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

Most research on vegetation in mountain ranges focuses on elevation gradients as climate gradients, but elevation gradients are also the result of geological processes that build and deconstruct mountains. Recent findings from the Luquillo Mountains, Puerto Rico, have raised questions about whether erosion rates that vary due to past tectonic events and are spatially patterned in relation to elevation may drive vegetation patterns along elevation gradients. Here we use airborne light detection and ranging (LiDAR) technology to observe forest height over the Luquillo Mountain Range. We show that models with different functional forms for the two prominent bedrock types best describe the forest height-elevation patterns. On one bedrock type there are abrupt decreases in forest height with elevation approximated by a sigmoidal function, with the inflection point near the elevation of where other studies have shown there to be a sharp change in erosion rates triggered by a tectonic uplift event that began approximately 4.2 My ago. Our findings are consistent with broad geologically mediated vegetation patterns along the elevation gradient, consistent with a role for mountain building and deconstructing processes

Regional differences in phosphorus budgets in intensive soybean agriculture

Author Posting. © American Institute of Biological Sciences, 2013. This article is posted here by permission of University of California Press for personal use, not for redistribution. The definitive version was published in BioScience 63 (2013): 49-54, doi:10.1525/bio.2013.63.1.10.Fertilizer-intensive agriculture has been integral to increasing food production over the past half century but has been accompanied by environmental costs. We use case studies of phosphorus fertilizer use in the world’s most productive soybean-growing regions, Iowa (United States), Mato Grosso (Brazil), and Buenos Aires (Argentina), to examine influences of management and soil type on agriculture’s most prevalent phosphorusrelated environmental consequences: eutrophication and consumption of Earth’s finite phosphorus reserves. With increasing phosphorus inputs, achieving high yields on tropical soils with high phosphorus-binding capacity is becoming more common. This system has low eutrophication risks but increases demands on phosphorus supplies. In contrast, production in traditional breadbaskets, on soils with lower phosphorus-binding capacities, is being sustained with decreasing phosphorus inputs. However, in these regions, historical overuse of phosphorus may mean continued eutrophication risk even as pressures on phosphorus reserves diminish. We focus here on soybean production but illustrate how achieving sustainable agriculture involves an intricate optimization of local, regional, and global considerations.SP is supported by the Andrew Mellon Foundation, and CN and SHR’s work in Mato Grosso was funded by National Science Foundation grant no. NSF-DEB-0640661 and through collaboration with the Instituto de Pesquisa Ambiental da Amazonia

Structure and composition of altered riparian forests in an agricultural Amazonian landscape

Author Posting. © Ecological Society of America, 2015. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 25 (2015): 1725-1838, doi:10.1890/14-1740.1.Deforestation and fragmentation influence the microclimate, vegetation structure, and composition of remaining patches of tropical forest. In the southern Amazon, at the frontier of cropland expansion, forests are converted and fragmented in a pattern that leaves standing riparian forests whose dimensions are mandated by the Brazilian National Forest Code. These altered riparian forests share many characteristics of well-studied upland forest fragments, but differ because they remain connected to larger areas of forest downstream, and because they may experience wetter soil conditions because reduction of forest cover in the surrounding watershed raises groundwater levels and increases stream runoff. We compared forest regeneration, structure, composition, and diversity in four areas of intact riparian forest and four areas each of narrow, medium, and wide altered riparian forests that have been surrounded by agriculture since the early 1980s. We found that seedling abundance was reduced by as much as 64% and sapling abundance was reduced by as much as 67% in altered compared to intact riparian forests. The most pronounced differences between altered and intact forest occurred near forest edges and within the narrowest sections of altered riparian forests. Woody plant species composition differed and diversity was reduced in altered forests compared to intact riparian forests. However, despite being fragmented for several decades, large woody plant biomass and carbon storage, the number of live or dead large woody plants, mortality rates, and the size distribution of woody plants did not differ significantly between altered and intact riparian forests. Thus, even in these relatively narrow forests with high edge : area ratios, we saw no evidence of the increases in mortality and declines in biomass that have been found in other tropical forest fragment studies. However, because of the changes in both species community and reduced regeneration, it is unclear how long this relative lack of change will be sustained. Additionally, Brazil recently passed a law in their National Forest Code allowing narrower riparian buffers than those studied here in restored areas, which could affect their long-term sustainability.This research has been supported by a grant from the U.S. Environmental Protection Agency's Science to Achieve Results (STAR) program (Award #: FP-91749001-0). Additional support was provided by NSF Award # DEB 0949370 and the São Paulo Research Foundation (FAPESP)

