Does the strength of facilitation by nurse shrubs depend on grazing resistance of tree saplings?

Facilitation of tree regeneration by nurse shrubs that offer protection against large herbivores is an important driver of wood-pasture dynamics. Here we asked whether the response to facilitation by nurse shrubs depends on the grazing resistance of the protégé saplings. We experimentally tested the protective effects of the thorny Rosa rubiginosa on browsing frequency, survival, and biomass change of saplings of two species-groups, presumably differing in grazing resistance: the coniferous Abies alba and Picea abies and the deciduous Acer pseudoplatanus and Fagus sylvatica saplings. The saplings were planted under and outside (1.5 m) planted nurse shrubs, under zero, low and high grazing intensity. In total, 1920 young saplings were transplanted to 60 blocks and followed for 1 year.Although the number of saplings browsed did not differ between species-groups, the coniferous saplings showed lower resistance to cattle browsing (i.e. lower survival and growth rates) than the deciduous saplings. The less resistant coniferous saplings benefited significantly more from nurse shrubs than the more resistant deciduous species in terms of growth of the surviving saplings, but not in terms of overall survival. This was likely due to herbivory on the nurse shrubs causing incidental browsing on protégé saplings and differences in biomass off-take. At high grazing intensity facilitative effects of the nurse shrubs decreased, especially for the coniferous species.These results have important management implications for the endangered wood-pastures in Western Europe. For a sustainable management and conservation that allows tree recruitment, grazing intensity should remain low to best promote facilitation processes for all tree species, but in particular for the less resistant conifer saplings.     

Short-Term Effects of Cattle Browsing on Tree Sapling Growth in Mountain Wooded Pastures

Browsing by livestock has been identified as an important factor preventing tree regeneration in wooded pastures. Two field experiments were performed to investigate the effects of cattle browsing on tree sapling growth in a mountain-wooded pasture. Two size classes (ca. 12–17 cm and 41–59 cm) of each of 4 species (Picea abies, Abies alba, Acer pseudoplatanus and Fagus sylvatica) were exposed to zero, low and high grazing intensities. We measured the proportion of saplings browsed and the effect of browsing on growth. A mowing treatment within the zero grazing intensity treatment showed no significant effect on sapling growth. One percent of the large saplings but 25% of the small saplings escaped browsing. Saplings were better protected when surrounded by taller vegetation. The proportion of saplings browsed was not significantly different among species although evergreen tree saplings lost a larger proportion of biomass than deciduous species when browsed. Under grazing, average current-year shoot production and total aboveground biomass of all species were significantly reduced. Browsing effects tended to be smaller at the lower grazing intensity. Because the deciduous species were least reduced in aboveground growth, especially under the low grazing intensity, we conclude that at least in short-term, deciduous species are less affected by cattle browsing.     

From protégé to nurse plant: establishment of thorny shrubs in grazed temperate woodlands

Question: Thorny shrubs play keystone roles in grazed ecosystems by defending non‐protected plants against herbivores, but their establishment in grazed ecosystems is poorly understood. Which factors control establishment of recruits of thorny nurse shrubs in grazed temperate woodlands? Location: Ancient grazed temperate woodlands (52°32′N, 6°36′E), The Netherlands. Methods: We surveyed biotic and abiotic factors for saplings of thorny nurse shrubs in plots with and without saplings. To disentangle these factors, we performed a transplantation experiment over two growing seasons with nurse shrub saplings (Prunus spinosa and Crataegus monogyna) planted in two dominant vegetation types – tall unpalatable swards and short grazed lawns – half of them protected from herbivory via exclosures. Results: Plots with shrub saplings had taller surrounding vegetation, higher soil pH and higher soil moisture than plots without saplings. These plots predominantly contained unpalatable sward species, while plots without saplings mainly contained palatable lawn species. After transplantation, sapling survival was higher in exclosures than in the open, and higher in sward exclosures than in lawn exclosures. Sapling growth was higher in swards than in lawns, higher inside than outside exclosures, and higher for Prunus than Crataegus, while browsing on saplings was higher in lawns. Conclusion: Unpalatable swards form essential establishment niches for thorny shrubs in grazed temperate woodlands: they protect against herbivores before thorns fully develop in saplings, and sapling growth is better due to improved micro‐environmental conditions. Once established and thorny, shrub saplings grow out of the protective range of the swards and in turn facilitate tree seedlings, which are essential for long‐term persistence of grazed temperate woodlands. This study shows that nurse plants may start as protégés before becoming facilitators for other plants in a later life stage. This may be common for nurse plants in various ecosystems. We argue that improved understanding of establishment of nurse plants and their constraining factors is crucial for effective conservation and restoration in various ecosystems.     

