For the first blog of the year I wanted to talk about some new management we are having to undertake. The observant visitor will have noticed some quite different management on site to what has gone before, particularly if they’ve had a peek out of Paddy’s hide in recent weeks. We have had a contractor on site rotovating large areas of the Central Wet Grassland viewable from the hide. More usually at this time of year, we would be using our sluices and ditches to direct and collect water to create seasonal flood conditions in this area to benefit our winter ducks. This year we have taken out some of those sluice boards and are allowing the rain (when we get any) to run off this area as best we can; we are keeping it dry intentionally.
But why the change? Won’t it be bad for the birds? Unfortunately the site has recently been colonised by Crassula helmsii, or New -Zealand Pygmy weed. Also known as Australian swamp stonecrop, this plant is a non-native invasive species that was first reported as naturalised in the UK, in a pond in Essex in 1956. I have read that as far back as 1927 it was being sold as one of the ‘Six Best Oxygenating Aquatics for the outdoor pool’ by Perry’s Hardy Plant Farm in Enfield, Middlesex.
Crassula helmsii is a non-native invasive plant introduced in to the UK in the first half of the 20th Century
Crassula, as it is widely known (there are other species of Crassula), grows in a variety of freshwater and riparian habitats. It can grow in free-floating, submerged, emergent and even terrestrial forms. It does produce flowers though is not thought to have any pollinators in the UK and reproduction is asexual, through the fragmentation of the plant. As fragments break off from the plant, they can disperse by floating, and spread around the water body or possibly be taken further afield by wildfowl and other birds using the wetland.
Crassula at Saltholme is thriving in the draw-down zone where waders need mud in order to feed
On the wet grassland at Saltholme, with a regime of flooding and drawing down water levels through the spring and summer, it is doing best in the draw-down zone and is growing where there is seasonally shallow water and muddy edges. Splashy areas and muddy edge are exactly what we are trying to maximise as we look to provide optimal conditions for waders to feed - whether breeding birds, migrants looking to re-fuel part way through a long and energetically demanding journey or birds that are overwintering. Given time, Crassula will creep into all these feeding areas and smother them, making the mud inaccessible as it builds up a thick mat of succulent leaves and stems.
We first noticed the presence of Crassula a couple of years ago and have been monitoring it since. There was uncertainty about how well it would do with the mildly brackish conditions at Saltholme, but it has spread and now some quite large areas are carpeted.
Crassula has quickly become well established in certain areas at Saltholme (photo credit: Gavin Thomas, RSPB Ecology)
There are not many practical options available to us when it comes to management, but it is clear we need to do something before the mats of Crassula become too thick and make the mud dwelling invertebrates inaccessible. In the past, some sites have used a hot foam to knock the Crassula back, but this has not been massively effective. Being a wetland plant, it does less well in dry conditions and so we have decided to dry out a significant section of the wet grassland at Saltholme to weaken the plant. We will aim to maintain dry conditions over the worst affected areas into Spring and through Summer and Autumn in order to have maximum effect. This clearly deviates from our usual management, and we have had to agree the changes with Natural England as the site is all within the Teesmouth and Cleveland Coast SSSI. Usually at this time of year the area would be filled with water for winter ducks, waders and gulls. The usual elevated water levels in winter mean we start Spring with high water levels enabling prolonged periods with lots of splash and lots of mud through Spring for the year's breeding waders. This year will be quite different but our hope is that by drying the area out, much of the plant will die. The plant is broken up by the rotovating, and is turned over from the root up (it is quite shallow rooted) which should cause the plant to dry out in the wind and in the sunnier periods. As we head towards Spring in a couple of months’ time, the more fine weather we have, the greater the impact should be on the Crassula. It may be interesting to see how Lapwing respond as they will often take advantage of a cultivated area of bare ground for nesting. If we can maintain dry conditions through summer, hopefully it should be severely knocked back. We won’t be rid of it however, and unfortunately, we will likely be locked into a cycle of repetitive management where we keep knocking it back, but it keeps returning.
An aerial view part way through the rotovation works (photo credit: Gavin Thomas)
There should come an additional benefit from rotovating the areaas the soil will be aerated and organic matter will be worked in to the soil. Both these things should improve conditions for the early stages of various invertebrate species, and lead to their increased abundance which in turn should lead to even better foraging conditions for birds the following year.
With any luck lapwing will make use of rotovated areas for nesting
References:
Smith and Buckley (Feb 2020) 'Biological Flora of the British Isles: Crassula helmsii' Journal of Ecology
Kelly, J., and Maguire, C.M., (2009) 'New Zealand Pigmyweed (Crassula helmsii) Invasive Species Action Plan' Prepared for NIEA and NPWS as part of Invasive Species Ireland