Pioneer Shrub Research Articles

Recruitment responses of shade‐tolerant and heliophilous trees in degraded areas: The necessity of knowing the recruitment autecology of species for effective reforestation decisions

AbstractDirect seeding is a reforestation technique that allows introducing a great variety of tree species in degraded areas with less logistics, but tree establishment rates are usually low because developing plants do not tolerate the high sun exposure, extreme temperatures, and low soil moisture that prevail in these habitats. This leads to the proposal that seeds should be planted beneath shrubs or herbaceous plants that ameliorate harsh abiotic conditions, although this can also trigger interspecific competition and impair the performance of developing trees. This study aimed to test these proposals in dry forests in northwest Argentina. For this, seeds of two shade‐tolerant trees and a heliophilous tree, native to this region, were sowed beneath the canopy of pioneer shrubs and in open spaces with and without herbaceous plants. After 2 years, our findings indicated that shade‐tolerant trees performed better beneath shrub canopies, while the heliophilous tree achieved higher establishment rates in the open spaces. Nevertheless, it seems that herbaceous plants compete with the tree seedlings and, therefore, they should be removed before conducting reforestation programs. Thus, we propose that direct seeding could be an efficient reforestation strategy, but the regeneration autecology of the tree species selected for this task, as well as their competitive ability in the face of the pioneer vegetation, must be carefully evaluated before seeding.

Trophic facilitation in forest restoration: Can Nothofagus trees use ectomycorrhizal fungi of the pioneer shrub Leptospermum?

The benefits of plant-to-plant facilitation in ecological restoration are well recognized, yet the potential for indirect trophic facilitation remains understudied. Nothofagus (southern beech; Nothofagaceae) is an iconic southern hemisphere tree genus that is frequently the focus of ecological restoration efforts. One aspect of Nothofagus ecology that may limit restoration success is the availability of appropriate ectomycorrhizal fungi. It has been suggested that pioneer dual-mycorrhizal hosts such as Leptospermum species (Myrtaceae) could facilitate Nothofagus establishment by providing fungal inoculum, but the capacity for Nothofagus to use Leptospermum ectomycorrhizal fungi is unknown. To investigate potential indirect facilitation, we conducted a common garden pot trial to determine if Nothofagus cliffortioides (mountain beech) can use symbionts from Leptospermum scoparium (mānuka) ectomycorrhizal communities. Nothofagus and Leptospermum seedlings were grown in monoculture and mixed pairs with reciprocal "home" and "away" soil fungal inoculum. ITS2 metabarcoding of eDNA from hyphal ingrowth bags revealed that Nothofagus and Leptospermum inoculum contained different ectomycorrhizal fungal communities, but that half of the common ectomycorrhizal taxa identified were found in both soil types, suggesting generalist fungi exist. Nothofagus was able to form associations with some fungal species originating from Leptospermum inoculum, however, probable spore contamination meant that the proportion of root colonization associated with those species was ambiguous. Root ectomycorrhizal colonization rates were positively associated with seedling biomass, and there was some evidence of a home soil inoculum advantage in Nothofagus, but these effects were minor. Additionally, we found evidence that home inoculum provides a protective advantage against drought stress for Leptospermum seedlings. Our results indicate the potential for using Leptospermum to promote Nothofagus establishment in restoration plantings and highlight the possible benefits of considering fungal mutualists in ecological restoration projects.

Compost promotes biocrust and plant growth in greenhouse cocultures for dryland restoration

Biocrusts and plants are the main ground cover and provide multiple ecosystem functions in arid and semiarid areas. As land degradation worsens and waste management is ever more pressing, the use of composted waste appears as a sustainable restoration practice to enhance vegetation reestablishment, but its effects on biocrusts still remain largely understudied. Moreover, developing appropriate methods for the co‐introduction of plants and biocrusts can be an emerging approach to reconstruct the biocrust/plant interaction and their associated functions. We evaluated the effect of different compost types on biocrust and plant cocultivations. We performed a greenhouse mesocosm experiment using urban organic solid waste and sewage sludge composts to grow a moss‐dominated biocrust and seedlings of two pioneer shrubs (Senecio filaginoides and Acaena splendens) in the Patagonian steppe. Both compost types successfully enhanced biocrust cover and chlorophyll a development, with cultured biocrusts being compositionally similar to the natural inoculum. Only the addition of composted sewage sludge enhanced seedling growth, and biocrusts had significant but opposite effects (positive/negative) on the two plant species we used for experimentation. This difference in the biocrust/plant interaction outcome was likely mediated by the significant influence of biocrusts on the relative availability of ammonium and nitrate and possible plant species‐specific preferences for different inorganic nitrogen forms. We conclude that the application of sewage sludge compost effectively improves the success of biocrust and plant cocultures, but species‐specific differences in the performance of seedlings growing in biocrusted soils impose limits on attempts to promote biocrust and plant joint restoration.

