Yew (Taxus baccata L.) is a European temperate species (Preston & Hill, 1997), widely distributed in regions with mild winters and high humidity. When climatic conditions become limiting factors, such as extreme temperatures, or severe drought, yew is restricted to favorable niches that are protected from these extreme conditions (Thomas & Polwart, 2003). Consequently, in the Mediterranean region, yew trees usually grow in isolated populations in shady mountain ravines and on cliffs, or as a sub-canopy tree in a ­variety of forest communities (Cortés et al., 2000; Piovesan et al., 2009). Mediterranean yew stands were probably isolated into small populations during the last glaciations (Bennet et al., 1991) and are now endangered by current land-use changes and fire spread. Hence, the Mediterranean yew habitat is recognized as a priority habitat for biodiversity conservation within the European Union (Habitat 9580*, European Council Directive 43/92/EEC, European Commission, 1992).

Facilitation among plants occurs when the recruitment and survival of one species is enhanced by another species by making the physical environment under its canopy more suitable for the beneficiary (Bertness & Callaway, 1994; Verdú & García-Fayos, 1996; Garcia et al., 2000). However, the net result of plant interactions may depend on the stress induced by the direct environment, which could be positive in some conditions but become negative under others (Iskulo et al., 2012). The need to identify appropriate management to maintain or enhance the resilience of forest ecosystems under the projected impacts of climate change has been highlighted by different authors (e.g. Lindner, 2000; Messier et al., 2013; Coll, 2014). From a conservation point of view, the identification of facilitation mechanisms may be useful for the management of threatened plants, such as the yew, or for the enhancement of the resilience of ecosystems to the projected impacts of the climate change (e.g. Batllori et al., 2009).

Although yew is an extremely shade-tolerant species (Niinemets & Valladares, 2006), the competition for light has been indicated as the main factor in explaining the decline of yew populations under a dense canopy (Ruprecht et al., 2009; Schwendtner, 2011). Recently, Devaney et al. (2015) reported a reduced number of juvenile yews in shade conditions in the southwest Ireland and suggested that light availability may be a limiting factor in the recruitment of yew to later demographic stages. However, some discrepancies may be found in the literature examining light responses in controlled environments. Hence, whereas Iszkulo & Boratyński (2006) showed that yew plants are able to establish under extremely dark conditions, with less than 2% photosynthetic photon flux density, but cannot grow to adult size, Perrin & Mitchell (2013) under experimental light conditions did not find mortality of yew saplings at heavy shade (<3% day-light). In Mediterranean conditions, while a shortage of light can compromise the survival of seedlings, juvenile yews may take advantage of growing in slight shade which provides a reduction of evaporative demand during drought periods.

Yew is a medium drought-tolerant species (Niinemets & Valladares, 2006). It usually grows in localities with a mean annual rainfall higher than 600 mm, though optimally being around 850 mm. The yew root system is mostly superficial, with an extensive horizontal root system; but roots penetrating deeply into the fissures of limestone and chalk pavements have been described in Irish yew woods (Kelly, 1981). In dense Mediterranean evergreen woodlands, the competition for soil water may be severe between yew plants and other species, such as pines or oaks, that present a higher ability to extract water at lower potentials (e.g. Martínez-Vilalta et al., 2014). In summary, water stress in yew trees may mostly depend on the net balance between the competition for soil water with neighbors and the facilitation by the same neighbors’ cover, which may reduce the overall evaporative demand. These effects may be especially relevant in the juvenile stage, whether or not they allow for the plants’ transition to adult status.

The aim of this study is twofold, first, to analyze the structure of forest with yew in the North-Eastern region of the Iberian Peninsula at a regional scale. Then, at an individual scale, it attempts to analyze the role of the surrounding woody neighbors in the performance of the yew juveniles. Specifically, it was hypothesized that in mixed Mediterranean forest, (1) high-density neighbors may negatively affect the growth of juvenile yews, but (2) the effect of moderately dense neighbors covering yew crowns may attenuate the negative effects of water deficit on yew performance.

