毛乌素沙地优势灌丛对两种松树幼苗的保育作用

选择毛乌素沙地优势灌丛柠条(Caragana intermedia)、紫穗槐(Amorpha fruticosa)和沙柳(Salix psammophila)作为保育植物,选择2年生油松(Pinus tabulaeformis)和樟子松(Pinus sylvestris)幼苗作为目标植物,将其分别种植在开阔地、柠条、紫穗槐和沙柳4种微生境下,连续3a监测油松和樟子松幼苗的存活和生长动态,从而确定毛乌素沙地优势灌丛是否对植物幼苗的定居有促进作用。结果表明:柠条、紫穗槐和沙柳冠层下光照强度、大气温度和土壤温度都显著低于开阔地,致使土壤湿度增高,油松幼苗总存活率均显著高于开阔地,而樟子松幼苗总存活率仅紫穗槐冠层下显著高于开阔地(P0.05)。紫穗槐冠层下油松和樟子松幼苗总存活率均最高(P0.05),外来种紫穗槐可选为保育植物。保育灌丛下松树幼苗的主枝生长良好,并未受到抑制,但第1年主枝生长长度最小。光照强度和土壤湿度是决定主枝生长长度的重要因素。保育灌丛高度和冠幅面积与油松和樟子松幼苗存活率呈显著正相关(P0.05)。固氮灌丛(紫穗槐和柠条)对松树幼苗存活有持续的促进作用,但柠条冠幅结构紧密,可能因为对光的竞争,减弱了促进作用。沙柳灌丛下松树幼苗的存活率较低。油松幼苗具有一定程度的耐阴性,樟子松幼苗喜光性强,保育灌丛下油松幼苗存活率显著高于樟子松幼苗(P0.05)。总的来说,保育灌丛对松树幼苗的定居有促进作用,保育植物技术可作为一种有效的恢复措施,应用在毛乌素沙地的植被恢复中来。但是,保育灌丛的选择要考虑是本土种或外来种,以及灌丛的高度、冠幅大小、冠幅结构和化学效应,恢复物种的选择要考虑其自身的生物学特性及其与保育植物的相互作用。     

Restoration of Dry Afromontane Forest Using Pioneer Shrubs as Nurse-Plants for Olea europaea ssp. cuspidata

Shrubs are often considered competitive barriers for seedlings planted in reforestation programs, although they can facilitate tree recruitment, especially in ecosystems under high abiotic stress. An alternative reforestation technique using pioneer shrubs as nurse‐plants for Olea europaea ssp. cuspidata was tested in exclosures in northern Ethiopia. Seedlings were planted in three different microhabitats, and their survival was monitored. The microhabitats were bare soil patches between shrubs, patches under the dominant shrub Acacia etbaica, and patches under Euclea racemosa, an evergreen shrub, which supports the majority of naturally established Olea recruits. The ability of shrubs to offer protection against browsing goats was tested experimentally. Controlled shading was used to determine whether solar irradiation causes seedling mortality in environments without water stress. Data were analyzed using Kaplan–Meier survival analysis, Kruskal–Wallis analysis of variance (ANOVA), and one‐way ANOVA. Olea survival was significantly higher and shoot damage by goats was lower when planted under shrub cover compared to bare soil patches, particularly under Euclea canopies, although high shade levels reduced seedling performance. Reduction of solar radiation by shrub canopies and thus control of soil–water evaporation and seedling transpiration most likely controlled the observed facilitation. Planting under shrubs may increase seedling survival and assist regeneration of dry Afromontane vegetation. Preserving pioneers also reduces soil erosion and conserves biodiversity. Excluding livestock is essential for Olea woodland restoration and allows persistent but morphologically modified Olea shrubs to develop vigorous regrowth. Facilitative processes are guiding principles for assisted forest restoration, but above‐average rains may be critical to restore higher biomass levels in semiarid areas.     

