Tree diversity in a human modified riparian forest landscape in semi-arid Kenya

Forest Ecology and Management 433 (2019) 645–655

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Tree diversity in a human modified riparian forest landscape in semi-arid Kenya

T

Christine B. Schmitta,b, , Daniel Kisangauc, Kennedy W. Mathekad ⁎

a

Center for Development Research (ZEF), University of Bonn, Genscherallee 3, 53113 Bonn, Germany Institute of Nature Conservation and Landscape Ecology, University of Freiburg, Tennenbacher Str. 4, 79106 Freiburg, Germany c Department of Biology, South Eastern Kenya University, P.O Box 170 – 90200, Kitui, Kenya d East African Herbarium, Botany Department, National Museums of Kenya, Museum Hill Road, P.O. Box, 40658 – 00100, Nairobi, Kenya b

ARTICLE INFO

ABSTRACT

Keywords: Riverine forest Wooded grassland Deforestation Invasive species Restoration Conservation

Riparian forests in tropical drylands support high biodiversity and provide crucial ecosystem services. Yet, fertile soil, water availability and trees as a source of charcoal and timber make them a favourable place for settlements and subsistence agriculture. The present study aimed at evaluating the floristic diversity of riparian forest remnants in semi-arid Kenya as a basis for developing conservation and management strategies. Plant diversity was assessed along the Nzeeu and Kalundu rivers in Kitui County, Eastern Province, where riparian forest patches were intermingled with agricultural and grazing lands and invasive thickets dominated by Lantana camara. Diameter at breast height (DBH) and height of woody species (DBH > 5 cm) were recorded in a total of 74 transects (50 m × 10 m) laid out perpendicular to the rivers on both sides at 300 m intervals. In each transect, the distribution of six land cover types was mapped out and the distance of each plant individual from the river bank was noted. Overall, 631 individuals were recorded representing 85 woody species, of which 12 were exotic timber and fruit trees. Human activities mostly reached within 10 m of the river margin; indigenous vegetation covered only 12% of the transect area but had 188 tree individuals and 49 tree species (including 3 exotics), whereas agricultural land had a mean cover of 52%, 168 individuals and 39 species, including 9 exotics. Ordination and multi-level pattern analysis showed that Euphorbia bicompacta Bruyns, endemic to Kenya, and Commiphora samharensis Schweinf. were characteristic of indigenous vegetation, whereas Acacia species dominated in invasive thicket, grazing land and agricultural land. Only two species, Shirakiopsis elliptica (Hochst.) Esser and Rauvolfia caffra Sond., were clearly associated with the river bank, while the others represented a mix of riparian species with a broader ecological amplitude and typical dryland species. The study highlights that the area still supports viable remnants of indigenous riparian vegetation, whereas tree diversity on agricultural land is strongly shaped by human preferences and shows lack of recruitment. Targeted management interventions could support the maintenance of indigenous tree diversity with positive effects for overall biodiversity, soil protection and livelihood diversification. For instance, it is recommended to facilitate natural tree regeneration and to plant a variety of indigenous tree species, especially on the river banks. Further research is necessary to assess the status of riparian vegetation along similar dryland rivers in Kenya and Africa to adequately manage these important areas for biodiversity and ecosystem services.

1. Introduction Permanent and seasonal rivers are lifelines in the global dryland landscapes, which cover 41% of the world’s land area (UNCCD, 2017). They maintain vital water and ground water sources and often support forest vegetation in areas otherwise dominated by woodland or grassland. They also act as ecological corridors, provide a habitat for a variety of animal species (Bennett et al., 2014) and constitute important

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zones of nutrient and carbon processing and water retention (Owen et al., 2015). Riparian forests along tropical dryland rivers typically harbor a unique suit of plant species that may also occur in tropical moist forest areas, or at higher altitudes. The particular vegetation type in a riparian zone depends on the regional climate, especially drought seasonality, hydrology, geomorphology and disturbance regime (Larned et al., 2010, Richardson et al., 2007). Next to their biodiversity value, rivers and riparian forests provide many ecosystem services to

Corresponding author. E-mail addresses: [email protected] (C.B. Schmitt), [email protected] (K.W. Matheka).

https://doi.org/10.1016/j.foreco.2018.11.030 Received 25 July 2018; Received in revised form 14 November 2018; Accepted 18 November 2018 0378-1127/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Location of Kitui town in East Africa (A), Black dots indicate transect locations (10 m × 50 m) at both sides of the sampled rivers (B), End and start points of transects shown for part of the Nzeeu river (C).

humans, such as water, fertile soil, timber, wood and useful plant species (Kremen and Miles, 2012). For this reason, riparian forests worldwide are strongly affected by forest degradation and by conversion to other land uses with severe impacts on species diversity and ecosystem functions (DeClerck et al., 2010, Kominoski et al., 2013). Across Africa, there is ample evidence for the ongoing conversion of riparian forests to settlements, agricultural lands, grazing areas and other land uses (e.g., Badjana et al., 2017, McCarthy et al., 2015, Mligo, 2017). Intensive land use in riparian areas can lead to soil erosion with negative effects on aquatic ecosystems as well as agricultural productivity, especially in dryland areas (Gray et al., 2015, Rickson, 2014). Furthermore, riparian species like Phragmites mauritianus Kunth which used to form thickets, provide habitat for wildlife like birds and act as insurance against drought and famines both for humans and wildlife no longer exist or have been depleted to a bare minimum (Lötter, 2014). Finally, disturbed and open habitats along the dryland rivers, are quickly invaded by exotic species, such as Lantana camara L. and Prosopis juliflora (Sw.) DC. that negatively affect native plant species diversity (Richardson et al., 2007, Muturi et al., 2013). Considering the negative impacts of climate change projected for African dryland areas, the adequate management of rivers and river margins becomes ever more important (Rockström and Falkenmark, 2015, Mekonnen and Hoekstra, 2016). Today, few riparian forests remain in the drylands of Africa. In Kenya, the Tana River still supports tall riparian forests along a stretch of ca. 400 km that traverses the semi-arid Tana River District prior to reaching the Indian Ocean. They extend up to 3 km on either side of the

