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Disentangling the effects of CO2 fertilization, nutrient limitation and water availability on forest ecosystem processes: Estimating their long-term consequences on SW European forests

Periodic Reporting for period 1 - DENDRONUTRIENT (Disentangling the effects of CO2 fertilization, nutrient limitation and water availability on forest ecosystem processes: Estimating their long-term consequences on SW European forests)

Reporting period: 2016-03-01 to 2018-02-28

European forests will experience novel growth conditions for which no historical reference exist. To correctly predict the impact of global change we have to understand the long-term response of forest ecosystems to management under climate change. To do so, it is necessary to have long-term data from real forests combined with ecological models. Therefore, the overall objective of this project was to improve our understanding of future growth patterns on European forests. To do so, two Scots pine stands in northern Spain representing continental and Mediterranean climates have been used. These sites have provided long-term field data to produce three major data series:

My results show that the response of SW European forests to changing growing conditions will depend on both the intensity of the change and the particular edaphic-topographic conditions of each forest stand. In Mediterranean, low-elevation forests, a reduction in growth is expected because the CO2 fertilization effect is not enough to compensate the growth reduction due to water stress. However, at these sites, the most limiting factor is nutrient availability, as forests stands are already acclimated to summer droughts. On the other hand, continental, high-elevation forests could benefit from increasing temperatures that will extend the growing season, although such positive effect could be limited by reductions in water availability, with CO2 fertilization being almost negligible (Figure Carbon). At these poor sites, as trees are already adapted to low availability of nutrients, water scarcity will be the most important limiting factor. The trees at the research forests have also shown the capacity to increase their efficiency of using nutrients to grow depending on the availability and temporal change of such nutrients (Figure Nitrogen). In fact, my work has shown that the changes in nutrient availability that trees experience during their lives are recorded in their wood, which can be used as a record of annual nutrient change (Figure PKCaMg). All things considered, my results indicate that SW European forestry needs to be locally adapted to take advantage of increasing growth potential in cool sites but at the same time reducing the negative effects of global change in warm sites.
I organized the work along three major research lines:

1. Generating the longest field record of nutrient flows and nutrient use efficiency in Scots pine forests in SW Europe.
Soil carbon flows, soil micrometeorology, soil nutrient status and biological activity were measured in spring and autumn in 2016 and 2017 in two research forests in the SW Spanish Pyrenees (a continental-subalpine forest and a Mediterranean-montane forest). A trend in decreasing soil organic matter and its associated carbon, nitrogen and phosphorus content was found, indicating the depletion of the organic matter input from the original thinning activities in 1999, and consequently an increase in nutrient limitation over the years. In addition, litterfall mass and nutrient content were monitored in autumn 2016 and 2017 and data added to previous records to reach an 18-year full record of litterfall. The last work done to extend field data records were tree inventories which were carried out in winter 2017-2018. Such activity allowed us to estimate standing tree biomass and combined with the results from the activities indicated above, the estimation of the change in nutrient use efficiency over an 18-year period. My results showed that there is higher efficiency in the use of nutrients at the high elevation forests than at the low elevation site. Thinning treatments over the years did not have a significant effect on nutrient use efficiency, but a trend of trees to increase their efficiency over time was found.

2. Quantifying the CO2 fertilization effect with a detailed battery of forest growth simulations for Scots pine forests in SW Europe under different climate scenarios.
The FORECAST Climate model was calibrated for the research sites during winter 2017 using field data collected during my research work and archived data from UPNA´s Ecology and Environment research team. In addition, the empirical relationships between different growing factors recorded in the forest during this fellowship and previous research done at the host institution were also included in the model. A factorial battery of simulations carried out with the FORECAST Climate ecological software to isolate the individual influence of three limiting factors on tree growth: nutrient availability (represented by N available in soil), climate (water availability and growing season length) and carbon (atmospheric CO2 concentration). The simulations were designed to switch off alternatively each limiting factors, and then running the simulations with all of them on or off simultaneously.

3. Developing the most detailed annual profiles of phosphorus, potassium, calcium and magnesium content in Scots pine wood for SW European forests.
Wood samples were collected from dominant trees at both sites in May 2017. Such samples were divided into a set for X-ray scanning during my secondment at CETEMAS and another set for traditional chemical analysis. Wood sample preparation and chemical analysis were carried out during summer 2017, generating a record of annual nutrient content in wood for the period 1997-2017. My work at CETEMAS generated the longest and most detailed annual records of P, K, Ca, and Mg content in Scots pine wood from SW European forests, covering the period 1980-2017.
I quantified for the first time the importance of different growth limiting factors on the accumulation of ecosystem C in SW European Scots pine forests. My results indicate that the CO2 fertilization effect, while positively affecting ecosystem C accumulation, it can counteract only a minor part of the growth limitation imposed by nutrients and climate. In addition, the ranking in importance of nutrients and climate is directly inverse to the present importance of such limitation. In other words, in forests which are poor in nutrient but precipitation is not a limiting factor, soil moisture could account for the biggest portion of growth limitation under climate change condition. Similarly, in forests where enough nutrients are available but not the soil moisture, trees have acclimated to water limitation and therefore they are less ready to deal with nutrient limitation. In other words, trees focus their efforts on overcoming the current limiting factors, but that strategy leaves trees exposed to other events that would make a factor usually non-limiting to become scarce in the future. Such situation is expected due to the global change generated by human activities.

In conclusion, my results highlight the need to review management plans, especially for Mediterranean mountain areas. At these sites, a well-planned thinning could alleviate part of the water stress, which in combination with the slight CO2 fertilization effect could offset part of the negative effects of climate change, particularly if an increase in nutrient availability is achieved as a consequence of increased deposition. On the other hand, in continental zones the need for change in management practices is lower, as a slight increase in the productivity of these forests can be expected as climate change will extend the growing season, therefore increasing their C sequestration capacity.