Understanding the Role of Slope Aspect in Shaping the Vegetation Attributes and Soil Properties in Montane Ecosystems

Tropical Ecology 59(3): 417–430, 2018 ISSN 0564-3295 © International Society for Tropical Ecology www.tropecol.com

Understanding the role of slope aspect in shaping the vegetation attributes and soil properties in Montane ecosystems

SHIPRA SINGH

School of Environmental Sciences, Jawaharlal Nehru University, New Delhi-110067, India

Abstract: Montane ecosystems are a remarkably diverse ecosystem, mainly characterised by hillslope asymmetry, i.e., hillslope variation as a function of slope angle and slope aspect. Slope angle, aspect, and elevation are considered as the primary factors creating spatial variation, causing differences in vegetation growth and distribution, ecosystem functioning, and processes. Differences in slope aspect cause differences in air and soil temperature, moisture content, evaporation, which creates microclimatic zonation, and that these differences are closely associated with alterations in vegetation structure and composition. North-facing shady slopes are supported with thick and dense vegetation with nutrient-rich soil, whereas, thin and scattered vegetation along with weaker soil development with higher erosion rates in south- facing sunny slope. However, the information related to the effect of the slope aspect in different latitudinal areas is still unclear. In this review, an effort has been made to collate the studies dealing with different facets of the slope and aspect in varied latitudinal regions and how it influences ecosystem processes and soil properties in the montane ecosystem. The present literature survey shows that the slope aspect has its maximum effect in the mid-latitudinal region. In the low-latitudinal region, the slope aspect shows no consistent differences between the opposite aspects. The effect could be seen in its lowest limit in the higher-latitudinal region. The slope aspect is a crucial parameter in montane forest ecosystems and for sustainable forest management measures, which cannot be neglected for studying topography related vegetation dynamics. Moreover, water and soil conservation measures should be applied precisely on drier and warmer aspects.

Key words: Biodiversity, elevation, insolation, slope angle, slope aspect.

Introduction is the most important one among the three (Busing 2005), and these three factors together determine There has been a keen interest among plant the microclimate and spatial distribution patterns ecologists to ponder slope aspect related issues in of vegetation. Within one elevation, slope aspect, vegetation communities since the 1850s region slope angle, and soil type are responsible for forest (Gilliam et al. 2014). The effects are more dynamics (Shank & Noorie 1950). Alexander von pronounced in mountainous regions, which are set Humboldt was the principal individual to portray apart by steeper slopes (Pepin et al. 2015). The the latitudinal as well as a corresponding three main topographic features, which rule over elevational gradient to be majorly responsible for the patterns and trends of vegetation in the fundamental understanding of species mountainous regions are elevation, slope aspect, composition and diversity along an altitudinal and slope angle (Titshall et al. 2000). The elevation gradient (Fischer et al. 2011). Changes in