river and are characterized by different vegetation types depending on local edaphic and hydrological conditions (Maingi and Marsh, 2006). Although some are protected, the Tana riparian forests are threatened by conversion to other land uses and changes in the hydrological system caused by upstream dam construction (Maingi and Marsh, 2006, Leauthaud et al., 2013). While the Tana River is relatively well studied, much less information is available on its smaller, intermittent tributaries that originate in the semi-arid Eastern Province. Only recently, Habel et al. (2018) documented the massive conversion of riparian vegetation into settlements and agricultural lands along two rivers in Kitui County since the 1960 s. The strong population increase in Kitui County is typical of the Eastern Province and other dryland areas in Kenya (KNBS, 2010) and Africa (UNCCD, 2017). The related land use pressure combined with the effects of global climate change puts additional stress on dryland rivers. Considering their value for biodiversity and ecosystem services, it is key to assess to what extent these rivers still maintain riparian forest vegetation and characteristic tree species as a basis for developing conservation and management concepts. Hereby, it is important to consider that the presence of tree species in human-modified landscapes strongly depends on land use type and management decisions of local farmers (Schmitt et al., 2010, Tabuti et al., 2011). The overall goal of this study was to assess the patterns of tree species diversity and land use types along the Nzeeu and Kalundu rivers in Kitui County, Eastern Province, Kenya. In particular, the study aimed to evaluate the species diversity of the remnant forest vegetation, and to explain overall tree species distribution patterns and population 646

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structure in relation to different land use types, such as agriculture, grazing and areas dominated by L. camara. A special focus was on the potential of the landscape to maintain indigenous tree species of particular biodiversity and use value. The results are relevant for understanding forest diversity distributions along the East African dryland rivers and for designing management strategies that help maintaining riparian biodiversity and ecosystem services. Moreover, the study helps shed light on the fate of the riparian forest vegetation along the many smaller rivers in the dryland areas of Africa that have so far been little documented.

(n = 32) and the Nzeeu (n = 42) rivers at 300 m intervals; the land use pattern was mapped for each transect. At the Nzeeu river, the sampling was entirely systematic, while at the Kalundu river, we deliberately sampled the few remaining forest fragments even if we had to deviate from the pre-defined 300 m intervals (compare Fig. 1). Thus, the proportion of indigenous vegetation at the Kalundu river may be slightly overestimated. Land use types were defined as follows: 1 = indigenous vegetation (without presence of invasive shrubs); 2 = mixed vegetation (mixture of indigenous vegetation and invasive shrubs, such as L. camara, Tithonia diversifolia (Hemsl.) A.Gray and Xanthium strumarium L.); 3 = invasive thicket (dominated by L. camara); 4 = agricultural land (including agricultural fields (crops: maize, beans, pigeon peas, cowpeas) and other areas that form part of the agricultural landscape, such as field margins, paths, small areas for brick making, fallow farmland (also grazed) and small patches of degraded soil); 5 = grazing land (areas without indication of cultivation activities); 6 = woodlots (planted tree plantations). The width of the river margin indicates the distance of the agricultural land, grazing land and woodlots from the river. Hence, river margin vegetation can be indigenous, mixed or invasive thicket. The t-test function in R was used to test the difference in mean land use cover as well as mean species and individual numbers per transect between the two rivers. Difference in population structure (DBH, height) between land use types and rivers was tested using the one-way and two-way ANOVA function and the Tukey post-hoc test in R. Nonmetric Multidimensional Scaling (NMDS) of species data was performed with the metaMDS function in the R package vegan using the autotransform option and 1000 permutations. Only species with more than three individuals and transects with more than one individual were used, reducing the data matrix to 40 species and 65 transects. Subsequently, the environmental data were fitted to the ordination diagram and p-values calculated based on 10,000 permutations. The NMDS was run for one, two and three dimensions. The lowest stress level was observed for three dimensions (stress = 0.14, stress type 1); hence, the ordination was considered as usable (Clarke, 1993). Nonmetric fit (R2 = 0.982) was better than linear fit (R2 = 0.876) in the Shepard diagram. Woodlots had very low area cover, and were therefore combined with agricultural land for the purpose of the ordination analysis (labeled “cultivation”). Multi-level pattern analysis (package 'indicspecies') was used to determine species associated with the six land use types and with the river bank (0–20 m distance from river versus 20–50 m distance from river).

2. Material and methods 2.1. Study area The study area is located in the semi-arid Kitui County, Eastern Province, Kenya (1.421017°S, 38.024145°E) and covers parts of the Nzeeu and the Kalundu rivers that are located East and West of Kitui town, respectively; the two study sites were around 3 km apart from each other (Fig. 1). Both rivers are tributaries of the Tiva river, and ultimately the Tana river. The study area is flat and has an altitude of about 1064 m a.s.l. (own GPS data). In Kitui town, the 1982–2012 mean annual temperature was 21.4 °C; mean rainfall was about 1068 mm with a bimodal rainfall pattern (https://en.climate-data.org/), but the county experiences increasing rainfall variability and rising temperatures due to climate change (Jaetzold et al., 2006). Geologically, Kitui County is located within the Mozambique belt and is generally occupied by the basement complex system consisting mainly of highly impermeable metamorphic rocks (Nyamai et al., 2003). The sandy river beds usually consist of well sorted and mature sand without or with minimal silt/clay content; hence these sands are ideal in tapping and retaining water in their sub-surface pores (Kitheka, 2016). The Nzeeu and Kalundu rivers are characterized by low flows during the dry season and high flows during the two rainy seasons from March to May and October to December (Ondieki and Kitheka, 2017). According to the Atlas of Potential Natural Vegetation in Kenya (Kindt et al., 2014), the study area consists of Dry Combretum wooded grassland and Acacia-Commiphora deciduous bushland and thicket, whereby the atlas resolution is not high enough to show the riparian vegetation belts. The demographic pressure in Kenya is generally very high. The human population in Kitui district more than doubled from 216,547 inhabitants in 1969 (Jaetzold et al., 2006) to 447,613 inhabitants in 2009 (KNBS, 2010). People mostly depend on subsistence rain-fed agriculture, while periodical precipitation and limited fertility of the strongly weathered soils restrict agricultural productivity (Jaetzold et al., 2006). This is locally managed through fallow periods and further extension of agricultural land. Along the Nzeeu river almost 60% of the riparian forest vegetation has been converted into agricultural land since the 1960 s; land use change was slightly less drastic at the Kalundu river (Habel et al., 2018). Fallow land and areas along the river were often covered by monodominant L. camara thicket and other invasive shrubs. In addition to agricultural and grazing land, charcoal production, brick making, sand mining and withdrawal of irrigation and drinking water were anthropogenic activities observed along both rivers at the time of study. While the value of riparian zones is legally recognized in Kenya, fragmented legislation, overlapping mandates and lack of coordination between different institutions impede the enforcement of protection measures (Matunda, 2015).

