Base saturation, nutrient cation, and organic matter increases during early pedogenesis on landslide scars in the Luquillo Experimental Forest, Puerto Rico

ZARIN, D. J. 1993. Nutrient accumulation during succession in subtropical lower montane wet forests, Puerto Rico.
Ph.D. dissertation. University of Pennsylvania, Philadelphia, Pennsylvania.
———, AND A. H. JOHNSON. 1995a. Nutrient accumulation during primary succession in a montane tropical forest,
Puerto Rico. Soil Sci. Soc. Am. J. 59: 1444–1452.
———, AND ———. 1995b. Base saturation, nutrient cation, and organic matter increases during early pedogenesis
on landslide scars in the Luquillo Experimental Forest, Puerto Rico. Geoderma 65: 317–330.

We evaluate data from a chronosequence study of landslide scars, ranging in age from 1-55 + yr, in the Luquillo Experimental Forest (LEF) a subtropical lower montane wet forest (sensu Holdridge) in eastern Puerto Rico. Surface mineral soil (0-10 cm) base saturation index (BSI) values increase during primary succession in the LEF (R=0.85, P<0.001 ). Both BSI values and major nutrient cation concentrations are extremely low on new landslide scars. During the course of the 55 + yr chronosequence, major nutrient cation concentrations are positively correlated with soil organic matter (SOM) content (P = 0.079) and not with clay content (P = 0.794). When data collected from plots representing the two dominant late-succession vegetation associations are added to the analysis, nutrient cation concentrations correlate significantly with both SOM (P= 0.001) and clay content (P=0.033). Our data show that when initial conditions are oligotrophic, both nutrient cation pools and BSI values can increase in the surface mineral soil during early pedogenesis. We discuss exogenic litter input, substrate weathering, and precipitation as potential sources for nutrient cations in these soils. We further suggest that production and decomposition of SOM is the dominant process controlling capture, retention and intra-ecosystem cycling of nutrient cations in these forests.

Ecosystem Development and Plant Succession on Landslides in the Caribbean

Ecosystem Development and Plant Succession on Landslides in the Caribbean
Lawrence R. Walker, Daniel J. Zarin, Ned Fetcher, Randall W. Myster and Arthur H. Johnson
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 566-576

Landslides are common in mountainous regions of the Caribbean and are triggered by heavy rains and earthquakes, and often occur in association with human disturbances (e.g., roads). Spatially heterogeneous removal of both substrate and vegetation is responsible for a variety of patterns of ecosystem development and plant successional trajectories within Caribbean landslides. Soil nutrient pools in exposed mineral soils reach levels comparable to mature forest soils within 55 yr but soil organic matter recovers more slowly. Plant colonization of landslides depends on the availability of propagules and suitable sites for germination, soil stability, and the presence of residual or newly deposited soil organic matter and associated nutrients. Once initial colonization occurs, the rate and trajectory of plant succession on landslides is strongly affected by plant/plant interactions. We present two conceptual models of landslide succession that summarize the major processes and pathways of ecosystem development and plant succession on landslides. Additional work is needed to characterize interactions between spatially heterogeneous zones, controls over soil development, impacts of key plant species, and the role of animals on Caribbean landslides.

Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008

Ren, D., et al., 2009. Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008. Geophysical Research Letters 36, L05401.

We have applied a scalable and extensible geo-fluid model that considers soil mechanics, vegetation transpiration and root mechanical reinforcement, and hydrological processes to simulate two dimensional maps of the landslides occurrence following the 2008 Wenchuan earthquake. Modeled locations and areas generally agree with observations. The model suggests that the potential energy of earth was lowered by 1.52×1015 J by these landslides. With this, the vegetation destroyed transfer ~235 Tg C to the dead respiring pool and transforms 5.54×10-2 Tg N into unavailable sediments pools and the atmosphere. The cumulative CO2 release to the atmosphere over the coming decades is comparable to that caused by hurricane Katrina 2005 (~105 Tg) and equivalent to ~2% of current annual carbon emissions from global fossil fuel combustion. The nitrogen loss is twice as much as that released by the 2007 California Fire (~2.5×10-2 Tg). A significant proportion of the nitrogen loss (14%) is in the form of nitrous oxide, which can affect the atmospheric ozone layer.

