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Currently, Mediterranean forests are experiencing the deleterious effects of global warming, which mainly include increased temperatures and decreased precipitation in the region. Relict Abies pinsapo fir forests, endemic in the southern Iberian Peninsula, are especially sensitive to these recent environmental disturbances, and identifying the genes involved in the response of this endangered tree species to climate-driven stresses is of paramount importance for mitigating their effects. Genomic resources for A. pinsapo allow for the analysis of candidate genes reacting to warming and aridity in their natural habitats. Several members of the complex gene families encoding late embryogenesis abundant proteins (LEAs) and heat shock proteins (HSPs) have been found to exhibit differential expression patterns between wet and dry seasons when samples from distinct geographical locations and dissimilar exposures to the effects of climate change were analyzed. The observed changes were more perceptible in the roots of trees, particularly in declining forests distributed at lower altitudes in the more vulnerable mountains. These findings align with previous studies and lay the groundwork for further research on the molecular level. Molecular and genomic approaches offer valuable insights for mitigating climate stress and safeguarding this endangered conifer.

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The emergence of new plant diseases is an increasingly important concern. Climate change is likely to be among the factors causing most of the emerging diseases endangering forest and tree heritage around the world. Such diseases may be caused by latent pathogens or microorganisms cryptically associated with plants. The shift from a non-pathogenic to a pathogenic stage may depend on physiological alterations of the host, environmental changes, and/or stress factors. In some woods of the Salento Peninsula (Apulia Region, Italy), sudden declines of holm oak plants (Quercus ilex L.) have been observed since 2016. The morphological and molecular characterization of representative fungal isolates associated with cankers and necrosis in declining plants indicated that these isolates belong to the Botryosphaeriaceae family, and the most frequent species were Diplodia corticola and Diplodia quercivora, followed by Neofusicoccum vitifusiforme. In artificially inoculated young holm oak plants, both D. corticola and D. quercivora species produced intense and severe subcortical and leaf margin necrosis. N. vitifusiforme, although less aggressive, induced the same symptoms. Our research, in addition to confirming the involvement of D. corticola in olm oak decline, represents the first report of D. quercivora as a new pathogen of Q. ilex in Italy. Furthermore, to the best of our knowledge, we also found N. vitifusiforme as a new pathogen of Q. ilex.

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Episodic tree mortality can be caused by various reasons. This study describes climate-driven tree mortality and tree growth in the Black Forest mountain range in Germany. It is based on a 68-year consistent data series describing the annual mortality of all trees growing in a forest area of almost 250 thousand ha. The study excludes mortality caused by storm, snow and ice, and fire. The sequence of the remaining mortality, the so-called "desiccated trees," is analyzed and compared with the sequence of the climatic water balance during the growing season and the annual radial growth of Norway spruce in the Black Forest. The annual radial growth series covers 121 years and the climatic water balance series 140 years. These unique time series enable a quantitative assessment of multidecadal drought and heat impacts on growth and mortality of forest trees on a regional spatial scale. Data compiled here suggest that the mortality of desiccated trees in the Black Forest during the last 68 years is driven by the climatic water balance. Decreasing climatic water balance coincided with an increase in tree mortality and growth decline. Consecutive hot and dry summers enhance mortality and growth decline as a consequence of drought legacies lasting several years. The sensitivity of tree growth and mortality to changes in the climatic water balance increases with the decreasing trend of the climatic water balance. The findings identify the climatic water balance as the main driver of mortality and growth variation during the 68-year observation period on a landscape-scale including a variety of different sites. They suggest that bark beetle population dynamics modify mortality rates. They as well provide evidence that the mortality during the last 140 years never was as high as in the most recent years.

