Search

Bastin et al.’s estimate (Reports, 5 July 2019, p. 76) that tree planting for climate change mitigation could sequester 205 gigatonnes of carbon is approximately five times too large. Their analysis inflated soil organic carbon gains, failed to safeguard against warming from trees at high latitudes and elevations, and considered afforestation of savannas, grasslands, and shrublands to be restoration., Funding: Supported by the Texas A&M Sid Kyle Global Savanna Research Initiative (T.W.B.); Swiss National Science Foundation (20FI20_173691) (N.B.); Centre National pour la Recherche Scientifique CNRS PICS 2018-2020 (RESIGRASS) (E.B.); CNPq (Brazil, 303179/2016-3) (G.D.); CNPq (Brazil) (G.W.F.); CNPq (Brazil, 303988/2018-5) (A.F.); NASA award NNX17AK14G (F.F.); NSF award 1354943 (W.A.H.); Fundação de Amparo à Pesquisa do Estado de Minas Gerais (Brazil, 2016/13232-5) (S.L.S.); the Office of the Royal Society (IC170015) (C.E.R.L.); CNPq (Brazil, 310345/2018-9) (G.E.O.); the Spanish Government (FIROTIC, PGC2018-096569-B-I00) (J.G.P.); the National Research Foundation (ACCESS, 114695) (N.S.); CNPq (Brazil, 303568/2017-8) (F.A.O.S.); NSF awards 1342703 and 1926431 (C.J.S. and D.M.G.); NSF award EAR-1253713 (C.A.E.S.); Deutsche Forschungsgemeinschaft grant 5579 POEM (V.M.T.); and USDA-NIFA Sustainable Agricultural Systems Grant 12726253 (J.W.V.)

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

International audience

Studies of biodiversity and terrestrial ecosystem functioning have concentrated almost exclusively on temperate grasslands. To broaden the reach of biodiversity-functioning research, five fast growing species, comprising three eudicot trees and two congeneric palms (none symbiotic with nitrogen-fixing microorganisms), were grown for 13 yr in a replacement-series mixture and monocultures on a fertile soil in a high-rainfall area of lowland Costa Rica. The mixture accrued more biomass and had greater net productivity than the average, but not the most productive, monoculture. Relative Land Output (a measure of comparative yield) increased steeply. The combined evidence points to an increase in resource partitioning or facilitation among species over time. Spatial partitioning of aboveground space (for light capture) and soil (possibly for retrieval of deep nitrogen), and facilitation of phosphorus availability by one species, are mechanisms that may account for the inferred complementarity. Extending the generalized findings on biodiversity–productivity relationships from well-studied grasslands to tropical forests is warranted. Mixtures of fast growing trees can out-perform the average of their component monocultures, whether the metric is biomass accrual or productivity. The modular growth of long-lived structure enables arborescent species to retain crown space previously captured and may lead to increased spatial partitioning and facilitation of resources over time. Resumen Aumento en la diferencia de crecimiento a traves del tiempo entre una mezcla y monocultivos de arboles tropicales Los estudios de biodiversidad y funcionamiento de ecosistemas terrestres se han concentrado casi exclusivamente en pastizales templados. A fin de ampliar el alcance de la investigacion biodiversidad-funcionamiento, durante 13 anos se estudio en el tropico humedo de Costa Rica el comportamiento de cinco especies de crecimiento rapido en monocultivos y en una mezcla donde se mantuvo la misma densidad de individuos que en los monocultivos. Las especies fueron tres arboles eudicotas y dos palmeras del mismo genero (ninguna en simbiosis con microorganismos fijadores de N), y el experimento se condujo en un sitio fertil de alta pluviometria. La mezcla de especies acumulo mas biomasa y tuvo mayor productividad neta que el promedio de los monocultivos, pero no mayor que la especie mas productiva. El RLO (una medida de rendimiento comparativo en base al area representada por cada especie) se incremento marcadamente. Los resultados apuntan a un aumento en el tiempo de la particion de recursos o la facilitacion entre especies. Los mecanismos que podrian explicar la complementariedad que se infiere son la particion del espacio aereo (para captar luz) y del suelo (posiblemente para recuperar N de los horizontes mas profundos), y la facilitacion de la disponibilidad de P por una de las especies. Por lo tanto, estos resultados justificarian la extrapolacion a bosques tropicales de las relaciones biodiversidad-productividad comunmente encontradas en estudios de pastizales. Las mezclas de arboles de crecimiento rapido pueden superar al promedio de sus respectivos monocultivos, ya sea como biomasa acumulada o productividad. El crecimiento modular de las estructuras longevas le permite a las especies arborescentes retener el espacio ocupado previamente por la copa, lo que puede conducir al aumento con el tiempo de la particion espacial y de la facilitacion de recursos.

The effects of biodiversity on ecosystem functioning generally increase over time, but the underlying processes remain unclear. Using 26 long-term grassland and forest experimental ecosystems, we demonstrate that biodiversity–ecosystem functioning relationships strengthen mainly by greater increases in functioning in high-diversity communities in grasslands and forests. In grasslands, biodiversity effects also strengthen due to decreases in functioning in low-diversity communities. Contrasting trends across grasslands are associated with differences in soil characteristics.