Effects of drought and nutrient deficiencies on the allocation of recently fixed carbon in a plant-soil-microbe system.

Carbon (C) allocation plays an important role in plant adaptation to water and nutrient stresses. However, the effects of drought and nutrient deficiencies on the allocation of recently fixed C in the plant-soil-microbe system remain largely unknown. Herein, we studied the response of C allocation of Sophora moorcroftiana (an indigenous pioneer shrub in Tibet) to drought, nitrogen (N) deficiency and phosphorus (P) deficiency using a microcosm experiment. The 13CO2 continuous labeling was used to trace C allocation in the plant-soil-microbe system. We found that drought significantly reduced plant 13C, but it increased 13C accumulation in soil. The decreased plant 13C under drought was attributed to the decrease of 13C in stem and root rather than that in leaf. The excess 13C fraction in the microbial biomass (MB13C) was reduced by N deficiency, but it was not affected by the combination of drought and N deficiency, indicating that drought weakened the effects of N deficiency on MB13C. By contrast, MB13C increased under the combination of drought and P deficiency, suggesting that drought enhanced the effects of P deficiency on MB13C. Drought and nutrient deficiencies regulated the belowground 13C allocation. Specifically, drought and P deficiency increased the allocation of 13C to root and N deficiency regulated the allocation of 13C to microbial biomass C and dissolved organic C in soil. Notably, soil 13C decreased with increasing plant 13C, while MB13C first decreased and then increased with increasing plant 13C. Overall, our study demonstrated that drought and nutrient deficiencies interactively affected C allocation in a plant-soil-microbe system and provided insights into C allocation strategies in response to multiple resource (water and nutrient) stresses under environmental changes.

Effect of stabilization time and soil chromium concentration on Sesbania virgata growth and metal tolerance

Sesbania virgata is a pioneer shrub from the Fabaceae family, native to riparian environments in northeast of Argentina, southern of Brazil and Uruguay. In peri-urban riparian soils, metal contamination is a frequent problem, being its bioavailability partly determined by the stabilization time and frequency of contamination events. The effect of time elapsed between chromium (Cr) soil enrichment and plant seeding and Cr doses on S. virgata tolerance and metal absorption were evaluated. Treatments were developed by adding Cr (80–400 ppm) to the soil and allowing two days or fifteen months to elapse before sowing, and a control treatment without Cr addition. After 150 days from seeding, bioaccumulation and translocation factors, growth parameters (dry biomass and its aerial/radical allocation pattern, stem length and its elongation rate), morphological parameters (root volume and leaf area), and physiological parameters (chlorophyll content) of the specimens were determined. The emergence of S. virgata was inhibited since 150 ppm when Cr was added to the soil two days before seeding, with Cr accumulation in roots starting at 80 ppm (17.4 ± 2.5 mg kg−1). Under 15 months of metal stabilization, S. virgata plants survived across the entire range of Cr doses tested, with accumulation in roots since 100 ppm (35.5 ± 0.2 mg kg−1) and metal translocation to aerial tissues only under 400 ppm. The results obtained showed that S. virgata did not have high BCF and TF values, suggesting that it cannot be classified as bioaccumulator of Cr under the tested conditions. However, its presence in environments contaminated with Cr can be beneficial, as it helps to stabilize the metal in the soil.