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Global forests are increasingly being threatened by altered climatic conditions and increased attacks by pests and pathogens. The complex ecological interactions among pathogens, microbial communities, tree hosts and the environment are important drivers of forest dynamics. Little is known about the ecology of forest pathology and related microbial communities in temperate forests of the southern hemisphere. In this study, we used next-generation sequencing to characterize sapwood-inhabiting fungal communities in North Patagonian Nothofagus forests and assessed patterns of diversity of taxa and ecological guilds across climatic, site and host variables (health condition and compartment) as a contribution to Nothofagus autecology. The diversity patterns inferred through the metabarcoding analysis were similar to those obtained through culture-dependent approaches. However, we detected additional heterogeneity and greater richness with culture-free methods. Host species was the strongest driver of fungal community structure and composition, while host health status was the weakest. The relative impacts of site, season, plant compartment and health status were different for each tree species; these differences can be interpreted as a matter of water availability. For Nothofagus dombeyi, which is distributed across a wide range of climatic conditions, site was the strongest driver of community composition. The microbiome of N. pumilio varied more with season and temperature, a relevant factor for forest conservation in the present climate change scenario. Both species carry a number of potential fungal pathogens in their sapwood, whether they exhibit symptoms or not. Our results provide insight into the diversity of fungi associated with the complex pathobiome of the dominant Nothofagus species in southern South America.

Los bosques del mundo están cada vez más amenazados por las condiciones climáticas alteradas y el aumento de los ataques de plagas y patógenos. Las complejas interacciones ecológicas entre los patógenos, las comunidades microbianas, los árboles hospedantes y el medio ambiente son impulsores importantes de la dinámica forestal. Poco se sabe sobre la ecología de la patología forestal y las comunidades microbianas relacionadas en los bosques templados del hemisferio sur. En este estudio, utilizamos la secuenciación Illumina para caracterizar las comunidades de hongos que habitan en la albura en los bosques de Nothofagus de la Patagonia Norte y evaluamos los patrones de diversidad de taxones y gremios ecológicos a través de variables climáticas, de sitio y de hospedante (identidad, condición de salud y compartimento) como una contribución a la autoecología de los Nothofagus. Los patrones de diversidad inferidos a través del análisis metabarcoding fueron similares a los obtenidos a través de enfoques dependientes de cultivo. Sin embargo, detectamos mayor heterogeneidad y mayor riqueza con métodos independientes de cultivo. La especie hospedante fue el modelador más fuerte de la estructura y composición de la comunidad fúngica, mientras que el estado de salud del hospedante fue el más débil. El impacto relativo del sitio, la estación, el compartimento y el estado de salud fueron diferentes para cada especie de árbol; estas diferencias pueden interpretarse en clave de disponibilidad de agua. Para N. dombeyi, que se distribuye a lo largo de una amplia gama de condiciones climáticas, el sitio fue el principal modelador de la composición de la comunidad. El micobioma de Nothofagus pumilio varió más con la estación y la temperatura, un factor relevante para la conservación de los bosques en el escenario actual de cambio climático. Ambas especies portan una serie de patógenos fúngicos potenciales en su albura, ya sea que muestren síntomas o no. Nuestros resultados brindan una idea de la diversidad de hongos asociados con el complejo patobioma de las especies dominantes de Nothofagus en el sur de América del Sur.