The reality confronting ecosystem managers today is one of heterogeneous, rapidly transforming landscapes, particularly in the areas more affected by urban and agricultural development. A landscape management framework that incorporates all systems, across the spectrum of degrees of alteration, provides a fuller set of options for how and when to intervene, uses limited resources more effectively, and increases the chances of achieving management goals. That many ecosystems have departed so substantially from their historical tra-jectory that they defy conventional restoration is not in dispute. Acknowledging novel ecosystems need not constitute a threat to existing policy and management approaches. Rather, the development of an integratedapproach to management interventions can provide options that are in tune with the current reality of rapid ecosystem change. Fil: Hobbs, Richard J.. University of Western Australia. School of Plant Biology; Australia Fil: Higgs, Eric. University of Victoria. School of Environmental Studies; Canadá Fil: Hall, Carol M.. University of Victoria. School of Environmental Studies; Canadá Fil: Bridgewater, Peter. Beijing Forestry University. School of Nature Conservation; China Fil: Chapin III, F. Stuart . University of Alaska. Institute of Arctic Biology; Estados Unidos Fil: Ellis, Erle C.. University of Maryland. Geography and Environmental Systems; Estados Unidos Fil: Ewel, John J.. University of Florida. Department of Biology; Estados Unidos Fil: Hallett, Lauren M.. University of California. Department of Environmental Science, Policy and Management; Estados Unidos Fil: Harris, James. Cranfield University. School of Energy, Environment and Agri-Food; Reino Unido Fil: Hulvey, Kristen B.. University of Western Australia. School of Plant Biology; Australia. Utah State University. Department of Wildland Resources and the Ecology Center; Estados Unidos Fil: Jackson, Stephen T.. DOI Southwest Climate Science Center; Estados Unidos Fil: Kennedy, Patricia L.. Oregon State University. Department of Fisheries and Wildlife and Eastern Oregon Agriculture and Natural Resource Program; Estados Unidos Fil: Kueffer, Christoph. Swiss Federal Institute of Technology. Department of Environmental Systems Science. Institute of Integrative Biology; Suiza Fil: Lach, Lori. James Cook University. Centre for Tropical Environmental and Sustainability Science; Australia Fil: Lantz, Trevor C.. University of Victoria. School of Environmental Studies; Canadá Fil: Lugo, Ariel E.. International Institute of Tropical Forestry; Puerto Rico Fil: Mascaro, Joseph. American Association for the Advancement of Science; Estados Unidos Fil: Murphy, Stephen D.. University of Waterloo. Department of Environment and Resource Studies; Canadá Fil: Nelson, Cara R.. University of Montana. College of Forestry and Conservation. Department of Ecosystem and Conservation Sciences; Estados Unidos Fil: Perring, Michael P.. University of Western Australia. School of Plant Biology; Australia Fil: Richardson, David M.. Stellenbosch University. Department of Botany and Zoology. Centre for Invasion Biology; Sudáfrica Fil: Seastedt, Timothy R.. University of Colorado. Department of Ecology and Evolutionary Biology; Estados Unidos Fil: Standish, Rachel J.. University of Western Australia. School of Plant Biology; Australia Fil: Starzomski, Brian M.. University of Victoria. School of Environmental Studies; Canadá Fil: Sudin, Katherine N.. University of California. Department of Environmental Science, Policy and Management; Estados Unidos. University of Colorado. Department of Ecology and Evolutionary Biology; Estados Unidos Fil: Tognetti, Pedro Maximiliano. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Métodos Cuantitativos y Sistemas de Información; Argentina Fil: Yakob, Laith. London School of Hygiene & Tropical Medicine. Department of Disease Control; Reino Unido Fil: Yung, Laurie. University of Montana. College of Forestry and Conservation. Resource Conservation Program; Estados Unidos

Plant or community longevity can strongly influence soil fertility, yet it is seldom among the functional traits considered in studies of biodiversity and ecosystem functioning. For 11 years we tracked the influences of plant longevity, life-form richness, and tree species identity on 12 soil chemical properties in model ecosystems on an allophanic Andisol in the humid lowlands of Costa Rica. The design employed three levels of plant longevity: 1 year and 4 years (trees cut without biomass removal and replanted to same species), and uncut; two levels of life-formdiversity (tree alone, or tree plus palm plus giant perennial herb); and three eudicot, non-nitrogen(N)-fixing tree species. The site?s Andisol proved remarkably resistant to treatment-induced loss of fertility. Although themagnitude of changeswas low, most properties declined during the early phases of plant growth, then stabilized or increased. The greatest declines occurred in stands of shortest life span, where organic matter inputs were low and leaching rates were high. In contrast, massive depositions of organic matter every 4 years sustained or augmented surface-soil cation concentrations, pH, organic carbon (SOC), and extractable phosphorus (P). An increase in diversity from one life form to three led to more SOC and calcium (Ca), whereaspotassium(K) decreaseddue to a species effect: high K uptake by the giant herb. The most notable treespecies effects concerned P: It increased under the species that had thehighest litterfall andmay facilitate apatite weathering; it decreased under the species of highest tissue-N concentrations. Through its effects on soil exposure and organic matter returns, plant longevity exerted greater influence on more soil properties than either diversity or species identity. Fil: Ewel, John J.. University Of Florida. Department Of Chemistry; Estados Unidos Fil: Mazzarino, Maria Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación En Biodiversidad y Medioambiente; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche; Argentina Fil: Celis, Gerardo. University Of Florida. Department Of Chemistry; Estados Unidos

Tropical forests are an important component of the global carbon balance, yet there is considerable uncertainty in estimates of their carbon stocks and fluxes, which are typically estimated through analysis of aboveground biomass in field plots. Remote sensing technology is critical for assessing fine-scale spatial variability of tropical forest biomass over broad spatial extents. The goal of our study was to evaluate relatively new technology, small-footprint, discrete-return lidar and hyperspectral sensors, for the estimation of aboveground biomass in a Costa Rican tropical rain forest landscape. We derived a suite of predictive metrics for field plots: lidar metrics were calculated from plot vertical height profiles and hyperspectral metrics included fraction of spectral mixing endmembers and narrowband indices that respond to photosynthetic vegetation, structure, senescence, health and water and lignin content. We used single- and two-variable linear regression analyses to relate lidar and hyperspectral metrics to aboveground biomass of plantation, managed parkland and old-growth forest plots. The best model using all 83 biomass plots included two lidar metrics, plot-level mean height and maximum height, with an r 2 of 0.90 and root-mean-square error (RMSE) of 38.3 Mg/ha. When the analysis was constrained to plantation plots, which had the most accurate field data, the r 2 of the model increased to 0.96, with RMSE of 10.8 Mg/ha (n = 32). Hyperspectral metrics provided lower accuracy in estimating biomass than lidar metrics, and models with a single lidar and hyperspectral metric were no better than the best model using two lidar metrics. These results should be viewed as an initial assessment of using these combined sensors to estimate tropical forest biomass; hyperspectral data were reduced to nine indices and three spectral mixture fractions, lidar data were limited to first-return canopy height, sensors were flown only once at different seasons, and we explored only linear regression for modeling. However, this study does support conclusions from studies at this and other climate zones that lidar is a premier instrument for mapping biomass (i.e., carbon stocks) across broad spatial scales.

