Regino Zamora

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.

Canopy vs. soil effects of shrubs facilitating tree seedlings in Mediterranean montane ecosystems

Abstract Question: Is the facilitative effect of nurse shrubs on early recruitment of trees mediated by a ‘canopy effect’ (microclimate amelioration and protection from herbivores), a ‘soil effect’ (modification of soil properties), or both? Location: Two successional montane shrublands at the Sierra Nevada Protected Area, SE Spain. Method: Seedlings of Quercus and Pinus species were planted in four experimental treatments: (1) under shrubs; (2) in open interspaces without vegetation; (3) under shrubs where the canopies were removed; (4) in open interspaces but covering seedlings with branches, mimicking a shrub canopy. Results: Both effects benefited seedling performance. However, microclimatic amelioration due to canopy shading had the strongest effect, which was particularly pronounced in the drier site. Below-ground, shrubs did not modify soil physical characteristics, organic matter, total N and P, or water content, but significantly increased available K, which has been shown to improve seedling water-use efficiency under drought conditions. Conclusions: We propose that in Mediterranean montane ecosystems, characterised by a severe summer drought, pioneer shrubs represent a major safe site for tree early recruitment during secondary succession, improving seedling survival during summer by the modification of both the above- and below-ground environment. Nomenclature: Castroviejo et al. (1986–2001) for Quercus and Pinus species, and Molero-Mesa et al. (1992) for shrub species.

Beyond species loss: the extinction of ecological interactions in a changing world

Summary 1. The effects of the present biodiversity crisis have been largely focused on the loss of species. However, a missed component of biodiversity loss that often accompanies or even precedes species disappearance is the extinction of ecological interactions. 2. Here, we propose a novel model that (i) relates the diversity of both species and interactions along a gradient of environmental deterioration and (ii) explores how the rate of loss of ecological functions, and consequently of ecosystem services, can be accelerated or restrained depending on how the rate of species loss covaries with the rate of interactions loss. 3. We find that the loss of species and interactions are decoupled, such that ecological interactions are often lost at a higher rate. This implies that the loss of ecological interactions may occur well before species disappearance, affecting species functionality and ecosystems services at a faster rate than species extinctions. We provide a number of empirical case studies illustrating these points. 4. Our approach emphasizes the importance of focusing on species interactions as the major biodiversity component from which the ‘health’ of ecosystems depends.

Impact of vertebrate acorn- and seedling-predators on a Mediterranean Quercus pyrenaica forest

We have experimentally investigated the impact of biotic factors, acting at the seed and seedling stages, on a Quercus pyrenaica forest in the Sierra Nevada mountains (SE Spain). We monitored the natural establishment of the oak for 3 years in two forest plots and two shrubland plots, by counting seedlings and juveniles. In addition, we established several experiments in these plots to examine acorn and seedling survival, while also considering the microhabitat effect on survival probability. Dispersed acorns were quickly consumed by several species of predators, particularly wild boar (Sus scrofa) and woodmouse (Apodemus sylvaticus). Less than 4% of the experimental acorns survived to produce seedlings, even when they were buried 4 cm in soil, simulating caches. No effect of microhabitat was found on predation, and thus no safe site appears to exist for Q. pyrenaica acorns in the study area. Some 98% of the 1000 experimental seedlings were killed by herbivores, notably woodmice, wild boar, and domestic and wild ungulates. Seedling survival varied spatially, being significantly higher under shrubs (4%) than in any other microhabitat (less than 0.5%). Both acorn and seedling survival were much lower in the shrublands than in the forests. In shrubland plots, the main agent of seedling mortality was trampling by domestic ungulates foraging in herds. This study suggests that the regeneration of Q. pyrenaica forests in Mediterranean mountains can be limited by herbivores acting at several life-history stages. # 2002 Elsevier Science B.V. All rights reserved.

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.

GENERALIZATION VS. SPECIALIZATION IN THE POLLINATION SYSTEM OF HORMATHOPHYLLA SPINOSA (CRUCIFERAE)

This study describes the interaction between Hormathophylla spinosa, a crucifer shrub, and its pollinators, analyzing the spatiotemporal variability of the pollinator assemblage as well as the foraging behavior and effectiveness of the common pollinators. The study was carried out in the high mountains of the Sierra Nevada, Spain, over four years (1988–1991). We selected three populations of H. spinosa located along a wide altitudinal gradient. This plant species was visited during the four years of the study by at least 70 species of insects belonging to five orders and 19 families. The assemblage of floral visitors of H. spinosa was composed mainly of ants and different species of solitary bees and flies, although most pollinators were comparatively rare. There were striking similarities between most pollinator species in the distributions of flight distance between consecutive flowers, which were leptokurtic and highly skewed. Moreover, almost all visitors acted as pollinators, depositing pollen grains...

Top‐Down Effects in a Tritrophic System: Parasitoids Enhance Plant Fitness

We determined the direct and indirect top—down forces in a tritrophic system composed of a guild of three parasitoid species (Necremnus tidius, N. folia, and Aprostocestus sp.), a single weevil seed—predator species (Ceutorhynchus sp. nov.), and the host plant, Hormathophylla spinosa, a woody crucifer of the high—mountain elevations of the Sierra Nevada (southeast Spain). By Means of observational and experimental approaches, we focused on the possible regulatory effects of parasitoids on the weevil population as well as on the analysis of the sign and strength of the resulting indirect effects on plant fitness. Weevill attacks 21.7% of the fruit produced by H. spinosa during the study, selecting fruits having the most seeds. Seeds eaten by weevils were significantly bigger than uneaten ones. The number of seeds per fruit after being depredated by Ceutorhynchus was significantly smaller than for fruit not attacked. Thus, weevils affect the fitness of the host plant both by decreasing the number of seeds (quantitative effect) and by reducing the average size of surviving seeds per fruit (qualitative effect). When parasitoids were excluded experimentally, the percentage of attacked fruit (which corresponds to the number of weevil larvae) rose from 20 to 43%. We assume that the increase in the experimental weevil population may be because excluding parasitoids eliminates not only death by parasitoid oviposition for some Ceutorhynchus larvae, but also death by direct predation of parasitoids on weevil larvae. We have also experimentally verified that parasitoid activity benefits the entire plant, by way of parasitoids contributing to a higher average number of seeds dispersed per fruit in each plant, by influencing both the abundance of the weevis and the behavior of the weevil larvae.

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...