Laure Barthes

This study extends the Hi-sAFe agroforestry model by incorporating the effects of elevated atmospheric CO₂ on tree growth. Hi-sAFe is a process-based biophysical model that represents tree-crop interactions, but until now lacked a mechanism to simulate CO₂-driven physiological responses of trees. We introduced CO₂ sensitivity into the Light Use Efficiency (LUE) module and simulated the growth of Juglans nigra (black walnut) under current and future climate conditions (550 ppm CO₂, + 3 °C, - 10% precipitation). Elevated CO₂ increased tree height, diameter and root development in both forestry and agroforestry systems, but responses were stronger and more persistent in forestry. In agroforestry, CO₂ effects were more variable over time and were strongly modulated by competition with crops, while belowground responses indicated greater root system plasticity. Climate change reduced tree growth in both systems, but CO₂ partially offset these effects, particularly in forestry. These results demonstrated the importance of explicitly representing CO₂ fertilisation in agroforestry models to realistically capture vegetation-climate interactions and assess the resilience of tree-crop systems under global change.

A 2-years assessment of nitrogen (N) stocks, tree N safety nets (from a ¹⁵N labeling experiment), and biological N fixation was conducted in a Mediterranean agroforestry system in southern France. The study aimed to quantify N retention in agroforestry. The study area is characterized by a skeletic rhodic luvisol soil, a mean annual temperature of 15.5 °C, and an average annual precipitation of 556 mm. N and ¹⁵N were quantified across all system components, which are N-fixing black locust trees, crops, weed plants, understory vegetation strips (UVS), rhizospheric soil, and soil microbial biomass. A split-plot experimental design was used, with agroforestry (AF) and monoculture (MC) plots. ¹⁵N labeling was applied to track N fluxes and determine tree nutrient absorption over time. We hypothesized that tree root growth and nutrient leaching would enhance the safety-net effect, improving N retention. Results showed that crops in AF had lower N stocks (13–30% less than MC), but tree and UVS contributions compensated for the deficit, leading to a 62% increase in total plot-level N stock in 2022. The Relative Nitrogen Content was 0.97 in 2021 and 1.63 in 2022. While no ¹⁵N was detected in trees in the first year (70 days after labeling), 2% of applied ¹⁵N was recovered in trees 14 months post-labeling. % of N derived from atmospheric N₂ (%Ndfa) ranged from 52 to 68%, with trees fixing 14–18 kg N ha⁻¹. We confirmed that agroforestry enhances N retention, but further research is needed to quantify leaching and gaseous losses of nitrogen.

The origin of N which contributes to the synthesis of N reserves of in situ forest trees in autumn and to the growth of new organs the following spring is currently poorly documented. To characterize the metabolism of various possible N sources (plant N and soil N), six distinct 20-year-old sessile oaks were ¹⁵N labelled by spraying ¹⁵NH₄¹⁵NO₃: (i) on leaves in May, to label the N pool remobilized in the autumn for synthesis of reserves, (ii) on soil in the autumn, to label the N pool taken up from soil and (iii) on soil at the beginning of the following spring, to label the N pool taken up from soil in the spring. The partitioning of ¹⁵N in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of ¹⁵N into the soil–tree system; more than 30 % of the administered ¹⁵N was recovered. Analysis of the partitioning clearly revealed that in autumn, roots' N reserves were formed from foliage ¹⁵N (73 %) and to a lesser extent from soil ¹⁵N (27 %). The following spring, ¹⁵N used for the synthesis of new leaves came first from ¹⁵N stored during the previous autumn, mainly from ¹⁵N reserves formed from foliage (95 %). Thereafter, when leaves were fully expanded, ¹⁵N uptake from the soil during the previous autumn and before budburst contributed to the formation of new leaves (60 %).

The origin of the N which contributes to the synthesis of N reserves of in situ forest trees in autumn, and to the growth of new organs the following spring, is currently poorly documented. To characterize the metabolism of various possible N sources (plant N and soil N), six distinct 20 year-old sessile oaks were ¹⁵N labelled by spraying ¹⁵NH₄¹⁵NO₃: (i) on leaves in May, to label the N pool remobilized in the autumn for synthesis of reserves; (ii) on soil in the autumn, to label the N pool taken up from soil; (iii) on soil at the beginning of the following spring, to label the N pool taken up from soil in the spring. The partitioning of ¹⁵N in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of ¹⁵N in the soil-tree system; more than 30 % of the administered ¹⁵N was recovered. Analysis of the partitioning clearly revealed that in autumn, roots’ N reserves were formed from foliage ¹⁵N (73 %) and to a lesser extent from soil ¹⁵N (27 %). The following spring, ¹⁵N used for the synthesis of new leaves came first from ¹⁵N stored during the previous autumn, mainly from ¹⁵N reserves formed from foliage (95 %). Thereafter, when leaves were fully expanded, ¹⁵N uptake from soil during the previous autumn and before budburst contributed to the formation of new leaves (60 %).