Tropical dry forest soils: global change and local-scale consequences for soil biogeochemical processes

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Tropical dry forest soils: global change and local-scale consequences for soil biogeochemical processes

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Vı´ctor J. Jaramillo*, Guillermo N. Murray-Tortarolo Instituto de Investigaciones en Ecosistemas y Sustentabilidad, Universidad Nacional Auto´noma de Me´xico, Morelia, Michoaca´n, Me´xico * Corresponding author.

ABSTRACT This chapter provides an overview of drivers of global change that affect soils in the tropical dry forest (TDF) biome. It identifies climate and land use change as major global change drivers for TDF soils. Key climate variables include rainfall seasonality and inter-annual variability, whereas land use change includes deforestation, fire and agricultural land use. It provides maps of the trends in key variables globally such as temperature, precipitation, soil moisture, fire, nitrogen deposition and deforestation for a 30-year period (1985e2015). It covers a review of relevant literature related to the local-scale effects of the identified variables on TDF soils and discusses mechanisms by which soil nutrients are affected by global change drivers, both locally and regionally. Finally, it offers a projection of how the interaction among the drivers, and their effects at local scales, may affect TDF soils in the future.

Introduction Tropical dry forest (TDF) occurs in tropical regions where several months of severe or absolute drought are present (Mooney et al., 1995). The strong seasonality of rainfall in TDF regions determines temporal patterns of biological activity while also constraining geographical distributions of organisms (Murphy and Lugo, 1986). Plants in TDF are generally drought-deciduous although evergreen species may be dominant (Rundel and Boonpragob, 1995) and even succulent plants become part of TDF with varying proportions along rainfall gradients (e.g. Dura´n et al., 2002). Globally, TDF occurs on all continents, with more than half in South America, representing about 42% of the intratropical vegetation (Murphy and Lugo, 1995). Its distribution spans tropical and subtropical (23
e30
; sensu Corlett, 2013) regions. There is, however, a current controversy concerning the global extent of TDF, since there are claims concerning the uncertainty of TDF occurring outside the Neotropics (Dexter et al., 2018). Due to widespread deforestation and forest conversion to agriculture, only 44% of the primary forest remains compared with cover estimates from 1900 (Hurtt et al., 2011; Fig. 7.1) comprising an estimated area of between 1,048,700 km2 (Miles et al., 2006) and 1,079,000 km2 (Bastin et al., 2017), although a degree of uncertainty still exists concerning this recent estimate (Schepaschenko et al., 2017). The densest cover of TDF occurs in regions of South America and the Yucata´n Peninsula in Mexico with variable amounts of forest cover elsewhere (Fig. 7.1). Global Change and Forest Soils. https://doi.org/10.1016/B978-0-444-63998-1.00007-0 Copyright Ó 2019 Elsevier B.V. All rights reserved.

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FIG. 7.1 Tropical dry forest distribution in the years 2015 (green) and 1900 (gray); the maps include both tropical dry forest and dry woodlands, following the distribution of Bastin et al. (2017).

Tropical and subtropical dry forests are found in frost-free areas where mean annual temperature is above 17
C, with annual rainfall ranging between 250 and 2000 mm and a ratio of potential evapotranspiration to precipitation from 1 to 2 (Murphy and Lugo, 1995), indicative of dry soils. Interannual rainfall variability is a common and relevant feature of TDF and extreme years may be significant determinants of TDF ecosystem properties (Murphy and Lugo, 1986). Such variability may, for example, consist of a fivefold difference between the wettest and driest year in a 50-year sequence of rainfall in TDF (Maass and Burgos, 2011). The high inter-annual variability is a consequence of anomalous rainfall events during the dry season, which may result in the growing season varying between 3 and 7.5 months, with potentially wide-ranging ecosystem effects. For example, the length of the wet season may, in part, determine soil organic C (SOC) storage, with longer wet seasons associated with more SOC (Rohr et al., 2013), or the distribution and abundance of plant species (Engelbrecht et al., 2007). Other key features of the rainy season in dry forest regions are the unpredictability and variability of rainfall amount and distribution (Sampaio, 1995), with soil water availability strongly mediating ecosystem biogeochemical processes (Jaramillo and Sanford, 1995). Recent analyses have shown an increase in the variability of seasonality over different regions in the dry tropics (Feng et al., 2013), however, areas of the dry forest biome such as North-Eastern Brazil, Southern Mexico and parts of India and Southeast Asia, all have shown a decreasing seasonality trend between 1930 and 2002. Although drought in TDF regions is a function of precipitation and temperature, a key variable such as soil moisture is rarely documented. Soil physical structure and topography play key roles in determining soil water availability. Soils in these regions are heterogeneous and may vary over short distances; soil depth, in particular, is an important factor for soil water availability and can vary greatly even within standard soil classes (Sampaio, 1995). As examples, soil conditions may allow certain vegetation types such as dry woodlands to extend into arid areas in Africa (Menaut et al., 1995). In Thailand, soil structure may act as a key determinant in forest type distribution (Rundel and Boonpragob, 1995). Soil type variation associated with changes in topography and geomorphology may be quite significant (Cotler et al., 2002). For example, TDF in Mexico occurs on at least seven different soil types, which vary in texture, organic matter and nutrient contents even within a small geographic area. Based on the discussion above, we identify two dominant global change drivers affecting TDF soils: climate and land use. In this chapter we use two approaches to provide an overview of these drivers; first, we map the trends in key variables globally for a 30-year period (1985e2015) and second, we

