Next Article in Journal
Optimization of Skid Trails and Log Yards on the Amazon Forest
Next Article in Special Issue
N Isotope Fractionation in Tree Tissues During N Reabsorption and Remobilization in Fagus crenata Blume
Previous Article in Journal
Leaf Enzyme and Plant Productivity Responses to Environmental Stress Associated with Sea Level Rise in Two Asian Mangrove Species
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Forests
Volume 10
Issue 3
10.3390/f10030251
forests-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Carbon Isotopes of Riparian Forests Trees in the Savannas of the Volta Sub-Basin of Ghana Reveal Contrasting Responses to Climatic and Environmental Variations

by
Emmanuel Amoah Boakye
1,2,
Aster Gebrekirstos
3,
Dibi N’da Hyppolite
1,
Victor Rex Barnes
2,
Stefan Porembski
4 and
Achim Bräuning
5,*
1
West African Science Service Centre on Climate Change and Adapted Land Use Graduate Research Program in Climate Change and Biodiversity, Université Felix Houphouët Boigny, 01 BP V34 Abidjan, Cote D’Ivoire
2
Faculty of Renewable Natural Resources, Kwame Nkrumah University of Science & Technology, 00000 Kumasi, Ghana
3
World Agroforestry Center, 30677 Nairobi, Kenya
4
Department of Botany, Institute of Biosciences, University of Rostock, 18051 Rostock, Germany
5
Institute of Geography, Friedrich-Alexander-University Erlangen-Nürnberg, 91058 Erlangen, Germany
*
Author to whom correspondence should be addressed.
Forests 2019, 10(3), 251; https://doi.org/10.3390/f10030251
Submission received: 24 January 2019 / Revised: 6 March 2019 / Accepted: 7 March 2019 / Published: 12 March 2019
(This article belongs to the Special Issue Stable Isotopes in Forest Ecosystem Research)
Browse Figures
Review Reports Versions Notes

Abstract

:
Stable isotopes of tree rings are frequently used as proxies in climate change studies. However, species-specific relationships between climate and tree-ring stable isotopes have not yet been studied in riparian forests in the savannas of West Africa. Four cross-dated discs, each of Afzelia africana Sm. (evergreen) and Anogeissus leiocarpus (DC.) Guill. & Perr. (deciduous) in the humid (HSZ) and dry (DSZ) savanna zones of the Volta basin in Ghana were selected from a larger tree-ring dataset to assess the relationships between the tree-ring carbon isotope composition (δ13C values) and climatic parameters. The atmospherically corrected δ13C values of both studied species showed that A. africana was enriched in 13C compared to A. leiocarpus. Strong correlations were found between δ13C values of A. africana and A. leiocarpus with temperature, but weak correlations with precipitation. Spatial correlation analysis revealed significant relationships between δ13C values of both tree species and Sea Surface Temperatures in the Gulf of Guinea in the southern Atlantic Ocean. The results suggest that the carbon isotope composition of riparian trees in the Volta river basin has a potential to reconstruct climate variability and to assess tree ecological responses to climate change.

1. Introduction

Riparian forests in arid and semi-arid regions are strips of woody vegetation growing along waterways [1,2]. Because they form linkages between terrestrial and aquatic ecosystems, riparian forests play an important role as ecological corridors and provide a variety of ecosystem services. They serve as important habitats, moderate stream temperatures for aquatic life and act as “ecological shelter” against desertification in the adjacent drylands [1,3]. Riparian forests trap seeds and filter sediment and nutrients transported from adjacent land areas into waterbodies [3]. This enables riparian areas to support higher plant productivity and biomass growth compared to non-riparian areas in the surrounding drier landscape [2,4]. Due to these and additional ecosystem functions, many riparian forests are protected by the Ramsar Convention and other legal acts by national regulations [3,4].
Riparian forests are controlled by hydrological processes [3] and as water is the most limiting resource to biological activity in drylands, the timing, and magnitude of ecosystem production, evapotranspiration and nutrient cycling are closely linked to precipitation [1,2,5]. Changes in hydroclimate will affect the size, frequency and seasonal timing of precipitation and moisture inputs into rivers. It is therefore anticipated that climate change will have a huge impact on vegetation distribution, growth and ecosystem functions of riparian forests in drylands [5,6]. However, despite their ecological relevance and climate change threats on riparian forests, this unique ecosystem is under-studied in the savannas of West Africa [1].
Just as trees in temperate regions, tropical tree species can also be interpreted as historical records of climatic signals because numerous species show common patterns in ring width variations that can be cross-dated [7,8,9,10,11,12]. Many studies have already successfully applied tree ring stable isotope analyses in tropical regions, most of them working on an annual resolution [6,13,14,15], and some studies reported stronger climatic signals in stable isotope chronologies compared to tree ring-width variations [6]. A strong negative correlation between annual precipitation and tree-ring δ13C values was found for several broadleaved tree species under various tropical climate regimes [6,13,14,16]. Gebrekirstos et al. [16,17] found significant negative correlations of annual δ13C values with humidity and positive correlations with temperature in the West African Sahel woodland. Because of such correlations, carbon isotope composition in tree rings is used for reconstruction of past climatic variations extending back into the pre-instrumental era and is therefore of special relevance in areas where instrumental climate records are short. Carbon isotope composition is also useful for evaluating the relative importance of natural variability and anthropogenic impacts on the global climate [18], and for predicting trees ecological responses to future climate conditions [19,20,21,22,23] by characterizing species water use efficiency and strategy [13,17].
The carbon isotope composition in tree rings results from CO2 fractionation during photosynthesis at leaf level [24,25]. Carbon isotope fractionation is mainly due to (i) diffusion effect, when external CO2 is transported through the boundary layer and the stomata into the internal gas space to carboxylation sites into the chloroplast, and (ii) at carboxylation sites because of the enzyme ribulose-1,5-bisphosphate carboxylase which discriminates against 13CO2 due to the lower intrinsic reactivity of 13C [26,27].
Leaf habit of trees influences carbon isotope fractionation. Evergreen trees partly show higher δ13C values compared to deciduous species because they maintain their leaves during drier conditions, and stomata closure under dry conditions reduces the discrimination of 13CO2 [6,28,29]. In the present study, we examined the tree rings of two riparian tree species, Afzelia africana (Fabaceae, evergreen) and Anogeissus leiocarpus (Combretaceae, deciduous) growing in the savannas of the Volta River basin of Ghana to document climatic variations. The species were selected because of their contrasting ecological attributes as well as their contribution to the livelihood and food resources of forest-dependent communities. Both species are widely distributed along riparian forests, have clearly detectable growth ring boundaries and are under threat from over-exploitation [11,30]. The Volta river basin is divided into three main agro-ecological zones which are aligned along a gradient of increasing moisture conditions from north to south: the Sudano-Sahelian, the Sudanian, and the Guinean savannas [31,32,33].
To assess the response of tree vegetation in the Volta river basin to climate and environmental changes, we studied two riparian forests in two ecological zones, one in the Guinean zone and one in the Sudanian zone. Those riparian forest ecosystems are embedded in the humid savanna zone (HSZ) and dry savanna zone (DSZ), respectively. The specific objectives of this study are (1) to determine if annual δ13C values of tree rings of A. africana and A. leiocarpus are related to functional differences in leaf phenology (evergreen or deciduous); (2) to investigate the coherence of 13C tree-ring signals between different sites (HSZ and DSZ); and (3) to assess whether similar climatic signals are recorded within the tree-ring 13C chronologies of the two species.

