Precipitation variability and landslide occurrence in a subtropical mountain ecosystem of NW Argentina

Landslides are common on the steep slopes of the subtropical montane forests in Northwestern (NW) Argentina (Yungas). Instrumental and tree-ring records from this region indicate that rainfall has increased during the second half of the 20th century and there has also been an increase in landslide events. We used dendroecological techniques to date the occurrence of landslides during the past 50 years and examine the relationships with regional precipitation trends. Alnus acuminata H.B.K. is the dominant species in the upper montane forest and colonizes the bare areas exposed by landslides. Landslide dating was based on the identification of suppression-release patterns in ring-width series from trees growing along the landslide scarps, in combination with age determination of trees growing on the landslide failure or depositional surfaces. We cored A. acuminata in three areas that span the latitudinal range of the montane forest in NW Argentina: Los Sosa (27°S), Hualinchay (26°S) and Yala (24°S). The results show that landslide occurrence (and therefore probability) is more frequent during summers with abundant rainfall. As General Circulation Models for subtropical South America predict an increase in summer precipitation during the 21st century, increased precipitation could induce changes in landslide regime that would lead to important environmental changes in these montane ecosystems.     

城市山地保护：美国俄亥俄州辛辛那提市案例研究

记录了辛辛那提市自成立以来的发展历史,包括那些成为城市及其居民的长期财政负担的资源开发型的土地利用模式和不理智的工程实践。强调了过去40年里该市为缓解坡地不稳定性而采取的步骤,并提出了其他一些该市可以采取的关于未来山地开发的更为积极的措施。还记录了山地信托基金会（Hillside Trust）的工作,该基金会是一个成立于1976年的非营利组织,旨在倡导慎重利用和保护辛辛那提地区的山地。     

Catastrophic and background disturbance of tropical ecosystems at the Luquillo Experimental Forest

The forests of the Luquillo Experimental Forest Long-Term Ecological Research site are subject to low-intensity, widespread disturbance that establishes levels of background mortality that contrast with periodic catastrophic mortality resulting from hurricanes and landslides. Although catastrophic mortality is more dramatic, background mortality is still more important in determining population turnover. However, catastrophic mortality may still be an important agent in determining ecosystem structure. Catastrophic disturbances affect forest function in many ways besides mortality, some of which are only apparent in the context of long-term studies. Since most ecosystems are subject to some form of catastrophic disturbance, general principles can be derived from comparative studies of disturbance in different systems.     

Root system architecture of Quercus pubescens trees growing on different sloping conditions

BACKGROUND AND AIMS: Plant roots' growth direction has important implications for plant development and survival; moreover it plays an effective and vital role in stabilizing weathered soil on a steep slope. The aim of this work was to assess the influence of slope on the architecture of woody root systems. METHODS: Five mature, single-stemmed Quercus pubescens trees growing on a steep slope and five on a shallow slope were excavated to a root diameter of 1 cm. A very precise numeric representation of the geometry and topology of structural root architecture was gained using a low-magnetic-field digitizing device (Fastrak, Polhemus). Several characteristics of root architecture were extracted by macros, including root volume, diameter, length, number, spatial position and branching order. KEY RESULTS: The diameter at breast height (dbh) was the best predictor of the root volume but had no correlation with length and number of roots. The slope affected the root volume for each branching order, and the basal cross-sectional area (CSA), number and length of the first-order roots. Number and length of the second- and third-order laterals were closely related in both conditions, although this relationship was closer in the shallow trees, suggesting the influence of a genetic control. Sloping trees showed a clustering tendency of the first- and second-order lateral roots in the up-slope direction, suggesting that the laterals rather than the taproots provide much of the anchorage. In a steep-slope condition, the taproot tapering was positively correlated with the asymmetry magnitude of first-order roots, indicating compensation between taproot and main lateral roots' clustering tendency. CONCLUSIONS: These results suggest that on a slope, on clayey soils, root asymmetry appears to be a consequence of several environmental factors such as inclination, shallow-slides and soil compactness. In addition, this adaptive growth seems to counteract the turning moment induced by the self-loading forces acting in slope conditions, and as a consequence improves the tree stability.     

Leaf Litter Decomposition and Substrate Chemistry of Early Successional Species on Landslides in Puerto Rico1

Decomposition is a critical process for nutrient release and accumulation of soil organic matter in disturbed soils, such as those found on landslides. I conducted a decomposition experiment on five landslides in the Luquillo Mountains of Puerto Rico as part of an investigation of the successional roles of two of the most common plant colonists to landslides, Cecropia schreberiana Miq. (Cecropiaceae) a pioneer tree species, and Cyathea arborea (L.) Sm. (Cyatheaceae) a pioneer tree fern. I compared leaf litter decomposition over one year and the initial and 1‐yr chemistry for both species. Initial litter chemistry differed between the two species, as Cecropia had slightly higher nitrogen (9.2 mg/g) than Cyathea (8.2 mg/g) and higher lignin (28.6%) than Cyathea (26.0%), but water‐soluble carbon and nonpolar extractable carbon (fats and oils, waxes, chlorophylls) were higher in Cyathea than Cecropia. Total carbon, acid‐soluble carbon, total phosphorus, and pH did not differ significantly between leaf litter species. Across all five landslides, Cyathea (k= 0.93 ± 0.06) leaves decomposed significantly faster than Cecropia (k= 0.68 ± 0.06). The differences in these species leaf litter decomposition rates and chemical composition could potentially influence organic matter dynamics and nutrient cycling rates in these early successional systems.     