*Corresponding Author; e-mail: [email protected] 418 SLOPE ASPECT SHAPES VEGETATION ATTRIBUTES IN MONTANE ECOSYSTEMS vegetation pattern as well as soil properties are the lower latitudinal area remains unclear very pronounced along the elevation gradient (Méndez‐Toribio et al. 2016). because of the increased climatic variation over Beaty & Taylor (2001) and Sternberg & relatively lesser distances (Dahlgren et al. 1997; Shoshany (2001) have made observations that the Lomolino 2001). Increased vegetation cover can northern slope with dry regions have improved lead to improved slope soil shear strength, soil vegetation in the northern hemisphere. This may quality, reduction in soil erosion (Chau & Chu perhaps not be valid for regions that do not 2017; Liu et al. 2014; Yuan et al. 2016) thus experience water limitations in the northern facilitating ecosystem restoration. Differences in hemisphere. Dearborn & Danby (2017) in his study microclimatic conditions also create a difference in in Kluane Region, southwest Yukon, Canada soil types influencing its development and (60.7°N) suggested that vegetation attributes processing (Egli et al. 2006) as well as affect the varied more as a function of slope angle and slope soil stability (Schoorl et al. 2004). Additionally, it aspect rather than elevation. South-facing slopes also affects the soil temperature, air temperature showed increased soil temperatures and deeper and moisture content (Bennie et al. 2006; Sidari et active layers in comparison to north-facing slopes, al. 2008). The effect of topographical positions while differences related to slope angle occurred (slope aspect and slope angle) on vegetation only on north‐facing slopes. While few studies composition and structure has been extensively suggest the effect of elevation to be more studied in temperate forests (Sharma et al. 2010a, pronounced as compared to the slope aspect in b, c), temperate grasslands (Gong et al. 2008), such regions (Zhang et al. 2013). This review Mediterranean or tropical humid communities builds on to enhance the comprehension of the (Gilliam et al. 2014), tundra ecosystem (Dearborn diverse impact of slope aspect on vegetation and & Danby 2017) and boreal forest ecosystems soil properties in different latitudinal regions; and (Astrom et al. 2007). However, the relationship extract information in regards to the plantation between vegetation structure and composition as and other practices for the local communities that well as soil properties in the Montane ecosystem are reliant directly or indirectly on it. Thus, an as a result of the effect of slope angle, aspect and attempt has been made to increase awareness elevation is still a matter of debate. There is a among policy-makers and conservators, which will complex relation between vegetation communities, be directly helpful in an effective overall sustai- soil properties and slope aspect in the Montane nable forest management. region, which requires local specific study to fully understand the relation between them. Effect of slope aspect on vegetation attributes There is still a lack of knowledge and in the montane ecosystem understanding of how the slope aspect causes changes in forest properties and how this effect Vegetation, defined as an array of plant varies along latitudinal degrees in both the species and the dominant life forms, include trees, hemisphere. It remains unclear what is the most shrubs, grasses, and forbs (Burrows 1990). Forest critical factor that is maximally affected by is a unit of vegetation having physiognomic and changes in the slope aspect (Warren 2010). For structural features so articulated that it appears example in the low-latitudinal area, Maass et al. distinct from other units (Champion & Seth 1968). (2005) found no significant difference in the tree Each forest group is related to an aspect, usually height between northern and southern slopes in having a southern and a northern counterpart, the seasonally dry forest of Chamela in West which differs in species composition (Singh & Mexico (19.5°N). Also, tree density was found to be Chaturvedi 2017). The two different aspects form a significantly higher in the southern slopes change in patterns and trends of structure and whereas, Gallardo-Cruz et al. (2009) in his study composition of the vegetation (Chmura 2008). The on the seasonally dry tropical landscape (Mount maximum effect of topography (slope angle, aspect, Cerro Verde, Oaxaca (17°N) suggested that and elevation) tends to occur in montane forest vegetation structure did not show significant ecosystems characterised by stratification with changes linked to the topographical positions, respect to elevation. In low latitudinal regions, although a perceptible floristic composition vegetation cover shows specifically no consistent difference was noticed along with the topographic differences between the northern and southern inclination. Therefore, the effect of topography in aspects. In mid-latitudinal regions, the northern SHIPRA SINGH 419 aspect shows increased vegetation density in such as livestock