José A. Hódar

APPLYING PLANT FACILITATION TO FOREST RESTORATION: A META-ANALYSIS OF THE USE OF SHRUBS AS NURSE PLANTS

After a millenarian history of overexploitation, most forests in the Medi- terranean Basin have disappeared, leaving many degraded landscapes that have been re- colonized by early successional shrub-dominated communities. Common reforestation tech- niques treat these shrubs as competitors against newly planted tree seedlings; thus shrubs are cleared before tree plantation. However, empirical studies and theory governing plant- 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 d 1 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 spe- cies and thus can positively affect reforestation success in many different ecological settings.

Pine processionary caterpillar Thaumetopoea pityocampa as a new threat for relict Mediterranean Scots pine forests under climatic warming

The wide distribution of Scots pine (Pinus sylvestris L.) in Europe includes two relict populations in southern Spain (Sierra Nevada and Sierra de Baza), belonging to the subspecies nevadensis. These populations are isolated in high mountains, which tends to protect them from the attack of a severe defoliating Mediterranean pest, the pine processionary moth Thaumetopoea pityocampa. However, as a consequence of climate change, the pine processionary caterpillar has increasingly attacked populations of this pine in recent years. This work describes the detrimental effects of defoliation by the pine processionary caterpillar in P. sylvestris subsp. nevadensis in Sierra Nevada. Defoliation strongly reduced pine growth as well as all the reproductive parameters measured (female and male cone production, mature cone size, seed production and seed weight), in addition to some deaths. If winter temperatures continue to increase, the pine processionary caterpillar will have a dangerous impact in these relict pine populations, by further reducing the pines weak regeneration capacity. We suggest some mitigation actions based on managing habitat structure, in order to reduce or avoid such negative impacts.

Age structure of Juniperus communis L. in the Iberian peninsula : Conservation of remnant populations in Mediterranean mountains

Abstract We examined the demographic structure of Juniperus communis L. populations in the Mediterranean high mountains of southern Spain in order to analyse its population viability. For this, we compared the age structure of these Mediterranean populations with those of northern Spain (Atlantic), and, on a local scale, the populations of different habitats within the Sierra Nevada. The populations from northern Spain showed age structures with high proportions at young stages. In contrast, the Mediterranean populations proved to be dominated by adult and senescent individuals, except for the few habitats with higher water availability during the summer, which have larger proportions of seedlings and juveniles. Dry habitats showed a lower seedling survival rate than did wet ones, mainly due to summer drought. Both the difference between Atlantic and Mediterranean categories, and the difference between habitats in Sierra Nevada reflect a serious limitation on recruitment of J. communis in Mediterranean mountains due to climatic stress imposed by summer drought. The Mediterranean populations provide a clear example of remnant dynamics, surviving for long periods due to great individual longevity which partially offsets losses to unfavourable environmental conditions. The low ability to recover after anthropic disturbances emphasizes the need for conservation of J. communis populations in south-eastern Spain.

Yew (Taxus baccata L.) regeneration is facilitated by fleshy-fruited shrubs in Mediterranean environments

Yew Taxus baccata is catalogued as a species endangered and prone to extinction in the Mediterranean mountains of southern Spain, due to the small size and senescent status of most populations. In this paper, we study the effects of herbivory and the protective role of woody shrubs in the regeneration ability of the yew in the Sierra Nevada. The estimated density of the yew in the study plot was 287.9 individuals/ha, more than 90% being juveniles (seedlings and saplings), which were mostly located under fleshy-fruited shrubs. Saplings suffered serious herbivore damage when unprotected by shrubs. Thus, fleshy-fruited shrubs proved to be the best habitat for seedling establishment and sapling survival and growth. The abundance of fleshy-fruited shrubs in our study site provides a yew population characterized by an active regeneration under natural conditions. We suggest that the maintenance of healthy populations of yew in Mediterranean mountains is strongly dependent on the conservation of well-developed fleshy-fruit understories and their community of avian dispersers.

Facilitation of tree saplings by nurse plants: Microhabitat amelioration or protection against herbivores?

Abstract Question: Positive interactions are predicted to be common in communities developing under high physical stress or high herbivory pressure due to neighbour amelioration of limiting physical and consumer stresses, respectively. However, when both stress sources meet in the same community, the relative importance of the two facilitation mechanisms is poorly understood. We ask: What is the relative importance of abiotic vs. biotic mechanisms of facilitation of tree saplings by shrubs in Mediterranean mountain forests? Location: Sierra Nevada, SE Spain (1800–1850 m a.s.l.) Methods: Saplings of four tree taxa (Acer opalus ssp. granatense, Quercus ilex, Pinus nigra ssp. salzmanii and P. sylvestris var. nevadensis) were planted following a 2 × 2 factorial design: two levels of herbivory (control and ungulate exclusion) and two microhabitats (under shrubs and in open areas). Sapling survival and growth were monitored for five years. Results: Shrubs had positive effects on sapling survival both in control...