Landslides and Their Contribution to Land‐cover Change in the Mountains of Mexico and Central America1

Landsliding is a natural process influencing montane ecosystems, particularly in areas with elevated rainfall and seismic activity. Yet, to date, little effort has been made to quantify the contribution of this process to land‐cover change. Focusing on the mountains of Mexico and Central America (M‐CA), we estimated the contribution of landsliding to land‐cover change at two scales. At the scale of M‐CA, we classified the terrain into major landforms and entered in a GIS historical data on earthquake‐ and rainfall‐triggered landslides. At the scale of the Sierra de Las Minas of Guatemala, we investigated Landsat TM data to map rainfall‐triggered landslides. During the past 110 yr, >136,200 ha of land in the mountains of M‐CA have been affected by landslides, which translates into disturbance rates exceeding 0.317 percent/century. In Sierra de Las Minas, rainfall associated with hurricane Mitch affected 1765 ha of forest, or equivalently, landslides triggered by storms of this magnitude transformed between 0.196 (return time of 500 yr) and 1.290 (return time of 75 yr) percent of forest/century. Although landsliding results in smaller rates of land‐cover change than deforestation, we hypothesize that it has a stronger impact on ecosystems, both in qualitative and quantitative terms, given its influence on vegetation and soil. Moreover, interactions between landsliding and deforestation may be altering the expression of this complex process such that the few protected areas in the mountains of M‐CA may represent the only possibility for the conservation of this process.     

Bird Perches Increase Forest Seeds on Puerto Rican Landslides

Abstract Landslides result in the loss of vertical vegetative structure, soil nutrients, and the soil seed bank. These losses impede timely recovery of tropical forest communities. In this study we added bird perches to six Puerto Rican landslides with three types of surfaces (bare, climbing fern, grass) in an effort to facilitate inputs of forest seeds through bird dispersal and to accelerate plant succession. Numbers of bird‐dispersed forest seeds were significantly higher in plots beneath introduced perches than in control plots. Perches did not increase forest seedling densities compared with control plots. Seven different species of birds were observed on introduced perches. Because 99% of the seed inputs to controls and perch plots in the six landslides were wind‐dispersed seeds (mostly graminoids), perches can improve landslide restoration if woody plants establish and shade out the dominant graminoid and climbing fern ground cover. Although increasing seed inputs from forest species is a critical step in accelerating revegetation of landslides, we suggest that supplemental restoration techniques be applied in addition to bird perches to promote forest recovery.     

Disturbance regimes,gap‐demanding trees and seed mass related to tree height in warm temperate rain forests worldwide

For tropical lowland rain forests, Denslow (1987) hypothesized that in areas with large‐scale disturbances tree species with a high demand for light make up a larger proportion of the flora; results of tests have been inconsistent. There has been no test for warm temperate rain forests (WTRFs), but they offer a promising testing ground because they differ widely in the extent of disturbance. WTRF is dominated by microphylls sensu Raunkiaer and has a simpler structure and range of physiognomy than tropical or subtropical rain forests. It occurs in six parts of the world: eastern Asia, New Zealand, Chile, South Africa, SE Australia and the Azores. On the Azores it has been mostly destroyed, so we studied instead the subtropical montane rain forest (STMRF) on the Canary Islands which also represents a relict of the kind of WTRF that once stretched across southern Eurasia. We sought to find whether in these six regions the proportion of tree species needing canopy gaps for establishment reflects the frequency and/or extent of canopy disturbance by wind, landslide, volcanic eruptions (lava flow and ash fall), flood or fire. We used standard floras and ecological accounts to draw up lists of core tree species commonly reaching 5 m height. We excluded species which are very rare, very localized in distribution, or confined to special habitats, e.g. coastal forests or rocky sites. We used published accounts and our own experience to classify species into three groups: (1) needing canopy gaps for establishment; (2) needing either light shade throughout or a canopy gap relatively soon (a few months or years) after establishment; and (3) variously more shade‐tolerant. Group 1 species were divided according the kind of canopy opening needed: tree‐fall gap, landslide, lava flow, flood or fire. Only some of the significant differences in proportion of Group 1 species were consistent with differences in the extent of disturbance; even in some of those cases other factors seem likely to have had a major determining influence during evolution. We also sought to determine whether the species that are at least ‘short‐term persistent’ in the soil seed bank (lasting 2–4 years) are all species needing canopy gaps for establishment. The answer was negative; large numbers of seeds of some shade‐tolerants accumulate in the soil, and these species are able to benefit from soil disturbance in deep shade. We found a significant and strong positive relationship in Japan between mean seed mass and mature tree height, a weak positive relationship in New Zealand and no relationship in any of the other four regions. When comparing the seed mass values of Group 1 and Group 3 species we obtained different answers depending on whether or not we confined ourselves to taxonomically controlled contrasts. In only two of the four regions with an appreciable number of species in Group 1 is the mean seed mass of such species significantly lower than that of Group 3 species when taxonomic relatedness is ignored.