grazing, extraction of firewood, comparison to the southern aspect. The effect is are supposed to be higher on the lower reaches of much pronounced in such regions. hill slopes, particularly those with moderate Ohsawa et al. (1985) in his study on Mount steepness (<7%) in Brazilian Savanna Steppe Kerinci, Sumatra (1.6°N) suggested that the (14°S) (da Silva et al. 2014). There is a decrease in latitude is the primary factor causing altitudinal forage availability with increasing slope angle, and zonation patterns, creating the most complex also, lower availability of vegetation suggests vegetation pattern on subtropical mountains lower foraging (Sigua & Coleman 2010). The slope where the two floristic realms, the Boreal and aspect may lead to successional changes after Palaeotropical, meet. The role of slope aspect in disturbances, such as clear-cutting and felling Mt. Kilimanjaro (3°S) was found to be less humid (Astrom et al. 2005; Okland et al. 2003). Such and warmer on the north-east slopes than south vegetation contrasts provide habitat edges that slopes (Pepin et al. 2017). Vegetation zonation in also influence wildlife. Studies conducted on montane forests of Mt. Meru (3°S) showed an scorpion diversity concerning slope aspect and extension of about 750 m lower on the wetter altitude in Soutpansberg, South Africa (22°S) southerly than on the drier northerly slopes. suggested that scorpions were found to be higher on Upper limits of the forest and moorland commu- the arid-northern slope. The diversity was also nities, however, showed little altitudinal variation found to be higher at lower than upper elevation. between the windward and the leeward sides of Few endemic species were found in the middle to this and the other East African mountains the higher elevation of the north slope (Foord et al. whereas, in Fiji Islands at (15-20°S), forests are 2015). This suggests an upward shifting of species usually found on the wetter (>5000 mm/year) studied through surveys, field analysis, and remote southeast slopes, and either grassland or open sensing. Galicia et al. (1999) mentioned the effect of shrubby vegetation grows on the drier (<2000 slope aspect in the tropical forest, Mexico (23°N) mm/year) northwest slopes (Smith 1951). Marler & stating that north-facing slopes receive less solar Moral (2018) in his study on Mount Pinatubo, radiation and have higher soil water content for a Philippines (15.14°N) found an increased vege- more extended period in the lower elevation zone. tation cover from 153 to 245% on the northern In contrast, higher elevation zonation is characte- aspect, and from 174 to 230% on southern aspect. rised by greater soil radiation intensity and lower Laurance et al. (2010) studied the influence of soil soil water content. Vegetation structure in the lower and topography on Amazonian tree diversity in elevation zone showed an increment in diameter of Brazil (14°S). He found topography to be the tree species at the northern aspect having greater strongest predictor of tree diversity, with diversity leaf area than a higher elevation zone. increasing with higher soil fertility and water The slope aspect is considered to be the most availability. Mendez-Toribio et al. (2016) studied influential driver in the forest ecosystem of the effect of slope aspect and topographic position environmental conditions in temperate regions. in Southern Mexico (14.32°N). He found that mean North facing slopes in the northern hemisphere annual temperature, evapotranspiration rate and having latitude of around 30 to 55o tends to receive potential energy income varied with slope aspects less direct sunlight than the south-facing slopes and topographic positions. (Searcy et al. 2003). Sloping lands facing toward the Vegetation structure and abundance was equator tend to receive a more significant amount of found to be greater in the upper portion of south- solar radiation with a higher timespan (Fig. 1). The facing slopes but decreased with elevation in radiation is supposed to be 50% greater on southern north-facing slopes. Conversely, species diversity slopes than on northern slopes in the northern was found to be higher in south-facing slopes and hemisphere (Gilliam et al. 2014). The northern increased from lower to upper elevation. The study aspect slopes remain shaded for day long due to showed that the vegetation attributes did not show relatively lower angle of the sun. They can likewise a homogenous response to topographical factors. produce feedbacks with atmosphere, soils, and Structural attributes and alpha-diversity were disturbances that influence the vegetation found to be more closely related to human distur- distribution (Pelletier et al. 2018). The pioneer bances and soil moisture, particularly on south- investigations of the montane forest ecosystem by facing slopes but the anthropogenic effect was not Turesson (1914) suggested slope exposure to be the related to topography. Anthropogenic activities controlling factor for the distribution of Douglas fir. 420 SLOPE ASPECT SHAPES VEGETATION ATTRIBUTES IN MONTANE ECOSYSTEMS