3. Results 3.1. General tree species diversity In total, 631 plant individuals were recorded in 74 transects representing 85 woody species, of which 12 were exotic timber and fruit trees planted on agricultural land and in woodlots (Annex 1 and 4). The recorded species belonged to 29 plant families; Fabaceae (n = 20) and Euphorbiaceae (n = 9) contained most species, while the most common genera were Acacia (n = 6), Combretum and Ficus (both n = 5). The most abundant species were Acacia polyacantha Willd. (n = 71), Lannea schweinfurthii (Engl.) Engl. (n = 42) and Euphorbia bicompacta Bruyns (n = 37), which is endemic to Kenya and Red listed as vulnerable (WCMC, 1998); two further species are regional endemics (Bridelia taitensis Vatke & Pax, Euphorbia friesiorum (Hassl.) S.Carter). Conspicuous species observed outside the transects included the endemic scandent shrub Combretum tanaense J. J. Clark, the endangered African sandalwood (Osyris lanceolata Hochst. & Steud. ex A. DC.) (Orwa et al., 2009), and baobab trees (Adansonia digitata L.) that occurred on farmland. At the Kalundu river, we recorded 368 individuals and 67 woody

2.2. Methods Field surveys were conducted during the dry season in February and March 2016. Diameter at breast height (DBH), height and distance from river were recorded for woody species (DBH ≥ 5 cm) in transects (50 m × 10 m) laid out perpendicular to both sides of the Kalundu 647

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Table 1 Mean values for percentage land use type, number of species and width of river margin in transects at the Nzeeu (n = 42) and Kalundu (n = 32) rivers. Significant differences indicated by letters (t-test, p < 0.05 for a, b, p < 0.01 for c, d). River

Nzeeu Kalundu

Mean proportion of land use type in transect (%)

Mean number in transects

Indigenous

Mixed

Invasive

Agriculture

Grazing

Woodlot

Individuals

Species

Exotic species

Margin

5.2a 21.7b

13.6 17.5

8.3 6.5

60.3a 41.7b

9.6 9.6

3.0 3.1

6.3c 11.5d

3.7c 6.2d

0.7 0.4

8.0a 15.4b

species in 32 transects, while at the Nzeeu river, we recorded 263 individuals and 55 woody species in 42 transects. The average number of individuals and species per transect was significantly higher at the Kalundu than at the Nzeeu river (Table 1). Overall, 53% of the species had less than 4 individuals and 74% had a frequency of below 5%, indicating high variability in the species composition of the transects. The pockets of remaining vegetation were sanctuaries of species diversity with 15 tree species in a transect of remnant forest at the Kalundu river.

agricultural land, respectively. The highest numbers of exotic species and exotic individuals were recorded on agricultural land and in woodlots. It is noteworthy that invasive thicket and grazing land virtually had the same proportion of land cover, but species diversity was much higher in the latter (10 and 28 indigenous species, respectively) (Table 2). 3.3. Population structure Mean DBH of the recorded individuals was 16.4 cm and mean height was 7.0 m. Maximum height was 22 m in A. polyacantha and Acacia robusta Burch., while maximum DBH was 87 cm in Ficus sycomorus L.. Overall, two-way ANOVA and Tukey post hoc test indicated that mean DBH was significantly higher on agricultural land (20.4 cm) than in indigenous vegetation (14.6 cm), on grazing land (12.8 cm) and in woodlots (11.5 cm). Moreover, the mean height of woody individuals was significantly lower on grazing land (4.8 m) than in any of the other land use types. It is noteworthy that planted exotic species such as lemon, guava and Grevillea were common in the smallest DBH size class (5 to < 10 cm) on agricultural land (compare Table 2). Concurrently, Fig. 2 highlights the lack of indigenous tree regeneration on agricultural land. The number of indigenous species in the smallest DBH size class was highest in indigenous vegetation (n = 32), followed by mixed vegetation (n = 23), grazing land (n = 21), agricultural land (n = 17), invasive thicket (n = 9) and woodlots (n = 5). Woodlots were not included in Fig. 2 because 68% of the individuals were exotic and mean land cover was very low (Table 2). The majority of the common indigenous tree species (n > 5) had more than one third of their individuals in the smallest DBH size class (Annex 1). E. bicompacta, Commiphora samharensis Schweinf. and Shirakiopsis elliptica (Hochst.) Esser exemplify species characteristic for indigenous vegetation with a balanced population structure (Fig. 3); the first two are described as small trees with a maximum height of up to 9 m. By contrast, species such as Rauvolfia caffra Sond., F. sycomorus and Kigelia africana (Lam.) Benth. displayed a bias towards older individuals, whereby the latter two mostly occurred on agricultural land (Annex 1).

3.2. Land use types The dominant land use type along the bank of the Nzeeu river was mixed thicket (in 57% of the transects), followed by indigenous vegetation and invasive thicket (both 18%). There were few transects where grazing (n = 2) or agriculture (n = 1) occurred up to the river bank. Along the bank of the Kalundu river, the dominant land use type was mixed thicket (in 48% of the transects), followed by indigenous vegetation (33%). There were some transects where invasive thicket (n = 1), grazing (n = 3), agriculture (n = 1) or woodlots (n = 1) occurred up to the river bank. The average width of the river margin vegetation was much wider at the Kalundu than at the Nzeeu river (Table 1), while the width of the river margin was below 5 m in 32% and 18% of the transects at the Nzeeu and the Kalundu river, respectively. There was a significant difference between the two rivers with regard to the land uses encountered in the transects (Table 1). On average, the proportion of indigenous vegetation was higher along Kalundu (22%) than Nzeeu river (5%). Overall, there were 3 and 1 transects at the Kalundu, and the Nzeeu river respectively, that had 100% of indigenous vegetation. Personal observation indicated that the higher proportion of indigenous vegetation along the Kalundu river was related to the higher frequency of very steep river banks that were not suitable for livestock grazing or agricultural activities. In the following, the species and land use data for the two rivers were combined because both were located within the same ethnic and ecological zone. Overall, indigenous vegetation contained 58% of all species but made up only 12% of the total land cover (Table 2). By contrast, the agricultural land use type contained 46% of all species, despite covering 52% of the sampled area. If exotic species were not included in the equation, the results were even more drastic: 54% and 35% of all indigenous species in indigenous vegetation versus

