Progress and challenges in using stable isotopes to trace plant carbon and water relations across scales

table isotope analysis is a powerful tool for assessing plant carbon and water relations and their impact on biogeochemical processes at different scales. Our process-based understanding of stable isotope signals, as well as technological developments, has progressed significantly, opening new frontiers in ecological and interdisciplinary research. This has promoted the broad utilisation of carbon, oxygen and hydrogen isotope applications to gain insight into plant carbon and water cycling and their interaction with the atmosphere and pedosphere. Here, we highlight specific areas of recent progress and new research challenges in plant carbon and water relations, using selected examples covering scales from the leaf to the regional scale. Further, we discuss strengths and limitations of recent technological developments and approaches and highlight new opportunities arising from unprecedented temporal and spatial resolution of stable isotope measurements

Carbon isotope discrimination as a tracer of functional traits in a mediterranean macchia plant community

Werner C, Maguas C. Carbon isotope discrimination as a tracer of functional traits in a mediterranean macchia plant community. FUNCTIONAL PLANT BIOLOGY. 2010;37(5):467-477.Characterising functional plant groups with simple robust parameters of structural and functional traits is an important tool in ecological research. The reliability of carbon isotope discrimination (Delta C-13) as an indicator of functional types was assessed in a highly diverse mediterranean macchia comprising drought semi-deciduous malacophylls, evergreen sclerophylls and a gymnosperm. Pronounced differences in Delta C-13 of 4% occurred: semi-deciduous species (Cistus sp. L.) showed the highest and the gymnosperm (Juniperus sp. L.) the lowest Delta C-13 (20.3 +/- 0.5% and 16.2 +/- 0.18%, respectively). Across all studied species, Delta C-13 was correlated with (i) phenology (length of growing period) and (ii) leaf structure (leaf mass and N per area). The correlation of Delta C-13 with leaf water potentials, an indicator of drought stress, was species-specific and only 6 out of 11 species exhibited a significant relationship. Thus, leaf phenology governs seasonal responsiveness of Delta C-13 to drought, which constrains its applicability as an indicator of water use efficiency, particularly in evergreen species with short growing periods. Principal components analysis indicated the robustness of Delta C-13 for the classification of functional groups yielding similar results based on multiple leaf traits or solely on Delta C-13. Hence Delta C-13 provides an ecological tracer of different functional types, integrating structural, functional and phenological attributes

On the use of phloem sap delta C-13 as an indicator of canopy carbon discrimination

Grieve Rascher K, Maguas C, Werner C. On the use of phloem sap delta C-13 as an indicator of canopy carbon discrimination. Tree Physiology. 2010;30(12):1499-1514.In this study we measured delta C-13 in various carbon pools along the basipetal transport pathway in co-occurring Pinus pinaster and Acacia longifolia trees under Mediterranean climate conditions in the field. Overall, species differences in photosynthetic discrimination resulted in more enriched delta C-13 values in the water-conserving overstory P. pinaster relative to the water-spending understory invasive A. longifolia. Post-photosynthetic fractionation effects resulted in differences in delta C-13 of water-soluble organic matter pools along the plant axis with progressive depletion in delta C-13 from the canopy to the trunk (similar to 6.5% depletion in A. longifolia and similar to 0.8% depletion in P. pinaster). Regardless of these fractionation effects, phloem sap delta C-13 in both terminal branches and the main stem correlated well with environmental parameters driving photosynthesis for both species, indicating that phloem sap delta C-13 has potential as an integrative tracer of changes in canopy carbon discrimination (Delta C-13). Furthermore, we illustrate that a simple model based on sap flow estimated canopy stomatal conductance (G(S)) and phloem sap delta C-13 measurements has significant potential as a tool for estimating canopy-level carbon assimilation rates

Understory Invasion by Acacia longifolia Alters the Water Balance and Carbon Gain of a Mediterranean Pine Forest

Grieve Rascher K, Grosse-Stoltenberg A, Maguas C, Werner C. Understory Invasion by Acacia longifolia Alters the Water Balance and Carbon Gain of a Mediterranean Pine Forest. Ecosystems. 2011;14(6):904-919.In water-limited ecosystems, where potential evapotranspiration exceeds precipitation, it is often assumed that plant invasions will not increase total ecosystem water use, because all available water is evaporated or transpired regardless of vegetation type. However, invasion by exotic species, with high water use rates, may potentially alter ecosystem water balance by reducing water available to native species, which may in turn impact carbon assimilation and productivity of co-occurring species. Here, we document the impact of invasion by an understory exotic woody species (Acacia longifolia) in a semi-arid Mediterranean dune pine forest. To quantify the effects of this understory leguminous tree on the water use and carbon fixation rates of Pinus pinaster we compare an invaded and a non-invaded stand. A. longifolia significantly altered forest structure by increasing plant density and leaf area index in the mid-stratum of the invaded forest. A. longifolia contributed significantly to transpiration in the invaded forest (up to 42%) resulting in a slight increase in stand transpiration in the invaded relative to non-invaded forest. More importantly, both water use and carbon assimilation rates of P. pinaster were significantly reduced in the invaded relative to non-invaded stand. Therefore, this study shows that exotic plant invasions can have significant impacts on hydrological and carbon cycling even in water-limited semi-arid ecosystems through a repartitioning of water resources between the native and the invasive species