Change in the Germination Performance of Tamarix Ramosissima Seeds Following Storage

The small seeds ofTamarix ramosissima, a pioneer shrub or small tree used for sand fixation and afforestation in desert areas, are suspected to have short longevity and as such not be suitable for storage. However, this has not been experimentally verified. In this investigation, germination of T. ramosissima seeds stored under different temperatures (room temperature (25°C), 5°C and -18°C) for different periods (0, 30, 60, 90 and 120 days) were tested for moisture content and germination capacity. Seed moisture content and germination percentage decreased with increase of storage period and storage temperature. For seeds stored at room temperature, germination percentage decreased to 58% after 30 days and to 0% by 120 days. However, for seeds stored at -18°C for 120 days, germination percentage was 53%. These results indicate that T. ramosissima seeds have short longevity and are not suitable for long-term storage, particularly at room temperature. of plant adaptation to changing environments (Rajjou and Debeaujon, 2008). Although seeds of many plants can survive for long periods, some small seeds have short longevity (Baskin and Baskin, 2014). The rate of loss of seed viability is dependent on various biotic and abiotic factors, such as seed characteristics (Sanoet al., 2015; Kundu et al ., 2020),

Tropical Cyclone Disturbances Induce Contrasting Impacts on Forest Structure, Plant Composition, and Soil Properties in Temperate Broadleaf and Coniferous Forests

Knowledge of forest recovery processes after severe disturbances, such as tropical cyclones, is essential for understanding the mechanisms maintaining forest diversity and ecosystem functioning. However, studies examining the impact of tropical cyclones on forest dynamics are still rare, especially in Northeast Asia. Here, we explore the complex responses of vegetation and soil chemistry to severe tropical cyclone disturbances in Hallasan National Park in South Korea. Vegetation and soil were examined five years before and five years after passages of tropical cyclones in 2012 in natural broadleaf and coniferous forests along an elevation gradient from 950 to 1770 m a.s.l., including the largest population of endemic Abies koreana. Tropical cyclones caused abundant tree mortality, resulting in a 46% decrease in stem basal area. Tropical cyclone disturbances triggered the spread of pioneer trees and shrubs and intense clonal propagation of graminoids, including dwarf bamboo, resulting in less diverse understory vegetation, especially in coniferous forests. In contrast, broadleaf forests at lower elevations experienced only minor disturbance. Opening of canopies after tropical cyclone disturbance led to the decrease in soil cations and doubling of available soil phosphorus stock, which likely contributed to the increased coverage of clonal graminoids, especially in coniferous forests (from 36 to 66%). Hence, increased competition of graminoids and reduction in seed fall and seedling recruitment prevented A. koreana regeneration. The subalpine forest dominated by A. koreana is forecasted to gradual loss due to the fact of temperature increases, intensified tropical cyclones and, particularly, due to the altered competition between dwarf bamboo and fir seedlings.

Water use characteristics of different pioneer shrubs at different ages in western Chinese Loess Plateau: Evidence from δ2H offset correction

Water use characteristics of different pioneer shrubs at different ages in western Chinese Loess Plateau: Evidence from δ2H offset correction

Radial Stem Growth of the Clonal Shrub Alnus alnobetula at Treeline Is Constrained by Summer Temperature and Winter Desiccation and Differs in Carbon Allocation Strategy Compared to Co-Occurring Pinus cembra

Green alder (Alnus alnobetula) is currently the most expanding shrub species in the Alps. Because dense thickets impair tree establishment, understanding how climate affects shrub growth is essential for predictions of treeline dynamics. We evaluated ring width data from >50 A. alnobetula stems sampled at treeline on Mt. Patscherkofel (Central European Alps, Austria) to identify main climatic drivers and influence of climate warming on radial stem growth (RG). We also compared RG of A. alnobetula with RG of the co-occurring treeline conifer Swiss stone pine (Pinus cembra). We addressed our questions through calculation of response functions and evaluation of climate in years showing exceptional growth deviations. Response function analyses and evaluation of growth trends during 1991–2020 revealed that RG of A. alnobetula is significantly and directly related to summer temperatures. Precipitation in January also showed a direct relationship to RG, indicating effects of frost drought on RG. Surprisingly, nitrogen fixing A. alnobetula showed strikingly lower RG compared to P. cembra, and the latter also responded more strongly to the increase in summer temperature in the course of climate warming. We explain these findings by different carbon allocation strategies, i.e., preference of “vertical” stem growth in late successional P. cembra vs. favoring “horizontal” spread in the pioneer shrub A. alnobetula.