Fig. 1. Incoming solar radiation at different latitudes governing the role of slope aspect.

Cottle (1932), in a study conducted in South- Regarding the regenerative behavior of the forest, western Texas (32.8°N), found the vegetation cover Cd value was reported to be highest for the to be twice more rich on north slopes as compared to southeast aspect (0.74) and lowest for the north- south slopes. Braun-Blanquet (1932) mentioned the west aspect (0.32). The contradictory results for the presence of heliophytes light-loving plants to reach Cd value of sapling and tree species may be due to their maximum limits in southern slopes in the the regenerating nature of the forest. It may also be Swiss Alps (46.56°N). Cantlon (1953), in his studies, due to enough light penetration in the south aspect, suggested therophytes be restricted towards the allowing saplings to grow appropriately, which may south and geophytes to be found more in the north, not be possible due to dense nature in the north with differences being predominantly associated aspect (3.9 trees/100 m2). Further, Sharma et al. with relative importance. He also found the (2010b) studied the effect of slope aspect in diverse maximum differences occurred in the herbaceous natural forests of Garhwal Himalaya, suggesting layer. This has also been confirmed in the Smoky the northern aspect to have higher values for total Mountains of North Carolina (35.6°N) (Whittaker basal cover (74.4 m2 ha-1), Cd (0.85) and tree 1956), the Siskiyou Mountains of Oregon and diversity (1.81). In different disturbing gradient California (42°N) (Whittaker 1960), and Santa zone (highly, moderate, and least) a study was Catalina Mountains of Arizona (32.4°N) (Whittaker conducted to understand the role of the slope aspect & Niering 1968) suggesting slope aspect to be the in the Garhwal Himalayan region, and it was found major factor controlling vegetation dynamics. that maximum species of northern aspect showed There has been an extensive range of studies good regeneration capacity. Functional regeneration being conducted in the Himalayan region. Sharma capacity was found to be maximum in moderate et al. (2010b, c) conducted a phytosociological study disturbance gradient (23.96%). At the same time, in the Garhwal Himalayan forest (30.8°N) to the southern aspect showed functional regeneration understand the growth behavior of Abies capacity to be 16.67%, maximum inadequate pindrow along varying elevational gradient. The regeneration capacity (34.09%), and none regene- basal cover for the same species was reported to be rating capacity (24.43%) is the highly disturbed highest on the north-east aspect (5099.6 cm2/100 zone (Prasad et al. 2017). m2), having a 26° slope angle and lowest on the The slope aspect also influences carbon (C) southeast aspect (3092.7 cm2/100 m2) having 50° uptake rates by altering soil moisture, tempe- slope angle. Also, the north-east aspect showed the rature, and incoming light intensity (Eamus 2003). highest concentration of dominance (Cd) (0.53) and Sharma et al. (2010b) studied the effect of the southeast showed the lowest Cd value, i.e. 0.41. slope aspect in the temperate forest region of SHIPRA SINGH 421

Garhwal Himalaya (31°N). They found maximum threshold of tolerance level (Astrom et al. 2005). total C density in the north-east aspect (291.6 Mg This may be followed by fire conditions as the ha-1) for moist Cedrus deodara forest and minimum aspect faces dryness and hotness, and chances of total carbon density for south-east aspect (77.3 Mg catching fire are higher, especially in the pine ha-1) for Quercus leucotrichophora forest in forest (Sharma & Rikhari 1997). Forest fire is Garhwal Himalaya. Gairola et al. (2012), in his more prone to the southern aspect, where there is studies on the effect of altitude on the Garhwal the presence of dry leaves and litter (Bradstock Himalayan forests, found a negative correlation 2010). The combined effect of high elevation and between density and species relation with respect cooler aspect produce the most extended fire to altitude and slope. Desta et al. (2004) conducted intervals as warmer aspects are twice as likely to a study in eastern North American deciduous burn than cooler ones (Rogeau & Armstrong 2017). forest (40°N), and found a 25–50% increment in Regions having regular fire frequency (every year) biomass, 5.5 °C cooler mean annual temperature, in south aspect showed reduction in the number of and 37% lower vapor pressure on the shady slope seedlings (360 individuals ha-1) and saplings (370 in comparison to sunny slopes. Regions where individuals ha-1) whereas in northern aspect, temperatures act as a limiting factor, the slope where the fire occurred once in a five year showed aspect is responsible for altering the number of an increment in the number of seedlings (610 growing days for photosynthesis and the C- individuals ha-1) and sapling (530 individuals ha-1) assimilation rate. This is valid for both the leaf count (Joshi et al. 2013). Studies on the effect of scale through photosynthesis, transpiration, as aspect on the flight activity of two alien pine bark well as at the ecosystem level through annual net beetles in pine forests in New Zealand (40°S) primary productivity (Chen et al. 2007). suggested an increase in the number of beetles on In the case of bryophytes, the mean density of the northern aspect. The results were found to be mosses and liverworts were found to be 88% and helpful for pest management and identification of 74% for forest and clear-cut plots in the north areas having comparatively lower pest for different aspect whereas, the percentage reduced to 79% and purposes. Warmer temperature supported an 33% for southern aspect, respectively. There was increase in the abundance of bark beetles and also also an increment in the number of bryophytes increased flight activity (Brockerhoff et al. 2017). species in the north aspect for a clear-cut plot in Species richness of rodents showed a decreasing South Central Sweden (58-59°N) (Astrom et al. pattern on the southern aspect and a hump-shaped 2005). The slope aspect is also crucial in assessing pattern in the northern aspect with increasing photosynthetic and morphological characters. The elevation (Shuai et al. 2017). net photosynthetic rate was suggested to be Heybrock (1934) investigated the altitudinal significantly lower on shady slopes and higher on limits of forest vegetation and alpine pasture in upper slopes than lower. The study conducted by Western Europe and found that, on average, the Zhao et al. (2011) in the middle of the Inner tree line is 348 m lower on north-facing than on Mongolian Plateau in China (43°N) on Caragana south-facing slopes. Vegetation patterns in microphylla showed a reduced concentration of net Jamtland (63°N), are described by Kilander (1965), photosynthetic rate, stomatal conductance, and who documents differences between the upper intercellular CO2 concentration and ratio of both on limits of species' ranges on north-facing and south- shady slopes. Furthermore, aspect is also essential facing slopes. For example, Vaccinium myrtillus in successional changes and the rate of biological reaches to 1350 m on the southerly slopes of Mt. invasion. For instance, in Kraków-Częstochowa Helago and 1200 m on its northerly flanks. An Upland (50°N), the south aspect is mostly invaded investigation of the flora of south-facing cliffs in by alien species than the north due to favorable Lappland (65°N), revealed several species which temperatures for the growth of the new species could be found on the ground level in the north and (Chmura 2008). This may be basically due to an were found to be some degrees of latitude farther increase in the amount of radiation falling on the in the south (Lundqvist 1965). Northern and surface with lower canopy cover that leads to an southern extremes in the Montane ecosystems of increase in the amplitude of most of the forest Polar regions tend to remain dark throughout organisms in the southern than in the northern winter and receive insolation throughout summer aspects. with the minor deviation between the slopes Open canopy leads to a higher insolation rate during spring and autumn (Krizek & Mida 2013). in the southern aspect region, crossing the It influences the vegetation in one way or the other 422 SLOPE ASPECT SHAPES VEGETATION ATTRIBUTES IN MONTANE ECOSYSTEMS

