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Phosphorus nutrition in trees is deteriorating

Deterioration of phosphorus nutrition in trees: a signal confirmed at the European scale

Mathieu Jonard (Catholic University of Louvain) takes the floor

Summary

In Europe, forest productivity has increased over the last few decades due to the combined effect of increasing CO2 concentrations in the atmosphere and atmospheric nitrogen deposits. Nitrogen deposits have remained high in different parts of Europe and with atmospheric CO2 concentrations continuing to increase, productivity could increase further, unless it is slowed by the limited availability of other resources, most notably nutrients.

The objectives of the present study were to describe leaf nutritional status for the main forest species in Europe (beech, sessile and pedunculate oak, spruce, Scots pine, silver fir), to identify growth-limiting nutrient levels for each of these species, and to detect changes in foliar nutrition over time.

The study used foliar analysis data collected from 1992 to 2009 on plots in the level II forest monitoring network of the IPC Forests programme. This unique data set covers two decades of data for the whole of Europe, collected according to standardised methods respected by the different countries involved.

The nitrogen nutrient levels for broadleaves were good (sub-optimal to optimal), but around half of the coniferous plots were below deficit levels. For phosphorus, on the contrary, a deficiency was found for a considerable portion of the plots regardless of species (from 22 to 74%). Furthermore, all species showed deficiencies in base cations (calcium, magnesium, potassium) though the proportion of plots concerned was lower (from 5 to 40%).

Analysing trends over time revealed a significant increase in foliar mass for beech (leaves) and Norway spruce (needles). Concerning foliar nutrient concentrations, a decreasing trend was detected in almost all cases (20 out of 22).

Among the trends of highest concern, most species showed a clear degradation in phosphorus nutrition; this confirms previously reported evidence on plots in France, Wallonia and Luxemburg (Jonard et al., 2009). Nitrogen, sulphur and potassium concentrations are also significantly declining in certain species. Concerning calcium and magnesium, trends declined for broadleaves and increased for conifers (cf. table).

Temporal evolution of foliar nutrients of the main European forest species. Upward trend '+' or downward '-', significance: P <0.1: (+) or (-), P <0.05: + or -, P <0.01: ++ or -, P <0.001: +++ or ---.
Temporal evolution of foliar nutrients of the main European forest species. Upward trend '+' or downward '-', significance: P <0.1: (+) or (-), P <0.05: + or -, P <0.01: ++ or -, P <0.001: +++ or ---. © Mathieu Jonard / Université catholique de Louvain

Several explanations for these trends are possible. Declining concentrations could reflect a dilution effect due to increasing foliar mass.

Indeed, the increased productivity of European forests (fertilisation effect of increasing atmospheric CO2 concentrations and nitrogen deposits) implies stronger demands for nutrients, which the soil may not be able to supply. The decrease in atmospheric sulphur deposits and, to a lesser extent, in nitrogen deposits certainly contributes somewhat to the decrease in the foliar concentrations of these elements. Since sulphates and phosphates are absorbed by the same type of exchangers in the soil, the decrease in sulphate concentrations in the soil solution (a consequence of the reduction in sulphur deposits) could cause phosphates to remain more strongly bonded to the solid phase of the soil, thus decreasing their bioavailability.

Stand age, though possibly also involved, cannot explain such a degree of decline.

This study shows that tree mineral nutrition is deteriorating in Europe and that the response of our forest ecosystems to global changes may become more and more constrained by nutrient availability. It is therefore crucial to consider these nutritional constraints in global carbon balance modals in order to avoid overestimating our forests' capacity to capture and store carbon.

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