Soils are the foundation of all terrestrial ecosystems1. However, unlike for plants and animals, a global assessment of hotspots for soil nature conservation is still lacking2. This hampers our ability to establish nature conservation priorities for the multiple dimensions that support the soil system: from soil biodiversity to ecosystem services. Here, to identify global hotspots for soil nature conservation, we performed a global field survey that includes observations of biodiversity (archaea, bacteria, fungi, protists and invertebrates) and functions (critical for six ecosystem services) in 615 composite samples of topsoil from a standardized survey in all continents. We found that each of the different ecological dimensions of soils-that is, species richness (alpha diversity, measured as amplicon sequence variants), community dissimilarity and ecosystem services-peaked in contrasting regions of the planet, and were associated with different environmental factors. Temperate ecosystems showed the highest species richness, whereas community dissimilarity peaked in the tropics, and colder high-latitudinal ecosystems were identified as hotspots of ecosystem services. These findings highlight the complexities that are involved in simultaneously protecting multiple ecological dimensions of soil. We further show that most of these hotspots are not adequately covered by protected areas (more than 70%), and are vulnerable in the context of several scenarios of global change. Our global estimation of priorities for soil nature conservation highlights the importance of accounting for the multidimensionality of soil biodiversity and ecosystem services to conserve soils for future generations.
Professor Scott holds the Maurice K. Goddard Chair in Forestry and Environmental Resource Conservation, a Pennsylvania State University Endowed Chair based in the College of Agricultural Sciences and housed in the Department of Ecosystem Science and Management. Goddard Chair since July 2021, Prof. Scott is also Professor of Ecosystem Science and Management. His research, extension, and engagement focus on natural resource conservation and policy, climate-smart forestry, watershed science and management, the water-energy-food nexus, interstate and transboundary waters, climate adaptation and resilience, and energy transitions, with emphasis on the Appalachians, Alleghenies, and international efforts in the Andes and Himalayas.
The Goddard Chair focuses on goals broadly related to outreach connected to natural resource conservation, policy development and professional development, student engagement and achievement, and ensuring a legacy of resource sustainability. The Chair position attests to the University's commitment and dedication to the conservation, allocation and protection of natural resources.
The dry tropical forests are among the most vulnerable ecosystems of the world and, however, are relatively understudied. These forests provide various ecosystem services, and are progressively being converted into patches of dry scrubs, savanna and marginal cropland systems, due to various anthropogenic perturbations. Soils of these regions are relatively nutrient poor with a patchy nutrient and water distribution pattern. Therefore, the variability in these natural resources imposed by the present climate change scenario may affect the forest plant community of dry tropics via its impact on seedling growth and recruitment. Seedlings are considered as the most sensitive stage of plant lifecycle, and therefore, understanding of seedling regeneration may help in restoration of forest ecosystems. Seedling growth is majorly regulated by various naturally occurring resources (such as light, water, nutrient, etc.) and disturbances (such as defoliation, grass competition, fire, etc.). Therefore, efforts on the regeneration of these forest systems are highly necessitated. In the present study, we critically reviewed the studies on seedling survival and growth under different resource and disturbance regimes with a special focus to dry tropical environment. We found that water, light, nutrients, herbivory, and grass competition majorly regulates recruitments, growth, and establishment of the tree seedling in dry tropical environment. Most of the studies are limited to observe the effect of one or two factors over the seedling survival and growth. However, the resources and disturbances may have an interactive effect over seedling growth. Therefore, studies encompassing the interactions of various growth factors (resources and disturbances) under different climatic conditions are urgently needed for the successful regeneration of tree seedlings and for the restoration of plant community. Moreover, it will improve our ability to manage the tropical vegetation under changing climatic scenario.
