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Tree mallow control on Craigleith 2014 An evaluating of management effects on the vegetation

This report summarises the main work and findings of the ecological monitoring conducted for

SNH in 2014 put in context of prior monitoring to facilitate and evaluate the success of tree

mallow management on the island.

1. Current monitoring set-up

Monitoring takes place largely as agreed in the Craigleith Management Plan, focussing on the

implications of cutting tree mallow in two large areas initiated in September 2006. One area is

the low lying ‘Glen’ on the south side of the island which has been covered in tall growing tree

mallow for a long time, leaving little space for either native plants or puffins. The second area

(‘Slopes’) is higher up on the north-eastern side of the island and has a shorter history of tree

mallow occurrence with some puffin breeding habitat that continued to be available. In both

areas, 8 monitoring plots of 5m × 5m each were laid out directly after the first cut in autumn

2006 to follow the response of the vegetation over time. The results below are largely based on

measurements in these plots. Island-wide patterns are captured through vegetation mapping and

additional ecological investigations.

2. Response of tree mallow to cutting

Tree mallow cover in July 2014 was for a second year well below 1% in both Glen and Slopes,

and thus no obstacle to puffins during the breeding season (which is already the case since 2010).

The number of tree mallow seedlings was also at an all-time low, most likely due rabbit grazing,

and on average 7 per plot (25m2) in both areas. Tree mallow cover in Sept 2014 had reached

11% in the Glen (38% in 2013) and 7% on the Slopes (8% in 2013).

3. Recovery of native vegetation

Fig. 1. The percentage of

ground covered by

different vegetation

components in late

June/early July for the

Glen (left) and Slopes

(right). Data are based on

estimations of live plant

cover only. The most

important pattern of

perennial plants is

indicated by larger closed

circles.

Perennial plant cover more or less stagnated in the Glen and dropped on the Slopes (Fig. 1 -

black dots), the latter likely due to the greater rabbit grazing pressure on the vegetation. The

composition thereof in the Glen is not one of grasses but largely Common Nettle (average cover

of 22%) and Small Nettle Elder (17%). Grasses in general in the Glen are clearly on the way out,

possibly due to increasing grazing pressure by rabbits but also indicating insufficient seed flow

and absence of a perennial grass seed bank. Much of the remaining vegetation was – just like last

Glen Slopes

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two years – the annual Chickweed (24%), taking advantage of ample bare ground earlier in the

year. Thus, the vegetation in the Glen remains transient with only tall competitive and grazing

insensitive perennials gradually gaining ground. In fact, it is hard to imagine what perennial

species may colonise this heavily disturbed area (by its former tree mallow history as well as

seals (lower parts) and rabbits) (see also section 5).

Perrenial plant cover in the Slopes was 27%, thereby breaking the pattern of gradual increase.

The same held for annual plants and thus total live plant cover in July 2014; plant litter &

standing dead was responsible for these lower estimates. Indeed, both Red fescue (14%) and

Yorkshire fog (13%) showed reduced cover, likely at least partially due to rabbits grazing their

swards but also due to gulls having impact on the vegetation. There remains a lot of transient

species such as Wall barley (15%) and Chickweed (20%) and thus opportunities for tree mallow

seedlings to stick up their heads. Small nettle was recorded in 5 out 8 plots but reached very low

cover (<1%). Common nettle was recorded (as 1%) in only one of the plots.

Fig. 2. The percentage of

ground covered by rock, bare

soil and plant litter & standing

dead vegetation for the two

study areas separate. The most

important pattern of bare soil is

indicated by larger closed

circles.

4. Soil erosion and rabbits

This years’ estimates of bare ground (Fig. 2 – black dots) was again around 10% in both areas,

whilst litter was considerably higher. Yet, across the island there were numerous small bare

patches, merely resulting from rabbit (and sometimes gull) activity.