Conditional outcomes in plant–herbivore interactions: neighbours matter

Spatial distribution of palatable and unpalatable plants can influence the foraging behaviour of herbivores, thereby changing plant‐damage probabilities. Moreover, the immediate proximity to certain plants can benefit other plants that grow below them, where toxicity or spines act as a physical barrier or concealment against herbivores. This paper presents the results of a multi‐scale experiment performed to test the effect of shrubs as protectors of tree saplings against herbivores and the mechanism involved in Mediterranean ecosystems. We performed a factorial design in two mountain ranges, similar in physiognomy and vegetation, planting saplings of a palatable tree, the maple (Acer opalus subsp. granatense), and an unpalatable tree, the black pine (Pinus nigra), under three different types of shrubs. We considered four experimental microhabitats: highly palatable shrub (Amelanchier ovalis), palatable but spiny shrub (Crataegus monogyna or Prunus ramburii), unpalatable spiny shrub (Berberis vulgaris subsp. australis) and control (gaps of bare soil without shrubs). Three main factors were found to determine the probability of sapling attack: sapling palatability, experimental microhabitat and plot. Palatable saplings (maples) were browsed much more than unpalatable ones (pines). The degree of protection provided by the shrub proved greater as its palatability decreased with respect to sapling palatability, the unpalatable spiny shrub being the safest microhabitat for palatable saplings and bare soil for unpalatable ones. The differences found in number of attacked saplings between plots may be attributable to differences in herbivore pressure. The community context in which interaction takes place, namely the characteristics of the neighbours and the intensity of herbivore pressure, are determining factors for understanding and predicting the damage undergone by a target plant species. The mechanism that best explains these results is associational avoidance of saplings that grow near to unpalatable shrubs. It is necessary to introduce this neighbour effect in theoretical models and food‐web approaches that analyse the plant–herbivore relationships, since it can strongly determine not only the intensity of the interaction, but also the spatial distribution and diversity of the plant community.     

Shrub facilitation drives tree establishment in a semiarid fog‐dependent ecosystem

Questions

The exceptional occurrence of tall rain forest patches on foggy coastal mountaintops, surrounded by extensive xerophytic shrublands, suggests an important role of plant–plant interactions in the origin and persistence of these patches in semi‐arid Chile. We asked whether facilitation by shrubs can explain the growth and survival of rain forest tree species, and whether shrub effects depend on the identity of the shrub species itself, the drought tolerance of the tree species and the position of shrubs in regard to wind direction.

Location

Open area–shrubland–forest matrix, Fray Jorge Forest National Park, Chile.

Methods

We recorded survival after 12 years of a ~3600 tree saplings plantation (originally ~30‐cm tall individuals) of Aextoxicon punctatum, Myrceugenia correifolia and Drimys winteri placed outside forests, beneath the shrub Baccharis vernalis, and in open (shrub‐free) areas. We assessed the effects of neighbouring shrubs and soil humidity on survival and growth along a gradient related to the direction of fog movement.