Dry tropical ecosystems experience a more arduous and less anticipated environment, thus resulting into its proneness to environmental stress during the successional process of plant community development (Murphy and Lugo 1986). In India, most of the dry tropical vegetation occur in nutrient poor soils (Singh et al. 1989), which may have a tendency to conserve the nutrients (via immobilization in microbial biomass) and act as potential C sinks (Srivastava et al. 2016). Forest composition in such soils consists of species varying in their life history traits, such as leaf types (i.e., broad-leaved species and fine-leaved species), successional statuses (i.e. pioneer and non-pioneer), N2 fixation ability, tree size, and habitat preference (Chaturvedi et al. 2011). These species, therefore, respond differently to the resource availability and disturbance gradients (Chapin et al. 2003). For example, pronounced spatio-temporal variability in resources, such as light, nutrient (Raghubanshi et al. 1990), and water (Kottek et al. 2006; Singh and Ranade 2010; Chaturvedi et al. 2011, 2013, 2014), as well as the disturbances, such as herbivory (Staver et al. 2009; Chaturvedi et al. 2012; Juan-Baeza et al. 2015) and fire (Russell-Smith et al. 2003; Otterstrom and Schwartz 2006), has been reported in dry tropical environment. Therefore, a better understanding of tree plants, especially seedling response under varying environmental conditions (i.e., resources and disturbances), may hold considerable importance in the restoration of these degrading forest ecosystems.
Several abiotic and biotic growth regulators are known to determine the vegetation structure and composition of dry tropical ecosystems, especially via its impact at the seedling stage of the plant (Fig. 2). These can be broadly classified as: (1) resources, such as water (Reich and Borchert 1984; Chaturvedi et al. 2013; Vadigi 2013; Barbosa et al. 2014), light (Ceccon et al. 2006; Tripathi and Raghubanshi 2014), and soil nutrients (Huante et al. 1995; Ceccon et al. 2006; Chaturvedi et al. 2012; Tripathi and Raghubanshi 2014) and (2) disturbances, such as fire (Khurana and Singh 2001; Otterstrom and Schwartz 2006, Pluchon et al. 2014), herbivory (Higgins et al. 2000; Chaturvedi et al. 2012; Norghauer and Newbery 2014; Juan-Baeza et al. 2015; Torres and Renison 2015), grass competition (Riginos 2009; Ortega-Pieck et al. 2011; Griscom et al. 2014), and atmospheric CO2 (Khurana and Singh 2002). A series of experiments have been conducted across the dry tropical environment, which suggest that tree-seedling survival and establishment are highly susceptible to water stress, nutrient shortage, shade, herbivory, and competition with grasses (Table 1). In the later section of this review, we have given a brief insight on the effect of various resources and disturbances individually as well as interactively on tree-seedling growth and establishment to understand the involved ecology for management perspective under dry tropical environment.
The nutrient poor soils of dry tropical environment generally show strong seasonal variability in nutrient release (Singh et al. 1989; Raghubanshi et al. 1990; Singh et al. 2009; Powers et al. 2015). These soils are characterized by the inherent patchy distribution of nutrient and water availability (Roy and Singh 1994; Chaturvedi et al. 2011). This distribution pattern is also governed by many external factors, such as fire, fertilizer addition, herbivore density, and forest degradation (Elmqvist et al. 2007; Balvanera et al. 2011). Water availability and soil nutrient availability are closely interrelated (Van der Waal et al. 2009; Sardans and Peñuelas 2013). In the presence of grasses, an increase in soil nutrient availability induces water stress, which further leads to a lower growth performance in the tree seedlings (Hu and Schmidhalter 2005, Akıncı and Lösel 2012). Interestingly, a more constrained seedling establishment is reported under higher soil fertility in dry tropical ecosystems. It is attributed to an increased competition of tree seedlings with grasses for t resources in the soil (Kraaij and Ward 2006; Griscom et al. 2005; Sankaran et al. 2008; Van der Wall et al. 2011; Mills et al. 2013). Therefore, it is imperative to consider the impact of soil nutrients under the combined effect of water and grass competitions on tree species recruitment for a holistic understanding of the drivers of tree-seedling growth.
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