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Cork oak (Quercus suber L.) is an evergreen tree native to SW Europe and NW Africa. It covers 2·106 ha in the western Mediterranean basin, forms heterogeneous forest ecosystems and represents an important source of income derived from cork production. While in Iberia, Italy, Tunisia and Algeria, drought and several endemic pathogens have been associated with cork oak decline (Moricca et al. 2016; Smahi et al. 2017), in Morocco there is no evidence, apart from overgrazing and human intervention (Fennane and Rejdali 2015), of a pathogen associated with oak decline. In December 2019, extensive dieback and mortality of 60-year-old cork oak trees were observed in a natural stand of ca 150 ha located 5 km east from Touazithe, in Maâmora forest, Morocco (34°13'38''N, 6°14'51''W - 87 m a.s.l.). Two years before, Q. suber seedlings from a local nursery were planted to increase tree density. Symptoms in trees and planted seedlings included chlorosis, reddish-brown discoloration of the whole crown and dieback starting in the upper crown. Root rot and lack of fine roots were observed. Tree mortality was estimated at ca 30%, and disease incidences of trees and seedlings were 45 and 70%, respectively. A Phytophthora species was consistently isolated from the rhizosphere of 3 symptomatic trees randomly selected at the site using leaves as bait (Jung et al. 1996). On carrot agar Phytophthora colonies were uniform and cottonwool-like. Sporangia were typically terminal, with ovoid, and obpyriform shape, mostly papillate, measuring 30.7 ± 4.7 µm length and 22.7 ± 4.1 µm wide. Oogonia were produced in single culture, and they were globose to subglobose, elongated to ellipsoid, 32.1 ± 2.9 µm in diameter and 46.1 ± 4.8 µm in length. Oospores were usually spherical, thick-walled, and measured 28.1 ± 2.4 µm. Antheridia were paragynous, mostly spherical, measuring 12.2 ± 1.4 µm. Isolates had minimum and maximum temperatures of 5 °C and 30 °C, respectively, and a growth optimum at 20 °C. Apart from the small size of sporangia, features were typical of Phytophthora quercina Jung. The identity of a representative strain (TJ1500) was corroborated by sequencing the ITS and mitochondrial cox1 gene regions, and BLAST search in GenBank showed 100% homology with sequences of the ex-type culture of P. quercina (KF358229 and KF358241 accessions, respectively). Both sequences of the representative isolate were submitted to GenBank (accessions OP086243 and OP290549). The strain TJ1500 is currently stored within the culture collections of the Mendel University in Brno and the University of Sassari. Its pathogenicity was verified and compared with a P. cinnamomi strain in a soil infestation test with one-year-old cork oak seedlings (Corcobado et al. 2017). Five months after inoculation, the symptoms described were observed in the seedlings, and fine root weight of plants inoculated with the TJ1500 strain and P. cinnamomi was reduced by 19 and 42%, respectively, in relation to non-inoculated controls. The pathogen was re-isolated from the necrotic roots, thus fulfilling Koch's postulates. So far, P. quercina has been reported associated with chronic mortality of cork oak in new plantations in Spain (Martín-García et al. 2015; Jung et al. 2016) and natural forests in Italy (Seddaiu et al. 2020). To our knowledge this is the first report of P. quercina in Morocco. Givenat Morocco is an important cork producing country, our finding warns about the risk this pathogen poses to Q. suber and other North African oaks.

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Phytophthora species can be found in multiple substrates. Due to dormancy of resting structures and presence of faster-growing antagonists, direct isolation of Phytophthora can be difficult to achieve, and indirect baiting methods often reach higher isolation frequencies. In this chapter, different methodologies are described for sampling and for the successful isolation of Phytophthora species from natural ecosystems. Sampling methods for soil, roots, bark cankers, and waterbodies are described. Agar recipes and guidance on the selection of suitable tissue to perform direct isolations are provided. A range of different baiting techniques are described for isolation of Phytophthora from different substrates. Purification methods to obtain clean and non-mixed cultures and conservation methods for pure cultures are shown. Two pathogenicity test methods for soil infestation and for under-bark inoculation, respectively, are described in detail.

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This paper presents research results on forest decline in Serbia. The results were obtained through monitoring defoliation of 34 tree species at 130 sample plots during the period from 2004 to 2018. This research aimed to determine whether the occurrence of defoliation and tree mortality were caused by drought. Defoliation was assessed in 5% steps according to the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) methodology. All the trees recorded as dead were singled out, and annual mortality rates were calculated. To determine changes in air temperature and precipitation regimes during the study period, we processed and analysed climatic data related to air temperature and precipitation throughout the year and in the growing season at 28 main weather stations in Serbia. Tree mortality patterns were established by classifying trees into three groups. The first group of trees exhibited a gradual increase in defoliation during the last few years of monitoring, with dying as the final outcome. The second group was characterised by sudden death of trees. The third group of trees reached a higher degree of defoliation immediately after the first monitoring year, and the trees died after several years. Tree mortality rates were compared between years using the Standardised Precipitation Evaporation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI), the most common methods used to monitor drought. The most intensive forest decline was recorded during the period from 2013 to 2016, when the largest percentage of the total number of all trees died. According to the annual mortality rates calculated for the three observation periods (2004-2008, 2009-2013, and 2014-2018) the highest forest decline rate was recorded in the period from 2014 to 2018, with no statistically significant difference between broadleaved and coniferous tree species. As the sample of coniferous species was small, the number of sample plots should be increased in order to achieve better systematic forest condition monitoring in Serbia. The analysis of the relationship between defoliation and climatic parameters proved the correlation between them. It was noted that the forest decline in Serbia was preceded by an extremely dry period with high temperatures from 2011 to 2013, supporting the hypothesis that it was caused by drought. We therefore conclude that these unfavourable climatic conditions had serious and long-term consequences on forest ecosystems in Serbia.