Overview of relevant global change drivers for TDF soils: climate and land use

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review pertinent literature related to the local-scale effects of the identified variables on TDF soils. We then provide a synthetic view and projection of how the interaction among the drivers and their effects at local scales, have determined recent trends and may affect TDF soils in the future. Briefly, our approach was to use the distribution data from Bastin et al. (2017) as a basis for all analyses, which includes both dry forest and dry woodlands. We then employed it to select the area of TDF distribution based on grids with more than 20% primary forest from data of Hurtt et al. (2011). Deforestation (change in primary cover) was estimated by comparing current cover with that estimated for 1900 (Hurtt et al., 2011). For physical variables, we plotted the average climatic conditions and the mean gridded trend for the last 30 years for observed precipitation (CRU4.0, Harris et al., 2014), observed temperature (CRU4.0, Harris et al., 2014), and soil moisture from observed and modeled data (Dorigo et al., 2015; Sitch et al., 2015). For land use, we analyzed the means and trends of fires and N inputs from fertilizer for the period 1980e2015 using data from Hurtt et al. (2011) for land cover type, nitrogen inputs from Nishina et al. (2017) and burnt area from Randerson et al. (2018, GFED4.1). Finally, to explore the potential change in future conditions, we employed data from a construct of nine Earth System Models (Taylor et al., 2012) to predict trends in future temperature, soil moisture and primary forest cover in four different representative concentration pathways (RCPs), as selected in Murray-Tortarolo et al. (2016a). The four RCPs span the range of year 2100 radiative forcing values found in the open literature, i.e. from 2.6 to 8.5 W m2 (van Vuuren et al., 2011). RCP2.6 represents scenarios that lead to very low greenhouse gas concentration levels, with a peak and a later decline to a radiative forcing level of 2.6 W m2. RCP4.5 is a scenario in which total radiative forcing is stabilized shortly after 2100, without overshooting the long-run target level. RCP6 is also a stabilization scenario in which total radiative forcing is stabilized shortly after 2100, without overshoot, by the application of technologies and strategies for reducing greenhouse gas emissions. In RCP8.5 greenhouse gas emissions increase over time, leading to high greenhouse gas concentration levels and a radiative forcing of 8.5 W m2.

Overview of relevant global change drivers for TDF soils: climate and land use The dominant role that rainfall seasonality, distribution and variability play in TDF regions and consequently in patterns of soil moisture, indicate that climate variables related to precipitation and its potential changes play a key role as biophysical drivers of change in TDF soils. Recognized sources of heavy rainfall globally include convective storms, hurricanes, and typhoons (Lin et al., 2011), which are particularly common during the growing season in TDF regions, such as the Pacific coast of Mexico (Garcı´a-Oliva et al., 1995). Concurrent changes in temperature have consequences for key soil processes such as soil respiration (Bond-Lamberty and Thomson, 2010). Clearly, any precipitation changes affecting terrestrial primary productivity imply hydrological impacts on soil biogeochemical processes. Projections from global models concerning changes in precipitation drivers and its characteristics (i.e., amount, seasonality, variability) are central to hypothesizing future trends for TDF soils. Importantly, the seasonality and variability in precipitation are predicted to vary with climate change and will likely affect soil moisture, both temporally and spatially. The other key global change driver of terrestrial ecosystems, and TDF in particular, is land use change. A recent study (Song et al., 2018) attributes 60% of all land changes to direct human activities,

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with the largest area of net tree canopy loss occurring in the tropical dry forest biome, closely followed by the tropical moist deciduous forest. The primary driver of these trends is identified as deforestation for the expansion of agriculture. Murphy and Lugo (1986), in an extensive review of the ecology of TDF, identified a human preference for environments where TDF occurs and suggested biological or ecological reasons for such preferences. These relate to the relative ease of clearing for agriculture, the suitability for livestock production, greater soil fertility, and a diminished impact of human diseases, when compared to rainforest regions. Thus, human exploitation of TDF is proportionally higher than other tropical forest biomes, and includes fuelwood, slash-and-burn practices for agriculture and grazing, and thin-stem extraction as plant support stakes for crops (e.g., to provide support for tomatoes; Rendo´n-Carmona et al., 2009). Agricultural practices may also lead to increases in N inputs, mostly as fertilizer, with potential consequences for soil biogeochemical processes. For example, N fertilization of tropical forest soils resulted in greater NO3 leaching, higher NO and N2O fluxes (Hietz et al., 2011) and greater gross nitrification rates (Silver et al., 2005), but the capacity of tropical agricultural soils to store soil NO3 below the rooting zone determines the size of hydrologic N losses (Jankowski et al., 2018). The consequences of disturbance in TDF (harvesting, browsing, fire and drought) have been documented as being more severe with increasing soil moisture stress (Chaturvedi et al., 2017), which supports a link between changes in land use and water availability. Finally, abandonment of land used for productive purposes has resulted in a mosaic of secondary successional forests worldwide, which may or may not restore soil properties and function (Powers and Marı´n-Spiotta, 2017).