We examined the growth response of Acacia koa Gray (koa) in dense (10 000 trees/ha), ~25-year-old, second-growth stands on Hawai‘i to crown release (thinning) and grass control. Koa diameter increment increased within 2 months of thinning across the range of residual densities (200–900 trees/ha). After 3 years, diameter increment in the most intense thinning treatment was 180% greater than in the control treatment (1.4 vs. 0.5 cm/year); in the least intense thinning treatment diameter increment was 100% greater than the control (1.0 vs. 0.5 cm/year). Koa’s growth response was independent of all measures of tree size (diameter at breast height, height, and leaf area) across the range of tree sizes sampled. A grass control treatment to determine if reduced competition for soil water would improve koa growth showed no improvement in growth response. Koa phyllode δ13C values, which represent an integrated measure of tree water status, showed no evidence of tree water stress (mean δ13C = –28.3‰) and were not significantly different between the grass removal and control treatments. These results demonstrate that koa is capable of responding quickly and positively to crown release despite years of suppressed growth and suggest that landowners interested in managing koa forests may have considerable flexibility in the timing and application of thinning treatments to koa stands.

Four tree species were harvested periodically over a 13-year period from plantations in the humid lowlands of Costa Rica: Cedrela odorata, Cordia alliodora, Hyeronima alchorneoides, and Euterpe oleracea. The soil was a well-drained, volcanic alluvium, and high fertility coupled with 4 m of annual rainfall and high temperatures led to rapid growth rates; at age 13 many individual were >30 cm dbh and >30 m tall. Harvested trees were dissected into their component parts: leaves, rachises (for Cedrela and Euterpe), branches, boles, and coarse roots (i.e., >0.5 cm diameter). Roots of small trees were excavated in their entirety; those of large trees were harvested from a cylinder 1.0 m in diameter, immediately beneath the trunk. Large numbers of trees were sampled: 258–379 per species. Size classes sampled ranged from seedlings too small to have a dbh to trees of � 30 cm dbh. Two separate allometric equations (one for trees having only a basal diameter and another for trees having a dbh), with diametersquared times height as the metrics, were developed for each component of each species. Based on breaks in linear trends of ln–ln plots and deviations of predicted from actual values, we developed separate allometric equations, by component, for trees of different sizes. The resulting 40 equations (with one exception, involving very small trees) fit the data well and enable the user to predict biomass, by component, for each of the four species. A single (non-allometric) linear equation, combining all plant parts of all three dicot species, also fit the data well, but it would not provide either the detail or the accuracy provided by the species-specific, plant-part-specific equations. Large sample sizes, a 13-year run of data collection, and the economic and ecological importance of the species studied make this data set uniquely useful for biomass estimations and for understanding the inherent heterogeneity of tree structure in dynamic tropical environments. # 2006 Elsevier B.V. All rights reserved.

Among the many factors that potentially influence the rate at which nitrogen (N) becomes available to plants in terrestrial ecosystems are the identity and diversity of species composition, frequency of disturbance or stand turnover, and time. Replicated suites of investigator-designed communities afforded an opportunity to examine the effects of those factors on net N mineralization over a 12-year period. The communities consisted of large-stature perennial plants, comprising three tree species (Hyeronima alchorneoides, Cedrela odorata, and Cordia alliodora), a palm (Euterpe oleracea), and a large, perennial herb (Heliconia imbricata). Trees were grown in monoculture and in combination with the other two life-forms; tree monocultures were subjected to rotations of one or four years, or like the three-life-form systems, left uncut. The work was conducted on fertile soil in the humid lowlands of Costa Rica, a site with few abiotic constraints to plant growth. Rates of net N mineralization and nitrification were high, typically in the range of 0.2-0.8 microg x g(1) x d(-1), with net nitrification slightly higher than net mineralization, indicating preferential uptake of ammonium (NH4+) by plants and microbes. Net rates of N mineralization were about 30% lower in stands of one of the three tree species, Hyeronima, than in stands of the other two. Contrary to expectations, short-rotation management (one or four years) resulted in higher net rates of N mineralization than in uncut stands, whether the latter were composed of a single tree species or a combination of life-forms. Neither additional species richness nor replenishment of leached N augmented mineralization rates. The net rate at which N was supplied tended to be lowest in stands where demand for N was highest. Careful choice of species, coupled with low frequency of disturbance, can lead to maintenance of N within biomass and steady rates of within-system circulation, whereas pulses, whether caused by cutting and replanting or by the phenological traits of the species selected or combined, subject N supplies to leaching loss.

We e xplore the issues relevant to those types of ecosystems containing new combinations of species that arise through human action, environmental change, and the impacts of the deliberate and inadvertent introduction of species from other regions. Novel ecosystems (also termed ‘emerging ecosystems’) result when species occur in combinations and relative abundances that have not occurred previously within a given biome. Key characteristics are novelty, in the form of new species combinations and the potential for changes in ecosystem functioning, and human agency, in that these ecosystems are the result of deliberate or inadvertent human action. As more of the Earth becomes transformed by human actions, novel ecosystems increase in importance, but are relatively little studied. Either the degradation or invasion of native or ‘wild’ ecosystems or the abandonment of intensively managed systems can result in the formation of these novel systems. Important considerations are whether these new systems are persistent and what values they may have. It is likely that it may be very difficult or costly to return such systems to their previous state, and hence consideration needs to be given to developing appropriate management goals and approaches.

