Krunoslav Sever

Recovery of trees from drought depends on belowground sink control

Climate projections predict higher precipitation variability with more frequent dry extremes1. CO2 assimilation of forests decreases during drought, either by stomatal closure2 or by direct environmental control of sink tissue activities3. Ultimately, drought effects on forests depend on the ability of forests to recover, but the mechanisms controlling ecosystem resilience are uncertain4. Here, we have investigated the effects of drought and drought release on the carbon balances in beech trees by combining CO2 flux measurements, metabolomics and 13CO2 pulse labelling. During drought, net photosynthesis (AN), soil respiration (RS) and the allocation of recent assimilates below ground were reduced. Carbohydrates accumulated in metabolically resting roots but not in leaves, indicating sink control of the tree carbon balance. After drought release, RS recovered faster than AN and CO2 fluxes exceeded those in continuously watered trees for months. This stimulation was related to greater assimilate allocation to and metabolization in the rhizosphere. These findings show that trees prioritize the investment of assimilates below ground, probably to regain root functions after drought. We propose that root restoration plays a key role in ecosystem resilience to drought, in that the increased sink activity controls the recovery of carbon balances.

In vitro pollen germination and pollen tube growth differences among Quercus robur L. clones in response to meteorological conditions

Abstract The impact of meteorological conditions on in vitro pollen germination and pollen tube growth during the initial phases of the development of male flowers in the Pedunculate Oak, Quercus robur, is studied. Phenological observations of male flowers and pollen sampling were performed on the field trial established with grafted Pedunculate Oak clones. During the investigation, weather conditions (absolute minimum and maximum daily air temperature, minimum absolute relative humidity of air and amount of precipitation) were recorded by an automatic meteorological station installed at the field trial. Influence of meteorological conditions on pollen germination and pollen tube growth was studied in the following stages of male flower: (I) during the last ten days of flower bud dormancy, (II) during swelling of the buds, (III) during bud burst and beginning of male catkins elongation, (IV) during the final stage of male flower catkins elongation. High temperatures and low relative air humidity during the bud burst and beginning of the male catkins elongation reduced pollen germination and pollen tube growth. Weather conditions did not significantly affect pollen germination and pollen tube growth during the swelling of flower buds, or in the final stage of male catkins elongation.

Forest plant community changes in the Spačva lowland area (E Croatia)

Abstract The forests in the Spačva Basin are a complex of lowland forests in the region of Slavonia (eastern Croatia). The present state of the forests is strongly influenced by intensive exploitation and hydro-ameliorative activities carried out in the past. The aim of this study was to consider the extent of changes in species composition, and the extent of environmental changes in forest communities of the Spačva Basin in relation to research conducted between 1969 and 1971. The species composition of four communities (Carpino betuli-Quercetum roboris, Genisto elatae-Quercetum roboris aceretosum tatarici, and caricetosum remotae, and Leucojo-Fraxinetum angustifoliae) is studied by comparing 41 old and 57 new relevés. Changes were estimated using ordinations (RDA, CCA, DCA) and changes in species frequency and cover. A general trend of moisture reduction was noticeable among all communities. All vegetation types are becoming floristically more similar. An increase in frequency and cover of flood intolerant woody species (such as Carpinus betulus, Cornus sanguinea, Tilia tomentosa, and Acer tataricum) as well as a decrease of vernal species related to wet habitats are particularly evident. The succession of all studied communities is resulting in loss of the mosaic community pattern characteristic of lowland alluvial forests.

Decomposition of Fine Woody Debris from Main Tree Species in Lowland Oak Forests

ABSTRACT Decomposition is an important carbon flux that must be accounted for in estimates of forest ecosystem carbon balance. Aim of this research is to provide estimate of fine woody debris decomposition rates for different tree species and sample sizes also taking into account the influence of specific microsite meteorological conditions on decomposition rates. In this paper we present results of the first two years of the experiment designed to last six years. Study was conducted in managed lowland oak forest in central Croatia. Decomposition rates (k) of fine woody debris (diameter 0.5–7 cm) for four species (Querus robur L., Carpinut betulus L., Alnus glutinosa Gaernt., Fraxinus angustifolia L.) in four size classes were estimated using litter bag method and mass loss equation of Olson (1963). Overall average k in our study was 0.182 ± 0.011 year-1. Results indicate that decomposition rate is affected by the size of the debris, with the smallest diameter branches (<1 cm) decomposing is significantly faster (k = 0.260 ± 0.018, P <0.05) than the larger one. Tree species from which debris had originated also affected decomposition, although to a lesser extent, with hornbeam samples having significantly (P <0.05) higher average decomposition rate (0.229 ± 0.028), compared to that of ash samples (0.141 ± 0.022). Proportion of variability in k explained by variables ‘species’ and ‘size class’ was assessed with general linear model (R2 = 0.644) also taking into account variables like soil temperature and soil water content. Sample size class explained 22.2%; species explained only 9.4%, while soil water content and temperature combined explained 32.8% of the variance of k. Rate constants obtained within this study might be useful in modelling ecosystem carbon balance for similar lowland forest ecosystems in Europe.

Nedestruktivna procjena koncentracije fotosintetskih pigmenata u lišću hrasta lužnjaka (Quercus robur L.)

U radu je predstavljena nedestruktivna procjena koncentracije fotosintetskih pigmenata (ukupnih klorofila i karotenoida) u lišću hrasta lužnjaka pomoću prijenosnog optičkog klorofilmetra CCM-200. Istraživanje je provedeno 2015. godine na biljnom materijalu izloženom različitim tretmanima vlažnosti tla. U kontrolnom tertmanu sadržaj vlage u tlu održavan je iznad poljskog vodnog kapaciteta, dok je u sušnom tretmanu biljkama voda bila uskraćena 112 dana (1. 4. – 21. 7.). U drugom dijelu vegetacijskog razdoblja (22. 7. – 22. 10.) u oba tretmana vlaga tla održavana je iznad poljskog vodnog kapaciteta. Na temelju lišća uzorkovanoga istovremeno u oba tretmana i to sredinom jeseni (27. 10.) izrađene su kalibracijske jednadžbe koje opisuju odnos indeksa relativnog sadržaja klorofila izmjerenoga pomoću klorofilmetra i stvarne koncentracije fotosintetskih pigmenata (utvrđene laboratorijski) pred sam kraj vegetacijskog razdoblja. Jednadžbe su konstruirane posebno za kontrolni, a posebno za sušni tretman. S obzirom na takav dizajn pokusa i vrijeme uzorkovanja, primarni cilj rada bio je ispitati utjecaj dugotrajnog sušnog razdoblja na homogenost parametara kalibracijskih jednadžbi (Y-odsječaka i nagiba pravaca). Na temelju dobivenih rezultata zaključeno je da parametri kalibracijskih jednadžbi nisu bili utjecani sušnim tretmanom. Prema tome, rezultati istraživanja ukazuju na to da klorofilmetar CCM-200, uz primjenu odgovarjućih kalibracijskih jednadžbi, predstavlja pouzdan alat za nedestruktivnu procjenu koncentracije ukupnih klorofila i karotenoida u lišću hrasta lužnjaka, bez obzira na različite režime vlažnosti tla kojima je biljni materijal bio izložen tijekom vegetacijskog razdoblja. Uz to, u radu je utvrđeno da bi preciznost procjene stvarne koncentracije fotosintetskih pigmenata bilo moguće i dodatno poboljšati korekcijom klorofilnog indeksa sa specifičnom lisnom masom.