Luitgard Schwendenmann

Spatial and temporal variation in soil CO2 efflux in an old-growth neotropical rain forest, La Selva, Costa Rica

Our objectives were to quantify and compare soil CO2 efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO2 release. We measured soil CO2 efflux from eight permanent soil chambers on six Oxisol sites. Three sites were developed on old river terraces (‘old alluvium’) and the other three were developed on old lava flows (‘residual’). At the same time we measured soil CO2 concentrations, soil water content and soil temperature at various depths in 6 soil shafts (3 m deep). Between ‘old alluvium’ sites, the two-year average CO2 flux rates ranged from 117.3 to 128.9 mg C m−2 h−1. Significantly higher soil CO2 flux occurred on the ‘residual’ sites (141.1 to 184.2 mg C m−2 h−1). Spatial differences in CO2 efflux were related to fine root biomass, soil carbon and phosphorus concentration but also to soil water content. Spatial variability in CO2 storage was high and the amount of CO2 stored in the upper and lower soil profile was different between ‘old alluvial’ and ‘residual’ sites. The major factor identified for explaining temporal variations in soil CO2 efflux was soil water content. During periods of high soil water content CO2 emission decreased, probably due to lower diffusion and CO2 production rates. During the 2-year study period inter-annual variation in soil CO2 efflux was not detected.

Soil water uptake by trees using water stable isotopes (δ2H and δ18O)−a method test regarding soil moisture, texture and carbonate

AimsStable isotopes of oxygen and hydrogen are often used to determine plant water uptake depths. We investigated whether and to what extend soil moisture, clay content, and soil calcium carbonate influences the water isotopic composition.MethodsIn the laboratory, dried soil samples varying in clay content were rewetted with different amounts of water of known isotopic composition. Further, we removed soil carbonate from a subset of samples prior to rewetting. Water was extracted from samples via cryogenic vacuum extraction and analysed by mass spectrometry.ResultsThe isotopic composition of extracted soil water was similarly depleted in both 18O and 2H with decreasing soil moisture and increasing clay and carbonate content. Soil carbonate changed the δ18O composition while δ2H was not affected.ConclusionsOur results indicate that soil carbonate can cause artifacts for 18O isotopic composition of soil water. At low soil moisture and high carbonate content this could lead to conflicting results for δ18O and δ2H in plant water uptake studies.

Linking variability in soil solution dissolved organic carbon to climate, soil type, and vegetation type

Lateral transport of carbon plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. There is, however, a lack of information on the factors controlling one of the main C sources of this lateral flux, i.e., the concentration of dissolved organic carbon (DOC) in soil solution across large spatial scales and under different soil, vegetation, and climate conditions. We compiled a database on DOC in soil solution down to 80 cm and analyzed it with the aim, first, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil, and vegetation types at global scale and second, to identify potential determinants of the site-to-site variability of DOC concentration in soil solution across European broadleaved and coniferous forests. We found that DOC concentrations were 75% lower in mineral than in organic soil, and temperate sites showed higher DOC concentrations than boreal and tropical sites. The majority of the variation (R2 = 0.67–0.99) in DOC concentrations in mineral European forest soils correlates with NH4+, C/N, Al, and Fe as the most important predictors. Overall, our results show that the magnitude (23% lower in broadleaved than in coniferous forests) and the controlling factors of DOC in soil solution differ between forest types, with site productivity being more important in broadleaved forests and water balance in coniferous stands.

Aboveground tree growth varies with belowground carbon allocation in a tropical rainforest environment.

Young secondary forests and plantations in the moist tropics often have rapid rates of biomass accumulation and thus sequester large amounts of carbon. Here, we compare results from mature forest and nearby 15–20 year old tree plantations in lowland Costa Rica to evaluate differences in allocation of carbon to aboveground production and root systems. We found that the tree plantations, which had fully developed, closed canopies, allocated more carbon belowground - to their root systems - than did mature forest. This increase in belowground carbon allocation correlated significantly with aboveground tree growth but not with canopy production (i.e., leaf fall or fine litter production). In contrast, there were no correlations between canopy production and either tree growth or belowground carbon allocation. Enhanced allocation of carbon to root systems can enhance plant nutrient uptake, providing nutrients beyond those required for the production of short-lived tissues such as leaves and fine roots, and thus enabling biomass accumulation. Our analyses support this deduction at our site, showing that enhanced allocation of carbon to root systems can be an important mechanism promoting biomass accumulation during forest growth in the moist tropics. Identifying factors that control when, where and for how long this occurs would help us to improve models of forest growth and nutrient cycling, and to ascertain the role that young forests play in mitigating increased atmospheric carbon dioxide.