Climate Climate trends between 1985 and 2015: potential implication for TDF soils Tropical dry forests are located in regions with a mean annual temperature of 23  5.2
C, a mean annual precipitation of 575  334 mm, and a mean annual soil moisture content of 17  6% (Fig. 7.2). For the 30-year trend, we found a positive temperature increment of þ0.6
C within the TDF distribution area, with the exception of small patches in South America and Australia. This trend is consistent with previous estimates by Malhi and Wright (2005) who found an average warming of 0.26
per decade since 1970 for most of the TDF, with a small negative trend of 0.1
C per decade in South America. Interestingly, mean precipitation and soil moisture have remained steady through time (þ1.4  3.0 mm year1 decade1 and þ2%  0.1 decade1). However, important regional differences exist. For example, there was an increase in precipitation and soil moisture over most of the African and Indian TDF, but a decline in rainfall and soil moisture across large areas of South America, where the best preserved and densest TDF exists. Similar spatial patterns were reported by Sitch et al. (2015) and by Malhi and Wright (2005), with small trends in total precipitation for TDF, but with large regional differences. Critically, dry seasons across the TDF seem to be having higher rainfall (þ3 mm year1 decade1), with no apparent change in wet season precipitation (Fig. 7.3) and thus seasonality is decreasing, consistent with results of Feng et al. (2013). This is also similar to the results of Murray-Tortarolo et al. (2017) who showed wetter dry seasons globally, but no change in length. This tendency seems to be affected particularly by El Nin˜o Southern Oscillation (ENSO) variability, which controls how the annual rainfall is distributed across the tropics (Condit et al., 2004; Feng et al., 2013). Thus, despite a relatively unchanged mean annual precipitation over the TDF biome, there are important spatial and particularly temporal changes occurring to rainfall.

FIG. 7.2 Mean climatic conditions (left panel) and their linear change (right panel) over the last 30 years (1985e2015) in the tropical dry forest biome. The maps include temperature (top), precipitation (middle) and soil moisture (bottom). Average conditions and trends are presented in the bottom left section of each plot.

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FIG. 7.3 Mean seasonal precipitation anomaly for the wet (left) and dry (right) seasons across the whole TDF ecosystem. Basal period was the mean for 1985e90. The seasonal threshold was defined as months with higher or fewer than 100 mm of rain. Linear regressions were fitted to both seasons and results are shown when statistically significant.

Wetter dry seasons may be particularly relevant, as dry seasons exert a strong control on primary productivity and C cycling (Murray-Tortarolo et al., 2016b). For example, the dry season plays a key role in soil methane (CH4) uptake, and TDF has been identified as an ecosystem type with the highest CH4 soil uptake rates (Murguı´a-Flores et al., 2018). Dry and hot conditions during the dry season in TDF regions generate optimum conditions for methane uptake, explaining why mean rates of 602 mg CH4 m2 year1 are well above the mean global value of 229 mg CH4 m2 year1. In fact, the largest uptake rates ever recorded (0.52 mg CH4 m2 h1) were for TDF during the dry season.

Site-level consequences of precipitation seasonality and variability on soil carbon and nutrient dynamics As with other arid and semiarid ecosystems, rainfall exerts a central control on soil water availability, which in turn controls TDF soil biogeochemical processes. Soil moisture has been well recognized as the direct link between precipitation and ecosystems (Weltzin et al., 2003), but suggests that at the site scale soil moisture, and thus soil biogeochemical processes in TDF, respond more to the size of individual rainfall events, seasonality, and the inter-annual variability of precipitation amount, rather than to total annual rainfall. Knapp et al. (2002) showed, for example, that higher variability in rainfall and soil moisture variability, independent of precipitation quantity, affected ecosystem C inputs from aboveground net primary productivity and the soil CO2 flux in a mesic grassland. Also importantly, when deciduous species are dominant in seasonally dry forests, the soil surface litter layer, which accumulates during the dry season (Martı´nez-Yrı´zar, 1995; Anaya et al., 2012), retains a fraction of the water input from rain and mediates the soil moisture response to a given rainfall event. For example,