Herbivory and climatic warming: a Mediterranean outbreaking caterpillar attacks a relict, boreal pine species

Climate change can harm many species by disrupting existing interactions or by favouring new ones. This study analyses the foreseeable consequences of climatic warming in the distribution and dynamics of a Mediterranean pest that causes severe defoliation, the pine processionary caterpillar Thaumetopoea pityocampa, and the effects upon the relict Andalusian Scots pine Pinus sylvestris nevadensis in the Sierra Nevada mountains (southeastern Spain). We correlated a set of regional data of infestation by T. pityocampa upon Scots pine, from a broad ecological gradient, with climatic data for the period 1991–2001, characterized by alternating warm and cold winters. Defoliation intensity shows a significant association with previous warm winters, implying that climatic warming will intensify the interaction between the pest and the Scots pine. The homogeneous structure of the afforested pine woodlands favours the outbreak capacity of the newcomer, promoting this new interaction between a Mediterranean caterpillar pest and a boreal tree at its southern distribution limit.

Effect of browsing by ungulates on sapling growth of Scots pine in a Mediterranean environment: consequences for forest regeneration

The impact of mammalian herbivory on Scots pine (Pinus sylvestris L.) sapling performance was quantified in three native forests located in two Mediterranean mountains, the Sierra Nevada and Sierra de Baza (SE Spain). More than 98% of the damage to terminal shoots was caused by goats and Spanish ibex in Sierra Nevada and sheep in Sierra de Baza. Some 72% of the tagged saplings (na 619) were browsed during at least 1 year of monitoring (1995‐1997). There were between-forest differences in herbivory pressure. Moreover, the herbivory pressure was significantly higher during a dry year (1995) than during wet ones (1996 and 1997). Overall, when browsing a sapling, ungulates consumed almost 30% of its apical shoots, and 85% of saplings were browsed more than once after establishment. As a consequence, ungulates severely affected the Scots pine sapling growth rate, and therefore browsed saplings grew slower than saplings unbrowsed by ungulates. Thus, according to exponential growth equations, the time necessary to attain a height threshold to escape from mammalian herbivores (150 cm height in our study forests) and start reproduction was retarded by the herbivory up to 12 years. Ungulates are a major factor hindering the natural regeneration and conservation of the last relict forests of Scots pines in SE Spain. # 2001 Elsevier Science B.V. All rights reserved.

Experimental study of pollination by ants in Mediterranean high mountain and arid habitats

In this paper, we report the results of an experimental study on ant pollination of three plant species inhabiting the Mediterranean high mountains (Alyssum purpureum, Arenaria tetraquetra and Sedum anglicum) and four species inhabiting the aridlands (Lepidium subulatum, Gypsophyla struthium, Frankenia thymifolia and Retama sphaerocarpa) of South-eastern Spain. We determined several plant and ant traits, as well as the composition and abundance of the pollinator assemblage. Insects belonging to 29 families and five orders visited the flowers of the plant species studied. In all but two, L. subulatum and G. struthium, the ants comprised 70–100% of the flower visitors. The results clearly show that five out of seven of these plant species were pollinated by ants. The role of the ants as pollinators seems to depend heavily on the relative abundance of the ants with respect to the other species of the pollinator assemblage, ant pollination becoming evident when ants outnumber other floral visitors. The ant-pollination systems analysed in this study may be the result of prevailing ecological conditions more than an evolutionary result of a specialized interaction.

Alleviation of summer drought boosts establishment success of Pinus sylvestris in a Mediterranean mountain: an experimental approach

We performed an irrigation experiment to study the impact of summer drought on Pinus sylvestris establishment at its southernmost distribution limit. Watering was done during the first growing season simulating mesic summer conditions, and we monitored the consequences for survival and growth during the first growing season and the delayed consequences on the second growing season. In addition, we considered the heterogeneity created by the microhabitats, where seeds are found after dispersal (bare soil, under shrubs, and under adult pines). Summer drought was the main mortality factor in all the microhabitats. Watering increased emergence and doubled seedling survival compared to non-watered control sites. Differences were even higher when the cumulative effect on emergence and survival was considered, with an overall recruitment of 22.4% in watered plots vs. 7.9% in control. Irrigation increased growth in bare soil and under shrubs, but had scant effect on growth under pines, suggesting that radiation was the limiting factor in this microhabitat. The positive effect of irrigation on growth parameters persisted during the second growing season despite water was not added the second year, showing delayed consequences of drought on seedling performance. Summer drought thus limits Pinus sylvestris establishment in these southernmost forests by reducing both recruitment and growth. This might lead to the development of a remnant dynamic in these relict populations under the current regional increase in dryness and rainfall variability associated with global warming.

Host utilisation by moth and larval survival of pine processionary caterpillar Thaumetopoea pityocampa in relation to food quality in three Pinus species

Abstract 1. The work reported here analysed host utilisation by the pine processionary moth Thaumetopoea pityocampa (Lepidoptera: Thaumetopoeidae), the relationship between moth oviposition patterns and larval performance, the chemical characteristics of the plant in relation to the performance of different larval instars, and the role of these factors in the outbreak capacity of the species. In order to do this, a combination of field and laboratory techniques was used to study three pine species differing in nutritional characteristics.