Biotic and abiotic controls on diurnal fluctuations in labile soil phosphorus of a wet tropical forest

The productivity of many tropical wet forests is generally limited by bioavailable phosphorus (P). Microbial activity is a key regulator of P availability in that it determines both the supply of P through organic matter decomposition and the depletion of bioavailable P through microbial uptake. Both microbial uptake and mineralization occur rapidly, and their net effect on P availability varies with soil moisture, temperature, and soil organic matter quantity and quality. Exploring the mechanisms driving P availability at fine temporal scales can provide insight into the coupling of carbon, water, and nutrient cycles, and ultimately, the response of tropical forests to climate change. Despite the recognized importance of P cycling to the dynamics of wet tropical forests and their potential sensitivity to short-term fluctuations in bioavailable P, the diurnal pattern of P remains poorly understood. This study quantifies diurnal fluctuations in labile soil P and evaluates the importance of biotic and abiotic factors in driving these patterns. To this end, measurements of labile P were made every other hour in a Costa Rican wet tropical forest oxisol. Spatial and temporal variation in Bray-extractable P were investigated in relation to ecosystem carbon flux, soil CO2 efflux, soil moisture, soil temperature, solar radiation, and sap-flow velocity. Spatially averaged bi-hourly (every two hours) labile P ranged from 0.88 to 2.48 microg/g across days. The amplitude in labile P throughout the day was 0.61-0.82 microg/g (41-54% of mean P concentrations) and was characterized by a bimodal pattern with a decrease at midday. Labile P increased with soil CO2 efflux and soil temperature and declined with increasing sap flow and solar radiation. Together, soil CO2 efflux, soil temperature, and sap flow explained 86% of variation in labile P.

Comparison of tree water use characteristics in reforestation and agroforestry stands across the tropics

In the tropics, reforestations and agroforestry become increasingly important and may help mitigate climate change. However, high water use by trees may deplete water resources for associated crops or other purposes. Choice of tree species might reduce water use rates to acceptable levels, but available information on species-specific water use characteristics is scarce. We addressed the following questions: 1) do species differ in xylem sap flux response to fluctuating environmental conditions, 2) are there species-specific differences in quantities of water used, and specifically 3) do universal rules relating tree size to water use apply? This chapter combines data on tree sap flux and water use gathered in Indonesia, Panama and the Philippines. These studies applied the same methods and were conducted in recently established stands (5–12 years old when studied) characterised by small diameter trees and relatively simple stand structure. We analyse data from more than 100 trees belonging to 17 species using a simple sap flux model. Model application suggested species-specific differences in parameters such as maximal sap flux velocity and responses to radiation and vapour pressure deficit. With respect to the quantity of water used per tree, we observed a strong correlation between tree diameter and tree water use, which confirms earlier publications. However, e.g. in the stands in the Philippines where tree diameter explained 65% of observed variation, some species clearly followed distinct trajectories. For a given diameter, up to twofold differences in tree water use among species were observed. Our findings thus support the idea that species selection can be used to control tree water use of future reforestations and within agroforestry Teja Tscharntke, Christoph Leuschner, Edzo Veldkamp, Heiko Faust, Edi Guhardja, Arifuddin Bidin (editors): Tropical rainforests and agroforests under global change: Ecological and socio-economic valuations. Springer Berlin 2010, pp systems. This will be especially relevant in areas where water resources are limited already or where climate scenarios predict decreasing precipitation.

Soil properties of boreal riparian plant communities in relation to natural succession and clear-cutting, Peace River lowlands, Wood Buffalo National Park, Canada

Thirteen study sites on the floodplain and terraces ofthe lower Peace River, Wood Buffalo National Park,Canada, were examined for the effects of naturalforest succession and logging on physical and chemicalsoil properties. The principal changes in substratecharacteristics along the natural successional pathwaywere (i) development of a forest floor and (ii) burialof organic layers by floodwater deposition. Thephysical soil parameters of the forest floor andmineral soil were closely related to floodingfrequency, soil texture, stand closure, and forestfloor build-up. Significant differences in a number ofchemical soil properties between early (Barren Silt,Establishing) and late (Mature, Old-growth)successional stages were evident. The rising influenceof the developing plant community on chemical soilparameters was obvious. Organic carbon, totalnitrogen, cation exchange capacity, exchangeablecalcium, magnesium and potassium and extractablephosphorus level of forest floor and mineral soilswere highest in Mature sites; whereas pH and basesaturation were lowest. Physical and chemical soilproperties at three clear-cut sites were compared tosix undisturbed Mature/Old-growth stands. Removingvegetation had an effect on the physical soilproperties. A significant decrease in forest floornitrogen, cation exchange capacity, calcium andphosphorus occurred due to clear-cutting. Nosignificant change of chemical soil parameters wasobserved in the mineral soil at 0–20 cm depthfollowing harvesting. Significant impacts on chemicalsoil characteristics appeared to be limited to theforest floor. The absence of post-cutting sitepreparations makes the study area a useful benchmarkfor comparison with logged and scarified borealriparian sites elsewhere.