In the NE Iberian Peninsula, the habitat of yew has been mapped mostly in northward facing forest stands, widely distributed through the coastal range and the southern foothills of the Pyrenees, in a wide range of altitudes (between 300 – 1,600 m a.s.l.) and in Mediterranean or Submediterranean bioclimatic regions (Font, 2014 and unpublished data). In these conditions, yew is found in a wide spectrum of forest structures, from clear stands to dense forests as indicated by the total stocking density (ranging from 149 stems ha^–1 to 4,584 stems ha^–1) and the total basal area (ranging from 0.85 m² ha^–1 to 75.6 m² ha^–1).

In general, the density of recruits and juveniles (625 ± 104 recruits ha^–1, 259 ± 55 juveniles ha^–1) found in the monitored stands was higher than the ones reported in other Mediterranean forests in which yew appears (e.g. García et al., 2000; but see Farris & Filigheddu, 2008) and similar to those from more temperate European areas (Hulme, 1996; García & Obeso, 2003; Dhar et al., 2006; Piovesan et al., 2009). However, and similar to other regions, the recruitment of yew is low or nonexistent in yew dominated stands (181±88 individuals ha^–1). In Mediterranean localities and under these conditions, low yew recruitment may be explained, at least partially, by (i) severe droughts (ii) high seed-predation rates (Sanz & Pulido, 2014), (iii) high browsing pressure caused by herbivores due to the absence of the protection of nursery plants (sensu Garcia & Obeso 2003; Mendoza et al., 2009) and (iv) the extremely dark conditions under the canopy of the mature yews (Iszkulo & Boratyński, 2006; Perrin et al., 2006).

Light is one of the main factors structuring forests and affecting the composition of species. In Mediterranean climate areas, where water supplies vary intra- and inter-annually, the plant water stress has also an important role in structuring the forests. When water is limited, adjacent plants may compete for soil water. In such cases, the competitive advantage of one species over another is triggered by the capacity to extract water from the soil at lower potential or the capacity to reduce the water losses by evapotranspiration. Drier conditions increase the WUE of plants, which culminates in higher δ¹³C values of plant tissues (e.g. Donovan & Ehleringer, 1994, Lloret et al., 2004). Hence, higher δ¹³C values in leaves of adult yews (data not shown) compared with those in juveniles growing in the same Mediterranean sites suggest that adults are more water-stressed than young ones. Although no differences in δ¹³C were detected among juvenile yews growing in Mediterranean or Submediterranean localities, the regression model predicts that Mediterranean juveniles were more water stressed than Submediterranean ones. Even though other factors related with micro-site characteristics (e.g. soil depth) can also modulate the observed patterns, our results indicated that the yew juveniles benefit from the canopy of neighbors covering yew crowns. The cover of yew by the neighbors’ crowns probably reduces the evapotranspiration of juvenile yew during periods with high atmospheric evaporation demand. However, a negative effect was found when the density and size of neighbors growing close to the yew juveniles is too high, likely due to an increase in the competition for soil water between the yews and the other woody species.

Management practices such as traditional selective thinning of Mediterranean oak coppices have been demonstrated to reduce the consequences of long drought periods compared with unthinned plots (Cotillas et al., 2009). However, it is worth highlighting that these authors also stated that the intensity and frequency of this management should be considered to ensure the efficacy of the treatments and to avoid non-desired effects. In Slovakia, for example, Saniga (2000) found that a reduction of around 18-20% of volume in the surrounding density in a stand dominated by Fagus and Picea abies improved the height growth of yew, whereas a reduction by 7-8% resulted in no growth above the control plots (cited in Thomas & Polwart, 2003).

In conclusion, the lack of yew recruitment in dense mature forest and the juvenile performance in less dense structures may constrain yew conservation in the midterm. As water availability in the Mediterranean region is likely to decrease in the near future (IPCC, 2013) and forest densification is expected to continue to rise, selective cuttings addressing the reduction of basal area of neighbors surrounding the target yew should be promoted since they may increase soil moisture and encourage juvenile growth. However, it would be suggested that removing neighboring woody plants be done with care in order to prevent excessive exposure of the juvenile yew crown to light. As other factors at the tree scale may be relevant, further studies should focus on the thresholds of thinning management.