5. Tree mallow seedlings

An investigation of a subset of such small bare areas (150 randomly selected) across the island

by UoA Master student Emma Williams revealed that tree mallow seedlings were predominantly

found around the transitioning between bare ground and vegetation, with almost 5 times more

seedlings than in areas of either bare ground or vegetated ground (Fig 3). Whereas rabbits are

known to promote seedling appearance by breaking up the sward, the bulk of the seedlings thus

does not establish in the open but on the edge of the vegetation where moisture conditions (and

perhaps also levels of herbivory by rabbits) are more favourable.

6. Experimental grass sowing to determine its utility as possible management tool

With the aim to investigate whether the sowing for grass seed could limit the number of tree

mallow seedlings emerging, 10 plots (1m x 1m) were established on bare soils in September

2013 (with adjacent an 0.5m x 1m area as control). Fescue seed was sown upon establishment

and thereafter (April & July 2014); and a small hanging basket was placed upside down to keep

rabbits out from (a very small) part of the sown plot. Although a fine grass mat established (or at

least partially) early on in parts of the plots this was largely within the cages. Rabbits wrapped

Fig. 3. Mean

number of seedlings

along a 1m transect

(sketch vegetation

along the transect in

lower panel).

Points 0 & 20 lie in

bare ground, 40 &

60 in edge

vegetation, and 80

& 100 in vegetated

areas. Highest

seedling numbers

are in edge

vegetation, at points

40 & 60.

up the newly emerging grasses effectively in those tiny areas. Two out of 10 plots turned into

dense Yorkshire fog stands (not sown!), pointing out that some bare areas, notably in the

northern part of the island, may re-vegetate (leading to a sward made up of perennial vegetation)

naturally. In early summer chickweed colonised most other plots and smothered the grass,

leaving only some plots with a more or less established Fescue sward (in the small cages only).

Thus, the Fescue sowing experiment proved not overly fruitful but drew out that: 1) there is a

severe grass seed limitation in the soil seed bank (though ideally this would be investigated also

directly from soil samples taken); 2) large part of established grass in bare areas is likely

smothered by Chickweed, which evidently has ample representation in the soil seedbank; 3) and

rabbits will put pressure on the establishing swards. Because of point 1 it would be worthwhile

considering a larger-scaled seed sowing experiment with more competitive grasses mixed in (e.g.

Yorkshire fog and Fescue combined, or competitive agricultural mixtures) as there seems to be

little else that may come up (Note: over time it is not inconceivable that Sea campion (Silene

maritima), currently covering ~18m2, will occupy some bare areas in the southern part of the

island, where it is gradually expanding; this species requires rabbit grazing pressure as otherwise

overtopped by grasses – see my long term exclosures on the Isle of May).

Careful inspection of these small experimental plots (with cages) revealed that tree mallow

seedlings were clearly eaten by rabbits into June, with seedling numbers were still low (on

average 3 per m2 but 10x higher in the cages). Largely because of greater emergence but with

some influence of reduced consumption by rabbits, seedling density increased rapidly thereafter

reaching 65 per m2 in late July and 103 per m2 in early September (with similar densities now

found in the cages).

7. Island wide vegetation change

Fig. 4. Tree mallow cover on Craigleith in September 2006, just before the onset of large-scale tree mallow

management, August/Sept 2010-2014. The 2006 map is based on interpretation of video images taken from the air

(and hence rather coarse); the other three years are based on ground mapping.

The large-scale management efforts since September 2006 have transformed the island. Whereas

the island wide cover of tree mallow in late summer was (crudely) estimated as 81% in 2006, for

the years 2010-14 this was 19%, 18%, 22%, 13% and 11% respectively. Thus, tree mallow

cover across the island was particularly low during the last two years, something which is also

visible in Fig 4 (overall lighter shades of grey meaning lower mallow cover). Like last year, the

largest part of the island is under ‘target vegetation’ (i.e. perennial grass; Fig. 5), though much of

the southern half continues to hold transient vegetation.

Fig. 5. The spatial extent and position of target vegetation on Craigleith in August/September 2010-14. Table 1 Breakdown of the main vegetation types recorded (thereby attributing mixed communities to its dominant

component) in 2014. Estimated cover in 2011-13 are provided for comparison.