Results

B. vernalis had a clear facilitative effect on tree establishment and survival since, after ~12 years, saplings only survived beneath the shrub canopy. Long‐term survival strongly depended on tree species identity, drought tolerance and position along the soil moisture gradient, with higher survival of A. punctatum (>35%) and M. correifolia (>14%) at sites on wind‐ and fog‐exposed shrubland areas. Sites occupied by the shrub Aristeguietia salvia were unsuitable for trees, presumably due to drier conditions than under B. vernalis.

Conclusions

Interactions between shrubs and fog‐dependent tree species in dry areas revealed a strong, long‐lasting facilitation effect on planted tree's survival and growth. Shrubs acted as benefactors, providing sites suitable for tree growth. Sapling mortality in the shrubland interior was caused by lower soil moisture, the consequence of lower fog loads in the air and thus insufficient facilitation. While B. vernalis was a key ecosystem engineer (nurse) and intercepted fog water that dripped to trees planted underneath, drier sites with A. salvia were unsuitable for trees. Consequently, nurse effects related to water input are strongly site and species specific, with facilitation by shrubs providing a plausible explanation for the initiation of forest patches in this semi‐arid landscape.     

The influence of competition from herbaceous vegetation and shade on simulated browsing tolerance of coniferous and deciduous saplings

The ability of saplings to tolerate browsing (i.e. the ability to persist with reduced biomass and to compensate for biomass loss) is influenced by the level of stress and their growth strategies. Ultimately, insight into species‐specific responses of saplings to browsing, shade and competition from neighbours will help explain diversity, structure and function of grazed ecosystems such as the endangered wood‐pasture systems. We measured the survival, whole‐sapling biomass and compensatory growth responses of two coniferous (Picea abies and Abies alba) and two deciduous (Acer pseudoplatanus and Fagus sylvatica) tree species to simulated summer browsing (one single clipping event), shade (installation of a shade cloth) and neighbour removal (mowing surrounding vegetation to ground level) treatments and the interactions between them after two‐growing seasons. For all species, there were interacting effects on growth of browsing and environmental condition (shade and neighbours). Simulated browsing resulted in relatively smaller growth losses when plants were growing slowly due to competitive conditions related to herbaceous neighbours. Although none of the clipped saplings could fully compensate for their biomass losses, the saplings were closer to compensation under high competitive conditions than under low competitive conditions. Survival of the clipped saplings remained relatively high and was only significantly reduced for Picea and Acer. Picea was least tolerant of competition and was the only species for which growth was not negatively affected by strong irradiance of a mountain pasture. Surprisingly, the tolerance of saplings to herbivory as browsing tolerance was enhanced under conditions that negatively affected sapling performance (i.e. survival and growth). Apparently, the relative impact of browsing at the early sapling stage is linked to tree life history characteristics such as competition and shade tolerance and will be lower in situations with intense competitive interactions and/or strong irradiance.     

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.     

Facilitation by herbivore-mediated nurse plants in a threatened tree, Taxus baccata: local effects and landscape level consistency

We investigated the regeneration of a threatened tree, the yew Taxus baccata, in relation to the presence of fleshy‐fruited woody plants acting as seed dispersal foci as well as protecting yew recruits against ungulate herbivores. We seek to determine if local facilitative effects are consistent across landscape in the Cantabrian range (NW Spain). Yew seed rain by birds mostly concentrated under yew trees and beneath hollies Ilex aquifolium. Seedling emergence distributed similarly to seed rain, but first‐year seedling survival was higher beneath hollies. In one site where woody vegetation was structured as nucleation centres (multispecific patches of fleshy‐fruited plants acting as foci for seed rain) yew recruits mostly occurred in yew‐dominated centres, suggesting dispersers‐mediated facilitation. However, holly was the main nurse plant for most of these recruits, considering the nurse as the species whose canopy covered directly the yew recruit. Living beneath nurse plants reduced herbivore damage on saplings and enhanced seedling survival. A planting experiment with yew rooted‐cuttings beneath different spiny shrubs corroborated this effect. Additional evidence on yew recruitment limitation by herbivory emerged from one population where ungulates were fence‐excluded. Our results suggest that nurse plants mitigate the negative effect of herbivores on yew regeneration, by providing defence against browsing and trampling. Shelter ability related to nurse structure, cone‐shaped shrubs with branches at their bases acting better as a barrier. Paradoxically, this structure resulted from heavy browsing on nurse plants. The study of yew regeneration and habitat structure in seven sites provided evidence for the consistency of facilitation by holly at the landscape level, since local values of yew recruitment positively related to nurse ground cover. Range‐scale yew management must consider the local functioning of the interaction among avian seed‐dispersers, nurse fleshy‐fruited plants and ungulate herbivores, in combination with regional measures, targeting the habitats where facilitation emerges.     