Map showing susceptibility to earthquake-induced landsliding, San Juan Metropolitan Area, Puerto Rico

Santiago Marilyn, Map showing susceptibility to earthquake-induced landsliding, San Juan Metropolitan Area, Puerto Rico, 2 plate.em>

Analysis of slope angle and rock type using a geographic information system indicates that about 66 percent of the San Juan metropolitan area (SJMA) has low to no susceptibility to earthquake-induced landslides. This is at least partly due to the fact that 45 percent of the SJMA is constructed on slopes of 3 degrees or less, which are too gentle for landslides to occur. The areas with the highest susceptibility to earthquake-induced landslides account for 6 percent of the surface area. Almost one-quarter (23 percent) of the SJMA is moderately susceptible to earthquake-induced landslides. These areas are mainly in the southern portions of the SJMA where housing development pressures are currently high because of land availability and the esthetics of greenery and hillside views. The combination of new development and moderate earthquakeinduced landslide susceptibility may indicate that the southern portions of the SJMA may be at greatest risk.

Luquillo Mountains Puerto Rico A water energy and biogeochemical budgets program site

Larsen MC, Stallard RF. Luquillo Mountains, Puerto Rico–a water,
energy, and biogeochemical budgets program site. US geological
survey fact sheet. Washington, D.C.7 U.S. Geological
Survey; 2000. p. 163– 99.

The Puerto Rico research site consists of the 113 square-kilometer Luquillo Experimental Forest (LEF), administered by the U.S. Forest Service, and the nearby Río Grande de Loíza drainage basin, an urbanized and agriculturally- developed watershed. This combined region serves as a terrestrial laboratory for the study of issues related to the global loss of tropical forest, and the associated changes in land-use practices. Findings from the WEBB research help scientists understand how vegetation, landscape, and people interact to affect the quantity and quality of water and the erosion of the landscape. The results of this work can be applied not only to Puerto Rico, but also to many other regions, where deforestation and rapid land-use change are issues.

The Frequency and Distribution of Recent Landslides in three MontaneTropical Regions of Puerto Rico

Larsen, Matthew C.; Torres-Sanchez, Angel J. 1998. The frequency and distribution of recent landslides in three montane tropical regions of Puerto Rico. Geomorphology 24 :309-331.

Landslides are common in steep mountainous areas of Puerto Rico where mean annual rainfall and the frequency of intense storms are high. Each year, landslides cause extensive damage to property and occasionally result in loss of life. Average population density is high, 422 peoplerkm2, and is increasing. This increase in population density is accompanied by growing stress on the natural environment and physical infrastructure. As a result, human populations are more vulnerable to landslide hazards. The Blanco, Cibuco, and Coamo study areas range in surface area from 276 to 350 km2 and represent the climatologic, geographic, and geologic conditions that typify Puerto Rico. Maps of recent landslides developed from 1:20,000-scale aerial photographs, in combination with a computerized geographic information system, were used to evaluate the frequency and distribution of shallow landslides in these areas. Several types of landslides were documented— rainfall-triggered debris flows, shallow soil slips, and slumps were most abundant. Hillslopes in the study area that have been anthropogenically modified, exceed 128 in gradient, are greater than 300 m in elevation, and face the east-northeast, are most prone to landsliding. A set of simplified matrices representing geographic conditions in the three study areas was developed and provides a basis for the estimation of the spatial controls on the frequency of landslides in Puerto Rico. This approach is an example of an analysis of the frequency of landslides that is computationally simple, and therefore, may be easily transferable to other settings.

Landslides Triggered by Hurricane Hugo in Eastern Puerto Rico, September 1989

Larsen, M. C., and Torres-Sánchez, A. J., 1992, Landslides triggered by Hurricane Hugo in eastern Puerto Rico, September 1989: Caribbean Journal of Science, vol. 28, no. 3-4, p. 113-125.

On the morning of September 18, 1989, a category-four hurricane struck eastern Puerto Rico with a sustained wind speed in excess of 46 m/s. The 24-h rainfall accumulation from the hurricane ranged from 100 to 339 mm. Average rainfall intensities ranging from 34 to 39 mm/h were calculated for 4 and 6 h periods, respectively, at a rain gage equipped with satellite telemetry, and at an observer station. The hurricane rainfall triggered more than 400 landslides in the steeply sloping, highly dissected mountains of eastern Puerto Rico. Of these landslides, 285 were mapped from aerial photography which covered 6474 ha. Many of the mapped landslides were on northeast- and northwest-facing slopes at the eastern terminus of the mountains, nearest the hurricane path. The surface area of individual landslides ranged from 18 m2 to 4500 m2, with a median size of 148 m2. The 285 landslides disturbed 0.11% of the land surface in the area covered by aerial photographs. An approximate denudation rate of 164 mm/1000 y was calculated from the volume of material eroded by landsliding and the 10-y rainfall recurrence interval.

