The effect of volcanism on submontane rainforest vegetation composition: Paleoecological evidence from Danau Njalau,Sumatra (Indonesia)

Abstract

Volcanic processes might have played an important role in the vegetation history of Sumatra, one of the largest and most tectonically active region in Southeast Asia. Palynological and macro-charcoal analysis results from Lake Njalau in the Kerinci Seblat National Park (KSNP) in Sumatra (Indonesia) provide an understanding of interactions between the volcanic deposition and vegetation in the past 5000 years. The deposition of volcanic material in the depression of the Lake Njalau (5100–4400 cal. yr BP) led to the dominance of pioneer species of Casuarina and Myrica, which grow in deforested land and volcano slopes (volcanophile taxa). The formation of the modern forest composition took several centuries after the volcanic deposition in the soil ended (ca. 900 years at ca. 2400 cal. yr BP). This suggests that the vegetation changes were not driven by a successional pattern, and soil formation was the most important environmental factor explaining this slow change in composition. The palynological records show no evidence for prehistoric human–landscape interactions in the area despite the close proximity to known megalith sites. The local fire regime reconstructed using macro-charcoal analysis indicates that fire was rare for the last 5000 years, and the average fire return interval was ca. 500 years. Phases of increased fire frequency could not be linked to either any of the vegetation phases or regional climatic changes, suggesting that fire occurrences were stochastic events. Our results overall suggest that volcanism has acted as one important driver of changes in the rainforests of the KSNP.

Keywords

Holocene Kerinci Seblat National Park palynological analysis Sumatra volcanic deposition volcanophile

Introduction

Volcanic activities and high biodiversity coexist in Southeast (SE) Asia, where the second largest area of rainforest in the world is found (Whitmore, 1998) in the middle of the so-called Pacific Ring of Fire (Kozák and Cermák, 2010). The high tectonic activity resulting from the intercrossing of the Eurasian plate and the Indo-Australian plate gave rise to numerous active volcanoes (Francis, 1993; Figure 1a). One of the largest islands and the most tectonically active place in SE Asia is Sumatra (Salisbury et al., 2012), where 35 volcanoes are listed as active (Smithsonian Institution, 2013; Figure 1a). In the past 100 years alone, more than 13 volcanic eruptions were reported to have occurred in this island (Salisbury et al., 2012; Smithsonian Institution, 2013). Due to this, the island has been known as arc of volcanoes. It is well known that volcanic eruptions are a major natural disturbance (Gómez-Romero et al., 2006), largely affecting species composition, competition, diversity, and succession of the vegetation around volcanoes or on volcanic islands (Del Moral and Grishin, 1999; Krebs, 2008). It follows that volcanic processes must have played an important role in the history of the vegetation in Sumatra (Laumonier, 1997). The west coast of Sumatra where the arc of volcanoes is found (Figure 1a) is dominated by the Barisan Mountains that run for the whole length of Sumatra (Whitten et al., 2000). A significant part of this mountain is located within the boundaries of the largest national park in Sumatra, the Kerinci Seblat National Park (KSNP; ca. 1.4 million hectares; Bramley et al., 2004).

Figure 1.

Maps of the study area: (a) the location of the studied region, the KSNP in Sumatra Island, Indonesia. The red triangle shows active volcanoes along the active tectonic faults in the islands and (b) the study site, Danau Njalau (Lake Njalau), marked with a yellow star. The red volcano symbol represents active volcanoes during the Holocene, and the gray volcano symbol is for an ancient Pleistocene volcano. The archeological sites (Bonatz, 2012) mentioned in the text are also shown. Data source for Digital Elevation Model (DEM): ASTER GDEM Version 2 from METI and NASA and volcanoes from Global Volcanism Program 2013 (Volcanoes of the World, v.4.5.3. Venzke, E. Smithsonian Institution. Accessed 19 December 2016. http://dx.doi.org/10.5479/si.GVP.VOTW4-2013).

While most of Sumatran rainforests have been rapidly converted to plantations and agricultural fields in the past 30–40 years (Drescher et al., 2016), the forests of the KSNP have been little affected and have remained relatively pristine. Well embedded in this rich and diverse protected montane rainforests, more than five active volcanoes, including the highest peak in Sumatra, Mount Kerinci (ca. 3805 m), are found (Ohsawa et al., 1985; Figure 1a). The area of Kerinci is, therefore, perfectly suitable to study the effect of volcanic eruption in the valleys of the Barisan montane range.

In the KSNP, when looking at the centennial to millennial timescale, volcanism can be considered as one of the most important disturbance factors. Volcanic eruptions produce a large amount of ash which accumulates in screes on top of volcanoes. These products are barren, pervious, sterile, and unstable and tend to move downhill, particularly during heavy rains (Whitten et al., 2000). The effect and damage that the ash deposition can exert on the vegetation depend on the thickness of the ash layer and size of the ash grain among other things (Hotes et al., 2004; Mack, 1981). The effect might be of increase susceptibility to forest fires and the establishing of harsh condition for certain plants in those areas most affected by tephra deposition such as topographic depressions (Titus and Tsuyuzaki, 2003). We hypothesize that this must have had a critical influence on the composition, structure, and dynamics of the rainforest vegetation in the KSNP.

To test this hypothesis, we carried out paleoecological multi-proxy analysis from a sediment core taken in a tectonic depression where a small lake is formed (Danau Njalau) in close vicinity to the active volcanoes in the KNSP (Figure 1b). Pollen and spore analyses are used to reconstruct the vegetation dynamics around the site. We carried out macro-charcoal analysis to reconstruct the local fire regime and used Botryococcus and Glomus concentrations as proxies for disturbance. We compared these results with the composition of the sediments in order to assess long-term effect of volcanic deposition and disturbance on the vegetation history of the site.

This study provides a different picture from the previous palynological and paleoecological studies in Sumatra (e.g. Flenley and Butler, 2001; Maloney, 1980, 1985; Maloney and McCormac, 1995; Morley, 1982; Newsome and Flenley, 1988; Stuijts et al., 1988). For the first time, paleoecological analyses are used to investigate the interactions between volcanic deposition and the rainforests ecosystems of western Sumatra, thus helping improving our understanding of their disturbance regimes in the long term.