100% 90% 80% 70%

Table 2 Land use type and species diversity in the study area (all transects, n = 74). Land use type

Indigenous Mixed Invasive Agriculture Grazing Woodlot

Mean width (m)

Mean cover in transects (%) 12 15 8 52 10 3

Number of (Percentage of total n) Individuals (n = 631) 188 (30%) 120 (19%) 44 (7%) 168 (27%) 64 (10%) 47 (7%)

Species (n = 85) 49 (58%) 40 (47%) 12 (14%) 39 (46%) 29 (34%) 13 (15%)

Exotic species (n = 12)

Exotic individuals (n = 92)

3 3 2 9 1 6

3 11 2 43 1 32

40+ cm

60% 50%

20- <40 cm

40%

15- <20 cm

30%

10- <15 cm

20%

5- <10 cm

10% 0% Indigenous (185)

Mixed (109)

Invasive Agriculture (42) (125)

Grazing (63)

Fig. 2. Proportion of indigenous tree individuals in five land use types by size class for diameter at breast height (DBH); total number of indigenous tree individuals in brackets. 648

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Table 3 Multi-level pattern analysis for species and land use types; mixed vegetation and woodlots did not have significant indicator species.

90% 80% 70% 40+ cm

60%

20- <40 cm

50%

15- <20 cm

40%

Land cover type and combination

Species

Indicator value

p-value

Indigenous

Euphorbia bicompacta Commiphora samharensis Acacia tortilis Acacia nilotica Acacia polyacantha

0.603 0.577

0.002** 0.004**

0.552 0.547 0.663

0.015* 0.015* 0.003**

10- <15 cm

30%

5- <10 cm

20%

Invasive & Grazing

10%

Invasive & Agriculture & Grazing

0% Euphorbia Commiphora Shirakiopsis Rauvolfia caffra Ficus Kigelia africana bicompacta samharensis (7) ellipticum (27) (13) sycomorus (12) (6) (37)

Fig. 3. Proportion of individuals in five size classes (diameter at breast height, DBH) for six indigenous tree species in the study area; total number of individuals in brackets; for more information see text.

4. Discussion

3.4. Relationship between species diversity and land use types

In this study, 12 out of 85 woody species were exotic timber and fruit trees and the remaining 73 species comprised species known from riparian forest as well as dryland vegetation types (Kindt et al., 2014). Overall woody species diversity (n = 85) was relatively high when compared to assessments from other woodland areas. For example, 44 tree species were sampled in the Mutomo District of Kitui County, located in Acacia-Commiphora bushland and thicket (Ndegwa et al., 2016). In fact, species numbers are often higher in disturbed habitats and ecotones than in a comparable single vegetation type (Shekhar and Azim, 2010). Some of the more common species in this study were also recorded in the Tana River gallery forest, notably A. robusta, Acacia tortilis (Forssk.) Hayne, F. sycomorus, K. africana and Maerua triphylla A.Rich. (Maingi and Marsh, 2006); Tabernaemontana ventricosa Hochst. ex A.DC. and Acacia xanthophloea Benth. are described as characteristic riparian forest species (Kindt et al., 2014). Moreover, two tall tree species were clearly associated with the river bank, and thus presumably high ground water tables: S. elliptica, known from Afromontane forest and swamp, riverine, moist and dry forest, and R. caffra, known from riverine and swamp forest as well as Dry Combretum wooded grassland (Beentje, 1994, Kindt et al., 2014). These two species highlight the rather broad ecological amplitude of many riparian species that are able to adapt to a variety of environmental conditions. Similarly, A. polyacantha, F. sycomorus and K. africana are characteristic of riparian forest, but may also occur in Dry Combretum wooded grassland (Kindt et al., 2014, Beentje, 1994). In addition, many of the

4.1. Distribution of riparian and dryland vegetation

Nonmetric Multidimensional Scaling (NMDS) confirmed that transects with a high cover of cultivated land also had a high number of exotic species, such as Grevillea, guava, lemon, Eucalyptus and Senna siamea (Lam.) H.S.Irwin & Barneby (Fig. 4). Those transects differed strongly from transects with a high proportion of grazing land that had virtually no exotic species (compare Table 2). Transects with a high proportion of indigenous vegetation had higher individual numbers and higher species numbers. By contrast, transects with a high proportion of invasive thicket had lower numbers of species and individuals and were strongly associated with A. polyacantha (compare Table 3). The mixed vegetation type did not have a high explanatory power in terms of species diversity. The multi-level pattern analysis highlighted only few species that were significantly related to one or several land use types (Table 3). This was due to the fact that most species occurred with only few individuals in a land use type (Annex 1). Three Acacia species were characteristic of agricultural land, invasive thicket and grazing land (Table 3), while A. robusta was also common in indigenous vegetation (see Annex 1). Multi-level pattern analysis also indicated that there were only two indicator species for the river margin (0–20 m distance from the river), namely S. elliptica and R. caffra, which occurred almost exclusively in indigenous and mixed vegetation (Annex 1).

Fig. 4. Nonmetric Multidimensional Scaling (NMDS) of 65 transects and 40 species represented as points and with abbreviated names where space allows (for full names see Annex 1). Arrows indicate proportion of land use type (cultivation = agriculture and woodlots combined, see Methods section) and diversity information for transects; diversity information is with asterisk: number of individuals, number of species and number of exotic species; the longer the arrow, the higher vector significance (see Annex 2).