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

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. grana‐tense, 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 and ungulate excluded plots. This effect was species‐specific, with shrubs increasing the survival of Acer opalus and Quercus ilex three and twofold, respectively, but having a minor effect on the Pinus species. Herbivory damage was also species‐specific, being much higher for Acer opalus than for any other species. Shrubs did not protect saplings of any species against ungulates. Thus, all Acer saplings (the most damaged species) suffered herbivory outside the exclosures, which largely reduced sapling height. Conclusions: Protection from abiotic stress (summer drought and winter frost) was much more relevant than protection from biotic stress (herbivory). However, we propose that the final balance between the two mechanisms can be expected to vary strongly between sites, depending on the relative magnitude of the different sources of stress and the intrinsic traits (e.g. palatability) of the species interacting.     

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.     

Direct versus indirect facilitation (herbivore mediated) among woody plants in a semiarid Chaco forest: A spatial association approach

In arid environments, direct facilitation (microhabitat amelioration) and indirect facilitation (‘associational resistance’ via protection from herbivory) among plants of different species may act simultaneously. Little is known about their relative effects. One way to disentangle the effects is by evaluating spatial associations. We examined the relative importance of these two mechanisms of facilitation in the semiarid Chaco vegetation of north‐central Argentina, through an eight‐way observational study in which we quantified the degree of spatial association between saplings of each of two key tree species, Schinopsis lorentzii (Anacardiaceae) and Aspidosperma quebracho‐blanco (Apocynaceae), with shrub neighbours either possessing spines or without spines and in both an ungrazed site and a site with a long history of cattle grazing. We analysed data across 400 subparcels at each site with spatial analysis by distance indices. Saplings of both tree species showed positive spatial associations with spiny shrubs in the grazed site but not in the ungrazed site, and never with non‐spiny shrubs. This result suggests that spiny shrubs may indeed provide associational resistance for saplings of key tree species in grazed habitats in these dry subtropical forests, that is, that indirect facilitation may predominate over direct facilitation. If confirmed by experimental studies, this result can have implications for the silvopastoral management of rapidly expanding ranches in the semiarid Chaco, where current practice includes the near elimination of native shrubs.     

Interactive effects of grazing and shrubs on the annual plant community in semi‐arid Mediterranean shrublands

Question: We studied the interactive effects of grazing and dwarf shrub cover on the structure of a highly diverse annual plant community. Location: Mediterranean, semi‐arid shrubland in the Northern Negev desert, Israel. Methods: Variation in the biomass and plant density of annual species in the shrub and open patches was monitored during four years, inside and outside exclosures protected from sheep grazing, in two contrasting topographic sites: north and south‐facing slopes that differed in their dominant dwarf shrubs species: Sarcopoterium spinosus and Corydothymus capitatus, respectively. Results: Above‐ground biomass, density and richness of annual species were lower under the canopy of both shrub species compared to the adjacent open patches in the absence of grazing. Grazing reduced the biomass of annuals in open patches of both topographic sites, but not in the shrub patches. On the north‐facing slope, grazing also reduced plant density and richness in the open patches, but increased plant density in the shrub patches. At the species level, various response patterns to the combined effects of grazing and patch type were exhibited by different annuals. Protection against the direct impacts of grazing by shrub cover as well as species‐specific interactions between shrubs and annuals were observed. A conceptual mechanistic model explaining these interactions is proposed. Conclusion: In semi‐arid Mediterranean shrublands grazing and dwarf shrub cover interact in shaping the structure of the annual plant community through (1) direct impacts of grazing restricted to the open patches, (2) species‐specific facilitation/ interference occurring in the shrub patches and (3) subsequent further processes occurring among the interconnected shrub and open patches mediated through variation in seed flows between patches.     