Environmental settings of the study site

The study site is located in the southern part of the KSNP within the Barisan mountain range in Sumatra (Figure 1a). A small marginal depression where the Lake Njalau is found (Danau Njalau, also known as Danau Nyalo; 2.27°S, 101.56°E; 1040 m a.s.l.) was chosen as the catchment area is small (1–2 ha), and it is located in a remote area at the end of a valley side of Mount Kerinci. The depression probably originated from tectonic activity as it is located in the southwestern edge of the Rift Valley. A volcanic origin as an extinct crater is also possible (Flenley and Butler, 2001) as a quiescent volcano Mount Kunyit (2151 m a.s.l.) lies some 20 km in the western side of the depression. This volcano is part of a series of active Holocene and older Pleistocene volcanoes surrounding the Lake Njalau depression (Figure 1b).

A ring of grasses and sedges surrounds the lake, but vegetation quickly changes to forest on the relatively steep slopes around it. The mountainous areas surrounding the lake are dominated by submontane vegetation (800–1400 m), where dominant families are Fagaceae, Lauraceae, and Myrtaceae (Laumonier, 1997). Most of the forest is still in old grown condition, although some clearing occurred to plant cinnamon (Cinnamomum burmannii) in the northern part of the lake and dry rice (Oryza sativa; Flenley and Butler, 2001). Additionally, embedded in the forest, gardens from local farmers can be seen. These are small monocultural ‘estates’ (Laumonier, 1997) for home gardening (in Indonesian language: berkebun), usually for planting annual crops and vegetables such as chili (Capsicum spp.), corn (Zea mays), eggplant (Solanum melongena), and fruit such as banana (Musa sp.).

Central Sumatra is characterized by wet tropical climate as it lies within the Intertropical Convergence Zone (ITCZ). The mean annual rainfall for the area of the KSNP is about 2990 mm and annual temperatures average 20°C (Karger et al., 2016; http://chelsa-climate.org/; see Appendix 1, Figure 6a). Rainfall seasonality is not usually marked, but a long rainy season of 9–10 months is alternated with a shorter drier season of 2 or 3 months from June to August (see Appendix 1, Figure 6b). This seasonality is the results of monsoon dynamics over the region where the wet northwest (NW) monsoon peaks from December to February (DJF) and the dry southeast (SE) monsoon from June to August (JJA; see Appendix 1, Figure 6b; Kalnay et al., 1996). Inter-annual variability in rainfall pattern is controlled by the changes in the phase of El Niño-Southern Oscillation (ENSO) and particularly in western Sumatra, by the Indian Ocean Dipole (IOD; Abram et al., 2007; Saji et al., 1999; Webster et al., 1999).

Materials and methods

A 491-cm-long sediment core (DN) was collected in 2013 on the shore of the Lake Njalau, in a swamp depression using a Russian peat corer (Jowsey, 1966). The core was photographed and described lithologically at the University of Jambi, Sumatra, using sediment and attributes including color, texture, and plant part composition. Afterward, the core was transported to the Department of Palynology and Climate Dynamics, University of Göttingen, for further analyses. Eight samples consisting of plant materials and organic bulk sediment were sent to Erlangen Laboratory in Germany, Poznan Radiocarbon Laboratory in Poland, and the NTUAMS Laboratory in Taiwan for accelerator mass spectrometry (AMS) radiocarbon dating (Table 1).

Table 1.

List of accelerator mass spectrometry radiocarbon dates from DN core. Calibration done with R script in CLAM 2.2, calibration curve is the Southern Hemisphere SHCal13.14C (Hogg et al., 2013).

Depth (cm)	Laboratory code	Material	pMC	¹⁴C BP	Cal. yr BP
98	Erl-19237	Plant material (bark, stalk, and part of leaf)	97.48 ± 0.42	205 ± 34	208 ± 77
140	NTUAMS-2703	Plant material (bark, wood, seed)	88.87 ± 0.66	948 ± 7	737 ± 64
218	Erl-19236	Plant material (stalk)	73.93 ± 0.43	2427 ± 47	2522 ± 222
290	NTUAMS-2704	Plant material (wood)	69.44 ± 0.37	2930 ± 16	3261 ± 119
373	Erl-19235	Organic bulk sediment	62.55 ± 0.33	3769 ± 43	4535 ± 300
422	NTUAMS-2705	Organic bulk sediment	62.12 ± 0.32	3825 ± 20	4675 ± 98
467	Poz-81603	Organic bulk sediment	NA	4255 ± 30	4694 ± 69
489	Erl-19234	Organic bulk sediment	59.11 ± 0.32	4223 ± 44	5314 ± 273

Palynological analysis

A total of 32 subsamples for palynological analysis were collected along the core at different intervals to account for changes in sediment accumulation rate. The subsamples were processed for pollen and spores using standard techniques (Faegri and Iversen, 1989) including HF 48% treatment and acetolysis. Each subsample consisted of 0.5 cm³ of sediment. One tablet of Lycopodium clavatum spores was added to each subsample to estimate palynomorph concentrations (Stockmarr, 1971). Residues were mounted in glycerol jelly for pollen visualization, identification, and counting. Pollen and spore analyses were carried out using light microscopy. All identified pollen and spore types were photographed using Leica photomicroscope with a 1000× magnification. Pollen and spores were identified using the tropical pollen reference collections of the Department of Palynology and Climate Dynamics which includes specimen collected from the KSNP area. Additional resources used include pollen key and atlases for SE Asia (Bulalacao, 1997; Flenley, 1976; Garrett-Jones, 1979; Huang, 1972; Jones and Pearce, 2015; Poliakova and Behling, 2016; Powell, 1970; Stevenson, 1998; Wang et al., 1995) and online database (Australasian Pollen and Spore Atlas (APSA) from Australian National University, Canberra – available at http://apsa.anu.edu.au and the Pollen and Spore Image Database of the University of Goettingen – available at http://gdvh.uni-goettingen.de/).

