McDowell W.H.

Effects of nutrient availability and other elevational changes on bromeliad populations and their invertebrate communities in a humid tropical forest in Puerto Rico

RICHARDSON, BARBARA A.; RICHARDSON,M. J.; SCATENA, F. N.; MCDOWELL, W. H. 2000. Effects of nutrient availability and other elevational changes on romeliad populations and their invertebrate communities in a humid tropical forest in Puerto Rico. Journal of Tropical Ecology 16:167±188.

Abstract: 
Nutrient inputs into tank bromeliads were studied in relation to growth and productivity, and the abundance, diversity and biomass of their animal inhabitants, in three forest types along an elevational gradient. Concentrations of phosphorus, potassium and calcium in canopy-derived debris, and nitrogen and phosphorus in phytotelm water, declined with increasing elevation. Dwarf forest bromeliads contained the smallest amounts of debris/plant and lowest concentrations of nutrients in plant tissue. Their leaf turnover rate and productivity were highest and, because of high plant density, they comprised 12.8% of forest net primary productivity (0.47 t ha-1 y-1), and contained 3.3 t ha-1 of water. Annual nutrient budgets indicated that these microcosms were nutrient-abundant and accumulated < 5% of most nutrients passing through them. Exceptions were K and P in the dwarf forest, where accumulation was c. 25% of inputs. Animal and bromeliad biomass/plant peaked in the intermediate elevation forest, and were positively correlated with the debris content/bromeliad across all forest types. Animal species richness showed a signi®cant mid-elevational peak, whereas abundance was independent of species richness and debris quantities, and declined with elevation as forest net primary productivity declined. The unimodal pattern of species richness was not correlated with nutrient concentrations, and relationships among faunal abundance, species richness, nutrient inputs and environment are too complex to warrant simple generalizations about nutrient resources and diversity, even in apparently simple microhabitats.

Distribution of Nitrous Oxide and Regulators of Its Production across a Tropical Rainforest Catena in the Luquillo Experimental Forest, Puerto Rico

MCSWINEY, CLAIRE P.; MCDOWELL, WILLIAM H.; KELLER, MICHAEL 2001. Distribution of nitrous oxide and regulators of its production across a tropical rainforest catena in the Luquillo Experimental Forest, Puerto Rico. Biogeochemistry 56: 265-286.

Abstract: 
Understanding of N2O fluxes to the atmosphere is complicated by interactions between chemical and physical controls on both production and movement of the gas. To better understand how N2O production is controlled in the soil, we measured concentrations of N2O and of the proximal controllers on its production in soil water and soil air in a field study in the Rio Icacos basin of the Luquillo Experimental Forest, Puerto Rico. A toposequence (ridge, slope-ridge break, slope, slope-riparian break, riparian, and streambank) was used that has been previously characterized for groundwater chemistry and surface N2O fluxes. The proximal controls on N2O production include NO−3 , NH+4 , DOC, and O2. Nitrous oxide and O2 were measured in soil air and NO−3 , NH+4 , and DO were measured in soil water. Nitrate and DOC disappeared from soil solution at the slope-riparian interface, where soil N2O concentrations increased dramatically. Soil N2O concentrations continued to increase through the flood plain and the streambank. Nitrous oxide concentrations were highest in soil air probes that had intermediate O2 concentrations. Changes in N2O concentrations in groundwater and soil air in different environments along the catena appear to be controlled by O2 concentrations. In general, N processing in the unsaturated and saturated zones differs within each topographic position apparently due to differences in redox status.

Short-Term Disappearance of Foliar Litter in Three Species Before and After a Hurricane'

Short-Term Disappearance of Foliar Litter in Three Species before and after a Hurricane
Neal H. Sullivan, William B. Bowden and William H. McDowell
Biotropica
Vol. 31, No. 3 (Sep., 1999), pp. 382-393

