Ground-based and remotely sensed nutrient availability across a tropical landscape

Abstract

Tropical soils often are assumed to be highly weathered and thus nutrient-depleted, but this prediction applies primarily to geomorphically stable surfaces. Topography complicates the assumption of nutrient depletion, because erosion can enhance the supply of nutrients to tropical ecosystems. Consequently, understanding nutrient availability across landscapes requires a spatially explicit assessment of the relative strength of depletion and enhancement. We document the relationship between foliar nutrients and topographic position across a 20-km 2 , 4-to 5-million-year-old eroded landscape in Kaua'i, Hawai'i, and use this relationship to build a bottom-up map of predicted nutrient availability across this landscape. Only Ϸ17% of the landscape is nutrient-poor, mostly on stable uplands; nutrient availability on slopes and valley bottoms is much higher, in some cases similar to the most fertile montane forests in the Hawaiian Islands. This pattern was corroborated by top-down remote sensing of area-integrated canopy phosphorus concentrations.Hawai'i ͉ remote-sensing ͉ topography ͉ erosion ͉ nutrient availability T errestrial biogeochemistry has been guided by the paradigm that as soils age they become increasingly weathered and nutrient-depleted (1-4). For example, Wardle et al. (5) evaluated six soil age sequences and determined that productivity and phosphorus (P) availability decline with age. However, to isolate the effects of aging, all six chronosequences were deliberately placed on relatively flat, geomorphically stable sites. In the only tropical sequence, in Hawai'i, plots were located on minimally eroded surfaces of Hawaiian shield volcanoes (6).Most landscapes, however, are comprised of both stable and eroding surfaces. While aging results in P depletion at the oldest (4.1 million years old) Hawaiian site, the surrounding landscape has experienced substantial erosion (7,8). Erosion rejuvenates the supply of nutrients by means of rock weathering on slopes, where the availability of rock-derived nutrients can approach that of younger, fertile, uneroded surfaces (9, 10). The resulting patchwork of stable and eroding surfaces is not unique to Hawai'i. A data set of 90-m cell digital elevation models (Shuttle Radar Topography Mission) shows that, globally, Ϸ10% of the tropics (Ϸ4.5 ϫ 10 6 km 2 ) have slopes Ͼ10°, with higher percentages in Central America, Asia, parts of South America, and some Pacific Islands (11).To understand the landscape-level interaction between depletion and rejuvenation, we applied two techniques, field-based determination of nutrient availability at different landscape positions and a newly developed technique for remote sensing of canopy nutrient content (12), to an upland forest landscape on the island of Kaua'i, Hawai'i.Kaua'i was formed Ϸ4-5 million years ago by basaltic eruptions emanating from the Hawaiian hotspot (7), and the landscape is underlain by a single bedrock type of a single age. After formation, Kaua'i was carried off the hotspot by movement of the Pacific Plate, a...

Abrupt Change in Forest Height along a Tropical Elevation Gradient Detected Using Airborne Lidar

“…While the results of erosion rate on foliar chemistry have been explored both on the ground [23] and from aircraft [24], this is the first effort that uses LiDAR to detect the effects of erosion rate on forest structure in a tropical forest. Thus, this work indicates another mechanism by which differences across elevation lead to differences in forest structure and function, one of the longest standing observations in ecosystem science and biogeography [25].…”

Prediction of Macronutrients at the Canopy Level Using Spaceborne Imaging Spectroscopy and LiDAR Data in a Mixedwood Boreal Forest

“…P, K, Mg and Ca concentrations have been examined in savannah grasses [27] as well as for willow, olive, grass and heather foliage [28]. At airborne scales, AVIRIS data were used to estimate area-integrated P concentrations in tropical forests of Hawaii [29] and African savannah using HyMap data [30]. Mirik et al [31] found a statistically significant relationship between a simple ratio vegetation reflectivity index (1129 nm/469 nm) and P concentration on an area basis in the forage vegetation of Yellowstone National Park using PROBE-1 data.…”

Stream Nutrient Concentrations on the Windward Coast of Hawai‘i Island and Their Relationship to Watershed Characteristics

“…Over time, atmospheric nitrogen is added to soils by fixation or wet /dry deposition, which is why older Hawaiian soils (Kohala) tend to have more nitrogen than younger Hawaiian soils (Mauna Kea) ( Vitousek et al 2003, Vitousek 2004, Porder et al 2005. By this logic one would expect Mauna Loa streams to have lower nitrate c oncentrations than Mauna Kea streams.…”