In Early September 2014 only 5% was island’s vegetation was tree mallow. Indeed, a low cover

of Tree mallow vegetation in the last two years is one of the noticeable island-wide vegetation

Primary level Secondary level Tertiary level m2 % of vegetated area

in 2014 in 2013 in 2012 in 2011

HORDEUM vegetation (Wall barley) 23.9 14.3 7.7 7.9

Hordeum Festuca 80868.89 17.4

Hordeum Tree_mallow 11209.54 2.4

Hordeum Stellaria 8048.84 1.7

Hordeum 6646.59 1.4

Hordeum Urtica_urens 4067.18 0.9

HOLCUS vegetation (Yorkshire fog) 19.6 21.3 13 14

Holcus 65400.54 14.1

Holcus Festuca 12574.90 2.7

Holcus Galeopsis 9836.13 2.1

Holcus Tree_mallow 1337.96 0.3

Holcus Rumex 1072.23 0.2

Holcus Urtica_urens 658.67 0.1

FESTUCA vegetation (Red fesque) 14.0 29.9 26.5 18.1

Festuca 30742.49 6.6

Festuca Hordeum 15193.35 3.3

Festuca Holcus 7137.10 1.5

Festuca Hordeum Atriplex 6170.33 1.3

Festuca Hordeum Urtica_urens 3761.71 0.8

Festuca Atriplex 1114.10 0.2

Festuca Stellaria 1004.35 0.2

STELLARIA vegetation (Chickweed) 12.7 3.3 0.6 2.4

Stellaria Urtica_urens Tree_mallow 26925.69 5.8

Stellaria ? 14553.16 3.1

Stellaria Hordeum Festuca 10946.85 2.4

Stellaria Holcus Urtica_urens 6253.35 1.3

URTICA vegetation (Nettle) 11.6 1.8 3.8 1.8

Urtica_dioica 18585.76 4.0

Urtica_urens Hordeum 17925.92 3.9

Urtica_urens 15587.71 3.4

Urtica_urens Stellaria 1375.36 0.3

Urtica_urens Tree_mallow 502.04 0.1

LAVATERA vegetation (Tree mallow) 5.3 8.8 26.3 24.7

Tree_mallow Urtica_urens Atriplex 16178.42 3.5

Tree_mallow 5300.09 1.1

Tree_mallow Atriplex 2993.56 0.6

ATRIPLEX vegetation (Orache) 2.3 5.4 0.8 1.8

Atriplex Hordeum 3815.80 0.8

Atriplex Urtica_urens 2090.51 0.5

Atriplex Tree_mallow 2015.90 0.4

Atriplex Stellaria 1401.11 0.3

Atriplex 1273.54 0.3

OTHER vegetation 10.6

Sambucus 10282.72 2.2

Bare Stellaria Urtica_urens 6828.59 1.5

Bare 4767.30 1.0

Cabbage 5460.25 1.2

Camomile_false 1191.43 0.3

Galeopsis Holcus 5644.58 1.2

Galeopsis 6468.60 1.4

Oat 2900.02 0.6

Rock 2329.70 0.5

Rumex 3081.33 0.7

LOLIUM 0.0 2.8 6.7 16.3

changes (in 2011/2 this was still a quarter of the island (Table 1)). Almost a third of the island is

now Red fescue or Yorkshire fog vegetation, but without much evidence for further expansion of

these perennial grass-dominated vegetation types. Over the years there has been a gradual

disappearance of Italian ryegrass (a short-lived plant) vegetation, and an increase (over the

period 2011-14) in the extent of both Wall barley dominated and Chickweed dominated

vegetation. Chickweed is a bare soil indicator (which it opportunistically colonises when

temperatures increase whilst soils remain damp). The expansion both vegetation types is likely

due to greater rabbit grazing and digging (pressures which are now exerted to an increasing

extent across the island) as this breaks up perennial swards and facilitates transient species to

invade.

The most obvious vegetation change this year was the omnipresence of Small nettle across the

island (Fig 6); almost 8% of the island was Small nettle vegetation and although the stems are

evidently eaten by rabbits, its sudden explosion is very likely a rabbit-induced vegetation change.