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encountered species are clearly associated with wooded grassland and bushland (e.g., Combretum molle G.Don, Combretum schumanii Engl., Steganotaenia araliacea Hochst., Terminalia brownii Fresen.) (Kindt et al., 2014, Beentje, 1994), or were also recorded in Acacia-Commiphora bushland and thicket, such as Acacia nilotica (L.) Willd. ex Delile, A. tortilis, Combretum collinum Fresen., Commiphora africana (A.Rich.) Engl., Ficus thoningii Blume and L. schweinfurthii (Ndegwa et al., 2016). The remnants of tall forest vegetation at the Kalundu river that covered between 37 and 50 m of the transect length indicate that seasonal rivers are able to support a riparian forest belt. In fact, riparian forests can occur up to 3 km on either side of a river depending on size of the river, annual floods and seepage (Maingi and Marsh, 2006, Stave et al., 2006). Yet, riparian vegetation structure and composition may vary considerably depending on the depth of the below-ground water table, which is in turn influenced by the underlying soil types. Furthermore, rocks may locally prevent trees from reaching ground water with their tap roots. Given the fact that there was no detailed hydrological account available for the study area, it is not possible to conclusively interpret species distributions in relation to the riparian habitat. Besides, ordination and multi-level pattern analysis indicated a strong influence of human land use activities on species composition.

latter could also help determine which species are maintained because of their use value, and which are threatened by overuse. 4.3. Exotic species and invasive thicket The number of exotic tree species was highest on agricultural land and in woodlots because farmers plant these species mostly for timber and fruits, but also for a larger variety of local uses (Annex 4). Other studies highlighted that most trees planted by local communities are exotic species selected for their nutritional and commercial value and their relatively short gestation period (Ndegwa et al., 2017). None of the exotic tree species recorded in the study area are classified as invasive species, at least not in the East African drylands (Foxcroft et al., 2010). However, eucalyptus is known to induce soil degradation, decline of groundwater level, and may inhibit the growth of understory plant species (Yong et al., 2009). Guava can outcompete native plants in some areas (Obiri, 2011), and in this study it spread spontaneously in indigenous and mixed vegetation, albeit at few individual numbers. Next to L. camara, there were two further exotic shrubs T. diversifolia and X. strumarium present in mixed vegetation that are classified as invasive in East Africa and other parts of the world (CABI, 2018, BioNET-EAFRINET, 2011). At the time of study, they did not form mono-dominant thickets and there was still potential for the regeneration of indigenous species (Fig. 2). However, the mixed vegetation type that is mostly found along the river bank may develop into invasive thicket in the future if no management interventions are taken (Meek et al., 2013). Mono-dominant L. camara thickets were present along the river and on abandoned agricultural fields that were not cultivated due to a fallow period or because owners had moved away (personal comments, March 2016). The shrub is a widely distributed invader in the drylands of East Africa; it can form dense thickets which are difficult to eradicate once established, making extensive areas unusable and inaccessible, and suppressing the regeneration of the native flora (Simba et al., 2013, Furukawa et al., 2011). Invasive thicket had the lowest number of indigenous species (n = 10), whereby two thirds of the individuals were made up of Acacia species (Table 3), illustrating the negative effect of L. camara on species diversity. Local farmers, however, did not consider the shrub as a severe problem. Several farmers stated that L. camara can easily be cleared and burned after the required fallow period. The plant has uses in folklore medicine (Kokwaro, 1993) and can have positive effects on soil properties (Fan et al., 2010, Osunkoya and Perrett, 2011). L. camara thickets were also shown to provide surrogate habitat for understorey bird species (Habel et al., 2016). Thus, the temporary invasion of L. camara on fields during fallow periods may not have negative effects, whereas L. camara invasion in areas that are not managed locally, such as along the river or on permanently abandoned fields may suppress native species diversity. Finally, grazing land is another land use type with drastic effects on tree diversity and plant population structure. Still, grazing lands maintain relatively high indigenous species numbers as compared to agricultural land because regeneration is not suppressed by local farmers and thorny species create pockets of vegetation that provide shelter from the browsing livestock. There are three Acacia species characteristic for invasive thicket and grazing areas (Table 3). Apparently, they manage to regenerate even under dense cover of L. camara and they are not too much affected by livestock browsing due to their thorns. A. tortilis, an indicator species of grazing land and invasive thicket, occurs abundantly throughout East Africa, especially along rivers and streams, and is typically dispersed by animals as its seed pots are an attractive feed during the dry season (Kindt et al., 2014, Stave et al., 2006). Digestion enhances the germination of A. tortilis seeds because it breaks seed-dormancy and removes the seed-predating bruchid beetles (Stave et al., 2006). This explains why A. tortilis is most abundant in areas where livestock browsing is or was common. Besides the shading effect of L. camara might be positive for seedling

4.2. Human uses and tree diversity Despite the strong anthropogenic influence, the pockets of indigenous vegetation in the study area still harbor relatively high species numbers, including endemic and threatened species. Ecological competition may explain the fact that the Acacia species, dominant in the other land use types, only occurred with low individual numbers in indigenous vegetation (Tefera et al., 2008). Most of the more common indigenous species had a viable population structure with a high number of regenerating individuals (Annex 1). However, the current tree species diversity cannot be maintained if the ongoing land use change continues unabated. This is exemplified by the fact that the agricultural land use type covered more than 50% of the transect area, but harbored only 35% of the indigenous species. Furthermore, there was a much lower proportion of regenerating individuals on agricultural land (Fig. 2), because large trees are maintained as shade trees, while regenerating individuals are removed as an integrative part of crop management (Oeba et al., 2012). For instance, local farmers are unlikely to remove sacred Ficus trees, which are amongst the largest tree individuals in this study. The lack of regenerating individuals seems to be a common feature in Ficus species related to their particular pollination requirements and long-distance seed dispersal (Harrison, 2005). By contrast, low recruitment in K. africana (Fig. 3) could be caused by management interventions and may be a problem for the regeneration of this tree species in the study area, which is valued locally for the use of its bark in local brews (Annex 3). Finally, the most abundant tree species on agricultural land, A. polyacantha, has wood that is of little use for local farmers who mostly maintain the species as a shade tree. By contrast, the other Acacia species may be less abundant because farmers are keen to use their hard wood for charcoal, furniture and other purposes (Annex 3). For instance, A. tortilis has been identified as one of the major high value tree species in the arid and semi-arid areas, especially for charcoal making, and is therefore likely to be over-exploited from the wild (Endale et al., 2017). Annex 3 indicates that virtually all of the indigenous tree species encountered on agricultural land have local uses. Hence, for the many indigenous species with few individuals in this study it is not possible to conclude if ecological effects, such as dispersal and regeneration requirements, human management interventions, or simply stochastic effects impede regeneration. This would require a more comprehensive study, including the assessment of seedlings and saplings, fencing experiments to allow for natural regeneration in managed areas and a survey of management preferences amongst the local farmers. The 650

