Modelling the dynamics of Pinus sylvestris forests after a die-off event under climate change scenarios.

In recent decades, die-off events in Pinus sylvestris populations have increased. The causes of these phenomena, which are usually related to local and regional extreme hot droughts, have been extensively investigated from a physiological viewpoint. However, the consequences of die-off process in terms of demography and vegetation dynamics have been less thoroughly addressed. Here, we projected P. sylvestris plot dynamics after a die-off event, under climate change scenarios, considering also their early demographic stages (i.e., seedlings, saplings and ingrowth from the sapling to adult class), to assess the resilience of P. sylvestris populations after such events. We used Integral Projection Models (IPMs) to project future plot structure under current climate, and under RCP4.5 and RCP8.0 climate scenarios, using climatic suitability - extracted from Species Distribution Models - as a covariable in the estimations of vital rates over time. Field data feeding IPMs were obtained from two successive surveys, at the end of the die-off event (2013) and four years later (2017), undertaken on populations situated across the P. sylvestris range of distribution in Catalonia (NE Spain). Plots affected by die-off experienced a loss of large trees, which causes that basal area, tree diameter and tree density will remain lower for decades relative to unaffected plots. After the event, this situation is partially counterbalanced in affected plots by a greater increase in basal area and seedling recruitment into tree stage, thus promoting resilience. However, resilience is delayed under the climate-change scenarios with warmer and drier conditions involving additional physiological stress, due to a reduced abundance of seedlings and a smaller plot basal area. The study shows lagged effect of drought-induced die-off events on forest structure, also revealing stabilizing mechanisms, such as recruitment and tree growth release, which enhance resilience. However, these mechanisms would be jeopardized by oncoming regional warming.

On the road: Anthropogenic factors drive the invasion risk of a wild solitary bee species.

Complex biotic networks of invaders and their new environments pose immense challenges for researchers aiming to predict current and future occupancy of introduced species. This might be especially true for invasive bees, as they enter novel trophic interactions. Little attention has been paid to solitary, invasive wild bees, despite their increasing recognition as a potential global threat to biodiversity. Here, we present the first comprehensive species distribution modelling approach targeting the invasive bee Megachile sculpturalis, which is currently undergoing parallel range expansion in North America and Europe. While the species has largely colonised the most highly suitable areas of North America over the past decades, its invasion of Europe seems to be in its early stages. We showed that its current distribution is largely explained by anthropogenic factors, suggesting that its spread is facilitated by road and maritime traffic, largely beyond its intrinsic dispersal ability. Our results suggest that M. sculpturalis is likely to be negatively affected by future climate change in North America, while in Europe the potential suitable areas at-risk of invasion remain equally large. Based on our study, we emphasise the role of expert knowledge for evaluation of ecologically meaningful variables implemented and interpreted for species distribution modelling. We strongly recommend that the monitoring of this and other invasive pollinator species should be prioritised in areas identified as at-risk, alongside development of effective management strategies.

Species climatic niche explains post-fire regeneration of Aleppo pine (Pinus halepensis Mill.) under compounded effects of fire and drought in east Spain.

Fire and drought are two major agents that shape Mediterranean ecosystems, but their interacting effects on forest resilience have not been yet fully addressed. We used Pinus halepensis to investigate how compound fire-drought regimes determine the success of post-fire regeneration. We measured the density of P.halepensis newly established individuals following fire in forty-three sites along the Spanish east coast, the wetter region of the species distribution. The climatic niche of P.halepensis was characterized by considering their populations across its Spanish distribution range. We used yearly values (1979-2013 period) of accumulated precipitation, mean temperature and the warmest quarter values of these two variables to generate the climatic space or climatic niche occupied by the species. Kernel density estimates were then applied to determine the niche centroid, which would correspond to the species' climatic optimum within its Spanish distribution range. Then, we computed the pre- and post-fire climatic deviations of each sampling site as the difference between site-specific climate conditions respect to the species niche centroid, and assessed their relationship with the success of post-fire regeneration. We found highly variable patterns of post-fire regeneration density of P.halepensis over the studied sites, ranging from 7 to 42,822 tree pines ha-1. Generalized linear models indicated a positive relationship between fire severity and the density of P.halepensis regeneration. Positive temperature deviations - warm conditions - before fire were positively related to pine regeneration. This effect increases under higher fire severity. By contrast, warm temperatures after fire showed a negative effect on the density of pine trees. Positive precipitation deviations - wet conditions - after fire enhanced pine regeneration, while precipitation before fire did not had any significant effect. Though P.halepensis is considered a species adapted to fire and drought, the interaction between these two disturbances can alter the success of its post-fire recovery patterns limiting the species' resilience in the future.

Relationship between heatwave-induced forest die-off and climatic suitability in multiple tree species.

In recent decades, many forest die-off events have been reported in relation to climate-change-induced episodes, such as droughts and heat waves. To understand how these extreme climatic events induce forest die-off, it is important to find a tool to standardize the climatic conditions experienced by different populations during a specific climatic event, taking into account the historic climatic conditions of the site where these populations live (bioclimatic niche). In this study, we used estimates of climatic suitability calculated from species distribution models (SDMs) for such purpose. We studied forest die-off across France during the 2003 heatwave that affected Western Europe, using 2,943 forest inventory plots dominated by 14 single tree species. Die-off severity was estimated by Normalized Difference Vegetation Index (NDVI) loss using Moderate-resolution Imaging Spectroradiometer remote sensor imagery. Climatic suitability at the local level during the historical 1979-2002 period (HCS), the episode time (2003; ECS) and suitability deviance during the historical period (HCS-SD) were calculated for each species by means of boosted regression tree models using the CHELSA climate database and occurrences extracted from European forest inventories. Low HCS-SD and high mean annual temperature explained the overall regional pattern of vulnerability to die-off across different monospecific forests. The combination of high historical and low episode climatic suitability also contributed significantly to overall forest die-off. Furthermore, we observed different species-specific relationships between die-off vulnerability and climatic suitability: Sub-Mediterranean and Mediterranean species tended to be vulnerable in historically more suitable localities (high HCS), whereas Euro-Siberian species presented greater vulnerability when the hot drought episode was more intense. We demonstrated that at regional scale, past climatic legacy plays an important role in explaining NDVI loss during the episode. Moreover, we demonstrated that SDMs-derived indexes, such as HCS, ECS and HCS-SD, could constitute a tool for standardizing the ways that populations and species experience climatic variability across time and space.