Agricultural intensification: will land spared from farming be land spared for nature?
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Agriculture↗
Biomedical subjects
Peter M Vitousek
Publications and source records attributed to Peter M Vitousek.
Temperature influences carbon accumulation in moist tropical forests.
Evergreen broad-leaved tropical forests can have high rates of productivity and large accumulations of carbon in plant biomass and soils. They can therefore play an important role in the global carbon cycle, influencing atmospheric CO2 concentrations if climate warms. We applied meta-analyses to published data to evaluate the apparent effects of temperature on carbon fluxes and storages in mature, moist tropical evergreen forest ecosystems. Among forests, litter production, tree growth, and belowground carbon allocation all increased significantly with site mean annual temperature (MAT); total net primary productivity (NPP) increased by an estimated 0.2-0.7 Mg C x ha(-1) x yr(-1) x degrees C(-1). Temperature had no discernible effect on the turnover rate of aboveground forest biomass, which averaged 0.014 yr(-1) among sites. Consistent with these findings, forest biomass increased with site MAT at a rate of 5-13 Mg C x ha(-1) x degrees C(-1). Despite greater productivity in warmer forests, soil organic matter accumulations decreased with site MAT, with a slope of -8 Mg C x ha(-1) x degrees C(-1), indicating that decomposition rates of soil organic matter increased with MAT faster than did rates of NPP. Turnover rates of surface litter also increased with temperature among forests. We found no detectable effect of temperature on total carbon storage among moist-tropical evergreen forests, but rather a shift in ecosystem structure, from low-biomass forests with relatively large accumulations of detritus in cooler sites, to large-biomass forests with relatively smaller detrital stocks in warmer locations. These results imply that, in a warmer climate, conservation of forest biomass will be critical to the maintenance of carbon stocks in moist tropical forests.
Biomass↗
Persistence of rock-derived nutrients in the wet tropical forests of La Selva, Costa Rica.
We used strontium isotopes and analysis of foliar and soil nutrients to test whether erosion can rejuvenate the supply of rock-derived nutrients in the lowland tropical rain forest of La Selva, Costa Rica. We expected that these nutrients would be depleted from soils on stable surfaces, a result of over one million years of weathering in situ. In fact, trees and palms in all landscape positions derive a relatively high percentage (> or =40%) of their strontium from bedrock, rather than atmospheric, sources. The fraction that is rock-derived increases on slopes, but with no detectable effect on plant macronutrient concentrations. These results differ from those in a similar ecosystem on Kauai, Hawaii, where plants on uneroded surfaces derive almost all of their foliar Sr from atmospheric, rather than bedrock, sources. The results from La Selva challenge the assumption that tropical Oxisols in general have low nutrient inputs from bedrock, and support the hypothesis that erosion can increase the supply of these nutrients in lower landscape positions.
Costa Rica↗
Ground-based and remotely sensed nutrient availability across a tropical landscape.
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 approximately 17% 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.
Ecosystem↗
Remote analysis of biological invasion and biogeochemical change.
We used airborne imaging spectroscopy and photon transport modeling to determine how biological invasion altered the chemistry of forest canopies across a Hawaiian montane rain forest landscape. The nitrogen-fixing tree Myrica faya doubled canopy nitrogen concentrations and water content as it replaced native forest, whereas the understory herb Hedychium gardnerianum reduced nitrogen concentrations in the forest overstory and substantially increased aboveground water content. This remote sensing approach indicates the geographic extent, intensity, and biogeochemical impacts of two distinct invaders; its wider application could enhance the role of remote sensing in ecosystem analysis and management.
Biodiversity↗
Interactive effects of elevated CO2, N deposition and climate change on plant litter quality in a California annual grassland.
Although global changes can alter ecosystem nutrient dynamics indirectly as a result of their effects on plant litter quality, the interactive effects of global changes on plant litter remain largely unexplored in natural communities. We investigated the effects of elevated CO2, N deposition, warming and increased precipitation on the composition of organic compounds in plant litter in a fully-factorial experiment conducted in a California annual grassland. While lignin increased within functional groups under elevated CO2, this effect was attenuated by warming in grasses and by water additions in forbs. CO2-induced increases in lignin within functional groups also were counteracted by an increase in the relative biomass of forbs, which contained less lignin than grasses. Consequently, there was no net change in the overall lignin content of senesced tissue at the plot level under elevated CO2. Nitrate additions increased N in both grass and forb litter, although this effect was attenuated by water additions. Relative to changes in N within functional groups, changes in functional group dominance had a minor effect on overall litter N at the plot level. Nitrate additions had the strongest effect on decomposition, increasing lignin losses from Avena litter and interacting with water additions to increase decomposition of litter of other grasses. Increases in lignin that resulted from elevated CO2 had no effect on decomposition but elevated CO2 increased N losses from Avena litter. Overall, the interactions among elements of global change were as important as single-factor effects in influencing plant litter chemistry. However, with the exception of variation in N, litter quality had little influence on decomposition over the short term.