The pollen grains were counted to a minimum of 300 grains per subsample, except for the two subsamples at 475 and 485 cm core depth, where a count of 300 could not be achieved due to low pollen concentrations. The identification was conducted at low taxonomic level as far as possible. However, some pollen grains are morphologically indistinguishable under the light microscopy and were summed together. These include pollen from family Moraceae and Urticaceae (Moraceae/Urticaceae) excluding Ficus, family Melastomataceae and Combretaceae (Melastomataceae/Combretaceae), pollen from Casuarina (Casuarinaceae), and Myrica (Myricaceae; Casuarina/Myrica), as well as pollen from Lithocarpus and Castanopsis (Fagaceae; Lithocarpus/Castanopsis).

Pollen counts were standardized to percentages based on the total pollen sum. Fern spores were counted along with the pollen grains and are expressed as percentage of the pollen and spores total sum. Pollen and spore taxa were grouped into submontane rainforest taxa including primary rainforest taxa and secondary rainforest taxa, volcanic indicator taxa (volcanophile), open and disturbance (anthropogenic) taxa and fern according to their ecology, habitus, and distribution (Flora Malesiana collection: http://portal.cybertaxonomy.org/flora-malesiana/; Prosea collection: http://proseanet.org; Laumonier, 1997). Concentrations are calculated using the Lycopodium clavatum marker and are expressed as number per cubic centimeter of sediment. The algae Botryococcus and the fungal spores of Glomus were also counted and concentrations calculated (number per cubic centimeter of sediment).

The software C2 was used for calculation of percentages and plotting of diagrams (Juggins, 2007). Local pollen assemblage zones are defined via constrained cluster analysis using the software CONISS (Grimm, 1987, 1993). All pollen and spores taxa are included in the analysis.

Macro-charcoal analysis

Macro-charcoal particles (>150 µm) were counted in contiguous subsamples at 2-cm intervals along the sediment core (250 samples). Subsamples of 2 cm³ were prepared following the methods for macro-charcoal analysis (Rhodes, 1998; Stevenson and Haberle, 2005). Low concentration of hydrogen peroxide (6% H₂O₂) was applied to partially digest and bleach organic material in the sediment samples. The macro-charcoal particles were counted under a binocular dissecting microscope. Results are expressed as the number of charred particles per cubic centimeter.

Numerical data analyses

Palynological diversity index (PDI) was estimated via ‘rarefaction analysis’ (Siegel, 1986). A small sample (low count, i.e. 10) was used as studies suggest that this pollen type diversity index correlates to the landscape diversity around the deposit (Matthias et al., 2015).

Principal curves (PCs) reduce the complex multivariate data in one-dimensional data space, and they are a powerful technique to summarize taxa compositional changes in the stratigraphic sequence. PC is the smooth, one-dimensional curve best fitted to the data in a number of dimensions (De’ath, 1999; Hastie and Stuetzle, 1989; Simpson and Birks, 2012). Pollen percentage data were transformed using the Hellinger transformation. The sample scores of the first correspondence analysis (CA) axis were used as starting points, and smoothing splines are fitted through these points. The penalty that determines the degree of smoothness was set to 2 because of the small dataset (31 subsamples; De’ath, 1999). Sample age was used as the sole covariate in the model, and the PC scores were extracted for each fit and arranged based on the time order. The curve is then modified through several iterations using a local averaging to reduce the sum of orthogonal distances between the PC and the observed data (De’ath, 1999; Simpson and Birks, 2012). To avoid overfitting of the curve, general cross-validation was done at each iteration step, and the degree of smoothness is allowed to vary between the different pollen taxa (Herzschuh et al., 2016). Additionally, the distance along the PC is used as an expression of the rate of change (RoC) per 1000 years (kyr) between samples for the pollen and spores dataset.

The PC and RoC were implemented using R version 3.3.2 (R Core Team, 2015) using the following packages: vegan 2.4-1 (Oksanen et al., 2013) and analogue 0.17-0 (Simpson, 2007; Simpson and Oksanen, 2016).

Fire regime characteristics were reconstructed using the software CharAnalysis (Higuera, 2009). The raw charcoal data were converted to charcoal accumulation rates (CHAR; particle/cm²/yr), and the CHAR time series was interpolated according to the median temporal resolution. Background CHAR was defined by smoothing the CHAR record over a 1000-year window with a lowest smoother robust to outliers. A Gaussian mixture model was used to separate the noise from the significant fire-related peaks, which exceeded locally defined thresholds in the noise distributions. The local signal-to-noise index (SNI), as developed by Kelly et al. (2011), was used to verify that the separation between the identified charcoal peaks and the noise distribution of the charcoal series was statistically significant (Kelly et al., 2011).

Multivariate statistical analysis was done to highlight the most important environmental factors driving vegetation dynamics in time. All identified pollen and spore percentage data were included in the analysis. The calculated DCA (with detrending by segments and Hill’s scaling) returned a length for the longest axis of 1.5 standard deviation (SD) units, thus linear ordinations were carried out. First, we used an unconstrained ordination (principal component analysis (PCA)) to highlight patterns in compositional change of pollen and spore assemblage. Rare taxa were not down-weight and the Hellinger transformation was applied (Simpson and Birks, 2012). Environmental variables are used as supplementary explanatories and are type of sediment (as factors: ash and lapilli and nonvolcanic), soil thickness (expressed as vertical distance along the sequence from the volcanic deposit), fire frequency as obtained from the CharAnalysis results as indicator of fire regime changes, and nonpollen palynomorphs as indicators of soil erosion and nutrient supplies (Glomus and Botryococcus; Kołaczek et al., 2013; Smittenberg et al., 2005; Van Geel et al., 2011). Following this, we carried out a constrained ordination (redundancy analysis (RDA)) in order to extract the variation that is directly explained by the environmental variables. PCA and RDA were performed using CANOCO 5 (Ter Braak and Smilauer, 2002).

