spatial variability

Spatial dependence and the relationship of soil organic carbon and soil moisture in the luquillo experimental forest, puerto rico

Wang H, Hall CAS, Cornell JD, Hall MHP.
2002. Spatial dependence and the relationship
of soil organic carbon and soil moisture in Luquillo experimental forest. Landsc.
Ecol. 17:671–84

We used geo-spatial statistical techniques to examine the spatial variation and relationship of soil organic carbon (SOC) and soil moisture (SM) in the Luquillo Experimental Forest (LEF), Puerto Rico, in order to test the hypothesis that mountainous terrain introduces spatial autocorrelation and crosscorrelation in ecosystem and soil properties. Soil samples (n = 100) were collected from the LEF in the summer of 1998 and analyzed for SOC, SM, and bulk density (BD). A global positioning system was used to georeference the location of each sampling site. At each site, elevation, slope and aspect were recorded. We calculated the isotropic and anisotropic semivariograms of soil and topographic properties, as well as the cross-variograms between SOC and SM, and between SOC and elevation. Then we used four models (random, linear, spherical and wave/hole) to test the semivariances of SOC, SM, BD, elevation, slope and aspect for spatial dependence. Our results indicate that all the studied properties except slope angle exhibit spatial dependence within the scale of sampling (200 – 1000 m sampling interval). The spatially structured variance (the variance due to the location of sampling sites) accounted for a large proportion of the sample variance for elevation (99%), BD (90%), SOC (68%), aspect (56%) and SM (44%). The ranges of spatial dependence (the distances within which parameters are spatially dependent) for aspect, SOC, elevation, SM, and BD were 9810 m, 3070 m, 1120 m, 930 m and 430 m, respectively. Cross correlograms indicate that SOC varies closely with elevation and SM depending on the distances between samples. The correlation can shift from positive to negative as the separation distance increases. Larger ranges of spatial dependence of SOC, aspect and elevation indicate that the distribution of SOC in the LEF is determined by a combination of biotic (e.g., litterfall) and abiotic factors (e.g., microclimate and topographic features) related to elevation and aspect. This demonstrates the importance of both elevation and topographic gradients in controlling climate, vegetation distribution and soil properties as well as the associated biogeochemical processes in the LEF.

Topographic control of soil microbial activity: a case study of denitrifiers

Florinsky, 1. V., S. McMahon, and D. L. Burton. 2004.
Topographic control of soil microbial activity: a case study of
denitrifiers. Geoderma 119:33-53.

Topography may affect soil microbial processes, however, the use of topographic data to model and predict the spatial distribution of soil microbial properties has not been widely reported. We studied the effect of topography on the activity of denitrifiers under different hydrologic conditions in a typical agroecosystem of the northern grasslands of North America using digital terrain modelling (DTM). Three data sets were used: (1) digital models of nine topographic attributes, such as elevation, slope gradient and aspect, horizontal, vertical, and mean land surface curvatures, specific catchment area, topographic, and stream power indices; (2) two soil environmental attributes (soil gravimetric moisture and soil bulk density); and (3) six attributes of soil microbial activity (most probable number of denitrifiers, microbial biomass carbon content, denitrifier enzyme activity, nitrous oxide flux, denitrification rate, and microbial respiration rate). Linear multiple correlation, rank correlation, circular–linear correlation, circular rank correlation, and multiple regression were used as statistical analyses. In wetter soil conditions, topographically controlled and gravity-driven supply of nutritive materials to microbiota increased the denitrification rate. Spatial differentiation of the denitrification rate and amount of denitrifying enzyme in the soil was mostly effected by redistribution and accumulation of soil moisture and soil organic matter down the slope according to the relative position of a point in the landscape. The N2O emission was effected by differentiation and gain of soil moisture and organic matter due to the local geometry of a slope. The microbial biomass, number of denitrifiers, and microbial respiration depended on both the local geometry of a slope and relative position of a point in the landscape. In drier soil conditions, although denitrification persisted, it was reduced and did not depend on the spatial distribution of soil moisture and thus land surface morphology. This may result from a reduction in soil moisture content below a critical level sufficient for transient induction of denitrification but not sufficient to preserve spatial patterns of the denitrification according to relief. Digital terrain models can be used to predict the spatial distribution of the microbial biomass and amount of denitrifying enzyme in the soil. The study demonstrated a feasibility of applying digital terrain modelling to investigate relations of other groups of soil microbiota with topography and the system ‘topography–soil microbiota’ as a whole.