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the litter layer in the Chamela TDF may intercept 100% of small rainfall events (less than 5 mm; Maass and Burgos, 2011) and this fraction diminishes with larger events, so that soil microbes in the top 5 cm of soil respond to a 30 mm, but not a 10 mm, event (Campo et al., 1998). This is relevant to any discussion of TDF soils because there may be a soil moisture threshold required for the release of available soil nutrients from microbial activity as described below. The key role of water availability for soil nutrient transformations and availability in ecosystems subjected to low or markedly seasonal rainfall is well recognized (Austin et al., 2004; Campo et al., 1998; Davidson et al., 1993; Lodge et al., 1994; Wardle, 1992). Available nutrients may be taken up by vegetation and/or by soil microbes, moved within the soil profile, or lost through runoff or leaching. Singh et al. (1989) and Raghubanshi et al. (1990) both have suggested that soil microbial biomass serves as a nutrient source for initiation of plant growth during the rainy season and a conservation sink during the dry season in TDF. Nutrient release with the onset of rains may occur through two possible mechanisms: microbial cell lysis due the sudden increase in soil water potential and by growth and expansion of a microbivorous community in soil (Raghubanshi et al., 1990; Srivastava, 1992). Field studies in Indian TDF have shown highly significant negative correlations (from 0.895 to 0.973) between soil moisture and microbial C, N, and P (Srivastava, 1992). Net N mineralization potential in Mexican TDF soil changed from 2.53 mg g1 in the late dry season (May) to 20.94 mg g1 with the onset of rains (June; Garcı´a-Me´ndez et al., 1991). Wetting of dry soil with a simulated 20 mm rainfall event resulted in rapid and large pulses of NO, N2O and CO2 (Davidson et al., 1993; Vitousek et al., 1989). Moreover, experimental work with intact litter and soil cores from the same TDF in Mexico showed a pulsed release of microbial P with a 30 mm simulated rainfall event representing 70% of the annual litterfall P return (calculated from data in Campo et al., 1998). More recent studies have shown that ammonium (NH4), dissolved organic C (DOC), and microbial C and N in TDF soils are higher toward the end of the dry season than in the rainy season, and are actually negatively correlated with soil moisture (Montan˜o-Arias et al., 2007). In these cases, microbial C and N concentrations consistently increased across the dry season while decreasing during the rainy season (Lodge et al., 1994; Montan˜o-Arias et al., 2007; Saynes et al., 2005; Srivastava, 1992). Experimental work in Chamela, Mexico suggested similar soil moisture thresholds exist for both the increase in the soil N2O flux (Garcı´a-Me´ndez et al., 1991) and for microbial nutrient release (Gonza´lez-Ruiz, 1997). A positive impact of the early rainfall events on nutrient availability in TDF soils may result from nutrient release from litter decomposition (Kundu, 1990), including leaching of soluble C and N forms from surface litter (Anaya et al., 2007). Concentrations of water soluble organic C and N in litter during the late dry season (AprileMay) in the Chamela TDF can be up to 5 (C) and 3.6 (N) times higher than in the early-rainy period (JuneeJuly; Anaya et al., 2007). Importantly, these soluble forms move easily from litter to soil with the onset of rains. Further, potential C mineralization rates in litter collected during the late dry season were substantially higher than in the rainy season. Anaya et al. (2007) concluded that seasonal N dynamics in surface litter of the Chamela TDF was interactively controlled by rainfall amount and labile C availability. Both litter and soil autotrophic nitrification increase as the rainy season progresses, because labile C availability limits heterotrophic microbes (Anaya et al., 2007; Montan˜o-Arias et al., 2007). In general, research conducted in TDF has indicated that surface litter and soil N and P dynamics, as well as soil C availability, respond to the seasonal pattern of precipitation. The onset of the rainy season drives total water input to the soil, which affects soil microbial activity, including heterotrophic C decomposition, N mineralization and P release to the soil.

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Changes in the seasonality of rainfall, especially the consequences of increased or reduced length of the dry season on nutrient availability, has been recently addressed through meta-analyses. One such study documented the consequences of experimental rainfall manipulation on soil N cycling across six biomes, including tropical and temperate forest, grassland and shrubland (Homyak et al., 2017). They concluded that N supply indices (e.g., net N mineralization) were not sensitive to precipitation reductions. Another global meta-analysis on the effects of climate on soil P in soils with primary or secondary (>10 years) vegetation cover focused on soil P Hedley fractions (Hou et al., 2018). They concluded that when aridity is driven by reductions in precipitation, aridity will have a minor effect on soil P availability. However, climate effects were contrasting in low- and high-sand (<50% sand) soils, with sandy soils more prone to P losses through leaching and runoff in response to high precipitation. This could be a factor in TDF growing on predominantly sandy soils, including TDF sites in the Caatinga of Brazil, western Mexico (Garcı´a-Oliva and Jaramillo, 2011), Thailand (Rundel and Boonpragob, 1995) and India (Raghubanshi, 1991; Singh et al., 1997). Inter-annual variability in precipitation in TDF is another key driver of soil moisture change (Allen et al., 2017). For example, variability was three- to fourfold when the driest and the wettest years were compared over a 33-year period (Maass and Burgos, 2011). Inter-annual variability is driven by atmospheric events that affect the amount of rainfall during the rainy season, such as hurricane incidences or El Nin˜o and La Nin˜a phenomena, which may also trigger precipitation events during the otherwise dry periods. Dry-season precipitation rates have increased globally in recent decades, linked regionally to positive changes in modeled net primary productivity (Murray-Tortarolo et al., 2017). If this trend continues in the following decades, we suggest that the proposed mechanism by which nutrients are released upon wetting of dry litter and soil may provide a feedback to boost NPP in certain regions of the TDF biome. In support of this hypothesis, high levels of productivity have been documented in TDF during years when dry-season rainfall is well above average (Martı´nez-Yrı´zar et al., 2018). Hurricanes, as a source of inter-annual variability in rainfall, may have important consequences for tropical ecosystem processes and biogeochemical cycles, especially when they make landfall. This has been the case several times in Puerto Rico (e.g., Silver et al., 1996 and many others), in the Yucatan peninsula of Mexico (e.g., Whigham et al., 1991 and other studies), and recently in the Chamela region ´ lvarez-Ye´piz et al., 2018) and in the northern Caribbean (Eppinga of Mexico along the Pacific coast (A and Pucko, 2018). Within the limited number of hurricane impact studies on TDF, few have examined soil nutrients or their outward flow in runoff or streams. Results from studies in wet tropical and subtropical forest in Puerto Rico and Taiwan indicate that hurricanes can cause an increase in groundwater nitrate (NO3), NH4 and dissolved organic N concentrations (McDowell et al., 1996), as well as in soil NH4 and NO3 concentrations (Lin et al., 2011; Silver et al., 1996). Hurricane impact in the TDF of Chamela, Mexico also resulted in a threefold increase in soil NH4 and a sixfold increase in soil NO3 (Gavito et al., 2018). In all cases, values returned to pre-disturbance levels following disturbance, although duration of elevated nutrient status varied across studies. Nutrient concentrations and fluxes in storm runoff or in stream flows have shown higher nitrate and dissolved and particulate P concentrations after hurricane disturbance for the wet tropical forests of Puerto Rico (Lin et al., 2011; McDowell et al., 1996), while dissolved organic C, N and especially P have shown large increases following hurricanes in Mexican dry forests (Jaramillo et al., 2018). The mid- and long-term consequences of these outputs for ecosystem productivity will likely depend on the frequency with which TDF is directly or indirectly impacted by such intense disturbances. This is particularly relevant since tropical hurricane intensity is predicted to increase with global warming (Knutson et al., 2015).