Facilitation by nurse shrubs of resprouting behavior in a post-fire shrubland in northern Patagonia,Argentina

Abstract. The nurse-plant syndrome is a widely recognized example of positive (facilitative) influences of plant species on the establishment and growth of other species. Most studies of the nurse-plant syndrome have been on species that reproduce mainly from seed rather than vegetatively. In this study, we experimentally compared the influences of two species of nurse shrubs, Schinus patagonicus and Berberis buxifolia, on the survival and growth of vegetatively reproducing herbaceous and woody plants in a post-fire shrubland in northern Patagonia, Argentina. The vegetation beneath shrubs was removed by clipping and, in a paired-sample design, one half of the canopy of each shrub was removed. We determined species richness, counted number of resprouts, and measured photon flux density and soil moisture beneath cut and uncut halves of each shrubs. Abundances of resprouts were several times greater beneath the uncut vs. the cut shrubs, as was the mean number of species. Thus, shrubs have a strong facilitating influence as measured by resprout densities and the number of species. Numbers of resprouts and of species were twice as high beneath Schinus as beneath Berberis, implying important differences in the facilitative effects of the two shrubs species. Microsites beneath Schinus were characterized by lower and more heterogeneous light levels but by greater soil moisture. Even though the reproductive mode in this post-fire shrubland is overwhelmingly vegetative, facilitation by nurse shrubs is important and differentially effective for different species of nurse shrubs.     

Coarse woody debris can reduce mammalian browsing damage of woody plant saplings in box‐gum grassy woodlands

The critically endangered box‐gum grassy woodlands of south‐east Australia face numerous threats including the failure of woody plant regeneration caused by over‐browsing. In the Australian Capital Territory, over‐browsing of tree and shrub saplings is likely caused by dense populations of Eastern Grey Kangaroo (Macropus giganteus) found in many nature reserves free of livestock. One possible way to protect these saplings is using coarse woody debris (CWD) as a browsing deterrent. We tested this idea by planting palatable Red Stemmed Wattle (Acacia rubida) saplings among manually applied CWD, among naturally fallen CWD, and in the open, in five woodland reserves. We recorded the proportion of saplings browsed, the number of weeks to first browsing and the browsing severity (sapling height lost). Applied CWD protected saplings from being browsed only at relatively low‐to‐moderate kangaroo browsing pressure (as measured by faecal pellet counts). At relatively high browsing pressure, the probability of a sapling being browsed among applied CWD was 100%, similar to the probability in the open treatment (no CWD). Natural CWD, in contrast, provided some protection even at high browsing pressures. Time to browsing was most affected by browsing pressure, although CWD cover also had an influence. Browsing severity was similar between the three treatments and was only affected by browsing pressure. These results indicate that without protection, palatable woody plant saplings have a high chance of being browsed by kangaroos in woodland reserves, and therefore, some protection is needed for successful regeneration. The CWD being applied to reserves has a limited capacity to protect regenerating saplings. If more protection is wanted a CWD structure more resembling natural fallen timber should be used. This could be done by artificially placing branches around plantings. However, the most important action to facilitate regeneration is to manage kangaroo populations to reduce overall browsing pressure.     

Safe sites for tree regeneration in wooded pastures: A case of associational resistance?