Modeling the spatial and temporal variability in climate and primary productivity across the Luquillo Mountains, Puerto Rico

Wanga, Hongqing; Halla, Charles A.S.; Scatenab, Frederick N.; Fetcherc, Ned; Wua, Wei 2003. Modeling the spatial and temporal variability in climate and primary productivity across the Luquillo Mountains, Puerto Rico.. Forest Ecology and Management 179 :69-94l.

There are few studies that have examined the spatial variability of forest productivity over an entire tropical forested landscape. In this study, we used a spatially-explicit forest productivity model, TOPOPROD, which is based on the FORESTBGC model, to simulate spatial patterns of gross primary productivity (GPP), net primary productivity (NPP), and respiration over the entire Luquillo Experimental Forest (LEF) in the mountains of northeastern Puerto Rico.We modeled climate variables (e.g. solar insolation, temperature, rainfall and transpiration) using a topography-based climate model, TOPOCLIM. The simulated GPP ranged from 8 to 92 t C/ha per year with a mean of 51 t C/ha per year. The simulated NPP ranged from 0.5 to 24 t C/ha per year with a mean of 9.4 t C/ha per year. The simulated plant respiration ranged from 31 to 68 with a mean of 42 t C/ha per year. Simulated GPP and respiration declined with increased elevation whereas simulated NPP increased from low to middle elevation but decreased from middle to high elevations. Statistical analyses indicate that variation in solar insolation, which decreases with increase in elevation, is the most important factor controlling the spatial variation of forest productivity in the LEF. Validation with the limited spatial empirical data indicated that our simulations overestimated GPP by 2% for a middle elevation test site, and by 43% for a mountain peak site. Our simulations also overestimated NPP in the middle elevation Colorado forest and higher elevation Dwarf forest by 32 and 36%, respectively, but underestimated NPP in the Tabonuco and Palm forests at low to middle elevations by 9–15% and 18%, respectively. Simulated GPP and NPP would decrease under CO2 doubling as projected temperatures increase and precipitation decreases. Different forest types respond differently to potential climate change and CO2 doubling. Comparison with other tropical forests suggests that the LEF as a whole has higher GPP (51 tC/ha per year versus 40 t C/ha per year) but lower NPP (9.4 t C/ha per year versus 11 t C/ha per year) than other tropical rain forests.

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.

Throughfall in a Puerto Rican lower montane rain forest: A comparison of sampling strategies

Holwerda, F.; Scatena, F.N.; Bruijnzeel, L.A. 2006. Throughfall in a Puerto Rican lower montane rain forest: A comparison of sampling strategies. Journal of Hydrology 327, :592- 602.

During a one-year period, the variability of throughfall and the standard errors of the means associated with different gauge arrangements were studied in a lower montane rain forest in Puerto Rico. The following gauge arrangements were used: (1) 60 fixed gauges, (2) 30 fixed gauges, and (3) 30 roving gauges. Stemflow was measured on 22 trees of four different species. An ANOVA indicated that mean relative throughfall measured by arrangements 1 (77%), 2 (74%), and 3 (73%) were not significantly different at the 0.05 level. However, the variability of the total throughfall estimate was about half as high for roving gauges (23%) as for fixed gauges (48–49%). The variability of stemflow ranged from 36% to 67% within tree species and was 144% for all sampled trees. Total stemflow was estimated at 4.1% of rainfall, of which palms contributed about 66%. Comparative analysis indicated that while fixed and roving gauge arrangements can give similar mean values, least 100 fixed gauges are required to have an error at the 95% confidence level comparable to that obtained by 30 roving gauges.
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