Regime shifts of Mediterranean forest carbon uptake and reduced resilience driven by multidecadal ocean surface temperatures

Global warming is shifting the relationships between fire weather and realized fire-induced CO2 emissions in Europe

Fire activity has significantly changed in Europe over the last decades (1980–2020s), with the emergence of summers attaining unprecedented fire prone weather conditions. Here we report a significant shift in the non-stationary relationship linking fire weather conditions and fire intensity measured in terms of CO₂ emissions released during biomass burning across a latitudinal gradient of European IPCC regions. The reported trends indicate that global warming is possibly inducing an incipient change on regional fire dynamics towards increased fire impacts in Europe, suggesting that emerging risks posed by exceptional fire-weather danger conditions may progressively exceed current wildfire suppression capabilities in the next decades and impact forest carbon sinks.

Drought Drives Growth and Mortality Rates in Three Pine Species under Mediterranean Conditions

Drought constrains tree growth in regions with seasonal water deficit where growth decline can lead to tree death. This has been observed in regions such as the western Mediterranean Basin, which is a climate-warming hotspot. However, we lack information on intra- and inter-specific comparisons of growth rates and responses to water shortage in these hotspots, considering tree species with different drought tolerance. We sampled several sites located in north-eastern Spain showing dieback and high mortality rates of three pine species (Pinus sylvestris, Pinus pinaster, Pinus halepensis). We dated death years and reconstructed the basal area increment of coexisting living and recently dead trees using tree ring data. Then, we calculated bootstrapped Pearson correlations between a drought index and growth. Finally, we used linear mixed-effects models to determine differences in growth trends and the response to drought of living and dead trees. Mortality in P. sylvestris and P. pinaster peaked in response to the 2012 and 2017 droughts, respectively, and in sites located near the species’ xeric distribution limits. In P. halepensis, tree deaths occurred most years. Dead trees showed lower growth rates than living trees in five out of six sites. There was a strong growth drop after the 1980s when climate shifted towards warmer and drier conditions. Tree growth responded positively to wet climate conditions, particularly in the case of living trees. Accordingly, growth divergence between living and dead trees during dry periods reflected cumulative drought impacts on trees. If aridification continues, tree drought mortality would increase, particularly in xeric distribution limits of tree species.

Disentangling the Legacies of Climate and Management on Tree Growth

Legacies of past climate conditions and historical management govern forest productivity and tree growth. Understanding how these processes interact and the timescales over which they influence tree growth is critical to assess forest vulnerability to climate change. Yet, few studies address this issue, likely because integrated long-term records of both growth and forest management are uncommon. We applied the stochastic antecedent modelling (SAM) framework to annual tree-ring widths from mixed forests to recover the ecological memory of tree growth. We quantified the effects of antecedent temperature and precipitation up to 4 years preceding the year of ring formation and integrated management effects with records of harvesting intensity from historical forest management archives. The SAM approach uncovered important time periods most influential to growth, typically the warmer and drier months or seasons, but variation among species and sites emerged. Silver fir responded primarily to past climate conditions (25–50 months prior to the year of ring formation), while European beech and Scots pine responded mostly to climate conditions during the year of ring formation and the previous year, although these responses varied among sites. Past management and climate interacted in such a way that harvesting promoted growth in young silver fir under wet and warm conditions and in old European beech under drier and cooler conditions. Our study shows that the ecological memory associated with climate legacies and historical forest management is species-specific and context-dependent, suggesting that both aspects are needed to properly evaluate forest functioning under climate change.

Drought, axe and goats. More variable and synchronized growth forecasts worsening dieback in Moroccan Atlas cedar forests