2. Materials and Methods

2.1. Study Area

The study area was located in the Volta basin of Ghana. The studied sites were situated in protected riparian forest reserves along the Afram and Tankwidi Rivers which flow through the humid savanna zone, HSZ (Guinean) and dry savanna zone, DSZ (Sudanian), respectively (Figure 1). Water levels of the two rivers are significantly reduced during the dry season which lasts from November–April in the Tankwidi catchment and from November–February in the Afram catchment. The two rivers are both 8–12 m wide. The dominant soil type in the floodplains is Luvisol [34], which is characterized by a sub-surface accumulation of clay and organic matter, and is composed of high activity clays with high base saturation. The soil physical structure is characterized by high porosity, good drainage and aeration [34]. The dominant woody species in the Afram riparian forests belong to the family Fabaceae and Combretaceae, whereas in the Tankwidi riparian forests, dominant families are Fabaceae and Rubiaceae [35,36].
The seasonal movement of the inter-tropical convergence zone (ITCZ) and the related occurrence of the northeast trade winds determine the temporal and spatial distribution patterns of precipitation in the Volta Basin. Besides, atmospheric disturbances in the tropical Atlantic Ocean (TAO) determine the occurrence of wetter and drier years in the study region. Sea surface temperatures (SSTs), both in the tropical Atlantic and the equatorial Pacific Oceans modulate frequency and strength of TAO disturbances [37]. The climate data covering the period 1961–2012 were obtained from the Navrongo (3 km to study site, Figure 2a) and Ejura (1 km to study site, Figure 2b) Meteorological Stations for the HSZ and DSZ, respectively. These were the climatic stations closer to the catchment areas as meteorological stations are sparse in Ghana. Mean annual maximum temperature in the HSZ reach 32 °C. Mean annual precipitation amounts 1100 mm. The, rainy season lasts from March to October, while precipitation rates occur in July and August. In the DSZ, mean annual maximum temperature reach 36 °C. Mean annual precipitation amounts 800 mm, and the rainy season lasts only seven months, from April to October. Correlation between precipitation and temperature data of both weather stations, Ejura (r = −0.41, p = 0.33) and Navrongo (r = −0.13, p = 0.56), showed weak negative correlations. Whereas a weak positive correlation was observed between mean annual precipitation data from Ejura (r = 0.31, p = 0.03) and Navrongo (r = 0.33, p = 0.04) weather stations with annual sea surface temperatures (SSTs) from 1961 to 2012. Similarly, mean annual maximum temperature showed highly significant correlations with SSTs for Ejura (r = 0.59, p = 0.00) and Navrongo weather stations (r = 0.51, p = 0.00) [11].

