Tree guards and weed mats: is protection worth the cost?

Back when I started as a fresh-faced lecturer at Lincoln University in the mid 90s I contributed to a third year class called Wildlife Management.  Graham Hickling was the main lecturer and provided the heft of wildlife management. He had lots of hands-on experience in working with possums, stoats, rabbits and so on. I provided the more theoretical and conservation side of things, like how do we make a decision on which species to save.

Each year I would ask a question of the class. Which is the lesser of two evils: Pushing ahead with a method or technique when we haven’t done enough research to know whether it may help a species in question (and may even hinder it), or waiting to implement a method till after we know that it is a good thing to do (but risk losing the species or population)? Over eight years I surveyed about 240 students before the course was lost in a periodic restructuring. About 60% wanted to rush in while the other 40% wanted to be cautious and get it right. The proportions were about the same from year to year.

This always formed the basis of a good discussion between the two groups. Neither option is a good outcome. Most people in the rushing in group felt that doing something was always preferable to doing nothing. They also felt that most of the time we had a reasonably good feel for what would work, so why wait? The cautious group felt that getting it wrong would make things even worse, and possibly seal the fate of the species and that taking the time to get it right would usually be worth it.

Is there a correct answer here? Obviously it depends. Sometimes, things are so desperate that you need to do something right now. Usually, however, we have more time than we think. I have also seen plenty of examples of where testing really obvious ideas have found that, actually, things are not so obvious after all.

I was reminded of this by a study by Lincoln University’s Rebecca Dollery, Nick Dickinson and Mike Bowie. The trio had noticed that when people planted trees, especially in dry Canterbury, that they often put tree guards (a kind of plastic sleeve) around the trunk and a weed mat around the base. It is believed that the guards and mats protect the young tree from frosts and drying out as well as reducing weed growth close to the plant. There might also be some protection from mammal browsing.

Placing a guard and a mat on each young tree adds significant cost and labour to planting. With New Zealand wanting to plant 1 billion trees this becomes a much more important question. It is thought that putting the guards and mats on is a cost effective process because it decreases tree mortality.  Seems obvious. Rebecca decided to test this.

In a dry site in North Canterbury at Te Whenua Hou Rebecca was able to plant about 1000 one year old kanuka and 180 Pomaderris plants. She tried two different tree guards, with and with out weed mats made from wool waste fibre. She also had plants that had no protection. Rebecca measured mortality over a year as well as soil moisture and temperature. Rebecca has published her PhD work in Ecological Management and Restoration.

Tree guards did seem to make a difference but not because of herbivory or frosts. It seemed that the the guards did stop the plants from drying out during summer. Weed mats, on the other hand, did not seem to do much at all. Rebecca was able to look at the costs associated with the different approaches. With the right tree guard, and factoring in survival of the kanuka, the cost was a third of doing nothing. However, with a different type of tree guard the cost was almost twice the cost of doing nothing! So, some obvious results and some not so obvious results.

With a billion trees to plant (at least!), these are not inconsequential savings or expenses (a billion times anything is a lot!). Rushing in could have very serious implications for the long term. In this case I am pleased that Rebecca, Nick and Mike have taken their time and tested the ‘obvious’.

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