Results

Core descriptions and chronology

The DN core consists mainly of two deposits: the lower volcanic layer (491–375 cm) and the upper peaty organic layer (375–0 cm). The volcanic layer consists of coarser volcanic lapilli embedded in an organic-rich matrix (491–426 cm), followed by finer volcanic ashes embedded in an organic-rich matrix with fine fibers (426–375 cm). The upper peat layer can be divided into two sublayers. The first one consists of hemic peat (375–29 cm), while the second represents the top little decomposed fibric peat layer (29–0 cm). A detailed description of the stratigraphic lithology of the core is shown in Appendix 2, Figure 7.

The chronology of the DN core is obtained from eight AMS radiocarbon dates (Table 1). The calibration of the radiocarbon dates is performed using Clam 2.2 (Blaauw, 2010) script in R (R Core Team, 2015) using the Southern Hemisphere SHCal13.14C calibration curve (Hogg et al., 2013). The top of the core (29–0 cm) is characterized by organic material rich in roots, and it is probably contaminated by modern material (Hogg, 1982). This part is, therefore, excluded from the model, and the modern date for the age–depth model is fixed at 29 cm. The dates are fitted into a locally weighted spline (LOESS) model. The results from the age–depth model indicate that the DN core records the last ca. 5000 years.

The depth versus age relationship (Figure 2) suggests an irregular sediment accumulation through time. The volcanic deposit corresponds to ca. 700 years of deposition (491–375 cm; 5100–4400 cal. yr BP; average sedimentation rate 1.7 mm/yr). A synchronous or slump deposition is not considered likely due to the stratigraphic characteristics of this deposition (from bottom to top: coarser to finer). Additionally, the organic material is embedded in the volcanic material suggesting a slow accumulation through time. Peat accumulated at the site for the past ca. 4400 years and a total of ca. 4 m of deposit are now found. The peat accumulation rate is initially low (375–110 cm; 4400–400 cal. yr BP; average 0.70 mm/yr) and subsequently progresses to a more rapid rate of accumulation (110–29 cm; 400 cal. yr BP–present; average 1.7 mm/yr).

Figure 2.

Age–depth profile of the DN core. A locally weighted spline (LOESS) is the best fitted model with extrapolated basal point and surface age set at −63 cal. yr BP (AD 2013, year of the coring) starting at 29 cm.

Palynological results

In total, 72 pollen taxa and 15 different spore taxa are identified in the 32 sediment subsamples (3 rare pollen taxa and 4 spore taxa remain unknown). Pollen and spore grains are well preserved, and average concentration is high along the core (average 70,000 grains/cm³). The bottom part (491–475 cm; 5100–5000 cal. yr BP) records low pollen concentration (average pollen concentration 50,000 grain/cm³). The pollen concentration values then significantly increased (475–355 cm; 5000–4200 cal. yr BP; average pollen concentration 110,000 grains/cm³) and subsequently decreased to about 44,000 grains/cm³ (355–290 cm; 4200–3300 cal. yr BP). Afterward, the pollen concentration shows an increasing trend up to the top of the sediment core (290–0 cm; 3300 cal. yr BP–present; average pollen concentration 64,000 grain/cm³).

The pollen diagram illustrates percentages of the dominant and most important taxa, which are grouped based on their ecology and habitat occurrences in the KSNP (Figure 3a and b; see Appendix 3, Table 3). ‘Others’ corresponds to pollen grains of taxa that can be described as generalists as they occur in all habitats considered. Based on the cluster analysis, the DN record is divided into two palynological zones (Figure 3a and b) which are described below.

Figure 3.

Palynological diagram of the DN core: (a) most important pollen and spore taxa represented as percentage of the total sum of pollen (pollen taxa) and pollen plus spores (for Pteridophyta), non-pollen palynomorph concentrations (Botryococcus and Glomus); and (b) percentage of the dominant and most important pollen and spore taxa for each ecological group. CharAnalysis results (fire peaks, peaks magnitude, and fire frequency) and palynological numerical analysis results including the principal curve (PCurve), palynological diversity index (diversity/ET₍₁₀₎), and rate of compositional change/velocity (RoC).

Zone DN-1 (491–275 cm; ⩾5100–3100 cal. yr BP; 12 subsamples)

This zone marks the dominance of volcanophile taxa in particular Casuarina/Myrica and can be divided into two subzones based on the change in volcanophile composition: subzone DN-1a and subzone DN-1b.

At the beginning of subzone DN-1a (491–425 cm; 5100–4700 cal. yr BP; four subsamples), where volcanic lapilli are deposited, the DN core records high volcanophile pollen taxa Casuarina/Myrica (average 38%) and fern spores such as Phymatosorus (average 9%), while other montane rainforest pollen and spore taxa are low. Subzone DN-1b (425–275 cm; 4700–3100 cal. yr BP; eight subsamples) starts when the deposition shifted to volcanic ash. The volcanophile pollen grains markedly decreased but remained dominant (average 17%). At the same time, other rainforest pollen taxa, that is, Ficus, start to increase (average 11%) and fern spores decrease (average 2%).

Zone DN-2 (275-0 cm; 3100 cal. yr BP–present; 20 subsamples)

This zone shows a gradual increase in rainforest pollen and spore taxa and a decline of volcanophile taxa. This zone can be divided into two subzones: subzone DN-2a and subzone DN-2b. In subzone DN-2a (275–120 cm; 3100–500 cal. yr BP; 14 subsamples), Ficus pollen marked an increase in primary rainforest taxa (average 13%) in parallel with the increase in secondary rainforest taxa such as Saurauia (average 14%), while Casuarina/Myrica started to decline before it finally almost disappears from the record (average 1%). The subzone DN-2b (120–0 cm; 500 cal. yr BP–present; six subsamples) is characterized by an increase in primary rainforest taxa Ficus (average 16%), mirrored with the decrease in secondary forest taxa Saurauia (8%). Additionally, the algae colonies of Botryococcus increased markedly in this zone (152–95 cm; 900–300 cal. yr BP; average 30,000 grains/cm³). A high peak of the fungal Glomus spores was recorded at 2 cm (2000 spores/cm³).

