Moisture
The direct Association between Boron and Moisture.
(by improving the latter you directly improve the former, in your forest)
Both of these products complement each other, and it is important to understand how one can effect the other. For example, in drought conditions, boron is not freely made available to growing plants. Unlike the macro nutrients, which are freely taken up by the plant root system, boron is passively taken up. Meaning, when the tree takes up water, it also takes up boron. If it is not taking up water, it will not be taking up boron, therefore boron deficiency occurs.
There are a few good management practices, which can help to alleviate boron deficiency, without actually applying the boron product. This is not a remedy to fix a boron deficiency problem, it just delays the need to apply it.
What we have found is that weed control can drastically influence the levels of boron in the plant. This is directly related to available moisture. Bare earth we have found, is four times wetter, than an adjacent area that is fully grassed, and up to ten times wetter, than an adjacent area that has actively growing brush weed present, in particular gorse, broom, or bracken fern.
To better explain this, lets look at an example:-
If you have a site that has abundant grass growth, and your trees, are say two years old, a foliage analysis taken from these trees, which are grown in an unsprayed, grassy sight, would reveal boron levels around 8 parts per million.(8ppm)
If an adjacent row of trees which had received a one sq metre spot spray, the boron levels in this foliage would be around 12 ppm. If the spot was increased to a two sq metre, the boron levels would increase up to 15 ppm. Meaning, boron levels in trees is directly related to the available moisture around the tree roots. If on the other hand the weed problem is brush weed, then you are dealing with a more severe problem.
Lets go back to our original example, where we have a row of unsprayed trees, and this time instead of grass, lets say it is a mixture of gorse, and broom. If the boron levels in this foliage equate to about 8 ppm, there will be no difference in the boron levels in adjacent rows which have been spot sprayed.
The reason for this is that, a one sq metre, or even a 2 sq metre spot, cannot impede the actively growing root systems of such things as gorse, broom, or bracken fern. Broom for example growing in between the rows, can completely desiccate the already sprayed spot, by tapping the moisture from this area because of its vigorous, underground root system.
To understand the complex issues surrounding brushweed growth and its influence on initial tree growth you, have to understand the issues involved in transpiration. Transpiration is the ability of a plant to absorb moisture from the ground, through the roots, and expel it through the leaves through small openings called stomata. For example, on a hot, dry day in Canterbury, with a typical Nor-Wester blowing, relative humidity is quite low, and plants vigorously transpire. Some brushweeds like broom, cannot fully close off their stomata. So during hot, dry, Nor- west winds they can transpire continuously, and eventually kill them selves through desiccation.
This is quite common, if you have a look along the river bank in Canterbury, where there is minimal top soil, and broom is actively growing in alluvial soils. In the hot dry conditions, the soil cannot retain moisture, the bush continually transpires, and eventually dies. Needless to say all other plants growing in the same vicinity will be desiccated as well.
Gorse on the other hand, can actually close off its stomata in these conditions, and therefore have less of a drying out effect on the site, compared with other bush weeds such as broom and bracken fern. For this reason, spot spraying in a lot of cases where these weeds are actively growing around your trees, is not a suitable operation., because of the weeds ability to out grow the recently planted seedlings and subsequently dry out the sprayed spot.
An aerial blanket release is the only thing that should be considered.
There is a formulation available now, which we can apply from the air over a plantation killing the brush weeds, and grasses and leaving the trees unharmed. The older the stand, the more costly the operation. On many occasions I have seen a site completely desiccated by brush weeds in Canterbury, and the forester has literally got to start again. Forward planning is imperative when establishing a woodlot or a plantation.
The first step, and most important step, is to kill all weeds on that site prior to planting. This is a must, and the best way to achieve this, is by an aerial pre plant spray, followed by roller crushing of the standing brush weed. This will form a mulch retaining all soil moisture, and nutrients. Once the trees have been established, and it is obvious that a brush weed problem will exist, then an aerial release the same year as planting is by far the best option, and very economical. If however this is not undertaken, and the brush weeds are left to grow for even one growing season, the cost for aerial releasing, and saving this crop can be quite high.
Because boron levels in your planted crop, can increase with increased moisture, depending on the regime you take, there is a possibility that you will not need a boron application until your crop reaches year three. If on the other hand, the tree stocks came out of the nursery boron deficient, and are planted into an area of actively growing weeds, whether grass, or brushweeds, and these weeds are not successfully removed, then boron deficiency can show up after the first year.
Usually when we talk of boron deficiency here in Canterbury, it means the tips die back as previously explained in other articles. This of course, has a drastic effect on the butt log. This is where the bulk of volume is, (as well as the value), therefore, it is imperative, that initial growth, at least for the first six metres, is unimpaired either by weed competition, or boron deficiency.
I would like to reiterate the importance of suitable weed control, and its association with optimum boron levels.