Browsing by Author "Leira, Manel"
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- Drought, fire and grazing precursors to large-scale pine forest declinePublication . Connor, Simon; Araújo, João; Boski, Tomasz; Gomes, Ana; Gomes, Sandra D.; Leira, Manel; Freitas, Maria da Conceição; Andrade, Cesar; Morales‐Molino, César; Franco‐Múgica, Fátima; Akindola, Rufus B.; Vannière, BorisAim Temperate forests are currently facing multiple stresses due to climate change, biological invasions, habitat fragmentation and fire regime change. How these stressors interact with each other influences how, when and whether ecosystems recover, or whether they adapt or transition to a different ecological state. Because forest recovery or collapse may take longer than a human lifetime, predicting the outcomes of different stressor combinations remains difficult. A clearer vision of future forest trajectories in a changing world may be gained by examining collapses of forests in the past. Here, we use long-term ecological data to conduct a post-mortem examination of the decline of maritime pine forests (Pinus pinaster Ait.) on the SW Iberian Peninsula 7000-6500 years ago. Location Portugal and Spain. Methods We compared four palaeoecological records-two with pine declines and two without-using a multiproxy approach. Bioclimatic differences between the four sites were explored. Proxies for past vegetation and disturbance (fire and grazing) were compared with independent palaeoclimatic records. We performed functional traits analysis and used phase plots to examine the causes of pine decline. Results The pine decline represents a critical transition in SW Iberia, which lies close to maritime pine's bioclimatic limits. Prolonged drought likely killed trees and suppressed the fires that normally stimulate pine germination and pinewood recovery. Increased grazing pressure facilitated the rapid spread of resprouter shrubs. These competed with pine trees and ultimately replaced them. Our data highlight complex interactions between climate, fire, grazing and forest resilience. Main Conclusions The pine decline occurred at least a century after post-fire resprouters overtook obligate seeders in the vegetation, constituting an early-warning signal of forest loss. Fire suppression, resprouter encroachment and grazing may threaten the persistence of Mediterranean forests as droughts become more frequent and extreme.
- Holocene sea level and climate interactions on wet dune slack evolution in SW Portugal: a model for future scenarios?Publication . Leira, Manel; Freitas, Maria C.; Ferreira, Tania; Cruces, Anabela; Connor, Simon; Andrade, Cesar; Lopes, Vera; Bao, RobertoWe examine the Holocene environmental changes in a wet dune slack of the Portuguese coast, Poco do Barbarroxa de Baixo. Lithology, organic matter, biological proxies and high-resolution chronology provide estimations of sediment accumulation rates and changes in environmental conditions in relation to sea-level change and climate variability during the Holocene. Results show that the wet dune slack was formed 7.5 cal. ka BP, contemporaneous with the last stages of the rapid sea-level rise. This depositional environment formed under frequent freshwater flooding and water ponding that allowed the development and post-mortem accumulation of abundant plant remains. The wetland evolved into mostly palustrine conditions over the next 2000 years, until a phase of stabilization in relative sea-level rise, when sedimentation rates slowed down to 0.04 mm yr(-1), between 5.3 and 2.5 cal. ka BP. Later, about 0.8 cal. ka BP, high-energy events, likely due to enhanced storminess and more frequent onshore winds, caused the collapse of the foredune above the wetlands' seaward margin. The delicate balance between hydrology (controlled by sea-level rise and climate change), sediment supply and storminess modulates the habitat's resilience and ecological stability. This underpins the relevance of integrating past records in coastal wet dune slacks management in a scenario of constant adaptation processes.
- The role of climate, marine influence and sedimentation rates in late-Holocene estuarine evolution (SW Portugal)Publication . Costa, Ana Maria; Freitas, Maria da Conceicao; Leira, Manel; Costas, S.; Costa, Pedro J. M.; Andrade, Cesar; Bao, Roberto; Duarte, Joao; Rodrigues, Aurora; Cachao, Mario; Araujo, Ana Cristina; Diniz, Mariana; Arias, PabloEstuaries are sensitive to changes in global to regional sea level, to climate-driven variation in rainfall and to fluvial discharge. In this study, we use source and environmentally sensitive proxies together with radiocarbon dating to examine a 7-m-thick sedimentary record from the Sado estuary accumulated throughout the last 3.6 kyr. The lithofacies, geochemistry and diatom assemblages in the sediments accumulated between 3570 and 3240 cal. BP indicate a mixture between terrestrial and marine sources. The relative contribution of each source varied through time as sedimentation progressed in a low intertidal to high subtidal and low-energy accreting tidal flat. The sedimentation proceeded under a general pattern of drier and higher aridity conditions, punctuated by century-long changes of the rainfall regime that mirror an increase in storminess that affected SW Portugal and Europe. The sediment sequence contains evidence of two periods characterized by downstream displacement of the estuarine/freshwater transitional boundary, dated to 3570-3400 cal. BP and 3300-3240 cal. BP. These are intercalated by one episode where marine influence shifted upstream. All sedimentation episodes developed under high terrestrial sediment delivery to this transitional region, leading to exceptionally high sedimentation rates, independently of the relative expression of terrestrial/marine influences in sediment facies. Our data show that these disturbances are mainly climate-driven and related to variations in rainfall and only secondarily with regional sea-level oscillations. From 3240 cal. BP onwards, an abrupt change in sediment facies is noted, in which the silting estuarine bottom reaches mean sea level and continued accreting until present under prevailing freshwater conditions, the tidal flat changing to an alluvial plain. The environmental modification is accompanied by a pronounced change in sedimentation rate that decreased by two orders of magnitude, reflecting the loss of accommodation space rather than the influence of climate or regional sea-level drivers.