Abstract: 
Litter disappearance was examined before (1989) and after (1990) Hurricane Hugo in the Luquillo Experimental Forest, Puerto Rico using mesh litterbags containing abscised Cyrilla racemiflora or Dacryodes excelsa leaves or fresh Prestoea montana leaves. Biomass and nitrogen dynamics were compared among: (i) species; (ii) mid- and high elevation forest types; (iii) riparian and upland sites; and (iv) pre- and post-hurricane disturbed environments. Biomass disappearance was compared using multiple regression and negative exponential models in which the slopes were estimates of the decomposition rates subsequent to apparent leaching losses and the y-intercepts were indices of initial mass losses (leaching). Cyrilla racemiflora leaves with low nitrogen (0.39%) and high lignin (22.1%) content decayed at a low rate and immobilized available nitrogen. Dacryodes excelsa leaves had moderate nitrogen (0.67%) and lignin (16.6%) content, decayed at moderate rates, and maintained the initial nitrogen mass. Prestoea montana foliage had high nitrogen (1.76%) and moderate lignin (16.7%) content and rapidly lost both mass and nitrogen. There were no significant differences in litter disappearance and nitrogen dynamics among forest types and slope positions. Initial mass loss of C. racemiflora leaves was lower in 1990 but the subsequent decomposition rate did not change. Initial mass losses and the overall decomposition rates were lower in 1990 than in 1989 for Dacryodes excelsa. Dacryodes excelsa and C. racemiflora litter immobilized nitrogen in 1990 but released 10-15 percent of their initial nitrogen in 1989, whereas P. montana released nitrogen in both years (25-40%). Observed differences in litter disappearance rates between years may have been due to differences in the timing of precipitation. Foliar litter inputs during post-hurricane recovery of vegetation in Puerto Rico may serve to immobilize and conserve site nitrogen.

Effects of Hurricane Disturbance on Stream Water Concentrations and Fluxes in Eight Tropical Forest Watersheds of the Luquillo Experimental Forest, Puerto Rico

SCHAEFER, DOUGLAS. A.; McDOWELL, WILLIAM H.; SCATENA, FREDRICK N.; ASBURY,CLYDE E. 2000. Effects of hurricane disturbance on stream water concentrations and fluxes in eight tropical forest watersheds of the Luquillo Experimental Forest, Puerto Rico. . Journal of Tropical Ecology 16:189-207

Abstract: 
Stream water chemistry responds substantially to watershed disturbances, but hurricane effects have not been extensively investigated in tropical regions. This study presents a long-term (2.5-1 1 y) weekly record of stream water chemistry on eight forested watersheds (catchment basins) in the Luquillo Mountains of Puerto Rico. This includes a period before and at least 2 y after the disturbance caused by the 1989 Hurricane Hugo. Nitrate, potassium and ammonium concentrations increased after the hurricane and remained elevated for up to 2 y. Sulphate, chloride, sodium, magnesium and calcium showed smaller relative significant changes. Average stream water exports of potassium, nitrate and ammonium increased by 13.1, 3.6 and 0.54 kg ha-' y-' in the first post-hurricane year across all watersheds. These represent increases of 119, 182 and 102% respectively, compared to the other years of record. The increased stream outputs of potassium and nitrogen in the first 2 y post-hurricane are equivalent to 3% (potassium) and 1% (nitrogen) of the hurricanederived plant litter. Effects of hurricanes on tropical stream water potassium and nitrogen can be greater than those caused by canopy gaps or limited forest cutting, but less than those following large-scale deforestation or fire.

Characterizing nitrogen dynamics, retention and transport in a tropical rainforest stream using an in situ 15N addition

Merriam, J. L. , McDowell, W. H. , Tank, J. L. , Wollheim, W. M. , Crenshaw, C. L. and Johnson, S. L. (2002), Characterizing nitrogen dynamics, retention and transport in a tropical rainforest stream using an in situ15N addition. Freshwater Biology, 47: 143–160. doi: 10.1046/j.1365-2427.2002.00785.x

Abstract: 
1. This study was part of the Lotic Intersite Nitrogen eXperiment (LINX); a series of identical 15NH4 tracer additions to streams throughout North America. 15NH4Cl was added at tracer levels to a Puerto Rican stream for 42 days. Throughout the addition, and for several weeks afterwards, samples were collected to determine the uptake, retention and transformation pathways of nitrogen in the stream. 2. Ammonium uptake was very rapid. Nitrification was immediate, and was a very significant transformation pathway, accounting for over 50% of total NH4 uptake. The large fraction of NH4 uptake accounted for by nitrification (a process that provides energy to the microbes involved) suggests that energy limitation of net primary production, rather than N limitation, drives N dynamics in this stream. 3. There was a slightly increased 15N label in dissolved organic nitrogen (DON) the day after the 15NH4 addition was stopped. This DO15N was < 0.02% of DON concentration in the stream water at the time, suggesting that nearly all of the DON found in-stream is allochthonous, or that in-stream DON production is very slow. 4. Leptophlebiidae and Atya appear to be selectively feeding or selectively assimilating a very highly labelled fraction of the epilithon, as the label found in the consumers became much higher than the label found in the food source. 5. A large spate (>20-fold increase in discharge) surprisingly removed only 37% of in-stream fine benthic organic matter (FBOM), leaves and epilithon. The fraction that was washed out travelled downstream a long distance (>220 m) or was washed onto the stream banks. 6. While uptake of 15NH4 was very rapid, retention was low. Quebrada Bisley retained only 17.9% of the added 15N after 42 days of 15N addition. Most of this was in FBOM and epilithon. Turnover rates for these pools were about 3 weeks. The short turnover times of the primary retention pools suggest that long-term retention (>1 month) is minimal, and is probably the result of N incorporation into shrimp biomass, which accounted for < 1% of the added 15N.

