Trees are beautiful complex adaptive organisms. As we see them standing immobile, deeply rooted to the ground and unable to move, we may think that they are defenceless to any change happening to their surrounding environment. In the case of drought, trees have evolved to use different strategies for facing water stress. However, intense and repeated droughts can induce changes in carbon allocation, regeneration rates and mortality. As the frequency and intensity of droughts are expected to increase in many regions due to climate change, scientists are trying to quantify growth responses to drought and to propose adaptation strategies. Read more about what different researchers recently found on this topic.

The growth response to drought

Growth is among the main processes impacted by drought. Tree growth can also be reduced for several years after a severe drought event. Xylogenesis (the formation of wood) requires a certain temperature and soil moisture to allow for cell division, and it stops when water potential is too low. As a consequence, in some regions trees show a bimodal growth pattern: they stop growing in winter due to low temperature but also during part of the summer because of water limitations. This is the case, for example, of Scots pine (Pinus sylvestris) growing in a continental Mediterranean climate. Droughts affect wood formation through the reduction in photosynthetic rates due to stomatal closure, reducing the amount of carbohydrates available for building new cells.

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Many regions have already experienced increases in drought and this has been shown to have negative consequences on forest ecosystems. Frequency and intensity of drought events are expected to continue intensifying in the future, thus there is a strong need for better understanding tree responses to drought.

Stand density and drought vulnerability

In a recent study led by Alessandra Bottero at the University of Minnesota, researchers investigated the role of tree population density on the response to drought. The used tree-ring data from long-term forest plots of two pine species, ponderosa pine (Pinus ponderosa) and red pine (Pinus resinosa). The experiments were distributed in different geographical areas in the USA and they covered a large aridity gradient. They quantified growth responses at the population level to express both resistance and resilience to drought in relation to the relative tree population density, finding out that reducing densities would enhance both growth responses to drought. Trees growing in denser populations were more negatively impacted by drought and this has been shown in all three biogeographical areas.

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Alessandra Bottero, now at WSL Birmensdorf (Switzerland), said about their results published in the Journal of Applied Ecology:

“In our study, the vulnerability to drought of different forest types was lowered by the reduction of tree population densities, independently of stand age. The wide range of climatic conditions represented by the long-term experiments examined in this study suggests that our results about the benefits of silvicultural thinning are likely applicable to many coniferous temperate and subtropical forest ecosystems”.

This study confirms once more that the vulnerability of monospecific coniferous forests to increasing drought can be reduced through thinning interventions, which represent a viable adaptation strategy under climate change. Forests growing in arid and semi-arid locations and at their dry limits are particularly vulnerable to climate change, and can therefore benefit from the effects of silvicultural thinning. However, the relationship between tree population density and drought vulnerability in other forest ecosystems merits further investigation.

Species diversity and drought resistance

In mixed species forests things are a little more complicated. On one hand, forests composed by many tree species often are more productive than monocultures. On the other hand, faster growing forests can have greater evapotranspiration rates and this may induce higher water stress under drought conditions. Thus, studies on mixed forest need to disentangle the effect of species composition (i.e., mixing effects) from the effect of stand density on growth response to drought.

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A study focusing drought responses in mixed forests was recently published by David Forrester (University of Freiburg, Germany; now at WSL Birmensdorf) and colleagues in the framework of the EU FunDivEUROPE project. They investigated the drought response of 16 individual tree species in different regions of Europe and evaluated if this was related to species diversity and stand density. Based on previous findings indicating that combining species with complementary characteristics is more important than simply increasing species diversity to cope with drought, their results indicate that species growing in a mixture are not always less water stressed than those growing in monoculture.

David Forrester said about their results:

“When predicting how a species might respond to drought, tree-species richness is not useful. Instead, the identity of the other species in the mixture needs to be considered. However, the same species mixture in one region can sometimes respond differently in different regions.”

Overall, this study demonstrates that the effects of species diversity on tree drought stress are hard to generalize. Investigating these effects at the level of species identity (i.e., different combinations of species) is more advisable than doing it at the level of species richness (i.e., abundance of species), because different mixtures respond differently depending on the region. If we consider that different provenances of the same species can show different adaptation strategies to cope with drought, the situation may be even more complex.

Often studies that report a clear and sharp result have a greater impact than others who conclude that their results “depend on…”. However, I believe that when the findings of an investigation “depend on” something, they are also very meaningful results. They should stimulate us thinking that forests are complex systems whose properties and functioning cannot be easily generalized, but it depends…

Sources:

Bottero A, D’Amato AW, Palik BJ, Bradford JB, Fraver S, Battaglia MA, Asherin LA (2017) Density-dependent vulnerability of forest ecosystems to drought. Journal of Applied Ecology. Doi: 10.1111/1365-2664.12847

Forrester DI, Bonal D, Dawud S, Gessler A, Granier A, Pollastrini M, Grossiord C (2016) Drought responses by individual tree species are not often correlated with tree species diversity in European forests. Journal of Applied Ecology 53:1725-1734. Doi: 10.1111/1365-2664.12745

This post was also inspired by a recent study on tree growth response to droughts, led by the author of the post:

Mina M, Martin-Benito D, Bugmann H, Cailleret M (2016) Forward modeling of tree-ring width improves simulation of forest growth responses to drought. Agricultural and Forest Meteorology 221:13-33. Doi: 10.1016/j.agrformet.2016.02.005

Main photo: Trees have even adopted to survive water stress on the Mojave Desert. Photo taken at: Red Rock Canyon National Conservation Area. Author of the photo: Rafal Chudy