The role of nurse successional stages on species-specific facilitation in drylands: Nurse traits and facilitation skills

Exploring the most important indicators for environmental condition assessment using structural equation modeling and InVEST habitat quality model

Land degradation threatens the social welfare of human societies. In order to identify the most important indicators for land degradation assessment, this article quantified 36 vegetation and soil indicators. Ecosystem condition was determined based on the ecosystem threats using the InVEST habitat quality model, dividing the region to five degradation classes, i.e., negligible, little, medium, high, and very high degradation classes. The structural equation modeling showed that vegetation indicators were more important than soil indicators for land degradation assessment. Climate had a significant mediation on the relationships between soil and vegetation indicators and degradation (P < 0.05). Warning indicators were identified for each degradation stage. The mean changes of degradation indicators were 18, 35, 56, and 78% in little, medium, high, and very high degradation classes, respectively. Cold and semi-arid climates were more influenced by vegetation indicators which had the most variations in the early stages of degradation. Warm and arid regions were more affected by soil indicators, which had the most variations in the high and very high degradation stages. This approach provides comprehensive and necessary information about the condition of ecosystems by determining the severity of degradation in an area, the most important warning indicators of degradation, and the deviation of ecosystems from normal condition at each degradation classes, which helps a lot to managers to choose appropriate restoration plans.

Nurse logs: A common seedling strategy in the Permian Cathaysian flora

Nurse logs are common in modern forests from boreal to temperate and tropical ecosystems. However, the evolution of the nurse-log strategy remains elusive because of their rare occurrence in the fossil record. We report seven coniferous nurse logs from lowermost to uppermost Permian strata of northern China that have been colonized by conifer and sphenophyllalean roots. These roots are associated with two types of arthropod coprolites and fungal remains. Our study provides the first glimpse into plant—plant facilitative relationships between late Paleozoic gymnosperms and sphenopsids. Detritivorous arthropods and fungi appear to have been crucial for the utilization of nurse logs in Permian forests. The phylogenetically distant roots demonstrate that nurse-log interaction was a sophisticated seedling strategy in late Paleozoic humid tropical forests, and this approach may have been adopted and developed by a succession of plant groups leading to its wide representation in modern forest ecosystems.

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Diverse nurse-log interactions are documented in the Permian Cathaysian flora.

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Conifer wood served as substrates for seed plants and sphenopsids at the time.

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Arthropods and fungi were crucial for utilization of nurse logs in ancient forests.

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Nurse logs probably first occurred in swamps then expanded to various habitats

Restoring Mexican Tropical Dry Forests: A National Review

Deforestation is the dominant threat to tropical dry forests (TDFs) in Mexico. Its causes include agriculture, tourism, and mining. In some cases, unassisted forest regeneration is sufficient to return diverse forest cover to a site, but in other cases, changes in land use are so severe that active restoration is required to reintroduce tree cover. The ecological and social constraints on TDF restoration in Mexico are poorly understood. To address this knowledge gap, we synthesized relevant restoration literature for Mexico published between January 1990 and February 2020. We examined 43 unique articles about TDF restoration practices in Mexico to identify (1) the national distribution of TDF restoration projects, (2) restoration objectives, and (3) factors contributing to TDF restoration success or failure. The largest number of restoration sites were in the Yucatan Peninsula, and the most common objective was to restore dry forest vegetation on lands that had been used for agriculture or impacted by fires. Planting seedlings was the most widely reported restoration strategy, and plant survival was the most frequently monitored response variable. Maximum annual temperature and the Lang Aridity Index were the best predictors of plant survival, which ranged from 15% to 78%. This synthesis highlights how national restoration inventories can facilitate the development of a restoration evaluation framework to increase the efficacy of restoration investments.

High forest stand density exacerbates growth decline of conifers driven by warming but not broad-leaved trees in temperate mixed forest in northeast Asia