Restoration of tropical montane cloud forest in bracken dominated pastures: The role of nurse shrubs

Bracken (Pteridum arachnoideum) is an invasive fern species that slows or inhibits forest regeneration over vast areas worldwide, affecting biodiversity, ecosystem functions and productivity. To overcome arrested succession, effective restoration interventions are necessary. We examined the effects of planting pioneer shrubs on establishment success in mid- to late-successional tree species transplants, and their combined effects on bracken dominance in abandoned pastures of former cloud forest in Mexico. We hypothesised that the rapid development of nurse shrubs will reduce bracken growth and facilitate mid- to late-successional tree species establishment. We analysed the effects of planting cuttings of the shrubs Tithonia diversifolia and Sambucus nigra on bracken and on the seedling performance of Juglans pyriformis, Quercus insignis, Meliosma alba and Tapirira mexicana, in three sites under four treatments: (1) trees without shrubs, (2) trees with T. diversifolia, (3) trees with S. nigra, and (4) trees with both T. diversifolia and S. nigra. Increased transplant performance was expected under mixed shrub species, compared to single shrub species plantings, due to complementarity in the use of resources (niche separation). After two years, plantings with shrubs and trees reduced bracken cover by >60% and bracken live rhizome biomass by >50%. Shrubs effects were species-specific, reflecting a complex combination of facilitation and competition mechanisms. In comparison to the trees without shrubs, survival of M. alba and Q. insignis significantly increased with T. diversifolia, T. mexicana survival decreased with S. nigra and J. piriformis survival decreased with T. diversifolia and the mixture. The results support the use of cuttings of T. diversifolia or S. nigra separately, rather than in combination, and the inclusion of mid- to late-successional trees, to assist recovery of degraded cloud forest areas where succession has been stalled by bracken. An integrated management approach that considers synergistic factors causing plant mortality in bracken sites needs to be addressed.

Dwarf Mistletoe and Drought Contribute to Growth Decline, Dieback and Mortality of Junipers

Rising temperatures and aridification, combined with the stressing effect of some hemiparasitic plants such as mistletoes, may contribute to reduce vigour and growth of trees and shrubs leading to dieback and increasing mortality. This has been rarely explored in pioneer shrubs such as junipers, which are assumed to be more drought tolerant than coexisting trees. To test these ideas, we reconstructed radial growth patterns of common junipers (Juniperus communis L.) with different crown cover and infestation degree by dwarf mistletoe (Arceuthobium oxycedri (DC.) M. Bieb.) in two sites with contrasting aspect and water availability located in north-eastern Spain. We used dendrochronology to study the response of junipers’ radial growth to climatic factors (temperature, precipitation, and soil moisture), an index of drought severity, and mistletoe infestation. Juniper growth was constrained by elevated temperatures and low precipitation leading to drought during the growing season. Infestation by dwarf mistletoe contributed to a short-term growth decline in junipers. The interaction between low summer precipitation and high dwarf mistletoe infestation constrained juniper growth, particularly in the north-oriented wetter site, where hosts presented higher growth rates during wet periods. The negative impact of low summer precipitation on juniper growth overrides the effects due to dwarf mistletoe infestation. Aridification and mistletoe infestation could trigger dieback and mortality of shrubs slowing down successional dynamics and delaying shrub encroachment into former croplands and grasslands.

Effect of Root and Mycelia on Fine Root Decomposition and Release of Carbon and Nitrogen Under Artemisia halodendron in a Semi-arid Sandy Grassland in China.

Plant fine root turnover is a continuous process both spatially and temporally, and fine root decomposition is affected by many biotic and abiotic factors. However, the effect of the living roots and the associated mycorrhizal fungal mycelia on fine root decomposition remains unclear. The objective of this study is to explore the influence of these biotic factors on fine root decomposition in a semi-arid ecosystem. In this study, we investigated the effect of fine roots and mycelia on fine root decomposition of a pioneer shrub (Artemisia halodendron) in Horqin sandy land, northeast China, by the ingrowth core method combined with the litterbag method. Litterbags were installed in cores. Results showed that core a allowed the growth of both fine roots and mycelia (treatment R + M), core b only allowed the growth of mycelia (treatment M), and in core c the fine root and mycelia growth were restricted and only bulk soil was present (treatment S). These findings suggest that the process of root decomposition was significantly affected by the living roots and mycelia, and carbon (C) and nitrogen (N) concentration dynamics during root decomposition differed among treatments. Mycelia significantly stimulated the mass loss and C and N release during root decomposition. Treatment R + M significantly stimulated the accumulation of soil total C, total N, and organic N under litterbags. The mycelia significantly stimulated the accumulation of the inorganic N (ammonium-N and nitrate-N) but the presence of fine roots weakened nitrate-N accumulation. The presence of living roots and associated mycelia strongly affected the process of root decomposition and matter release in the litter-soil system. The results of this study should strengthen the understanding of root-soil interactions.