The establishment and management of tropical plantations has the potential to significantly alter patterns in nitrogen (N) cycling relative to old-growth tropical forests, which are generally characterized by high N availability and large fluxes of nitrous oxide (N2O), an important greenhouse gas. We used 15 N tracer additions to examine the effects of plantation establishment and management on gross N-cycling rates, N retention via microbial assimilation and dissimilatory nitrate (NO 3 2 ) reduction to ammonium (NH4 1 ) (DNRA), and N losses via N2O emissions. In general, plantations had lower rates of N cycling and increased potential for N losses compared to old-growth forests, but there were few differences between very short (one-year) rotation and 10-yr-old uncut plantations. Gross N mineralization declined by almost 50% in the plantations compared to the old- growth forests, and much of the mineralized N was nitrified at all sites. Gross nitrification rates were more variable and did not differ between old-growth forests and unfertilized plantations; however, fertilization increased gross nitrification by a factor of 6 in short- rotation forests, signaling a potential mechanism for increased N losses via leaching and gaseous emissions. Old-growth forests had significantly higher microbial biomass N and NH4 1 assimilation rates. No microbial N assimilation was measured in the plantation soils, nor was there evidence of gross NH4 1 immobilization from estimates of NH4 1 consumption and nitrification. Plantations and old-growth forests had similar DNRA rates (0.23 mg·g 21 ·d 21 ), which retains N in the ecosystem, and plantations had lower N2O emissions. Nitrous oxide fluxes from plantations were highly sensitive to reducing conditions, high- lighting the potential for high rates of N 2O losses. Our results show that plantation estab- lishment can decrease rates of N cycling, but once forests are converted to plantations, internal N-cycling pathways and N2O fluxes are relatively resistant to disturbance associated with short rotation length.

We studied the relationships among plant and arbuscular mycorrhizal (AM) fungal diversity, and their effects on ecosystem function, in a series of replicate tropical forestry plots in the La Selva Biological Station, Costa Rica. Forestry plots were 12 yr old and were either monocultures of three tree species, or polycultures of the tree species with two additional understory species. Relationships among the AM fungal spore community, host species, plant community diversity and ecosystem phosphorus-use efficiency (PUE) and net primary productivity (NPP) were assessed. Analysis of the relative abundance of AM fungal spores found that host tree species had a significant effect on the AM fungal community, as did host plant community diversity (monocultures vs polycultures). The Shannon diversity index of the AM fungal spore community differed significantly among the three host tree species, but was not significantly different between monoculture and polyculture plots. Over all the plots, significant positive relationships were found between AM fungal diversity and ecosystem NPP, and between AM fungal community evenness and PUE. Relative abundance of two of the dominant AM fungal species also showed significant correlations with NPP and PUE. We conclude that the AM fungal community composition in tropical forests is sensitive to host species, and provide evidence supporting the hypothesis that the diversity of AM fungi in tropical forests and ecosystem NPP covaries.

Blanket condemnation of alien species in restoration efforts is counterproductive. Where their presence does not unduly threaten surrounding ecosystems, alien species can be tolerated or even used to good advantage, if they provide essential ecological or socioeconomic services. By speeding restoration or making it more effective, non-native species can provide economic and ecological payoffs. Risk is always an issue when alien species are involved, but greater risk taking is warranted where environmental conditions have been severely modified through human activity than where reassembly of a biological community is the sole goal of restoration.

The tropical emergent tree Hyeronima alchorneoides has large decreases in leaf size with tree age: 1200 cm(2) at 1 yr, 900 cm(2) at 3 yr, 200 cm(2) at 11 yr, and 80 cm(2) in old (>30 yr) individuals. We tracked leaf growth and physiological attributes on trees of three different ages (1, 3, and 11 yr) to determine the developmental basis and functional consequences of this variation. Leaves on young trees grew faster and sustained maximum rates of leaf expansion longer than leaves on older trees. Leaf mass per area (LMA) did not differ among age classes. Maximum photosynthetic rates reflected differences in leaf nitrogen concentration, in which leaves from the lower crown of younger trees outperformed those at a comparable crown position in older trees. One-year-old trees had the lowest stomatal conductance and the greatest instantaneous water use efficiency. Ontogenetic plasticity in mature leaf size, structure, and physiology may be a balance between the advantages conferred by rapid height growth when trees are young and the benefits derived from producing branches that increase light harvesting ability as trees reach the canopy.

Extensive areas of species-rich forests in the tropics have been replaced by tree monocultures over the last two decades, and the impact on biogeochemical cycles is unclear. We characterized effects on soil carbon dynamics of species identity and rotation frequency in experimental plantations containing three native, non-N-fixing tree species, Hyeronima alchoreoides, Cedrela odorata, and Cordia alliodora, grown in monocultures and in polycultures with two monocot species, Euterpe oleracea and Heliconia imbricata. Over all treatments, change in total soil organic carbon (TSOC, 0–15 cm) after 10 years ranged from a loss of 24% (0.9 mg/ha in 1-yr rotation of Cedrela) to an increase of 14% (0.6 mg/ha under Hyeronima polycultures). Species differed in their effects on quantities of TSOC (P = 0.038), but differences were more pronounced in light particulate organic matter (LPOM; P = 0.001), a biologically active, sand-size soil fraction that constituted 6% of TSOC. Effects of rotation frequency were strong; in ...

In the humid tropics large quantities of nutrients can be rapidly leached when the soil is unprotected by actively growing vegetation. We established experimental plan- tations of three indigenous tree species on a fertile Andisol in Costa Rica and managed them under 1- or 4-year cutting cycles with uncut stands as controls. Our goals were to test whether nutrient leaching was greatest under a regime of frequent disturbances that returned modest amounts of biomass to the soil surface (cutting and replanting on a 1-yr cycle) or less frequent disturbances that returned significantly greater amounts of plant tissues to the soil (cutting and replanting on a 4-yr cycle), and to compare those cutting cycles with nutrient leaching from uncut stands. Leaching of NO3 (over 9 yr), Ca2+, Mg2+, and K+ (over 4 yr) from upper soil horizons was monitored. Water balance was determined by linking Penman-Monteith evaporation with changes in soil water storage modeled from soil physical parameters. Drainage water for solute measurement was sampled from porous ceramic cups at 1.1 m depth in the soil. Disturbance frequency proved to be an important determinant of NO3- leaching. Average long-term NO3- leaching losses from stands on a 1-yr cutting cycle were extraordinarily large: 442 mmol_-m-2.yr-1 (62 kg-ha-1.yr-1 of N), compared to 187 mmolc.m-2.yr-1 under a 4-yr cutting cycle and 71 mmolc.m-2.yr-1 from uncut stands. Elevated NO3- leaching was primarily due to increased concentration in the soil solution (rather than increased water drainage), because cutting usually resulted in a reduction of

