Juniperus communis: victim of the combined action of climate warming and nitrogen deposition?

Abstract

Research on the combined effects of climate change and nitrogen deposition on reproductive traits, and especially on the production of viable seeds, is still scarce despite their importance for population persistence and expansion. Hence, in this study we set out to investigate the direct and indirect effects of the above-mentioned global change drivers on seed viability in the coniferous shrub Juniperus communis L. In many parts of its European range, juniper is increasingly threatened, partly because of a lack of sexual reproduction. We hypothesised that this regeneration failure is partly due to poor seed viability. Using data from 39 populations throughout Europe, we were able to demonstrate that a strong, triangular-shaped relationship exists between the percentage of viable seeds produced and the percentage of juniper seedlings occurring in a population, which indicates that the species is indeed partly seed limited. Furthermore, based on an extended dataset of 42 populations, we found that seed viability was negatively affected by temperature, measured as mean annual growing degree-days, and nitrogen deposition (but not by drought). Suggestions are made about the processes behind the observed patterns, but more research is required. Nevertheless, our results do raise serious concerns for the conservation of juniper in light of the predicted rise in temperature and global nitrogen emissions. Furthermore, it is likely that similar patterns can also be observed for other species. Unravelling all these interacting effects of GC requires either well-designed, but inevitably complex, experiments or large-scale observational studies covering broad environmental gradients (cf. Sagarin et al. 2006). In this study, we adopt the latter approach to investigate the direct and indirect effects of GC drivers on seed viability in the coniferous shrub Juniperus communis L. A good understanding of the factors explaining the variation in seed viability in juniper is of particular interest, as poor seed viability may be linked with strong population declines that are actually occurring in large parts of its range, including northwest European lowlands and Mediterranean mountain regions. In Flanders (i.e. the northern part of Belgium), the number and sizes of juniper populations have declined dramatically in the past two decades, and at present only three populations remain that contain more than 100 individuals (Adriaenssens S., Baeten L., Crabbe S., Verheyen K., Ghent University, Ghent, unpublished results). Also, in surrounding regions, including Wallonia (southern Belgium; The main reasons for the juniper decline probably include habitat destruction, habitat degradation and very limited recruitment in remaining populations (e.g. Verheyen et al. 2005). Due to its protected status, the destruction of juniper habitat is at present less common, and for the same reason, actions are being undertaken to prevent further habitat degradation in extant populations. Degradation of juniper habitat is mostly related to progress...

Males and females of Juniperus communis L. and Taxus baccata L. show different seasonal patterns of nitrogen and carbon content in needles

“…These species also occur in different habitats; juniper is a pioneer species, occurring in dry and nutrient poor habitat while yew persists into late-successional stages where it grows in fertile habitats. Yew is an endangered species and certain woodland communities containing T. baccata have received priority habitat status EU Habitat Directive 92/43/EEC (Thomas and Polwart 2003;Thomas and Garcia-Marti 2015) although, as recently shown, juniper populations are also reducing (Thomas et al 2007;Verheyen et al 2009). …”

Genetic diversity of Lithuanian populations of Juniperus communis L. in relation to abiotic and biotic factors

“…Recent ongoing severe climate fluctuations create new unfavorable survival scenarios for weaker individuals of the species (Verheyen et al, 2009). Comparison of 42 populations of J. communis across Western Europe showed negative correlation between vitality of the seeds and mean temperature of vegetation season.…”

“…Occurring in four continents with its European range spanning from Ireland to Russia and from Spain up to Scandinavia (Thomas et al, 2007), J. communis is threatened in several regions in Europe (Verheyen et al, 2009;Gruwez et al, 2013). It is one out of three conifer species naturally growing in Eastern Baltic States and is important from both ecological and economical points of view (Marozas et al, 2007;Vaitkevičiūtė et al, 2011b;Labokas & Loziene, 2013).…”

PAS-gebiedsanalyse in het kader van herstelmaatregelen voor BE2200043 Bosbeekvallei en aangrenzende bos- en heidegebieden te As-Opglabbeek-Maaseik

“…Niet van toepassing (Verheyen et al 2009, Gruwez et al 2014, Gruwez et al 2017. Geschikte beheeroplossingen liggen echter niet voor de hand: bekalkingsexperimenten om de verzuring tegen te gaan leiden tot een verhoogde kieming (Lucassen et al 2011, Verheyen et al 2009), maar heeft echter als keerzijde dat de nutriëntenbeschikbaarheid op lange vermindert, waardoor de kans dat een zaailing kan uitgroeien tot een adulte boom net lijkt te verkleinen door het bekalken.…”

“…De bevindingen over de herstelmaatregelen zijn soms tegenstrijdig wat betreft bekalken, opslag verwijderen, branden, begrazen en plaggen (zie bv. Haveman 2005, Hommel et al 2013, Gruwez et al 2017, Jeneverbesgilde 2017, Lucassen et al 2011, Smits et al 2012, Verheyen et al 2009 …”