Fig. 2. Effect of topography (slope aspect, elevation and topographic position) on vegetation and snow cover.

(Sidari et al. 2008; Yimer et al. 2006); the snow The slope aspect governs the spatial distribution of remains for a more extended timeframe on the land incoming solar radiation, temperature, preci- in the northern aspect which affects when the land pitation, nutrient concentration, which results in will be vegetated, and there is also an effect on the the formation of micro-climatic conditions with soil properties (Fig. 2). However, sufficient studies variation in soil properties and nutrient concen- were not found for the higher latitudinal region to tration (Agren & Andersson 2011; Gutierrez-Jurado support the assumption that higher latitudinal et al. 2013). Microclimate formation due to different regions are least affected by the slope aspect. slope aspects is an output of variation in incoming Pelletier et al. (2018) said that at relatively higher solar radiation, which causes change in latitudes and elevation, equatorial-facing hillslopes temperature, moisture, humidity, and evaporation, tend to be steeper whereas, the pattern reverses at thus affecting vegetation dynamics and soil proper- lower latitudes and elevations. The most dominant ties (Eisenlohr et al. 2013). factor at higher latitudes and elevations is the Abbe (1905) reported differences in soil temperature limitation on vegetation growth. At temperature between different aspect to be as high higher latitudes and elevations, pole-facing as 30 to 40°F at a depth of 15 cm. Gail (1921), in hillslopes tend to have gentle slopes with less his study on soil, showed lower relative humidity vegetation cover, higher erosion rates, thus (22%), more evaporation (50%), 44°F greater air reversing the pattern of asymmetry found at lower temperature range, and higher wind velocities for latitudes and elevations. This was also supported by the south aspect. Cottle (1932) found lower soil Dearborn & Danby (2017). moisture by 5 to 16%, greater evaporation by 24 to 44%, increased soil temperature by 10 to 20°F at a Effect of slope aspect on abiotic factors depth of 5 cm, increased atmospheric humidity by The slope aspect governs the input of solar 5 to 11% and increased wind velocity for south energy at the landscape level, forming distinct slopes. Cantlon (1953) found increased solar microclimatic conditions (Agren & Andersson 2011). radiation and reduced moisture on south-facing These are marked with temperature changes, and slopes in Central New Jersey in North America. water content variability, which prompts nutrient The major aspect-related variation in the variation on different aspects (Wang et al. 2011). vegetation and associated abiotic factors is mainly The slopes facing southward direction in the driven by the disparity in net incoming solar Northern Hemisphere and northward in the radiation (insolation), which is suggested to be 50% Southern Hemisphere are comparatively warmer higher on southern slopes in the northern hemi- than the other slopes (Cantlon 1953; Pook 1966). sphere (Geiger et al. 2009). The ascend is chiefly SHIPRA SINGH 423