Understanding how climate warming and land-use changes determine the vulnerability of forests to drought is critical. However, we still lack: (i) robust quantifications of long-term growth changes during aridification processes, (ii) links between growth decline, changes in forest cover, stand structure and soil conditions, and (iii) forecasts of growth variability to projected climate warming. We investigated tree-ring records over the past 400-700 years, quantified changes in grazing area and forest cover during the 20th century, sampled current stand structure, and analyzed soil organic carbon δ¹³C and total nitrogen δ¹⁵N of Atlas cedar (Cedrus atlantica (Endl.) Manetti ex Carrière) Moroccan forests to characterize their dieback. Atlas cedar forests experienced massive dieback after the 1970s, particularly in the xeric High Atlas region. Forest cover increased in the less xeric regions (Middle Atlas and Rif) by almost 20%, while it decreased about 18% in the High Atlas, where soil δ¹³C and δ¹⁵N showed evidences of grazing. Growth declined and became more variable in response to recent droughts. The relative growth reduction (54%) was higher in the Middle Atlas than elsewhere (Rif, 32%; High Atlas, 36%). Growth synchrony between forests located within the Middle and High Atlas regions increased after the 1970s. Simulations based on a worst-case emission scenario and rapid warming forecast a stronger limitation of growth by low soil moisture in all regions, but particularly in the Middle Atlas and after the mid-21st century. Climate warming is expected to strengthen growth synchronization preceding dieback of conifer forests in xeric regions. The likelihood of similar dieback episodes is further exacerbated by historical degradation of these forests.

Recent advances in understanding and improving photosynthesis

Since 1893, when the word "photosynthesis" was first coined by Charles Reid Barnes and Conway MacMillan, our understanding of the elements and regulation of this complex process is far from being entirely understood. We aim to review the most relevant advances in photosynthesis research from the last few years and to provide a perspective on the forthcoming research in this field. Recent discoveries related to light sensing, harvesting, and dissipation; kinetics of CO₂ fixation; components and regulators of CO₂ diffusion through stomata and mesophyll; and genetic engineering for improving photosynthetic and production capacities of crops are addressed.

Climate change, ecosystems and abrupt change: science priorities

Ecologists have long studied patterns, directions and tempos of change, but there is a pressing need to extend current understanding to empirical observations of abrupt changes as climate warming accelerates. Abrupt changes in ecological systems (ACES)-changes that are fast in time or fast relative to their drivers-are ubiquitous and increasing in frequency. Powerful theoretical frameworks exist, yet applications in real-world landscapes to detect, explain and anticipate ACES have lagged. We highlight five insights emerging from empirical studies of ACES across diverse ecosystems: (i) ecological systems show ACES in some dimensions but not others; (ii) climate extremes may be more important than mean climate in generating ACES; (iii) interactions among multiple drivers often produce ACES; (iv) contingencies, such as ecological memory, frequency and sequence of disturbances, and spatial context are important; and (v) tipping points are often (but not always) associated with ACES. We suggest research priorities to advance understanding of ACES in the face of climate change. Progress in understanding ACES requires strong integration of scientific approaches (theory, observations, experiments and process-based models) and high-quality empirical data drawn from a diverse array of ecosystems. This article is part of the theme issue 'Climate change and ecosystems: threats, opportunities and solutions'.

Droughts and climate warming desynchronize Black pine growth across the Mediterranean Basin

The effects of climate change on forest growth are not homogeneous across tree species distribution ranges because of inter-population variability and spatial heterogeneity. Although latitudinal and thermal gradients in growth patterns have been widely investigated, changes in these patterns along longitudinal gradients due to the different timing and severity of regional droughts are less studied. Here, we investigated these responses in Mediterranean Black pine (Pinus nigra Arn.). We built a tree-ring width dataset comprising 77 forests (1202 trees) across the Mediterranean Basin. The biogeographical patterns in growth patterns and the relationships between growth and mean temperature, precipitation, drought and atmospheric circulations patterns (NAO -North Atlantic Oscillation-, SOI -Southern Oscillation Index- and MOI -Mediterranean Oscillation index-) were analyzed. Then, we evaluated the spatial and temporal growth synchrony between and within east and west populations. We found different growth and climate patterns in west vs. east Black pine populations, although in both regions growth was driven by similar temperature and precipitation variables. MOI significantly influenced tree growth, whilst NAO and SOI showed weaker effects. Growth of east and west Black pine populations desynchronized after the 1970s when several and uncoupled regional droughts occurred across the Mediterranean Basin. We detected a climate shift from the 1970s to the 1980s affecting growth patterns, changing growth-climate relationships, and reducing forest growth from west to east Black pine forests. Afterwards, climate and growth of east and west populations became increasingly more divergent. Our findings imply that integral bioclimatic and biogeographical analyses across the species distribution area must be considered to adequately assess the impact of climate change on tree growth under warming and more arid conditions.