Effects of pioneer shrubs on the recruitment of the fleshy‐fruited tree Olea europaea ssp. cuspidata in Afromontane savanna

Question: Is seedling recruitment of a fleshy‐fruited tree in degraded Afromontane savanna dependent on shelter from pioneer shrubs, and is shelter availability related to shrub traits? Location: Degraded montane savanna in northern Ethiopia (13°36′N, 39°21′E). Method: Nurse plants of Olea europaea ssp. cuspidata seedlings were recorded using T‐square plotless sampling and clustered according to shrub traits, using Ward's method after Principal Components Analysis. Facilitation was further examined through experimental planting and Kaplan‐Meier survival analysis. Results: Both in grazed and protected areas, Olea recruits were found exclusively under shrubs, primarily under Euclea racemosa although Acacia etbaica was more abundant. Olea recruitment is distributed randomly at landscape scale, but depends on shelter at patch scale. Shelter ability is related to shrub shape and species identity. Dense multi‐stemmed shrubs with a wide base and crown on a mulch‐rich mound are key recruitment foci. Euclea shrubs have these favoured traits and probably act as preferential perching sites for avian seed dispersers. Soil and organic matter accumulation under Euclea shrubs may also create favourable conditions for Olea germination and survival. Experimentally planted seedlings had a better chance for survival under Euclea. Conclusions: Olea regeneration is probably subject to both passive (disperser‐mediated) and active facilitation. Small changes of shrub traits can alter the suitability of a patch for Olea recruitment. Protection of shrubs can increase facilitation for seedlings, while pruning may reduce competition for saplings and thus enhance forest succession. Planting of raised Olea seedlings under Euclea shrubs in years with a good rainy season may further assist forest restoration.     

Non‐Native Grass Removal and Shade Increase Soil Moisture and Seedling Performance during Hawaiian Dry Forest Restoration

Invasive non‐native species can create especially problematic restoration barriers in subtropical and tropical dry forests. Native dry forests in Hawaii presently cover less than 10% of their original area. Many sites that historically supported dry forest are now completely dominated by non‐native species, particularly grasses. Within a grass‐dominated site in leeward Hawaii, we explored the mechanisms by which non‐native Pennisetum setaceum, African fountain grass, limits seedlings of native species. We planted 1,800 seedlings of five native trees, three native shrubs, and two native vines into a factorial field experiment to examine the effects of grass removal (bulldozed vs. clipped plus herbicide vs. control), shade (60% shade vs. full sun), and water (supplemental vs. ambient) on seedling survival, growth, and physiology. Both grass removal and shade independently increased survival and growth, as well as soil moisture. Seedling survival and relative growth rate were also significantly dependent on soil moisture. These results suggest that altering soil moisture may be one of the primary mechanisms by which grasses limit native seedlings. Grass removal increased foliar nitrogen content of seedlings, which resulted in an increase in leaf‐level photosynthesis and intrinsic water use efficiency. Thus in the absence of grasses, native species showed increased productivity and resource acquisition. We conclude that the combination of grass removal and shading may be an effective approach to the restoration of degraded tropical dry forests in Hawaii and other ecologically similar ecosystems.     

Establishing Native Grasses in a Big Sagebrush–Dominated Site: An Intermediate Restoration Step

