title: Comparing growth phenology of co-occurring deciduous and evergreen conifers exposed to drought.
abstract: Plant phenological events are influenced by climate factors such as temperature and rainfall. To evaluate phenological responses to water availability in a Spring Heath-Pine wood (Erico-Pinetum typicum), the focus of this study was to determine intra-annual dynamics of apical and lateral growth of co-occurring early successionalLarix deciduaandPinus sylvestrisand late successionalPicea abiesexposed to drought. The effect of reduced plant water availability on growth phenology was investigated by conducting a rainfall exclusion experiment. Timing of key phenological dates (onset, maximum rate, end, duration) of growth processes were compared among species at the rain-sheltered and control plot during 2011 and 2012. Shoot and needle elongation were monitored on lateral branches in the canopy atc. 16 m height and radial growth was recorded by automatic dendrometers atc. 1.3 m height of > 120 yr old trees. Different sequences in aboveground growth phenology were detected among the three species under the same growing conditions. While onset of radial growth in April through early May was considerably preceded by onset of needle growth inLarix decidua(5 - 6 weeks) and shoot growth inPinus sylvestris(c. 3 weeks), it occurred quite simultaneously with onset of shoot growth inPicea abies. Low water availability had a minor impact on onset of aboveground growth, which is related to utilization of stored water, but caused premature cessation of aboveground growth. At the control plot mean growing season length was 130 days inPinus sylvestris, 95 days inLarix deciduaand 73 days inPicea abiessupporting the hypothesis that early successional species are resource expenders, while late successional species are more efficient in utilizing resources and develop safer life strategies. High synchronicity found in culmination of radial growth in late spring (mid-May through early June) prior to occurrence of more favourable environmental conditions in summer might indicate sink competition for carbohydrates to belowground organs. This is supported by completion of apical growth in mid June in all species, except for needle growth ofPinus sylvestris, which lasted until early August. Phenological observations of conifers exposed to drought revealed that tree water status early during the growing season determines total annual aboveground growth and besides temperature, species-specific endogenous and/or environmental factors (most likely photoperiod and/or different threshold temperatures) are involved in controlling apical and lateral growth resumption after winter dormancy.
authors.affiliation: Institute of Botany, Leopold-Franzens-University of Innsbruck, Sternwartestrasse 15, A-6020 Innsbruck, Austria.
authors.forename: Irene
authors.initials: I
authors.lastname: Swidrak
authors.forename: Roman
authors.initials: R
authors.lastname: Schuster
authors.forename: Walter
authors.initials: W
authors.lastname: Oberhuber
keywords: aboveground growth; drought; intra-annual growth; mixed conifer forest; phenology; tree growth