Release of Juniperus thurifera woodlands from herbivore-mediated arrested succession in Spain

Abstract

Question: Do abiotic constraints maintain monospecific
woodlands of Juniperus thurifera? What is the role of
biotic (livestock) versus abiotic (climate) drivers in the
recruitment and growth of the different tree species?
Location: Cabrejas range, Soria, north-central Spain,
1200m altitude.
Methods: Stand history was reconstructed using dendroecology
and spatial pattern analysis, combined with historical
data of livestock abundances and climatic records.
Results: J. thurifera establishment occurred in two distinct
pulses, with a tree component establishing in the late 1800s
to early 1900s. Quercus ilex and Pinus sylvestris establishment was evident only from the late 1970s onward.
Recruitment events were related to reductions in livestock
browsing. J. thurifera spatial structure was clumped and Q.
ilex showed a short-scale aggregation to J. thurifera trees
and saplings. Radial growth trends of J. thurifera saplings,
Q. ilex and P. sylvestris were negatively related to livestock density. Summer drought limited the radial growth of all the study species, and P. sylvestris and Q. ilex grew faster than J. thurifera even after considering an age effect.
Conclusions: The differences in radial growth patterns and
recruitment pulses between species indicate that livestock
browsing and not abiotic factors is the main factor
controlling plant succession and structural development.
In this process, J. thurifera acts as a nurse plant, facilitating the establishment of other tree species. Under the current low pressure from herbivores, formerly pure J.
thurifera woodlands will change towards dense stands of
mixed species composition.