Abstract
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.
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1 Introduction
Globally, tropical forests harbor considerably higher genetic, species, and ecosystem-level diversity, and play an important role in global C cycle as compared to temperate forests (Hubbell and Foster 1983; Singh and Singh 1988; Raven et al. 1993). These forests share only 7 % of total land surface of the Earth while supports 50 % of the plant life forms (Wilson 1988). In addition to their higher species richness, these forests provide several ecosystem services for the well-being of human and other animals. Moreover, these forests have a considerable impact on the global carbon cycle due to its higher net primary productivity (Raven et al. 1993). Tropical forest is categorized as moist and dry tropical forests, depending upon the annual temperature and precipitation regimes. Among tropical forests, tropical dry forest shares about 42 % of the total global coverage (Holdridge 1967; Murphy and Lugo 1986), which represents the most vulnerable and least protected ecosystems of the Earth’s land surface (Murphy and Lugo 1986). These forests are mostly located in India, Kenya, Zimbabwe, Egypt, and Brazil (Raven et al. 1993). In this review, we critically reviewed the studies of the dry tropical environment due to its importance in global climate change by considering dry tropical forests of India as a representative of such environment.
In India, dry deciduous forests represent the largest forest type (Singh and Singh 1988), and share about 38.2 % of the total forested area (MoEF 1999). These forests occur in comparatively warm to hot climate having evaporation to precipitation ratio (e/r) of more than 1 (Raven et al. 1993). These regions have a prominent seasonality in temperature and rainfall pattern with a regular drought period of 2–6 months in each year (Murphy and Lugo 1986). The mean annual temperature in dry tropical region is more than 17 °C (Holdridge 1967; Olivares and Medina 1992; Crews et al. 1995; Dirzo et al. 2011). Comparatively high human population density in dry tropical region as than other regions of the world is also one of the characteristic features of these ecosystems (Murphy and Lugo 1986; Sanchez-Azofeifa et al. 2005). Currently, the dry tropical vegetation is experiencing severe degradation due to increased human interferences (Jha and Singh 1990; Chazdon 2003; Morris 2010; Popradit et al. 2015) and consequent ingression of invasive plant species (Sharma et al. 2005; Raghubanshi and Tripathi 2009). These events have led to a progressive conversion of species-abundant dry tropical forest into species-poor dry deciduous scrub, savanna, grasslands, and adjacent cropland systems (Fig. 1), since past several decades (Champion and Seth 1968; Jha and Singh 1990; Singh et al. 1991). The vegetation of dry tropical environment can be better considered as a mixed form of closed-to-open forests and savanna ecosystems. Therefore, in the preceding section of the review, we have considered the studies conducted for dry tropical forests as well as savanna vegetation of the world with a focus on the studies from India.
Current ecological scenario of dry tropical forest ecosystems
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.
The restoration of dry tropical vegetation is severely limited due to problems associated with the regeneration of tree seedling (i.e., survival, growth and development), which is highly sensitive and most important stage of plant’s life (Grubb 1977). It is reported that a change in the regeneration pattern of tree seedlings can influence the plant community structure and composition of dry tropical ecosystems (Wiegand et al. 2006; Van Langevelde et al. 2011; Chaturvedi et al. 2012). It may have serious ecological consequences, particularly under changing climate. Studies suggest that tree-seedling survival and establishment in dry tropical ecosystems are constrained mainly by water availability (Khurana and Singh 2001; Kraaij and Ward 2006; Cardoso et al. 2016), nutrient availability (Bardgett and Wardle 2003; Vadigi and Ward 2013), shade (Gerhardt 1996; Khurana and Singh 2001), and grass competition (Griscom et al. 2014). Therefore, tropical tree species may have considerably poor productivity under resource-limited conditions (e.g., inadequate water, light, and soil nutrients) and under grass competition. Tree seedling traits vary significantly across the dry tropical ecosystems to get adapted with the variability in abiotic and biotic growth regulators (e.g., light, temperature, soil nutrient regimes, distribution and amount of rainfall, and intensity of predation and disturbance) (Jha et al. 2000; Khurana and Singh 2001; Fredericksen 2011; Tripathi and Raghubanshi 2014). However, the complex mechanism that affects the tree species establishment in the dry tropical environments is still unclear (Khurana and Singh 2002). So far, such mechanistic studies have been done with a limited range of species and factors (Kambatuku et al. 2011; Ward and Esler 2011; Tripathi and Raghubanshi 2014). Therefore, a comprehensive understanding of the factors influencing the tree-species-seedling recruitment and the ecology of a set of species representing important plant communities are needed to improve our ability to manage the dry tropical environment and for the prediction of future change in plant community dynamics. The objective of the present study was to identify the seedling survival and growth under individual resources and to observe the interactive effects of various environmental variables (resources and disturbances) on seedling survival and growth in the dry tropical environment (forest and savanna vegetation), based on the available literature. The findings of the study may help to better predict and devise the measures for restoration of dry tropical environment.