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Subsurface Solute Patterns

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Subsurface Solute Patterns
William H. McDowell, William B. Bowden and Clyde E. Asbury
Biogeochemistry
Vol. 18, No. 2 (1992), pp. 53-75

Abstract: 
Nitrate, ammonium, dissolved organic N, and dissolved oxygen were measured in stream water and shallow groundwater in the riparian ones of two tropical watersheds with different soils and geomorphology. At both sites, concentrations of dissolved inorganic N (DIN: NH-4+ and NO-3-N) were low in stream water lt 110 mu-g/L). Markedly different patterns in DIN were observed in groundwater collected at the two sites. At the first site (Icacos watershed), DIN in upslope groundwater was dominated by NO-3-N (550 mu-g/L) and oxygen concentrations were high (5.2 mg/l). As groundwater moved through the floodplain and to the stream, DIN shifted to dominance by NH-4+-N (200-700 mu-g/L) and groundwater was often anoxic. At the second site (Bisley watershed), average concentrations of total dissolved nitrogen were considerably lower (300 mu-g/L) than at Icacos (600 mu-g/L), and the dominant form of nitrogen was DON rather than inorganic N. Concentrations of NH-4+ and NO-3- were similar throughout the riparian zone at Bisley, but concentrations of DON declined from upslope wells to stream water. Differences in speciation and concentration of nitrogen in groundwater collected at the two sites appears to be controlled by differences in redox conditions and accessibility of dissolved N to plant roots, which are themselves the result of geomorphological differences between the two watersheds. At the Icacos site, a deep layer of coarse sand conducts subsurface water to the stream below the rooting zone of riparian vegetation and through zones of strong horizontal redox zonation. At the Bisley site, infiltration is impeded by dense clays and saturated flow passes though the variably oxidized rooting zone. At both sites, hydrologic export of nitrogen is controlled by intense biotic activity in the riparian zones. However, geomorphology appears to strongly modify the importance of specific biotic components.

Effects of Hurricane Disturbance on Groundwater Chemistry and Riparian Function in a Tropical Rain Forest

Effects of Hurricane Disturbance on Groundwater Chemistry and Riparian Function in a Tropical Rain Forest
William H. McDowell, Claire P. McSwiney and William B. Bowden
Biotropica
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 577-584

Abstract: 
The long-term response of shallow groundwater chemistry to the canopy disturbance and defoliation associated with Hurricane Hugo was studied at two sites in the Luquillo Experimental Forest, Puerto Rico. The sites differed in bedrock type, dominant vegetation, and availability of pre-hurricane data. At the primary study site, the Bisley catchment, hurricane disturbance resulted in increased concentrations of NO3 -, NH4 +, dissolved organic N, base cations, Cl-, and SiO2 in groundwater within 5 mo of the hurricane. The largest relative change in concentration occurred for K+, which increased from 0.7 to as high as 13 mg/L, concentrations were still 1.3 mg/L 5.5 yr after the hurricane. Most other solutes had returned to background levels within 1-2 yr of the hurricane. At the secondary study site, the Icacos catchment, NO3 - concentrations peaked at 1.1 mg/L one yr after the hurricane and decreased to nearly zero 5.5 yr after the hurricane. At both sites, NO3 - concentrations were higher in upslope wells than in those closer to the stream. Overall, riparian processes appear to reduce but not eliminate hydrologic losses of N following catastrophic disturbance. The nature of the long-term biogeochemical response to disturbance in this tropical rain forest ecosystem is similar to that observed in some montane temperate forests, and the time course of recovery appears to be associated with the speed with which vegetation regrows following disturbance.

