Longitudinal vegetation turnover in an eastern Rift Valley riparian corridor

East African riparian corridors are biodiversity hot spots that occur in catchments degrading under deforestation and overgrazing. Quadrats belt transects were employed to investigate plant succession along the River Gilgil, Kenyan Rift Valley. The study found most species (total 365) broadly distributed across tropics, but Valley endemics 12.5%, 4%–18% exotic. Plant composition varied with altitude, lithology, soils rainfall. Agglomerative (Jaccard) hierarchical clustering Non-metric Dimensional Scaling identified two three vegetation groups, for quadrats transects, respectively, correlated lithology soils. Indicator analysis highlighted upstream portion of catchment was characterised by forest taxa few exotics. downstream included woodland adapted mesic/xeric conditions Longitudinal change (β diversity) between sites assessed taking into consideration contribution ‘spatial turnover’ ‘nestedness’, resulting from replacement loss, respectively. β diversity corridor 0.40 spatial turnover accounted 80%–85% this, while rest attributed nestedness component. Riverine native trees increased low regeneration. Afromontane extended its distribution towards lower altitudes. Les riverains d'Afrique de l'Est sont des hauts lieux la biodiversité qui se trouvent au sein bassins versants en déclin sous l'effet déforestation et du surpâturage. Des bande ont été utilisés afin d'étudier végétale le long rivière dans vallée Kenya. L'étude a révélé que plupart espèces (365 total) étaient largement réparties régions tropicales. Cependant, les endémiques représentaient 12,5 % d'entre elles, 4 à 18 ces mêmes exotiques. La variait selon l'altitude, lithologie, sols précipitations. Le regroupement aggloméré hiérarchique mise l'échelle dimensionnelle non métrique permis d'identifier deux trois groupes végétation, pour respectivement, corrélation avec lithologie sols. L'analyse indicatrices mis évidence partie amont bassin versant était caractérisée par taxons forestiers comprenant peu d'espèces aval comprenait forestières adaptées aux mésiques ou xériques changement longitudinal (diversité β) entre évalué tenant compte l'influence « rotation spatiale » l'« imbrication », résultant respectivement remplacement perte d'espèces. diversité végétation riverain 0,40 représentait 80 85 cette même diversité, tandis reste attribué l'imbrication. L'abondance d'arbres autochtones augmenté aval. Toutefois, leur degré régénération faible. répartition afromontagnarde s'est étendue vers basses Riparian ecotones adjacent aquatic terrestrial ecosystems. Their high richness (α is influence multiscale controls (Sarr & Hibbs, 2007), combination specialised riverine as well up-valley penetrate zones advantage favourable microclimatic developing riverbanks. In gradient streams particularly steep (Sabo al., 2005). High rates biotopes achieved within extremely small scales Soykan, 2006). Species also tends be streams, altitudinal rainfall temperature, because changes geological background (Nucci 2012). structure supports important ecological functions regulating temperature regimes (Garner 2017; Trimmel 2018), retaining sediments nutrients (Boz Gumiero, 2016; Gumiero Boz, 2017), their availability (Naiman 1993), stabilising banks (Sabater 2003; Sweeney 2004), diversifying habitats other species, providing refuge during drought preferential dispersal routes. Compositional functional strongly ability provide multiple ecosystem services (Grizzetti Naiman 1993; Vidal-Abarca Gutiérrez Suárez Alonso, 2013). Patterns research objectives setting conservation priorities globally recognised spots. Our area Eastern Arc Mountains, part Biodiversity spot, (Critical Ecosystem Partnership Fund, 2012; Mittermeier presence significantly increases regional lists, river basins undergoing significant hydrological (Guzha 2018). Replacement pristine slopes crops tree plantations causes loss biotic integrity makes local climate