Fagus sylvatica L. Peripheral Populations in the Mediterranean Iberian Peninsula: Climatic or Anthropic Relicts?

Looking at the modern landscape of submediterranean Greece through a palaeoecological lens

The importance of understanding the long-lasting legacy of past land use on modern ecosystems has long been acknowledged. However, the magnitude and persistence of such legacies have been assessed only occasionally. Northern Greece has been a gateway of farming into mainland Europe during the Neolithic, thus providing a perfect setting to assess the potential impact of land-use history on present-day ecosystems. Additionally, the marked Holocene climatic variability of the southern Balkans makes it possible to investigate climate-vegetation-land use interactions over long timescales. Here, we have studied a sediment record from Limni Vegoritis (Northern Greece) spanning the past ∼9000 years using palaeoecological proxies (pollen, spores, stomata, microscopic charcoal). We aimed to reconstruct long-term vegetation dynamics in submediterranean Greece, to assess the environmental factors controlling them and to establish the legacies of the long history of land use in the modern landscape. We found that the Early Holocene afforestation, mainly oak woodlands, was delayed because of suboptimal moisture conditions. Later, colder and drier conditions during the rapid climate change centred around the '8.2 ka event' triggered woodland opening and the spread of wooded (Juniperus) steppe vegetation. First indicators of farming activities are recorded during this period, but their abundances are too low to explain the concurrent large deforestation episode. Later, pinewoods (probably dominated by Pinus nigra) with deciduous Quercus spread and dominated the landscape for several millennia. These forests experienced repeated multi-centennial setback-recovery episodes associated with land-use intensification, but pines eventually declined ∼2500-2000 years ago during Classical times under heavy land use comprising intense pastoralism. This was the starting point for the present-day landscape, where the main 'foundation' taxon of the ancient forests (Pinus cf. nigra) is missing, therefore attesting to the strong imprint that historical land use has left on the modern landscape.

Divergent spatio-temporal tree growth trends in Pinus pinaster Ait. in South-Western European forests

Climate change influences forest ecosystems in several ways, such as modifying forest growth or ecosystem functionality. To fully understand the impact of changing climatic conditions on forest growth it is necessary to undertake long-term spatiotemporal analyses. The main purpose of this work is to describe the major trends in tree growth of Pinus pinaster in Spain over the last 70 years, differentiating homogeneous ecological units using an unsupervised classification algorithm and additive modelling techniques. We also aim to relate these growth trends with temporal series for precipitation and temperature, as well as forest variables. We leverage information from a large data set of tree cores (around 2200) extracted during the field campaign of the Fourth Spanish National Forest Inventory. An unsupervised algorithm classified the plots into five classes, which were consistent in ecological terms. We also found a general decline in growth in three of the five ecoregions since the 1970s, concomitant with an increase in temperature and a reduction in precipitation. However, this tree growth decline has not been observed in the Atlantic influenced ecoregion, where the cooler, more humid climatic conditions are more stable. Certain stand features, such as low basal area through forest management practices, may have alleviated the impact of harsh climatic conditions on some areas of inner Spain, while denser stands display a more pronounced decline in tree growth. We concluded that Southern populations show some degrees of growth decline and low growth trends while Northern populations did not exhibit growth decline and have the largest growth rates. Under a forecasted increment of temperatures, the growth decline can be expanded.

Tree ring segmentation using UNEt TRansformer neural network on stained microsections for quantitative wood anatomy

Forests are critical in the terrestrial carbon cycle, and the knowledge of their response to ongoing climate change will be crucial for determining future carbon fluxes and climate trajectories. In areas with contrasting seasons, trees form discrete annual rings that can be assigned to calendar years, allowing to extract valuable information about how trees respond to the environment. The anatomical structure of wood provides highly-resolved information about the reaction and adaptation of trees to climate. Quantitative wood anatomy helps to retrieve this information by measuring wood at the cellular level using high-resolution images of wood micro-sections. However, whereas large advances have been made in identifying cellular structures, obtaining meaningful cellular information is still hampered by the correct annual tree ring delimitation on the images. This is a time-consuming task that requires experienced operators to manually delimit ring boundaries. Classic methods of automatic segmentation based on pixel values are being replaced by new approaches using neural networks which are capable of distinguishing structures, even when demarcations require a high level of expertise. Although neural networks have been used for tree ring segmentation on macroscopic images of wood, the complexity of cell patterns in stained microsections of broadleaved species requires adaptive models to accurately accomplish this task. We present an automatic tree ring boundary delineation using neural networks on stained cross-sectional microsection images from beech cores. We trained a UNETR, a combined neural network of UNET and the attention mechanisms of Visual Transformers, to automatically segment annual ring boundaries. Its accuracy was evaluated considering discrepancies with manual segmentation and the consequences of disparity for the goals of quantitative wood anatomy analyses. In most cases (91.8%), automatic segmentation matched or improved manual segmentation, and the rate of vessels assignment to annual rings was similar between the two categories, even when manual segmentation was considered better. The application of convolutional neural networks-based models outperforms human operator segmentations when confronting ring boundary delimitation using specific parameters for quantitative wood anatomy analysis. Current advances on segmentation models may reduce the cost of massive and accurate data collection for quantitative wood anatomy.

Historical ecology identifies long-term rewilding strategy for conserving Mediterranean mountain forests in south Italy

In the context of global decline in old-growth forest, historical ecology is a valuable tool to derive insights into vegetation legacies and dynamics and develop new conservation and restoration strategies. In this cross-disciplinary study, we integrate palynology (Lago del Pesce record), history, dendrochronology, and historical and contemporary land cover maps to assess drivers of vegetation change over the last millennium in a Mediterranean mountain forest (Pollino National Park, southern Italy) and discuss implications in conservation ecology. The study site hosts a remnant beech-fir (Fagus sylvatica-Abies alba) mixed forest, a priority habitat for biodiversity conservation in Europe. In the 10th century, the pollen record showed an open environment that was quickly colonized by silver fir when sociopolitical instabilities reduced anthropogenic pressures in mountain forests. The highest forest cover and biomass was reached between the 14th and the 17th centuries following land abandonment due to recurring plague pandemics. This rewilding process is also reflected in the recruitment history of Bosnian pine (Pinus heldreichii) in the subalpine elevation belt. Our results show that human impacts have been one of the main drivers of silver fir population contraction in the last centuries in the Mediterranean, and that the removal of direct human pressure led to ecosystem renovation. Since 1910, the Rubbio State Forest has locally protected and restored the mixed beech-fir forest. The institutions in 1972 for the Rubbio Natural Reserve and in 1993 for Pollino National Park have guaranteed the survival of the silver fir population, demonstrating the effectiveness of targeted conservation and restoration policies despite a warming climate. Monitoring silver fir populations can measure the effectiveness of conservation measures. In the last decades, the abandonment of rural environments (rewilding) along the mountains of southern Italy has reduced the pressure on ecosystems, thus boosting forest expansion. However, after four decades of natural regeneration and increasing biomass, pollen influx and forest composition are still far from the natural attributes of the medieval forest ecosystem. We conclude that long-term forest planning encouraging limited direct human disturbance will lead toward rewilding and renovation of carbon-rich and highly biodiverse Mediterranean old-growth forests, which will be more resistant and resilient to future climate change.