Abstract :
[en] Warmer and drier conditions are expected globally and regionally in the future due to climate change. In Belgium, a 50% decrease in summer precipitations and an increase in winter precipitations are expected. The drier conditions have the potential to influence forest ecosystem functioning, both aboveground and belowground. Further, more drought events could result in more frequent soil drying-rewetting events, which could influence the CO2 emissions (“Birch effect”). Mature forests are already experiencing adverse effects of climate change on tree growth, tree mortality and species composition. Studies suggested that ecosystems with higher tree species diversity could be more resistant and resilient to stress conditions, such as drought. Therefore, mixed planting may be an option to mitigate drought effects to forest ecosystems. The overall objective of this thesis was to assess the combined effects of tree species admixture and drought on different above- and belowground processes in a young forest plantation.
In this study, combined effects of mixed planting and precipitation reduction (hereafter “drought”) on different above- and belowground processes around central oak (Quercus robur. L.) and beech (Fagus sylvatica L.) trees were investigated in 6-7 years old plantations (FORBIO). Oak and beech were planted in monocultures or admixed with 1-3 other functionally dissimilar tree species (tree species admixture gradient from 1 to 4). Three drought and three control plots (3 m × 3 m) around oak and beech trees were established at every admixture levels. Rainout shelters were installed in drought plots to intercept about 50% of the precipitation. Different aboveground variables such as plant growth, leaf traits, crown damage and belowground variables such as soil organic matter decomposition, nutrient availability, microbial biomass, C and N mineralization were measured over two growing seasons. Further, tree species admixing and drought legacy effects on soil microbial biomass and activity were measured under oak.
Tree species admixing influenced different variables but the effects were not consistent between oak and beech. Tree species admixing reduced the height and increased the specific leaf area of beech, but did not influence any of the belowground variables measured. For oak, reduced crown damage from pests and pathogens, decreased leaf Al, Na and Zn contents, and increased soil N and total exchangeable cations were observed with higher tree species admixture level. Further, tree species admixing to oak influenced soil microbial community composition, and drought legacy reduced soil microbial biomass, soil respiration and bacterial growth but not fungal growth rates. Upon soil drying and rewetting, tree species admixing to oak did not influence bacterial growth and soil respiration rates but drought legacy reduced both bacterial growth and soil respiration.
There were significant interactions between tree species admixing and drought for several variables such as organic matter decomposition, leaf Cu and N contents, net nitrification, potential respiration, MBC, metabolic potential of bacteria, qmic fungal to bacterial growth ratio etc. These interactions were mainly driven by the drought effects on monocultures and two species mixtures, indicating that tree species admixtures had offset the drought effects in three and four tree species mixture.
Results from this study suggest that both tree species admixing and drought have the potential to influence ecosystem processes in young forests. Results also indicate that tree species admixing could offset the effects of drought on different ecosystem processes, thus stabilize ecosystem process under drought. Further, drought and drought legacy may reduce soil microbial activity, hence drought followed by precipitation (drying-rewetting) would not increase the CO2 emissions from soils. Overall, both oak and beech could benefit from admixing of functionally dissimilar tree species. Therefore, planting oak and beech in mixtures could be an option for forest management in the face of climate change.