Fig. 3. Exposure induced changes in vegetation. as a function of angle of incidence of insolation, which suggests the south-facing aspect to be more oriented directly with the sun, thus, receive the Fig. 4. General trends for slope aspects on higher intensity of insolation as well as for more vegetation and soil in the northern hemisphere. significant duration (Geiger et al. 2009) and vice- much warmer than a shaded east-facing slope versa. It also causes significant differences in (Bennie et al. 2006) (Fig. 3). temperature and moisture concentration (Warren 2008). Such differences endorse differential rates of Effect of slope aspect on soil properties soil weathering, thus, favoring increased soil weathering on the south aspect as compared to the The soil and vegetation have a composite north aspect (Rech et al. 2001). However, interrelation because they develop together over a contradictory results have been found by Anderson more extended period (Sharma et al. 2010b; Singh et al. (2014), suggesting an increased weathered et al. 2017). The successional changes taking place profile on north-facing slopes. Another main effect in vegetation and also their development of contrasting slope aspect is on incoming short- influences the properties of soil (Li et al. 2013) and wave radiations. Short-wave radiations intercepting soil microorganism count (Haiyan et al. 2016) and, on a horizontal surface of the Earth is likely to be because of their development taking place together decreasing with latitude. Shaded slopes intercept for over a more extended episode, the different less direct radiation and vice-versa (Anderson & species having the requirement of macro- as well Rock 2013). It also varies with change in season as micro-nutrients and the process of feedback with maximum radiation in the winter when the mechanism by them, alter soil characteristics solar elevational angle is very low to a minimum in (Gairola et al. 2012; Singh et al. 2017). However, summer when the solar elevational angle is high. the overall positive effect of having a healthy Anderson et al. (2014) mentioned in his study that vegetation status improves the soil structure, Hinckley et al. (2014) found a 50% increment in infiltration rate, water holding capacity, hydraulic direct solar radiation in Gordon Gulch during conductivity, and aeration. There is a variation in spring season when the snow starts melting in the physicochemical properties of forest soils spatially south aspect. During the same period, the average and temporally because of variation in topography air temperature was found to differ by 2°C (slope aspect), climate, and weathering of parent (Anderson et al. 2014). Rorison et al. (1986) found a rock materials, forest cover, and microbial difference of 2.5–3°C annual mean temperature activities (Fig. 4). Although physical, chemical, and between adjacent slopes in the British calcareous biological properties of soil and its functions have landscape. Pepin et al. (2017) suggest the been well defined; it is difficult to visibly split soil temperature difference of 4 to 5°C during dawn functions into physical, chemical, and biological (sunrise) in the rainforest zone. Studies have also processes because of the very complex, webbed, been conducted on east and west slopes suggesting and dynamic properties of its mechanism (Parras- that the solar radiations are tilted to west during Alcántara et al. 2015). the peak day time in the afternoon, therefore Due to the higher incidence of solar radiation, in maximum cases, a west-facing slope tends to be there is a notable difference in moisture content; 424 SLOPE ASPECT SHAPES VEGETATION ATTRIBUTES IN MONTANE ECOSYSTEMS thus, the spatial distribution of vegetation asso- carbon content to be higher by 11–12% on the ciated with microclimate (Holland & Steyn 1975). north aspect than east aspect. The variation in Differences in the aspect and slope angle create a values obtained might be due to differences in significant difference in the availability of water litter supply and the decomposition rates of (Badano et al. 2005). This in link affects the organic matter, which are also found to be higher nutrient content of the soil as the plant might be on the north aspect due to increased moisture bound to absorb specific nutrients and return them content (Yimer et al. 2006). to the soil (Singh et al. 1986). There is a decrement Contradictory results were found for equatorial in moisture content as well as water-holding