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were encountered in the study area. For most of these species, seedlings are not easily available in nurseries and wild populations of African Blackwood and African sandalwood are reported to be endangered. Hence, further study is required on tree species regeneration, population management and commercialization. It must be recognized, however, that the agricultural land in the study area is owned privately and local land owners will need a strong incentive to change their current land use practice. Above all, local farmers gain immediate revenues from agriculture and exotic tree plantations whereas the subtle effects of land use change on ecosystem services such as ground water availability and pollination may be more long-term and not that obvious. In Central Kenya, increased tree retention on farm was directly related to higher levels of formal employment, income, education and technical skills (Oeba et al., 2012). Hence, universities, NGOs and government authorities need to collaborate closely with local farmers and farmer cooperative to develop visions for the future use and management of the riparian zones in dryland areas. This also holds to true for waterway management where sand mining, withdrawal of irrigation water, livestock watering and the washing of clothes and vehicles contribute to water pollution, river bank erosion and a decline in ground water tables (Pitchaiah, 2017).

establishment (Stave et al., 2006). 4.4. Outlook and recommendations Overall, the results indicate that human land use activities lead to a strong decline in tree diversity and an increase in Acacia individuals in the riparian zone of dryland rivers. In addition, there is a decrease in the number and diversity of regenerating indigenous trees on agricultural land and in invasive thicket. Hence, indigenous tree diversity will further decline in the study area if land use change continues at the current rate. This is exacerbated by the fact that local farmers selectively maintain only certain indigenous tree species on agricultural land and in agro-forestry systems (Tabuti et al., 2011, Shumi et al., 2018). A decline in the indigenous tree species diversity is likely related to a decline in the associated insect, bird and small mammal communities (Terborgh, 2015). In central Kenya, low tree diversity on agricultural lands was already shown to have negative effects on pest management, pollination, and the associated faunal diversity (FAO, 2007). In addition, the decline in forest and tree cover can lead to river bank erosion and declining ground water tables (Ilstedt et al., 2016). To counteract the negative effects of land use and land use change, it is recommended to protect the remaining pockets of indigenous vegetation as a refuge for plant and animal species and as a source of indigenous seeds and seedlings. It is also crucial to promote indigenous tree regeneration, e.g., by using fencing methods on agricultural land, along the river banks and in grazed areas. This is especially important at the Kalundu river that has steep river banks and where the effects of erosion are already visible. Furthermore, collective action should be taken to remove L. camara in abandoned areas that are not subject to temporary fallow periods. Finally, the planting of indigenous tree species can make a contribution to soil protection, biodiversity conservation and income diversification for local farmers. In other Kenyan dryland areas, tree nurseries, mango orchards and woodlots proved to be viable local enterprises, although their success was constrained by disorganized market linkages, high transport costs, access to credit and low technical knowhow (Andika et al., 2014). So far, most of the successfully commercialized tree species are exotic, which is also reflected by the variety of exotic species in this study. However, there is a growing body of experience in planting indigenous species in tree plantations for fuel wood production (e.g., Acacia ssp., Oduor et al., 2012). In addition, there are wild fruit enterprises that use tamarind Tamarindus indica L., Thespesia garckeana F.Hoffm., baobab and Vitex species for juice and jam production (Andika et al., 2014, Chiteva et al., 2016, Muok et al., 2000). Further commercially interesting indigenous species are African Blackwood Dalbergia melanoxylon Guill. & Perr. (hard wood, carvings), T. brownii (medicinal, household tools), L. schweinfurthii (medicinal, timber, household tools), Albizia anthelmintica Brongn. (dewormer) and African sandalwood O. lanceolata (oil perfume production), all of which

5. Conclusions The study exemplifies that the riparian zones of dryland rivers constitute groves of biodiversity that sustain crucial ecosystem services but are under severe land use pressure. Further study is needed to better understand biodiversity patterns along these rivers, dispersal and regeneration requirements of riparian species and the hydrological functions of riparian zones. As global climate change will put additional pressure on dryland rivers and the adjacent agricultural areas, concerted action is required to ensure that riparian biodiversity and ecosystem services are maintained. This can only be achieved through transdisciplinary approaches whereby government authorities, NGOs and universities collaborate closely with local land users. Acknowledgement We acknowledge support from Rebekka Honecker (data collection, data analysis), Denis Sahler (data collection), Tobias Kirchbaur (data analysis), Carl Anderson (GIS) and PD Dr. Jan Habel (facilitation of field work). This work was carried out under the project “Reconciling human livelihood needs and nature conservation” funded by the DAAD Quality Network Biodiversity. We also received support from “BiomassWeb - Improving food security in Africa through increased system productivity of biomass-based value webs” [grant number 031A258A] funded by the German Federal Ministry of Education and Research (BMBF).

Annex 1. Alphabetical list of tree species (DBH ≥ 5 cm) recorded in the study area and abundance in six land use types; species with asterisk are exotic (see Annex 4); regeneration is the percentage of individuals in the smallest DBH size class (5 – < 10 cm) calculated for indigenous species with more than 5 individuals Species Acacia brevispica Harms Acacia nilotica (L.) Willd. ex Delile Acacia polyacantha Willd. Acacia robusta Burch. Acacia tortilis (Forssk.) Hayne Acacia xanthophloea Benth. Albizia amara (Roxb.) Boivin Albizia anthelmintica Brongn. Albizia gummifera (J.F.Gmel.) C.A.Sm. Allophylus rubifolius (Hochst.) Engl. *Annona squamosa L.