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Drought-related tree mortality is a global phenomenon that currently affects a wide range of forests. Key functional variables on plant hydraulics, carbon economy, growth and allocation have been identified and play a role in tree drought responses. However, tree mortality thresholds based on such variables are difficult to identify, especially under field conditions. We studied several Aleppo pine populations differently affected by an extreme drought event in 2014, with mortality rates ranging from no mortality to 90% in the most severely affected population. We hypothesized that mortality is linked with high levels of xylem embolism, i.e., hydraulic dysfunction, which would also lead to lower tree resistance to drought in subsequent years. Despite not finding any differences among populations in the vulnerability curves to xylem embolism, there were large differences in the hydraulic safety margin (HSM) and the hydraulic dysfunction level. High mortality rates were associated with a negative HSM when xylem embolism reached values over 60%. We also found forest weakening and post-drought mortality related to a low hydraulic water transport capacity, reduced plant growth, low carbohydrate contents and high pest infestation rates. Our results highlight the importance of drought severity and the hydraulic dysfunction level on pine mortality, as well as post-drought conditions during recovery processes.

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Afforestation is among the most effective means of preventing and controlling desertification. Silver poplar (Populus alba) is commonly planted tree species for afforestation of the Horqin Sandy Land of China. However, this species has exhibited some drawbacks such as top shoot dieback, premature senescence and mortality, and soil and ecosystems degradation. In contrast, Siberian elm (Ulmus pumila) rarely experiences these problems in the same regions. Ecological stoichiometry plays a vital role in exploring ecological processes and nutrient cycle relationships in plant-litter-soil systems. To explore the differences in the carbon (C), nitrogen (N), and phosphorus (P) balance, the stoichiometry characteristics and stoichiometric homeostasis in elm and poplar plantations in the Horqin Sandy Land, we measured C, N, and P concentrations in leaves, branches, roots, litter, and soils and analyzed N and P resorption efficiencies in the two plantations. The results showed that soil C and N concentrations, C:P, and N:P were greater in the elm plantation than in the poplar plantation. The leaf and root C:P and N:P during summer and litter N and P concentrations were greater, whereas N and P resorption efficiencies were lower, in the elm plantation than in the poplar plantation. Generally, elm exhibited greater N:P homeostasis than poplar. N and N:P homeostasis were greater in roots than in leaves and branches in the elm plantation, but they varied with soil N concentration and N:P in the poplar plantation. These findings indicate that poplar exhibited more developed internal nutrient conservation and allocation strategies but poor nutrient accumulation in soil, which may contribute to degradation of poplar plantation. In contrast, elm tended to return more nutrients to the soil, showing an improved nutrient cycle in the plant-litter-soil system and increased soil C and N accumulation in the elm plantation. Therefore, compared with poplar, elm may be a more suitable afforestation tree species for the Horqin Sandy Land, in terms of promoting the accumulation of soil nutrients and enhancing nutrient cycling in the plant-litter-soil system.

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Declining forests usually face uncertain regeneration dynamics and recovery trajectories, which are challenging to forest management. In this study, we investigated the decline pattern of Castanopsis fargesii and examined the effects on conspecific seedling regeneration. We found that 61.45% of adult individuals were in decline and the smaller DBH size classes of trees (10-40 cm) had a greater probability of decline. Most of the intermediate decline (94.52%) and nondecline individuals (95.23%) did not worsen, and the crowns of 21.91% of the intermediate decline trees were recovered during 2013-2018. Adult tree decline had a negative effect on seed production (mean mature seed density of nondecline, intermediate decline, and high decline individuals was 167.3, 63.3, and 2.1 seeds/m2, respectively), but no effect on key seed traits. The seed survival rate of declining trees was greater than that of nondeclining trees at both the seed production and seed dispersal stages. The seed to seedling transition rates in canopy gaps, decline habitats, and nondecline habitats were 7.94%, 9.47%, and 109.24%, respectively. The survival rate and height growth of newly germinated seedlings were positively correlated with the light condition, which was notably accelerated in the canopy gaps. Taken together, these results indicate that the reduction in seed production of some adult trees had a weakly negative effect on new seedling recruitment, while the improved environmental condition after the decline significantly enhanced the survival and growth of both advanced and new germinated seedlings. Looking at the overall life history, the short-term defoliation and mortality of some C. fargesii adult trees can be regarded as a natural forest disturbance that favors conspecific seedling regeneration. High-intensity management measures would be unnecessary in cases of an emerging intermediate decline in this forest.