2.2. Tree-Ring Stable Isotope Analyses

We selected a total number of 16 discs (Table 1) from a larger tree-ring data set of 31 stem discs of A. africana and A. leiocarpus from both the DSZ and HSZ [11]. The discs were collected between January and February 2014, and the four oldest and well cross dated individual trees were selected for stable isotope analyses which were conducted at the tree-ring laboratory of the Friedrich-Alexander University of Erlangen-Nuremberg, Germany. Ring widths of all samples were measured on 2–4 radii and properly cross-dated before annual rings were separated with a scalpel for stable isotope analyses. Due to the diffuse porous wood anatomy of the studied species, the whole annual growth ring was used, resulting in an annual resolution of the final 13C chronologies. The last year (2013) of each series was excluded from analysis as ring width formation was not yet complete at the time of sampling [11].
Earlier studies have used bulk wood for the isotope analysis of tree rings. However, different wood components may differ in their isotopic composition [28,38]. This variation in the chemical composition of the wood may influence δ-values. Cellulose extraction has disadvantages, such as the labour intensiveness and costly laboratory procedure, which sometimes limit the number of samples that can be processed in a research project [38]. It is worth mentioning that recent studies comparing the isotopic composition of different wood components (cellulose, lignin and bulk wood) have found a high correlation between cellulose and bulkwood δ13C values as well as between cellulose and lignin δ13C values [16,39,40,41]. In some cases, bulkwood δ13C values was found to be equally suitable and sometimes even showed higher correlations with climate parameters than cellulose δ13C values [42,43,44]. To test the effect of cellulose extraction on stable isotope variability, we separated the annually resolved wood samples for isotope measurements into samples of bulk wood and α-cellulose. Between 12 to 16 annual rings from the outer parts of each cross-dated disc of A. africana and A. leiocarpus in each savanna type (HSZ and DSZ) were used for this experiment. This allows testing if the offsets of δ13C values remained constant over time thus helping to decide the effectivity of 13C archives of whole wood for paleoclimatic reconstruction.
For stable isotope measurements, powdered wood samples were produced along two radii of each disc using a micro drill with a diameter of 0.5 mm. The powdered samples were pooled into tin capsules and homogenized with a metal stick to represent the whole ring following the method described by Gebrekirstos et al. [6]. Subsamples from each year were weighed into tin capsules (0.4–0.5 mg). The cellulose extraction process was carried out according to the methods described in Wieloch et al. [45]. Accordingly, resin, fatty acids, etheric oils, and hemicellulose were extracted with a solution of 5% NaOH for 2 h at 60 °C. This operation was repeated twice. Then, lignin was extracted with 7% NaClO2 solution for 40 h at 60 °C. Hemicelluloses were then extracted with 17% NaOH for 2 h at room temperature. A washing procedure was interposed between the different steps. Finally, samples were washed once with 1% HCl and three times with boiled de-ionized water (until pH 7 ± 1) and transferred from the filter funnels into Eppendorf tubes with 1 mL de-ionized water. Following Laumer et al. [46], ultrasonic homogenization was carried out for 15 s with a UP200s (Hielscher Ultrasonics GmbH, Berlin, Germany). After freeze-drying for 72 h in an ALPHA 1-4/2-4 LSC lyophilisation unit, the dried cellulose was then weighed to determine the yield and finally measured by isotope ratio mass spectrometer (Delta V Advantage, Thermo Electron, Bremen, Germany) coupled to a HekaTech Elemental Analyzer with a precision and accuracy of up to ±0.1% relative. The results are given in δ-notation, which is the relative deviation from the PDB (Pee Dee Belemnite) standards:
δ13C = [(13C/12C) sample/(13C/12C) PDB) − 1] × 1000‰

2.3. Statistical Analysis

Tree-ring isotope data may contain trends unrelated to climate but related to the decline in atmospheric δ13C values caused by burning of 13C-depleted fossil fuel. This trend was removed by following the procedure of McCarroll and Loader [28]. Analysis of variances was done to determine whether δ13C values differed across the different species; and the significant differences were tested through Tukey’s pair-wise comparison. Pearson correlations were conducted between the 13C chronology of each species (A. africana and A. leiocarpus) of the HSZ and DSZ and total monthly and annual averages of precipitation and monthly maximum temperature. The average monthly minimum temperature did not show correlations with δ13C values and are therefore not presented. The influence of regional SSTs on the 13C chronologies was evaluated by spatial correlation analysis using the KNMI Climate Explorer (http://climexp.knmi.nl/). Highly significant correlations of regional SSTs and the 13C chronology of each species were presented in the results.

3. Results

3.1. Relationship between 13C Chronologies of Whole Wood and Cellulose

Comparative analyses (Figure 3) of A. africana and A. leiocarpus from the HSZ and DSZ showed that the δ13C values of whole wood were lower than δ13C values of α-cellulose. δ13C values of whole wood and cellulose showed significant positive correlations for both species. The highest correlation was recorded for A. africana in the HSZ (r = 0.88, p = 0.00) followed by A. africana in the DSZ (r = 0.84, p = 0.00). A. leiocarpus of the HSZ (r = 0.59, p = 0.03) followed up third with A. leiocarpus in the DSZ (r = 0.40, p = 0.20) having the weakest correlations (Figure 3a,b). The mean offset of the δ13C values of whole wood and cellulose for A. africana in the HSZ was small (0.63‰), A. africana in the DSZ was 1.95‰, whereas A. leiocarpus in the DSZ and HSZ were 1.93‰ and 1.50‰ respectively.
There was a significant difference in cellulose δ13C values among the species (F(3, 53) = 34.3, p = 0.00). Species-specific δ13C values (Table 1) showed that in the DSZ, the A. africana (evergreen) individuals are significantly (p < 0.05) enriched in 13C compared with A. leiocarpus (deciduous). This was, however, not the case for the HSZ despite A. africana having a higher δ13C values than A. leiocarpus. Comparison between the two savanna types also showed that the cellulose δ13C values of A. africana and A. leiocarpus of the DSZ are enriched in 13C (p < 0.05) compared to the same species in the HSZ. Correlation coefficients between the δ13C values of A. africana and A. leiocarpus collected from the HSZ (r = 0.45, p = 0.001) were significant, whereas they were weak in the DSZ (r = 0.10, p = 0.68). Significant correlations were also found between δ13C values of A. africana from the HSZ and the DSZ (r = 0.41, p = 0.001). In contrast, we found no significant correlation for A. leiocarpus between the HSZ and DSZ (r = 0.13, p = 0.40). There was a weak correlation between species-specific ring widths and 13C chronologies for both HSZ and DSZ (Table 1).

3.2. Relationships between Tree-Ring δ13C Values and Climatic Parameters

Despite site-specific differences, temperature and precipitation generally showed significant positive and negative correlations with δ13C values of the studied trees (Figure 4). In the DSZ, A. africana showed positive correlations with temperature from January to July and September whereas A. africana in the HSZ showed positive correlations with temperatures throughout the year (Figure 4a). A. africana in the DSZ showed strong negative correlations with precipitation in July and October (Figure 4a). However, A. africana in the HSZ had a significant negative correlation with precipitation in January and March despite the low precipitation during those months (less than 30 mm of rains).
A. leiocarpus in the DSZ showed positive correlations with temperature from January to March and November. In October, the species showed negative relationships with temperature. A. leiocarpus in the HSZ however, showed a positive correlation with temperature throughout the year (Figure 4b). A. leiocarpus in the DSZ had a significant negative relationship with precipitation in September. In contrast, A. leiocarpus in HSZ had a significant negative correlation with precipitation in January and March.
Tree-ring δ13C values of A. africana and A. leiocarpus from the HSZ showed a positive correlation with gridded sea surface temperatures (SST) in the El Niño region of the equatorial Pacific Ocean and the Gulf of Guinea for the average values of the period, March to February (Figure 5). In the DSZ, both A. africana and A. leiocarpus showed a negative correlation with SSTs in the Gulf of Guinea and weak correlations with SSTs the Pacific region.