2 Resources and disturbances
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.
Illustrated representation of key environmental resources and natural disturbances governing tree-seedling survival and growth
2.1 Water
Soil water availability is one of the key factor influencing the survival and growth of plant communities in dry tropical ecosystems (Khurana and Singh 2001). Increasing soil water availability promotes the survival and growth rate of juvenile tree seedlings (Khurana and Singh 2004; Yavitt and Wright 2008). However, soil water limitation may increase the seedling mortality (Yavitt and Wright 2008; Bingham and Simard 2011), reduce photosynthesis (Chaves et al. 2002; Ashraf and Harris 2013; Chaturvedi et al. 2013), and increase the vulnerability of forests to fire (Nepstad et al. 2002; Van Mantgem et al. 2013). Dry tropical ecosystem often experiences a drought period of 2–6 months, during which increased water scarcity causes the dehydration or wilting of tree seedlings (Chaves et al. 2002; Khurana and Singh 2004). It intensifies the chances of seedling mortality for the species growing under moist habitats than those of under drier habitats due to better adaptation of later to the water stress (Engelbrecht et al. 2006). Furthermore, extended dry periods during the drought months are found perilous to the survival of tree seedling in the dry tropical ecosystems (Ward and Esler 2011; Comita and Ehgelbrecht 2014).
2.2 Nutrient availability
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.
2.3 Light
Light limits the tree-seedling recruitment under various forest canopies (Way and Pearcy 2012), due to its primary importance in photosynthesis. In general, tropical plant species have a wider amplitude of light requirements (Whitmore 1975; Vieira and Scariot 2006a, b; Markesteijn et al. 2007). The importance of light for the growth of trees seedlings of tropical ecosystem has been well documented (Turner 2001; Vieira and Scariot 2006a; Tripathi and Raghubanshi 2014). Seedlings generally grow slowly under high canopy or deep shade relatively utilizes less or no added nutrients to the soil (Baker et al. 2003). However, canopy covering by the adult trees may have certain advantages to the understory seedlings, such as it allows the access to higher soil moisture for longer periods to the tree seedlings (Holmgren et al. 1997; Phillips and Barnes 2002; Bertacchi et al. 2016) due to hydraulic lift (Ludwig et al. 2004) or decreases temperature and evapo-transpiration (Bernhard-Reversat 1982). It has been reported that all benefit at the seedling stage was observed in the light gaps (Augspurger 1984; Huante et al. 1993). It is found that canopy openings created by tree-fall gaps are important sites for the growth and establishment of tree seedlings (Sapkota and Odén 2009). These gaps are characterized by higher levels of light, which leads to an increased seedling recruitment and, thus, higher species diversity (Canham 1988; Schnitzer and Carson 2001; Schnitzer et al. 2008). These gaps are generally characterized by comparatively greater herbaceous vegetation (Sagar et al. 2012). Such vegetation growth under canopy gaps is attributed to its favorable light and nutrient conditions on one hand and the reduced competition from the tree on the other (Sagar et al. 2008, 2012). Therefore, studies are required to further elucidate such effects of light on the tree-seedling growth under dry tropical environment in addition to grass competition.
2.4 Grass competition
In general, grasses pose severe competition to tree-seedling survival, growth, and establishment (Griscom et al. 2009), mainly by affecting their recruitment (Khurana and Singh 2001; Thaxton et al. 2012). Most of the studies have explained tree-seedling growth with grass in terms of competition for above- (e.g., light) and below-ground resources (e.g., water and nutrients) between them (Hardwick et al. 2000; Hoffmann and Franco 2003). Grasses suppress the seedling growth of tree species due to the depletion of resources (i.e., water or nutrient) under dry tropical environments (Sankaran et al. 2004; Kambatuku et al. 2011). In a study under dry ecosystems, Donzelli et al. (2013) found that grasses compete more for water, whereas trees are the better competitor for soil nutrients. However, studies highlighting the interactive effects of water, light, nutrients, and grass competition on the survival and growth of tree seedling are limited under dry tropical environment (Gerhardt 1996; Cabin et al. 2000; Tripathi and Raghubanshi 2014).