A rainfall intensity duration threshold for landlisdes in a humid-tropical environment, puerto rico

A Rainfall Intensity-Duration Threshold for Landslides in a Humid-Tropical Environment, Puerto Rico
Matthew C. Larsen and Andrew Simon
Geografiska Annaler. Series A, Physical Geography
Vol. 75, No. 1/2 (1993), pp. 13-23

Landslides are triggered by factors such as heavy rainfall, seismic activity, and construction on hill-slopes. The leading cause of landslides in Puerto Rico is intense and/or prolonged rainfall. A rainfall threshold for rainfall-triggered landsliding is delimited by 256 storms that occurred between 1959 and 1991 in the central mountains of Puerto Rico, where mean annual rainfall is close to or in excess of 2,000 mm. Forty one of the 256 storms produced intense and/or prolonged rainfall that resulted in tens to hundreds of landslides. A threshold fitted to the lower boundary of the field defined by landslide-triggering storms is expressed as I = 91.46 D-0.82 where I is rainfall intensity in millimeters per hour, and D is duration in hours. Landslide-producing storms occurred at an average rate of 1.2 per year. In general the landslides triggered by short-duration, high-intensity rainfall events were mainly shallow soil slips and debris flows, while the long-duration, low-intensity rainfall produced larger, deeper debris avalanches and slumps. For storms that had durations of up to 10 h, landsliding did not occur until rainfall intensity was as much as three times as high as the rainfall intensity reported as sufficient to trigger landsliding in temperate regions. As storm durations approach 100 h, the rainfall conditions necessary to initiate landsliding in Puerto Rico converge with those defined for temperate regions. A comparison of the Puerto Rico threshold with rainfall data from other humid-tropical regions suggests that the threshold developed for Puerto Rico may be applicable to other similar environments throughout the world.

Mass Wasting and Sediment Storage in a Small Montane Watershed: an Extreme Case of Anthropogenic Disturbance in the Humid Tropics

LARSEN, M.C. and SANTIAGO-ROMA´ N, A., 2001. Mass wasting and
sediment storage in a small montane watershed: an extreme case
of anthropogenic disturbance in the humid tropics. In: DORAVA,
(eds.), Geomorphic Processes and Riverine Habitat. American Geophysical
Union Monograph, pp. 142–170.

By the peak of land-use conversion for subsistence cropping and plantation agriculture in Puerto Rico in the 1940's, 94 percent of the original forest cover had been eliminated. In a small (26.4 km2) upland watershed that typifies this land-use history, field surveys and examination of aerial photographs indicate that more than 2,000 landslides have occurred since about 1820 when forest clearing began. The landslides are attributable to a combination of three factors: a highly weathered bedrock (Cretaceous granodiorite), episodic heavy rainfall, and almost two centuries of intense land-use practices. On average, landslide scars number 140/km2 in the Cayaguás watershed, equal to 80 landslide scars/km2/100 y. The volume of hillslope material eroded by landsliding is estimated at 660,000 m3/km2 (870,000 Mg/km2). If all colluvium was transported from the catchment, then the volume is equivalent to a mean surface lowering of the entire watershed by 660 mm, or 3.8 mm/y. Soil augering, field observations at construction sites, road cuts and stream banks, mapping from aerial photographs, and GIS-based estimates of the surface area of footslopes, indicate that colluvium may total 149,000 Mg/km2. If mobilized, this would be sufficient stored material to supply the annual average fluvial sediment yield for as long as 129 years. The great availability of colluvial and alluvial sediment on footslopes, floodplains, and in channels will maintain high sediment yield well into the 21st century in spite of government efforts to reforest hillslopes and institute other hillslope soil conservation measures.

Evaluation of temporal and spatial factors that control the susceptibility to rainfall-triggered landslides

Larsen, M.C., 2001, Evaluation of temporal and spatial factors that control the susceptibility to rainfall-triggered landslides, in Gruntfest, E., and Handmer, J., eds., Coping with Flash floods: Kluwer Academic Publishers, p. 277-288.

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