the landscape have been naturally selected for late flowering, though I don’t place much confidence in this. As far as I know, our first-planted trees are derived from nuts collected in our own seed zone. I actually believe lateness in bud emergence (and thus flowering) runs counter to the natural trend, and that earliness is of evolutionary advantage, but this is nothing we shall prove at the farm. What we will test is the heritability of date of bud break in the 25 lines selected this year, and see whether there is any consistent lateness between maternal and F1 lines.

The Triumph of the Tree

John Stewart Collis, in his lyrical prose in the book of the same name, begins: We were nursed into life by trees. It is to trees that we owe the development of a physiology which made Man possible – that is to say, made conceptual thought possible. JSC was a remarkable writer, more a natural philosopher, a conscript to WW II who requested to spend it on the land, and who served in an agricultural brigade as a result (he’d actually served in the conventional sense in WW I, so one cannot question his motive). Out of this came The Worm Forgives The Plough (1973) among others, the first book of his I read. His writing had far more in common with the blogging style of today than with the writing of the time http://transitionculture.org/book-reviews/the-worm-forgives-the-plough-john-stewart-collis/). Again, in The Triumph of the Tree (1950), in fact it is not unlikely that in the course of the next few decades after we have had some really rude shocks – we will gradually return (this time on the plane of consciousness) into the Order of Nature, as one factor of the whole. If this happens, the turning point will undoubtedly have been this century. It seems that he was probably just a decade out, a remarkable insight into the perils facing us at a time when global warming was unknown. He is able to conjure questions enough to make a tree laugh, but in spite of its lyricism this is very somber reading, because, sixty years on, one can really see how much further out we now actually exist on the edge, and all, if we accept JSC’s arguments, because we lost our wonder (perhaps fear) of trees, and fled the Era of Mythology. I would like to think that the Era of Science could restore some of this wonder (perhaps respect).

(There have been various publishers of The Triumph of the Tree, starting with William Sloane Associates, and House of Stratus. Most are now extinct, but I see that the used book is still available through the Internet).

2007 Nut Report

This week I completed the analysis of the nut samples from the trees selected for on-planting in 2007: I had collected all the nuts from these trees, cleaned and dried them in the usual way, then selected three nuts at random per tree for later cracking. These three-nut samples were then forced-air dried in paper bags at outdoor ambient temperatures for about six weeks before being transferred indoors, and left to stabilize in a warm, low-humidity environment for another six weeks. All nuts were cracked with a Master nutcracker. Nuts were cracked as replicates, i.e. one nut per tree was cracked until all trees were assessed, then the second nut in the same way, and finally the third nut from each tree. The order of cracking within each replicate was random.

Before presenting the results, it is important to note the characteristics of the 2007 harvest. A very late spring frost (May 19^th) caused widespread damage to emerging flowers. Nutting was considerably reduced when compared to 2006. Selection of trees was made initially on low expressed leaf-disease scores, then adjusted in total number for the number of trees that actually gave a sample big enough to ensure sufficient nuts for on-planting. The area selected for planting of this F1 material was big enough for a randomized complete block study of 25 lines replicated 11 times. The planted sample (two nuts per location) thus consisted of nuts from the 25 maternal trees with characteristics (traits) considered most interesting (low disease scores, high nut number in a low nut number year, precocious trees – nutting at a small size/young age, range in nut sizes, etc). As no cracking analysis had been done to date on-farm, there was no prior knowledge of kernel weights or percentages.

The mean nut weight (NW) was 15g, and the mean kernel weight was 2.5g. By themselves, these values don’t tell much about the sample. The following histogram indicates the distribution of KW. NW ranged from about 7.6g to 18.8g. Kernel weight (KW) similarly ranged from 1.7g to 3.9g (line 1-1-17), though NW and KW are not directly correlated, i.e. lowest NW did not exhibit lowest KW, nor highest NW the highest KW.

But I’ve left the best to last. The sample showed a mean kernel percentage (K%) of 22%, calculated from KW as a percentage of NW. Also shown as a histogram, we can see that almost 25% of the trees showed a very respectable K% of 25% or more. The highest recorded K% was over 28% (line 1-9-23). This was far higher than I expected to find in this sample. I had expected a mean K% of about 20%, with perhaps the top line approaching 24%. These expectations are the results of earlier surveys I’ve done off-farm where I’ve had many more trees in the sample, and mainly where lowest K% was far lower ( ~12%). But it is actually quite exciting to find 28% kernel, because the probability is that we shall eventually find higher values. Again, K% is not correlated with NW or KW. In fact, the highest K% was found in a line with mean NW <10g,>

If we complete the analysis by calculating kernel yield per tree (KY, by multiplying KW by the number of nuts per tree; no of nut data is lacking for two lines), we once again find a re-ordering of lines. Line 1-1-14 yielded 1.34kg of kernel. The tree with highest KW (1-1-17) yielded 0.45kg, and the tree with highest K% (1-9-23) yielded 0.32kg. As economic yield will be more closely related to KY, the other parameters of KW and K% become less critical in our selection focus.