Macro-charcoal and fire regime

Macro-charcoal analysis resulted in a low number of counting along the DN sediment core. Macro-charcoal concentration marked relatively high values within subzone DN-1a (491–425 cm; 5100–4700 cal. yr BP) in the volcanic lapilli deposit (Figure 3b). Subzone DN-1b (425–355 cm; 4700–4200 cal. yr BP) initially shows a decrease in macro-charcoal concentration and then it started to increase until the end of the zone (355–275 cm; 4200–3100 cal. yr BP). In zone DN-2 (275–0 cm; 3100 cal. yr BP–present), macro-charcoal concentrations are more stable (minimum concentration: 16 particles/cm³ and maximum concentration: 64 particles/cm³).

Using the CharAnalysis software (Higuera, 2009), the raw charcoal data are interpolated into 22 years (the median temporal resolution), and the peak signals of fire episodes are modeled while removing the background noise. A total of 12 fire peaks are detected along the core. The local SNI values for the DN macro-charcoal data are fluctuating above 3, indicating significant signals for fire peaks (Kelly et al., 2011). The mean fire return interval (FRI) is 523 with mean frequency of fire of 2 peaks/1000 years (maximum 5 peaks/1000). Fire frequency within zone DN-1 is low (ca. 5 peaks/1000). Zone DN-2 is marked by an increasing trend, and the highest fire frequencies are found within this zone (7 peaks/1000). High magnitude of peak (>100 particles/peak) are found in zone DN-1 (1 peak), while in zone DN-2 are detected lower in magnitude peaks but higher fire frequency (5 peaks; Figure 3b).

PDI, PC, and RoC

PDI (PDI/ET₍₁₀₎) values are initially low in subzone DN-1a (37 pollen and spore types). They show an increase at 425 cm depth (4700 cal. yr BP), to remain stable afterward (Figure 3b).

The PC using CA as a starting point converged after five iterations, and the final curve explains 51% of the variance. The distance along the gradient of the subsample composition shows three main phases of compositional change: from 4700 to 4400 cal. yr BP, from 3300 to 2400 cal. yr BP, and from 500 cal. yr BP to present. By scaling these changes per unit of time (RoC per 1000 years; Figure 3b), it can be seen that of these three phases, the ones characterized by the most rapid changes, are (in order) the oldest change (from 4700 to 4400 cal. yr BP) and the most recent (from 500 cal. yr BP to present).

The individual response curves of the six most important pollen taxa are shown in Figure 4a–f. In summary, the DN record shows a high response in terms of compositional changes only for the volcanophile taxa Casuarina/Myrica, while the other most represented pollen taxa show no marked change in time but rather slow changes.

Figure 4.

Fitted response curves for the six most abundant pollen taxa in the DN records as estimated using a PC with CA as the initial starting point. Open circles are the observed proportional abundance, and the solid line is the optimized smoother from the final iteration of the PC: (a) Ficus, (b) Moraceae/Urticaceae, (c) Mallotus/Macaranga, (d) Lithocarpus/Castanopsis, (e) Saurauia, and (f) Casuarina/Myrica.

PCA and RDA

The result from the PCA indicates that 47% of the total variance is explained by the first (39%) and second (8%) axes (Figure 5a and b). Pseudo-canonical correlation for the supplementary environmental variables shows high correlations with the first (0.95) and second (0.96) axes, respectively. The results from the RDA reveal that 51% of the variation can be explained with the environmental variables and show which environmental variables significantly explain the variance in pollen and spore composition (Table 2). The simple effect model (i.e. the independent effects of individual environmental variables) shows that all environmental variables are significantly correlated with the variance in taxa composition (p < 0.005 adjusted from false discovery rate) except for ash, erosion (Glomus spore), and fire. However, the largest part of the variance (34.5%) is explained by soil thickness (distance from the volcanic deposition; Table 2, RDA – simple term effects). The conditional effect model summarizes the conditional effect of each environmental variable after accounting for the effect of the variable placed above each (in order of their decreasing explained variation). The results of this model indicate that only soil thickness (34% variation explained) and lapilli (7% variation explained) are significant (p < 0.005 adjusted from false discovery rate; Table 2, RDA – conditional term effects).

Figure 5.

Principal component analysis (PCA) of all percentage data of identified pollen and spore taxa after Hellinger transformation. Environmental supplementary variables are type of sediment (peat, ash, and lapilli as nominal variables), fire frequency from the results of the CharAnalysis, non-pollen palynomorph concentrations for erosion and nutrient status (Botryococcus and Glomus), and the soil thickness (vertical distance in centimeter from the volcanic deposit). First two axes are shown (39% and 8% of the variation in composition). (a) Pollen and spore taxa scatterplots and (b) sample scatterplots represented as pie charts of the relative abundance of pollen and spore taxa groups. Numbers in gray are estimated age of each sample (cal. yr BP).

Table 2.

Results from the redundancy analysis (RDA) with the (a) simple term effect and (b) conditional term effect (significant results with p < 0.005 in bold).