Fig. 6 The extent of Small nettle

vegetation in 2013 and 2014.

The expansion of the perennial Common nettle is certainly due to the high pressure of rabbits on

those species that are –unlike Nettle – not defended against grazing. In September 4% of the

island was Common nettle and much of the increase in extent was due to expansion of existing

patches (Fig. 7).

8. Conclusions and recommendations for management

In last years’ report the following prediction regarding the increase of Craigleiths’ rabbit

population was made: i) expansion of grazing insensitive species (such as nettle); ii) expansion

of bare ground (part of which will be covered by Chickweed for part of the year) in which tree

mallow seedlings will emerge from the seed bank; and iii) the consumption of tall tree mallow

over winter. All three predictions seem have come out this year.

The most notable aspect this year was that the southern part of the island had a rather ‘nettle-like

appearance’. Rabbits will eat small quantities of nettles when food is scarce (See Tayler 2009 – J

Ecol) and some snails and slugs consume this plant preferentially. The latter two groups of

organisms are either scarce or absent. Hence, grazing pressure on this plant is unlikely to become

substantial. Therefore, unless the rabbits die back, or for another ecological surprise to occur,

Common nettle stands will almost inevitable grow bigger. Those stands tend to be rather

impenetrable, perhaps even more so than was the case for Tree mallow stands, and thus not good

habitat for many ground-breeding birds. Larger stands of Nettle are mostly found where soils are

deepest, however, and thus overlap with puffin breeding colonies is so far limited but not absent.

Hence, a discussion should be held whether some of the larger stands of Common nettle

requires control, though the critical question that needs posing is what vegetation is meant to

come up in its place. Indeed, the only factor that would control Common nettle in the longer

term is the presence of an established sward of perennial species. Therefore, discussion on the

virtues of a larger-scaled seed sowing experiment with competitive species is recommended,

alongside an investigation of the natural seed bank of the southern part of the island, to

determine which species may be lingering in the soil.

Extensive occurrence of Small nettle, the clear winner of this year, compounded the Common

nettle situation and gave the impression of a Tree mallow-free but nevertheless not overly suited

vegetation for puffins on part of the island. It remains to be seen whether the pressure on the

stems of this plant by rabbits over-winter and in spring will keep it at bay. Most likely, however,

a considerable amount may be present during summer again because other most other plant

species (notably grasses) will be supressed to an even far greater level. Yet, the species is mostly

transient and may be replaced by another annual species (see the gradual disappearance of once

successful Italian ryegrass vegetation from the island – Table 1).

The greater levels of disturbance (notably by rabbits but arguable also by gulls and locally

possibly also by seals) continuous to enable tree mallow seedlings to show up, reaching greatest

densities on transitions between bare and vegetated patches notably in late summer. Part of the

seedlings is being consumed by rabbits, and larger plants will likely fall victim of rabbits.

Indeed, one wonders whether the grazing pressure on the island is now such that rabbits

can control tree mallow without further management. Again, this is something to discuss;

additional monitoring of tree mallow from early spring onwards would be needed to judge

whether rabbits indeed exert sufficient pressure on this target species to warrant cessation of

large-scaled cutting efforts on the island.

Large part of the island has now been in good condition during the puffin breeding season and

although still only partially under perennial plant cover, and seeing this cover slightly regress

likely due to rabbit grazing, it is likely that rabbits alone will stay on top of tree mallow here.

Continued monitoring and island wide surveillance may suffice as management instruments. It is

very important, however, to remain vigilant so that no larger-scaled tree mallow flowering

will re-occur.

Were a hands-of approach deemed inappropriate by the Craigleith management group at this

point in time, then a minimum of a single cut in all areas need conducting to prevent flowering

and hence new seed formation; cutting in autumn in areas with little tree mallow and working

towards a low abundance of tree mallow by the end of March will provide light to the expanding

perennial grass communities and present puffins with ample breeding habitat. Areas with

extensive mallow cover might be best cut in winter/early spring as autumn cutting brings little

additional benefit.

20 October 2014

René van der Wal

University of Aberdeen

r.vanderwal@abdn.ac.uk