Grafted named selections in the US have yielded K% of up to 39%. Our experience in 2007 indicated the existence on the farm of lines of up to about 75% of this potential K% in common walnut obtained from our own seed zone. Far less has been published on nut yield per tree. We have planted an F1 trial of these 25 lines to examine heritability in K%, and the other traits measured on each tree. Almost all of the 25 lines had good disease expression scores (lower incidence); it will be interesting to observe heritability in this trait.

The premise underlying our biomass approach to nut production is that there will be enough trees in the overall population with traits of sufficient value to exploit multiple income streams. To date we can characterize these streams as kernel, shell, and sequestered carbon. We have no intention of focusing on a single trait in our selection program but have yet to construct the functional model which will guide us in optimizing multiple selection criteria. All in all, our economic yield is likely to be defined by how we market the different carbon streams partitioned by the tree. So this is an interim report.

A bit more about cultivars

I should probably qualify my last remarks, because there are undoubtedly some mid-Westerners getting a bit hot under the collar right now. What I think it is important to add is ‘especially at the northern fringe of the species range’, i.e. where I am, where graft unions are very prone to winter damage, ultimately leaving one with an expensive tree growing from the rootstock (which, by the way, will have been a common black walnut seedling grown from a nut, sacrificed for the greater good of the scion). This, also by the way, is the Third Law of Nut Production: Rootstocks Make Very Expensive Nut Trees. The following are the First and Second Laws: Use Selected and Named Varieties, and, Grafted Trees Don’t Like Marginal Conditions. If you read them in order, there is a modicum of sense to them. So, no matter how good your soils are, if you live in the same climate zone as we do, you can take the conditions as marginal.

Now let’s address the energetics of nut production. A grafted tree will produce nuts at a very young age, because it grows from a scion which is already mature wood. A common black walnut will take some years to reach physiological maturity, and the only way to speed that up is to create the best growing conditions possible so that the tree grows as fast as it can. Still you will not know when to expect nutting, but my goal is to minimize this period (more on this later). But we must still deal with the issue of the tree’s ability to fill all those nuts it is going to produce, which will be related to the canopy size for the number of nutting sites within it. A young grafted tree will still not produce an ‘economic’ yield, because it will be too small to do so. Pushing a tree to nut at an early age creates the risk that too much energy will go into multiple shell production, leaving insufficient resources for the kernels to fill them. Cumulative solar energy incident on the canopy declines with increasing latitude, so, once again, there is a risk that a tree genetically driven to produce large quantities of nuts, will, at our latitude, leave those shells empty. The tree’s root system grows in tandem with its above-ground structure. Resource capture by the roots is also important here, as the tree requires nutrients to produce fruit. Small root systems will not sustain heavy nut loads.

So, all in all, my original post was fair even if it wasn’t fully explanatory. Just remember the three Laws of Nut Production, and make your choice.

As I walk the walnuts - 3

As I walk the walnuts, or more accurately, pass through on snowshoes, as we had about 25cm of the white stuff on Friday, I look ahead to the coming season and wonder what’s in store for us. Maple syrup production has turned into a big gamble, because of early sap flow when the temperature spikes, and a friend of mine wonders whether it will ever be worthwhile tapping the trees again. He’s slowly building a new sugarhouse, in case he becomes so inclined, but it doesn’t look as though it will be worth his time hurrying this year. The corollary for a nut producer is those aggravating late frosts in May, which can sweep the trees clean of the emerging flowers. So far, the walnuts will take 2^oC up until the end of the first week of May, after which it is a complete gamble. Last year, a frost on May 19^th did widespread damage, though it didn’t come close to the two consecutive nights of -5 ^oC we recorded about the 8^th May in 2005.

Common sense says we need to track the phenology of flowering, because later emerging flowers stand a better chance of missing the frost. However, this is a laborious task, which would require scoring all our trees. Well, you say, you already score for disease expression and nut production, why not phenology? Well, I say, if a tree bears nuts after a frost in the critical period I mentioned, there’s a good chance it did so because it flowers late, i.e. I’d rather score the result than the cause. However, now that I have this indicator, it would be a good idea to track the phenology of flowering on the 25 trees from which I on-planted last fall. My hypothesis is that all valuable traits have at least some measurable heritability from the maternal line, and that I need to determine whether the average expression of each trait in the population of offspring is at least marginally better than that same trait expressed in the maternal tree, i.e. that I can make genetic gain by selecting under our conditions for nut yield, kernel quality or whatever other characteristic I believe valuable. However, it takes that first effort at on-farm selection to know whether it is worthwhile (equivalent to concentrating beneficial characteristics in a smaller population than the one from which the selections originated).