4. Discussion

4.1. Variation of δ13C Values of Bulk Wood and Cellulose in Tree Rings

Various wood components show different isotopic signals, and removal of wood constituents such as lignin and resins enhance the inter-annual variability of δ13C values in cellulose [43,47]. The means of the δ13C values of whole wood and cellulose of A. africana and A. leiocarpus in this study suggest (Table 1; Figure 3) that cellulose is enriched in 13C compared to whole wood, as already found in other studies [42,43,47,48,49]. Furthermore, the offset (~1–2‰) between whole wood and cellulose δ13C values of both species (Table 1) are in the range reported from the literature [40,42]. Because both study species (except A. leiocarpus in DSZ) showed constant differences in δ13C values of the whole wood and cellulose pattern, they are well suited as climate proxies [40]. Accordingly, whole wood was adopted for further exploration of the relationship of δ13C values of the species with climatic variables, thus simplifying the preparation procedure by skipping time-consuming cellulose extraction.

4.2. Patterns and Variations in Tree Rings δ13C Values in Different Tree Species

The higher enrichment of 13C in cellulose and whole wood of the evergreen A. africana in both the HSZ and DSZ zones indicates that the species may be more stressed at higher temperatures than A. leiocarpus (Table 1; Figure 3). During long periods of high temperature, the deciduous A. leiocarpus avoids heat stress by shedding its leaves [29,50,51]. Because A. africana is evergreen, it can extend the length of the photosynthetic season. This presents challenges to the plants as they must tighten stomata closure to reduce water loss [6,51,52]. After the closure of the stomata, 13CO2 which remains in the leaf air spaces and is less preferred by the RUBISCO enzyme is relatively less discriminated in the process of photosynthesis, thus increasing the proportion of 13C in the leaf [53]. A. africana and A. leiocarpus in the DSZ (Table 1) had higher enrichment in 13C compared to the same species in the HSZ probably because of enhanced water stress and stomatal limitations to photosynthesis. Stomata regulation is the strategy adopted by trees in drylands for minimizing evaporative water loss which could influence the enrichment of 13C in dry savanna trees [21,51,54,55].
The low correlation between δ13C values of A. leiocarpus and A. africana in the DSZ could have partly resulted from the differences in rooting depth or leaf architecture that cause the species to respond differently to climatic conditions. Evergreen species (A. africana) have the ability to reach the deeper layers of soil to access ground water to overcome dry seasons limitations than deciduous trees (A. leiocarpus) [51]. This was, however, not the case for the A. leiocarpus and A. africana in the HSZ, probably because of reduced spatial variability in nutrients and soil moisture of this riparian area [2].

4.3. Relationship Between Tree-Ring δ13C and Climatic Parameters

The months during which the highest correlations occurred between tree-ring δ13C values and climatic variables varied between the studied species. In most of the cases, temperature had a positive correlation with δ13C values of A. africana (Figure 4a) and A. leiocarpus (Figure 4b). In dry years, leaf stomata may close and hence 13C may be less discriminated. This increases the enrichment of 13C in the tree rings and results in a reduction of tree growth [6,14].
The δ13C values of the trees species showed weak sensitivity to precipitation, probably because of the influence of underground water or discharges from the upstream that controls the stream flow (Figure 4a,b). The correlation between precipitation and δ13C values of A. africana in the HSZ and A. leiocarpus in the DSZ showed a better relationship to the rainy season (July–October), when trees are photosynthetically active and growth rates are high. The lack of general similarities in the response of the different species may be due to their differences in leaf phenology of the trees [14].
High SSTs in the Gulf of Guinea (Figure 5) are associated with high precipitation over large parts of West Africa, enhancing vegetation productivity [37,56,57]. In the DSZ, δ13C values of A. leiocarpus and A. africana showed a negative correlation with the SSTs at the Gulf of Guinea probably because of the contribution to precipitation in the region and the impact in recharging the rivers to support the growth of trees in such dry landscape. A. leiocarpus and A. africana in the HSZ (which is much wetter than DSZ) showed a positive correlation with SSTs in the Gulf of Guinea, perhaps because excess precipitation can cause flooding in this kind of humid riparian forests. The long duration of flooding limits nutrient availability and gas exchange of plants which hinders the growth of trees [11,58]. Information on historic flooding episodes for the HSZ were not available for verification.

5. Conclusions

The present study is the first to assess the potential of tree-ring carbon isotope variations of riparian forest tree species for paleo-ecological research in Ghana. The δ13C values of the riparian trees in the humid and dry savanna zones revealed differences in their responses to climatic and environmental fluctuations as the trees could be employing different strategies to overcome seasonal water limitations. A. leiocarpus strategy of leaf shedding and conserving water enables it to avoid temperature and water limitation stress better than A. africana. Increasing dry periods and decreasing precipitation in the dry savanna zone induce water stress among A. africana and A. leiocarpus growing in the riparian forests. Our study has also shown that 13C signals of riparian trees in the humid and dry savanna zones of Ghana can be linked to climatic fluctuations. The analyses of the trees’ carbon isotopes at an annual resolution should, however, be expanded in sample size and length for a robust climatic reconstruction. For management purposes, carbon isotopes can be used for screening species for the restoration of degraded riparian landscapes in the savannas.