Ecosystem nutrient use efficiency–the ratio of net primary productivity to soil nutrient supply–is an integrative measure of ecosystem functioning. High productivity and nutrient retention in natural systems are frequently attributed to high species diversity, even though some single-species systems can be highly productive and effective at resource capture. We investigated the effects of both individual species and life-form diversity on ecosystem nutrient use efficiency using model tropical ecosystems comprised of monocultures of three tree species and polycultures in which each of the tree species was coplanted with species of two additional life forms. Tree species significantly influenced nutrient use efficiency by whole ecosystems in monocultures; however, in polycultures, the additional life forms interacted with the influence exerted by the dominant tree. Furthermore, the presence of the additional life forms significantly increased nutrient uptake and uptake efficiency, but in only two of the three systems and 2 of the 4 years of the study period. These results indicate that the effect of life-form diversity on ecosystem functioning is not constant and that there may be temporal shifts in the influence exerted by different components of the community. Furthermore, although species (and life forms) exerted considerable influence on ecosystem nutrient use efficiency, this efficiency was most closely related to soil nutrient availability. These findings demonstrate that ecosystem nutrient use efficiency is an outcome not only of the characteristics of the species or life forms that comprise the system but also of factors that affect soil nutrient supply. The results argue against the simple upward scaling of nutrient use efficiency from leaves and plants to ecosystems.

The silent invasion of Hawaii by insects, disease organisms, snakes, weeds and other pests is the single greatest threat to Hawaii’s economy and natural environment.... Even one new pest-like the brown tree snake--could forever change the character of our islands. (Coordinating Group on Alien Pest Species 1996, P. 1). Reforestation in the tropics is so vastly behind deforestation that we cannot wait to fully appraise all the potential negative elements of domestication. Weediness is of consequence perhaps in Honolulu, but not in Addis or Delhi. (James Brewbaker, quoted by Hughes 1994, p. 244 ).

Natural ecosystems, whose components are the results of natural selection, are sustainable; most are productive, responsive to pests, and retentive of nutrients. Thus, they are appropriate models on which to base the design of new systems of land use. Abiotic and biotic stressors are related non-linearly; the nadir of total stress being mid-way along a gradient of environmental harshness. Superimposing the stress functions on Holdridge's life zone chart yields four broad categories of environments for agriculture: climates where annual rainfall is similar to potential evapotranspiration, plus three other categories that are either too cold, too arid, or too wet. Extremely cold lands have no potential for agriculture. Lands that are arid or infertile can be used successfully, although the cost of compensating for environmental limitations increases exponentially with increasing abiotic stress. Grazing animals (which act as trophic buffers between people and environment) have proven successful in dry and infertile environments. The humid tropical lowlands epitomise environments of low abiotic stress but overwhelming biotic intricacy. Here it pays to imitate natural systems rather than struggle to impose simplicity on ecosystems that are inherently complex. The keys to success are to (i) channel productivity into outputs of nutritional and economic importance, (ii) maintain adequate diversity to compensate for losses in a system simple enough to be horticulturally manageable, (iii) manage plants and herbivores to facilitate associational resistance and not associational susceptibility, and (iv) use perennial plants to maintain soil fertility, guard against erosion, and make full use of resources.

The Hawaiian archipelago, the most isolated on Earth, has proven to be especially vulnerable to invasions by non-indigenous species. Alien species now outnumber natives and the threat they pose is in part responsible for the fact that approximately 25% of Hawaii's native flora, 90% of which is endemic, has been listed as threatened or endangered. To assess the effect of stand characteristics on the colonization of the plantations by alien and native species, we measured basal area, leaf area index (LAI), litter depth and standing litter mass in 26- to 32-year-old plantations of Eucalyptus saligna, Flindersia brayleyana and Fraxinus uhdei. The plantations are surrounded by native Hawaiian rainforest which has been invaded by numerous non-indigenous species. The basal area of the planted Flindersia was 45.2 m2/ha, three to four times that of the other two plantation species. Nevertheless, when colonizing species were included, total stand basal area and LAI did not differ significantly across the three plantation types. Litter depth ranged from 3.7 cm for Flindersia to 4.7 cm for Eucalyptus, and litter mass ranged from 7.4 Mg/ha for Fraxinus to 10.5 Mg/ha for Eucalyptus. Of 51 species identified in the plantation understories, 23 (45%) were common to all three plantation species. Eucalyptus plantations had 42 species, seven of which were uniquely found there, Fraxinus had 39 species, six of which were unique and Flindersia had 36 species, only one of which was unique. The most abundant understory species under Fraxinus were Cibotium glaucum and Metrosideros polymorpha, two dominant components of native Hawaiian rainforest. In contrast, the most abundant understory species in the Eucalyptus plantations was the alien tree, Psidium cattleianum and the understory of Flindersia was dominated by its own offspring. Average species richness (number of species per plot) of both native species and all species combined was significantly greater under Fraxinus than under the other two plantation species, but all plantations contained approximately equal numbers of non-indigenous species. Average density (number of individuals/m2) was greatest under Flindersia, although the density of native species alone was greatest under Fraxinus. Species richness and plant density were not correlated with litter depth or mass, but total species richness and density of native species did decrease with increasing LAI. In general, abundance of native plants in the understory decreased with increasing abundance of non-indigenous species and increasing basal area of plantation species.

The diversity of plant life forms in tropical forests affords the opportunity for assembly of plant combinations, both natural and managed, that make full use of resources and sustain high productivity. The influence of combining life forms on productivity and resource use was evaluated using three fast-growing tree species (Hyeronima alchorneoides, Cedrela odorata, and Cordia alliodora); each species was grown alone and with two perennial, large-stature, self-supporting monocots (Euterpe oleracea and Heliconia imbricata). Aboveground net primary productivity was extremely high in all stands. The monocots did not contribute significantly to the productivity of the Hyeronima polycultures, which was 4.5–8.4 g·m−2·d−1 between 18 and 36 mo. In contrast, the monocots accounted for 57% of the productivity (9.7 g·m−2·d−1) of Cedrela polycultures and 67% of the productivity (6.8 g·m−2·d−1) of the Cordia polycultures, by age 3 yr. The leaf area and density of fine roots in the Cedrela and Cordia polycultures were ...