warmer after (Hesslerová Pokorný, 2011). Extensive logging farming have severely affected forests outside protected areas gazetted managed Kenya Forestry Service, often only remaining forested portions highly fragmented landscape (Enanga 2011; Everard 2002; Lelo 2005; Mathooko 2009; Shivoga 2007). Despite relevance terms enhancement services, stream draining escarpment poorly studied. Descriptions made lowland floodplains (Hughes, 1990; Medley, 1992), highland received little scientific attention (Mathooko Kariuki, 2000). present focussed on community representing typical features characterise running waters central We analysed turnover, highlighting patterns related climate, background, anthropogenic influence. Community origin preferred habitat. seek establish whether coherent potential surrounding or forms taxa, whose may dominate change. Equatorial monsoonal rainy seasons (March–May October–December; Lamb 2003). Gilgil originates confluence tributaries stem springs Bahati Highlands, just below Equator. Starting springs, system flows southwards 58 km before entering Lake Naivasha at 1889 m a.s.l. (Figure 1). approximately 2750 a.s.l., above Gwakiongo Dam (Mirangine location), receive average annual ca. 1300 mm (Kibona, 2000), mouth, reaches 610 year−1 (Lamb 2003) 17°C (Odongo 2016). basin covers about 402 km2, flow 24 × 106 m3 per year (Everard 2002). upper cool well-watered, it becomes semi-arid Town (2010 a.s.l.). incident radiation an mean evaporation, which corresponds double precipitation 2015). At peak dry season, remain many days; frequency such events has been increasing last 20 years (D.M. Harper, N. Pacini, personal observation). differ both geology climate. consists Tertiary Quaternary (Pliocene) basalt igneous rocks (Saemundsson, 2008), overlain deep fertile volcanic nitisols. valley floor underlain Pre-Cambrian Basement complex, consisting weathered gneisses, schists granulites covered deposits. North lacustrine deposited Pleistocene, when present-day lakes Naivasha, Elementaita Nakuru used form single (Richardson Richardson, 1972). narrow deeply cutting tributaries: Kiriundu (formerly “Karindo”) Morendat “Oleobar”), flowing several sections inaccessible grazing mammals. riverbed step-pool morphology frequent cascades. catchment, floodplain broadens channel alternates short riffles glides incised alluvial experiences wide seasonal variations, overbank flooding. Livestock watering, damming, wetland drainage, charcoal burning, littering water abstraction widespread catchment. Alien (Acacia mearnsii De Wild., Eucalyptus globulus Labill. Hesperocyparis lusitanica (Mill.) Bartel) cultivated plots (cabbage, kale, potatoes maize) extend sometimes up riverbank, causing fragmentation. Daily collection water, cattle watering washing common. (ca. 2.5 km2) receives drainage wetlands source Morendat, minor dams second largest dam Town. section downstream, Town, points fords, where trampling degrades cover interrupts continuity corridor. This sub-catchment human population density, roads infrastructure. Signs degradation common, narrowing, incision eroded banks. A Potential Natural Vegetation map Africa developed project (VECEA project, van Breugel detailed documentation types provided sister publication (Kindt According VECEA Project maps, sources classified ‘Afromontane bamboo (B)’ group. Just sources, undifferentiated (Fb)’. category ‘Evergreen semi-evergreen bushland thicket (Be)’. formation typically ‘Upland Acacia wooded grassland (We)’ ‘Acacia-Commiphora deciduous (Wd)’ consequence fire 2011), common our no reliable game ranching (Jaetzold lowest site, situated VECEA’s ‘Riverine (R)’. location (Nakuru County), density 178 inhabitants km−2; urban centre 60,000 (Kenya National Bureau Statistics, 2020). distinction above-mentioned categories confirmed visual assessment field activities. These compared mapped (van 2015), assess extent land-use strategies: take account fact processes acting Sarr An initial survey took place