Weathering and Soils

Estimating soil turnover rate from tree uprooting during hurricanes in Puerto Rico

Lenart, Melanie T.; Falk, D.A.; Scatena, F.N.; Osterkamp, W.R. 2010. Estimating soil turnover rate from tree uprooting during hurricanes in Puerto Rico. Forest Ecology and Management. 259:1076-1084.

Soil turnover by tree uprooting in primary and secondary forests on the island of Puerto Rico was measured in 42 study plots in the months immediately after the passage of a Category 3 hurricane. Trunk basal area explained 61% of the variability of mound volume and 53% of the variability of mound area. The proportion of uprooted trees, the number of uprooted trees, or the proportion of uprooted basal area explained 84–85% of the variation in hurricane-created mound area. These same variables explain 79–85% of the variation in mound volume. The study indicates that the soil turnover period from tree uprooting by Puerto Rican hurricanes is between 1600 and 4800 years. These rates are faster than soil turnover by landslides and background treefall in the same area and provide a useful age constraint on soil profile development and soil carbon sequestration in these dynamic landscapes.

Controls on soil organic matter content within a northern hardwood forest

Johnson, K.D.; Scatena, F.N.; Johnson, A.H.; Pan, Y. 2009. Controls on soil organic matter content within a northern hardwood forest. Geoderma. 148(3-4): 346-356.

Forest soils can act as both sinks and sources for atmospheric CO2 and therefore have an important role in the global carbon cycle. Yet the controls on forest soil organic matter content (SOM) distribution at the scale of operational land management scales within forest types are rarely quantified in detail. To identify factors that influence the spatial distribution and accumulation of SOM in forests, soils and stand composition data from 42 even-aged northern hardwood forest plots were analyzed using multiple linear regression and non-parametric statistical approaches. The analysis included three layers of SOM pools (forest floor, 0–20 cm mineral soil, and 20+ cm mineral soil) over three spatial scales (point, plot and regional). The largest amounts of total SOM (mean = 289, std dev = 70 Mg ha− 1) occurred in deep and well drained soils located on gently grading slopes. When soil layers were analyzed separately, the following relationships were observed: 1) highest forest floor SOM occurred under mixed species composition as opposed to stands dominated by sugar maple, 2) highest 0–20 cm mineral SOM occurred at high elevations (greater than 450 m) in moderately well drained soils, and 3) highest 20+ cm mineral SOM also occurred at high elevations and when soils were deeper. Further analysis of 0–20 cm mineral layer revealed that lower rock volume and finer soil texture resulted in higher SOM at a single point. When SOM that was predicted from models based on plot-specific attributes (soils series, slope and aspect) were compared to soil survey SOM estimates, the mean SOM values for both approaches were similar (253 and 269 Mg ha− 1 respectively). Easily identifiable characteristics such as mixed stand composition, the presence of forest floor and E horizon thickness may be used as field indicators of SOM storage. The variety of controls identified in this study should be considered when assessing soil carbon response to management options and future changes in climate.

Asynchronous fluctuation of soil microbial biomass and plant litterfall

Ruan, H.H., Zou, X.M., Scatena, F.N., Zimmerman, J.K.,
2004. Asynchronous fluctuations of soil microbial
biomass and plant litterfall in a tropical wet forest.
Plant Soil 260, 147–154.

Carbon availability often controls soil microbial growth and there is evidence that at regional scales soil microbial biomass is positively correlated with aboveground forest litter input. We examined the influence of plant litterfall on annual variation of soil microbial biomass in control and litter-excluded plots in a tropical wet forest of Puerto Rico. We also measured soil moisture, soil temperature, and plant litterfall in these treatment plots. Aboveground plant litter input had no effect on soil microbial biomass or on its pattern of fluctuation. Monthly changes in soil microbial biomass were not synchronized with aboveground litter inputs, but rather preceeded litterfall by one month. Soil microbial biomass did not correlate with soil temperature, moisture, or rainfall. Our results suggest that changes in soil microbial biomass are not directly regulated by soil temperature, moisture, or aboveground litter input at local scales within a tropical wet forest, and there were asynchronous fluctuation between soil microbial biomass and plant litterfall. Potential mechanisms for this asynchronous fluctuation include soil microbial biomass regulation by competition for soil nutrients between microorganisms and plants, and regulation by below-ground carbon inputs associated with the annual solar and drying-rewetting cycles in tropical wet forests.

Litter Dynamics Along Stream, Riparian and Upslope Areas Following Hurricane Hugo, Luquillo Experimental Forest, Puerto Rico

Litter Dynamics Along Stream, Riparian and Upslope Areas Following Hurricane Hugo, Luquillo Experimental Forest, Puerto Rico
K. A. Vogt, D. J. Vogt, P. Boon, A. Covich, F. N. Scatena, H. Asbjornsen, J. L. O'Harra, J. Perez, T. g. Siccama, J. Bloomfield and J. F. Ranciato
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 458-470

Litterfall (fine and coarse) due to Hurricane Hugo and subsequent fine annual litterfall inputs (1, 2 and 5 yr after Hugo) were determined for two sites (El Verde and Bisley) in the Luquillo Experimental Forest in Puerto Rico. Litter transfers into streams, riparian and upslope areas were determined within each catchment. The recovery rate of aboveground fine litterfall (leaf, fine wood <1 cm diameter, and other miscellaneous inputs) to predisturbance levels were determined 1, 2, and 5 yr after Hurricane Hugo. The amount of total litter transfers and their individual components into the riparian and upslope areas due to Hurricane Hugo varied significantly by catchments within the Luquillo Experimental Forest. At El Verde, 26-39 percent, 31-35 percent, 14-35 percent and 7-12 percent of the total litter transfers were contributed by leaf litter, fine wood, coarse wood and fine roots, respectively. At Bisley, 28-31 percent, 26-29 percent, 33-35 percent and 8-10 percent of the litter transfers were contributed by the same categories. Differential decay rates contributed to the relative importance of fine and coarse litter inputs. The recovery of fine aboveground litterfall to pre-hurricane levels after 5 yr varied by topographic location (streams had the slowest recovery, upslope areas the highest) and catchment (El Verde: 55-77%; Bisley: 39-82% of pre-hurricane values).

Suppression of methanogenesis by dissimilatory Fe(III)- reducing bacteria in tropical rain forest soils: implications for ecosystem methane flux

Teh, Y.A., Dubinsky, E.A., Silver, W.L., and Carlson, C.M.
(2008) Suppression of methanogenesis by dissimilatory Fe
(III)-reducing bacteria in tropical rain forest soils: implications
for ecosystem methane flux. Glob Change Biol 14:

Tropical forests are an important source of atmospheric methane (CH4), and recent work suggests that CH4 fluxes from humid tropical environments are driven by variations in CH4 production, rather than by bacterial CH4 oxidation. Competition for acetate between methanogenic archaea and Fe(III)-reducing bacteria is one of the principal controls on CH4 flux in many Fe-rich anoxic environments. Upland humid tropical forests are also abundant in Fe and are characterized by high organic matter inputs, steep soil oxygen (02) gradients, and fluctuating redox conditions, yielding concomitant methanogenesis and bacterial Fe(III) reduction. However, whether Fe(III)-reducing bacteria coexist with methanogens or competitively suppress methanogenic acetate use in wet tropical soils is uncertain. To address this question, we conducted a process-based laboratory experiment to determine if competition for acetate between methanogens and Fe(III)-reducing bacteria influenced CH4 production and C isotope composition in humid tropical forest soils. We collected soils from a poor to moderately drained upland rain forest and incubated them with combinations of C-13-bicarbonate, C-13-methyl labeled acetate ((CH3COO-)-C-13), poorly crystalline Fe(III), or fluoroacetate. CH4 production showed a greater proportional increase than Fe2+ production after competition for acetate was alleviated, suggesting that Fe(III)-reducing bacteria were suppressing methanogenesis. Methanogenesis increased by approximately 67 times while Fe2+ production only doubled after the addition of (CH3COO-)-C-13. Large increases in both CH4 and Fe2+ production also indicate that the two process were acetate limited, suggesting that acetate may be a key substrate for anoxic carbon (C) metabolism in humid tropical forest soils. C isotope analysis suggests that competition for acetate was not the only factor driving CH4 production, as C-13 partitioning did not vary significantly between (CH3COO-)-C-13 and (CH3COO-)-C-13 + Fe(III) treatments. This suggests that dissimilatory Fe(III)-reduction suppressed both hydrogenotrophic and aceticlastic methanogenesis. These findings have implications for understanding the CH4 biogeochemistry of highly weathered wet tropical soils, where CH4 efflux is driven largely by CH4 production.

Soil Oxygen Availability and Biogeochemistry along Rainfall and Topographic Gradients in Upland Wet Tropical Forest Soils

Silver W, Lugo AE, Keller M (1999) Soil oxygen availability and
biogeochemistry along rainfall and topographical gradients in
upland wet tropical forest soils. Biogeochemistry 44:301–

We measured soil oxygen concentrations at 10 and 35 cm depths and indices of biogeochemical cycling in upland forest soils along a rainfall and elevation gradient (3500– 5000 mm y−1; 350–1050 masl) and along topographic gradients (ridge to valley, 150 m) in the Luquillo Experimental Forest, Puerto Rico. Along the rainfall gradient, soil O2 availability decreased significantly with increasing annual rainfall, and reached very low levels (<3%) in individual chambers for up to 25 consecutive weeks over 82 weeks of study. Along localized topographic gradients, soil O2 concentrations were variable and decreased significantly from ridges to valleys. In the valleys, up to 35% of the observations at 10–35 cm depth were <3% soil O2. Cross correlation analyses showed that soil O2 concentrations were significantly positively correlated along the topographic gradient, and were sensitive to rainfall and hydrologic output. Soil O2 concentrations in valley soils were correlated with rainfall from the previous day, while ridge sites were correlated with cumulative rainfall inputs over 4 weeks. Soils at the wettest point along the rainfall gradient had very high soil methane concentrations (3–24%) indicating a strong influence of anaerobic processes.We measured net methane emission to the atmosphere at the wettest sites of the rainfall gradient, and in the valleys along topographic gradients. Other measures of biogeochemical function such as soil organic matter content and P availability were sensitive to chronic O2 depletion along the rainfall gradient, but less sensitive to the variable soil O2 environment exhibited at lower elevations along topographic gradients.

Nutrient availability in a montane wet tropical forest: Spatial patterns and methodological considerations

Silver, W.L., F.N. Scatena, A.H. Johnson, T.G. Siccama, and M.J.
Sanchez. 1994. Nutrient availability in a montane wet tropical forest: Spatial patterns and methodological considerations. Plant Soil 164:129–145.

Soils and forest floor were sampled quantitatively from a montane wet tropical forest in Puerto Rico to determine the spatial variability of soil nutrients, the factors controlling nutrient availability to vegetation, and the distribution of nutrients in soil and plants. Exchangeable cation concentrations were measured using different soil extracting procedures (fresh soil with NH4C1, air-dried and ground soil with KC1, and a Modified Olsen solution) to establish a range of nutrient availability in the soil, and to determine the relationship between different, but commonly used laboratory protocols. The availability of exchangeable Ca, Mg, and K was significantly lower in soils extracted fresh with NHaCI than from soils which were dried and ground prior to extraction with KCI or a modified Olsen solution. Soil nutrient availability generally decreased with depth in the soil. Several soil properties important to plant growth and survival varied predictably across the landscape and could be viewed in the context of a simple catena model. In the surface soils, exchangeable base cation concentrations and pH increased along a gradient from ridge tops to riparian valleys, while soil organic matter, exchangeable Fe and acidity decreased along this gradient. On the ridges, N, P, and K were positively correlated with soil organic matter; on slopes, N and P were positively correlated with organic matter, and Ca, Kg, and pH were negatively correlated with exchangeable Fe. Nutrient availability in the upper catena appears to be primarily controlled by biotic processes, particularly the accumulation of organic matter. The Ca, K, and P content of the vegetation was higher on ridges and slopes than in the valley positions. Periodic flooding and impeded drainage in the lower catena resulted in a more heterogeneous environment. A comparison of the Bisley, Puerto Rico soils with other tropical montane forests (TMF) revealed that the internal heterogeneity of soils in the Bisley Watersheds is similar to the range of average soil nutrient concentrations among TMF's for Ca, Mg, and K (dry/ground soils). Phosphorus tended to be slightly higher in Bisley and N was lower than in other TMFs.


Larsen, M.C., Torres-Sánchez, A.J., and Concepción, I.M., 1998, Slopewash, surface runoff, and fine-litter transport in forest and landslide scars in humid-tropical steeplands, Luquillo Experimental Forest, Puerto Rico [abs] EOS, Transactions American Geophysical Union, vol. 80.

Slopewash, surface runoff, and fine-litter transport in forest and landslide scars in humid-tropical steeplands, Luquillo Experimental Forest, Puerto Rico Rainfall, slopewash (the erosion of soil particles), surface runoff, and fine-litter transport at humid-tropical steepland sites in the Luquillo Experimental Forest, Puerto Rico (18° 20' N, 65° 45' W) were measured from 1991 to 1995. Hillslopes underlain by: 1) Cretaceous tuffaceous sandstone and siltstone in subtropical rain (tabonuco) forest with vegetation recovering from Hurricane Hugo (1989); and underlain by 2) Tertiary quartz diorite in subtropical lower montane wet (colorado and dwarf) forest with undisturbed forest canopy were compared to recent landslide scars. Monthly surface runoff on these very steep hillslopes (24° to 43°) was only 0.2 to 0.5 percent of monthly rainfall. Slopewash was higher in sandy loam soils whose parent material is quartz diorite (averaging 46 g m-2 a-1) than in silty-clay loam soils derived from tuffaceous sandstone and siltstone where the average was 9 g m-2 a-1. Annual slopewash of 100 to 349 g m-2 on the surfaces of two recent, small landslide scars was measured initially but slopewash decreased to only 3 to 4 g m-2 a-1 by the end of the study. The mean annual mass of fine litter (mainly leaves and twigs) transported downslope at the forested sites ranged from 5 to 8 g m-2 and was lower at the tabonuco forest site, where post-Hurricane Hugo recovery is still in progress. Mean annual fine-litter transport was 2.5 g m-2 on the two landslide scars.


Worldwide, suspended sediment flux entering the oceans each year is increasing due to human activities. Land development in Puerto Rico is causing the island’s mean annual suspended sediment flux to increase. These high suspended sediment yields are unfavorable for human recreation, water treatment, as well as habitat protection. This study attempts to determine if land development in eastern Puerto Rico is significantly impacting suspended sediment (SS) levels. In eastern Puerto Rico lies the Luquillo Experimental Forest (LEF), which has been protected from land development since the 1940s. With 75 percent of the Island’s virgin forests, seven streams (Quebrada 1, Quebrada 2, Quebrada 3, Rio Mameyes Puente Roto, Quebrada Sonadora, Quebrada Guaba, and Rio Icacos) were chosen to represent “natural” suspended sediment conditions. Examining the relationships between drainage area, discharge, suspended sediment concentrations, slope and suspended sediment loads, these seven streams were compared to one stream outside of the national forest, Rio Mameyes Rt. 3, whose watershed is partially used for agriculture and urban purposes. Results indicate that humans are not significantly impacting suspended sediment levels in the Rio Mameyes Rt. 3. Because land development in the Rio Mameyes Rt. 3 watershed is relatively small, less than 20 percent, it is feasible that the agriculture and urban development are not drastically affecting SS levels or the sampling period did not capture the full range of variability. A principal component analysis of drainage area, suspended sediment load, discharge, slope and temperature of the eight streams shows that only one factor, which contains all components, can be extracted.

Morphology and sedimentation in the Caribbean montane streams: examples from Jamaica and Puerto Rico

Ahmad R, Scatena FN, Gupta A. 1993. Morphology and sedimentation in Caribbean montane streams: examples from Jamaica and Puerto Rico. Sedimentary Geology 85: 157–169.

This paper presents a summary description of the morphology, sedimentation, and behaviour of the montane streams of eastern Jamaica and eastern Puerto Rico. The area is located within a 200 km wide seismically active zone of Neogene left-lateral strike-slip deformation which defines the plate boundary between the Caribbean and North American Plates. Tropical storms, occasionally strengthening up to hurricane force, affect the region periodically. This is an area of steep, mass-movement-scarred hillslopes which supply a large amount of coarse sediment to the rivers. From the description presented, we have constructed a model for the rivers of this region controlled by both neotectonics and periodic large floods. The drainage density is low with a near-rectangular stream network. The gradients are steep with boulder accumulations in the channels, their location at times related to the presence of large past landslides on hillslopes. Narrow, steep and confined channels occur in the mountains, but in wider sections and lower down near coastal plains, flood depositional forms appear in coarse valley alluvium. Small-scale deviations from the general pattern occur locally, controlled by variations in lithology, neotectonism, seasonality in flow, etc. This model for Caribbean montane streams differs considerably from the standard descriptions of alluvial rivers for which a number of detailed studies are available.
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