Islands have much to offer our understanding of novel ecosystems, for it is there that novel assemblages of species have developed most rapidly and dramatically. This chapter on island ecosystem novelty considers non-continental land masses currently surrounded by ocean, whether that has been true throughout their geological history or not. It briefly discusses the three main factors that influence island ecosystem novelty: physical geography, biogeography and human ecology. It aims to provide some context for development of the main topic of interest in keeping with the principal subject matter of this book: when and how to intervene in novel ecosystems. The topic of intervention is discussed in relation to need, barriers and feasibility. Finally, the chapter presents five lessons from islands and elsewhere which have particularly widespread applicability.

An ecosystem containing a mixture of species that differ in phenology, morphology, and physiology might be expected to resist leaching of soil nutrients to a greater extent than one composed of a single species. We tested the effects of species identity and plant-life-form richness on nutrient leaching at a lowland tropical site where deep infiltration averages (2 m year -1 . Three indigenous tree species with contrasting leafing phenolo- gies (evergreen, dry-season deciduous, and wet-season deciduous) were grown in monoculture and together with two other life-forms with which they commonly occur in tropical forests: a palm and a giant, perennial herb. To calculate nutrient leaching over an 11-year period, con- centrations of nutrients in soil water were multiplied by drainage rates estimated from a water balance. The effect of plant-life-form richness on retention differed according to tree species identity and nutrient. Nitrate retention was greater in polycultures of the dry-season deciduous tree species (mean of 7.4 kg ha -1 year -1 of NO3-N lost com- pared to 12.7 in monoculture), and calcium and magnesium retention were greater in polycultures of the evergreen and wet-season deciduous tree species. Complementary use of light led to intensification of soil exploitation by roots, the main agent responsible for enhanced nutrient retention in some polycultures. Other mechanisms included differences in nutrient demand among species, and avoidance of cat- astrophic failure due to episodic weather events or pest outbreaks. Even unrealistically simple multi-life-form mimics of tropical forest can safeguard a site's nutrient capital if careful attention is paid to species' characteristics and temporal changes in interspecific interactions.

Chance events such as seed dispersal determine the potential composition of plant communities, but the eventual assemblage is determined in large part by subsequent interactions among species. Postcolonization sorting also affects the ultimate composition of communities assembled by people for restoration, horticulture, or conservation. Thus, knowledge of the mechanisms controlling interspecific interactions in plant communities is important for explaining patterns observed in nature and predicting success or failure of utilitarian combinations. Relationships among species, especially those from studies of biological diversity and ecosystem functioning, are largely based on studies of short-lived, temperate-zone plants. Extrapolation to perennial plants in the humid tropics is risky because functional relationships among large-stature species change with time. Shifts in competitive relationships among 3 life forms—trees, palms, and perennial herbs—occurred during 13 yr in experimental tropical ecosystems. In 2 cases the novel competitive mechanism responsible for the shift was reduction in crown volume, and therefore light-capturing capability, of overtopping deciduous trees by intrusive growth from below a palm. In a third case, complementary resource use developed between 2 evergreen life forms (overstory tree and palm), probably because of differential nutrient acquisition. Species-level traits and adequate time for shifts in interspecific relationships to emerge are crucial for predicting community trajectories.

Some interactions between plants are uniquely conspicuous elements of certain tropical forests; the giant lianas that wend through the canopy and the epiphyte-laden branches of cloud forests are striking examples. Nevertheless, the fundamental processes involved are no different from those in extra-tropical communities, even though diverse, sometimes uniquely tropical, mechanisms may be involved. An individual of one plant species interacting with an individual of a second plant species can lead to any of the same five outcomes at any latitude, and these consist of all combinations of negative, positive and neutral effects (except the non-interaction described by the mutually neutral interaction, 0/0). But interactions among plants in forests seldom involve such simple one-on-one relationships. More commonly, multiple players are involved and the interactions change with time: the liana binds crowns of several trees, the fallen palm frond damages multiple seedlings, and the solum is shared by roots of many species. Furthermore, positive and negative interactions occur simultaneously, so the observer sees only an integrated net effect of multiple interactions (Holmgren et al. 1997).

Heterogeneity and ecosystem processes are closely linked, and together they have implications for conservation. At the scale of communities, life form and temporal patterns may exert stronger control over some ecosystem processes (such as fluxes of energy and materials) than do species. Where conservation involves restoration, every effort should be made to include the original community’s full complement of life forms, phenologies, and seasonality of fire occurrence and water flows.

Life-form was said by Warming (1909) to represent the sum of adaptive characters in a species, and thus is an expression of the harmony between a plant and its environment. This colorful perspective, although too broad to be useful in classification, does highlight the essential point that life-form groupings should be ecologically relevant.

As we seek to understand the relationship between biological diversity and ecosystem processes, we often turn to islands because of their unique biota — impoverished, disharmonic, and alien-infested though it may be, there are still those who love it. Islands can be viewed as natural experiments, and ecosystem processes thereon can be compared with those on continents or other islands having similar geology, topography, and climate. Such comparisons should do much to elucidate the relationships between biota and fluxes of materials and energy, as described in Chapter 14.