Many semiarid rangelands in the Great Basin, U.S.A., are shifting dominance to woody species as a consequence of land degradation including intense livestock grazing and fire suppression. Whereas past rehabilitation efforts in Big sagebrush (Artemisia tridentata) steppes removed the shrub and added introduced forage grasses to successfully shift communities from shrublands to grasslands, current consensus is that native species should be included in restoration projects and that retention of some woody plants is desirable. We examined the potential for interseeding grasses into dense shrub communities as a precursor to thinning shrubs and releasing grasses from shrub interference. We compared seedling establishment of the native grass, Bluebunch wheatgrass (Pseudoroegneria spicata), with that of the Eurasia grass, Crested wheatgrass (Agropyron desertorum), in dense Ar. tridentata stands. Shrubs may play an important role as nurse plants for seedling establishment (reduced solar radiation, “island of fertility” effect) but result in highly contrasting light environments and root interference for seedlings. In experimental plots, we examined effects of Ar. tridentata shade levels (0, 40, 70, and 90% reduction of solar radiation) and initial root exclusion (present/absent) on the establishment and growth of P. spicata and Ag. desertorum seedlings. With this design we evaluated the interference effects of Ar. tridentata on the two grasses and identified the most beneficial microsites for grass restoration in Ar. tridentata–dominated communities. We predicted seedling survival and growth to be greater under moderate shade (40% reduction) and limited root competition than under no or strong shade conditions (0 and 90%) and unrestricted root interactions. Fifty to 85% of the P. spicata and Ag. desertorum seedlings survived the dry summer months of 1995 and 1996 and the intervening winter. Neither shading nor root exclusion from Ar. tridentata affected final seedling survival of either species. Seedling biomass of both grass species was negatively affected by initial root interactions with Ar. tridentata. However, the analysis of seedling biomass variability (coefficient of variation) indicated that in all shade and root‐exclusion treatments, some seedlings of both species developed to large individuals to survive in Ar. tridentata–dominated rangelands. Thus, the use of interseeding techniques shows promise for restoring herbaceous species in dense Ar. tridentata stands and should be given further consideration when shrub retention is an important consideration.     

The importance of seed trees in the dioecious conifer Pilgerodendron uviferum for passive restoration of fire disturbed southern bog forests

Biological legacies are important for ecosystem recovery following disturbance as demonstrated by studies in the northern hemisphere. Southern bog forests dominated by the conifer Pilgerodendron uviferum in Northern Patagonia are a typical case of an ecosystem with low resilience to disturbance by fire, which kills most trees and seeds, on which the species depends for regeneration. In this study, we hypothesize that the natural recovery of P. uviferum populations in burned areas is limited by seed availability and this limitation may be exacerbated by the dioecy of the species. Using a multi‐scaled approach, we quantified the seed dissemination potential from P. uviferum seed trees, assessed the suitability of substrates for the germination of seedlings, and finally analysed the spatial distribution of seed trees of the species at the landscape level. Our results indicate that 70 years after a fire on Chiloé Island (43°S), natural regeneration from seed trees can assist the recovery of P. uviferum populations following large‐scale fire disturbance, but their effect is limited at a landscape level owing to a low number of reproductive female trees (0.3 trees ha^?1) and limited seed dispersal (<20 m). In this context, a mixed passive‐active restoration approach that takes into account the spatial pattern and sex of seed trees could be the most effective and efficient option to restore not only P. uviferum forests in North Patagonia, but also other heavily disturbed forests with few remnant seed trees, in particular of dioecious species.     

Legacy microsite effect on the survival of bitterbrush outplantings after prescribed fire: capitalizing on spatial variability to improve restoration

Restoration of shrubs in arid and semi‐arid rangelands is hampered by low success rates. Planting shrub seedlings is a method used to improve success in these rangelands; however, it is expensive and labor intensive. The efficiency of shrub restoration could be improved by identifying microsites where shrub survival is greater. Bitterbrush (Purshia tridentata Pursh DC) is an important shrub to wildlife that has declined because of conifer encroachment, excessive defoliation, wildfires, and low recruitment. We investigated planting bitterbrush seedlings in western juniper (Juniperus occidentalis ssp. occidentalis Hook) encroached shrublands after prescribed fire was used to control trees. Bitterbrush seedlings were planted in under (canopy) and between (interspace) former juniper canopies at five blocks and evaluated for three growing seasons. Bitterbrush survival was greater than 50% in the former canopy, but only 5% in the interspace microsite by the third growing season. Growth of bitterbrush was also greater in the former canopy compared with the interspace, potentially due to markedly less perennial vegetation in this microsite. Exotic annual grasses and annual forbs became prevalent in the former canopy in the second and third growing season, suggesting that soil resource availability was greater in this microsite. These results suggest that restoration success will vary by specific locations within a burned landscape and that this variability can be used to improve restoration efficiency. In this situation, bitterbrush restoration can be improved by planting seedlings in former canopy compared with interspace microsites.     

