Ghent University. Faculty of Bioscience Engineering
Abstract
With a distribution range that covers most of the northern hemisphere, common juniper (Juniperus communis L.) has one of the largest distribution ranges of all plant species on earth. The last decades, however, multiple studies are reporting a significantly declining size and plummeting number of common juniper populations in many regions, including in the western European lowlands and the Mediterranean mountain regions. Failing recruitment is one of the main reasons for this decline. In many of the remaining European juniper populations, the percentage of viable seeds is extremely low, thereby limiting the chances for successful regeneration. However, also other reasons, such as the absence of microclimate suitable for germination and seedling establishment, can be responsible for the lack of recruitment. It was suggested that climate warming as well as enhanced atmospheric depositions can negatively affect juniper’s seed viability. However, the mechanisms behind these effects remain unclear. The aim of this thesis was to further our understanding of the effects of climate change (changing temperature and precipitation) and air pollution (atmospheric depositions of nitrogen and sulphur) on different phases of the sexual reproductive phase of common juniper.
We focussed on three crucial seed phases: (seed phase 1, SP1) the early gametophyte development and pollination; (seed phase 2, SP2) further development of the gametophytes, fertilization and early embryo development; (seed phase 3, SP3) late embryo development. The seed viability of populations with and without recruitment was compared. Only after SP3, we found significant differences in seed viability between populations with and populations without recruitment. Hence, this made us conclude that late embryo development is the most crucial phase during seed development. However, anomalies during SP2 can have a delayed effect on embryo development. Therefore, both seed phases were retained for further research.
With both observational and experimental research we studied the effects of global environmental changes on seed viability. Overall, we found viability of SP2- and SP3-seedsto decrease with temperature. Enhanced atmospheric depositions were only negatively correlated with SP3-seed viability.
There are different pathways through which these global-change drivers possibly elaborate their negative effects. For example, increasing temperatures can negatively affect the development and growth of the female gamethophyte and the pollen tube, the male-female synchrony in the pre-fertilization phases, the development of the megagametophyte and the nutrition and growth of the embryo. On the other hand, both atmospheric depositions and warming can cause nutrient imbalances.
Although the ripening of the embryo (SP3) was identified as the most vulnerable phase, we found strong indications that the actual reasons for low seed viability should be searched in the preceding processes. For example, our transplant experiment along a latitudinal gradient clearly showed that significant differences in seed viability between the temperature treatments only occurred if the seeds also experienced higher temperatures during SP2. Hence, it is plausible that anomalies, induced by warming or enhanced atmospheric deposition, occur during the development of the female gametophyte (SP2). However, their effects only manifested themselves in SP3, for example, by leading to a malfunctioning megagametophyte that will not be able to nourish the developing embryo.
In addition, with a germination experiment, we assessed ideal microclimatic soil conditions for germination and seedling establishment. We found that it is highly important that juniper seeds are in contact with mineral soil, with a sufficient amount of loam or organic matter, for successful germination. For example, sod cutting can create these conditions.
To conclude, our findings highlight that common juniper, arguably one of the most iconic threatened tree species of Europe, is becoming a victim of global change. Successful recruitment in common juniper will only occur if the following conditions are met: enough viable seed must be available (e.g. during a colder period of several years) and enough microsites suitable for establishment must be present. The probability that microsites exist where these conditions (viable seeds and chances for establishment) occur at the same time increases in larger areas where the management is already suited for common juniper. Concerning the low seed viability, attention should be focused on locations where the microclimate is expected to be colder (e.g. northerly oriented slopes or the vicinity of small rivers) and where atmospheric depositions are lower (e.g. not too close to busy roads and/or farms).
This work is also an extra call for policy makers to increase their efforts in reducing the emissions of greenhouse gases and reactive nitrogen
