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A Comparative Analysis of Soil Characteristics in St. Croix’s Waterways: Introduction Waterways or guts channel rainwater downstream and connect marine terres- trial ecosystems. When forest is removed and bare soil is exposed, this often results in erosion. Guts with eroded or compacted soil have faster water flow and are less able to absorb rainfall and recharge ground water. When soil and other particles wash downstream, they are eventually depos- ited in coastal areas and can contaminate marine systems such as coral reefs. The purpose of this study is to examine the ef- fect that human land-use and land-cover change has on the health of St. Croix’s wa- terways. A recent land-cover change analy- sis by the University of the Virgin Islands, Agricultural Experiment Station (UVI-AES) found that one half of St. Croix is still for- ested and that 15% of the island’s surface experienced significant change during the 10-year study period (Technical Bulle- tin #13). This represents a relatively high percentage of change and is an important ecological measure because environmen- tal damage often occurs during land-cover change. This research project compares forested sites with changing sites in order to determine if there is a difference in soil quality, which is a strong indicator of over- all stream health. Comparing soil samples from guts in three land cover types (For- ested, Deforested and Reforested) pro- vides quantitative and descriptive differ- ences between them. We hypothesize that; stable forests have healthier waterways while deforested and recently reforested sites will have degraded soil conditions. Site Selection Methods The primary tool for this project was a comprehensive Geographic Information System (GIS) containing multiple spatial data layers and field measurements. The research team consisted of AES staff and student employees, who were trained and certified to use the GIS software, ArcMap 9.2 and GPS (Global Positioning System) A Student Research Bulletin from the University of the Virgin Islands Agricultural Experiment Station June, 2009 By Brian Daley, Tristian Muhammad and Hema Balkaran A Look at the Dirt in our Guts Table 1. Land-cover types and their descriptions Figure 1. Above, St. Croix’s waterways are categorized based on an AES land-cover study. Below are the 30 soil sampling points. Land-Cover Change Type Description Stable Forest (FF) Forests >15 yrs old. These are frequently comprised of multiple canopies, relatively high level of diversity and contain native species. Deforestation (FN) Classified as having no forest cover in 2002. Reforestation (NF) Sites that re-grew forest between 1992 and 2002. These forests tend to be young, with single-layer canopies under 15 ft tall, have relatively low species diversity and are dominated by invasive species such as tan-tan. units by the firm, Geographic Consulting. The GIS was used to organize the entire project with a single system and accurate- ly manage multiple data types. The base layer for this project is the land-cover change map from the previous AES project (Technical Bulletin #13). In that study, satellite images of St. Croix from 1992 and 2002 were categorized as Forest (F) and Non-forest (N) and then compared. A map was created identifying areas of reforestation (NF), deforestation (FN) and stable forest (FF) which we used as a se- lection tool when choosing sample sites (Table 1). A USGS (United States Geologi- cal Survey) layer showing waterways was overlaid on the categorized map and a 15 foot buffer was created on each side. The area within the buffered guts was catego- rized or classified based on the surround- ing land-cover type (Figure 1). We created 30 random points within the buffered gut areas, ten in each of the three land-cover types. The points were uploaded into GPS units and the field team navigated to each of them to collect soil samples. Soil Sampling Method St. Croix’s waterways are ephemeral; con- taining water only during rain events, so sampling water is logistically difficult. Soil samples are considered a proxy for water quality and also provide additional informa- tion on the waterways’ structure. Samples were collected using the “Soil Quality Test Kit” created by the USDA, Agriculture Re- search Service, Soil Quality Institute (Fig- ure 2). Observations on land management, erosion, forest structure and tree species diversity were recorded as appropriate. Slope, water infiltration and soil tempera- ture were measured in the field (Figure 3). Soil samples from each site were lab tested for soil moisture, bulk density, elec- Forest to forest (FF) Forest to non-forest (FN) Non-forest to forest (NF) Uncategorized waterways Watershed boundaries Figure 2. USDA Soil Quality Test Kit used to collect and analyze soil samples. tro-conductivity, and pH. Descriptions of the variables and their significance appear in Table 2. There were ten sample sites in each of three land cover categories. Three samples were collected at each site (from the center, left and right bank) for a total of 90 samples from 30 sites. Results and Discussion The results support our original hypothesis that soil in forested sites would have supe- rior characteristics to soil in change areas. Forested guts have a higher average slope and their banks are significantly steeper than deforested sites (Table 2). Although steep slopes are more prone to erosion, we observed less erosion in the forested sites. Water infiltration rate is influenced by sev- eral factors such as soil particle size and soil organic material, but water infiltrated soil in stable forest sites significantly faster than