Conifer radial stem growth in response to drought

Radial growth indices are known to be valuable long-term measures of overall tree vigor and dendroclimatological methods are frequently applied to identify the climatic factors most closely associated with variations in tree-ring parameters. Within dry inner Alpine valleys it is well established that tree growth is primarily limited by precipitation and severe drought during the growing season results in long-lasting growth reductions and increased mortality of susceptible or less competitive tree species. Because climate warming is expected to cause an increase in both the frequency and intensity of droughts and changes in seasonal precipitation pattern in the future, differences in species responses to drought under similar site and climate conditions is a basic ecological question. Within the dry inner Alpine valley of the Inn river (Tyrol, Austria) mixed stands of three major coniferous species in the Austrian Alps (Norway spruce [Picea abies], European larch [Larix decidua] and Scots pine [Pinus sylvestris]) are developed, which will provide the rare opportunity to focus on species-specific differences in drought tolerance on the same site and the degree to which these species are sensitive to long-term (inter-annual) and short-term (intra-annual) changes of climate variables. Specifically, dendroclimatological techniques will be applied to study the inter-annual relationship and long-term variability between climate and radial stem growth. Additionally, by applying ecophysiological methods the influence of tree water status and carbon supply on radial stem growth will be determined. Daily radial stem increments will be extracted from dendrometer traces and compared with cambial dynamics and xylogenesis, which will be determined by repeated cellular analyses of micro-cores. All recorded physiological parameters will be linked to meteorological variables, soil moisture dynamics and tree phenology to detect species-specific influences of climate and site conditions on intra-annual radial growth and the growing season. Hence, aim of the proposed study is to determine drought sensitivity of co-occurring native coniferous tree species, which differ in phenological and successional traits, by applying dendroclimatological, ecophysiological and histological techniques. Understanding tree growth responses to climatic stresses is an essential element of climate change research, because species-specific drought resistance will effect the development of forest ecosystems under a warmer and drier climate by changing species composition and inducing shifts in forest distribution, especially within dry inner Alpine valleys.

Climate warming is expected to cause an increase in both the frequency and intensity of drought events and changes in seasonal precipitation pattern in the future, which might differently affect tree species responses to drought. The main focus of this study therefore was to compare long-term and short-term growth-climate responses and dependence of radial stem growth on carbon gain and tree water status of major coniferous species in the Austrian Alps. Within the dry inner Alpine valley of the Inn river (Tyrol, Austria) mixed stands of (Norway spruce (Picea abies), European larch (Larix decidua) and Scots pine (Pinus sylvestris) are developed, which provided the opportunity to focus on species-specific differences in drought tolerance on the same site and the degree to which these species are sensitive to long-term and short-term changes of climate variables.Major findings of this study were that (i) lack of precipitation early during the growing season is the main climatic factor limiting radial growth, whereby growth of Scots pine mainly depended on April - May precipitation while precipitation in May - June favoured radialgrowth of Norway spruce and European larch, (ii) drought sensitivity of all three conifer species increased in recent decades, (iii) water stored in the stem is utilized for growth resumption in spring, (iv) air temperature sum rather than precipitation or soil temperature triggers start of radial growth, (v) maximum radial growth rates during the growing season peaked synchronously in all species in late spring before more favorable, i.e., moister conditions prevailed during summer, (vi) initiation of phloem formation advanced productionof xylem cells, (vii) carbon availability is not a limiting resource for radial stem growth in Pinus sylvestris , (viii) tree water status early during the growing season determines total annual above ground growth to a large extent, and (ix) different interactions of endogenous (phytohormonal regulation, carbon allocation) and environmental factors (temperature, photoperiod, drought) are involved in controlling seasonal dynamics of apical and lateral meristems in conifers exposed to drought.It may be concluded that increasing competition for scarce water and nutrients with increasing stand density explains higher sensitivity to precipitation and temperature of all species in recent decades. Furthermore, close coupling of radial tree growth to atmospheric conditions and marginal effects of moisture shortage on photosynthesis point to the importance of stem water status for intra-annual radial growth of drought-prone conifers. Considering temporal shift in onset and cessation of wood formation detected among co- occurring coniferous species, determining growth-climate responses of trees on short timescale is important to refine analyses gathered from long-term tree ring studies.