Critical loads of nitrogen deposition and critical levels of atmospheric ammonia for semi-natural Mediterranean evergreen woodlands

Nitrogen (N) has emerged in recent years as a key factor associated with global changes, with impacts on biodiversity, ecosystems functioning and human health. In order to ameliorate the effects of excessive N, safety thresholds such as critical loads (deposition fluxes) and levels (concentrations) can be established. Few studies have assessed these thresholds for semi-natural Mediterranean ecosystems. Our objective was therefore to determine the critical loads of N deposition and long-term critical levels of atmospheric ammonia for semi-natural Mediterranean evergreen woodlands. We have considered changes in epiphytic lichen communities, one of the most sensitive comunity indicators of excessive N in the atmosphere. Based on a classification of lichen species according to their tolerance to N we grouped species into response functional groups, which we used as a tool to determine the critical loads and levels. This was done for a Mediterranean climate in evergreen cork-oak woodlands, based on the relation between lichen functional diversity and modelled N deposition for critical loads and measured annual atmospheric ammonia concentrations for critical levels, evaluated downwind from a reduced N source (a cattle barn). Modelling the highly significant relationship between lichen functional groups and annual atmospheric ammonia concentration showed the critical level to be below 1.9 μg m−3, in agreement with recent studies for other ecosystems. Modelling the highly significant relationship between lichen functional groups and N deposition showed that the critical load was lower than 26 kg (N) ha−1 yr−1, which is within the upper range established for other semi-natural ecosystems. Taking into account the high sensitivity of lichen communities to excessive N, these values should aid development of policies to protect Mediterranean woodlands from the initial effects of excessive N

Community scale 15N isoscapes: tracing the spatial impact of an exotic N2-fixing invader

Grieve Rascher K, Hellmann C, Maguas C, Werner C. Community scale 15N isoscapes: tracing the spatial impact of an exotic N2-fixing invader. Ecology Letters. 2012;15(5):484-491.Plantplant interactions are key processes shaping plant communities, but methods are lacking to accurately capture the spatial dimension of these processes. Isoscapes, i.e. spatially continuous observations of variations in stable isotope ratios, provide innovative methods to trace the spatial dimension of ecological processes at continental to global scales. Herein, we test the usefulness of nitrogen isoscapes (d15N) for quantifying alterations in community functioning following exotic plant invasion. Nitrogen introduced by an exotic N2-fixing acacia could be accurately traced through the ecosystem and into the surrounding native vegetation by combining native species foliar d15N with spatial information regarding plant location using geostatistical methods. The area impacted by N-addition was at least 3.5-fold greater than the physical area covered by the invader. Thus, downscaling isoscapes to the community level opens new frontiers in quantifying the spatial dimension of functional changes associated with invasion and in resolving the spatial component of within-community interactions

Interpreting post-drought rewetting effects on soil and ecosystem carbon dynamics in a Mediterranean oak savannah

Unger S, Maguas C, Pereira JS, David TS, Werner C. Interpreting post-drought rewetting effects on soil and ecosystem carbon dynamics in a Mediterranean oak savannah. Agricultural and Forest Meteorology. 2012;154-155:9-18.A transient increase in soil carbon dioxide efflux after rewetting of previously dry soils, termed the Birch effect, can significantly influence the ecosystem carbon balance. This has generally been related to increased soil microbial respiration in response to a temporal increase in labile soil carbon. In order to quantify ecosystem carbon losses by the Birch effect and to trace the underlying biogeochemical processes, we monitored the effects of first natural rain pulses on soil and ecosystem carbon dioxide fluxes and their isotopic composition (delta C-13) after an extended summer drought in a Mediterranean oak woodland. While GPP was affected negatively, rain largely enhanced soil respiratory processes, which increased their relative contribution to NEE by up to 95%, resulting in a significant net carbon loss from the ecosystem. This was reflected by high correlation (r 0.91) between the two fluxes. Further, isotopic composition of soil respiration (delta C-13(S)) explained on average 71% of the isotopic composition of ecosystem respiration (delta C-13(R)). We found a strong relationship between soil moisture and the increase in soil respiration (R-S) and NEE, indicating that the Birch effect observed during this study did not result from a transient increase in labile soil carbon but from a gradual (several days) moisture response of size and/or activity of the soil microbial community. We also observed large variation in delta C-13 of soil and ecosystem respired CO2 that corresponded to the rain pulses with enrichment of up to 8 and 6%., respectively, and a subsequent depletion to initial values during the following dry days, which might be explained by increased relative contribution of soil microbial communities from deeper soil layers to overall soil respiration, a switch in the respired carbon source (e.g. anaplerotic carbon) and changes in apparent fractionation during the mineralization of soil organic matter after the depletion of labile carbon pools. (C) 2011 Elsevier B.V. All rights reserved