Use of Shrubs as Nurse Plants: A New Technique for Reforestation in Mediterranean Mountains

Common techniques currently used for afforestation in the Mediterranean basin consider the pre‐existing vegetation (mainly shrubs) as a source of competition for trees, and consequently it is generally eliminated before planting. Nevertheless, it has been demonstrated that woody plants can facilitate the establishment of understory seedlings in environments that, like the Mediterranean area, are characterized by a pronounced dry season. In this study, we experimentally analyze the usefulness of shrubs as nurse plants for afforestation of two native conifers, Pinus sylvestris L. (Scots pine) and Pinus nigra Arnold (black pine). Two‐year‐old seedlings were planted in four microhabitats: (1) open interspaces without vegetation (which is the usual method used in afforestation programs), (2) under individuals of Salvia lavandulifolia, (3) under the north side of spiny shrubs, and (4) under the south side of spiny shrubs. Pine survival was remarkably higher when planted under individuals of the shrub S. lavandulifolia (54.8% for Scots pine, 81.9% for black pine) compared with open areas (21.5% for Scots pine, 56.8% for black pine; chi square, p < 0.05). The survival of both pines was also higher when planted on the north side of spiny shrubs, although the survival on the south side was similar to that found in open areas. In addition, pine growth was not inhibited when planted in association with shrubs. This pattern appears to result from the combination of abiotic conditions imposed by the presence of a nurse shrub, which leads to improvement in seedling water status and therefore reduced summer mortality by drought. The results show that the use of shrubs as nurse plants is a technique that offers both economic and ecological advantages, in terms of savings in labor and plant material and reduced and even negligible impact on the pre‐existing vegetation.     

Benefits of Using Shrubs as Nurse Plants for Reforestation in Mediterranean Mountains: A 4-Year Study

Shrubs are commonly considered competitors of planted seedlings in reforestation programs. However, shrubs can facilitate the establishment of understory seedlings in environments that, like Mediterranean‐type ecosystems, are characterized by harsh environmental conditions. In 1997, an experiment was set up in the Sierra Nevada mountains (southeast Spain) to test the use of shrubs as nurse plants for an alternative reforestation technique. Two‐year‐old seedlings of Pinus sylvestris and Pinus nigra were planted in four microhabitats: (1) open interspaces without vegetation (which is the usual method employed in reforestation programs), (2) under individuals of the shrub Salvia lavandulifolia, (3) under the north side of spiny shrubs, and (4) under the south side of spiny shrubs. Seedlings were also distributed in plots with and without ungulates to test the effect of herbivore damage. We report here the results of survival and growth after four growing seasons, a time span long enough to draw robust conclusions concerning the suitability of this technique. Pine survival was remarkably higher when planted under individuals of S. lavandulifolia as compared with open areas (2.6 times for P. sylvestris and 1.8 for P. nigra). The survival of both pine species was also higher when planted on the north side of spiny shrubs, while mortality on the south side was similar to that found in open areas. The reduction of solar radiation by the canopy of shrubs was likely the main factor determining shrub facilitation. The growth of the pines differed among years. However, growth was not inhibited when planted with shrubs as compared with open areas in any of the years. Herbivore damage was low but was mostly concentrated in the leader shoot, exacerbating the deleterious effect of ungulate herbivores on pine growth. We conclude that the use of shrubs as nurse plants for reforestation is a viable technique to increase establishment success of reforestation in Mediterranean‐type ecosystems and that it might be similarly useful in other water‐stressed environments. In addition, this technique offers the advantage of following natural succession, thus minimizing the impact in the community.     

