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Early vegetation recovery after the 2008-2009 explosive eruption of the Chaitén Volcano

In May 2008, Chaiten volcano entered in eruptive process, one of the world largest in the last time decades. The catastrophic event left different type of disturbance and caused diverse environmental damage. Consequently, the biological legacies were distributed heterogeneously in surrounding areas of the volcano. We went to the field to assess the early vegetation responses to the eruption a year after, September 2009. Particularly in the lateral-blast disturbance zone. We distributed a set of plots in three disturbed sites, and one in an undisturbed site. In each site, in a plot of 1000m2 we marked all stand tree, recording whether they were alive, resprouting or dead. Later in 80 small-plots (∼4m2) we tallied the plants regenerating, its coverage, and the log-volume. For regenerating plants, we described whether the substrate was mineral or organic. The impacts in the blast-zone created a gradient of disturbance, where close to the crater we found high devastation marked by the no surviving species, scarce standing-dead trees and logs, as well as no regeneration. On the other extreme, trees with devastated crown were resprouting, small-plants regrowing and dispersed seedlings. The main regeneration strategy was resprouting from trunks or buried roots, while few seedlings were observed in the small plots and elsewhere in disturbed areas. The main findings of this study are: i) a mosaic of pioneering-wind dispersed species, scattered survivors regrowing and spreading from biological legacies, and plant species dispersed by frugivorous birds, likely favored by the biological legacies; (ii) the early succession is influenced by the interaction of the species-specific life history, altitudinal gradient and the different intensity of disturbance.


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Volcanic eruptions are large-scale disturbance that modify the composition, the structure and 43 ecosystems processes (Swanson et al., 2013;Turner and Dale, 1998;Turner et al., 1997). As 44 volcanoes enter in eruptive phase complex mechanism involve the events in a gradient of 45 multiple type of disturbances such as explosive blasts, thermal and toxic chemical waves, 46 landslides, glowing avalanche deposits, debris flows, lava flows, and air falls of volcanic tephra 47 glacio-fluvial outwash (Swanson et al., 2013). Vegetation within the influence area of the 115 Chaiten volcano is dominated by a dense old-growth temperate rainforest of northern 116 Patagonian and Valdivian types (Luebert and Pliscoff, 2006;Swanson et al., 2013;Veblen et 117 al., 1983;Veblen et al., 1996) We carried out the field work in September 2009. We first conducted a general inspection of 131 the study area (del Moral, 1981). Later, we selected four sites with different degrees of volcanic 132 impact following similar classifications of previous studies (del Moral, 1981). The first site 133 corresponds to a forest with low volcanic impact that only received few millimeters of tephra 134 deposition. This site was called "Old-growth forest". It was located 5 km from the center of the 135 new dome at an elevation of 600 m a.s.l. (Figure 1b). The second site was characterized by the 136 presence of abundant dead standing trees, and higher tephra deposition. This site was called 137 "Standing-dead" located at 3.5 km from the center of the new dome, at 160 m. The third site 138 was more affected by the eruption, and was characterized by high tephra deposition, where most 139 trees were broken and throw down by the eruption. This site was called "Blow down" and was 140 located at 2.3 km from the center of the new dome at an elevation of 300 m a.s.l. Finally, the 141 fourth and most affected site was in the border of the crater, at 1.5 km of the center of the new 142 dome and at 750 m a.s.l. This site presented the higher tephra deposition and all trees were blow 143 down by the eruption and heavily buried under volcanic ash. This site was called "Total 144 destruction". 145