Effect of sika deer on forest recovery after clear-cutting conifer plantations in warm temperate regions of western Japan: from a field experiment of deer exclosure and planting of Quercus acutissima

ABSTRACT A field experiment was conducted involving deer exclosures and planting of Quercus acutissima, a deciduous oak, to reveal forest recovery patterns and factors following clear-cutting at overcrowded conifer plantation sites. Sites were selected in a warm temperate region in western Japan, which is dominated by deciduous broad-leaved secondary forests and extensive conifer plantations. Thirteen years after clear-cutting in the study area, naturally regenerated stands were established with exclosures. The exclosures housed vegetation dominated by pioneer trees, such as Mallotus japonicus or Zanthoxylum ailanthoides. In contrast, naturally regenerated stands without exclosures had limited pioneer trees present. Vegetation dominated by pioneer shrubs (e.g. Aralia elata) was established in these stands. Reduced canopy height and total basal area were observed for vegetation outside the exclosures. In stands where Q. acutissima was planted immediately after clear-cutting, the vegetation that is dominated by this species in the canopy layer was established in both stands with and without exclosure. Most of tree species that emerged after clear-cutting across all stands were of the frugivore seed dispersal type. However, results from this study suggest that these frugivore seed dispersal type trees are more vulnerable to deer browsing than trees from the Fagaceae family. Natural forest recovery following plantation clear-cutting could be inhibited due to deer herbivory at relatively low deer density if a major source of tree regeneration in clear-cut plantations is non-Fagaceae trees.

Resource islands of Salix cupularis facilitating seedling emergence of the companion herbs in the restoration process of desertified alpine meadow, the Tibetan Plateau

Salix cupularis is a common shrub for ecological restoration of the desertified alpine meadow on the Tibetan Plateau. However, the effect of S. cupularis on spatial heterogeneity of soil resources (i.e., resource islands effect) has not been systematically evaluated, and the influence of shrub patches on the rehabilitation of understory herbs has also been unknown. In this study, we randomly selected S. cupularis individuals in the early restoration stage of desertified alpine meadow, where the three native forages (Elymus nutans, Elymus sibiricus and Festuca sinensis) were sown at different microsites around S. cupularis to explore the effects of S. cupularis on soil resources and emergence rates of the native forages. The results showed that S. cupularis significantly increased SWC (soil water content), C (carbon) and N (nitrogen) nutrients (p < 0.01) and enzyme activities (p < 0.05) under canopy compared with the bare land, and the improvement performed better in the topsoil (0–5 cm) than in the subtop-soil (5–15 cm). Moreover, the soil properties were affected significantly by microsites around S. cupularis, resulting in regular changes of SWC, nutrients and enzyme activities in different microsites (Shrub center > Middle of canopy radius > Bare land). In addition, there are significant regression relationships between emergence rates and enriching soil water, C and N nutrients, so the emergence rates of native forages under canopy may be improved significantly with the enriched soil resources, especially for E. nutans. As a result, S. cupularis is a suitable pioneer shrub for the vegetation restoration of desertified alpine meadow on the Tibetan Plateau, because it could not only shape the enrichment of soil resources under canopy, but also facilitate emergence of companion forages in the process of vegetation restoration.

A novel fast-vegetative propagation technique of the pioneer shrub Baccharis linearis on mine tailings by adding compost

Interest in Baccharis linearis has increased as an alternative for assisted phytostabilization due to its spontaneous colonization of tailings dumps. The search for a novel fast-vegetative propagation technique to accelerate its coverage on mine tailings is a promising research area for sustainable mine closure plans. In this study, we determined the optimal proportion of compost and tailings as growing media to promote fast B. linearis propagation through a compound layering technique. The assessed growing substrates were: 100% tailings, 70% tailings + 30% compost, and 50% tailings + 50% compost. After 84 days of growth, the change in number and height of layering branches, root and shoot dry mass, percentage of ground coverage, and substrate chemical properties were assessed. The main results showed that compound layering of B. linearis is possible with compost addition. The growth of new roots and layering branches was significantly improved by either 30% or 50% compost addition into tailings, due to chemical improvements of substrate (higher nutrients and pH and decreased copper bioavailability). The study confirms that the compound layering of B. linearis may be an effective and novel technique for speeding the reclamation of post-operative mine tailings, which is improved by the incorporation of compost.