Author Contributions

E.A.B.: Design, sample collection, analysis, and write-up; A.G.: Design, analysis and write-up; D.N.H.: sample collection and analysis; V.R.B.: sample collection and analysis; S.P. and A.B.: Design, analysis and write-up.

Funding

This work was supported by the West African Science Service Center on Climate Change and Adapted Land Use (WASCAL) project of the German Federal Ministry for Education and Research.

Acknowledgments

The authors wish to express their sincere gratitude to the technical staff, Melanie Viehauser, Iris Burchardt, and Roswitha Höfner-Stich in the dendrolab of the Institute of Geography at Friedrich-Alexander University Erlangen-Nuremberg, Germany for their support during the laboratory work of the corresponding author in Germany.

Conflicts of Interest

The authors declare that they have no conflict of interest.

References

  1. Sambare, O.; Bognounou, F.; Wittig, R.; Thiombiano, A. Woody species composition, diversity and structure of riparian forests of four watercourses types in Burkina Faso. J. Res. 2011, 22, 145–158. [Google Scholar] [CrossRef]
  2. Azihou, F.K.; Glele, K.R.; Ronald, B.; Brice, S. Distribution of tree species along a gallery forest–savanna gradient: Patterns, overlaps and ecological thresholds. J. Trop. Ecol. 2013, 29, 25–37. [Google Scholar] [CrossRef]
  3. Naiman, R.J.; Decamps, H. The Ecology of Interfaces: Riparian Zones. Annu. Rev. Ecol. Evol. Syst. 1997, 28, 621–658. [Google Scholar] [CrossRef]
  4. Gray, C.L.; Slade, E.M.; Mann, D.J.; Lewis, O.T. Do riparian reserves support dung beetle biodiversity and ecosystem services in oil palm-dominated tropical landscapes? Ecol. Evol. 2014, 4, 1049–1060. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  5. Intergovernmental Panel on Climate Change (IPCC). IPCC Fourth Assessment Report: Climate Change (AR5); IPCC: Geneva, Switzerland, 2013. [Google Scholar]
  6. Gebrekirstos, A.; Teketay, D.; Fetene, M.; Worbes, M.; Mitlohner, R. Stable carbon isotope ratios in tree rings of co-occurring species from semi-arid tropics in Africa: Patterns and climatic signals. Glob. Planet. Chang. 2009, 66, 253–260. [Google Scholar] [CrossRef]
  7. Schöngart, J.; Orthmann, B.; Hennenberg, K.J.; Porembski, S.; Worbes, M. Climate-growth relationships of tropical tree species in West Africa and their potential for climate reconstruction. Glob. Chang. Biol. 2006, 12, 1130–1150. [Google Scholar] [CrossRef]
  8. Schöngart, J.; Bräuning, A.; Barbosa, A.C.M.C.; Lisi, C.S.; de Oliveira, J.M. Dendroecological Studies in the Neotropics: History, Status and Future Challenges. In Dendroecology. Ecological Studies (Analysis and Synthesis); Amoroso, M., Daniels, L., Baker, P., Camarero, J., Eds.; Springer: Cham, Germany, 2017; Volume 231. [Google Scholar]
  9. Gebrekirstos, A.; Mitlohner, R.; Teketay, D.; Worbes, M. Climate–growth relationships of the dominant tree species from semi-arid savanna woodland in Ethiopia. Trees 2008, 22, 631–641. [Google Scholar] [CrossRef] [Green Version]
  10. Colombaroli, D.; Cherubini, P.; De Ridder, M.; Saurer, M.; Toirambe, B.; Zweifel, N.; Beeckman, H. Stable carbon and oxygen isotopes in tree rings show physiological responses of Pericopsis elata to precipitation in the Congo Basin. J. Trop. Ecol. 2016, 32, 213–225. [Google Scholar] [CrossRef]
  11. Boakye, E.A.; Gebrekirstos, A.; Hyppolite, D.N.; Barnes, V.R.; Kouamé, F.N.; Porembski, S.; Bräuning, A. Influence of climatic variation factors on the tree growth of the trees Afzelia africana and Anogeissus leiocarpus in riparian forests in the humid and dry savannas of the Volta basin, in Ghana. Trees 2016, 30, 1695–1709. [Google Scholar] [CrossRef]
  12. Mokria, M.; Gebrekirstos, A.; Abiyu, A.; Van Noordwijk, M.; Bräuning, A. Multi-century tree-ring precipitation record reveals increasing frequency of extreme dry events in the upper Blue Nile River catchment. Glob. Chang. Biol. 2017, 23, 5436–5454. [Google Scholar] [CrossRef]
  13. Brienen, R.J.W.; Wanek, W.; Hietz, P. Stable carbon isotopes in tree rings indicate improved water use efficiency and drought responses of a tropical dry forest tree species. Trees 2011, 25, 103–113. [Google Scholar] [CrossRef]
  14. Fichtler, E.; Helle, G.; Worbes, M. Stable-carbon isotope time series from tropical tree rings indicate a precipitation signal. Trees 2010, 66, 35–50. [Google Scholar] [CrossRef]
  15. Nakai, W.; Okada, N.; Sano, M.; Nakatsuka, T. Sample preparation of ring-less tropical trees for δ18O measurement in isotope dendrochronology. Tropics 2018, 27, 49–58. [Google Scholar] [CrossRef]
  16. Gebrekirstos, A.; Mitlohner, R.; van Noordwijk, M.; Bräuning, A. Tracing responses to climate variability from stable isotopes in tree rings of Anogeissus leiocarpus and Sclerocarya birrea from the Sahel zone, Burkina Faso. Trace 2012, 10, 6–12. [Google Scholar]
  17. Gebrekirstos, A.; Bräuning, A.; Van Noordwijk, M.; Mitlohner, R. Understanding past, present, and future climate changes from East to West Africa. Agric. Innov. Sustain. Dev. 2011, 3, 77–86. [Google Scholar]
  18. Huang, R.; Zhu, H.; Liu, X.; Liang, E.; Grießinger, J.; Braeuning, A.; Li, X.; Wu, G. Does increasing intrinsic water use efficiency (iWUE) stimulate tree growth at natural alpine timberline on the southeastern Tibetan Plateau? Glob. Planet. Chang. 2017, 148, 217–226. [Google Scholar] [CrossRef]
  19. Saurer, M.; Siegwolf, R.T.W. Human impacts on tree-ring growth reconstructed from stable isotopes. In Stable Isotopes as Indicators of Ecological Change; Dawson, T.E., Siegwolf, R.T.W., Eds.; Terrestrial Ecology; Elsevier: Amsterdam, The Netherlands, 2007; pp. 49–62. [Google Scholar]
  20. Tognetti, R.; Lombardi, F.; Lasserre, B.; Cherubini, P.; Marchetti, M. Tree-ring stable isotopes reveal twentieth-century increases in water-use efficiency of Fagus sylvatica and Nothofagus spp. in Italian and Chilean mountains. PLoS ONE 2014, 9, e113136. [Google Scholar] [CrossRef]
  21. Gebrekirstos, A.; Bräuning, A.; Sass-Klassen, U.; Mbow, C. Opportunities and applications of dendrochronology in Africa. Curr. Opin. Environ. Sustain. 2014, 6, 48–53. [Google Scholar] [CrossRef] [Green Version]