December 2017 January 2018 11 sampling main (three reach, reach five reach; Figure Following preliminary visits Google Earth imagery, each site determined according ease access avoidance degraded vegetation, settlements plots. we placed 10 15 quadrat bank, stretching edge include 15-m strip order accurately record all above-ground proceeded considering first herbaceous layer (<0.3 height), then shrubs (0.3–1 m) finally (>1 m). Each subdivided 3 (5 long) strips, one other. Taxa could not photographed, samples collected later processing. Transect surveys carried out February September 2019 recording 1 height 26 sites. laid, side channel. transect 100 wide, parallel active 2). total surface 3300 m2 (quadrats) 15,600 (transects) 88-km-long (Morendat 30 km, 32 km). number dictated available time resources. Regarding quadrats, limited accessibility implied ended having larger linear length. identification based Agnew (2013), Beentje (1994), al. (1954–2012) Maundu (2005). Nomenclature authorities derived Plants World Online (POWO). assigned following categories: (1) habitus (herb, shrub, tree); (2) (native, introduced); (3) chorotype (Rift Valley, Tropical Africa, Pantropical, Cosmopolitan); (4) habitat occurrence (Cultivation, Bushland, Woodland, Forest, Forest edge, Riverine) (1954–2012). data, considered separately, R v. 4.0.2 (R Core Team, Sample groups defined agglomerative (complete linkage) Jaccard distance, tested similarity further Multi-Dimensional ordination (Bray–Curtis rank similarities) Permanova (999 permutations) checking homogeneity multivariate dispersion. Altitude contour lines plotted NMDS axes space using ‘vegan package’ (Oksanen Lithology, land use can key determinants County atlas agro-ecological 2006) allowed us assign physiographic/lithological location. upstream, follows: UPB1 well-drained luvisols over ashes, overlying nepheline tuffs, inclusions gleyic soils; RB3 friable eutric nitisols basalt, phonolite tuffs. Two downstream: P1PC—imperfectly drained sandy cambisols mostly ancient lake laying pyroclastic rocks, PlU1—very moderately sodic solonetz plains. fitted vectors ordinations; vector significance envfit ‘vegan’ package Oksanen Strohbach 2009). indicspecies (De Cáceres 2020) function Indval, ranks specificity (predictive value indicator group) sensitivity (probability finding belonging same group). multipatt calculate Pearson's φ coefficient association, correlation index among alternative fidelity, measure concentration units (Chytrý gave additional confirmation. Indval reflects partitioning groupings set observer, obtained comparing dataset. less prone bias than relation 2002) data. As feature, association express given identify tend euryoecious; is, rather being associated particular group, they highest score origin, chorology, group Spearman altitude percentage category. Critical p values Holm–Bonferroni adjusted (type I error increase due testing). (γ sum added diversity; Gering Assessing separate components (species sites) lead insights gradients. methodology adopted obtain estimates (βsim) ‘nestedness’ (βnes) Simpson's Sørensen's (βsør) dissimilarity indices (βsør = βsim + βnes; Baselga, 2010), calculated betapart (Baselga, method focusses Averages pairwise comparisons (e.g. ‘upstream quadrats’ ‘downstream quadrats’). disregards shared pairs 2010; Legendre, 2014). So, recently introduced Fortin (2020a, 2020b; betadiv). statistical estimators Baselga’s (2010) work, them multiple-site calculations avoid produced populations different sizes (Fortin 2020a, 2020b). algorithm calculates α γ diversities. To heteroscedasticity unequal sample size, suggested (2020a) indicated uppercase subscripts) Welch's t test (Welch, 1947). modified. It mainly extensively farmland (44%) Dry (28%) extensive pasture 3). includes Bushland (14%), (4%), Intensively Wetlands (3%), Settlements (3%). undisturbed 0.32 km2 Juniperus procera Hochst. ex Endl. visited proximity M6 Morendat. mosaic Wooded grassland. Along whole basin, positioned riverbanks, farms directly beside recorded 348 (Appendix S1), 312 (93%) level 21 genus; remained unidentified reported. 