Individual study: The effects of sheep grazing on the growth and survival of juniper Juniperus communis seedlings at Aston Rowant National Nature Reserve, Oxfordshire, England
Fitter A.H. & Jennings R.D. (1975) The effects of sheep grazing on the growth and survival of seedling junipers (Juniperus communis L.). Journal of Applied Ecology, 12, 637-642
In the UK, juniper Juniperus communis has contracted markedly in its distribution over the last 100 years or so. Primary factors appear to be habitat loss and succession of scrub and woodland due to reduced grazing. In southern England juniper occurs mainly on grazed chalk grassland, and many colonies contain only old non-reproductive plants with no regeneration. This study looked at the effects of sheep grazing at different seasons of the year on the growth and survival of juniper seedlings at Aston Rowant National Nature Reserve in Oxfordshire.
Experimental plots were established on similar south-west facing chalk grassland slopes. Plots were surrounded by rabbit-proof fencing in 1964 and then grazed for three months in autumn (October-December), winter (January-March) or summer (April-June) at a rate of 1.2 sheep/ha, or were left ungrazed (control).
Juniper seedlings in each plot were labelled (autumn = 54; winter = 46; summer = 30; control = 10) in 1966 (except for summer-grazed plots which were labelled in in 1967). These plants were monitored twice a year for the first three years, with subsequent observations after five and seven years. The control plots comprised smaller fenced areas within the main plots (hence containing fewer juniper seedlings. Measurements were made of seedling height, crown diameter and stem diameter at 1 cm above the ground; visual estimates were made of vigour and grazing damage.
Survival: Juniper seedling survival was similar in autumn-grazed, winter-grazed and control plots (about 50% survival after 5 years). Survival on summer-grazed plots were also similary high initially low (sheep seemed to preferentially select other more palatable food available in this season). After 7 years, survival was highest in the control (50%) compared to autumn- and winter-grazed (30%). The summer-grazed plot fences were not maintained and plants were heavily grazed by European rabbits Oryctolagus cuniculus, thus no comparison could be made.
In the autumn- and winter-grazed plots most juniper seedlings died due to grazing or uprooting by sheep, with some appearing smothered by grasses (attributed to vigerous growth due to the manuring effect of sheep droppings). In the ungrazed plot the main cause of death was unknown but possibly due to vole damage (attacks on stem bases were much more common than in the grazed plots, as the thicker ground cover afforded good vole habitat). There were no seedling losses in the control after five years, but losses continued in the autumn-grazed and winter-grazed plots.
Height and crown diameter: Height and crown diameter of plants on the control and summer-grazed plots increased, whereas the height and diameter of plants on autumn- and winter-grazed plots decreased. Plants that survived on ungrazed plots were larger than those in the grazed plots.
Stem diameter: Stem diameter increased most in the control (55% increase) compared with autumn-grazed (23%) or winter-grazed (15%) plots. Over the 5-year recording period for the summer-grazed seedlings, average diameter increased by 41%.
Conclusions: This study shows that young juniper can be killed or growth retarded by sheep. It might be argued that regeneration would be enhanced if grazing was prevented. However, if ungrazed for too long, other more rapidly growing woody species will become dominant. Juniper regeneration also usually occurs only on bare ground or in very short turf (usually a result of high grazing intensity). Regeneration appears to have been widespread after myxomatosis in1954 (which killed many rabbits) when a heavy to light grazing transition last occurred. The authors suggest that juniper regeneration might thus be achieved by initial severe grazing to create bare ground or broken turf to encourage germination and seedling establishment, then light summer grazing to reduce competition from other vegetation without killing the junipers.
Note: The compilation and addition of this summary was funded by the Journal of Applied Ecology (BES). If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://links.jstor.org/sici?sici=0021-8901%28197508%2912%3A2%3C637%3ATEOSGO%3E2.0.CO%3B2-R