  22. Gebrekirstos, A.; Teketay, D.; Mitlöhner, R. Responses of Dobera glabra and Eight Co-occurring Species to Drought and Salinity Stress at a Savanna-Scrub Ecotone: Implications in the Face of Climate Change. Open J. For. 2014, 4, 327–337. [Google Scholar] [CrossRef]
  23. Hartl-Meier, C.; Zang, C.; Büntgen, U.; Esper, J.; Rothe, A.; Göttlein, A.; Dirnböck, T.; Treydte, K. Uniform climate sensitivity in tree-ring stable isotopes across species and sites in a mid-latitude temperate forest. Tree Physiol. 2015, 35, 4–15. [Google Scholar] [CrossRef]
  24. O’Leary, M.H. Carbon Isotopes in Photosynthesis: Fractionation techniques may reveal new aspects of carbon dynamics in plants. BioScience 1988, 38, 328–336. [Google Scholar] [CrossRef]
  25. Brugnoli, E.; Farquhar, G.D. Photosynthetic Fractionation of Carbon Isotopes. In Photosynthesis: Physiology and Metabolism; Leegood, R.C., Sharkey, T.D., Von Caemmerer, S., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000; pp. 352–434. [Google Scholar]
  26. Ehleringer, J.R.; Dawson, T.E. Water-Uptake by Plants: Perspectives from Stable Isotope Composition. Plant Cell Environ. 1992, 15, 1073–1082. [Google Scholar] [CrossRef]
  27. Farquhar, G.D.; Lloyd, J. Carbon and Oxygen Isotope Effects in the Exchange of Carbon Dioxide between Terrestrial Plants and the Atmosphere. In Stable Isotopes and Plant Carbon-water Relations; Ehleringer, J.R., Hall, A.E., Farquhar, G.D., Eds.; Academic Press: San Diego, CA, USA, 1993; pp. 47–70. [Google Scholar]
  28. McCarroll, D.; Loader, N.J. Stable isotopes in tree rings. Quat. Sci. Rev. 2004, 23, 771–801. [Google Scholar] [CrossRef] [Green Version]
  29. Poorter, L.; Markesteijn, L. Seedling traits determine drought tolerance of tropical tree species. Biotropica 2008, 40, 321–331. [Google Scholar] [CrossRef]
  30. Mensah, S.; Houehanou, T.D.; Sogbohossou, E.A.; Assogbadjo, A.E.; Glèlè Kakaï, R. Effect of human disturbance and climatic variability on the population structure of Afzelia africana Sm. ex pers. (Fabaceae-Caesalpinioideae) at country broad-scale (Bénin, West Africa). S. Afr. J. Bot. 2014, 95, 165–173. [Google Scholar] [CrossRef]
  31. UNEP-GEF. Volta River Basin, West Africa. Available online: http://www.iwmi.cgiar.org/assessment/files_new/research_projects/River_Basin_Development_and_Management/VoltaRiverBasin_Boubacar.pdf (accessed on 6 September 2015).
  32. De Condappa, D.; Lemoalle, J. Volta River Basin Atlas; CGIAR: Montpellier, France, 2009; p. 96. [Google Scholar]
  33. Callo-Concha, D.; Gaiser, T.; Ewert, F. Farming and Cropping Systems in the West African Sudanian Savanna. WASCAL Research Area: Northern Ghana, Southwest Burkina Faso and Northern Benin; ZEF Working Paper 100; University of Bonn: Bonn, Germany, 2012. [Google Scholar]
  34. Konate, S.; Kampmann, D. Biodiversity Atlas of West Africa; Goethe University: Frankfurt, German, 2010; Volume III. [Google Scholar]
  35. Boakye, E.A.; Hyppolite, D.N.; Barnes, V.R.; Porembski, S.; Thiel, M.; Kouame’, F.N.; Kone, D. Threat of agricultural production on woody plant diversity in Tankwidi riparian buffer in the Sudanian Savanna of Ghana. Int. J. Biodivers. Conserv. 2015, 7, 354–363. [Google Scholar]
  36. Boakye, E.A.; Hyppolite, D.N.; Barnes, V.R.; Porembski, S.; Thiel, M.; Kouamé, F.N.; Kone, D. Comparative analysis of woody composition of farmlands and forest reserve along Afram river in a tropical humid savanna of Ghana: Implications to climate change adaptation. In Innovation in Climate Change Adaptation, Climate Change; Filho, W.L., Ed.; Springer: Basel, Switzerland, 2016; pp. 195–209. [Google Scholar]
  37. Odekunle, T.O.; Eludoyin, A.O. Sea surface temperature patterns in the Gulf of Guinea: Their implications for the spatio-temporal variability of precipitation in West Africa. Int. J. Clim. 2008, 28, 1507–1517. [Google Scholar] [CrossRef]
  38. van der Sleen, P.; Zuidema, P.A.; Pons, T.L. Stable isotopes in tropical tree rings: Theory, methods and applications. Funct. Ecol. 2017, 31, 1674–1689. [Google Scholar] [CrossRef]
  39. D’Alessandro, C.M.; Guerrieri, M.R.; Saracino, A. Comparing carbon isotope composition of bulk wood and holocellulose from Quercus cerris, Fraxinus ornus and Pinus radiata tree rings. Forest 2004, 1, 51–57. [Google Scholar] [CrossRef]
  40. Sidorova, O.V.; Siegwolf, R.T.W.; Saurer, M.; Naurzbaev, M.M.; Vaganov, E.A. Isotopic composition (d13C, d18O) in wood and cellulose of Siberian larch trees for early Medieval and recent periods. J. Geophys. Res. 2008, 113, 148–227. [Google Scholar] [CrossRef]
  41. Krepkowski, J.; Gebrekirstos, A.; Shibistova, O.; Brauning, A. Stable carbon isotope labeling reveals different carry-over effects between functional types of tropical trees in an Ethiopian mountain forest. New Phytol. 2013, 199, 441–451. [Google Scholar] [CrossRef] [PubMed]
  42. Borella, S.; Leuenberger, M.; Saurer, M.; Siegwolf, R.T.W. Reducing uncertainties in δ13C analysis of tree rings: Pooling, milling and cellulose extraction. J. Geophys. Res. 1998, 103, 19519–19526. [Google Scholar] [CrossRef]
  43. Loader, N.J.; Robertson, I.; McCarroll, D. Comparison of stable carbon isotope ratios in the whole wood, cellulose and lignin of oak tree-rings. Palaeogeogr. Palaeoclim. Palaeoecol. 2003, 196, 395–407. [Google Scholar] [CrossRef]
  44. Schleser, G.H.; Anhuf, D.; Helle, G.; Vos, H. A remarkable relationship of the stable carbon isotopic compositions of wood and cellulose in tree-rings of the tropical species Cariniana micrantha (Ducke) from Brazil. Chem. Geol. 2015, 401, 59–66. [Google Scholar] [CrossRef]
  45. Wieloch, T.; Helle, G.; Heinrich, I.; Voigt, M.; Schyma, P. A novel device for batchwise isolation of a-cellulose from small-amount wholewood samples. Dendrochronologia 2011, 29, 115e–117e. [Google Scholar] [CrossRef]
  46. Laumer, W.; Andreu, L.; Helle, G.; Schleser, G.H.; Wieloch, T.; Wissel, H. A novel approach for the homogenization of cellulose to use micro-amounts for stable isotope analyses. Rapid Commun. Mass Spectrom. 2009, 23, 1934–1940. [Google Scholar] [CrossRef] [PubMed]