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Land use change Current global trend of land use change in TDF regions: 1985 to 2015 Land use change can impact TDF soils via three mechanisms: 1) deforestation (or change in primary cover); 2) changing fire dynamics; and 3) increased inputs of inorganic N via fertilizers. Most regions of TDF have been largely transformed through land-use activities, with only 44% of the primary forest remaining compared with cover estimates from 1900. In terms of the remaining 56% of the original TDF area, only 18% corresponds to secondary forest, while the rest (38%) is either croplands or pasture (Fig. 7.4). This decrease in forest cover is consistent with results from current global land change assessments (Song et al., 2018). The largest current extent of forest cover in the TDF biome is located in South America, with smaller remnant patches across India. These regions coincide with the largest rates of forest loss globally over the last 30 years, with an average loss rate of 1.2% decade1 (Fig. 7.5). Bolivian dry forests have also been reported as experiencing the highest loss rates (3%e 4.6% year1) in Latin America (Sa´nchez-Azofeifa and Portillo-Quintero, 2011). Natural fire has not been considered to occur frequently in tropical dry forests, since fires caused by lightning are normally extinguished by rain (Murphy and Lugo, 1986). One of the few reports documenting a natural fire was from Costa Rica (Middleton et al., 1997). In general, naturally occurring

FIG. 7.4 Mean annual land cover in the TDF biome from 1985 to 2015. Primary forest decreased from 48% to 44% (1% per decade). Secondary forest was reduced from 45% to 28% (6% per decade.). Croplands and pasture increased from 19% to 42% (þ7% per decade).

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fires are considered rare in Central America and possibly in the whole of the Neotropical TDF region (Otterstrom et al., 2006) and India (Schmerbeck and Fiener, 2015). It has been suggested that in seasonally dry regions of Africa fires may occur with a frequency of about 5e10 years, although not generally in dense dry forests (Menaut et al., 1995). A recent study identifies wildfires to be dominant drivers of forest disturbance in the northern forests of North America and Russia, but not as relevant a driver of change in TDF (Curtis et al., 2018). The largest distinction between savanna and TDF ecosystems is the recurrence of wildfire, with fire frequency being much higher in savanna systems, although it is recognized that TDF do experience fires sporadically (Dexter et al., 2018). Thus, fires in the TDF biome are mostly human-caused (Kondapani et al., 2008; Sa´nchez-Azofeifa and Portillo-Quintero, 2011; van der Werf et al., 2008), but may be fundamental to the maintenance of dry forest types such as the miombo in Africa (Menaut et al., 1995). However, miombo has been considered more savanna-like due to the grassy understory (Dexter et al., 2018). Our evaluation of fire occurrence across the globe indicates that the main burnt areas are mostly located across narrow latitudinal bands in Africa, where annual fires consume between 40% and 60% of the area in each 1
grid cell (Fig. 7.5); however, the TDF biome is remarkably resistant, as only around 6% of forested TDF burns annually. Miles et al. (2006) analyzed a three-year period and found that fire occurrence differed little among regions, but Africa did show the highest percentage (27%) of its area affected by fires. Another study identified the TDF biome among the highest in terms of fire activity when compared to other biomes globally (Pausas and Ribeiro, 2013). They indicated that high variability in fire activity occurs within biomes and argue that beyond the clear influence of anthropogenic drivers of fire in many ecosystems, climate and vegetation generate an “underlying pattern of variability in fire regimes”. Interestingly, our assessment shows a large decrease in burnt area (1% decade1 or 3%) over the last 30 years, with half as much area being impacted by fire today as 30 years ago. The decrease in fire activity is consistent with results of van der Werf et al. (2008), who showed a general decrease in fire detections in TDF between 1998 and 2006. In terms of N inputs from fertilization, TDF in India is most heavily affected by high input rates (Fig. 7.5), which can exceed 50 kg N ha1 year1, almost 50 times higher than the mean for TDF globally (1.7 kg N ha1 year1). Mean annual N inputs continue to be practically zero in the rest of the TDF biome.