Several experiments on the tree-seedling growth under grass competition in dry tropics have led to two schools of thoughts, depending upon the level of competition. Some studies showed a positive (Duncan and Chapman 2003; Anthelme and Michalet 2009) or negligible effects of grass presence on tree-seedling growth (Scariot et al. 2008). However, other studies reported negative effects of grass presence on growth of tree seedling (Cramer et al. 2007; Hooper et al. 2005; Grellier et al. 2012). Therefore, a dilemma still prevails in the identification of optimal growth condition for various resources under which grass presence either suppresses or facilitates the tree-seedling growth. Tree seedling shows greater mortality during dry season as compared to wet season, which may be attributed to its competition with grasses for nutrient resources (Chirara et al. 1999). On the contrary, under limited or lower resource conditions, grass competition has been reported to have a less negative or even a positive role on the establishment of tree seedling during dry season (Vieira and Scariot 2006a, b; Cardoso et al. 2016). Therefore, field experiments are needed to elucidate these conflicting observations and to understand how grass competition determines the tree recruitment in dry tropical environment under the effect of other set of environmental drivers.
2.5 Herbivory
Herbivory is widely recognized as an important driver of changes in vegetation, community structure (i.e., size and composition), and diversity (Augustine and McNaughton 1998; Olff and Ritchie 1998). Several studies reported that herbivory has a strong influence on the plant communities (Becerra 2007; Powers et al. 2015). It influences the plant productivity (Sanchez-Azofeifa et al. 2013; Turcotte et al. 2014) directly by affecting the plants via tissue removal, which results in a shift in plant nutrient allocation (Frost and Hunter 2008; Mikola et al. 2009). However, browsing and grazing have shown differential effects on the tree-seedling growth, which may vary significantly with the plant growth stages. It is reported that browsing severely affects the tree species at the juvenile stage, constraining the successful establishment of tree seedling (Campa et al. 1992; Biaou 2009). However, grazing may have an indirect facilitative effect on the tree seedlings via suppressing the grasses and, thus, reducing the later’s competitive effect in the dry tropical environment (Pandey and Singh 1992; Ward and Esler 2011; Grellier et al. 2012). In addition, controlled grazing may also have a positive effect (similar to fire) by creating gaps resulting in a better availability of above- and below-ground resources for the tree seedlings, helping in their establishment (Bush and Van Auken 1995; Jeltsch et al. 1996; Kraaij and Ward 2006). However, the impact of herbivory on the tree-seedling survival and the growth under the influence of water or nutrient availability in soil are still unexplored (Derroire et al. 2016).
2.6 Fire
Fire is an important factor determining the structure and composition of forest ecosystems (Murphy and Lugo 1986). The dry tropical vegetation are highly susceptible to fire during the drought period, which spans a period of several months (Saha and Hiremath 2003; Kennedy and Fontaine 2009; McDonald et al. 2010). Forest fire has both positive as well as negative impacts on the natural successional processes (Hardwick et al. 2000; Ceccon et al. 2006; Dirzo et al. 2011). The degradation of the dry tropical vegetation by fire encourages the woody regeneration in fields via reducing above-ground grass biomass, which almost eliminates the competitive environment for the growth of tree seedlings (Schultz et al. 1955; Kodandapani et al. 2008). Simultaneously, it also significantly suppresses the pastures growth and regeneration (Powers et al. 2009). The adverse effect of fire on the tree-seedling survival and growth has been well documented in some studies (Du Toit 1972; Hardwick et al. 2000). It greatly impacts the seedlings and juveniles lying on the ground surface (Bond and Keeley 2005). Moreover, the seedlings of dry tropical tree species possess several mechanisms for adaptations to survive or escape from fire mutilation, even under intensive fires. For example, many trees species have tendency to accumulate nutrient and other growth reserves in the underground swollen structures to escape from the influence of fires (Van Langevelde et al. 2003; Wigley et al. 2009; Bond and Parr 2010). On the return of favorable growth conditions, the accumulated reserves further help in the tree-seedling re-growth and establishment (Hoffmann et al. 2004; Wigley et al. 2009).