in deforested and reforested sites. Average soil moisture did not differ between sites, with most sites registering on the lowest end of the range. We attribute this to the prolonged dry-period prior to sampling and not to an accurate measure of soil organic matter. Average pH values in deforested sites were significantly higher than in other sites. This is likely due to erosion of pH neutral top- soil leading to exposure of the alkaline sub soil. There was also a trend of soil from for- ested sites having lower values for, electro- conductivity, temperature, and bulk density. These results are a trend only and did not meet the criteria for statistical significance. Electro-conductivity measurements for for- ested and reforested fell within the healthy range while deforested sites registered levels that begin to impede plant growth. Average relative soil temperatures were lowest for forested sites, but the results were not sig- nificant. This is somewhat surprising as we expected this simple and direct measure to provide strongly significant results. It is pos- sible that increased sampling density could more clearly reveal the trend. It is important to note that several fac- tors may have contributed to confounding these results. Primary among these is that St. Croix’s landscape is highly fragmented and the course of a gut passes through al- ternating forested and deforested sections. The conditions at the 30 data collection sites ranged from intact, native forest with deep organic soil, to exposed, treeless sites where the gut banks showed clear evidence of being previously leveled by a bull-dozer. The origi- nal classification of forested, deforested and Brian Daley, Agroforestry Research Specialist University of the Virgin Islands Agricultural Experiment Station RR 2, Box 10,000, Kingshill, VI 00850 bdaley@uvi.edu Phone: 340-692-4078 Fax: 340-692-4035 http://www.uvi.edu/sites/uvi/Pages/AES-Agroforestry-Home.aspx?s=RE reforested was completed in 2005. The 10 forested sites were still forested and the ap- pearance and species composition indicated they were stable sites. Conversely, four of the reforested sites had recently been cleared and four sites classified as deforested had young patches of secondary forest estab- lishing. These findings likely emphasize the variable and dynamic nature of land-cover change sites. Conclusion We conclude that land-cover change in St. Croix is directly linked to the degradation of ephemeral waterways or guts. Degradation can be measured by decreased water infil- tration rates and increased pH, bulk density and electro-conductivity. This is a prelimi- nary study and the relatively small number of samples contributed to high variability and inconclusive results for some variables. Sampling on a larger scale may verify the prediction that forest removal also leads to increased soil temperature, for example. To prevent damage to the island’s waterways, we support the Virgin Islands law (Title 12, Chapter 3 section 123) prohibiting cutting of any trees within 25 feet of the edge of a wa- tercourse. Enforcement of this code would simultaneously protect forests, fresh water, soil, and coastal marine systems. Acknowledgements This project was made possible through a grant from UVI-Water Resources Research Institute (WRRI), U.S. Geological Survey Award No. 06HQGR0125 and was conducted by AES staff and student employees. Special thanks to Geographic Consulting for provid- ing training and generous in-kind donations. Table 2. A description of the eight measurement collected, their importance and the results by land-cover change type. Values followed by different letters with the row are significantly different (* P<0.1, ***P<.001) Measurement Description Interpretation Average values by land-cover change type Stable Forest Reforestation Deforestation (FF) (FN) (NF) Bank slope * Percentage of the stream bank slope. High values indicate channelization. 27.0% a 21.0% a 10.9% b Hill slope * Percentage of the slope of the stream High numbers indicate streams on bed. steep hills and low numbers represent flatter land. 4.0% a 1.0% b 2.0% ab Infiltration The time it takes for a fixed volume of High numbers mean more water is rate *** water in a tube to be absorbed by the absorbed. This implies the soil has soil (cm3/hr.). large particle size, such as in sand or organic material. 597.4 a 462.0 b 271.0 bc Relative The difference between the full-sun A 00 value signifies the same temperature temperature measurement and a site temperature as full-sun, while negative numbers taken at the same time of the day (0 F) are cooler than full-sun. -10.25 a -9.36 a -8.32 a Soil moisture Measures the weight difference for High values can mean high organic samples before and after they are material content, but is highly oven-dried (g/g). influenced by rain/drought. 0.160 a 0.189 a 0.167 a Bulk density The ratio of dried soil mass to bulk This can be used as a measure of soil volume (g/cm3). Sand and loam will compaction that can inhibit root growth. have higher numbers than clay. Several factors influence this test, but 1.181 a 1.061 a 2.486 a Electro- Indicates the amount of salts in the higher values are expected near farms conductivity * sample. All soils have some salt, but with excess run-off, coastal areas, high amounts can impede plant growth. and very dry sites. 0.375 a 1.219 ab 2.486 b pH * The measure of acidity or alkalinity of a In the Virgin Islands, higher numbers may soil sample. indicate loss of top soil and exposure to alkaline parent material. 7.49 a 8.21 b 7.66 a Figure 3. AES research team members collect soil samples. This site containing young trees was categorized as reforested (NF).