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The central mountainous area of Japan is affected by air pollutant emissions from nearby countries such as China and Korea. Sharp increases in the consumption of fossil fuels in the early 21st century, associated with rapid industrialization in China, resulted in long-range transport of pollutants from East Asia and increases in the harmful effects of pollution. However, the air pollutants emissions have decreased since 2006, when air pollution countermeasures were implemented in China. Furthermore, climatic patterns during 2008-2013 reduced tropospheric ozone concentrations around Japan. Such major changes in the social and climatic environment may have had a significant impact on forests. To investigate this, long-term forest monitoring data obtained at Buna-daira (1190 m a.s.l.), Buna-zaka (1090 m a.s.l.) and Arimine (1350 m a.s.l.) were analyzed. Buna-daira and Buna-zaka forests face the continental side of Mt. Tateyama in Toyama Prefecture. In both stands, the girth growth rate of Fagus crenata was found to have increased after 2008; however, such a tendency was not detected at Arimine, which is surrounded by mountains. The growth rates of Cryptomeria japonica, a conifer resistant to air pollution, were found to remain unchanged or decrease. Here, regional long-range transport of air pollution (including ozone and sulfur oxide) has been demonstrated to influence mountain forests in Japan. In particular, recent decreases in regional air pollution may be an important factor controlling increases in F. crenata, likely through changes in interspecific relationships between species sensitive to and tolerant of air pollution.

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Scots pine (Pinus sylvestris L.) is a widespread tolerant forest tree-species; however, its adaptability to environmental change differs among sites with various buffering capacity. In this study, we compared the spatial effects of aridity index (AI) and nitrogen deposition (ND) on biomass density in natural and man-made pine stands of differing soil fertility using geographically weighted multiple lag regression. Soil fertility was defined using soil series as zonal trophic (27.9%), acidic (48.2%), gleyed (15.2%) and as azonal exposed (2.5%), maple (2.4%), ash (0.8%), wet (2.1%) and peat (0.9%) under pine stands in the Czech Republic (Central Europe; 4290.5 km2; 130-1298 m a.s.l.). Annual AI and ND in every pine stand were estimated by intersection between raster and vector from 1 × 1 km grid for years 2000, 2003, 2007 and 2010 of severe non-specific forest damage spread. Biomass density was obtained from a MODIS 250 × 250 m raster using the enhanced vegetation index (EVI) for years 2000-2015, with a decrease in EVI indicating non-specific damage. Environmental change was assessed by comparing predictor values at EVI time t and t+λ. Non-specific damage was registered over 51.9% of total forest area. Less than 8.8% of damaged stands were natural and the rest (91.2%) of damaged stands were man-made. Pure pine stands were more damaged than mixed. The ND effect prevailed up to 2007, while AI dominated later. Temporal increasing ND effect under AI effectiveness led to the most significant pine stand damage in 2008 and 2014. Predictors from 2000 to 2007 afflicted 58.5% of non-specifically damaged stands at R2 0.09-0.76 (median 0.38), but from 2000 to 2010 afflicted 57.1% of the stands at R2 0.16-0.75 (median 0.40). The most damaged stands occurred on acidic sites. Mixed forest and sustainable management on natural sites seem as effective remediation reducing damage by ND.