Facilitation and herbivory during restoration of California coastal sage scrub

Using nurse plants to facilitate native plant recruitment in degraded habitats is a common restoration practice across various arid and semiarid environments. Living trees or shrubs are typically considered nurse plants, whereas dead shrubs left in the landscape from prolonged drought are understudied prospective facilitators for native plant recruitment. The interaction between nurse plants and biotic pressures, such as herbivory, on native recruitment is also not well understood in semiarid plant communities. We investigated the effects of facilitation and herbivory on native seedling germination, growth, and survival in the restoration of degraded coastal sage scrub (CSS) habitat. Native shrub seedlings (Artemisia californica and Salvia mellifera) were planted, and native annual species (Amsinckia intermedia, Deinandra fasciculata, Phacelia distans, and Pseudognaphalium californicum) were sown in three Shrub Type treatments (live shrub, dead shrub, and exposed areas), with a nested Cage treatment (no cage and cage) in each Shrub Type treatment. Annual species grew equally well in all Shrub Type treatments; shrub seedlings grew largest in exposed areas. While there was little evidence of facilitation for all species tested, there were strong positive effects of caging on growth and establishment of all species. Caging palatable native species or planting species with anti‐herbivory traits around target plants may be more strategic approaches compared to using nurse plants in restoring degraded CSS after extended drought.     

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.     

Facilitation and interference underlying the association between the woody invaders Pyracantha angustifolia and Ligustrum lucidum

Questions: 1. Is there any post‐dispersal positive effect of the exotic shrub Pyracantha angustifolia on the success of Ligustrum lucidum seedlings, as compared to the effect of the native Condalia montana or the open herbaceous patches between shrubs? 2. Is the possible facilitation by Pyracantha and/or Condalia related to differential emergence, growth, or survival of Ligustrum seedlings under their canopies? Location: Córdoba, central Argentina. Methods: We designed three treatments, in which ten mature individuals of Pyracantha, ten of the dominant native shrub Condalia montana, and ten patches without shrub cover were involved. In each treatment we planted seeds and saplings of Ligustrum collected from nearby natural populations. Seedlings emerging from the planted seeds were harvested after one year to measure growth. Survival of the transplanted saplings was recorded every two month during a year. Half of the planted seeds and transplanted saplings were cage‐protected from rodents. Results: Ligustrum seedling emergence did not differ among treatments while growth was significantly higher in the absence of shrub cover. Sapling survival was significantly higher under the canopy of Pyracantha, intermediate under Condalia, and lowest in the absence of shrub cover. Caging did not affect growth but enhanced seedling emergence and sapling survival. Conclusion: The differential sapling survival in the shrub canopy treatments is consistent with natural sapling distribution. Pyracantha and, less so, Condalia, has a nurse‐plant effect on Ligustrum. This results from contrasting effects of the shrubs on different stages of the life cycle of Ligustrum: no effect on seedling emergence, negative on seedling growth, and positive on sapling survival. This suggests that efforts to control the expansion of Ligustrum over the landscape should tackle Pyracantha as well.     

Pinus monophylla establishment in an expanding Pinus‐Juniperus woodland: Environmental conditions,facilitation and interacting factors

