Seed limitation, not soil legacy effects, prevents native understorey plant species from establishing following the removal of invasive Rhododendron ponticum David Burslem Janet E. Maclean, Ruth Mitchell, David Genney, Jeanette Hall and Robin Pakeman Background Rhododendron ponticum: introduced to the UK in 1763 from Spain and/or Portugal planted widely as an ornamental plant naturalised across large areas of woodland threatening biodiversity in Atlantic Oak woods in Scotland, an EU Annex 1 priority habitat • Present in 2,238 10km2 grid cells out of a potential 3,844 across the British Isles • On SNH’s species action list • National Forest Estate to be clear from Rhododendron within 15 yrs (from 2011) What happens after Invasive Species are Removed? Pristine Oak Woodland Rhododendron Invasion Cleared Site What happens after Invasive Species are Removed? Pristine Oak Woodland Rhododendron Invasion Cleared Site What happens after Invasive Species are Removed? Pristine Oak Woodland Rhododendron Invasion Cleared Site What happens after Invasive Species are Removed? Pristine Oak Woodland Rhododendron Invasion Cleared Site ? What happens after Invasive Species are Removed? Pristine Oak Woodland Rhododendron Invasion Cleared Site Novel Community ? What happens after Invasive Species are Removed? Question 1: What happens to the native understory community as Rhododendron invades? Pristine Oak Woodland Rhododendron Invasion Cleared Site Novel Community ? What happens after Invasive Species are Removed? Question 2: What happens to the native understory community after Rhododendron has been cleared? Pristine Oak Woodland Rhododendron Invasion Cleared Site Novel Community ? Atlantic Oak Woodlands Plagiochila spinulosa Invaded woodland The impact of Rhododendron? Comparison of ground flora in woodlands with different densities of R. ponticum 56 Atlantic oak woodlands Ground flora % cover Survey Methods 20m Nine understory quadrats (1m X 1m) in a grid. Also measure pH and soil nutrients. 20m Nine birch trees. Epiphytic bryophyte quadrats (10cm X 30cm) at base and breast height. Nine oak trees. Epiphytic bryophyte quadrats (10cm X 30cm) at base and breast height. The impact of R. ponticum on ground flora Decline in species richness Decline in vegetation cover Maclean et al (2017) Journal of Applied Ecology, 55, 874-884 R. ponticum removal = Restoration? R. ponticum removal current control efforts cost c £8 million per year across the UK Aim to restore native woodland community Success of restoration rarely tested R. ponticum removal = Restoration? Chronosequence of 37 plots All plots previous had density R. ponticum Cleared between 1984- 2014 Compared to uninvaded sites Ground flora % cover R. ponticum removal ≠ Restoration Compared to uninvaded sites cleared sites have: • Lower cover of forbs • Lower cover of grass • Similar cover of bryophytes Even after 30 years sites have not recovered Maclean et al (2017) Journal of Applied Ecology, 55, 874-884 Ordination to Reveal Changes in Community Composition • Partial RDA coding time since removal as levels of a factor Uninvaded Dense • Classified plot control Rhodo diagram – shapes (time 0) delineate values belonging to each 10 5 time period. 20 15 • Clear pattern through time but 30 definitely NOT moving towards pristine sites. Removing R. ponticum ≠Restoration Why? R. ponticum ‘poisons’ the soil Lack of seed availability Bryophyte layer inhibits germination? Experimental site Arran (island off the west coast of Scotland) Oak & birch woodland Cleared of R. ponticum Experiment established September 2013 Results September 2015 Experiment 1: Chemical legacy effects in the soil prevent the establishment of native forbs and grasses? All plots seeded with native forbs & grasses Fertilizer Activated carbon All combinations Ground vegetation removed 8 treatments 10 replicates Chemical legacy effects in the soil ? VR = Vegetation removal F = Fertilizer addition C = addition of activated carbon N = No Significant effect of vegetation removal No effect of fertilizer or activated carbon Experiment 2: seed limitation and vegetation removal? Seed addition All Ground vegetation removed combinations 4 treatments Species selected: 10 replicates • Agrostis capillaris • Deschampsia flexuosa • Anthoxanthum odoratum • Hyacinthoides non-scripta • Potentilla erecta Seed limitation and vegetation removal No seed Seed added No vegetation removal No vegetation Seed added Vegetation removed Vegetation removed Vegetation Species not sown Species sown Conclusions and implications for practise The removal of invasive species ≠ restoration Additional management is required Addition of native seed and creation of a suitable germination sites is essential Rhododendron ponticum does not leave chemical legacy effects ‘poisoning’ the soil Addition of activated carbon to remove these possible legacy effects not required Thanks to: Scottish Natural Heritage for funding this work The National Trust for Scotland providing the field site 2011-2016 and 2016-2021 Strategic Research Programmes of the Scottish Government. Further information David Burslem: [email protected] Ruth Mitchell: [email protected] Maclean et al. (in press) Seed limitation, not soil legacy effects, prevents native understory from establishing in oak woodlands in Scotland after removal of Rhododendron ponticum. Restoration Ecology, in press. doi: 10.1111/rec.12664 Maclean et al (2018) Invasion by Rhododendron ponticum depletes the native seed bank with long-term impacts after its removal. Biological Invasions, 20, 375-384. Maclean et al (2018) Understorey plant community composition reflects its invasion history decades after invasive Rhododendron ponticum has been removed. Journal of Applied Ecology, 55, 874-884 Maclean et al (2017) The epiphytic bryophyte community of Atlantic oak woodlands shows clear signs of recovery following the removal of invasive Rhododendron ponticum. Biological Conservation, 212, 96-104 .