Our experience during the first field season at Marajoara, from October 1995 to February 1996, left several key impressions that strongly influenced research activities upon our return in September 1996. First, the forested landscape in southeast Pará feels essentially flat as you walk through it, and yet forest composition grades perceptibly between areas of higher vs. lower ground where seasonal streams flow. Interestingly, in this region mahogany trees are found almost exclusively on low ground near streams. Second, during the wet season large areas of low ground turn soggy to swampy during extended rainy periods, indicating that the below-ground water table rises to the soil surface during these periods. ‘Perched’ water tables can shape soil structural and chemical properties and may also influence forest composition, that is, the mix of plant species that can survive in grow within a given area. Because we wanted to understand why mahogany occurs mainly on low ground in this region, we decided to investigate soil hydrology through the seasons and years. This meant we needed to dig permanent wells as deep into the soil profile as possible, and then follow the movement of the water table as it rose and fell during successive wet and dry seasons.
Little published information is available about the soil water table (phreatic water) and the dynamics of subsurface drainage in this region. Askew et al. (1971) documented seasonal fluctuations in a water table in Mato Grosso that, at the tops of slopes rising 30 vertical meters, receded a corresponding distance below the ground surface during each year’s long dry season. They attributed soil differentiation across slopes in large measure to drainage effects created by the water table’s annual rise and fall. Topographic relief at Marajoara is gentler than this, rising only 5 to 20 m across slopes a kilometer or more wide.
To monitor seasonal changes in water table depths, we used 10-cm diameter bucket augers to dig narrow ‘wells’ to the water table in September 1996, near the end of that year’s dry season, at three slope positions (low ground, midslope, and high ground) across two ~200-m slopes grading from sandy low-ground soil to sandy clay high-ground soils. Elevation differences between low- and high-ground wells measured 5 and 6.5 m, respectively. 1.5-m iron extension rods allowed us to dig as deep as 15 m at high-ground positions before the soil became too waterlogged to extractl we collected soil samples at 1-m intervals to determine gravimetric moisture along the soil profile. Wells were lined with perforated 10-cm diameter PVC tubing, to roughly 11 m depth at high-ground well positions and to 6 m depth at low-ground positions, with midslope wells intermediate in depth. Vertical migrations of the water table were documented weekly through 4.5 years, until January 2001, by measuring the depth to water using a weighted buoy attached to a distance tape. All water depth readings were taken before mid-day.
Water table depths (data) across both slopes demonstrated cyclical vertical migrations that repeated annually, varying within years according to the seasonal timing, intensity, and quantity of rainfall. The overall pattern of subsidence through the dry season and recharge during the rainy season held across both slopes. Peak subsidence during the late dry season ranged from 9–12 m below the soil surface at high-ground positions to 5–7 m at low-ground positions; this re-established each year an essentially horizontal water table by mid to late dry season beneath topographic relief. Response by the water table to early wet season rains or mid wet season dry periods was consistent across the two slopes, with rises and dips in the water table generally mirrored from high- to low-ground positions. As groundwater re-charged through the wet season, the water table rose to near the soil surface at low-ground well positions, persisting there for many weeks during the especially heavy rainy season of 1996–1997, though less so during subsequent years with lower overall rainfall. At one of the two slopes, the water table at the high-ground position twice rose nearly to the soil surface during the heavy first rainy season.
Water tables began receding from the soil surface each year soon after the last significant rains in April or May. This can be seen comparing the dry season’s onset during the first and second years: water tables began subsiding four to five weeks earlier during 1997–1998, slowing briefly in mid May in response to a single heavy rainfall. Subsidence patterns during the early dry season demonstrate the effect of lateral surface flow across slopes: water tables at low-ground positions receded to depth more slowly than midslope and high-ground water tables, remaining within 2 meters of the soil surface into the dry season until July or August depending on the previous season’s rainfall totals and temporal pattern. Subsiding less relative to the soil surface at low-ground positions, and recharging there more rapidly to rainy season peak levels, the water table contributed to a wetter environment in low-ground forest in a temporal as well as an absolute sense.