Biomass↗
Erosion and landscape development affect plant nutrient status in the Hawaiian Islands.
We quantified variation in plant nutrient concentrations and provenance along catenas in landscapes of three different ages (0.15, 1.4, and 4.1 ma) in the Hawaiian Islands. Strontium (Sr) isotopes demonstrate that erosion provides a renewed source of rock-derived nutrients to slopes in landscapes of all ages, in some cases reversing a million years of ecosystem development in a distance of 100 m. However the effects of this input vary with landscape age. Plants on uneroded surfaces in a 0.15-ma landscape derive approximately 20% of their Sr from local bedrock (foliar 87Sr/86Sr approximately 0.7085), while on adjacent slopes this increases to approximately 80% (foliar 87Sr/86Sr approximately 0.7045). Despite this shift in provenance, foliar N and P do not vary systematically with slope position. Conversely, eroded slopes in a 4.1-ma landscape show smaller increases in rock-derived cations relative to stable uplands (foliar 87Sr/86Sr approximately 0.7075 vs 0.7090), but have >50% higher foliar N and P. These results demonstrate both that erosion can greatly increase nutrient availability in older landscapes, and that the ecological effects of erosion vary with landscape age. In addition, there can be as much biogeochemical variation on fine spatial scales in eroding landscapes as there is across millions of years of ecosystem development on stable surfaces.
Geological Phenomena↗
Rapid nutrient cycling in leaf litter from invasive plants in Hawai'i.
Physiological traits that contribute to the establishment and spread of invasive plant species could also have impacts on ecosystem processes. The traits prevalent in many invasive plants, such as high specific leaf areas, rapid growth rates, and elevated leaf nutrient concentrations, improve litter quality and should increase rates of decomposition and nutrient cycling. To test for these ecosystem impacts, we measured initial leaf litter properties, decomposition rates, and nutrient dynamics in 11 understory plants from the Hawaiian islands in control and nitrogen + phosphorus fertilized plots. These included five common native species, four of which were ferns, and six aggressive invasive species, including five angiosperms and one fern. We found a 50-fold variation in leaf litter decay rates, with natives decaying at rates of 0.2-2.3 year(-1) and invaders at 1.4-9.3 year(-1). This difference was driven by very low decomposition rates in native fern litter. Fertilization significantly increased the decay rates of leaf litter from two native and two invasive species. Most invasive litter types lost nitrogen and phosphorus more rapidly and in larger quantities than comparable native litter types. All litter types except three native ferns lost nitrogen after 100 days of decomposition, and all litter types except the most recalcitrant native ferns lost >50% of initial phosphorus by the end of the experiment (204-735 days). If invasive understory plants displace native species, nutrient cycling rates could increase dramatically due to rapid decomposition and nutrient release from invasive litter. Such changes are likely to cause a positive feedback to invasion in Hawai'i because many invasive plants thrive on nutrient-rich soils.
Analysis of Variance↗
Interactions of climate change with biological invasions and land use in the Hawaiian Islands: Modeling the fate of endemic birds using a geographic information system.
The Hawaiian honeycreepers (Drepanidae) represent a superb illustration of evolutionary radiation, with a single colonization event giving rise to 19 extant and at least 10 extinct species [Curnutt, J. & Pimm, S. (2001) Stud. Avian Biol. 22, 15-30]. They also represent a dramatic example of anthropogenic extinction. Crop and pasture land has replaced their forest habitat, and human introductions of predators and diseases, particularly of mosquitoes and avian malaria, has eliminated them from the remaining low- and mid-elevation forests. Landscape analyses of three high-elevation forest refuges show that anthropogenic climate change is likely to combine with past land-use changes and biological invasions to drive several of the remaining species to extinction, especially on the islands of Kauai and Hawaii.
Agriculture↗
Nitrogen and nature.
Anthropogenic changes to the global N cycle are important in part because added N alters the composition, productivity, and other properties of many natural ecosystems substantially. Why does added N have such a large impact? Why is N in short supply in so many natural ecosystems? Processes that slow the cycling of N relative to other elements and processes that control ecosystem-level inputs and outputs of N could cause N supply to limit the dynamics of ecosystems. We discuss stoichiometric differences between terrestrial plants and other organisms, the abundance of protein-precipitating plant defenses, and the nature of the C-N bond in soil organic matter as factors that can slow N cycling. For inputs, the energetic costs of N fixation and their consequences, the supply of nutrients other than N, and preferential grazing on N-fixers all could constrain the abundance and/or activity of biological N-fixers. Together these processes drive and sustain N limitation in many natural terrestrial ecosystems.
Animals↗