For 5 yr we monitored the fertility of a volcanic-ash derived Inceptisol at a site in the humid tropics of Costa Rica. After forest felling and burning, we established four treatments in a randomized block design with six blocks: a sequence of monocultures (two crops of maize [Zea mays] followed by cassava [Manihot esculenta], then the tree species Cordia alliodora), successional vegetation, a mimic of successional vegetation that was physiognomically similar to the model but shared no species with it, and a species-enriched version of successional vegetation. In addition, one plot was maintained free of vegetation. Species-rich successional vegetation was effective at maintaining soil fertility, although we observed general trends of soil-nutrient decline beneath all treatments, presumably because of plant uptake. It proved possible to imitate the fertility-maintaining characteristics of successional vegetation by creating an equally species-rich community of different floristic composition, but the maintenance of fertility was not enhanced by further species enrichment. Successive peaks of nitrate-nitrogen in soil solution, extractable phosphorus, and extractable potassium occurred during the 1st yr, perhaps driven by an early increment of organic matter from postburn debris and roots. Organic matter, total nitrogen, and extractable sulfur were remarkably stable during the 5-yr period. Depletions of cations, decreases in effective cation exchange capacity (CECe ), and increases in acid saturation were related to treatment in the following order: bare soil > monocultures > the three diverse, successional communities. In the bare-soil plot, fertility decreased dramatically: there was a net loss of exchangeable cations and inorganic nitrogen, the phosphorus-fixation capacity increased, and acid saturation reached a potentially toxic 86%. At the start of the study, three of the blocks had soil with lower pH, lower CECe , and higher acid saturation. During the study this less fertile soil lost proportionally more cations and increased more in acid saturation and phosphorus-fixation capacity. The less fertile soil under monocultures proved exceptionally vulnerable to loss of fertility; after 5 yr under monocultures, for example, acid saturation reached 38% in the more fertile soil and 75% in the less fertile soil. In the species-rich communities, however, changes in soil fertility were far less marked.

The effects of shoot damage on starch concentrations in large roots and lower boles of the tropical timber tree, Cedrela odorata, were measured over a 30-day period following mechanical shoot removal. The degree of damage was based on amounts typically destroyed by Hypsipyla grandella, a serious lepidopteran pest of C. odorata. From 14-monthold plantations at La Selva Biological Station, Costa Rica, three treatments were established based on the length of shoot and number of leaves removed: controls (no tissue removed), moderate damage, and severe damage. Initial starch concentrations were compared with concentrations observed 18 to 30 days following shoot removal using enzymatic starch hydrolysis and colorimetry. Root starch concentrations increased in undamaged trees, decreased slightly in the moderately damaged trees, and decreased markedly in severely damaged trees during the sampling period. Starch concentrations in lower boles did not decrease following shoot damage, suggesting a specific role of root starch reserves in responding to aboveground carbon requirements. The dry weights of regrown shoots were similar for the moderately and severely damaged trees, but severely damaged trees produced numerous basal shoots while moderately damaged trees produced only one or two terminal shoots near the point of excision. Decreases in root starch concentrations following shoot damage and rapid shoot regrowth suggest that starch remobilization in roots of C. odorata might provide a survival mechanism after attack by H. grandella.

Warmth, moisture, and constancy are the environmental factors responsible for the potentially high productivity of the humid tropical lowlands. Ironically, they are also the biome's greatest ecological constraints to agriculture. Chemical weathering increases with temperature and is three to six times faster in the tropics than in the temperate zone (125, 250). This frequently leads to the development of deeply weathered soil with scant opportunity for the addition of fresh supplies of nutrients from the bedrock. High temperatures also mean potentially lower net photosynthesis; respiration approximately doubles with a temperature increase of 100 (114, 122, 178), and the oxygen inhibition of photosynthesis in C3 plants increases with temperature (34, 166). Potential evapotranspiration at 280 is about 1.6 m yr-1 (106, 241), yet annual rainfall in the humid tropics commonly exceeds 3 m. The excess runs off across the surface or infiltrates the soil and, if the site is not fully vegetated, may carry nitrate and cations with it. Moreover, fungi, bacteria, and epiphylls, many of which are crop pests, thrive in warm, humid climates. The lack of a harsh season comparable to temperate-zone winter or the annual drought of the seasonally dry tropics means that life, including agricultural pests, can flourish year-round. A seasonal shutdown of growthwhich also depresses pest populationsis an important agricultural subsidy, one that is missing in the less seasonal tropics (36, 117, 242). Many of the constraints imposed by the humid tropical environment can be overcome by supplying fertilizers and pest controls (207), but these amenities require locally scarce and expensive fossil fuels. Given the reality of current

Herbivory by Atta cephalotes was measured in four plant communities of different complexity in Costa Rica. The four communities were a monoculture of cassava {Manihot esculenta) and three diverse assemblages, each 1.5 yr old: (1) successional vegetation, unmodified by the in- vestigators; (2) imitation of succession, a community of investigator-introduced species designed to mimic the unmodified succession; and (3) enriched succession, a successional vegetation that the investigators had augmented by propagule inputs. Each ant colony had access to all four community types simultaneously. The average herbivory rate (daily cutting of leaf per square metre of ground, all treatments combined) was ~ 150 mg (or 38 cm2). In communities with greater leaf area index, structural complexity, and species richness, A. ceph? alotes cut lower portions of total leaf area. Before harvest of the cassava monoculture, the herbivory rate of A. cephalotes was 87.9 cm2m~2d_1 in the monoculture, 21.4 in the imitation, 14.7 in the succession, and 6.8 in the enriched succession community. These amounts represented 0.3% of total leaf area in the monoculture and a mean of 0.03% of total leaf area in the three complex ecosystems. Cassava, which occurred in three of the four communities, was attacked most heavily (per unit leaf area) in the imitation successional community, least heavily in the enriched succession, and at inter- mediate intensity in the monoculture. In response to loss of their preferred forage (cassava) through harvesting by humans, the ants cut more leaf tissue in the three species-rich communities, especially the imitation. As cassava resprouted in the monoculture, A. cephalotes' rate of attack on the three diverse treatments reurned to preharvest levels. Atta cephalotes cut only 17 of 332 available plant species. They cut proportionally more woody than herbaceous species, more introduced species than natural colonizers, and species with below- average water contents. Plant relative abundance alone did not determine host plant selection, but most of the attacked species were cut in proportion to their total leaf area.