Abstract. The tree species comprising Pinus‐Juniperus woodlands are rapidly expanding into shrub‐grasslands throughout their range. Observational studies indicate that establishment is facilitated by nurse plants, but little information exists on the mechanisms involved. I examined both abiotic and biotic factors influencing Pinus monophylla establishment in Artemisia tridentata steppe with expanding populations of P. monophylla and Juniperus osteosperma. I determined soil water contents, temperatures, and nutrient characteristics for the primary establishment microhabitats, i.e. under Pinus, under Juniperus, under Artemisia, tree interspace and sage interspace, and evaluated the emergence and survival response of two seedling cohorts over a 3‐yr period for the different microhabitats. I also examined the effects of seed burial and predation on seedling establishment. Microhabitats under trees and shrubs had higher extractable P and K, higher organic matter, total nitrogen and cation exchange capacity than interspace microhabitats. Soil water contents (0–15 cm) were lower in interspaces than under shrubs or trees due to dry surface (0–5 cm) soils. Soil temperatures (at 1 and 15 cm) were lowest under trees, intermediate under shrubs, and highest in interspaces. Timing and rate of seedling emergence were temperature dependent with the order of emergence paralleling mean growing season temperatures: tree interspace = shrub interspace > under shrub > under Juniperus under Pinus. Seed burial was required for rooting and the highest emergence occurred from depths of 1 and 3 cm indicating that caching by birds and rodents is essential and that animals bury seeds at adequate if not optimal depths for emergence. Seedlings required microenvironmental modification for survival; all seedlings, including those that emerged from seeds and transplants, died within the first year in interspace microhabitats. Survival in under‐tree or under‐shrub microhabitats depended on soil water availability and corresponded closely to soil water contents over the 3‐yr study. Under‐shrub microhabitats had more favourable soil and micro‐environmental characteristics than under‐tree microhabitats and had the highest seedling life spans for the first‐year seedling cohort. Predation of Pinus seedlings by rodents was a significant cause of mortality with caged transplants exhibiting life spans that were 74% longer overall than uncaged transplants. Emergence and survival of P. monophylla within the expanding woodland were dependent upon a complex set of interacting factors including growing season conditions, microhabitat characteristics, and animal species.     

Feedbacks between canopy composition and seedling regeneration in mixed conifer broad‐leaved forests

To address the role of canopy‐seedling feedbacks in the structure and dynamics of mixed conifer broad‐leaved forests in the eastern US, we monitored seedling regeneration patterns and environmental conditions in the understorey of stands dominated by either hemlock (Tsuga canadensis) or red oak (Quercus rubra) for three years. Hemlock seedlings were favoured over other species’ seedlings in hemlock stands (a true positive feedback), due to a combination of high seed inputs, high seedling emergence and relatively high seedling survival during the growing season, which allowed hemlock to remain dominant under its own canopy. Red oak stands favoured a suite of mid‐successional broad‐leaved species over hemlock. A more even age structure of broad‐leaved species in red oak stands revealed that high seedling survival in such stands were driving this feedback. Canopy‐mediated variations in both understorey light availability (1.5% for hemlock vs 3.5% for red oak) and soil pH (3.9 for hemlock vs 4.4 for red oak) were found to be the primary correlates of stand‐level differences in seedling regeneration dynamics. In mixed temperate forests in the eastern US, canopy‐seedling feedbacks could act to slow successional trajectories and contribute to the maintenance of a stable landscape structure over many generations.     

Direct and indirect interactions among plants explain counterintuitive positive drought effects on an eastern Mediterranean shrub species

Coexistence of woody and herbaceous plants may be governed by a complex set of direct and indirect interactions, whose relative importance have been rarely assessed. We experimentally studied woody species establishment in a mixed plant community by disentangling the potential role of such biotic interactions and the effect of environmental variations on them. Seedling establishment of the common eastern Mediterranean shrub species Sarcopoterium spinosum was investigated under different rainfall and light conditions, combined with the effect of the presence of adult shrubs and annual neighbors. We predicted that seedlings will be directly affected by competition with annuals with increasing water availability, while direct effects of adult shrubs will be positive via amelioration of water stress. Indirect effects were expected beneath shrub canopies due to reduced water stressed and light availability for both annuals and shrub seedlings, which may intensify competition between annuals and shrub seedlings. To test these predictions we performed field and garden experiments in which we combined manipulation of shrub and annual presence with manipulations of water availability and light conditions to simulate the effect of shrub canopy. In contrast to our prediction, shrub seedling establishment was not facilitated but inhibited by adult shrubs because of light limitation. As expected, annuals had direct negative effects on shrub seedlings under wet conditions, which shifted to neutral or positive effects under dry conditions. Thus, interactions among shrubs and annuals, and in particular the release from competition during drought years, leads to a counterintuitive positive effect of drought on shrub seedling establishment. Our findings point to the importance of experimentally studying multidimensional interactions for coexistence of different life forms and to the underestimated role of light for success in water‐limited ecosystems.     