  47. Battipaglia, G.; Jäggi, M.; Saurer, M.; Siegwolf, R.T.W.; Cotrufo, M.F. Climatic sensitivity of δ18O in the wood and cellulose of tree rings: Results from a mixed stand of Acer pseudoplatanus L. and Fagus sylvatica L. Palaeogeogr. Palaeoclim. Palaeoecol. 2008, 261, 193–202. [Google Scholar] [CrossRef]
  48. Ferrio, J.P.; Voltas, J. Carbon and oxygen isotope ratios in wood constituents of Pinus halepensis as indicators of precipitation, temperature and vapour pressure deficit. Tellus B 2005, 57, 164–173. [Google Scholar] [CrossRef]
  49. Roden, J.S.; Farquhar, G.D. A Controlled Test of the Dual-Isotope Approach for the Interpretation of Stable Carbon and Oxygen Isotope Ratio Variation in Tree Rings. Tree Physiol. 2012, 32, 490–503. [Google Scholar] [CrossRef]
  50. Gebrekirstos, A.; Teketay, D.; Fetene, M.; Mitlohner, R. Adaptation of five co-occurring tree and shrub species to water stress and its implication in restoration of degraded lands. For. Ecol. Manag. 2006, 229, 259–267. [Google Scholar] [CrossRef]
  51. Hasselquist, N.J.; Allen, M.F.; Santiago, L.S. Water relations of evergreen and drought-deciduous trees along a seasonally dry tropical forest chronosequence. Oecologia 2010, 164, 881–890. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  52. Brodribb, T.J.; McAdam, S.A.M.; Jordan, G.J.; Martins, S.C.V. Conifer species adapt to low-rainfall climates by following one of two divergent pathways. Proc. Natl. Acad. Sci. USA 2014, 111, 14489–14493. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  53. Farquhar, G.D.; Ehleringer, J.R.; Hubick, K.T. Carbon isotope discrimination and photosynthesis. Annu. Rev. Plant Physiol. Plant Mol. Bioi. 1989, 40, 503–537. [Google Scholar] [CrossRef]
  54. Tomlinson, K.W.; Poorter, L.; Bongers, F.; Borghetti, F.; Jacobs, L.; van Langevelde, F. Relative growth rate variation of evergreen and deciduous savanna tree species is driven by different traits. Ann. Bot. 2014, 114, 315–324. [Google Scholar] [CrossRef] [Green Version]
  55. Dodd, I.C.; Ryan, A.C. Whole-Plant Physiological Responses to Water-Deficit Stress; John Wiley & Sons, Ltd.: Hoboken, NJ, USA, 2016. [Google Scholar]
  56. Balas, N.; Nicholson, S.E.; Klotter, D. The relationship of rainfall variability in West Central Africa to sea-surface temperature fluctuations. Int. J. Clim. 2007, 27, 1335–1349. [Google Scholar] [CrossRef] [Green Version]
  57. Fontaine, B.; Janicot, S. Sea surface temperature fields associated with West African rainfall anomaly types. J. Clim. 1996, 9, 2935–2940. [Google Scholar] [CrossRef]
  58. Schifman, L.A.; Stella, J.C.; Teece, M.; Volk, T.A. Plant growth and water stress response of hybrid willow (Salix spp.) among sites and years in central New York. Biomass Bioenergy 2012, 36, 316–326. [Google Scholar] [CrossRef]
Forests 10 00251 g001 550
Figure 1. Location of the two study sites in two river catchments situated in different climatic zones of the Volta basin in Ghana, West Africa.
Figure 1. Location of the two study sites in two river catchments situated in different climatic zones of the Volta basin in Ghana, West Africa.
Forests 10 00251 g001
Forests 10 00251 g002 550
Figure 2. Seasonal patterns of mean monthly precipitation and maximum temperature (averages of 1961 to 2012) of the dry (DSZ) (a) and humid (HSZ) (b) savanna zones.
Figure 2. Seasonal patterns of mean monthly precipitation and maximum temperature (averages of 1961 to 2012) of the dry (DSZ) (a) and humid (HSZ) (b) savanna zones.
Forests 10 00251 g002
Forests 10 00251 g003 550
Figure 3. Pattern of annual 13C chronologies in whole wood and cellulose of Afzelia africana (a) and Anogeissus leiocarpus (b) in the dry (DSZ) and humid (HSZ) savanna zones of Ghana.
Figure 3. Pattern of annual 13C chronologies in whole wood and cellulose of Afzelia africana (a) and Anogeissus leiocarpus (b) in the dry (DSZ) and humid (HSZ) savanna zones of Ghana.
Forests 10 00251 g003
Forests 10 00251 g004 550
Figure 4. Correlations of Afzelia africana (a) and Anogeissus leiocarpus (b) chronologies from the dry (DSZ) and humid (HSZ) savanna zones with maximum temperature (temp) and precipitation (prec). For DSZ, wet period starts from April to October, and dry period from November to March. The wet period in the HSZ start from March to October, and dry period start from November to February. Avg signifies yearly averages. Dashed horizontal lines indicate significance (p ≤ 0.05).
Figure 4. Correlations of Afzelia africana (a) and Anogeissus leiocarpus (b) chronologies from the dry (DSZ) and humid (HSZ) savanna zones with maximum temperature (temp) and precipitation (prec). For DSZ, wet period starts from April to October, and dry period from November to March. The wet period in the HSZ start from March to October, and dry period start from November to February. Avg signifies yearly averages. Dashed horizontal lines indicate significance (p ≤ 0.05).
Forests 10 00251 g004
Forests 10 00251 g005 550
Figure 5. Spatial correlation of Afzelia africana and Anogeissus leiocarpus in both the humid (HSZ) and dry (DSZ) savanna zones with sea surface temperatures.
Figure 5. Spatial correlation of Afzelia africana and Anogeissus leiocarpus in both the humid (HSZ) and dry (DSZ) savanna zones with sea surface temperatures.
Forests 10 00251 g005
Table
Table 1. Characteristics of carbon isotope composition of Afzelia africana and Anogeissus leiocarpus in the dry (DSZ) and humid (HSZ) savanna zones of Ghana.
Table 1. Characteristics of carbon isotope composition of Afzelia africana and Anogeissus leiocarpus in the dry (DSZ) and humid (HSZ) savanna zones of Ghana.
Dry Savanna ZoneHumid Savanna Zone
A. africanaA. leiocarpusA. africanaA. leiocarpus
Whole wood (δ13C)
Number of rings measured101419176
Mean (‰)−25.54−26.29−25.32−26.54
Std. Dev. (‰)0.310.340.220.41
Maximum (‰)−25.01−25.60−24.95−25.75
Minimum (‰)−26.09−26.71−25.72−27.19
Correlation (ring width and δ13C) −0.060.14−0.15−0.09
p-value of correlation0.520.390.180.44
Cellulose (δ13C)
Mean (‰)−23.59−24.36−24.69−25.04
Std. Dev. (‰)0.280.770.210.28
Number of rings measured17121414
Offset (Whole wood-Cellulose, ‰)1.951.930.631.50