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With the advent of Sentinel-2, it is now possible to generate large-scale chlorophyll content maps with unprecedented spatial and temporal resolution, suitable for monitoring ecological processes such as vegetative stress and/or decline. However methodological gaps exist for adapting this technology to heterogeneous natural vegetation and for transferring it among vegetation species or plan functional types. In this study, we investigated the use of Sentinel-2A imagery for estimating needle chlorophyll (Ca+b) in a sparse pine forest undergoing significant needle loss and tree mortality. Sentinel-2A scenes were acquired under two extreme viewing geometries (June vs. December 2016) coincident with the acquisition of high-spatial resolution hyperspectral imagery, and field measurements of needle chlorophyll content and crown leaf area index. Using the high-resolution hyperspectral scenes acquired over 61 validation sites we found the CI chlorophyll index R750/R710 and Macc index (which uses spectral bands centered at 680 nm, 710 nm and 780 nm) had the strongest relationship with needle chlorophyll content from individual tree crowns (r2 = 0.61 and r2 = 0.59, respectively; p < 0.001), while TCARI and TCARI/OSAVI, originally designed for uniform agricultural canopies, did not perform as well (r2 = 0.21 and r2 = 0.01, respectively). Using lower-resolution Sentinel-2A data validated against hyperspectral estimates and ground truth needle chlorophyll content, the red-edge index CI and the Sentinel-specific chlorophyll indices CI-Gitelson, NDRE1 and NDRE2 had the highest accuracy (with r2 values >0.7 for June and >0.4 for December; p < 0.001). The retrieval of needle chlorophyll content from the entire Sentinel-2A bandset using the radiative transfer model INFORM yielded r2 = 0.71 (RMSE = 8.1 µg/cm2) for June, r2 = 0.42 (RMSE = 12.2 µg/cm2) for December, and r2 = 0.6 (RMSE = 10.5 µg/cm2) as overall performance using the June and December datasets together. This study demonstrates the retrieval of leaf Ca+b with Sentinel-2A imagery by red-edge indices and by an inversion method based on a hybrid canopy reflectance model that accounts for tree density, background and shadow components common in sparse forest canopies.

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The natural distribution, habitat, growth and evolutionary history of tree species are strongly dependent on ecological and genetic processes in ecosystems subject to fluctuating climatic conditions, but there have been few experimental comparisons of sensitivity between species. We compared the responses of two broadleaved tree species (Fagus sylvatica and Quercus petraea) and two conifer tree species (Pinus sylvestris and Picea abies) to climatic transfers by fitting models containing the same climatic variables. We used published data from European provenance test networks to model the responses of individual populations nested within species. A mixed model approach was applied to develop a response function for tree height over climatic transfer distance, taking into account the climatic conditions at both the seed source and the test location. The two broadleaved species had flat climatic response curves, indicating high levels of plasticity in populations, facilitating adaptation to a broader range of environments, and conferring a high potential for resilience in the face of climatic change. By contrast, the two conifer species had response curves with more pronounced slopes, indicating a lower resilience to climate change. This finding may reflect stronger genetic clines in P. sylvestris and P. abies, which constrain their climate responses to narrower climatic ranges. The response functions had maxima that deviated from the expected maximum productivity in the climate of provenance towards cooler/moister climate conditions, which we interpreted as an adaptation lag. Unilateral, linear regression analyses following transfer to warmer and drier sites confirmed a decline in productivity, predictive of the likely impact of ongoing climate change on forest populations. The responses to mimicked climate change evaluated here are of considerable interest for forestry and ecology, supporting projections of expected performance based on "real-time" field data.

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Globally, nitrogen deposition increment has caused forest structural changes due to imbalanced plant nitrogen metabolism and subsequent carbon assimilation. Here, a 2 consecutive-year experiment was conducted to reveal the effects of canopy addition of nitrogen (CAN) on nitrogen absorption, assimilation, and allocation in leaves of three subtropical forest woody species (Castanea henryi, Ardisia quinquegona, and Blastus cochinchinensis). We hypothesized that CAN altered leaf nitrogen absorption, assimilation and partitioning of different plants in different ways in subtropical forest. It shows that CAN increased maximum photosynthetic rate (Amax), photosynthetic nitrogen use efficiency (PNUE), and metabolic protein content of the two understory species A. quinquegona and B. cochinchinensis. By contrary, for the overstory species, C. henryi, Amax, PNUE, and metabolic protein content were significantly reduced in response to CAN. We found that changes in leaf nitrogen metabolism were mainly due to the differences in enzyme (e.g. Ribulose-1,5-bisphosphate carboxylase, nitrate reductase, nitrite reductase and glutamine synthetase) activities under CAN treatment. Our results indicated that C. henryi may be more susceptible to CAN treatment, and both A. quinquegona and B. cochinchinensis could better adapt to CAN treatment but in different ways. Our findings may partially explain the ongoing degradation of subtropical forest into a community dominated by small trees and shrubs in recent decades. It is possible that persistent high levels of atmospheric nitrogen deposition will lead to the steady replacement of dominant woody species in this subtropical forest.