Name	Explains %	pseudo-F	p (adjusted)
(a) RDA – simple term effects
Soil thickness	34.5	15.3	0.00233
Peat	25.4	9.9	0.00233
Lapilli	20.7	7.6	0.0035
Nutrient (Botryococcus)	15.5	5.3	0.00233
Ash	7.9	2.5	0.0378
Erosion (Glomus)	3.5	1.1	0.33833
Fire (macro-charcoal)	3	0.9	0.425
(b) RDA – conditional term effects
Soil thickness	34.5	15.3	0.0035
Lapilli	6.7	3.2	0.0035
Ash	3	1.4	0.126
Peat	3	1.4	Unknown
Fire (macro-charcoal)	3.1	1.5	0.07933
Nutrient (Botryococcus)	2.2	1.1	0.4858
Erosion (Glomus)	1.9	0.9	0.67317

Discussion

Effects of volcanic deposition on the vegetation

Central Sumatra is possibly reckoned as the source area for the most frequent and large explosive eruptions in Sumatra (Salisbury et al., 2012). The volcano formation in Sumatra is formed either by the uplift of sedimentary deposits, that is, in the Bukit Barisan mountain range or by the volcanic activity, that is, Mount Kerinci, Mount Sinabung, and Mount Singgalang (Whitten et al., 2000). In Sumatra, volcanic activity might have been continuous for millions of years, with evidence of reduced activity only in the Late Miocene (Crow, 2005). The volcanoes of the Quaternary period (the last million years) are located in the mountainous areas and are usually associated with faults (Whitten et al., 2000). The products of explosive volcanic eruptions are commonly rich in plagioclase and acid (damatic to liparitic; Laumonier, 1997), and the main long-term effect on the ecosystem is due to the fallout of this volcanic material. The deposit thickness varies depending on the distance to the volcanic center, and it can interest large areas in the landscape (Del Moral and Grishin, 1999). There are several modes of impact of volcanic deposition on ecosystems, including physical impact of tephra on the vegetation (Bjarnason, 1991; Clarkson and Clarkson, 1994; Cook et al., 1981; Eggler, 1948; Wilcox, 1959), impact of tephra deposition on the hydrology (Crowley et al., 1994; Hotes et al., 2004), and chemical impact of tephra and tephra leachates (Smith et al., 1983; cf. Wissmar et al., 1982). At the same time, the extent of the impact of volcanic fallout on the ecosystem depends on various factors such as climatic conditions, soil characteristics, as well as grain size and structure of the tephra (Kilian et al., 2006). Vegetation recovery and succession can be affected by the differential survival modes of plants, erosion, concentration of tephra deposits, and many other factors (Del Moral and Grishin, 1999; Tsuyuzaki and Haruki, 2008; Wang et al., 2010). While in Sumatra, previous research on volcanic impacts on the vegetation has been very limited, there are sufficient evidences to suggest that changes on the vegetation were significant on the regional vegetation (Fesq-Martin et al., 2004; Kilian et al., 2006; Payne and Blackford, 2008).

Paleoecological analysis carried out in lakes and swamps located in valleys surrounded by active volcanoes can be extremely informative on the mode and timing of the effects of volcanic material on the montane ecosystems in Sumatra. The area of the KSNP represents the ideal location to investigate changes in vegetation and test for the most important drivers of change for the past thousands of years. The DN core was taken from a location surrounded by several active volcanoes (Figure 1b). The area is characterized by steep slopes where monsoon raining events can cause erosion and deposition of volcanic material from the peaks to the central valley and depressions, such as the one where the site is located (see Appendix 1, Figure 6b).

The results from the radiocarbon dating of the DN core indicate that there was a rapid initial deposition of volcanic coarse material at the beginning of the record (Figure 2). This is likely due to the erosion of volcanic material from the mountain slopes after the eruption event occurred. It is apparent that the DN archive does not include the initial volcanic episode but only the successive redeposition of volcanic material in the lake catchment area since ca. 5000 cal. yr BP. This is confirmed by the nature of the deposits themselves, which are not pure tephra layers but rather a mixture of volcanic and organic mud/peat (Kilian et al., 2006). A gradual decrease in the deposition occurred since 4700 cal. yr BP, when volcanic material deposited was finer and erosion from the slopes likely decreased.

The effect of this deposition of volcanic material in the depression of the Lake Njalau is shown from the results of pollen and spore analysis. Parallel to the presence of volcanic material in the soil (5100–4400 cal. yr BP; 491–375 cm) was the presence of a vegetation dominated by species of Casuarina and Myrica and the fern Phymatosorus which is often found growing on the volcanic lapilli deposit (Orwa et al., 2009; Pinyopusarerk, 1997). Few species can adapt to volcanic soil, while the majority of rainforest species cannot. The fact that the diversity of the vegetation in this phase was low is clarified by the results of the PDI which shows in this phase the lowest values for the entire 5000 years recorded (Figure 3b).

Ecological investigations indicate that Casuarina and Myrica are very successful under stressful condition and highly competitive (Gauthier et al., 1999; Orwa et al., 2009; Potgieter et al., 2014). This can be explained by a combination of traits and factors such as their ability to rapid grew, an early and prolific reproduction, avian seed dispersal, and symbiosis with the actinomycete Frankia (Potgieter et al., 2014; Sayed, 2011; Vitousek, 1990; Vitousek and Walker, 1989; Wang and Qiu, 2006; Zhong et al., 1994) so that they can colonize nitrogen-deficient deposits. As a consequence, it has been observed that in the KSNP, several species of Casuarina and Myrica act as pioneer species of both deforested land and volcano slopes (Vitousek and Walker, 1989). These volcanophile taxa slightly decreased when the sediment shifted into volcanic ash deposition and the sediment accumulation slowed down (4700–4400 cal. yr BP; 425–375 cm depth). However, after the volcanic deposition in the soil ended (4400 cal. yr BP; 375 cm depth), the volcanophile taxa still persisted in the vegetation with relatively high values (up to 22%) before slowly declining along the record (since ca. 3300 cal. yr BP; 290 cm depth). In parallel, other rainforest taxa such as Ficus started to increase although the composition of the modern forest took several centuries to complete (ca. 900 years at ca. 2400 cal. yr BP).