Facilitative Effects of Aloe Shrubs on Grass Establishment,Growth, and Reproduction in Degraded Kenyan Rangelands: Implications for Restoration

Under the harsh environmental conditions present in severely overgrazed, semiarid rangelands, facilitator plants offer a promising tool for ecological restoration. This study investigated facilitative effects of Aloe secundiflora—a native drought‐tolerant, unpalatable, thorny shrub—on grass establishment in degraded rangelands in Kenya. We planted native perennial grass seeds adjacent to three neighbor treatments: transplanted mature aloe shrubs, piles of thorn branches that provided similar physical protection to aloes, and control treatments with no facilitator. We monitored grass performance for three growing seasons. During the first growing season, association with aloe shrubs significantly improved seedling survival and plant size of individual grasses, whereas grass survival in thorn treatments was intermediate between aloe and control treatments. At the population level, aloe neighbor treatments were associated with the greatest grass abundance and cover in all three seasons and reproductive output in the second season. Control treatments were associated with the poorest grass performance for all three variables. The findings indicate that planting aloes can improve the effectiveness of grass reseeding for rangeland restoration, exceeding the benefits gained from the more common strategy of using thorn branch piles. The utility of aloes in particular is further enhanced by the economic value of these plants; medicinal sap can be sustainably harvested from aloes planted for restoration.     

Species Identity and Water Availability Determine Establishment Success Under the Canopy of Retama sphaerocarpa Shrubs in a Dry Environment

Seedling establishment in harsh environments is often enhanced by the proximity of adult shrubs. This information has been used in restoration work by placing seedlings of species being restored under the canopy of some shrubs. However, monitoring this process is often restricted to a single species, and comparisons with practices that protect planted seedlings against harsh conditions are scant. Similarly, few studies have supplied seedlings with water in the summer to observe the effects of water availability on the interaction. We compared sapling survival of three woody species (Olea europaea, Pistacia lentiscus, and Ziziphus lotus) under the leguminous shrub Retama sphaerocarpa and in gaps covered with piled branches that mimicked a shrub canopy. After 3 years, survival of saplings planted under Retama differed depending on species identity and water supply. Survival of Olea saplings placed under Retama shrubs was twice that under piled branches if not watered (35 ± 8 vs. 17 ± 2 %, respectively), whereas survival of saplings under Retama, if watered, was less (48 ± 11 vs. 68 ± 8%, respectively). Retama shrubs had a negative effect on Ziziphus; most saplings died under its canopy, whereas survival in piled branches ranged 10–54%. Pistacia was neither facilitated nor outcompeted by Retama. Facilitation of Olea by Retama shrubs was more apparent under dry conditions where watering increased competition and decreased facilitation. Overall, we conclude that Retama shrubs can help dry land restoration to a greater extent than artificial shade for Olea when not watered. The critical role played by water supply in determining nursing success rates warrants further study.     

Biting the hand that feeds: the invasive grass Schismus barbatus (Poaceae) is facilitated by,but reduces establishment of,the native shrub Ambrosia dumosa (Asteraceae)