Abstract. Question: Are tree saplings in wooded pastures spatially associated with specific nurse structures or plants that facilitate tree sapling survival? Location: Wooded pastures in the Jura Mountains, Switzerland. Methods: In two sites, 73 km apart, we sampled 294 plots of 4 m², systematically distributed on 1 ha. We recorded number and height of all established Picea abies saplings (> 1 a of age and up to 40 cm in height), and visually estimated cover of rocks, shrubs, tree stumps, overhanging tree branches and unpalatable plant species. Results: Despite differences in site characteristics, we found overall positive effects of cover of unpalatable plants, rocky outcrops and tree stumps on the density of Picea saplings. Plots with tree stumps and higher cover of rocky outcrops and unpalatable plants were more likely to contain Picea saplings. Conclusions: Unpalatable plants, rocky outcrops and tree stumps seem to form safe sites for Picea saplings in this grazed system, improving their establishment and survival. Our findings support the idea that associational resistance drives the dynamics of wooded pastures, but experimental evidence for this hypothesis is still required.     

Extreme drought stress shifts net facilitation to neutral interactions between shrubs and sub‐canopy plants in an arid desert

The stress gradient hypothesis (SGH) predicts that the importance or intensity of competition and facilitation will change inversely along abiotic stress gradients. It was originally postulated that increasing environmental stress can induce a monotonic increase in facilitation. However, more recent models predicted that the relationship between severity and interaction exhibits a hump‐shaped pattern, in which positive interactions prevail under moderate stress but decline at the extreme ends of stress gradients. In the present study, we conducted a field experiment along a temporal rainfall gradient for five consecutive years, in order to investigate interactions in a shrub‐herbaceous plant community at the southern edge of the Badain Jaran Desert, and, more specifically, investigated the effects of Calligonum mongolicum, a dominant shrub species, on both abiotic environmental variables and the performance of sub‐canopy plant species. We found that shrubs can improve sub‐canopy water regimes, soil properties, plant biomass, density, cover, and richness and, more importantly, that the positive effect of shrubs on sub‐canopy soil moisture during the summer diminishes as rainfall decreases, a pattern that partly explains the collapse of the positive interaction between shrubs and their understory plants. These results provide empirical evidence that the positive effect of shrubs on understory plant communities in extreme arid environments may decline and become neutral with increasing drought stress.     

The shrub Rhodomyrtus tomentosa acts as a nurse plant for seedlings differing in shade tolerance in degraded land of South China

Questions: What are the nurse effects of Rhodomyrtus tomentosa in degraded land of South China? Are canopy or soil factors responsible for the main nurse effect? Do facilitative effects increase with the shade tolerance of the target species? Location: Degraded shrubland in South China. Methods: Seedlings of three native climax woody species (Schima superba, Michelia macclurei, Castanopsis fissa) that differ in shade tolerance were subjected to four treatments by transplantation: (1) under the canopy of R. tomentosa shrubs; (2) in open interspaces without vegetation cover (control); (3) under the canopy of R. tomentosa from which canopies had been removed; and (4) in open interspaces without vegetation but covered by branches and leaves of R. tomentosa. Results: At low soil nutrient levels, increased canopy shade, soil porosity and soil moisture under the canopy of R. tomentosa increased seedling survival of the climax tree species S. superba, C. fissa and M. macclurei, and shoot height of S. superba. The nurse effect (a form of facilitation) of R. tomentosa depended more on canopy shade than on soil amelioration. The magnitude of facilitation or nurse effect was positively correlated with shade tolerance of the target species. Conclusions: Use of nurse plants in restoration differs from traditional reforestation (clearing and/or burning to reduce interspecific competition between target tree species and non‐target species) because it focuses on positive interactions between nurse plants and target plants that increase establishment of target species and reduce time required for restoration. Because nurse effects of R. tomentosa shrubs tended to be larger on target species with greater shade tolerance, shade‐tolerant plants are suggested as target species to accelerate restoration.     