Fine and coarse root mass and fine root surface area were studied during 5 yr following the felling and burning of a tropical forest near Turrialba, Costa Rica. Five experimental ecosystems were established: 1) natural successional vegetation, 2) successional vegetation enriched by seed applications, 3) imitation of succession (built by substituting investigator-selected species for natural colonizers), 4) monocultures (two maize crops followed by cassava andCordia alliodora), and 5) a bare plot. Fine roots grew rapidly in all treatments during the first 15 wk, at which time there were 75 gm−2 in the monoculture and 140 gm−2 in the enriched and natural successions. Subsequent growth was slower, and fine-root mass decreased during the first dry season. After 5 yr coarse root mass to a depth of 85 cm was about 800, 1370, and 1530 gm−2 in the succession, enriched succession and imitation of succession, respectively. At the final harvest, the 3.5 yr-oldC. alliodora plantation had 1000 g m−2 of coarse-root biomass. Roots

The vertical distribution of leaf area by species; transmission of photosynthetically active radiation; root biomass and fine-root surface area; and leaf damage were measured in nine tropical ecosystems: six in Costa Rica and three in Mexico. Ecosystems studied included monocultures of maize (young and mature) and sweet potato; year-old natural succession and vegetation designed to mimic succession; a 2.5-year-old mixture of three arborescent perennials (cacao, plantain, Cordia alliodora); 2.7-year-old plantation of Gmelina arborea; coffee shaded by Erythrina poeppigiana; and an old, diverse wooded garden. Leaf area index ranged from 1.0 in young maize to 5.1 in natural succession and the gmelina plantation. The vertical distribution of leaves was most uniform in diverse ecosystems, and most clumped in species-poor ecosystems. Light transmission was inversely proportional to leaf area, and two dense-canopied monocultures (sweet potato and gmelina) were nearly as effective at light capture as were some of the more diverse ecosystems. Optical density of the canopy ranged from 2.0 ( Large roots (> 5 mm diameter) accounted for most root biomass in the older ecosystems at a soil dept of 5–25 cm, and fine roots ( 2.0 m2 m−2 of ground. Total root surface area increased with age and diversity, and the monocultures — even those effective at light capture — had low root surface area. Herbivore damage on leaves of 35 species ranged from 16% of leaf area. Heavily damaged species contributed less to total ecosystem leaf area than did species damaged less than average. Ecosystem-level damage was not well correlated with age or diversity. Leaf damage in all ecosystems ranged from about 2 to 10% of leaf area, or 25 g m−2 of ecosystem. Young monocultures do not necessarily capture less light, provide less soil cover, and experience more herbivory than older, more diverse ecosystems. However, root surface area (and therefore possible nutrient-capture ability) is high only in ecosystems that are diverse or old, and this is an important design consideration for agroecosystems appropriate for the humid tropical lowlands.

Over the last few decades, soil scientists, hydrologists, foresters, and ecologists have become increasingly interested in monitoring downward water and nutrient fluxes in soils. There have been problems with various methods for measuring these fluxes, especially in the field studies on small plots. This paper compares three methods for measuring water and nutrient leaching in the field. These methods provide estimated flux using the Darcy flow equation, a water balance, and zero-tension lysimeters. The methods were compared in an Andept near Turrialba, Costa Rica, during two large storms (33 cm and 22 cm/wk), at two soil depths (25 and 110 cm), and in four kinds of 1- to 2.5-yr-old vegetation. The water balance was the most accurate method, because the other two techniques sample only parts of the flow. Zero-tension lysimeters were evaluated for use in measuring channeled flow under certain conditions.

To investigate the relationship between herbivory and floristic complexity, we measured losses to herbivores in four 0.1- to 4-yr-old tropical ecosystems: (1) unman? ipulated successional vegetation, (2) successional vegetation with higher plant diversity than the unmanipulated succession, (3) an ecosystem of investigator-controlled species composition, designed to imitate the physiognomy and species richness of the successional vegetation, and (4) monocultures of maize, cassava, and Cordia alliodora. We measured herbivory rates (loss of leaf area per day) on dominant plant species in each system and aggregated these over species to estimate rates for plant communities. Although herbivory rates varied widely among species, losses to herbivores in terms of mass of leaf tissue lost per unit of ground area were approximately equal in the four systems, 71.5to78.5gm-2- yr~ *. Ecosystems with greater plant species richness lost a lower proportion of available leaf area and exhibited lower temporal variability in herbivory. Species-rich ecosystems had relatively constant, predictable rates of herbivory due to counterbalancing of low rates on some species with high rates on others. The rate of herbivory on any species was strongly influenced by the nature of the surrounding vegetation. Although surrounding vegetation often conferred protection upon potential pest targets, in some cases a plant species ex- perienced increased susceptibility to herbivores through association with other species.

Impacts of felling, mulching, and burning on budgets of C, N, S, P, K, Ca, and Mg; rates of CO2 evolution from the soil; soil seed storage; and plant growth were evaluated. The felled tropical evergreen forest was 8-9 yr old, interspersed with patches of 70-yr-old forest and had a leaf area index of 6 and aboveground biomass of 5.2 kg/M2. Harvest of the largest trees removed 18% of the S, and >10% of all other elements except N. During the 1 1-wk mulching and drying period, 33% of the K and 13% of the P disappeared, but losses of other elements were 620'C in surface fuels, but soil temperatures were seldom >100IC at 1 cm or >50'C at 2 cm. The burn volatilized 1600 g/m2 C, 49 g/m2 N, and 13 g/m2 S. Postburn wind and water erosion of ash, plus leaching, removed 34 g/m2 N, 20 g/m2 K, 1 g/ m2 P, 39 g/m2 Ca, and 7 g/m2 Mg, but insignificant amounts of C and S. After the burn and onset of the rains, 57% of the initial amount of N and 39% of the initial C still remained because of conservation of the organic-rich upper 3 cm of soil. Soil CO2 evolution was greater from beneath the 11-wk-old slash (3.6 gC m-2 d-1) than from beneath the forest (2.5 gC M-2 d-1), probably because the slash conserved soil moisture better than the actively transpiring forest. After the burn both the burned field and forest soil evolved CO2 at 4.5 gC.m-2 d-i. At this rate, 154 d of decomposition and respiration would release as much C into the atmosphere as did the burn. Soil seed storage dropped from ;8000 seeds/M2 (67 species) in the forest, to 6000 seeds/M2 (51 species) after 11 wk of mulching, to 3000 seeds/m2 (37 species) after the burn. The seeds not killed by the burn, the survival of mycorrhizal fungi, and the release of nutrients resulted in vigorous and diverse postburn regrowth.