Local scale consequences of TDF conversion to agriculture on soil carbon and nutrients: fire and management The local-scale impacts of human disturbance on either above- or belowground processes and services in TDF have been addressed in several review papers (Balvanera et al., 2011; Garcı´a-Oliva and Jaramillo, 2011; Giardina et al., 2000b; Maass, 1995; Maass et al., 2002, 2005). Land use change in TDF, as in other tropical forests, involves several sequential steps, which may have diverse consequences for soil biogeochemical processes. These include forest clearing, the use of fire, and management to establish agricultural crops or pasture for cattle grazing. Thus, the consequences from forest conversion may be identified in the short-term (e.g. fire effects on soil nutrients and transformations) and in the longer term as a consequence of management (Garcı´a-Oliva and Jaramillo, 2011). The review by Giardina et al. (2000b) examined the effects of slash burning on soil temperatures and nutrients in diverse tropical biomes including TDF. They identified the crucial role that nutrients in

Mean gridded indicators of land use change and its impacts in the tropical dry forest biome. The maps show the mean area of primary forest (top), mean area burnt (middle) and mean nitrogen inputs from fertilizer (bottom), as well as their respective linear trends over the last 30 years (1985e2015). Average conditions and trends are included in the bottom left section of each plot.

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ash and ash loss (i.e. from wind) play with respect to ecosystem nutrients, as well as the little studied role of soil heating during slash fires in nutrient transformations and release from microbial biomass in the top 5 cm of soil. For example, fire disrupts soil organic C dynamics and organic C redistribution among physical-size fractions after slash burning and results in temporary increases in ammonium-N through a decrease in microbial immobilization and an increase in available P through thermal decomposition of organic P and P desorption of inorganic forms (Garcı´a-Oliva and Jaramillo, 2011; Giardina et al., 2000a). Results from a recent meta-analysis on the effects of fire on soil P, which included sites in the TDF biome, revealed that all soil P mineral fractions tend to increase with fire, with more variable results in organic P (Butler et al., 2018). This study suggests that fire may decouple C, N and P cycling, ease P limitation where present or amplify P cycling in sites with a conservative P cycle. The effect size would depend on physicochemical and biological mechanisms, as well as their interactions, which change among vegetation types. Despite the positive short-term effects of fire on soil nutrient availability, ash and mineralized soil nutrients may be lost after fire because of wind erosion and subsequent water erosion with the onset of the first storms, but before significant plant cover can develop (Maass, 1995). Rain-induced erosion potential increases significantly without the surface litter cover characteristic of primary TDF soils during the dry season (Martı´nez-Yrı´zar, 1995), because it may be completely consumed by fire during slash-burning practices (Kauffman et al., 2003). This is particularly important in TDF regions with topographically heterogeneous landscapes where loss of the protective forest floor layer can be an important cause of soil nutrient loss, further depleting the nutrient capital before crop or pasture establishment. Thus, soil fertility maintenance, a regulating service of great importance to societies that occupy TDF regions (Balvanera et al., 2011; Maass et al., 2005), may be substantially affected by slash burning in these warm, dry ecosystems. Among land uses, agricultural and grazing management have the most direct impact on soil, although few studies have quantified such effects in TDF. An experimental study (Maass et al., 1988) revealed that conversion of TDF to agriculture and pasture reduces soil cover and infiltration rates while increasing soil erosion, resulting in downstream sediment transport and nutrient losses, often well above the natural rates. The longer term impact of land use on soil C and nutrients has been considered in previous reviews (e.g. Murty et al., 2002) and the general results indicate that conversion to agriculture generally promotes soil nutrient loss more than conversion to pasture. Cultivation of TDF soil in India resulted in a greater than 50% loss of soil organic C and organic N and a 66% loss of soil organic P (Srivastava and Singh, 1991). Lawrence et al. (2007) showed that an increase in the number of cultivation cycles in TDF of the Yucatan Peninsula significantly decreased soil P availability. Also, there is evidence of consistent decreases in soil N with TDF conversion to agriculture in the Neotropics, but more inconsistent changes when conversion to pasture is considered (Garcı´a-Oliva and Jaramillo, 2011). These authors did suggest soil organic C would decrease with increasing pasture age in TDF. Recent studies on pastures with up to 40 years of management (Trilleras-Motha et al., 2015) indicate that repeated burning for pasture management reduces soil N concentration (25%) and C stocks (21%) in the top 10 cm of soil. Overgrazing in semi-arid soils in Brazil also led to decreases in soil C and N stocks when compared to forest sites (Schulz et al., 2016; Sousa et al., 2012). Emissions of N trace gases such as N2O and NO can also drive soil N losses following conversion of TDF to agriculture and pasture. However, limited evidence from Chamela, Mexico indicates that despite the elevated N2O emissions from maize fields compared to TDF, regional fluxes of N2O and NO are minimally altered by these land use practices