Water tables elevated across slopes during the rainy season, generating lateral subsurface water flow feeding surface streams at slope bottoms. Once atmospheric inputs cease during the dry season, this phreatic ‘tilt’ begins to level off as lateral flow releases hydraulic pressure across slopes. The onset of each rainy season is marked by a sharp rise in measured water table depths at slope bottoms. This sudden filling may not reflect actual water table depths if lateral flow above indurated pans after heavy early season rains accumulates in wells lined by perforated PVC tubing. As accumulating precipitation recharges groundwater, high-ground water tables re-elevate above low-ground levels, streams begin flowing again, and waterlogged surface soils occur where relief is too slight to rapidly move water out of the system, or where indurated subsurface horizons impede vertical drainage.
The behavior of deep-soil water through two relatively dry years following 1996–1997’s heavy rainy season was extremely interesting. Successive wet season precipitation totals of 1709 mm and 1636 mm were not sufficient to return water tables to the first year’s high levels. Highest and mean water table heights fell from one year to the next, suggesting that prolonged drought—for example, five to 10 years of below-average rainfall—could lead to community-wide moisture stress as available deep-soil water recedes below depths to which plants are capable of extracting it to maintain dry season transpiration. High rates of drought-related mortality occurred among common low-ground species nearing the end of the El Niño dry season of 1998. Drought conditions extending over several years may cause differential mortality between communities, opening large-scale growing space at low-ground positions at long return intervals.
Seasonal vertical migrations of the water table at Marajoara are more extreme than those described in the literature at South American woodland savanna sites. Dense evergreen forests transpiring through the dry season may contribute to this, tapping receding phreatic water with deep roots. Late dry season moisture content in deep soil above the water table is likely bound by matric forces against all but the most efficient plant uptake, forcing deeply rooting woody species to tap phreatic water; we recovered roots as deep as 12 m while augering water table wells at high-ground positions. At slope bottoms, though deep-soil water lies shallower by dry season’s end than at slope tops, hydromorphic colors and gleyed subsurface horizons indicate that anaerobic soil conditions persist there through substantial portion of the year. Water tables perched at the soil surface may force a shallow, laterally spreading rooting pattern on low-ground species, and indeed, we never observed taproots on windthrown trees in low-ground forest at Marajoara. This forced rooting habit may prove a handicap during the dry season when the water table plummets several meters below the soil surface, exposing trees to prolonged soil water deficits during drought years.
Askew GP, Moffatt DJ, Montgomery RF & Searl PL (1971) Soils and soil moisture as factors influencing the distribution of the vegetation of the Serra do Roncador, Mato Grosso. In: Ferri MG (ed.), III Simpósio sôbre o Cerrado, pp. 150-160. Editôra de USP, São Paulo, SP, Brasil.
Fetter CW (1988) Applied Hydrogeology (2nd Ed). Merrill Publishing Company, Columbus, OH, USA.
Grogan, JE (2001) Bigleaf mahogany (Swietenia macrophylla King) in southeast Pará, Brazil: a life history study with management guidelines for sustained production from natural forests. PhD dissertation, Yale University School of Forestry & Environmental Studies, New Haven, CT, USA.
Grogan J & Galvão J (2006) Physiographic and floristic gradients across topography in transitional seasonally dry evergreen forests of southeastern Amazonia, Brazil. Acta Amazonica 36: 483-496.
Nepstad DC, Carvalho CR, Davidson EA, Jipp PH, Lefebvre PA, Negreiros GH, Silva ED, Stone TA, Trumbore SE & Vieira S (1995) The role of deep roots in the hydrological and carbon cycles of Amazonian forests and pastures. Nature 372: 666-669.
Nepstad DC, Veríssimo A, Alencart A, Nobre C, Lima E, Lefebvre P, Schlesinger P, Potter C, Moutinho P, Mendoza E, Cochrane M, Brooks V (1999) Large-scale impoverishment of Amazonian forests by logging and fire. Nature 398: 505-508.
Poels RLH (1987) Soils, Water and Nutrients in a Forest Ecosystem in Suriname. Agricultural University, Wageningen, The Netherlands.