Impact of an exotic N-2-fixing Acacia on composition and N status of a native Mediterranean community

Hellmann C, Sutter R, Grieve Rascher K, Maguas C, Correia O, Werner C. Impact of an exotic N-2-fixing Acacia on composition and N status of a native Mediterranean community. Acta Oecologica. 2011;37(1):43-50.Nitrogen fixing plant species are among the most invasive species worldwide. However, field studies directly estimating the differential effect of native versus invasive exotic N-2-fixing plants on plant communities are lacking. The exotic leguminous shrub Acacia longifolia invades coastal dunes across Portugal often co-existing with the native N-2-fixer Stauracanthus spectabilis. Moreover, this co-existence with native species is possible due to a relatively low cover of Acacia species particularly in the south of Portugal where drought is intense. In this study we compare the impact of two different legume species (native and exotic) on the nitrogen status of a protected Mediterranean dune system. We document how presence of the exotic invader A. longifolia impacted community structure, soil properties and the foliar nitrogen concentrations and growth rates of native species. A. longifolia invaded areas had decreased biodiversity, fewer seedlings and altered soil properties (e.g., increased soil organic matter, NO3- and NH4+). A. longifolia presence was also associated with significant increases in foliar nitrogen content and delta N-15 of the endemic shrub Corema album while presence of the native legume Stauracanthus spectabilis had no discernible impact on C. album. Furthermore, a strong correlation between increased foliar nitrogen and enhanced growth rates in C. album indicate a facilitative effect of A. longifolia on this native shrub. We posit that the combination of nitrogen fixation, a rapid growth rate and production of a thick litter layer result in a very substantial impact of the invader in invaded ecosystems. (C) 2010 Elsevier Masson SAS. All rights reserved

Seasonal changes in water sources used by woody species in a tropical coastal dune forest

AimsOur aim was to investigate the water sources used by woody species under contrasting water availability and the extent of water-sources-use differentiation among dominant woody species in a tropical coastal dune forest.MethodsWe sampled 15 woody species in a Brazilian restinga forest and, through Bayesian isotope mixing models, we estimated the proportion of water sources used. We tested whether water-sources-use was (i) different between contrasting water availability conditions; (ii) dependent on growth form, plant size or crown illumination; and (iii) influenced by stand density, evenness or biomass.ResultsWe found a seasonal variation in water-sources-use, but no vertical soil-water partitioning among woody species. In wetter periods, plants used mainly water from top-soil, as a shallow water table limited water uptake to top-soil layers recharged with rainwater. Contrastingly, during drier periods, with the absence of rain and a deeper water table, plants generally relied on deeper (50 cm) soil layers. Only under less-wet conditions, a greater evenness and density implied higher water-uptake depth heterogeneity among plants. However, changes in the main water-sources used by plants were neither evoked in more dense or diverse plots, nor induced by plant size.ConclusionsOur study shows that restinga species have dynamic shifts in water-uptake depth caused by seasonal water availability changes, influenced by the combined effect of insufficient moisture at shallow soil layers and water-table lowering in drier periods. These temporal shifts are common among species, implying that restinga woody community has a homogeneous strategy of water-resources acquisition. This study enhances our understanding of the effects that water variations can have on water-resource use in restinga forests437438624154CAPES - Coordenação de Aperfeiçoamento de Pessoal e Nível SuperiorCNPQ - Conselho Nacional de Desenvolvimento Científico e TecnológicoFAPESP – Fundação de Amparo à Pesquisa Do Estado De São Paulosem informação403710/2012-02003/12595-7; 2012/51509-8; 2012/51872-5PPG - Ecologia, Instituto de Biologia, Universidade Estadual de Campinas; Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES) - Cristina Antunes PhD scholarship [PROEX 0229083]; Fundacao para a Ciencia e a Tecnologia (FCT)Portuguese Foundation for Science and Technology [PTDC/AAC-CLI/118555/2010, UID/BIA/00329/2013]; Brazilian National Research Council/CNPq (PELD) [403710/2012-0]; British Natural Environment Research Council/NERCNERC Natural Environment Research Council; State of Sao Paulo Research Foundation/FAPESP as part of the project Functional Gradient within the BIOTA/FAPESP Program - The Biodiversity Virtual Institute [2003/12595-7, 2012/51509-8, 2012/51872-5]; State of Sao Paulo Research Foundation/FAPESP as part of the project PELD/BIOTA within the BIOTA/FAPESP Program - The Biodiversity Virtual Institute [2003/12595-7, 2012/51509-8, 2012/51872-5]; State of Sao Paulo Research Foundation/FAPESP as part of the project ECOFOR within the BIOTA/FAPESP Program - The Biodiversity Virtual Institute [2003/12595-7, 2012/51509-8, 2012/51872-5