Mass suicide of trees in São Paulo city, Brazil

Trees are important elements in urban Adaptation to Global Warming / Climate Change.  São Paulo, capital of the state of São Paulo, Brazil, between 29 December 2014 and 13 January 2015, became less adapted to global warming/climate changes, once over a thousand trees of the city streets fell over. It was the forecasted for the full year 2015.

Why so many trees died in so short space of time?

There are explanations interesting for comment:
 

 Suffocation of the roots under the sidewalks

Nobody changed the sidewalks of São Paulo city in any radical way. They are the way they have been in many years. If the relationship between tree roots and sidewalks are partial explanations for the forecast of one thousand trees falling down in one year, it can not be invoked to explain the reason why the number of trees that fallen down between December 29, 2014 and January 13, 2015 was, in average, 24 times higher than the forecasted for one entire year.


 Pruning poorly done

As far as it is known the municipal authority of São Paulo city has not changed the practical of pruning the street trees in any radical way. They are pruned the same way they have been in many years. As in the case of the relationship between the sidewalks and the trees roots, if the method of pruning is partial explanation for the forecast of one thousand trees falling down in one year, it can not be invoked to explain the reason why the number of trees that fallen down between December 29, 2014 and January 13, 2015 was, in average, 24 times higher than the forecasted for one entire year.

There is an explanation that sounds rational, or there was a mass suicide of trees.

To search for an explanation for this change one must  look for something that have changed in these two weeks of high falling of trees. As a matter of fact, there were three great storms that hit the city along these two weeks, namely December 29 (2014), January 7 and January 12 (2015) . They brought winds far higher than usual. Winds up to 85 km/h and gusts up to 100 km/h.  As a pattern for this city, gusts of just 60 km/h were remarkable: I'm not saying that it was only the wind, but it sure is something going on in town, and the wind is the only thing that is standing out, that knocked down in 12, 13 days what was forecasted for half a year", said Secretary Ricardo Teixeira, of the Coordination of subdistricts.

If the higher winds are rejected as an explanation, another one may be raised. But should be based on other variable with changed behaviour  for these two weeks of massive lost of trees. One may think about as a mass suicide of trees. But, to advances on this one should propose and wait the development of of a new science: the Vegetable Psycology.

Better be rational, and pay attention to the changes that should be adopted in the way the urban trees are chosen and treated, taking consideration the changes forecasted for the wind speed in the extreme events while the climate changes are deepened.

It seems very important to take in consideration that besides the joke about the suicide of trees, all the reasons raised for the massive fall of trees that occurred in São Paulo city this year are relevance:

- the suffocation of the roots under the sidewalks;

- the way pruning is done; and

- the greater speeds of the winds.

The two firsts explanations taken together and the third one work like the upper and the lower blades of a scissor. What cuts the piece of paper, the upper or the lower blade. Of course both.

The clue for Adaptation to Global Warming / Climate Change seems to be in changing the methods of pruning. Lets see why.

As is in general stated: Reasons to prune plants include deadwood removal, shaping (by controlling or directing growth), improving or maintaining health, reducing risk from falling branches, preparing nursery specimens for transplanting, and both harvesting and increasing the yield or quality of flowers and fruits.

We see "reducing risk from falling branches" as a reason for pruning. As we observed falling trees because of higher speed winds, not only branches, let us include, why not, another reason: "reducing risk of falling trees". This objective could be achieved by shaping, but should be considered separated from just "shaping" because even being some type of shaping, it is a new and fundamental reason for pruning.

It is known that the force parallel to the soil forcing a tree to fall, given the wind speed, is function of the volume of the  tree canopy.  By reducing the volume of the tree canopy one decreases the risk of falling a tree. But nature has its own laws. There are limits to decrease the canopy of a tree already grown, as we find them in secular established cities. If a tree canopy became too little in comparison with the roots it already have developed, a part of them may become rotten, favouring the fall of the tree.

On the other side, the risk of falling a given tree for each volume of its canopy, may be reduced also by decreasing the average heigh of the canopy.

So, may be there is room for increase the resistance to the wind by changing adequately the method of pruning. In doing so, one should consider two different situations. One is when a city is expanding its area, and the streets may be planned taking in consideration the better pruning that satisfy the needs of the roots of the trees. Another is when the street and its side walks are already established. In both these different situations the risk of falling should be extremely low.