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iForest -Biogeosciences and Forestry
IntroductionAt scales ranging from organ to ecosystem, many processes, particularly those related to carbon cycling (productivity and growth), water (evapotranspiration and runoff), and nutrients (decomposition and mineralization), are directly mediated by phenology (Gu et al. 2003). Spring bud burst provides a particularly good example, as it involves various interactions between trees and the environment. The initiation of this phenological phase depends on early spring air and soil temperatures (Menzel 2000, Lupi et al. 2012 as well as other environmental attributes such as photoperiod (Wareing 1953, Basler & Körner 2012, precipitation and tree health. In turn, the seasonal cycle of leaf development determines many crucial aspects of forest canopy function, including photosynthetic uptake, radiation extinction, rainfall interception, and evapotranspiration (Bonan 1993, Saigusa et al. 2002. Foliage development, assessed at the stand level in terms of the leaf area index (LAI), typically increases rapidly in western European deciduous forests, from early May to mid-July, with the maximum around midJuly (Mussche et al. 2001). LAI affects the transmittance of photosynthetically active radiation (PAR) to the understory. Additionally, leaf development yields increases in transpiration (Střelcová et al. 2006).Tree phenology is dependent on the plant's storage and consumption of carbohydrates (Michelot et al. 2012). These are stored in all organs (i.e., leaves, branches, roots, and stems) in different concentrations, which can vary during the growing season based on the relative rates of production and use (Scartazza et al. 2013). New photoassimilates transported in phloem sap may be used for radial growth, with leaf phenology (i.e., budburst, leaf development and yellowing) thus influencing intra-annual wood formation (Čufar et al. 2008, Sass-Klaassen et al. 2011. The time lags between leaf and xylem growth phenologies vary among species, leading to differences in relative carbon allocation. In deciduous species, budburst generally occurs following radial growth for ring-porous species, whereas for diffuse-porous species, cambial division and consequent radial growth are initiated at almost the same time as budburst (Suzuki et al. 1996, Schmitt et al. 2000, Čufar et al. 2008. Nevertheless, even in diffuse-porous species, onset of cambial activity is sustained to a large degree by carbon accumulated in storage pools (Dietze et al. 2014).Given sufficient soil water and based on atmospheric evaporative demand, maximal transpiration rates occur when trees have full foliage (Meier & Leuschner 2008). However, numerous laboratory experiments on herbaceous plants and tree seedlings or saplings have shown that leaf coloring and leaf area reduction (i.e., events marking onset of particular phenological stages) are common responses to soil water shortage (Löf & Welander 2000). Loss of leaves results in a decrease of the transpiring surface area, avoiding a severe reduction of cell water p...