2.7 Atmospheric CO2
The concentration of CO2 in the atmosphere has been consistently increasing, since the industrial revolution and the current CO2 level have reached around 400 ppm (CO2now.org; Bala 2013; Srivastava et al. 2016). The ongoing change in climate with an increase in atmospheric CO2 may strongly affect the tree-seedling survival and growth (Cernusak et al. 2011). In addition, the increased atmospheric CO2 also affects various eco-physiological characteristics (such as photosynthesis, water-use efficiency, and nutrient-use efficiency) of the plants as well as other seedling growth parameters (Khurana and Singh 2004). Most of the studies conducted on the seedling growth in response to atmospheric CO2 in dry tropical environment revealed that relative growth rate (RGR), leaf area ratio (LAR), and total dry weight increase with an increase in the atmospheric CO2 (Khurana and Singh 2002). However, the response to an elevated atmospheric CO2 varies widely among the species (Khurana and Singh 2002; Cernusak et al. 2011). Therefore, future studies should focus on the response of different plant functional groups (such as leguminous vs. non-leguminous, pioneer vs. non-pioneer, and fine-leaved vs. braod-leaved) under varying CO2 concentrations to design the appropriate measures to conserve and regenerate the dry tropical forests.
3 Interactive effects of resources and disturbances on tree-seedling growth and establishment
Interaction among the various resources and disturbances has been found to regulate the survival, growth, and establishment of the tree seedlings (Rincón and Huante 1993; Khurana and Singh 2001; Ceccon et al. 2006; Tripathi and Raghubanshi 2014). Therefore, a thorough understanding of this interrelationship would help in a better management of the forests in dry tropical ecosystem. A brief account of the interactive effects of major regulatory variables on the seedling growth has been discussed below with a particular emphasis on dry tropical environment.
3.1 Interactive effect of light availability, nutrient addition, and grass competition
As stated earlier, the survival and growth of the tree seedlings are intensely determined by light availability (Rincón and Huante 1993; Poorter 2001; Khurana and Singh 2006; Tripathi and Raghubanshi 2014), nutrient availability (Khurana and Singh 2004; Tripathi and Raghubanshi 2014), and grass competition (Ludwig et al. 2004; Grellier et al. 2012) in the dry tropical environments. The plant diversity and distribution are mainly governed by the interaction among these factors in dry tropical ecosystems (see Table 1). Therefore, studies focusing on this complex interaction would help us to understand how dry tropical tree species responds to variable nutrient availability and grass competition across the irradiance levels. In general, nutrient addition significantly affects plant growth in high light availability than the low light availability (Ingestad and McDonald 1989; Rincón and Huante 1993; Tripathi and Raghubanshi 2014). Above- and below-ground resource competition by grasses may suppress the tree-seedling growth under adequate light conditions (Kambatuku et al. 2011). Moreover, grasses have been reported to facilitate the tree-seedling growth via moderating the microclimatic conditions (Vieira and Scariot 2006a, b; Barbosa et al. 2014; Guarino and Scariot 2014). The interactive grass-seedling growth dilemma still prevails, which further gets impetus under the resource variability. Therefore, studies covering tree-seedling growth under grass competition in the effects of nutrients and light are urgently required under the dry tropical environments for a clear understanding and restoration of such vegetation.
3.2 Interactive effects of water availability, nutrient addition, and grass competition
Globally, the frequent occurrence of severe droughts has necessitated a thorough understanding of the interaction among the availability of water, nutrient supplementation, and grass competition, particularly in the dry tropical environments (Peñuelas et al.2002; Khurana and Singh 2004; Poorter 2009; Salinas-Peba et al. 2014; Powers et al. 2015). An unpredictable rainfall pattern in dry tropical environment has been indicated in the present climate change forecasts, which may further lead to a greater occurrence of droughts in coming years (IPCC 2007). It is still unclear that how tree seedlings, co-existing with herbaceous vegetation in the dry tropical environments, would respond to the concomitant changes in the availability of soil moisture and nutrients (Hulshof et al. 2013). The current understanding reveals that the relative proportion of woody to delicate herb vegetation is mainly affected by water availability, whereas edaphic (e.g., texture and fertility) and natural disturbance factors (e.g., fire and herbivory) are of minor importance, especially under savanna vegetation (van Langevelde et al. 2003; Sankaran et al. 2008). In general, tree seedlings are negatively affected by the droughts during the moist season, as successful recruitment depends on the continuous water availability (Harrington 1991; Khurana and Singh 2004 Kraaij and Ward 2006; Ferreira et al. 2015). Furthermore, the co-existence of grasses with the woody plants is the characteristic features of dry tropical environments, and therefore, a vigorous competition for light, water, and nutrients exists, specifically in savanna. Such competitions can put an important obstacle in the tree-seedling establishment (Fetene 2003; Flory and Clay 2010). However, it is still a matter of debate that how the intensity of competition between grasses and tree seedlings changes across the water availability in dry tropical environments. Woody cover generally leads to an increase in soil water availability, whereas a decrease in nutrient availability in the dry tropical environment (Sankaran et al. 2008; Sagar et al. 2012).