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(Matson and Vitousek, 1995). A more recent study of N2O emissions in Brazilian Caatinga forest found no difference between estimates for forest and pasture (Ribeiro et al., 2016). Studies of the impact of pasture establishment on soil P availability are also scarce, but limited evidence suggests that soil available P decreases with pasture age (Garcı´a-Oliva and Jaramillo, 2011). A study in the Chamela region (Chirino-Valle, 2008) showed that inorganic and organic P in the NaHCO3 and NaOH fractions decreased and that P forms were re-distributed into less labile forms (i.e. NaOH fraction) in 28 year-old pastures compared with primary forest, suggesting a substantial decrease in soil P availability with management. In a study of deforestation and land use impacts in Mexican TDF (Jaramillo et al., 2003), initial losses of total ecosystem C and N from deforestation were caused primarily by burning of aboveground biomass. Impacts on belowground C and N stocks are likely to increase in importance with time, because these soil pools can represent up to 90% (C) and 98% (N) of the total ecosystem pools. This suggests that long-term management that results in soil nutrient loss will severely affect ecosystem productivity.

Trends in soil carbon and nutrients during secondary succession Large areas of degraded agricultural and pasture lands in the TDF biome are being abandoned, allowing for secondary succession with woody species to occur. Secondary forests with different times of recovery are considered the dominant tropical forest type in both wet and dry regions (Powers and Marı´n-Spiotta, 2017), although our assessment shows that croplands and pastures are becoming the dominant land cover in the TDF biome (Fig. 7.4). Secondary forests have received increasing attention in TDF regions (Campo and Va´zquez-Ya´nes, 2004; Lawrence and Foster, 2002; Sa´nchez-Azofeifa et al., 2009; Urquiza-Haas et al., 2007), but few have examined biogeochemical changes (e.g. Read and Lawrence, 2003; Saynes et al., 2005; Vargas et al., 2008). As a result, knowledge of the role that tropical secondary forests in seasonally dry regions play on biogeochemical cycles at the global or even regional scale is still scarce compared to other tropical biome types. The current status of research on ecosystem processes and biogeochemistry in secondary tropical forest succession has been recently reviewed by Powers and Marı´n-Spiotta (2017), with a broad focus including both wet and dry tropical secondary forests. The review found that soil C stocks may increase, decrease or not change with secondary succession, with prior land use or management identified as key drivers of the contrasting trends. The lack of a relationship between forest age and soil C stocks in secondary TDF that follow after agriculture and pasture was reported in a recent study as well (Mora et al., 2018), and is consistent with one of the trends identified by Powers and Marı´nSpiotta (2017). In contrast, soil C stocks in forest sites of the Brazilian Caatinga appear to increase significantly, but nonlinearly, with age (de Arau´jo Filho et al., 2018). Compared with studies of C, those examining trends for other soil elements, such as N and P, during secondary succession are more rare. Chronosequence studies are few in number and have been inconclusive for trends in N and P availability, perhaps because factors such as nutrient re-distribution with soil depth or from soil to plant biomass and changes in species composition, can all affect patterns in nutrient availability with succession (Powers and Marı´n-Spiotta, 2017). A recent review and synthesis showed that soil C, C/N, and inorganic N, but not soil available P, recovered through secondary succession in the TDF region of Chamela, Mexico (Ayala-Orozco et al., 2018). They suggested that woody species richness was a better indicator of soil recovery than forest age. Clearly, global change drivers such as climate

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CHAPTER 7 Tropical dry forest soils: global change and local-scale consequences

and sequential clearing of secondary forests for production affect the trends during succession, and as Powers and Marı´n-Spiotta (2017) indicate, the soil trends are less predictable than the trends of aboveground processes.