Indigenous 1 1 7 3 1 2 1 2

Mixed

Invasive

Agriculture

Grazing

1 3 6 12 3 1

2 17 7 5

2 38 6 9 4

9 6 2 8

1

1

1

651

1 1

Woodlot

3 1

Total abundance 1 17 71 34 29 6 3 1 2 3 1

Regenaration 41% 24% 35% 41% 0%

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C.B. Schmitt et al. Breonadia salicina (Vahl) Hepper & J.R.I.Wood Bridelia taitensis Vatke & Pax *Casuarina equisetifolia L. *Citrus sinensis (L.) Osbeck Clausena anisata (Willd.) Benth. *Cocos nucifera L. Combretum collinum Fresen. Combretum molle G.Don Combretum mossambicense (Klotzsch) Engl. Combretum schumanii Engl. Combretum spec. 3 Commiphora africana (A.Rich.) Engl. Commiphora edulis (Klotzsch) Engl. Commiphora samharensis Schweinf. Croton dichogamus Pax Croton megalocarpus Hutch. Dalbergia melanoxylon Guill. & Perr. Dichrostachys cinerea (L.) Wight & Arn. Dombeya kirkii Mast. Erythrina abyssinica DC. *Eucalyptus spec. Euclea divinorum Hiern Euphorbia bicompacta Bruyns Euphorbia friesiorum (Hassl.) S.Carter Euphorbia scheffleri Pax Euphorbia tirucalli L. Ficus ingens (Miq.) Miq. Ficus natalensis Hochst. Ficus sycomorus L. Ficus thonningii Blume Ficus vasta Forssk. *Grevillea robusta R.Br. Grewia bicolor Juss. Grewia plagiophylla K.Schum. Grewia similis K.Schum. Grewia tephrodermis K.Schum. Haplocoelum foliolosum (Hiern) Bullock *Jacaranda mimosifolia D.Don *Jatropha curcas L. Kigelia africana (Lam.) Benth. Lannea rivae (Chiov.) Sacleux Lannea schweinfurthii (Engl.) Engl. *Leucaena leucocephala (Lam.) de Wit Philenoptera bussei (Harms) Schrire Maerua kirkii (Oliv.) F.White Maerua triphylla A.Rich. *Mangifera indica L. Markhamia lutea (Benth.) K.Schum. Gymnosporia putterlickioides Loes. Nuxia oppositifolia (Hochst.) Benth. Ochna ovata O.Hoffm. Ormocarpum kirkii S.Moore Piliostigma thonningii (Schumach.) Milne-Redh. Pittosporum viridiflorum Sims Premna oligotricha Baker *Psidium guajava L. Psydrax schimperiana (A.Rich.) Bridson Rauvolfia caffra Sond. Rhus natalensis Krauss Rothmannia fischeri Bullock *Senna siamea (Lam.) H.S.Irwin & Barneby Senna singueana (Delile) Lock Senna spectabilis (DC.) H.S.Irwin & Barneby Shirakiopsis elliptica (Hochst.) Esser Steganotaenia araliacea Hochst. Tabernaemontana ventricosa Hochst. ex A.DC. Tamarindus indica L. Terminalia brownii Fresen. Thespesia garckeana F. Hoffm. Vangueria madagascariensis J.F.Gmel. Vitex strickeri Vatke & Hildebrandt Ximenia americana L. Zanthoxylum chalybdeum Engl. Ziziphus abyssinica A.Rich.

2 2

1

1

1

2 13

1 2 1 1

7 1 1 1 7 4 1 1 4

30 4 2 1 2

1 6 2 1 1 13 1 3 1 5

1 1

2 1 1 7

7 1 1 7

3 2

1

1 2 1

3

3

1 1 2

1 2 1 2

2 5

4

1

7 3

1

5 1 1

1

1

1 2 1

9

6 3 10

1 1

1

1

3

1 2

1 13 5 2 2 1

1

4 4

6 2 1 7 2

1

1 3

1

4

4

5 2 1

1

1

2

1 3

17 5

7

1

11 2

9

6

2

1

4 1 7 5

1

1

1

652

1 6

1 1 1 1

1 2 2 1 1 1

1

1

16 1

7

2 3 1 7 3 1 6 25 1 9 1 6 2 7 4 5 1 4 6 3 16 1 37 5 9 4 1 2 12 4 1 13 1 1 1 9 2 2 1 6 1 42 8 3 1 9 9 4 2 1 1 2 4 2 2 9 1 13 2 1 24 8 1 27 6 6 2 35 7 1 1 1 1 1

50% 35% 56% 67% 86%

83%

70% 89%

8%

67%

17% 21%

67%

8%

100% 44% 67% 50% 49% 58%

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C.B. Schmitt et al.

Annex 2. Significance of vectors overlaid onto to the NMDS diagram (Fig. 4) Vectors indicate land use type (cultivation = agriculture and woodlots combined, see Methods section) and diversity information for transects; diversity information is with asterisk: Number of individuals, number of species and number of exotic species Vectors

NMDS1

NMDS2

r2

Pr (> r)

Indigenous Mixed Invasive Grazing *Individuals *Species *Exotics Cultivation

0.83637 0.98293 0.17907 0.99841 0.74402 0.85349 −0.94010 −0.99856

0.54816 −0.18398 −0.98384 0.05631 0.66816 0.52111 0.34090 −0.05361

0.2557 0.0045 0.1197 0.0979 0.1157 0.2306 0.4852 0.3286

0.0005999*** 0.8662134 0.0193981* 0.0395960* 0.0253975* 0.0005999*** 9.999e−05*** 9.999e−05***

Significance codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1. Permutation: free. Number of permutations: 10,000.

Annex 3. Individual numbers and uses of indigenous species recorded on agricultural land (DBH ≥ 5 cm); only species with more than one individual listed Species

Local Kamba name

No. indiv.

Uses

Acacia polyacantha Willd. Acacia tortilis Hayne

Museelele Muaa

38 9

Combretum molle R.Br. ex G.Don Terminalia brownii Fresen. Acacia robusta Burch. Lannea schweinfurthii (Engl.) Engl Ficus sycomorus L. Azanza garckeana (F. Hoffm.) Exell et Hillc. Acacia xanthophloea Benth.

