10 research outputs found
The critical behavior of frustrated spin models with noncollinear order
We study the critical behavior of frustrated spin models with noncollinear
order, including stacked triangular antiferromagnets and helimagnets. For this
purpose we compute the field-theoretic expansions at fixed dimension to six
loops and determine their large-order behavior. For the physically relevant
cases of two and three components, we show the existence of a new stable fixed
point that corresponds to the conjectured chiral universality class. This
contradicts previous three-loop field-theoretical results but is in agreement
with experiments.Comment: 4 pages, RevTe
Piezoelectric and pyroelectric behavior of lead zirconate titanateâlead sulfideâglass nanocomposites
Applying plant facilitation to forest restoration in Mediterranean ecosystems: a meta-analysis of the use of shrubs as nurse plants
11 pages, 4 figures, 1 table, 75 references. We thank the ConsejerĂa de Medio Ambiente, Junta de
AndalucĂa, and the Direction of the National Park of Sierra
Nevada for permission to field work, constant support and
facilities. We are also especially grateful to Empresa de TransformaciĂłn Agraria S.A. (TRAGSA) for carrying out the experimental
reforestation. We thank Sergio de Haro and several
students for field assistance. David Nesbitt looked over the
English version of the manuscript.After a millenarian history of overexploitation, most forests in the Mediterranean
Basin have disappeared, leaving many degraded landscapes that have been recolonized
by early successional shrub-dominated communities. Common reforestation techniques
treat these shrubs as competitors against newly planted tree seedlings; thus shrubs
are cleared before tree plantation. However, empirical studies and theory governing plantâ
plant interactions suggest that, in stress-prone Mediterranean environments, shrubs can
have a net positive effect on recruitment of other species. Between 1997 and 2001, we
carried out experimental reforestations in the Sierra Nevada Protected Area (southeast
Spain) with the aim of comparing the survival and growth of seedlings planted in open
areas (the current reforestation technique) with seedlings planted under the canopy of
preexisting shrub species. Over 18 000 seedlings of 11 woody species were planted under
16 different nurse shrubs throughout a broad geographical area. We sought to explore
variation in the sign and magnitude of interactions along spatial gradients defined by altitude
and aspect. In the present work, we report the results of a meta-analysis conducted with
seedling survival and growth data for the first summer following planting, the most critical
period for reforestation success in Mediterranean areas. The facilitative effect was consistent
in all environmental situations explored (grand mean effect size d1 5 0.89 for survival and
0.27 for growth). However, there were differences in the magnitude of the interaction,
depending on the seedling species planted as well as the nurse shrub species involved.
Additionally, nurse shrubs had a stronger facilitative effect on seedling survival and growth
at low altitudes and sunny, drier slopes than at high altitudes or shady, wetter slopes.
Facilitation in the dry years proved higher than in the one wet year. Our results show that
pioneer shrubs facilitate the establishment of woody, late-successional Mediterranean species
and thus can positively affect reforestation success in many different ecological settings.This study was supported
by a PFPU-MECD grant to L. G. and PFPI-MCYT to E. B.
and projects FEDER 1FD97-0743-CO3-02 and REN 2001-
4552-E from MCYT.Peer reviewe