Changing ground water levels: the impact on an invasive species and native plant communities in a Mediterranean dune ecosystem
Final Report Abstract
The introduction and spread of invasive species are recognized to be a major threat to biodiversity and ecosystem functioning worldwide. Even though a full understanding of the impact of exotic plant invasions on ecosystem-level processes remains elusive, fluctuations in resource availability through either natural or anthropogenic disturbances are key factors controlling invasibility. Climate change is expected to enhance ecosystem invasibility through changes in resource availability (e.g. water) and the risks of desertification in Mediterranean areas. However, predicting the invasibility and responses of different habitats, even to the same plant invader, remains a challenge. In this project we evaluated specific traits of a characteristic woody plant invader towards competition for limited resources and the consequent alteration of community functioning under decreasing resource availablitiy. We took advantage of large-scale industrial extraction of groundwater in a protected Mediterranean coastal dune system. These systems are heavily invaded by the exotic Acacia longifolia, which is causing serious ecological problems in many mediterranean-climate coastal regions worldwide. Controlled experiments showed a high competitiveness and water spender strategy of A. longifolia. Interestingly, A. longifolia was facilitated by the low resource use of native species under limiting resouce conditions. Moreover, A. longifolia exhibited novel traits (e.g. high resource utilization, symbiotic N2-fixation, rapid growth, high seed production) all highlihgting the potential of this woody invader to disrupt co-evolved interactions between native species in invaded habitats. Groundwater extraction created a spatially heterogeneous mosaic of microsites varying in distance to the groundwater table due to non-uniform water extraction, the topography of the sand dunes and seasonal climate fluctuations. Correlating the groundwater dependence (assesed by δ18O of xylem water and water sources) of five different plant functional types with ecophysiolgical parameters revealed that A. longifolia was more plastic than the native species in its response to both seasonal drought and microsite variability in groundwater access. In general, the susceptibility to changing groundwater access determined by the microsite conditions and plant strategies. It was particularly large in deep rooted species and phreatophytes, which do typically not experience large fluctuations in groundwater depth. Further investigations showed that the impact of the invasive acacia extended beyond competition for groundwater sources: substantial system level impacts on ecosystem structure and functioning were identified by using a range of methods (e.g. plant demographics, phenology, ecophysiology, xylem sap flux, stable isotopes and modelling): A. longifolia presence significantly altered community structure, by creating a new vegetation layer, dominating the mid-stratum of invaded forests and co-dominating the canopy of dunes. This dramatic alteration to community structure had cascading effects on biodiversity and regeneration of native species, understory light climate, stand water use and stand carbon gain. A. longifolia presence in the understory affected total water use of pine forest. Water use rates of Pinus pinaster in an invaded stand were on average 25% lower than in an non-invaded stand, indicating that A. longifolia disrupted water cycling through a repartitioning of water resources. In contrast, the atmospheric N2 fixed by A. longifolia had a facilitative effect on the surrounding vegetation, with marked spatial dependency indicating that A. longifolia significantly enriched the system with nitrogen beyond its physical extent. Furthermore, A. longifolia had a disproportionately larger impact on ecosystem N-dynamics compared with a native N2-fixer (Stauracanthus spectabilis). To assess stand carbon gain, we developed a new method for modeling canopy-level carbon assimilation rates, by combining information from sap flux estimated canopy stomatal conductance, phloem sap δ13C, and meteorological data. This new approach has farreaching implications since it can be applied even in relatively remote field sites. It could be shown that A. longifolia presence reduced canopy carbon assimilation rates of P. pinaster in the invaded stand relative to a non-invaded stand, indicating that competition with A. longifolia can lead to decreased growth rates and reduced forest productivity. As a sideline project, new insights on post-photosynthetic C discrimination (i.e. internal xylem CO2-transport and stem photosynthesis; 3‰-deleption during phloem transport) emerged. Together, this study clearly demonstrate the significant impact of the resource-spending woody invader A. longifolia on vegetation structure and regeneration, nitrogen, carbon and water dynamics in pristine Mediterranean ecosystems. In addition the presented results significantly expand our general understanding of woody plant invasions in low resource ecosystems. http://www.uni-bielefeld.de/biologie/Oekosystembiologie/doc/oeko13.html
Publications
- (2008) Competitive balance between the alien invasive Acacia longifolia and native Mediterranean species. In: Plant Invasions: Human perception, ecological impacts and management. Eds. B. Tokarska-Guzik, J. Brock, G. Brundu, L. Child, C. Daehler, P. Pyšek, Backhuys Publishers, Leiden, The Netherlands, pp. 261-275
Werner C., R. Peperkorn, C. Máguas, W. Beyschlag
- (2009) Tracing Seasonal Changes in Water Use of an Invasive Acacia and a Native Pine in Southern Portugal by Measurement of Sap Flow. Acta Horticulturae, (ISHS) 846: 209-216
Rascher K.G., C. Máguas, O. Correia, C. Werner
- (2010) High competitiveness of a resource demanding invasive acacia under low resource supply. Plant Ecology, 206: 83- 96
Werner C., U. Zumkier, W. Beyschlag & C. Máguas
(See online at https://doi.org/10.1007/s11258-009-9625-0) - (2010) On the use of phloem sap δ13C as an indicator of canopy carbon discrimination. Tree Physiology, 30: 149-1514
Rascher K.G., C. Máguas, C. Werner
(See online at https://doi.org/10.1093/treephys/tpq092) - (2011) Acacia longifolia invasion impacts vegetation structure and regeneration dynamics in open dunes and pine forests. Biological Invasions
Rascher K.G., A. Große-Stoltenberg, C. Máguas, J.A.A. Meira-Neto, C. Werner
(See online at https://doi.org/10.1007/s10530-011-9949-2) - (2011) Diel variations in the carbon isotope composition of respired CO2 and associated carbon sources: a review of dynamics and mechanisms. Biogeosciences Discuss. 8: 1–49
Werner C. & Gessler A.
(See online at https://dx.doi.org/10.5194/bgd-8-1-2011) - (2011) Influence of an exotic N2-fixing Acacia on community composition and N status of native Mediterranean species. Acta Oecologia, 37: 43 – 50
Hellmann C., R. Sutter, K.G. Rascher, C Máguas, O. Correia, C. Werner
(See online at https://doi.org/10.1016/j.actao.2010.11.005) - (2011) Responses of woody species to spatial and temporal ground water changes in coastal sand dune systems, Biogeosciences Discuss., 8, 1591- 1616
Máguas, C., Rascher, K. G., Martins-Loução, A., Carvalho, P., Pinho, P., Ramos, M., Correia, O., and Werner, C.
(See online at https://doi.org/10.5194/bg-8-3823-2011)