Muama

7

Muuku Kithi Kyuasi

7 6 6

no charcoal value (soft wood) (pers. com.); fuelwood, remedy against snakebite (Orwa et al., 2009) browse in terms of leaf litter and nutritious seed pots (Stave et al., 2006); hard wood, good for charcoal making (Ndegwa et al., 2016); fuelwood, treating wounds (Orwa et al., 2009); dry pods edible, gum is edible by Maasai, Turkana and Pokot (Maundu and Tengnas, 2005) flowers useful in honey production, firewood, charcoal, medicinal (Orwa et al., 2009); firewood, charcoal, construction poles, bee forage, tool handles (Maundu and Tengnas, 2005) construction poles, bee hives, tool handles, utensils, soil conservation, shade, medicinal (Maundu and Tengnas, 2005) furniture, shelves and yokes, fire wood, animal feed, medicinal (Lemmens, 2006) no charcoal value (soft wood) (Ndegwa et al., 2016)

Kikuyu Kitoo

6 5

sacred tree (pers. com.); river bank stabilization, shade, soil improvement, fruits edible (Orwa et al., 2009) fruit sold on local markets (Muok et al., 2000); firewood, fiber from inner bark, bows, medicine (Orwa et al., 2009)

Musewa, Mweya Kiatine –

4

timber, firewood, medicine, gum edible, bee forage, fodder (Orwa et al., 2009)

4 4

Kithulu

3

Piliostigma thonningii (Schumach.) Milne-Redh. Ficus thonningii Blume

Mukolokolo

3

Muumo

3

Acacia nilotica (L.) Delile

Kyuasi

2

Commiphora africana (A.Rich.) Engl. Markhamia lutea (Benth.) K.Schum. Erythrina abyssinica Lam. ex DC.

Ikuu

2

Kyoo

2

used for local brew (pers. com.); medicinal (wound healing) (Agyare et al., 2013) medicinal (wounds and sore eyes), fruits edible, construction poles, making stools, beds and knife sheaths (Schmelzer, 2006) seeds incorporated in poultry feeds, fire wood, honey production, ornamental, shade, medicine (intestinal worms and whooping cough) (Orwa et al., 2009) fiber, dye, thirst-quencher, poles and timber for construction, firewood, charcoal, bee forage, medicine (wounds and ulcers) (Lemessa, 2010) no charcoal value (soft wood) (Ndegwa et al., 2016); medicine (diarrhea, urinary tract infections, diabetes mellitus, gonorrhea and respiratory infection) (Dangarembizi et al., 2013) fodder for livestock (Stave et al., 2006); hard wood, good for charcoal making (Ndegwa et al., 2016); medicinal (antimutagenic, spasmogenic, vasoconstrictor, anti-pyretic, anti-asthamatic, cytotoxic, anti-diabetic) (Ali et al., 2012) no charcoal value (soft wood) (Ndegwa et al., 2016); gum edible, medicinal (fruits chewed for tooth ache and gum), building poles, live fence (Orwa et al., 2009) construction, furniture, agroforestry, medicinal (toothache, stomachache, skin infection), bee forage (Maroyi, 2012)

Kivuti

2

Kigelia africana (Lam.) Benth. Tabernaemontana ventricosa Hochst. ex A.DC. Croton megalocarpus Hutch.

no charcoal value (soft wood) (Ndegwa et al., 2016); making carvings and household tools, soil conservation, live fence, dye, medicinal (peptic ulcers, epilepsy, malaria, blennorrhagia and schistosomiasis, diarrhea) (Aerts, 2008)

Annex 4. Individual number and uses of the twelve exotic species recorded in the study area (DBH ≥ 5 cm) Species

Local Kamba name

No. indiv.

Uses

Annona squamosa L.

Kitoomoko

1

Casuarina equisetifolia L. Citrus sinensis (L.) Osbeck (pro. sp.) Cocos nucifera L.

– Kisungwa

1 7

fruit (Andika et al., 2014); insecticidal, an anti-tumor agent, anti-diabetic, anti-oxidant, anti-lipidimic and anti-inflammatory agent, ulcers and aounds (Gajalakshmi et al., 2011) timber (Andika et al., 2014); soil fertility, antioxidant (Aher et al., 2009), fruit (Andika et al., 2014); fever, anti-microbial (Fisher and Phillips, 2008)

Munathi

1

Eucalyptus spectabilis F.M- Musanduku uell. Muviliti

16

fruit (Andika et al., 2014); oil extraction, oil used for cooking, coconut milk drunk fresh, thatching (Lim, 2012a); antihelminthic, anti-inflammatory, antinociceptive, antioxidant, antifungal, antimicrobial, and antitumor activities (Lima et al., 2015) tree nursery, poles, timber (Andika et al., 2014); anti-microbial (Swamy et al., 2016)

13

653

Forest Ecology and Management 433 (2019) 645–655

C.B. Schmitt et al. Grevillea robusta A. Cunn. ex R. Br. Jacaranda mimosifolia D.- Jakalanda Don Jatropha curcas L. Kyaiki kya kyeni Leucaena leucocephala (L- Lusina am.) DeWit. Mangifera indica L. Kiembe Psidium guajava Senna siamea

9 24

Kivela Muvengele

1

tree nursery, charcoal, firewood, poles, timber (Andika et al., 2014); bee forage, used to fuel locomotives and river steamers (Orwa et al., 2009) tree nursery, poles, timber (Andika et al., 2014); antioxidant antidepressant, antimicrobial, anticancer, anti-leishmanial, antiprotozoal, hypotensive and anti-hypertriglyceridemic activities (Mostafa et al., 2014) medicinal uses (skin infections) (Prasad et al., 2012); malarial fevers, diuretic, coughs, convulsions and fit (Manandhar, 2002)

8

timber (Andika et al., 2014); roasted seeds are used as an emollient, abortifacient (Lim, 2012b)

9

tree nursery, fruit (Andika et al., 2014); antioxidant, radioprotective, antitumor, immunomodulatory, anti-allergic, antiinflammatory, antidiabetic, lipolytic, antibone resorption, monoamine oxidase inhibiting, antiviral, antifungal antibacterial and antiparasitic properties (Wauthoz et al., 2007) fruit (Andika et al., 2014); intestinal worms, dysentery (Kareru et al., 2007) timber (Andika et al., 2014); antimicrobial, antimalarial, antidiabetic, anticancer, hypotensive, diuretic, antioxidant, laxative, anti-inflammatory, analgesic, antipyretic, anxiolytic, antidepressant, and sedative activities (Kamagaté et al., 2014)

2

Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.foreco.2018.11.030.

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