Species assemblage patterns around a dominant emergent tree are associated with drought resistance

Water availability has long been recognized as an important driver of species distribution patterns in forests. The conifer Agathis australis (D. Don) Lindl. (kauri; Araucariaceae) grows in the species-rich forests of northern New Zealand. It is accompanied by distinctive species assemblages, and during summer the soil beneath A. australis is often significantly drier than soils beneath surrounding broadleaved angiosperm canopy species. We used a shade house dry-down experiment to determine whether species that grow close to A. australis differed in drought tolerance physiology compared with species that rarely grow close to A. australis. Stomatal conductance (g(s)) was plotted against leaf water potential (ψ) to identify drought tolerance strategies. Seedlings of species that occur in close spatial association with A. australis (including A. australis seedlings) were most resistant to drought stress, and all displayed a drought avoidance strategy of either declining gs to maintain ψ or simultaneous declines in g(s) and ψ. The species not commonly occurring beneath A. australis, but abundant in the surrounding forest, were the most drought-sensitive species and succumbed relatively quickly to drought-induced mortality with rapidly declining gs and ψ values. These results were confirmed with diurnal measurements of g(s) and assimilation rates throughout the day, and leaf wilting analysis. We conclude that the varied abilities of the species to survive periods of drought stress as seedlings shapes the composition of the plant communities beneath A. australis trees. Furthermore, forest diversity may be impacted by climate change as the predicted intensification of droughts in northern New Zealand is likely to select for drought-tolerant species over drought-intolerant species.

Heavy metal soil pollution is influenced by the location of green spaces within urban settings

Heavy metals are naturally present in soils but are significantly altered by anthropogenic activity which can pose both environmental and human health risks. Sixty green space reserves were chosen in New Zealand’s largest city and separated into six site categories to determine the influence of site location, soil depth, underlying substrate, canopy cover, and distance from roads and central business district on soil heavy metals. Soils in native urban forests had the lowest pools (g m–2) of heavy metals compared with green spaces close to high-traffic, park, school, industrial and residential areas. Least variability in heavy metals was determined among forest sites, with variability increasing in the order: residential, school, industrial, park, and high-traffic sites. Using forest sites as a baseline, a ‘pollution index’ (PI) was established and deemed high for nickel, cadmium and copper and moderate for arsenic, zinc, chromium, lead and mercury. The mean ‘integrated’ PI was high at 3.3 (range 0.3–9.3), indicating elevated levels of soil pollution. The PI was considered a useful technique for interpreting data and complemented traditional ways of reporting concentrations of heavy metals through use of soil pools, which has been limited to date. This study provides important heavy metal data for use in determining where resources may be required to mitigate future risk of increased soil pollution.

Can deuterium tracing be used for reliably estimating water use of tropical trees and bamboo

Reliable estimates of water use by trees and other woody plants are crucial for an improved understanding of plant physiology and for water resource management. Since the 1980s, the thermal dissipation probe (TDP) method has been widely applied in trees, proved to be fairly accurate but is challenging in remote areas. Also in the 1980s, the deuterium (D(2)O or deuterium oxide) tracing method was proposed, which so far has less often been applied. However, deuterium tracing requires less sophisticated equipment in the field and new analytical methods reduce costs and increase sample throughput. The objectives of this study were (i) to compare plant water use estimates of the TDP and D(2)O method and (ii) to determine whether the D(2)O method is appropriate for assessing absolute magnitudes of plant water use. The two methods were employed on five tropical tree species and a bamboo species growing in a reforestation stand in the Philippines and an agroforestry system in Indonesia. For bamboo, an increase in D(2)O values in neighbouring, non-labelled culms suggests that injected D(2)O was partly redistributed among culms, which would seriously limit the accurate estimation of water use for the target culm. For trees, water use estimates resulting from the D(2)O tracing method were proportional to the TDP results (r(2) = 0.85, P < 0.001), but absolute values were, on average, about seven times higher. This overestimation may be due to the assumptions underlying the D(2)O tracing method, such as the conservation of tracer mass, not being met. Further, it cannot be excluded that underestimation of water use by the TDP method contributed partly to the observed difference. However, when considering known sources of error, a large part of the observed difference remains unexplained. Based on our results, the use of the D(2)O tracing method cannot be recommended without further experimental testing if absolute values of whole-plant water use are a major goal. However, the D(2)O tracing method appears suitable for answering other questions, such as relative differences in water use among trees, water redistribution among neighbours and internal water transport and storage processes in plants.