The results from the PC can help clarifying when most of the variation in taxa composition occurred at the site (Figure 3b). Major changes in the composition of pollen and spores occurred only three times in the recorded 5000 years. These episodes are recorded from 4700 to 4400 cal. yr BP, from 3300 to 2400 cal. yr BP, and in the last ca. 500 years. The first two episodes correspond to the changes from lapilli to ash in the sediments and when the volcanophile taxa decreased markedly around 3300 cal. yr BP (Figure 3b). When rescaling the PC per unit of time (1000 years), RoC analysis can show how fast those changes were. From this, we can discern that the first episode of change was much more rapid compared with the second decrease in volcanophile taxa, thus suggesting a slow pace of change in this second episode (Figure 4a–f). The PC from the most represented taxa in the record highlight an important trend in the vegetation dynamics. The only taxa which shows a major trend (decreasing) is Casuarina/Myrica, while the other most important taxa such as Moraceae/Urticaceae, Lithocarpus/Castanopsis, and Ficus show a slow increase through time. This suggests that following the deposition of volcanic material at the beginning of the record, the vegetation changes were not driven by a successional pattern which would be otherwise clear in the PC. The most likely explanation for this trend is that competition played the major role in shaping plant composition in the area. Species of Casuarina and Myrica were successful in dominating patches of the forest until other rainforest taxa (e.g. Ficus and Moraceae/Urticaceae), conquered back the space in the landscape, reducing the representation of Casuarina and Myrica markedly.

There are several environmental factors which might have played a role in driving this competition. The PCA and RDA show that the two most important and significant factors were the presence of lapilli in the soil at the beginning of the record and the subsequent formation of soil. The presence of lapilli in the first 400 years of the record explains 7% of the variation in the record. The deposition of this barren material likely marked the first expansion of the volcanophile Casuarina/Myrica. Although this cannot be tested with our data, it is likely that soil acidification might have played an important role favoring their expansion and, at the same time, causing dying off and/or impeding the development of the other rainforest taxa (Potgieter et al., 2014).

However, our results suggest that the most important factor explaining the majority of the variance in our record is soil thickness (34%). It is likely that the presence of volcanic material in the soil limited plant survival and growth by preventing root from reaching the below organic soil (Gómez-Romero et al., 2006). This suggests that the dominant presence of Casuarina and Myrica in the Lake Njalau depression was largely affected by the presence of volcanic material in the soil layer where trees have their roots. Once the peaty organic soil accumulated above such a layer, species of Casuarina and Myrica could no longer outcompete against other rainforests species and rapidly decreased.

Other possible drivers of change: Fire and human activities

Fire regime shifts can occur in tropical rainforests as a consequence of climate variability (e.g. ENSO, IOD; Gaveau et al., 2014; Kita et al., 2000; Page et al., 2002; Siegert et al., 2001; Van Der Werf et al., 2004; Wang et al., 2004) or due to human activities (Bowen et al., 2001; Cole et al., 2015). Our results for the high-resolution macro-charcoal analysis and CharAnalysis reveal that fire episodes were extremely rare at this site. In average, the FRI was of a fire episode per 523 years with only 12 major fire episodes detected over the past ca. 5000 years. The general low return interval as well as the lack of any trend in the fire frequency through time suggests that the occurrence of fires at the site were stochastic events, most likely as a consequence of occasional storms and lightening (Stolle et al., 2003). Additionally, the RDA indicates that there was no significant correlation between vegetation composition and fire frequency (Table 2), thus suggesting no link between vegetation changes and fire regime in the long term.

The first evidence of arrival of Homo sapiens in Sumatra trace back to 12,000 years ago at the beginning of the Holocene (Forestier et al., 2006) with the Hoabinhian hunter-gatherer civilization (12,000–7000 yr BP; Glover, 1979). In the KSNP area, palynological evidences seem to point toward an increase in deforestation and fires because of human activities already starting from ca. 7000 cal. yr BP (Flenley, 1988; Flenley and Butler, 2001; Morley, 1982). However, the first cultural evidence of human presence dates back to the Neolithic periods around 3400–2900 cal. yr BP when stone artifacts and obsidian tools are found in the Sungai Hangat and Renah Kemumu regions (Bonatz, 2012; Figure 1b).

Afterward, the settlement history in the Kerinci area is marked by the ‘Megalithic period’ which lasted from the late 10th century until the 14th century AD (900–1300 yr BP) and had its peak during the 12th century AD (1100 yr BP; Bonatz et al., 2006; Tjoa-Bonatz et al., 2009). What is left of this culture is outstanding stone megaliths and burial jars as well as remnant of single stones which are usually placed in connection to a house in a small-scale settlement. These megalithic remnants are found around the KSNP area in cultural clusters close to the villages of Renah Kemumu in Serampas region, Lolo Gedang in the Kerinci area, Sungai Hangat, and Bukit Arat region (Bonatz, 2012; Figure 1b).

Despite the close proximity of the DN site to these clusters of cultural human phases, no evidence of prehistorical human–landscape interactions was found in the palynological record from the Lake Njalau catchment area. As shown by the PC, RoC, PCA, and RDA, the major changes in vegetation composition can be explained with effect of volcanic deposition in the area.

However, a dynamical phase is recorded for the top part of the record from ca. 500 cal. yr BP (AD 1450) to present. This phase is characterized by rapid changes (PC and RoC, Figure 3b) and a composition of pollen and spores which was not found in any of the samples before as shown in the PCA (Figure 5a and b).

Three high in magnitude fire peaks were recorded at ca. −63 (present time), 47, and 157 cal. yr BP which might indicate anthropogenic burning activities. These fire episodes coincide with an increase in Glomus spores which can be used as an indicator of soil erosion (Kołaczek et al., 2013). Thus, the synchronous occurrence of fires and the increase in Glomus might be related to deforestation and burning of the surrounding forest. However, no evidence could be found of other activities such as Cinnamon and ‘berkebun’ cultivation. This suggests that either there was little human disturbance on the vegetation during the past 5000 years or the DN core was too remote and isolated to record even close by activities in the Kerinci area.

Conclusion

This study was conducted with the aim of investigating the effects of volcanic depositions on the highly diverse tropical montane ecosystems of the KSNP in Sumatra, Indonesia. The use of paleoecological analysis increased our understanding of this interaction, by adding a long-term temporal view to the modern understanding of these ecosystems (Simpson and Birks, 2012).