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In each one of these four areas was installed a 50 x 20 m permanent plot where we 147 measured the diameter at the breast height (dbh) of all trees ≥ 5cm. Every tree was identified at 148 the species level when possible and marked. We registered if the stems were alive, resprouting 149 after the eruption, or snag (standing dead). To estimate the presence and abundance of tree 150 regeneration, we traced four transects of 120 m long. Transects were distributed parallel, 151 separated 20 m from each other. In each transect, we mounted five circular plots of 1.5m radius 152 (4.71m 2 ) every 30 m. On these plots we counted all seedlings and saplings present, and 153 recording the substrate where they rooted (forest soil, logs, or volcanic tephra). We registered 154 other non-tree plant species in the circular plots, estimated its abundance as the relative cover, 155 and we also recorded the substrate where they rooted were organic or mineral (e.g., forest soil, 156 logs, or volcanic tephra). We assessed the volume of all exposed logs in the plot, until each 157 border within the plot. Assuming a cylindrical form of each log, we calculated the volume of logs present inside the plot and standardizing its measurements to m 3 of logs per m 2 of plot 159 surface. 160 161 2.3 Data Analysis 162 Trees seedlings, herbs and shrubs species richness was compared among sites using rarefaction 163 analysis (Colwell 2006   The eruption of the Chaitén volcano has an impressive effect on the forest. The eruption in the 181 study area produced a gradient of destruction. Around the crater all the trees were blown down and heavily buried by tephra. Other trees were partially buried, broken in half, with the trunks 183 covered with impacts of many little stones looking similar to the fire of gun shots (Fig x). 184 Moving away and down from the crater, trees were blown down and densely covered by ash (Fig  185 x), while further away trees were standing dead presenting progressively finer branches (Fig x). 186 Many small patches of the organic litter were exposed and sparsely distributed and protected 187 from the direct effect of the eruption by root discs and large trunks, or were exposed by small 188 ravines that flush away the volcanic ash. These small patches looked like clumps of organic 189 soil, with roots of ferns and other small plants that were not buried by the tephra. These clumps 190 were covered by plants that survived in the area affected by the eruption, and their frequency 191 increases when moving away from the crater towards the undisturbed forests. 192 In the surrounding, we did not observed evidence of fire caused by the eruption, but the trunk 194 and bark of several trees looked charred within the influence area. Trees were dead and dry by 195 the eruptive explosions, but some trees in Blow-down and Standing-dead areas maintained the 196 bark in the opposite side of the crater, perhaps with possibilities to resprouting hereafter. Tephra 197 buried the fallen trees in the whole study area, but principally in Blow-down and Total-198 destruction areas. Tephra was very compacted, seeming as pavement that covered the fallen 199 forests. Heterogeneous tephra deposition exposed some trunks, clumps and remnants with 200 organic-forest soil likely from the original forest. Shrubs and epiphytes were spread everywhere 201 in the Standing-dead area. Ferns, bamboo and tree seedlings were growing in remnants of 202 exposed organic soil, usually below or at one side of trunks or unearth roots. Surviving trees 203 also increases when moving away and down from the crater. 204 205 3.2 Species composition 206 We found 10 species of trees in the area, and 50 species of smaller plants, including seedlings, 207 herbs and ferns (Table 1). Rarefaction analysis for tree species showed that the Standing-dead 208 plot holds one more species than the Old-growth forest plot (Fig. 3). This same analysis showed 209 the Blow-down plot holds very few individuals, but the initial slope and shape of the curve was 210 similar to the other two curves. It indicates that at similar number of individuals, all curves 211 showed a similar number of tree species. Rarefaction analysis of small vascular plants showed 212 the Old-growth forest site had the richest species composition, while decreasing in the Standing 213 dead and Blow-down plots (Fig. 3). The Total-destruction plot showed practically no species. 214