Bioecological and Behavioral Interaction between Pollinating Bees and the Pioneer Shrub Ludwigia nervosa in Degraded Area Suggests an Exotic Bee as Its Major Pollinator.

Simple SummaryLudwigia nervosa plants are commonly found in humid environments in the process of regeneration; however, little is known about reproductive contribution to attracting bees in fragmented environments. Therefore, we aimed to identify the bees visiting the flowers, and whether during pollen and/or nectar collection they performed pollination. The absence of pollen limitation shows that there was sufficient and efficient deposition of pollen in L. nervosa flowers during the visit, guaranteeing the formation of larger fruits and seeds with high germinative potential.The flowers of plants of the genus Ludwigia are an important source of food for several species of bees. In the current study, we conducted an experiment with the aim to describe the reproductive biology and phenology of L. nervosa; to identify the species of visiting bees; analyze the foraging behavior of bees; and to investigate whether the reproductive success of the species is related to the foraging activity of bees. We found that the flowers received visits from several native bee species (n = 7), in addition of the exotic honey bees which came to be the dominant species. During visits the majority of the bees foraged in both resources, pollen and nectar. The significantly higher production of fruits in open pollinated pollination experiment compared to artificial cross pollination, suggests honey bees as effective pollinator of this plant species in the study site. Pollen deposition occurs efficiently, given the absence of pollen limitation. Despite massive visitation of honey bees, Ludwigia nervosa is attractive to native bees, and therefore it may help to sustain population of both native and exotic pollinators in fragmented humid areas.

Involvement of soluble proteins in growth and metabolic adjustments of drought-stressed Calligonum mongolicum seedlings under nitrogen addition.

The planting of seedlings is the most effective measure for vegetation restoration. However, this practice is challenging in desert ecosystems where water and nutrients are scarce. Calligonum mongolicum is a sand-fixing pioneer shrub species, and its adaptive strategy for nitrogen (N) deposition and drought is poorly understood. Thus, in a pot experiment, we studied the impacts of four N levels (0, 3, 6, 9gN·m-2 ·year-1 ) under drought or a well-watered regime on multiple eco-physiological responses of 1-year-old C. mongolicum seedlings. Compared to well-watered conditions, drought considerably influenced seedling growth by impairing photosynthesis, osmolyte accumulation and activity of superoxidedismutase and enzymes related to N metabolism. Nitrogen addition improved the productivity of drought-stressed seedlings, as revealed by increased water use efficiency, enhanced superoxidedismutase and nitrite reductase activity and elevated N and phosphorus (P) levels in seedlings. Nevertheless, the addition of moderate to high levels of N (6-9gN·m-2 ·year-1 ) impaired net photosynthesis, osmolyte accumulation and nitrate reductase activity. N addition and water regimes did not markedly change the N:P ratios of aboveground parts; while more biomass and nutrients were allocated to fine roots to assimilate the insufficient resources. Soluble protein in assimilating shoots might play a vital role in adaptation to the desert environment. The response of C. mongolicum seedlings to N addtion and drought involved an interdependency between soluble protein and morphological, physiological and biochemical processes. These findings provide an important reference for vegetation restoration in arid lands under global change.

Substrate-specialist plants for restoring vegetation in post-mining gypsum substrates

