Natural Regeneration After Volcanic Eruptions: Resilience of the Non-legume Nitrogen-Fixing Tree Parasponia rigida

Abstract

Volcanic eruptions disturb vegetation at a time it is needed for preventing mudflows. A resilient indigenous non-legume nitrogen-fixing tree that is adapted to the ash and spreads rapidly protects areas downstream in a volcanic landscape in Indonesia. Within the volcanic ring of fire both the long-term benefits (including densely populated, fertile agricultural soils) and short-term ecological disturbance of volcanic ash deposition are clear. Mount Kelud in East Java has erupted on a 15–37-years cycle for the past centuries, most recently in 2014, causing damage to settlements, agricultural land, agroforestry, and watershed protection forests, as the ash deposits caused tree mortality, restricted infiltration, and led to ash flows. Rapid “restoration” or recovery of tree-based vegetation with planted Legume trees (such as Calliandra spp.) has been attempted but is not very effective. However, the non-legume nitrogen-fixing Parasponia rigida, symbiotic with rhizobium bacteria, contrasted to its non-symbiotic sibling (Trema orientalis) has been studied in laboratory conditions, but not in its native environment. We mapped and sampled P. rigida in various locations (upper, middle, and lower elevation positions in ridge-slope-valley toposequences) on the Kelud complex starting 1 year after the latest eruption, estimated biomass development, and quantified P. rigida root nodules in relation to N availability in the ash/soil mixtures in these locations. P. rigida was found as a pioneer tree at elevations between 600 and 1,700 m a.s.l. (above sea level) along ridges, in slope, and valley positions. At lower elevations T. orientalis dominated. Within 3 years of the eruption, stem diameters were 3–10 cm. Up to 93% of P. rigida root nodules were found to be effective, based on the hemoglobin color on cross-sections. Rhizobium bacteria were found in root nodule tissue at densities of two to a hundred times higher than in rhizosphere soil. Between a total soil N content from 0.01 to 0.04% the density of effective nodules decreased from 1,200 to 200 m−2. P. rigida stands in the area, especially at ridges close to the crater deserve to be managed proactively as future seed sources, given the high frequency of eruption episodes, while recovery after eruptions on similar volcanoes can likely be facilitated by tactical assisted seed dispersal if effective seed collection and storage methods can be established.

Decomposition of Fine Roots and Aboveground Agroforestry Litter as Plant-soil Feedback After Volcanic Ash Deposition

“…The relationships were accepted at P<5%. To assess the effect of additional volcanic ash deposition on the soil on decomposition rate of above and below the soil surface, we compared our data with above-ground litter study conducted by Ishaq et al (2020a) which was conducted in the same research location. We performed analysis of variance to con rm the statistically signi cant between the factors (season, location, and root tree).…”

“…For several aboveground litter sources (only partially overlapping with our root study and including a number of species litter mixtures), decomposition rates were measured in the same study sites by Ishaq et al (2020a). When compared against the (L+P):N ratio, the mean residence time for aboveground litter was at least three times longer than that for ne roots (Figure 7).…”

“…The total tree count in the +Ash site was higher, with a lower tree basal area than the -Ash site. On average, 56% of trees in the +Ash site had a stem diameter of 5-10 cm, 34% of trees one of 10-30 cm and 10% a diameter of >30 cm (Ishaq et al 2020a), while in the -Ash site was 25%, 75% and 0, respectively. Beyond coffee, the tree species at the + Ash site consisted of 77% fruit trees, 2% timber trees, and 21% non-timber shade trees; while the types of trees in the -Ash location were 52% fruit trees, 45% timber trees, and 3% non-timber shade trees.…”

“…Volcanic ash from the eruption of Mount Kelud changes the physical, chemical, and biological properties of the soil (Ishaq et al 2020a). The C org content in the Kelud volcanic ash was very low, around 0.35%, with an acidic pH of around 4.33 and a low cation exchange capacity (Utami et al 2017).…”

“…<p>Mean residence time of tree leaves (Ishaq et al 2020a) and roots (this study) in the same study area</p><p> <br></p> Supplementary Files This is a list of supplementary les associated with this preprint. Click to download.…”