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Plant composition among Old-growth and Standing-dead plots showed some similarities (34%), 221 but less related with Blow-down (20%) and the Total-destruction plot (0%). Standing-dead and 222 Blow-down plots showed the highest similarities in species composition (Table 2). In the 223 disturbed plots, the plant species recorded were a subgroup of the species present in the Old-224 growth forest plot (Nestedness calculator T= 13.85º). The most frequent plant was the fern L. 225 quadripinnata that survived the direct impact of the eruption in small clumps of organic matter 226 protected by trunks or root disc of trees, and were not completely covered by tephra. Bamboo 227 and ferns were sprouting from roots in the remnants of exposed forest soil. 228 229 The forest structure of the Old-growth site showed individuals of all sizes, with several large 232 emergent trees (Fig. 4). Most trees in the Old-growth plot were alive and the snags most 233 probably represent trees that died before the eruption, while in the other plots most trees were 234 killed by the eruption. The Standing-dead plot showed similar dbh distribution than the Old-235 growth plot, with higher frequency of small dbh. The Blow-down plot showed a very small 236 number of standing trees, concentrated in intermediate size class, while the Total-destruction 237 plot practically had no individual standing trees (Fig. 4). 238 239 Figure 4. Diameter at the breast height (dbh) distribution for living and standing dead trees in the study sites influenced by the eruption of the Chaitén volcano.

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The basal area of the Old-growth plot was dominated by L. philippiana, followed by E. 242 cordifolia and C. paniculata (Table 3). In the Standing-dead and Blow-down plots W. 243 trichosperma and N. dombeyi were the dominant components of the basal area, but also with a 244 high number of unidentified trees (Table 3). We were not able to identify the species of the 245 standing dead trees in the Total destruction plot. The volume of fallen trees increased in the 246 Blow-down and in Standing-dead plots, but it was much lower in the Old-growth and the Total-247 destruction plots (Fig. 5). In the Total-destruction plot most logs were heavily buried by a dense 248 tephra layer. 249 250 Table 3. Total basal area in 1000 m 2 of tree species of the study plots in the Chaitén volcano area, classified as alive, resprouting after the eruption, or snag

Seedlings and small plants 254
We found a total of 90 seedlings from 5 trees and shrubs species in the disturbed plots, while 255 we registered 174 seedlings of 12 species in Old-growth plots. The Standing-dead plots had 83 256 seedlings from five species, while the Blow-down plots presented 7 seedlings from two species. 257 We did not find any seedling in the total destruction plot. In the Old-growth plots, seedlings 258 were largely dominated by the shade tolerant A. luma, M. parviflora and M. planipes. In the 259 Standing-dead plots, seedlings were dominated by C. paniculata (48% of the total), but also 260 present the shade-tolerant tree A. luma (24%), the pioneer tree species W. trichosperma and E. 261 coccineum (18% and 8% respectively). In the Blow-down plot, E. coccineum, a pioneer species, 262 and A. lanceolata were the only recorded species. 263 We found different mechanism of forest regeneration in the plots. In the Old-growth plot most 264 regeneration was from seeds, while in the Standing-dead plots most regeneration become from 265 resprout and few tree seedlings from seeds (Table 4). In the Old-growth plots, most of the tree 266 seedlings corresponded to species with fleshy fruits dispersed by birds, while in the Standing-267 dead plots most seeds were wind-dispersed. In the Blow-down plots, all seedlings came from 268 seeds (Table 4), but a half of the seedlings corresponded to species dispersed by birds (e.g. 269 Ribes magellanicum) and the other half were species dispersed by wind (e.g. E. coccineum). 270 Second, in all plots most seedlings were growing on organic substrata (Table 4), such as organic 271 soil on the ground, root discs and logs, while few seedlings were growing directly either on the 272 tephra or on the rocks. In the Blow-down plot we found 7 seedlings of E. coccineum, growing 273 just in remnants of organic litter; in Standing-dead plot 11 seedlings growing directly in the 274 tephra, while the other 72 seedlings were resprouting directly from the standing tree's trunks. 275 In the Old-growth plots we found just one species growing in an exposed rock while the rest of 276 the seedlings were growing in organic substrates ( Other small plant species in the studied plots (Table 1) showed differences along the disturbance 281 gradient (Table 2). For example, the Old-growth plot has species in all categories of plant cover, 282 with few species covering over 75% of the ground, and many species covering between 1-5% 283 (Fig. 6). In contrast, Standing-dead and Blow-down plots were covered by fewer species, and concentrated in the lower ranges of coverage (Fig. 6). In the Total destruction plot we only 285 found a non-vascular plant (Marchantia sp.) with a cover between 5-25 % (Fig. 6) growing on 286 a log exposed to the surface. The dominant understory species in the Old-growth forest were 287 the bamboo Chusquea spp. and the fern L. quadripinnata. These species were also present in 288 the Standing-dead and Blow-down plots, although scattered and with a low percent of coverage 289 in the disturbed plots. 290  Few studies have been conducted short after an eruption in the southern Andes, however 294 successional model predict that disturbed areas in low and mid altitude (<1000 m a.s.l.) forests 295 would be dominated by Nothofagus species due to the shade-intolerance (e.g. Veblen and 296 Ashton, 1978). However, in this study after the first year we did not register Nothofagus species 297 despite were abundant in the surroundings non disturbed areas. It might be due to cyclic seed 298 production with not constant seed rain along the years (Donoso, 1993;Veblen, 1985), and the 299 short-term and limited spatial extension we were able to monitor the vegetation. However, our 300 results pointed that regeneration occurs mainly due to an in situ development of plants 301 associated to biological legacies such as logs, soil clumps and individuals that survive to the 302 eruption. Other areas disturbed by Chaiten volcano presented similar patterns of types and 303 abundance of biological structures (Swanson et al., 2013). 304