It is stated that the establishment of woody seedlings in the presence of grass is poorly affected by soil fertility (Cohn et al. 1989; Kraaij and Ward 2006) and enhanced N deposition (Sankaran et al. 2008; Mo et al. 2015; Powers et al. 2015). Limited researches have been done to elucidate that how competitive ability of grass affects the establishment of tree seedlings in interaction with the water and nutrient availabilities (Coomes and Grubb 2000; Maass et al. 1995). This knowledge gap limits the response prediction of woody cover to the concomitant changes in nutrient and water availability in the presence of grasses. Water is considered as the most important intermediary resource (Debain et al. 2005), thus, under dry conditions it is supposed to exacerbate the negative effects of fertilizer addition by promoting competition between grasses and the tree seedlings. The increased competition of grasses with seedlings under nutrient-rich conditions has been reported, especially under intermediate or limited water availability which affects the seedling survival and growth (Cohn et al. 1989; Vieira and Scariot 2006a, b). However, further studies are needed to better understand the mechanism underlying such changes.
3.3 Interactive effects of herbivory, nutrient addition, and grass competition
Tree-seedling establishment is highly affected by grazers and browsers under different dry tropical forests and savanna. It is attributed to the ingestion of plant tissue and resulting change in soil nutrient condition (due to dung and urine fertilization and erosion-transport by trampling) which affects the competition between tree seedlings and grasses (Scholes and Archer 1997; Griscom et al. 2009). The herbivory may differs with plant functional traits. However, broad scale studies covering a group of tree species for deducing the relationship of the herbivory with tree-seedling establishment are limited in the dry tropical environment (Kraaij and Ward 2006; Gratani et al. 2003; Bonito et al. 2011; Pringle et al. 2012). Understanding the dynamics of seedling establishment of most of the dry tropical tree species with respect to herbivory in the effect of other environmental factors can help to predict the impending changes in the forest vegetation (Vadigi 2013).
It has been found that the success of tree seedlings is mainly controlled by their ability to cope with the herbivory (Becerra 2007; Powers et al. 2015). In addition to its evident negative effects, herbivores may also favor the tree-seedling growth under the dry tropical environment. For example, browsers have been observed to stimulate the shoot development under fertile soil condition (Du Toit et al. 1990; Gerhardt 1998; Santiago et al. 2012; Vadigi and Ward 2013), whereas grazers reduce grass competition and promote the seedling establishment (Briske 1996; Gunaratne et al. 2010). Moreover, ungulate herbivores indirectly regulate the tree community structure and dynamics mainly by their excretal inputs enriching the soil nutrient status (Van der Waal et al. 2011; Juan-Baeza et al. 2015). Such externally added nutrient via herbivores on one hand promotes the growth of tree seedlings, whereas enhances the grass competition on the other and, thus, may reduce seedling establishment (Van der Waal et al. 2009; Juan-Baeza et al. 2015).
Grass competition constrains the tree recruitment, mainly under high precipitation conditions (Hau and Corlett 2003; Hooper et al. 2005). On contrary, potential positive effects, such as protection from browsing and grazing, are also exerted by the presence grasses which help in tree-seedling establishment (Seymour 2008; Lagendijk et al. 2011; Vadigi and Ward 2013). Thus, grazers may facilitate the browsing of tree seedlings by eliminating the grasses. Moreover, grazing creates more harms than benefits by removing the grass competition. Furthermore, the interplays between browsers and grazers significantly determine the establishment success of trees in the tropical dry environment via their effect on soil nutrients and grass competition.
