Marjan Jongen

Precipitation variability does not affect soil respiration and nitrogen dynamics in the understorey of a Mediterranean oak woodland

Background and aimsFuture climate scenarios for the Mediterranean imply increasing precipitation variability. This study presents a large-scale water manipulation experiment simulating changes in precipitation variability, aiming at a better understanding of the effects of rainfall patterns on soil C and N cycling and understorey productivity in a Mediterranean oak woodland.MethodsWe used rain-out shelters to achieve (1) a normal dry period (7xa0days), and (2) a dry period increased three-fold (21xa0days), without altering total annual precipitation inputs.ResultsThe temporal patterns of soil respiration (Rs) and soil inorganic N were not affected by treatment. However, water infiltration and N leaching increased with large infrequent watering events. Rs and soil NH4+-N correlated with soil temperature, with soil NO3−-N being influenced by leaching.ConclusionsThe lack of significant treatment effects on either Rs or soil inorganic N can be explained by (1) minor differences in plant productivity between the treatments, suggesting equal plant N demand, and (2) the absence of moisture dependence of Rs and soil NH4+-N. Increased N leaching with large infrequent precipitation events may have longer-term consequences for ecosystem functioning. Our results contribute to an improved understanding of possible climate change effects on key ecosystem processes in Mediterranean ecosystems.

Consequences of Changing Precipitation Patterns for Ecosystem Functioning in Grasslands: A Review

Grassland ecosystems worldwide provide agricultural goods and important ecosystem services. Productivity and other ecosystem processes in grasslands are, in most cases, strongly linked to the ecosystems’ water status, a factor that is predicted to experience major alterations with global climate change. Future predictions include changes in the amount, distribution, frequency, and intensity of precipitation, which, particularly in grasslands, may have important consequences for ecosystem state and functioning. This review analyses the effects of experimental precipitation manipulation on plant productivity, species diversity, soil/ecosystem respiration, and soil nitrogen in grassland-type ecosystems over a wide range of climate types, synthesising the results from 72 studies.

Species-specific adaptations explain resilience of herbaceous understorey to increased precipitation variability in a Mediterranean oak woodland.

Abstract To date, the implications of the predicted greater intra‐annual variability and extremes in precipitation on ecosystem functioning have received little attention. This study presents results on leaf‐level physiological responses of five species covering the functional groups grasses, forbs, and legumes in the understorey of a Mediterranean oak woodland, with increasing precipitation variability, without altering total annual precipitation inputs. Although extending the dry period between precipitation events from 3 to 6 weeks led to increased soil moisture deficit, overall treatment effects on photosynthetic performance were not observed in the studied species. This resilience to prolonged water stress was explained by different physiological and morphological strategies to withstand periods below the wilting point, that is, isohydric behavior in Agrostis, Rumex, and Tuberaria, leaf succulence in Rumex, and taproots in Tolpis. In addition, quick recovery upon irrigation events and species‐specific adaptations of water‐use efficiency with longer dry periods and larger precipitation events contributed to the observed resilience in productivity of the annual plant community. Although none of the species exhibited a change in cover with increasing precipitation variability, leaf physiology of the legume Ornithopus exhibited signs of sensitivity to moisture deficit, which may have implications for the agricultural practice of seeding legume‐rich mixtures in Mediterranean grassland‐type systems. This highlights the need for long‐term precipitation manipulation experiments to capture possible directional changes in species composition and seed bank development, which can subsequently affect ecosystem state and functioning.

Effects of precipitation variability on carbon and water fluxes in the understorey of a nitrogen-limited montado ecosystem

To date the implications of greater intra-annual variability and extremes in precipitation on ecosystem functioning have received little attention. This study presents results on soil and vegetation carbon and water fluxes in the understorey of a Mediterranean oak woodland in response to increasing precipitation variability, with an extension of the dry period between precipitation events from 3 to 6xa0weeks, without altering total annual precipitation inputs. With prolonged dry periods soil moisture did breach the stress thresholds for ecosystem processes, which led to short-term treatment differences in photosynthesis, but not in system carbon losses, with subsequent short-term decreases in net ecosystem exchange. Independent of treatment, irrigation events rapidly increased carbon and water fluxes. However, contradicting the predictions drawn from the ‘bucket model’, over the course of the growing season no all-over treatment differences were found in system assimilation and respiration, nor in evapotranspiration and ecosystem water use efficiency. This lack of responsiveness is attributed to the ecosystem’s resilience to low soil moisture during the growing season of the herbaceous understorey, with temperature rather than soil moisture controlling key ecosystem processes. Moreover, severe nitrogen limitation of the studied ecosystem may explain the lack of moisture effects on net system carbon dynamics. Thus, although the bucket model predicts changes in soil water dynamics with increasing precipitation variability, ecosystem responses to more extreme precipitation regimes may be influenced by additional factors, such as inter-annual variability in nutrient availability.

Overwhelming effects of autumn-time drought during seedling establishment impair recovery potential in sown and semi-natural pastures in Portugal

Climate change-induced alterations to precipitation may affect the regeneration dynamics of plant species, with the occurrence of drought influencing germination and seedling establishment. In Mediterranean grasslands, typically dominated by C3 annual species, germination occurs rapidly once sufficient rain falls in autumn. However, a single large precipitation event may be followed by a dry spell, with possible consequences for vegetation composition and productivity. We investigated the effects of autumn-time drought on germination and seedling establishment, and the subsequent recovery potential, in semi-natural and sown pastures. Although the majority of grasslands in Portugal are semi-natural, sowing legume-rich seed mixtures is an increasingly common agricultural practice in Portugal. Our results show an overwhelming effect of autumn-time drought on seedling establishment, with the semi-natural pasture being more compromised than the sown pasture. However, after the recovery period, the semi-natural pasture exhibited tendentially higher aboveground biomass recovery than the sown pasture. The differential species sensitivity to autumn-time drought and subsequent implications for recovery potential may be important for the agricultural practice of sown pastures, justifying additional research for drought-tolerant cultivar improvement to maintain productivity with climate change.