Synthesis and future trends Among the drivers of soil biogeochemical processes, the seasonality and inter-annual variability of precipitation have clear local-scale consequences in the TDF biome. The rainfall pulse at the start of the rainy season is a dominant factor driving litter decomposition, the release and solubilization of litter and soil nutrients, and primary productivity in these forests. Nutrients accumulate during the dry season in fresh litter on the soil surface, but also in soil due in part to microbial resistance or even positive responses to desiccation during the dry season. Nutrient accumulation in the dry season is a prevalent feature of TDF soils in all regions. Changes in the seasonal distribution of rainfall through either increasing rainfall events during the dry season (i.e., the current decrease in seasonality) or, in the opposite case, due to intensification of the dry season will modify soil C and nutrient dynamics in TDF. Among sources of inter-annual variability, extreme rainfall events may result in nutrient losses, but are apparently more important for P than for C and N dynamics. Also, the increased frequency of high intensity hurricanes and typhoons with their impact on P losses is hypothesized to have longer-term consequences for forest productivity due to progressive soil P impoverishment (Jaramillo et al., 2018; Lin et al., 2011) in some TDF regions. Our review indicates that land use change, through deforestation and slash burning of TDF for agricultural production, promotes C and nutrient losses, particularly from crop establishment and after long-term pasture management. Land abandonment and the subsequent development of secondary forests may result in soil C and N recovery, but not soil P, although studies are still scarce. This poses a critical issue concerning soil P availability for TDF in the long-term, because of the additional impact of extreme climatic events such as hurricanes on soils can also result in P losses, as noted above. This represents a clear example in which the interaction (cumulative effect) between land use and climate change pose a threat for long-term sustainability in TDF regions. Evidence suggests that the relative roles these processes play in TDF soils differ among regions, due to the large spatial heterogeneity in the drivers of change across the TDF biome during recent decades, creating a mosaic of conditions among continents, which, in themselves, show a certain degree of variability regarding the trends of some drivers and threats (Table 7.1). Our assessment of trends indicates the soils in most of the TDF biome are exposed to increased interannual variability of precipitation and land use change for agriculture and pasture and thus to a decrease in primary forest cover, a general decrease in the seasonality of precipitation and a decrease in fire activity. While a decrease in forest cover (i.e., an increase in deforestation) should imply greater fire activity, we suggest that the increase in dry season rainfall (i.e., the decrease in seasonality) may be increasing fuel moisture during the dry season, so that fire activity has generally decreased. Soils in TDF regions of South America appear to be experiencing several concomitant negative changes due to the highest rates of deforestation (up to 10% per decade), decreasing water availability associated with lower precipitation and soil moisture, and an increase in fire activity and area burned. This last trend has also been reported by Araga˜o and Shimabukuro (2010) for Caatinga

Synthesis and future trends

123

Table 7.1 Qualitative representation of recent trends in drivers and threats for soils in the TDF biome. Drivers and threats*

N. Am.

S. Am.

Africa

India

S.E. Asia

Australia

Climate Temperature Precipitation total Interannual variability Seasonality Hurricanes* Soil moisture (rainy season) Primary forest cover* Agriculture and pasture* Fire* Nitrogen inputs*

þ þ þ  þ þ  þ  ¼

þ þ þ  þ þ  þ þ ¼

þ þ þ  x þ  þ  ¼

þ þ þ  þ þ  þ  þþ

þ þ þ  þ þ  þ  þ

þ þ þ  x þ ¼ x  ¼

 





Increasing (þ) and decreasing () trends may occur in parts the same region. Symbols: þ  regional increase or decrease; x not considered a threat; ¼ no change; þþ fast increase. The trends are extracted from the figures. N. Am. ¼ North America (includes Mexico and Caribbean), S. Am. ¼ South America. * denotes variables which are identified as threats.

and the Southern Amazon. In light of the processes above, these trends suggest that TDF soils in regions of South America are becoming more vulnerable to nutrient loss from extreme or dry-season rainfall events, likely triggered by ENSO, and are already in the process of sustaining significant nutrient loss from fire and land management. In contrast, TDF and woodland cover in Australia has not experienced major threats and changes in the last three decades. Across northern South America, the Caribbean and North America (Mexico), TDF has decreased in primary forest cover, precipitation patterns have changed, but soil water availability has not been strongly affected, while there has been a general decrease in fire activity. These trends suggest that the greater vulnerability to nutrient loss has been experienced from extreme precipitation events, for example from the increasing number and intensity of hurricanes. The TDF and woodlands in Africa, while experiencing a downward trend in fire activity are still threatened by burning from land conversion to agriculture and pasture, whereas TDF in India represents a case where, in contrast to other TDF regions, N inputs from fertilizer use have increased significantly, with impacts to soil C, N and P cycling in the region. To visualize the potential impacts of future changes to TDF soils resulting from global change, we computed the change in mean climate and forest cover by the year 2100 in four RCPs (Fig. 7.6). Our results showed a consistent picture of drying trends and deforestation across all scenarios, although there is the potential for higher forest area under RCP4.5. Thus, TDF soils would face degrading moisture conditions over the next century, in contrast to what has been observed in recent decades. Deforestation and loss of primary forest, presumably for agriculture and pasture, would continue to represent a major threat across the TDF biome. This is consistent with estimates by Miles et al. (2006), who proposed that only 3.3% of remaining TDF is not at a high risk of degradation by climate, fire and conversion to agriculture. The potential impacts are particularly worrisome for the TDF across the

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CHAPTER 7 Tropical dry forest soils: global change and local-scale consequences

FIG. 7.6 Modeled changes in temperature, soil moisture and forest cover for the tropical dry forest biome under four different future representative concentration pathways (RCPs). The bars represent the difference between the mean conditions for the year 2100 minus the mean conditions in 2015, for each variable.

Americas and Africa, as drying trends are very likely to occur as a result of the overturning in the Atlantic Meridional Circulation (Chen et al., 2018) and an increase in wildfire frequency and area burned due to a stronger and more frequent El Nin˜o (Fasullo et al., 2018).

Acknowledgments We thank Christian Giardina, Matt Busse and two anonymous reviewers for their careful reviews, insights and very useful recommendations, which helped us to significantly improve our chapter.

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