Share and Cite

MDPI and ACS Style

Boakye, E.A.; Gebrekirstos, A.; Hyppolite, D.N.; Barnes, V.R.; Porembski, S.; Bräuning, A. Carbon Isotopes of Riparian Forests Trees in the Savannas of the Volta Sub-Basin of Ghana Reveal Contrasting Responses to Climatic and Environmental Variations. Forests 2019, 10, 251. https://doi.org/10.3390/f10030251

AMA Style

Boakye EA, Gebrekirstos A, Hyppolite DN, Barnes VR, Porembski S, Bräuning A. Carbon Isotopes of Riparian Forests Trees in the Savannas of the Volta Sub-Basin of Ghana Reveal Contrasting Responses to Climatic and Environmental Variations. Forests. 2019; 10(3):251. https://doi.org/10.3390/f10030251

Chicago/Turabian Style

Boakye, Emmanuel Amoah, Aster Gebrekirstos, Dibi N’da Hyppolite, Victor Rex Barnes, Stefan Porembski, and Achim Bräuning. 2019. "Carbon Isotopes of Riparian Forests Trees in the Savannas of the Volta Sub-Basin of Ghana Reveal Contrasting Responses to Climatic and Environmental Variations" Forests 10, no. 3: 251. https://doi.org/10.3390/f10030251

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop