Siccama T.G.

Influence of landscape position and vegetation on long-term weathering rates at the Hubbard Brook Experimental Forest, New Hampshire, USA

Nezat, C. A., J. D. Blum, A. Klaue, C. E. Johnson, and T. G. Siccama
(2004), Influence of landscape positions and vegetation on long-term
weathering rates at the Hubbard Brook Experimental Forest, New
Hampshire, USA, Geochem. Cosmochim. Acta, 68(14), 3065– 3078.

Abstract: 
The spatial variability of long-term chemical weathering in a small watershed was examined to determine the effect of landscape position and vegetation. We sampled soils from forty-five soil pits within an 11.8-hectare watershed at the Hubbard Brook Experimental Forest, New Hampshire. The soil parent material is a relatively homogeneous glacial till deposited 14,000 years ago and is derived predominantly from granodiorite and pelitic schist. Conifers are abundant in the upper third of the watershed while the remaining portion is dominated by hardwoods. The average long-term chemical weathering rate in the watershed, calculated by the loss of base cations integrated over the soil profile, is 35 meq m2 yr1—similar to rates in other 10 to 15 ka old soils developed on granitic till in temperate climates. The present-day loss of base cations from the watershed, calculated by watershed mass balance, exceeds the long-term weathering rate, suggesting that the pool of exchangeable base cations in the soil is being diminished. Despite the homogeneity of the soil parent material in the watershed, long-term weathering rates decrease by a factor of two over a 260 m decrease in elevation. Estimated weathering rates of plagioclase, potassium feldspar and apatite are greater in the upper part of the watershed where conifers are abundant and glacial till is thin. The intra-watershed variability across this small area demonstrates the need for extensive sampling to obtain accurate watershed-wide estimates of long-term weathering rates.

At What Temporal Scales Does Disturbance Affect Belowground Nutrient Pools?

At What Temporal Scales Does Disturbance Affect Belowground Nutrient Pools?
Whendee L. Silver, Fred N. Scatena, Arthur H. Johnson, Thomas G. Siccama and Fiona Watt
Biotropica
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 441-457

Abstract: 
We monitored the effects of both harvesting aboveground biomass and Hurricane Hugo on soil chemical and physical properties, and live and dead root biomass over 6 yr in a subtropical wet forest in Puerto Rico. Our goal was to determine how belowground processes changed at different temporal scales including the immediate period prior to revegetation (9 wk), the intermediate period of initial regrowth (9 mo), and the longer-term reorganization of the vegetation and biogeochemical cycling (6 yr). Harvesting resulted in temporary increases in the availability of exchangeable nutrients, but forest floor and soil nutrient pools had generally returned to pre-harvest values over a 9 wk period. Significant amounts of K moved through the soil over this time period, amounting to 29-46 kg/ha-1, and resulting in a reduction in the size of the exchangeable soil K pool. The hurricane deposited approximately 845 kg/ha-1 of forest floor mass and considerable nutrients on the soil surface, and increased soil NO3-N and exchangeable K pools, but in all cases, pool sizes had returned to pre-hurricane values within 9 mo. Examination of the data on an annual time step over the 6 yr period revealed an increase in soil cation pools and a significant decrease in soil pH. No change in soil organic matter was detected at any time step following the disturbances. Live fine root biomass was dramatically reduced as a result of the hurricane, and was only beginning to show signs of recovery near the end of the 6 yr experiment.

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.

Abstract: 
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.

Biomass and Nutrient Content of the Bisley Experimental Watersheds, Luquillo Experimental Forest, Puerto Rico, Before and After Hurricane Hugo

Biomass and Nutrient Content of the Bisley Experimental Watersheds, Luquillo Experimental Forest, Puerto Rico, Before and After Hurricane Hugo, 1989
F. N. Scatena, W. Silver, T. Siccama, A. Johnson and M. J. Sanchez
Biotropica
Vol. 25, No. 1 (Mar., 1993), pp. 15-27

Abstract: 
The biomass and nutrient content of two steepland watersheds were estimated using allometric equations and nutrient concentrations derived from a subsample of the vegetation. Prior to the passage of Hurricane Hugo in September 1989, the watersheds had a total vegetative biomass of 301 tons/ha, 75 percent of which was aboveground. The total nutrient content of this vegetation was 907, 49, 644, 653, and 192 kg/ha for N, P, K, Ca, and Mg, respectively and varied with topographic setting. Concentrations per unit dry weight of P (0.16), K (2.49), Ca (2.13), and Mg (0.62) in aboveground vegetation were similar to other steepland tropical forests, while the concentration of N (2.9) was greater. Following the passage of Hurricane Hugo, the standing aboveground biomass was reduced to 113 t/ha and the aboveground nutrient content of the forest was reduced 45 to 48 percent.
Syndicate content