regions, showing increased carbon content for the capacity in the soil facing southern directions south aspect due to increased precipitation and (Sharma et al. 2010b). He found increased moisture reduced temperature (Sigua & Coleman 2010). Pu content (40.8%), water holding capacity (48.9%), et al. (2008) found higher organic carbon content organic carbon (3.8%), phosphorous (31.9 kg ha-1), on the windward side (east) as compared to the and nitrogen content (1.0%) values for northern leeward side (west) slope in Southwest China. The main reason attributed to this variation was aspect in Garhwal region of Indian Himalaya. Also, suggested to be increased precipitation on increased clay content and reduced sand content windward slopes (1735 mm) than the leeward side was found for north (63.4 %) and increased content (640 mm). Aspect influences soil moisture content for east (67.8%). Lenka et al. (2013) studied the and water retention capacity (Geroy et al. 2011), effect of slope aspect in soil carbon sequestration in hydraulic conductivity (Casanova et al. 2000) and Indian sub-tropics (22.8°N) suggesting that north hydrological processes in hillslopes (Hinckley et al. -1 aspect had 1.13 g 100 g greater soil moisture 2014). Kutiel (1992) studied the differences in soil content in top 15 cm soil depth and 0.75 g 100 g-1 properties in the Mediterranean region having higher soil moisture than the east aspect in 15–30 higher organic matter, sodium, chloride, cm soil depths. potassium, and different available forms of Higher cation exchange capacity and pH are nitrogen. However, the difference was so small in said to be associated with soils of the northern number and even insignificant in most cases. aspect (Sariyildiz et al. 2005), but few studies However, pH, available phosphorus, and calcium supported the opposite trend (Haiyan et al. 2016; carbonate were found to be significantly higher in Sewerniak et al. 2017). Higher clay content in soil the southern aspect. The study also showed an increases the sorption of organic carbon in deeper increased diversity in the southern slope due to the soil layers, which accounts for 25-98% of the presence of herbaceous plants. measured soil organic C stock in the soil (Basile- Temperate forests are said to be nitrogen- Doelsch et al. 2007). Shallower and well-developed limited forests (Fenn et al. 1998). The south aspect sub-soil characteristics are to be found where there shows increased weathering and erosion processes are soils with a prevailing windward aspect. At the (Rech et al. 2001), leading to variation in soil leeward side, there is more deposition of airborne nutrient concentration in different aspects as well particles. Due to an increase in deposited soils at as slope angles (Fig. 5). Soil water evaporation is the lower end of hills and mountains, there may be said to be more significant in the south, mainly due an increase in the amount of organic matter and to higher vegetation density in the north. The carbon stocks at the lower end (Lozano-Garcia et simulated soil water recharge is said to be more al. 2016). The study conducted by Lozano-Garcia et abundant in the north aspect (Wang et al. 2011). al. (2016) in native forests and reforested areas of The slope aspect also plays a significant role in soil Spain suggested northern aspect has increased soil respiration rates by altering the microclimatic organic carbon stocks for reforested areas (147.1 conditions in the post-fire environment. The study Mg ha-1) as well as native forests (110 Mg ha-1). In was conducted by Martinez-Garcia et al. (2017), contrast, the lowest value was found for southern which suggests the slope aspect to be primarily aspect (87 Mg ha-1 and 80.9 Mg ha-1, respectively, responsible for affecting soil respiration rates. The for reforested areas and native forests). Zhu et al. study conducted by Wang et al. (2011) between (2017) also found increased organic C on the north natural and regrown forest land suggests aspect. The values were found to be higher by 3.2, interception (173 and 154 mm, respectively) and 2.9, and 1.9 times than the south, southwest, and transpiration (338 and 327 mm) values to be higher west aspects. This was supported by Huang et al. on the south aspect. In contrast, the north aspect (2015) in the Chinese loess region and Lenka et al. showed reduced interception (219 and 203 mm) and (2013) in Indian sub-tropics who suggested organic vegetation transpiration rate (344 and 342 mm) for SHIPRA SINGH 425