4 Knowledge gaps and future recommendations
Studies appraising the growth response of individual tree species in relation to resource augmentation and disturbance are well presented (e.g., Ashton and Berlyn 1992; Navas and Garnier 2002; Gratani et al. 2003; Kraaij and Ward 2006; Bonito et al. 2011). However, comparative studies on the tree seedlings of important dry tropical tree species representing different functional groups, within or across the communities, are limited (Khurana and Singh 2001; Sanchez-Azofeifa et al. 2013; Powers et al. 2015). This critically limits our understanding of the dynamics of complex (multi-species) dry tropical forests, where species compete and facilitate each other in growth and establishment. Therefore, it is imperative to understand (1) the response variation of tree seedlings in dry tropical environments for differences in the local environmental (i.e., resources and disturbances) conditions (to explain their relative dominance in varied environmental conditions) and (2) how such response variations relate to functional traits of the studied tree species. The present article identified the following knowledge gaps in the studies on tree-seedling growth and establishment: (1) to identify the key determinants of tree-seedling growth under the dry tropical environment, specifically across the functional types, (2) to identify the relative importance of resources on seedling growth under the dry tropical environment, (3) how seedling growth performs under grass competition in the effect of other adjoining resources and disturbances, and (4) identification of adaptive responses of tree seedlings to defoliation or herbivory pressure.
Therefore, studies are required on tree-seedling survival, growth, and establishment under the aforementioned interactions in the dry tropical environment: (1) to fine-tune the management strategy for the conservation and restoration of these dry forests, (2) to improve the limited understanding on competitive inhibition versus resource limitation for their effect on the tree-seedling growth (Kraaij and Ward 2006), (3) to understand how variation in the nutrient availability (both natural and man-made) would affect the plant community dynamics under the influence of other set of environmental factors, and (4) to better understand the impacts of widespread human-induced fertilization, watering, and canopy cutting on the dry tropical ecosystems (Fig. 3).
Conclusive remarks on dry tropical tree-seedling growth strategies by multi-factorial studies for improving the provisioning of ecosystem services
In the dry tropical environment, the knowledge of early survival and growth response of native species tree seedlings under the combinations of temporal variations of nutrient, light, and grass competition is critically required for the management and restoration of dry tropical vegetation. These differences might affect the establishment of native tree species, which may subsequently change the composition and dynamics of dry tropical plant community. Ultimately, it may result in a significant variation in the functional structure of the tropical dry vegetation. However, the regeneration of plant communities in dry tropical environments, with a focus on species and functional group specific responses, would help in improving their provisioning services (Dhyani and Dhyani 2016).
5 Conclusion
The dry tropical ecosystems are under tremendous pressure, imposed by the ever-growing human population, which is resulting in their degradation. These degraded ecosystems are majorly comprised of closed-to-open forest and savanna vegetation along with adjacent sparse grasslands and marginal croplands. Seedlings are the most affected stage under these ecosystems, and thus, call for a better management perspective. The present review clearly indicates that the fresh experimental efforts are highly required to understand the interactive effects of various resources (such as light, water, nutrient, etc.) and disturbances (such as defoliation, grass competition, etc.) regimes on tree-seedling recruitment and establishment in dry tropical environment. The outcome might be helpful for the prediction of future community structure of dry tropical forest as well as savanna vegetation, which is continuously facing natural (e.g., herbivory and fire) and anthropogenic (e.g., lopping, climate change, and nitrogen deposition) perturbations. Such interactive studies would provide an improved understanding about the tree-seedling growth, which could be helpful in the management of the dry tropical environment. However, if not managed in a sound manner, the subsequent change in forest community dynamics due to such perturbations may have a considerable impact on global C cycle and climatic change.
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Acknowledgments
The authors are highly thankful to Emeritus Prof. J.S. Singh and Dr. Hema Singh, Department of Botany, Institute of Science, Banaras Hindu University, for their valuable suggestions and encouragement. Financial support from University Grants Commission (UGC), Council of Scientific and Industrial Research (CSIR), New Delhi, India and Banaras Hindu University, Varanasi, India is also acknowledged. We also thank the Handling editor and anonymous reviewers for their critical suggestions for improving the article.
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ASR and RB proposed the theme of the article. RB, RS, and PS wrote the article in consultation with ASR.
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Bhadouria, R., Singh, R., Srivastava, P. et al. Understanding the ecology of tree-seedling growth in dry tropical environment: a management perspective. Energ. Ecol. Environ. 1, 296–309 (2016). https://doi.org/10.1007/s40974-016-0038-3
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DOI: https://doi.org/10.1007/s40974-016-0038-3