Recovering plant biodiversity

Abstract

Studying recovering plant biodiversity on Mount Pinatubo may provide valuable insights that improve our understanding of recovery of other ecosystems following disturbances of all types. Ongoing sheet and rill erosion coupled with mass waste events in the unstable pyroclastic flow deposits persist, effectively re-setting primary succession at micro-landscape scale without affecting habitat level diversity. Spatial factors and micro-habitat diversity may exert more control over continued succession as the riparian systems become more deeply dissected and complex. The number of taxa within functional groups and conservation concerns are botanical issues that deserve further research.

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Introduction

The study of vegetation recovery on new substrates such as those present following volcanic eruptions is important to understand recovery of damaged managed or natural ecosystems. Therefore, we recently published an exhaustive list of vascular plants that have assembled on two disturbed riparian drainages on the eastern flanks of Mount Pinatubo.1 As the first paper of its kind on Mount Pinabuto, the results may aid in understanding tropical vegetation recovery following destruction or damage by other disturbances such as agriculture, landslides and forestry.2 Here I explore issues and observations that resulted from the field work and data analysis, focusing on potential drivers of floristic diversity and distribution.

Re-Setting of Primary Succession

One of the most striking characteristics of the mid-elevations of Mount Pinatubo is the continuing impact of sheet and rill erosion and mass wasting events. The volume of soil moved each year is surprising considering 20 years have elapsed since the eruption. New surfaces formed from these processes are rapidly vegetated because of the adjacent established vegetation that is in varied stages of succession. This process alters α-diversity without affecting β-diversity, therefore Mount Pinatubo is an ideal system for studying the re-setting of primary succession at various spatial scales.

Microhabitat Development

The homogeneous habitat characteristics of Mount Pinatubo during the initial years of primary succession have been replaced by deeply dissected gullies and canyons descending in all directions from the caldera. These descending river systems are characterized by canyon widths that decrease on average with elevation and canyon wall heights which are greatest in midelevations. The influence of canyon width and height on microclimatic variables will increase as these two east-west canyons continue to develop secondary canyons. Therefore, heterogeneity of microclimate factors such as timing and quantity of solar exposure, surface aspect, wetness and temperature will increase. Many of these factors are amenable to being quantified in attempts to disentangle the explanatory factors that drive continued succession of these post-eruptive surfaces.

Functional Redundancy

Research priorities may be informed by our results on functional group cover.1 For example, species that belong to functional groups with redundancies may provide ecosystem services that are duplicated by the other taxa in the functional group. The four large grasses in Mount Pinatubo provide a pertinent example. Saccharum spontaneum (local name, talajib) is the dominant species and accounts for most of the large grass cover on the mountain. But Miscanthus floridulus (local name, uyong) dominates on steep slopes, whereas Phragmites karka (local name, timbu) and Thysanolaena maxima (local name, bagoyboy) increase in abundance near wet habitats. These niche priorities among the four large grasses allow this functional group to maintain integrity of ecosystem services in diverse habitats. Conversely, ecosystem services provided by a species that lacks redundancies may be at greater risk during continued disturbances. The native banana Musa errans (local name, amukaw) is one example, as this herbaceous perennial possesses many distinctive characteristics.

Spatial Factors

Many of the vegetated habitats are along the edges of the canyons where the assemblage of taxa is distinct from that of the remaining habitats. Vegetation cover was correlated with distance from the edge of each canyon.1 The underlying causes of these patterns deserve further study for individual species. Two causal factors likely interplay, depending on the species of interest. Firstly, the grasses Miscanthus floridulus and Pogonatherum crinitum (local name, kalaput) occur in greater abundance closer to canyon edges. Proximity to propagule source is exclusively or partly causal, as these two grasses dominate the surfaces of the canyon walls that are too steep to support other species. As ubiquitous colonizers of the canyon walls, they provide direct deposits of seeds on the surfaces closest to the canyon edge. Secondly, the ferns Pityrogramma calomelanos (local name, pako-sapa) and Nephrolepis hirsutula (local name, pakodamulid) and some of the forb species thrive at the base of canyon walls, yet they are rarely seen growing on the vertical surfaces of the steep canyon walls or in the exposed surfaces in the middle of the canyons. The sheltered microclimate at the base of the canyon walls may provide a habitat that is most conducive to robust growth and this leads to the spatial segregation of these taxa. Reciprocal transplant studies or resource manipulation studies could disentangle the interplay between proximity to propagule source versus safe sites with beneficial microclimates that enable plant establishment and early vigor.

Zoochory

Much of the Mount Pinatubo surfaces remain barren to date, primarily due to continuing erosion and sediment deposition.3 The vegetated habitats may serve as preferential corridors for wildlife as they avoid the barren areas while accessing the channel for water. Thus, these vegetated habitats may facilitate recruitment of some zoochorous plant species as succession continues. For example, Martini and dos Santos4 reported higher numbers of animal-dispersed seeds in surfaces under intact vegetation than in surfaces in treefall gaps in Brazil.

The species Musa errans occurred in Pasig-Potrero River plots but was absent from Sacobia River plots.1 This pattern was likely a result of greater population density of native animal dispersers that vector the Musa seeds into the canyons from the surrounding forests, as we encountered wildlife and/or seed-laden scat throughout all elevations of the Pasig-Potrero River basin. In contrast, the animal dispersers we observed in the heavily disturbed Sacobia River habitats were restricted to elevations near the alluvial fan and were comprised of domesticated animals that tend to disperse weedy exotic species.

The study of zoochory in Mount Pinatubo may uncover the influences of animal dispersers on continuing succession. For example, the scat from civets (Paradoxurus philippensis) we found in the Pasig-Potrero habitats was laden with Musa errans and Ficus spp. seeds in every case. Direct encounters with red jungle-fowl (Gallus gallus) were also frequent during Pasig-Potrero field work, and these birds were consistently observed feeding on Lantana camara fruits. This native bird may therefore facilitate dispersal of this invasive plant species. Cattle (Bos primigenius) and water buffalo (Bubalus bubalis) are common free-range ungulates in the lower elevations of Mount Pinatubo, and they may be exerting profound influences on some plant species populations.

Conservation Issues

Conservation priorities may also inform ongoing research needs. The Philippines is part of the Malesian floristic region.5 Myers et al.6 list the country as one of three biodiversity hotspots worldwide deserving of the greatest conservation priorities and funding. This designation was rooted in the great number of endemic species. Madulid7 reported numerous endemic taxa that may have been imperiled by the eruption of Mount Pinatubo. This lengthy list is not surprising in light of the high endemism within the country. Continued delays in conducting surveys throughout all of the recovering habitats may waste crucial time for conservation efforts, as the cost of bringing severely diminished populations back to the levels at which they are not endangered may be severely high.8

Figures and Tables

Figure 1 Undercutting of the canyon walls by running water is one of the causes of ongoing mass waste events in the riparian systems of Mount Pinatubo (top left). The mass waste events that continue to occur two decades after the eruption present barren surfaces as seen in lower center (top right). These can become covered by plants rapidly due to the surrounding established vegetation. Saccharum spontaneum is the dominant large grass in the caldera as seen here (bottom) and throughout all elevations of the recovering ecosystem.

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