Question: We present a study of positive and negative interactions between the invasive grass Schismus barbatus (Poaceae) and Ambrosia dumosa (Asteraceae). Ambrosia facilitates seedling establishment, and such facilitation may accelerate invasion of exotic species, which, in turn, may reduce establishment of native plants. Location: Joshua Tree National Park, California, USA. Methods: During 2003‐2004, we used field surveys to characterize the natural spatial distribution of Schismus in relation to native shrubs, and experimentally manipulated seed rain of Ambrosia and Schismus at three distances from adult Ambrosia canopies. We measured percentage germination and individual performance of both species. Field data were complemented by a greenhouse experiment that measured competition between Ambrosia seedlings and Schismus planted at three densities and five relative abundances under controlled conditions. Results: Field surveys showed that the density of Schismus is independent of Ambrosia shrubs, but growth is enhanced near shrub canopies. In our field experiment Schismus is facilitated by adult Ambrosia. Under controlled conditions, Schismus does not respond to the density of Ambrosia seedlings, but changes in density of Schismus decreased performance of Ambrosia seedlings. Conclusion: Schismus invasion may be detrimental to native perennial plant populations. Although a reduction of seedling establishment is not usually expected to slow population growth of long‐lived perennials, recent unprecedented adult mortality in this community, and the well‐documented facilitative role of Ambrosia, suggest that Schismus invasion may be of high ecological significance.     

When do nurse plants stop nursing? Temporal changes in water stress levels in Austrocedrus chilensis growing within and outside shrubs

Question: Does the proximity of shrubs affect seasonal water stress of young Austrocedrus chilensis trees (a native conifer of the Austral Temperate Forest of South America) in xeric sites? Location: A. chilensis xeric forest in northwest Patagonia, Argentina. Methods: We examined the dependence of predawn twig water potential on tree development (seedling to adult) and proximity to nurse shrubs during spring and summer. We analysed spatial associations of seedlings, saplings and adult trees with nurse shrubs, and also evaluated if trees affected shrub canopy vitality. Results: Water stress in Austrocedrus trees was affected by shrub presence. Small trees (i.e.<0.5 m in height) growing in the open were most stressed, particularly in summer. Small trees growing within a shrub canopy had low water stress and little change between spring and summer. The opposite trend, however, was true for the medium‐height category (i.e. 0.5‐1.5 m in height); trees in this size category were more stressed when growing within the shrub canopy than in the open. Larger Austrocedrus trees (i.e.>2 m in height) were not affected by shrub presence. Austrocedrus trees were spatially associated with shrubs in all height classes; however, the percentage of living shrub canopy decreased with tree height. Conclusions: In xeric areas of northwest Patagonia, the strength and direction of interactions between A. chilensis and shrubs, in terms of tree water stress, are dynamic and modulated by tree size and environmental conditions. Overall, positive effects of shrubs on early developmental stages appear to be more important than subsequent negative interactions, since nursing effects could generate a spatial association of shrubs and Austrocedrus trees that persists through later successional stages. These findings shed light on mechanisms behind successional changes, and have important conservation and management implications.     

Restoring Quercus pyrenaica forests using pioneer shrubs as nurse plants

Question: How to improve reforestation success of Quercus pyrenaica. Location: 1800 m a.s.L, southern Spain. Methods: One‐year‐old Quercus pyrenaica seedlings were planted using two treatments: (1) bare soil, using a 30‐cm diameter augur bit (conventional technique) and (2) under the canopy of a pioneer shrub, Salvia lavandulifolia, using a 12‐cm diameter augur bit. Survival and growth were monitored for six years. Our hypothesis is that the use of shrubs as nurse plants is an alternative technique of reforestation with higher success than traditional techniques, in which pre‐existing vegetation is usually considered a source of competition. The rationale for the study was that for environments with a dry season, pre‐existing vegetation buffers summer drought stress, ameliorates the water status of seedlings and thus usually increases seedling recruitment. Results: Quercus survival was 6.3 × higher when planted under individuals of the pioneer shrub as compared to open areas. Quercus seedlings under shrubs also had shoots 1.8 X longer, while the number of shoots per plant did not differ among treatments. The first summer was the period with the highest mortality (49.1% of seedlings). Summer drought was the main cause of mortality. Conclusions: The use of shrubs as nurse plants for Q.pyrenaica reforestation is a viable technique to increase establishment success. The technique could be similarly useful in other environments with a dry period and for other Quercus species. In addition, this technique offers the advantage of following natural succession, thus minimizing the impact in the community.