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The Role of extreme drought and legacy EFfects of long-term manipulatiOn of water availability on growth and REproduction of ScoTs pine REFOREST

Periodic Reporting for period 1 - REFOREST (The Role of extreme drought and legacy EFfects of long-term manipulatiOn of water availability on growth and REproduction of ScoTs pine REFOREST)

Okres sprawozdawczy: 2017-11-01 do 2019-10-31

Scots pine (Pinus sylvestris) and deciduous oak (e.g. Quercus petrea, Q. robur and Q. pubescens) are widely distributed tree species in Europe. Several recent studies conducted in southern Europe have reported drought-induced dieback of Scots pine. Many of those forests once dominated by Scots pine are now shifting towards the dominance of oak. Drought stress has been identified as an important driving factor for this transition of forest composition from Scots pine to oak.

The future climate is expected to be associated with more frequent and longer drought events, which will challenge the acclimation potential (i.e. ability to grow and survive with environmental changes) of forest tree species. An important question in the debate on drought and acclimation is whether trees will be able to acclimate fast enough to cope with increased frequency and severity of droughts. It is therefore important to understand how tree growth responses to extreme droughts vary across sites with different productivity, since site productivity can modify trees growth and survival. In addition to adult tree responses to drought, the seedling growth and survival performances are key to forecast the future forest composition. Current global warming is happening too quickly for long-lived organisms such as trees to be able to adapt genetically to new living conditions through the process of evolution, which takes many generations. Fortunately, trees – like all plants – can respond very flexibly to their environment, for example by forming more roots in times of drought and more leaves (promoting strong growth) when conditions are good. However, these specific environmental adaptations are not handed down genetically to their offspring; the inherited genome only includes the ability for a tree to adapt itself.

Consequently, it was long believed that the abilities acquired by an individual tree were lost from one generation to the next. It is only in recent years that scientists have discovered mechanisms in animals and humans that allow them to pass on responses to environmental influences to their offspring. These mechanisms take the form of small molecules, known as methyl groups, which are attached to DNA building blocks and help determine the extent to which individual genes are deployed. This pattern of molecular groups is transmitted to offspring via egg cells, sperm or pollen.

In this context, the overall objectives of this study were to (1) quantify tree-level growth of the two common European tree species, Scots pine and deciduous oak along a latitudinal gradient from southern Spain to northern Germany with particular focus on the effects of severe drought events, and (2) quantify the potential role of mother trees environmental experiences to offspring growth and survival in variable growing conditions (water, temperature and shading).
The first objective was addressed using tree annual growth data from different sites along the latitudinal gradient from Spain to Germany. For Scots pine, 615 trees from 30 sites were used while 2061 trees from 119 sites were used for the three deciduous oak species. Tree annual growth and climate data (temperature and precipitation) of each site were provided by Swiss Federal Research Institute (WSL), Institute of Terrestrial Ecosystems ETH Zurich, Thünen Institute of Forest Ecosystems, Technische Universität München, IRSTEA Aix-en-Provence, Universite de Lorraine, Instituto Pirenaico de Ecologıa (IPE-CSIC), and Ecological and Forestry Applications Research Centre (CREAF).

The distinct role of mother tree’s environmental experience on seedling growth and survival performance was accomplished by applying transplanting experiments with seed from long-term irrigated trees (13 years) and trees growing under natural dry conditions. Two experiments were installed one in the field site and another in the greenhouse of the WSL, Zurich, Switzerland.

From the analysis of first objective, we found that the magnitude of tree growth resilience (i.e. growth after drought relative to growth prior to drought) of Scots pine to extreme drought decreased over the past three decades due to more frequent and severe droughts in the later period. Our study showed that although tree growth resilience to extreme drought of Scots pine was not dependent on geographic location, tree growth resilience of deciduous oak decreased with higher latitudes. However, oak trees had a higher basal area increment during 1996-2005 than the two previous decades indicating higher growth potential of oaks during the warmer years.

Addressing our second objective, we showed that during the lifetime, trees are not only able to adapt quickly to new conditions but can even pass on the 'memory' of such environmental changes to the next generation. This amazing ability of trees of Scots pine has been proved for the first time by this project. The findings provide hope that trees will be better at adapting to climate change than was originally feared.

We prepared four scientific manuscripts from this project and one of them has already been published in the journal Plant, Cell & Environment which can be accessible in this weblink: https://onlinelibrary.wiley.com/doi/abs/10.1111/pce.13729. The three other manuscripts are currently in the process of publication. In addition, the results of this project were presented on international conferences including EGU General Assembly 2019, Functional Ecology 2018, and Science Day of Swiss Federal Research Institute (WSL).
This is the first scientific experiment that identified the crucial role of “memory” that is the role of mother tree's environmental experience for offspring growth and survival. The most important part of this finding is that the current understanding is based on an idea that environment does not cause any changes in the DNA sequence of an organism but may cause changes in the phenotype such as production of heavier seeds. However, our results suggest that the effect of the parental environment on offspring growth and survival cannot be always explained by seed mass and thus indicate the role of epigenetic regulation of the DNA. The epigenetic regulation of DNA suggests that a long-term environmental experience can change the expression of the gene coded at DNA and therefore can change the cell functions of the organism. The impact of parental environmental memory on offspring’ performance could have a profound impact on species composition of the terrestrial ecosystems.

In addition, we understood that severe weather events can eradicate species such adaptation capacity. The field experiment that we used for the second objective was severely affected by the 2018 drought. In this experiment, we grew seedlings of Scots pine and 90% of germinated seedlings were died. We also understood from tree-level growth analysis that experiences of frequent drought events do not benefit tree’s acclimation potential rather makes them more vulnerable to upcoming drought events.

In the applied forest management context, foresters need to consider the location of seed sources (i.e. where parent trees are growing) prior to selecting their plantation materials (seedlings/saplings) from different seed orchards because a seedling stock may exhibit different phenotypic traits, which may respond differently to prevailing conditions, despite their genetic identity.

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