- The timing of the deglaciation in the Atlantic Iberian mountains: Insights from the stratigraphic analysis of a lake sequence in Serra da Estrela (Portugal)Publication . Hernández, Armand; Sáez, Alberto; Santos, Ricardo N.; Rodrigues, Teresa; Martin‐Puertas, Celia; Gil‐Romera, Graciela; Abbott, Mark; Carballeira, Rafael; Costa, Pedro; Giralt, Santiago; Gomes, Sandra D.; Griffore, Melissa; Ibañez‐Insa, Jordi; Leira, Manel; Moreno, João; Naughton, Filipa; Oliveira, Dulce; Raposeiro, Pedro M.; Trigo, Ricardo M.; Vieira, Gonçalo; Ramos, Alexandre M.Understanding the environmental response to the last glacial termination in regions located in transitional climate zones such as the Atlantic Iberian mountains is crucial to estimate potential changes in regions affected by current glacial melting. We present an 8.5 m-long, solid last deglaciation and Holocene chronostratigraphic record including detailed sediment analysis from Lake Peixao, a pro-glacial lake in the Serra da Estrela (Central Portugal). The age-depth model relies on a Bayesian approach that includes 16 AMS C-14 dates and Pb-210-(CS)-C-137 measurements, robustly dating the lake formation at 14.7 +/- 0.32 cal. ka BP. This chronological reconstruction shows an average sedimentation rate of ca. 0.07 cm yr(-1) (15 yr cm(-1)), enabling proxy analyses at decadal timescales. The sediment sequence is composed of five lithological units: (U1) coarse and unsorted fluvioglacial lacustrine deposits; (U2) massive fluvioglacial lacustrine deposits (863-790 cm below surface [bsf]; 14.7 +/- 0.32-13.8 +/- 0.12 cal. ka BP); (U3) water current fluvioglacial lacustrine deposits (790-766 cm bsf; 13.8 +/- 0.12-12.9 +/- 0.29 cal. ka BP); (U4) laminated/banded lacustrine deposits characterized by terrigenous deposits from ice-covered lake periods and episodic events of ice and snow melting (766-752 cm bsf; 12.9 +/- 0.29-11.7 +/- 0.15 cal. ka BP); and (U5) massive muddy lacustrine deposits (752-0 cm bsf; 11.7 +/- 0.15 cal. ka BP-present). The occurrence of U2 to U4 deposits defines the transition from glacial cold (U1) to net warm postglacial conditions (U5). These climate transitions are marked by changes in sediments and the presence of very low sedimentation rate periods, possibly related to the Intra-Allerod Cold Period and the coldest phase of the Younger Dryas. Our results support the previously proposed timing of the retreat of the Serra da Estrela glaciers ca. 13.8 +/- 0.12 cal. ka BP. The robust chronology of Lake Peixao highlights the potential of Iberian pro-glacial lakes for dating deglaciation processes and will lead to unprecedented decadal-to-centennial timescale palaeoclimate reconstructions in this region since the last glacial-interglacial transition.
- Understanding the Atlantic influence on climate and vegetation dynamics in western Iberia over the last 2000 yearsPublication . Santos, Ricardo N.; Rodrigues, Teresa; Naughton, Filipa; Schefuß, Enno; Oliveira, Dulce; Moreno, João; Raposeiro, Pedro M.; Gil-Romera, Graciela; Morgan, Alistair; Leira, Manel; Gomes, Sandra D.; Ladd, S. Nemiah; Trigo, Ricardo M.; Ramos, Alexandre M.; Hernández, ArmandPredicting the environmental impact of climate change in extremely sensitive areas, like western Iberia, requires an understanding of the long-term interactions between climate and vegetation. Here we present a novel hightemporal resolution multiproxy analysis, including plant-wax n-alkane isotope data, pollen analysis, macrocharcoal identification, chironomid and diatom records of sediments from a mountain lake in central Portugal. We examined the evolution of the Atlantic and Mediterranean climate influences over the last two millennia, exploring their connection with major atmospheric patterns and impacts on the climatic signal and vegetation dynamics in this understudied region. During the Roman Period (RP; ca. -200 – 500 AD), the study area was characterized by grass dominance, with high temperatures indicated by chironomid composition and microcharcoal content. The increase in plant-wax δ2 H values during this period suggests a shift from wet to dry conditions. The Early Middle Ages (EMA; ca. 500–900 AD) were characterized by colder and a transition to wetter conditions, as indicated by the vegetation and plant-wax n-alkane isotope data. The Medieval Climate Anomaly (MCA; ca. 900–1300 AD) was generally warm, with a short initial lake level drop. This period exhibits the maximum expansion of the Mediterranean forest over the last 2 ka and possibly proximal moisture sources. During the Little Ice Age (LIA; 1300–1850 AD), a reduction of the Mediterranean forest and a strong depletion of plant-wax δ2 H values suggest cold and wet conditions with strong influence of remote Atlantic moisture, with the coldest and wettest phase of the last 2 ka detected between 1550 and 1900 AD. The post-LIA period, from 1900 AD onwards, shows a change to the present warmer and drier conditions, in a highly anthropized landscape. We also demonstrate that major changes in climate have influenced vegetation patterns, with these changes mainly controlled by large-scale atmospheric dynamics. This underscores the sensitivity of western Iberian ecosystems to climate shifts, enriches the current regional understanding of climate-vegetation interplay, and offers valuable insights for future climate change projections.