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The operational monitoring of forest decline requires the development of remote sensing methods that are sensitive to the spatiotemporal variations of pigment degradation and canopy defoliation. In this context, the red-edge spectral region (RESR) was proposed in the past due to its combined sensitivity to chlorophyll content and leaf area variation. In this study, the temporal dimension of the RESR was evaluated as a function of forest decline using a radiative transfer method with the PROSPECT and 3D FLIGHT models. These models were used to generate synthetic pine stands simulating decline and recovery processes over time and explore the temporal rate of change of the red-edge chlorophyll index (CI) as compared to the trajectories obtained for the structure-related Normalized Difference Vegetation Index (NDVI). The temporal trend method proposed here consisted of using synthetic spectra to calculate the theoretical boundaries of the subspace for healthy and declining pine trees in the temporal domain, defined by CItime=n/CItime=n+1 vs. NDVItime=n/NDVItime=n+1. Within these boundaries, trees undergoing decline and recovery processes showed different trajectories through this subspace. The method was then validated using three high-resolution airborne hyperspectral images acquired at 40 cm resolution and 260 spectral bands of 6.5 nm full-width half-maximum (FWHM) over a forest with widespread tree decline, along with field-based monitoring of chlorosis and defoliation (i.e., 'decline' status) in 663 trees between the years 2015 and 2016. The temporal rate of change of chlorophyll vs. structural indices, based on reflectance spectra extracted from the hyperspectral images, was different for trees undergoing decline, and aligned towards the decline baseline established using the radiative transfer models. By contrast, healthy trees over time aligned towards the theoretically obtained healthy baseline. The applicability of this temporal trend method to the red-edge bands of the MultiSpectral Imager (MSI) instrument on board Sentinel-2a for operational forest status monitoring was also explored by comparing the temporal rate of change of the Sentinel-2-derived CI over areas with declining and healthy trees. Results demonstrated that the Sentinel-2a red-edge region was sensitive to the temporal dimension of forest condition, as the relationships obtained for pixels in healthy condition deviated from those of pixels undergoing decline.

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The frequently observed forest decline in water-limited regions may be associated with impaired tree hydraulics, but the precise physiological mechanisms remain poorly understood. We compared hydraulic architecture of Mongolian pine (Pinus sylvestris var. mongolica) trees of different size classes from a plantation and a natural forest site to test whether greater hydraulic limitation with increasing size plays an important role in tree decline observed in the more water-limited plantation site. We found that trees from plantations overall showed significantly lower stem hydraulic efficiency. More importantly, plantation-grown trees showed significant declines in stem hydraulic conductivity and hydraulic safety margins as well as syndromes of stronger drought stress with increasing size, whereas no such trends were observed at the natural forest site. Most notably, the leaf to sapwood area ratio (LA/SA) showed a strong linear decline with increasing tree size at the plantation site. Although compensatory adjustments in LA/SA may mitigate the effect of increased water stress in larger trees, they may result in greater risk of carbon imbalance, eventually limiting tree growth at the plantation site. Our results provide a potential mechanistic explanation for the widespread decline of Mongolian pine trees in plantations of Northern China.

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Insects, diseases, fire and drought and other disturbances associated with global climate change contribute to forest decline and mortality in many parts of the world. Forest decline and mortality related to drought or insect outbreaks have been observed in North American aspen forests. However, little research has been done to partition and estimate their relative contributions to growth declines. In this study, we combined tree-ring width and basal area increment series from 40 trembling aspen (Populus tremuloides Michx.) sites along a latitudinal gradient (from 52° to 58°N) in western Canada and attempted to investigate the effect of drought and insect outbreaks on growth decline, and simultaneously partition and quantify their relative contributions. Results indicated that the influence of drought on forest decline was stronger than insect outbreaks, although both had significant effects. Furthermore, the influence of drought and insect outbreaks showed spatiotemporal variability. In addition, our data suggest that insect outbreaks could be triggered by warmer early spring temperature instead of drought, implicating that potentially increased insect outbreaks are expected with continued warming springs, which may further exacerbate growth decline and death in North America aspen mixed forests.