Restoration in degraded gypsum areas is challenging due to physical-chemical soil characteristics that limit plant development. Facilitation mediated by the stress-adapted gypsophytes may be a key process in restoration because it potentially favors the establishment of lower stress-adapted species underneath, promoting plant succession. This study aimed to assess the suitability of the gypsophyte Gypsophila struthium Loefl. for restoring vegetation in gypsum mines, focusing on its role as a pioneer species that may act as an ecosystem engineer and hence as a nurse plant. We conducted a vegetation survey in a gypsum mine in NE Spain, in plots where the original topsoil was reinserted in different years after the mineral extraction to study plant community succession and reorganization through time. Complementary, we set a sowing and planting experiment in a gypsum spoil dump to assess how G. struthium may positively affect soil and micro-environmental conditions and, consequently, may act as a nurse species facilitating plant establishment and development. To test the nurse role of G. struthium, we sowed seeds and planted seedlings of Helianthemum squamatum (L.) Pers. and Stipa lagascae Roem. & Schultz under the canopy of adult individuals of G. struthium, and in the bare soil. We found that G. struthium appeared as a pioneer shrub in the restored gypsum mine and accumulated plant diversity in the local vicinity in mature plant communities. This shrub had a positive effect on seed germination and seedling growth of the test species in the experiment and improved soil and micro-environmental conditions under its canopy. Therefore, we recommend two non-exclusive actions to enhance restoration in post-mining gypsum substrates: (1) to promote the establishment of the pioneer plant G. struthium by improving the physical-chemical and biological soil properties through the re-insertion of the original topsoil that contains propagules of the former community; (2) to apply active restoration actions, using G. struthium as a nurse plant through sowing seeds or planting seedlings of other species of interest under its canopy.

Long‐term dynamics of shrub facilitation shape the mixing of evergreen and deciduous oaks in Mediterranean abandoned fields

Abstract Recovery of Mediterranean forests after field abandonment is a slow process, even without propagule limitations. This is mainly due to stressful conditions for seedling establishment. In this context, shrubs play a critical role in facilitating tree recruitment, but how this process unfolds after field abandonment is not entirely known. We evaluated the long‐term dynamics of facilitation by the nurse shrub Retama sphaerocarpa in the recruitment of two ecologically contrasting oaks, the evergreen Quercus ilex and the deciduous Quercus faginea. Thirty years after field abandonment, we dated shrubs and oaks established in an old field to estimate the annual recruitment rates and investigate temporal recruitment patterns. For oaks, we differentiated recruitment at two microsites: open areas or under shrub. To assess how nurse shrubs modulated environmental stressors, we modelled oak recruitment as a function of climatic variables. For the evergreen oak, we assessed these effects within each microsite. Finally, we estimated the annual interaction index between shrubs and evergreen oak juveniles as a function of climatic conditions. Each species showed different recruitment pulses during colonization. Recruitment rate was the highest for the shrub, followed by the evergreen oak. Oak colonization under shrubs was appreciable 20 years after field abandonment, when shrub cover reached 2.2%, and concentrated under medium and large shrubs older than 7 years. Shrubs not only accelerated evergreen oak colonization but also attenuated the fluctuations of recruitment pulses. For the evergreen oak, the interaction index indicated facilitation dominance in years with more arid summers (precipitation &lt; 47 mm and heat waves longer than 8 days) and competition in wetter summers. Synthesis. Oak colonization in Mediterranean abandoned fields progressed slowly, and only two decades after abandonment pioneer shrub population grew to a level in size and number that could effectively trigger facilitation and accelerate tree recruitment. The shrub nurse effect was unbalanced between oak species, only benefiting the evergreen oak, and it was more prevalent in arid years. Our study illustrates the build‐up of shrub facilitation during forest recolonization and the varying nature of this process among climatically different years and ecologically distinct species. This information provides insights for assessing and managing Mediterranean forest recovery.

Soil respiration dynamics in a semi-fixed sand dune under the pioneer shrub Artemisia halodendron Turcz. ex Bess. in the Horqin sandy land, northeastern China

Soil respiration is a major pathway by which carbon dioxide (CO2) fixed by terrestrial plants returns to the atmosphere. Studies of the dynamics of soil respiration in sandy land are rare and the contribution (Rc) of plant root respiration to the total soil respiration (Rt) is not well known. In this study, Rt in a semi-fixed dune planted with Artemisia halodendron was separated into autotrophic (root) respiration (Ra) and heterotrophic respiration (Rh) by the root-exclusion method. Rt and its two components were measured or calculated during 2013 on seven dates during the growing season when variation during the day was measured. Both Ra and Rh showed similar dynamics, with the maximum rates observed in July and with large daily variability observed on the earlier dates. Rc ranged from 39.3% to 53.1% during the growing season and exhibited a single peak in early August. Soil temperature at a depth of 20 cm was significantly correlated with soil respiration. Ra was more sensitive to soil temperature than Rh and the temperature sensitivity coefficients (Q10) were 3.39, 2.52, and 2.79 for Ra, Rh, and Rt, respectively. In summary, A. halodendron roots contributed an average of 46% to the total soil respiration, although this contribution varied during the growing season.