Increasing temperature over recent decades is expected to positively impact tree growth in humid regions. However, high stand density could increase the negative effects of warming-induced drought through inter-tree competition. How neighborhood competition impacts tree growth responding to climate change remains unclear. Here, we utilized the Changbai Mountain region in northeastern Asia as our study area. We quantified individual tree growth using tree-ring samples collected from three dominant tree species growing in three forest stand density levels. We estimated the effects of climate warming and forest stand density on growth processes and tested for a species-specific response to climate. Our results demonstrated that overall 25% of Korean pine, but only ~3% of Mongolian oak and ~ 4% of Manchurian ash experienced growth reduction. Increased forest density can also exacerbate growth reduction. We identified a climate turning point in 1984, where warming rapidly increased, and defined two groups, "enhance group" (EG) and "decline group" (DG), according to the individual tree growth trend after 1984. For the EG, climate warming increased temperature sensitivity, but the temperature sensitivity declined with increasing stand density for the whole study period. For the DG, tree growth sensitivity shifted from temperature to precipitation after 1984, driven by increased competition pressure under climate warming. Our study concludes that growth decline from warming-induced drought might be amplified by high forest stand density, was especially pronounced in conifer trees.

Shrubs as magnets for pollination: A test of facilitation and reciprocity in a shrub-annual facilitation system

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Insect-pollinated shrub Larrea tridentata increases the pollinator visitation to annuals.

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The effect of Larrea tridentata on pollinator visitation is inconsistent between years.

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Wind-pollinated shrub Ambrosia dumosa reduces the visitation duration of flies to annuals.

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Surrounding annuals reduce the visitation duration of pollinators to the shrub Larrea tridentata.

The magnet species hypothesis proposes that flowering plants that are attractive to pollinators can increase the relative pollination rates of neighbouring plants by acting as ‘magnets.’ Here, we test the hypothesis that insect-pollinated shrub species Larrea tridentata and wind-pollinated shrub species Ambrosia dumosa act as magnets for the pollinator visitation of understory annual plant species in an arid ecosystem. As an extension to the magnet species hypothesis, we propose the double magnet species hypothesis in which we further test for reciprocity by the floral island created in the understory of the benefactor shrubs as an additional pollinator magnet for the shrub itself. We used an annual plant placed near each shrub and the open to measure the effect of shrubs on annuals. The double magnet species hypothesis was tested using L. tridentata with and without surrounding annuals. We measured pollinator visitation and visit duration using video and in-situ observation techniques to test whether shrubs increase pollinator visitation to understory annual plants, if insect-pollinated shrubs act as better pollinator magnets than wind-pollinated shrubs (to determine the effects of the floral resource itself), and whether shrubs with annuals in their understory have higher pollinator visitation rates relative to shrubs without annuals. We found that insect-pollinated shrubs increased the visitation rate and duration of visits by pollinators to their understory plants and that wind-pollinated shrubs decreased the duration of visits of some insect visitors, but these relationships varied between years. While the presence of annuals did not change the visitation rate of all possible pollinators to L. tridentata flowers, they did decrease the visitation duration of specifically bees, indicating a negative reciprocal effect of the understory on pollination. Thus, the concentrated floral resources of flowers on insect-pollinated shrubs can act as a magnet that attract pollinators but that in turn provide a cost to pollination of the shrub. However, while wind-pollinated shrubs may provide other benefits, they may provide a cost to the pollination of their understory. These findings support the magnet species hypothesis as an additional mechanism of facilitation by insect-pollinated shrubs to other plant species within arid ecosystems.

The dark side of facilitation: native shrubs facilitate exotic annuals more strongly than native annuals

Positive interactions enhance biodiversity and ecosystem function, but can also exacerbate biological invasions. Facilitation of exotic invaders by exotic foundation species (invasional meltdown) has been studied extensively, but facilitation of exotic invaders by native foundation species has attracted less attention. Specifically, very few studies have examined the extent that native foundation species facilitate native and exotic competitors. Understanding the processes that mediate interactions between native and exotic species can help explain, predict, and improve management of biological invasions. Here, we examined the effects of native foundation shrubs on the relative abundance of the annual plant community – including native and exotic taxa – from 2015–2018 in a desert ecosystem at Carrizo Plain National Monument, California, USA (elevation: 723 m). Shrub effects varied by year and by the identity of annual species, but shrubs consistently enhanced the abundance of the annual plant community and facilitated both native (n=17 species) and exotic (n=4 species) taxa. However, at the provenance level, exotic annuals were facilitated 2.75 times stronger in abundance than native annuals, and exotic annuals were always more abundant than natives both near and away from shrubs. Our study reaffirms facilitation as an important process in the organisation of plant communities and confirms that both native and exotic species can form positive associations with native foundation species. However, facilitation by native foundation species can exacerbate biological invasions by increasing the local abundance of exotic invaders. Thus, the force of facilitation can have a dark side relevant to ecosystem function and management.