Mitja Ferlan

Chronological Sequence of Leaf Phenology, Xylem and Phloem Formation and Sap Flow of Quercus pubescens from Abandoned Karst Grasslands

Intra-annual variations in leaf development, radial growth, including the phloem part, and sap flow have rarely been studied in deciduous trees from drought-prone environments. In order to understand better the chronological order and temporal course of these processes, we monitored leaf phenology, xylem and phloem formation and sap flow in Quercus pubescens from abandoned karst grasslands in Slovenia during the growing season of 2014. We found that the initial earlywood vessel formation started before bud opening at the beginning of April. Buds started to open in the second half of April and full leaf unfolding occurred by the end of May. LAI values increased correspondingly with leaf development. About 28% of xylem and 22% of phloem annual increment were formed by the time of bud break. Initial earlywood vessels were fully lignified and ready for water transport, indicating that they are essential to provide hydraulic conductivity for axial water flow during leaf development. Sap flow became active and increasing contemporarily with leaf development and LAI values. Similar early spring patterns of xylem sap flow and LAI denoted that water transport in oaks broadly followed canopy leaf area development. In the initial 3 weeks of radial growth, phloem growth preceded that of xylem, indicating its priority over xylem at the beginning of the growing season. This may be related to the fact that after bud break, the developing foliage is a very large sink for carbohydrates but, at the same time, represents a small transpirational area. Whether the interdependence of the chronological sequence of the studied processes is fixed in Q. pubescens needs to be confirmed with more data and several years of analyses, although the ‘correct sequence’ of processes is essential for synchronized plant performance and response to environmental stress.

Effects of fire on carbon fluxes of a calcareous grassland

Frequent fires on drought-prone grasslands, such as pastures in the sub-Mediterranean region, can induce large post-fire variations of CO2 fluxes between the ecosystem and the atmosphere. Consequently, substantial changes in the C-cycle can be expected. In our research, we studied post-fire carbon fluxes (net canopy CO2 exchange (NE) and canopy respiration) on calcareous karst grassland after two spring fire events, a natural one in 2010 and a man-induced (anthropogenic) one in 2011. Canopy chamber measurements performed at short regular time intervals throughout the season revealed the rapid recovery of NE after the initial loss of C-sink strength. The long-term effects of the natural fire were largely masked by an early-season drought. In contrast, the burned areas did not reach the productivity of non-disturbed sites until the end of the season after the anthropogenic fire in 2011, when the post-burning period was characterised by favourable growing conditions. The similar NE values could be explained by a significant reduction of respiration at burned areas. Our research showed that C-sink strength of grassland after a fire disturbance can be re-established quite rapidly, after a month, although a full recovery (regeneration of plant cover, C fluxes) can take more time. The re-establishment is largely dependent on environmental conditions (soil water availability).

Intra-annual leaf phenology, radial growth and structure of xylem and phloem in different tree parts of Quercus pubescens

Basic knowledge of the intra-annual timings of leaf development and radial growth (including the phloem part) in different tree parts is generally missing although such rudimentary data are crucial to link the structure and function of vascular tissues at the whole tree level. To understand better the time course of leaf development and radial growth patterns in different tree parts, we studied leaf phenology and intra-annual xylem and phloem formation and structure in the stem and at two locations in branches of sub-Mediterranean Quercus pubescens in 2015. Onset and end dates of cambial cell production were synchronized at the two locations in branches, but were different at the stem base. The period of cell production was thus a month longer in the stem, resulting in 82.8 and 45.1% wider xylem and phloem increments, respectively. In addition, the xylem ring was wider than the phloem ring in all three parts. Thus, phloem ring widths in stem represented 24.8% and in branches 79.4% of the xylem ring width. Earlywood occupied 52.9% (stem) and 74.9% (branches) of the xylem ring, and early phloem 53.7% (stem) and 43.3% (branches) of the phloem ring. Most of the annual radial increment (i.e. xylem and phloem increments) in stem and branches was formed prior to full leaf development. Latewood and late phloem were formed in the period of full leaf unfolding. Our study confirmed that the temporal sequence of leaf development and radial growth are not contemporary in Q. pubescens. Different intra-annual patterns of radial growth in different tree parts result in different structures of xylem and phloem, which is in line with different roles of stem and branches in terms of tree functioning.