Topographic controls on soil nitrogen availability in a lowland tropical forest

Geomorphic position often correlates with nutrient cycling across landscapes. In tropical forests, topography is known to influence phosphorus (P) availability, but its effect on nitrogen (N) cycling has received less exploration, especially in lowland forests where widespread N richness is frequently assumed. Here, we report significant effects of topographic slope and landscape position on multiple aspects of the N cycle across a highly dissected lowland tropical forest on the Osa Peninsula, Costa Rica. A suite of N cycle metrics measured along a topographic sequence revealed a distinct gradient in N availability. Values of soil d15N, inorganic N pools, net nitrification rates, and nitrification potentials were all substantially lower on a flanking steep hillslope (;288) compared to a relatively flat ridge top (;68), indicating lower N availability and a less open N cycle in steep parts of the landscape. Slope soils also hosted smaller total carbon and nitrogen stocks and notably less weathered soil minerals than did ridge soils. These latter findings suggest that elevated N loss resulting from high rates of soil and particulate organic matter erosion could underpin the spatial variation in N cycling and availability. Expanding our analysis to the larger study landscape, a strong negative linear relationship between soil d15N values and surface slope angles was observed. N isotope mass balance models suggest that this pattern is most plausibly explained by an increase in N loss via erosive, non-fractioning pathways from steep zones, as most other variables commonly assumed to affect soil d15N values (such as temperature, precipitation, and vegetation type) did not vary across the sampled region. Together, these results reveal notable hillslope-scale variation in N richness and suggest an important role for nonfractionating N loss in the maintenance of this pattern. Such findings highlight the importance of geomorphology and the significant capacity of erosion to influence N availability in steepland ecosystems

Solute and sediment export from Amazon forest and soybean headwater streams

Author Posting. © Ecological Society of America, 2016. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 27 (2017): 193–207, doi:10.1002/eap.1428.Intensive cropland agriculture commonly increases streamwater solute concentrations and export from small watersheds. In recent decades, the lowland tropics have become the world's largest and most important region of cropland expansion. Although the effects of intensive cropland agriculture on streamwater chemistry and watershed export have been widely studied in temperate regions, their effects in tropical regions are poorly understood. We sampled seven headwater streams draining watersheds in forest (n = 3) or soybeans (n = 4) to examine the effects of soybean cropping on stream solute concentrations and watershed export in a region of rapid soybean expansion in the Brazilian state of Mato Grosso. We measured stream flows and concentrations of NO3−, PO43−, SO42−, Cl−, NH4+, Ca2+, Mg2+, Na+, K+, Al3+, Fe3+, and dissolved organic carbon (DOC) biweekly to monthly to determine solute export. We also measured stormflows and stormflow solute concentrations in a subset of watersheds (two forest, two soybean) during two/three storms, and solutes and δ18O in groundwater, rainwater, and throughfall to characterize watershed flowpaths. Concentrations of all solutes except K+ varied seasonally in streamwater, but only Fe3+ concentrations differed between land uses. The highest streamwater and rainwater solute concentrations occurred during the peak season of wildfires in Mato Grosso, suggesting that regional changes in atmospheric composition and deposition influence seasonal stream solute concentrations. Despite no concentration differences between forest and soybean land uses, annual export of NH4+, PO43−, Ca2+, Fe3+, Na+, SO42−, DOC, and TSS were significantly higher from soybean than forest watersheds (5.6-fold mean increase). This increase largely reflected a 4.3-fold increase in water export from soybean watersheds. Despite this increase, total solute export per unit watershed area (i.e., yield) remained low for all watersheds (<1 kg NO3− N·ha−1·yr−1, <2.1 kg NH4+-N·ha−1·yr−1, <0.2 kg PO43−-P·ha−1·yr−1, <1.5 kg Ca2+·ha−1·yr−1). Responses of both streamflows and solute concentrations to crop agriculture appear to be controlled by high soil hydraulic conductivity, groundwater-dominated hydrologic flowpaths on deep soils, and the absence of nitrogen fertilization. To date, these factors have buffered streams from the large increases in solute concentrations that often accompany intensive croplands in other locations.NSF Grant Numbers: DEB-0640661, DEB-0949370; Fundação de Amparo á Pesquisa do Estado de São Paulo Grant Number: FAPESP 03/13172-2; Watson Graduate Student Fellowship; Center for Latin American and Caribbean Studies at Brown Universit

Assessing Nutrient Limitation in Complex Forested Ecosystems : Alternatives to Large-Scale Fertilization Experiments