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The initial establishment of the tree species, beside of the disturbance gradient, may be 306 influenced by the interaction between life history of the species and the altitudinal gradient. 307 Life history attributes such as seed dispersal, flowering phenology, growth form, resprouting 308 ability or light-temperature stress resistance are a mechanism that influence the recovery 309 response of the post-disturbance vegetation ( Pausas and Lavorel, 2003;Tilman, 1997). In the disturbed plots, the principal 311 mechanism of regeneration was the resprouting capability of some species (Tables 1 and 4). 312 This is a frequent strategy in many plant species in response to disturbances of different types 313 and intensities (Bellingham and Sparrow, 2000;Bond and Midgley, 2001). The seed dispersion 314 syndrome could be also an important mechanism that influence the colonization after 315 disturbance. Chilean forests have one of the highest proportions of plant dispersed by 316 frugivorous birds compared to other temperate rainforests , Willson et al., 317 1996. Birds consume a high proportion of fruits and may disperse the seed to specific places, 318 such as to a perch (Armesto et al., 2001;Lindenmayer and Franklin, 2002). The seed availability 319 might increase in sites where remnant snags, shrubs or perching trees where available (e.g., 320 Bustamante-Sánchez, 2011). Finally, among the wind dispersed seeds, we only found seedlings 321 of E. coccineum in disturbed plots. While this tree is adapted to stressful condition, the seed is 322 able to disperse for long-distances (Donoso, 2006). 323 324 For non-tree species, Old-growth plot had more species than Standing-dead plot. On the 325 contrary, for tree species Standing-dead plot is richest than Old-growth, revealing the effect of 326 the eruption on the small plant species. On the other hand, in Blow-down and in Total-327 destruction plot, i.e. from mid to high disturbance intensity at mid to high altitude, and without 328 or sparse biological legacies, we found low species richness and coverage, and sparse tree 329 regeneration. This mosaic of legacies in the landscape offers opportunities for the 330 recolonization of many different species in the affected area. The different species composition 331 would promote different pathways to early succession with different communities, but also 332 different recovery rates. We propose that in low to mid elevation areas, the vegetation recovery 333 will be faster, dominated by species shade-tolerant to semi-tolerant and dispersed by birds, 334 mostly due to biological legacies. On the other hand, at higher altitude, the vegetation recovery 335 would be slower, recolonization made by shade-intolerant and seed dispersed by winds because 336 the most stressful conditions of temperature at higher altitudes.