75 families—Asteraceae (46 sp.), Poaceae (25 Fabaceae (20 Acanthaceae Lamiaceae (14 Malvaceae (16 sp.) Rubiaceae (11 had >10 taxa. 55. On average, 16 (4.7%) endemic eastern highlands 33 (9.8%) introduced. Transects 83 S2), seven Twenty-one these (indicated * Appendix S2). 76 belonged 35 families 59 genera. richest family (8 spp.), followed Anacardiaceae, Araliaceae, Euphorbiaceae, Salicaceae, contained four each. Seven Highlands KT7, Kiriundu, consisted monospecific plantation A. excluded any analysis. Overall, 1/4 1/2 regardless if known taxonomic literature throughout tropical Africa. ‘Riverine’ (i.e. growth corridors) represented 15% transects. Introduced amounted 13% 11% averaged 3% 10% Finally, pantropical cosmopolitan. occurring >75% following: Achyranthes aspera L. species; Hypoestes forskaolii (Vahl.) R. Br. widely distributed, toxic, Arabic peninsula, Zehneria scabra (L.f.) Sond. vine subtropical Old Gymnosporia heterophylla (Eckl. Zeyh.) Loes., spiny shrub peninsula. commonest Olea europaea subsp. africana P.S.Green, 3/4 ⅓ once. cluster dendrogram 4) eleven sampling: (quadrats K1, M2, K3, M4, K5, M6, 2700 2300 m); G7, G8, G9, G10, G11, 2100 1900 Almost half (153; 44.5%) exclusively six located 116 (34.7%) restricted sites; overall similar, fewer (65, 19.5%) (the frequencies differed those exclusive catchment; χ2 35.2, df 2, < .001). plot permutations, .001; 5) showed internally heterogeneous composition. ordisurf fitting data consistent relationship axis. 6): (MT1, MT2, MT3, MT4, MT8, KT1, KT2, KT4, KT5, 2500 midstream (MT5, MT6, MT7, MT9, MT10, KT3, KT6, KT8, KT9 2600 2200 (GT1 GT8, 7) slight overlap although subdivision Twelve (15.8%) groups. 70 either (34 36 downstream) scores; selection illustrated Table 1. Upstream Rhamnus prinoides L'Hér., Asplenium stuhlmannii Hieron. Sanicula elata Buch.-Ham. D.Don, corridors, Jasminum abyssinicum DC. Myrsine melanophloeos (L.) R.Br. Sweet. Downstream dominated Grewia similis K. Schum., Pavonia burchellii (D.C.) R.A.Dyer, Maerua triphylla Rich. Mystroxylon aethiopicum (Thunb.) Loes. overgrazed disturbed distinctive Sida tenuicarpa Vollesen, Acalypha volkensii Pax, Setaria verticillata P. Beauv. Ocimum gratissimum Strictly taxa; Senna didymobotrya (Fresen.) H.S.Irwin Barneby, Celtis Burm.f., Cyperus dichrostachyus A.Rich. exaltatus Retz. Further attributes euryoecious, least S3). nine combinations (1 downstream), ranking methods (Indval association) close match (Table E. scored importance Similarly, appeared H. midstream. Highly indicative Dombeya burgessiae Gerrard Harv. Sond., next xeric biotopes, M. triphylla, Tarchonanthus camphoratus L., Euclea divinorum Hiern., Euphorbia candelabrum Welw., G. Vachellia abyssinica (Hochst. Benth.) Kyal. Boatwr. Salix mucronata Thunb., Sesbania sesban Merr. Ficus sur Forssk. rank. Among S4), J. procera, O. Pittosporum viridiflorum Sims, headwaters downstream. There relationships distribution, (five categories), (six categories) (native/introduced) 3), contrasting results quadrats. ‘Forest’ positively ‘Woodland’ ‘Bushland’ negative chorotypes ‘Tropical Africa’ ‘Rift Valley’ ‘Native’ (just significance). (but quadrats) revealed pattern detected single-dominant (Fbj)’, large proportion modified accommodate cultivation. hosted strengthened positive torrida (J.F.Gmel.) P.Bamps, Vitex fischeri var. keniensis (Turrill) Meerts, gerrardii Benth. P.J.H. Hurter, Ethulia vernonioides (Schweinf.) M.G. Gilbert, Geranium kilimandscharicum Engl, Impatiens meruensis Gilg, Crotalaria agatiflora Schweinf. Dryopteris tricellularis J.P.Proux. Quadrat originating higher 3); reverse pattern. Exotic Several exotic herbs, macrophytes Calceolaria mexicana Benth., Cestrum aurantiacum Lindl., Erica arborea Galium spurium Passiflora mollissima (Kunth) L.H.Bailey, Phalaris arundinacea Persicaria nepalensis (Meisn.) H.Gross Polystichum sinense (Chris