Comparative ecophysiological responses to drought of two shrub and four tree species from karst habitats of southwestern China

Drought stress is one of the most important factors in limiting the survival and growth of plants in the harsh karst habitats of southwestern China, especially at the seedling establishment stage. The ecophysiological response to drought stress of native plants with different growth forms is useful for re-vegetation programs. Two shrub and four tree species were studied, including Pyracantha fortuneana (evergreen shrub), Rosa cymosa (deciduous shrub), Cinnamomum bodinieri (evergreen tree), and other three deciduous trees, Broussonetia papyrifera, Platycarya longipes, and Pteroceltis tatarinowii. The seedlings were randomly assigned to four drought treatments, i.e., well-watered, mild drought stress, moderate drought stress, and severe drought stress. Leaf water relations, gas exchange, chlorophyll fluorescence, and growth of the seedlings were investigated. Under severe drought stress, the two shrubs with low leaf area ratio (LAR) maintained higher water status, higher photosynthetic capacity, and larger percent biomass increase than the most of the trees. The two shrubs also had lower specific leaf area, greater intrinsic water use efficiency, and thermal dissipation than the trees. This suggested that the two shrubs had high tolerance to severe drought and were suitable for re-vegetation in harsh habitats. The evergreen C. bodinieri exhibited higher leaf mass ratio (LMR) and LAR than the deciduous species under mild and moderate stress. However, the low maximum quantum efficiency of PSII photochemistry (F _v/F _m) and net assimilation rate, and the sharp decreases of water potential, LMR, LAR, and biomass under severe stress indicated C. bodinieri’s weak tolerance to severe drought. In response to drought stress, the three deciduous trees revealed sharp reductions of biomass due to the large drought-induced decreases of gas exchange, LAR, and LMR. Under drought conditions, the deciduous trees minimized water loss by stomatal closure and by reducing transpiration leaf area and light harvesting through shedding leaves. This suggested that the three deciduous trees were more sensitive to water availability than the shrubs and used avoidance strategies against drought stress. However, the better growth performance of the deciduous trees than that of the shrubs under favorable conditions suggested that deciduous trees could be suitable for habitats with mild and temporary drought stress.     

Early spring leaf out enhances growth and survival of saplings in a temperate deciduous forest

Saplings of many canopy tree species in winter deciduous forests receive the major portion of their light budget for their growing season prior to canopy closure in the spring. This period of high light may be critical for achieving a positive carbon (C) gain, thus contributing strongly to their growth and survival. This study of saplings of Aesculus glabra and Acer saccharum in Trelease Woods, Illinois, USA, tested this hypothesis experimentally by placing tents of shade cloth over saplings during their spring period of high light prior to canopy closure in three consecutive years. Leaf senescence began 16 days (year 0) and 60 days (year 1) earlier for shaded A. glabra saplings than control saplings. No change in senescence occurred for A. saccharum. The annual absolute growth in stem diameter of both species was negligible or negative for shaded saplings, but positive for control saplings. Only 7% of the shaded A. glabra saplings were alive after 2 years, while all control saplings survived for 3 years; only 20% of the shaded A. saccharum saplings survived for 3 years, while 73% of control saplings were alive after the same period. Early spring leaf out is a critical mechanism that allows the long-term persistence of saplings of these species in this winter deciduous forest. Studies and models of C gain, growth, and survival of saplings in deciduous forests may need to take into account their spring phenology because saplings of many species are actually “sun” individuals in the spring prior to their longer period in the summer shade.     

Distribution of seedlings and saplings of Quercus robur in a grazed deciduous forest

Seedling and sapling distribution of the deciduous tree Quercus robur was, studied in a grazed, partly open forest, ca. 20 km S of Linkoping, Sweden. Numbers of seedlings and saplings were compared between open and shaded parts of the forest. Cover values for the shrub and tree layers, proportion of bare ground and of stones and soil water content were determined. Q. robur was shown to germinate equally well in open and shaded parts of the forest; however, most saplings were found in the shade. No correlation was found between seedlings or saplings and cover of shrub layer or proportion of stones in the soil surface or with soil water content.