Nothofagus dombeyi, or coihue, is a large tree that grows in the Andes. In the late 90s, northern Patagonia was hit with a pretty severe drought that killed many of these trees. So what was the difference between the trees that kicked it and the trees that are still going strong?
Some trees constantly live with higher water stress than others. This is usually due to where they’re growing. If a tree is on a particularly steep patch of ground, the water runs off before it can suck it up. If a tree is growing in shallow soil or on a very sunny slope, it’s going to be a lot thirstier than your average tree in the forest.
The authors of this study thought that these trees might be the ones to be hit hardest during the drought since they were already stressed. They also considered the possibility that the trees growing in more water stressed areas are different and better able to cope with drought. The trees used to the good life might have invested more in their tops than their roots, which could be bad news in a drought. A tree with a smaller root system might not be able to suck up enough water to support all its branches in a very dry year.
The authors also wondered if there were a way to look at a tree and use where and how its growing to predict whether or not it will survive a drought.
So, what did they find?
Where the tree grew didn’t really matter, unless the slope was very steep. Then mortality rates went up. Also, if a lot of bugs had been eating you, the drought was more likely to put you over the edge.
Smaller trees are in big trouble, but big trees are pretty tough. A measure called “climate sensitivity” was pretty good at predicting tree death in young trees. If a tree grows a lot in wet years and very little in dry years it is said to be climate sensitive. If it grows about the same amount no matter the weather, it is said to be climate insensitive. Climate sensitive juveniles generally did not make it.
Older trees were likely to die if they had slower growth rates. Older trees that made it had more crown dieback. So rather than try to keep the entire tree alive, they’d sacrifice a few limbs. Kind of like chopping off your arm when you can’t afford groceries. Except trees can grow new limbs and you can’t.
So why is studying drought mortality important, you ask? Number one reason: I think it’s cool. Other reasons: It’s nice to know how things will change during a drought – I like to know what to expect. Will so many of one type of tree die that the composition of the forest changes? Different trees and different aged trees support lots of different animals – if you know which trees are going to die, you might know which animals are going to be scarce. If you know a lot of trees are going to die, you can be more prepared for the sure to follow fires, too.
SUAREZ, M.L., GHERMANDI, L., KITZBERGER, T. (2004). Factors predisposing episodic drought-induced tree mortality in Nothofagus- site, climatic sensitivity and growth trends. Journal of Ecology, 92(6), 954-966. DOI: 10.1111/j.1365-2745.2004.00941.x