The DN core taken from this depression dates back to ca. 5000 years ago, and it revealed a secondary deposition of volcanic material at the beginning of the record. The results from the radiocarbon dating indicate that this redeposition occurred at a slow pace (ca. 700 years), while the results from pollen and spore analysis indicate that the presence of this barren material in the soil had a strong impact on the plant composition in the area. Species of Casuarina and/or Myrica were advantaged by this condition and dominated the depression in this phase, leading to low diversity. When the deposition of volcanic material decreased and finally ended ca. 4400 years ago, the presence of these volcanophile pioneer taxa remained frequent in the area. The establishment of the modern submontane rainforests took ca. 900 years. Such a long-time dynamic is unlikely to have been driven by succession, as this kind of recover pattern after volcanic eruption has been shown to have taken less than 100 years, for instance, in the island of Krakatoa in Indonesia (Whittaker et al., 1989). This is confirmed by the PCA of the pollen and spore data which highlight that this decreasing trend was not characterized by temporal succession of different taxa but by a continuous decrease in Casuarina/Myrica and a slow but continuous increase in several other rainforest taxa (i.e. Ficus, Moraceae/Urticaceae, and Mallotus/Macaranga). Several environmental factors might have contributed to this slow increase in submontane species including soil thickness, erosion, and disturbance and human activities. We tested for these different factors using multivariate statistical analyses on the pollen and spore assemblage. The environmental explanatory variables used were type of sediment (as factors: ash and lapilli and nonvolcanic), soil thickness (expressed as vertical distance along the sequence from the volcanic deposit), fire frequency as obtained from the CharAnalysis results as indicator of fire regime changes, non-pollen palynomorphs as indicators of soil erosion, and nutrient supplies. The constrained ordination (RDA) results indicate that among those factors which were significantly correlated with the decrease in volcanophile taxa, soil thickness was the one that explains most of the variance. We conclude that the presence of volcanic material in the soil layer where trees have their roots limited the survival and growth of most of submontane rainforests taxa rather than Casuarina/Myrica. Once the peaty organic soil accumulated above such a layer, species of Casuarina and Myrica could no longer outcompete against other rainforests species and rapidly decreased.

Other than the effect of volcanic material in the soil, disturbance caused by human activities, climatic variability, and fire regime changes can be considered strong drivers of ecosystem dynamics in the tropical montane rainforests in Indonesia (e.g. Biagioni et al., 2015, 2016; Haberle, 2007; Haberle et al., 2001; Hope, 1998; Kirleis et al., 2011; Maloney, 1980; Maloney and McCormac, 1995; Morley, 1982). Despite the close vicinity of the site to known archaeological sites, we did not find compelling evidence for prehistoric disturbance of the vegetation due to human activities such as deforestation and agriculture. Additionally, the high-resolution macro-charcoal analysis of local fire history at the Danau Njalau depression indicated that fire never had a strong influence on the vegetation for the past 5000 years. Fire episodes were extremely rare and, therefore, cannot be linked to any causality (i.e. human cultural phases and climate variability).

While it should be noted that this is a single case study and more records are needed to confirm these results, our record from the KSNP suggests that the effect of volcanism on the western side of the Sumatra island might be spatially and temporally more important than so far assumed in shaping the composition and structure of the diverse rainforests we see today.

Footnotes

Appendix 1

Appendix 2

Appendix 3

Table 3.

List of pollen and spore taxa identified in the DN core and grouping based on their ecological distribution: submontane rainforest taxa, volcanophile taxa, open and disturbance indicator taxa, and Pteridophyta. Other taxa correspond to pollen grains of taxa that could not be included in any of the defined group.

Submontane rainforest taxa		Volcanophile taxa	Open and disturbance indicator taxa	Other taxa	Pteridophyta
Acacia Acalypha Antidesma Arecaceae Bischofia Blumeodendron Canarium Diospyros Dipterocarpus Dysoxylum Elaeocarpaceae Engelhardtia Ericaceae Ficus Flacourtiaceae Gardenia Glochidion Gnetum Hamamelidaceae Homalanthus Lauraceae Leea Lithocarpus/Castanopsis Melastomataceae/Combretaceae Menispermaceae Moraceae/Urticaceae	Myristicaceae Myrsinaceae cf. Aegiceras Myrtaceae Nauclea Pandanus Phyllanthus Pinus Rutaceae Sapindaceae Saurauia Verbenaceae Zanthoxylum	Aglaia Casuarina/Myrica Celtis Dacrycarpus Podocarpus Pometia	Arenga Asteraceae Baccaurea Calophyllum Cyperaceae Ilex Liliaceae/Iridaceae Mallotus/Macaranga Mimosoideae Poaceae Prunus Trema Vernonia	Annonaceae Celastracaceae Euphorbiaceae indet. Fabaceae indet. Loranthaceae Malphigiaceae Malvaceae Oleaceae Randia Rosaceae Rubiaceae indet. Sapotaceae Tilioideae	Cyatheaceae Davallia Marattiaceae Ophioglossum Phymatosorus Polypodiaceae Pteris Schizaea Selaginella Stenochlaena palustris Thelypteris

Acknowledgements

The study was conducted using a research permit (no. 232/SIP/FRP/SM/VII/2013) from the Ministry of Research and Technology of Indonesia (RISTEK), collection permit (no. 3272/IPH.1/KS.02/X/2013) and export permit (no. 183/IPH.1/KS.02/I/2014) as recommended by the Indonesian Institute of Sciences (LIPI). We gratefully acknowledge the logistic support by the EFForTS coordination team and the Indonesian partner universities in Bogor and Jambi, Institut Pertanian Bogor (IPB) and University of Jambi (UNJA), the Ministry of Education in Jakarta (DIKTI), the Indonesian Institute of Sciences (LIPI), and the authorities of the KSNP for the help and cooperation during the fieldwork. We thank the anonymous reviewers for their comments for this manuscript.

Funding

Financial support is provided by the DFG Sonderforschungsbereich in the framework of the collaborative German-Indonesian research project CRC 990 (EFForTS) subproject A01 and Erasmus Mundus (Experts4ASIA) Program.

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