Export of Carbon, Nitrogen, and Major Ions from Three Tropical Montane Watersheds

Export of Carbon, Nitrogen, and Major Ions from Three Tropical Montane Watersheds
William H. McDowell and Clyde E. Asbury
Limnology and Oceanography
Vol. 39, No. 1 (Jan., 1994), pp. 111-125

Abstract: 
Annual exports of suspended sediment, dissolved and particulate C and N, dissolved N and P, and major cations and anions were measured in three montane tropical rain forest watersheds in Puerto Rico during 1983-1986. Organic C was primarily exported in the form of DOC, and DOC export (33-94 kg ha-1 yr-1) was similar to values in larger tropical watersheds with similar runoff. Particulate and dissolved organic N accounted for 60-70% of the 4-9 kg ha-1 yr-1 of total N exported. Export of base cations and rates of weathering varied with bedrock geology in the three watersheds. Concentrations of suspended sediment, particulate C and N, and DOC increased as a function of discharge in all three streams. NH4+, NO3-, SO42-, and K+ concentrations showed little or no response to variations in discharge; Na-, Ca2+, Mg2+, Cl-, HCO3-, and SiO2 all decreased at high flows. After removing the effects of discharge, residual NO, concentrations in each stream were inversely related to estimated rates of leaf fall. On a watershed basis, export and accumulation of N in biomass were greater than precipitation inputs, suggesting that unmeasured inputs (8-16 kg ha-1 yr-1) were large.

Disturbance and long-term patterns of rainfall and throughfall nutrient fluxes in a subtropical wet forest in Puerto Rico

Heartsill-Scalley, T.; Scatena,F.N.; Estrada,C.; McDowell,W.H.;Lugo,A.E. 2007. Disturbance and long-term patterns of rainfall and throughfall nutrient fluxes in a subtropical wet forest in Puerto Rico. Journal of Hydrology 333, :472- 485.

Abstract: 
Nutrient fluxes in rainfall and throughfall were measured weekly in a mature subtropical wet forest in NE Puerto Rico over a 15-year period that included the effects of 10 named tropical storms, several prolonged dry periods, and volcanic activity in the region. Mean annual rainfall and throughfall were 3482 and 2131 mm yr1, respectively. Average annual rainfall and throughfall fluxes of K, Ca, Mg, Cl, Na, and SO4–S were similar but somewhat larger than those reported for most tropical forests. Rainfall inputs of nitrogen were comparatively low and reflect the relative isolation of the airshed. More constituents had seasonal differences in rainfall fluxes (6 out of 12) than throughfall fluxes (4 out of 12) and all volume weighted throughfall enrichment ratios calculated for the 15-year period were greater than one. However, median weekly enrichment ratios were less than 1 for sea salts and dissolved organic carbon, between 1 and 2 for Mg, Ca, SiO2 and SO4–S, and greater than 10 for NH4–N, PO4–P, and K. Droughts tended to reduce enrichment ratios of cations and sea-salts, but increased enrichment ratios of NH4–N, PO4–P, and K. In the weeks following hurricanes and tropical storms, relative throughfall tended to be higher and enrichment ratios tended to be lower. Saharan dust and the activity of Caribbean volcanoes can also be detected in the time series. Nevertheless, the impacts of particular events are variable and modified by the magnitude of the event, the preand post-event rainfall, and the time since the previous event. Rainfall, throughfall, rainfall pH, and rainfall fluxes of seven constituents had decreasing trends over the 15-year period. However, these decreases were small, less than inter-annual and annual varia-tions, and not considered to be ecologically significant. These long-term observations indicate that physical and biological processes associated with water passing through the canopy act to buffer internal nutrient cycles from inter-annual and seasonal variations in rainfall inputs.

A nitrogen budget for late-successional hillslope tabonuco forest, Puerto Rico

A Nitrogen Budget for Late-Successional Hillslope Tabonuco Forest, Puerto Rico
Tamara J. Chestnut, Daniel J. Zarin, William H. McDowell and Michael Keller
Biogeochemistry
Vol. 46, No. 1/3, New Perspectives on Nitrogen Recycling in the Temperate and Tropical Americas (Jul., 1999), pp. 85-108

Abstract: 
Nitrogen budgets of late successional forested stands and watersheds provide baseline data against which the effects of small- and large-scale disturbances may be measured. Using previously published data and supplemental new data on gaseous N loss, we construct a N budget for hillslope tabonuco forest (HTF) stands in Puerto Rico. HTF stands are subject to frequent hurricanes and landslides; here, we focus on N fluxes in the late phase of inter-disturbance forest development. N inputs from atmospheric deposition (4-6 kg N/ha/yr) are exceeded by N outputs from groundwater, gaseous N loss, and particulate N loss (6.3-15.7 kg N/ha/yr). Late-successional HTF stands also sequester N in their aggrading biomass (8 kg N/ha/yr), creating a total budget imbalance of 8.3-19.7 kg N/ha/yr. We surmise that this imbalance may be accounted for by unmeasured inputs from above- and belowground N-fixation and/or slow depletion of the large N pool in soil organic matter. Spatial and temporal variability, especially that associated with gaseous exchange and soil organic matter N-mineralization, constrain the reliability of this N budget.
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