(59.6%), urease (52.6%), L-asparaginase (62.8%), and L-glutaminase (65.7%) activities for north aspect as compared to the south. Enzymatic activities were also found to be higher in the northern counterpart. Khalili-Rad et al. (2011) found similar results for biochemical and microbial properties for the two slope position. A study conducted by Haiyan et al. (2016) on soil bacterial and Arbuscular mycorrhizal fungal (AMF) communities in the Boreal forest suggested lowest Fig. 5. Effect of slope aspect on steep slope and bacterial diversity on the north aspect and lowest gentle slope Arbuscular mycorrhizal fungal diversity on a plane region. Soil nitrogen mineralisation rate was natural and regrown forest land, respectively. Soil higher in the south aspect. The mineralisation rate water evaporation was also suggested to be higher was faster due to a 2–9°C increment in soil on the south aspect (416 mm in regrown forestland temperature at the south. There was also found to and 373 mm in natural forestland) than the north be an ample inorganic nitrogen pool in the topsoil aspect (325 mm in regrown forestland and 330 mm due to lower uptake of nitrogen as a result of in natural forestland). The water recharge value slower growth of shrubs in north than grasses in was also higher on the north aspect (90 mm for south aspect (Zhao & Li 2017). Southern aspect regrown forestland and 104 mm for natural also shows increased light penetration due to lower forestland). This might be due to increased density soil organic matter, and also, a lower amount of in the north aspect. vegetation on the south allows the animal to move Slope aspect shows stronger effect in mid- to towards plant, so the penetration resistance values high-latitudinal areas having mean annual are found to be higher beneath the canopy position temperatures less than 5°C, however, it varies (Bayat et al. 2017). from their warmer counterparts (Eger & Hewitt Predictive models related to vegetation 2008; Egli et al. 2015). Southern aspect in higher dynamics assume climatic factors to be the primary elevation at Polar regions also tends to accumulate factor responsible for variation in species more significant organic matter content but does composition, structure and diversity (Heikkinen et not show signs of increased chemical weathering in al. 2006) especially in mountainous terrain. equatorial slopes. It might accumulate soluble Therefore, the inclusion of slope aspect would salts in drought-prone areas. Thus, the slope improve models as well as inventory studies aspect has not been proven to be a significant substantially, as slope aspect creates variation in driver in controlling soil nutrient properties in arid temperature and water content in such regions. In and semi-arid climatic conditions (Kutiel & Lavee particular, the effect should be studied on local and 1999). This also might be due to reduced regional landscape level to enhance the under- vegetation density in such areas. standing of assemblage of species. For instance, Soil temperature (Miller et al. 2009) and field-based inventories should be done considering moisture (Angel et al. 2010) are also known to the morphological and physiological traits of single regulate microbial community structure. This can species, in a wide range of climatic conditions. Such also be influenced by litter input from vegetation studies will surely help in understanding the cover (Smith et al. 2010) by feeding microbes with unambiguous effect of slope aspect, especially in photosynthetic products (Staddon et al. 2003). low- and high-latitudinal regions, where the effect is Therefore, soil microbial knowledge in different still not clear. aspects can lead to a better understanding of the role of the slope aspect. Nahidan et al. (2015) Conclusion and future scope studied the effect of slope aspect on soil properties in West Central Iran and found higher values for The general aim of this review was to highlight silt, clay, organic carbon, total nitrogen, and C: N the roles played by slope aspect in governing ratio on the north aspect. Additionally, there was montane vegetation attributes and soil properties. an increment in Microbial Biomass Carbon The slope aspect creates asymmetrical patterns (46.1%), alkaline phosphomonoesterase (65.9%), across the divides of major mountains varying acid phosphomonoesterase (58.6%), aryl-sulfatase from the steeper shady aspect on the equatorial 426 SLOPE ASPECT SHAPES VEGETATION ATTRIBUTES IN MONTANE ECOSYSTEMS region to steeper sunny aspects on poles. The Åström, M., M. Dynesius. K. Hylander & C. Nilsson. study confirms that the effect of the slope aspect 2007. Slope aspect modifies community responses to can majorly be seen in the mid-latitudinal region clear‐cutting in boreal forests. Ecology 88: 749–758. followed by Equatorial and Polar regions. In low- Badano, E. I., L. A. Cavieres, M. A. Molina-Montenegro latitudinal regions, there occurs no geographically & C. L. Quiroz. 2005. 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(Received on 28.05.2018 and accepted after revisions, on 23.10.2018)