title: Xylem and phloem phenology in co-occurring conifers exposed to drought.
abstract: CONCLUSIONS
  Variability in xylem and phloem phenology among years and species is caused by contrasting temperatures prevailing at the start of the growing season and species-specific sensitivity to drought.
  The focus of this study was to determine temporal dynamics of xylem and phloem formation in co-occurring deciduous and evergreen coniferous species in a dry inner Alpine environment (750 m a.s.l., Tyrol, Austria). By repeated micro-sampling of the stem, timing of key phenological dates of xylem and phloem formation was compared among maturePinus sylvestris,Larix deciduaandPicea abiesduring two consecutive years. Xylem formation inP. sylvestrisstarted in mid and late April 2011 and 2012, respectively, and in both years about 2 week later inP. abiesandL. decidua. Phloem formation preceded xylem formation on average by 3 week inP. sylvestris, andc. 5 week inP. abiesandL. decidua. Based on modeled cell number increase, tracheid production peaked between early through late May 2011 and late May through mid-June 2012. Phloem formation culminated between late April and mid-May in 2011 and in late May 2012. Production of xylem and phloem cells continued for about 4 and 5-6 months, respectively. High variability in xylem increment among years and species is related to exogenous control by climatic factors and species-specific sensitivity to drought, respectively. On the other hand, production of phloem cells was quite homogenous and showed asymptotic decrease with respect to xylem cells indicating endogenous control. Results indicate that onset and culmination of xylem and phloem formation are controlled by early spring temperature, whereby strikingly advanced production of phloem compared to xylem cells suggests lower temperature requirement for initiation of the former.
authors: Institute of Botany, Leopold-Franzens-University of Innsbruck, Sternwartestrasse 15, 6020 Innsbruck, Austria.
authors: Irene
authors: I
authors: Swidrak
authors: Institute of Botany, Leopold-Franzens-University of Innsbruck, Sternwartestrasse 15, 6020 Innsbruck, Austria.
authors: Andreas
authors: A
authors: Gruber
authors: Institute of Botany, Leopold-Franzens-University of Innsbruck, Sternwartestrasse 15, 6020 Innsbruck, Austria.
authors: Walter
authors: W
authors: Oberhuber
keywords: Cambium; Drought; Intra-annual radial growth; Phloem formation; Xylogenesis