Quantifying nutrient limitation of primary productivity is a fundamental task of terrestrial ecosystem ecology, but in a high carbon dioxide environment it is even more critical that we understand potential nutrient constraints on plant growth. Ecologists often manipulate nutrients with fertilizer to assess nutrient limitation, yet for a variety of reasons, nutrient fertilization experiments are either impractical or incapable of resolving ecosystem responses to some global changes. The challenges of conducting large, in situ fertilization experiments are magnified in forests, especially the high-diversity forests common throughout the lowland tropics. A number of methods, including fertilization experiments, could be seen as tools in a toolbox that ecologists may use to attempt to assess nutrient limitation, but there has been no compilation or synthetic discussion of those methods in the literature. Here, we group these methods into one of three categories (indicators of soil nutrient supply, organismal indicators of nutrient limitation, and lab-based experiments and nutrient depletions), and discuss some of the strengths and limitations of each. Next, using a case study, we compare nutrient limitation assessed using these methods to results obtained using large-scale fertilizations across the Hawaiian Archipelago. We then explore the application of these methods in high-diversity tropical forests. In the end, we suggest that, although no single method is likely to predict nutrient limitation in all ecosystems and at all scales, by simultaneously utilizing a number of the methods we describe, investigators may begin to understand nutrient limitation in complex and diverse ecosystems such as tropical forests. In combination, these methods represent our best hope for understanding nutrient constraints on the global carbon cycle, especially in tropical forest ecosystems

Surprisingly modest water quality impacts from expansion and intensification of large-scale commercial agriculture in the Brazilian Amazon-Cerrado region

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Tropical Conservation Science 10 (2017): 1-5, doi:10.1177/1940082917720669.Large-scale commercial cropping of soybeans expanded in the tropical Amazon and Cerrado biomes of Brazil after 1990. More recently, cropping intensified from single-cropping of soybeans to double-cropping of soybeans with corn or cotton. Cropland expansion and intensification, and the accompanying use of mineral fertilizers, raise concerns about whether nutrient runoff and impacts to surface waters will be similar to those experienced in commercial cropland regions at temperate latitudes. We quantified water infiltration through soils, water yield, and streamwater chemistry in watersheds draining native tropical forest and single- and double-cropped areas on the level, deep, highly weathered soils where cropland expansion and intensification typically occurs. Although water yield increased four-fold from croplands, streamwater chemistry remained largely unchanged. Soil characteristics exerted important control over the movement of nitrogen (N) and phosphorus (P) into streams. High soil infiltration rates prevented surface erosion and movement of particulate P, while P fixation in surface soils restricted P movement to deeper soil layers. Nitrogen retention in deep soils, likely by anion exchange, also appeared to limit N leaching and export in streamwater from both single- and double-cropped watersheds that received nitrogen fertilizer. These mechanisms led to lower streamwater P and N concentrations and lower watershed N and P export than would be expected, based on studies from temperate croplands with similar cropping and fertilizer application practices.The work described here was supported by National Science Foundation grants EF 1655432, IOS 1457662 and ICER 1342953 and grants from the Fundação de Amparo à Pesquisa do Estado de São Paulo

Remotely Sensed Canopy Nitrogen Correlates With Nitrous Oxide Emissions in a Lowland Tropical Rainforest

Tropical forests exhibit significant heterogeneity in plant functional and chemical traits that may contribute to spatial patterns of key soil biogeochemical processes, such as carbon storage and greenhouse gas emissions. Although tropical forests are the largest ecosystem source of nitrous oxide (N2O), drivers of spatial patterns within forests are poorly resolved. Here, we show that local variation in canopy foliar N, mapped by remote‐sensing image spectroscopy, correlates with patterns of soil N2O emission from a lowland tropical rainforest. We identified ten 0.25 ha plots (assemblages of 40–70 individual trees) in which average remotely‐sensed canopy N fell above or below the regional mean. The plots were located on a single minimally‐dissected terrace (km2) where soil type, vegetation structure and climatic conditions were relatively constant. We measured N2O fluxes monthly for 1 yr and found that high canopy N species assemblages had on average three‐fold higher total mean N2O fluxes than nearby lower canopy N areas. These differences are consistent with strong differences in litter stoichiometry, nitrification rates and soil nitrate concentrations. Canopy N status was also associated with microbial community characteristics: lower canopy N plots had two‐fold greater soil fungal to bacterial ratios and a significantly lower abundance of ammonia‐oxidizing archaea, although genes associated with denitrification (nirS, nirK, nosZ) showed no relationship with N2O flux. Overall, landscape emissions from this ecosystem are at the lowest end of the spectrum reported for tropical forests, consist with multiple metrics indicating that these highly productive forests retain N tightly and have low plant‐available losses. These data point to connections between canopy and soil processes that have largely been overlooked as a driver of denitrification. Defining relationships between remotely‐sensed plant traits and soil processes offers the chance to map these processes at large scales, potentially increasing our ability to predict N2O emissions in heterogeneous landscapes