Study

Tree-felling and soil-scarification enhances sand lizard Lacerta agilis habitat at Brattforsheden, Värmlands, Sweden

Summary

On the northern periphery of its range in central Sweden, loss of open habitat on sandy heaths due to succession to Scot's pine Pinus sylvestris woodland has resulted in declines of isolated relict populations of sand lizards Lacerta agilis. Over 16 years, habitat development and lizard populations were studied in an area of heathland, in conjunction with habitat restoration at two sites within this area.

 

Study area: The study was undertaken on a 11,000 ha area of sandy pine heath at Brattsforsheden in Värmlands, south-central Sweden. Afforestation and forest-fire suppression had resulted in stands of trees developing. In open, disturbed patches the moss Polytrichum piliferum (a pioneer coloniser of dry, sun-exposed sand) persisted, along with patches of old heather Calluna vulgaris.

Over 20 years, six populations of sand lizard occured at Brattsforsheden, isolated from each other by 2.5 to 10 km of unsuitable forested habitat. Two of these populations became extinct. Two of the extant populations were the subject of this study, conducted over 16 years (1988 to 2004). In 1988, the area of suitable lizard habitat was around 1 ha at each of these two sites (designated SB and FL). As this habitat was decreasing due to growth of pine trees, habitat restoration was undertaken.

Habitat restoration:  In the winter and spring of 1988 and 1992, clearance of trees (mostly Scot’s pine) and removal of humus by manual scraping of patches to create bare sandy areas was undertaken. In 1992, soil scarification was carried out using a tractor in two sections at both sites; this entailed turning about 0.4 x 1 m of the humus layer upside down, creating a series of open patches about 1 m apart. This management doubled the area of potentially suitable habitat to around 2 ha at each site. The new clearings remained open during the study period, but other areas became less suitable as the tree canopy developed. Overall suitable lizard habitat declined at both sites to around 1 ha, at SB by 1999 and at FL by 2001. To increase the extent of sand lizard habitat once again, further management was undertaken in the autumn of these years. Over a 10 ha area, trees were felled and removed, and the ground subject to patch scarification. Additionally, using an excavator, large 100-200 m² sand patches were also created (11 at FB and seven at FL).

Vegetation succession and habitat suitability: At two areas within site SB the effect of three different management treatments on vegetation development and sand lizard microhabitat features was compared in 2004. The treatments (eight replicates of each) were:

i) tree-felling and patch scarification;

ii) tree-felling only;

iii) no management.

Percentage vegetation cover, litter and bare ground was estimated within 2 x 2 m quadrats.

Sand lizard colonisation and population structure: Population changes in relation to management were assessed by recording the number of clutches, subadult and adult sand lizards within all large sand patches (actual or potential egg-laying sites) and a 30 m area around each. Both sites combined (17 sections) were compared between 1988-1999 (eight with no management; nine restored during 1988 and 1992). Between 1999 to 2004, 27 sections were compared; the nine restored during 1988 and 1992; 18 restored during 1999 (site SB) and 2001 (site FL).

Population size: Adult female population size (i.e. the highest number of females known to be alive or number of clutches) was assessed from 1988 to 2004.

Vegetation succession and habitat suitability: The most important habitat ground cover variables are summarised in Table 1 (attached). Heather cover (know to be an important constituent of sand lizard habitat) varied significantly between treatments. It was by far most abundant in the felled + scarified treatments (26 and 38%), whilst felling only and the controls had 6% or less cover. The moss P.piliferum was most abundant (3-4% cover) in scarified areas, unsurprisingly as it is a pioneer coloniser of bare sand. Reindeer lichens were the prevalent ground vegetation in control areas and prior to management (over 60% cover); felling trees alone had no significant impact on lichen cover, felling + soil-scarification reduced it to between c.20 to 35% cover.

Sand lizard colonisation and population structure: Between 1988-1999, the population sizes in the 1988 and 1992 restored sections increased or decreased as follows: overall clutches up by 50%; subadults up by 30%; adults down by 35%).  Those in unmanaged areas decreased by 60-80% (clutches down by 60%; subadults down by 75%; adults down by 80%). Between 1999-2004, populations in unmanaged areas had more-or-less been lost.

Population size: As sand lizards colonized restored sections, adult female population size over the first 10 years (1988-1997) declined by 9 and 11% with lows of 5 females (FL in 1998) and 6 females (SB in1997 and 1999) . However, over the subsequent six years up to 2004, they increased by 19 and 11% per year, (14 females at FL; 15 at SB) which partly coincided with the larger-scale habitat restoration.

Conclusions: In this area, it was apparent that successional processes (particularly tree growth and development of the canopy) resulted in loss of suitable sand lizard habitat, with population declines and eventual extirpation in unmanaged areas. Besides felling of trees, it is not obvious which techniques of a number that are available, are most effective in providing a suitable mosaic of different size sand patches and dense Calluna, as shown to be important for sand lizards. Burning has been used traditionally in heathland management in many areas but in terms of reptile conservation, this is risky as it may cause major reptile mortality, and it is unclear how suitable post-burn vegetation is in providing the required habitat structure. It was apparent that soil-scarification reduced the dominance of reindeer lichen and led to a mosaic of sandy patches

with P.piliferum and heather within 10 years. An advantage of the mechanical method used to create these patches is that it can be locally employed on areas adjacent to occupied sand lizards sites, thus extending suitable habitat available for colonisation, without impacting on lizard survival during the operation. It was also clear that vegetation litter and felled/fallen trees are also important habitat components, providing basking areas and refugia, thus it is recommended that when clearing trees, some cut trees and branches should be left lying on the ground.


Note: If using or referring to this study, quote the thesis.

Output references

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