Seeing The Wood For The Trees…Again

I posted a while back about the relative merits of grassland and woodland for food production. Here’s a little addendum to that post.

Suppose you can produce 170kg of beef from 1 hectare of grassland annually – quite a generous supposition, I think, if the cattle are being fed from the grass alone. That amounts to something like 1,960MJ of food energy.

Suppose alternatively that you have two oak trees and two crab apples on your hectare of woodland, producing something like 160kg of acorns and 100kg of crab apples annually. In practice, you’d probably have more than that, or at least you’d also have some other trees or shrubs producing something of food value, but frankly harvesting it all would be a pain so let’s take those figures as a realistic achievable harvest. That would yield something like 2,680MJ of food energy.

Now, I’m not saying that this crude exercise tells you anything very significant about whether you should choose woodland or grassland in any particular situation. And there are lots of additional factors to consider – other nutrients, processing inputs, fertility inputs and outputs, joint products, management issues, system redundancies, and palatability to name a few (a plate of roast beef or a plate of acorns – I know what I’d choose). Still, on the face of it this suggests to me that choosing to plant a woodland may not necessarily be inferior nutritionally to retaining permanent pasture, which is interesting.

In my earlier post I hedged my bets a bit, but I suppose my general drift assumed that a mature woodland was of less food value than permanent grassland. But now I’m not so sure that this is necessarily the case.  Well, that’s the beauty of a blog, I suppose. Yesterday’s thoughts disappear off the bottom of the page, and are easily replaced with today’s entirely different ones. And nobody will notice and give me any grief about it, with the possible exception of Paul Hillman. But I don’t expect he’s looking.

6 thoughts on “Seeing The Wood For The Trees…Again

  1. Thanks Ben – yes we’re certainly planning to do that with some of our plantations once we get the fencing etc organised. I think there’s a lot to be said for silvo-pasture. But in either/or situations I thought those figures may be of interest in relation to respective possible productivities.

  2. Interesting though these numbers are I wonder if they become a tad more interesting if you take the analysis further and seek to get the best of both worlds as Ben suggests.
    You are being a bit unfair on grass because by using cattle to convert the grass into edible material the energy value is lowered by the conversion inefficiency of the beast. The acorns and crab apples are also fairly inedible and would be better if converted by an animal such as a goat or a pig, which may be more efficient with the feeds in your example. I found a conversion rate for Iberian pigs eating acorns of 49%. This gives a recalculated yield of 1313MJ which is much lower than the cattle on grass.
    But then, as you pointed out, you are being unfair on the trees because you are not using the full area of land available. The oaks and the crabs would occupy(area of crowns) 10% of the hectare. If they were in pasture the yield from the grass would go down to 1764MJ (assuming no yield beneath the trees but no difference in the grass beyond the crown). But you can add back in the yield from the trees making a total of 3077MJ, a big improvement. If you take the calculation to its logical conclusion you could replace the pasture with oaks and crab apples and yield 13,130MJ but then you do have to feed the pigs something else before the autumn.

  3. Thanks Paul for those interesting additional calculations. I agree with you & Ben on the merits of silvopasture. I also agree that ideally it would be nicer to feed the acorns & crabs to livestock, but then it gets complicated as you mention because of seasonality (not to mention mast years etc) which wouldn’t be so much of a problem if we had a functioning smallholder economy, but we don’t! Grass surely wins on useful year-round productivity, but the thing is it’s pretty much completely inedible to humans so in that sense I’d argue that I wasn’t being unfair to it – crabs and acorns are more directly edible, though I admit that the temptation to import sugar and flour to make them more palatable would likely be overwhelming. The ecological literature seems to suggest that the overall trophic transfer efficiency of woodland is probably less than grassland so if the woodland biomass goes through another organism (especially a mammal) I think grassland would win. So what do you think? Silvopasture with a few oaks, crabs etc thrown in for famine food (plus birch & wild service to drown our sorrows?)

  4. Errh.. what do I think? I think that mixed land use is bound to serve the needs of localism better than grassland or woodland alone and that any discussion of land use practice should take place in the context of an clear understanding of what the land is intended to produce in total. Food energy is only part of the picture and a focus on it may lead to erroneous conclusions. But you knew that anyway.
    I also think any ecologist who writes papers about trophic transfer efficiency should be made to give the grant money back and find something useful to do.

  5. Oh come now Paul what’s a bit of jargon between friends – I’m sure you must have used a bit of it in your time. But if you insist, the overall ability of organisms feeding on grass to assimilate it into their own biomass is probably greater than that of organisms feeding on woodland. There look, you’ve made me waste 13 extra words…though actually on reflection I’m not entirely sure how relevant that is. Anyway, yes I figured that’s what you’d think, and of course I agree with it. But if you consider food energy to be an important part of the picture then I’d argue that exercises of this kind (albeit rather more sophisticated ones) are useful in figuring out the clear understanding of what you do intend the land to produce in total.

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