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Submediterranean forests are considered an ecotone between Mediterranean and Eurosiberian ecosystems, and are very sensitive to global change. A decline of Scots pine (Pinus sylvestris L.) and a related expansion of oak species (Quercus spp.) have been reported in the Spanish Pre-Pyrenees. Although this has been associated with increasing drought stress, the underlying mechanisms are not fully understood, and suitable monitoring protocols are lacking. The aim of this study is to bring insight into the physiological mechanisms anticipating selective decline of the pines, with particular focus on carbon and water relations. For this purpose, we performed a sampling campaign covering two growing seasons in a mixed stand of P. sylvestris and Quercus subpyrenaica E.H del Villar. We sampled seasonally twig xylem and soil for water isotope composition (δ18O and δ2H), leaves for carbon isotope composition (δ13C) and stems to quantify non-structural carbohydrates (NSC) concentration, and measured water potential and leaf gas exchange. The first summer drought was severe for both species, reaching low predawn water potential (-2.2 MPa), very low stomatal conductance (12 ± 1.0 mmol m-2 s-1) and near-zero or even negative net photosynthesis, particularly in P. sylvestris (-0.6 ± 0.34 µmol m-2 s-1 in oaks, -1.3 ± 0.16 µmol m-2 s-1 in pines). Hence, the tighter stomatal control and more isohydric strategy of P. sylvestris resulted in larger limitations on carbon assimilation, and this was also reflected in carbon storage, showing twofold larger total NSC concentration in oaks than in pines (7.8 ± 2.4% and 4.0 ± 1.3%, respectively). We observed a faster recovery of predawn water potential after summer drought in Q. subpyrenaica than in P. sylvestris (-0.8 MPa and -1.1 MPa, respectively). As supported by the isotopic data, this was probably associated with a deeper and more reliable water supply in Q. subpyrenaica. In line with these short-term observations, we found a more pronounced negative effect of steadily increasing drought stress on long-term growth in pines compared with oaks. All these observations confer evidence of early warning of P. sylvestris decline and indicate the adaptive advantage of Q. subpyrenaica in the area.

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RESUMEN

Forest decline is either caused by damage or else by vulnerability due to unfavourable growth conditions or due to unnatural silvicultural systems. Here, we assess forest decline in the Czech Republic (Central Europe) using fuzzy functions, fuzzy sets and fuzzy rating of ecosystem properties over a 1×1km grid. The model was divided into fuzzy functions of the abiotic predictors of growth conditions (Fpred including temperature, precipitation, acid deposition, soil data and relative site insolation) and forest biomass receptors (Frec including remote sensing data, density and volume of aboveground biomass, and surface humus chemical data). Fuzzy functions were designed at the limits of unfavourable, undetermined or favourable effects on the forest ecosystem health status. Fuzzy sets were distinguished through similarity in a particular membership of the properties at the limits of the forest status margins. Fuzzy rating was obtained from the least difference of Fpred-Frec. Unfavourable Fpred within unfavourable Frec indicated chronic damage, favourable Fpred within unfavourable Frec indicated acute damage, and unfavourable Fpred within favourable Frec indicated vulnerability. The model in the 1×1km grid was validated through spatial intersection with a point field of uniform forest stands. Favourable status was characterised by soil base saturation (BS)>50%, BCC/Al>1, Corg>1%, MgO>6g/kg, and nitrogen deposition<1200mol(H+)/ha·year. Vulnerable forests had BShumus 46-60%, BCC/Al 9-20 and NDVI≈0.42. Chronic forest damage occurs in areas with low temperatures, high nitrogen deposition, and low soil BS and Corg levels. In the Czech Republic, 10% of forests were considered non-damaged and 77% vulnerable, with damage considered acute in 7% of forests and chronic in 5%. The fuzzy model used suggests that improvement in forest health will depend on decreasing environmental load and restoration concordance between growth conditions and tree species composition.