Search

Extreme droughts are expected to increase in frequency and severity in many regions of the world, threatening multiple ecosystem services provided by forests. Effective strategies to adapt forests to such droughts require comprehensive information on the effects and importance of the factors influencing forest resistance and resilience. We used a unique combination of inventory and dendrochronological data from a long‐term (>30 years) silvicultural experiment in mixed silver fir and Norway spruce mountain forests along a temperature and precipitation gradient in southwestern Germany. We aimed at examining the mechanisms and forest stand characteristics underpinning the resistance and resilience to past mild and severe droughts. We found that (i) fir benefited from mild droughts and showed higher resistance (i.e., lower growth loss during drought) and resilience (i.e., faster return to pre‐drought growth levels) than spruce to all droughts; (ii) species identity determined mild drought responses while species interactions and management‐related factors strongly influenced the responses to severe droughts; (iii) intraspecific and interspecific interactions had contrasting effects on the two species, with spruce being less resistant to severe droughts when exposed to interaction with fir and beech; (iv) higher values of residual stand basal area following thinning were associated with lower resistance and resilience to severe droughts; and (v) larger trees were resilient to mild drought events but highly vulnerable to severe droughts. Our study provides an analytical approach for examining the effects of different factors on individual tree‐ and stand‐level drought response. The forests investigated here were to a certain extent resilient to mild droughts, and even benefited from such conditions, but were strongly affected by severe droughts. Lastly, negative effects of severe droughts can be reduced through modifying species composition, tree size distribution and stand density in mixed silver fir‐Norway spruce forests., We examined mechanisms and stand characteristics underpinning the growth resistance (Rt), recovery (Rc) and resilience (Rs) to past mild and severe droughts in mixed silver fir and Norway spruce mountain forests in southwestern Germany. We found that the forests investigated were to a certain extent resilient to mild droughts, and even benefited from such conditions, but were strongly affected by severe droughts (especially those stands with higher residual basal area and larger trees). Lastly, negative effects of severe droughts can be reduced through modifying species composition, tree size distribution and stand density in mixed silver fir‐Norway spruce forests.

As climate change should lead to an increase in the vulnerability and the sensitivity of forests to extreme climatic events, quantifying and predicting their response to more severe droughts remains a key task for foresters. Furthermore, recent works have suggested that tree diversity may affect forest ecosystem functioning, including their response to extreme events. In this study we aimed at testing whether the growth response of forest stands to stressful climatic events varied between mixed and monospecific stands, under various environmental conditions. We focused on beech-fir forests (Fagus sylvatica [L.] and Abies alba [L.]) and beech-oak forests (F. sylvatica [L.] and Quercus pubescent [L.]) in the French Alps. We used a dendrochronological dataset sampled in forest plots organized by triplets (one mixture and two monospecific stands) distributed in six sites along a latitudinal gradient. We tested (1) whether stand diversity (two-species stands vs monospecific stands) modulates the stands’ response to drought events in terms of productivity, (2) whether species identity may drive the diversity effect on resistance and recovery, and (3) whether this can be explained by interspecific interactions. We found that (1) interspecific differences in response to extreme drought events (possibly due to interspecific differences in hydraulic characteristics) can induce a mixture effect on stand growth, although it appeared (2) to be strongly depending on species identity (positive effect only found for beech-fir mixed stands), while (3) there were no significant non-additive effects of diversity on stand resistance and recovery, except for some specific cases. Overall, our study shows that promoting selected mixed stands management may buffer extreme drought effect on stand productivity.

Studying intra-annual growth dynamics through a modelling approach has proved useful for characterizing differences in wood phenology between species and deviations from the expected tree growth due to climate change.The aim of this paper was to determine the climatic factors and the inter-and intra-specific competition traits that drive intra-annual secondary growth dynamics in three Mediterranean tree species that naturally co-exist in the Spanish Northern Plateau (Pinus pinea L., Juniperus thurifera L., and Quercus ilex subsp. ballota (Desf.) Samp.). For this purpose, we obtained girth increment data from band dendrometers installed on 58 trees from May 2012 to April 2015. Half of them were located in a high-density plot and half were in a low-density plot. These data were analysed through a nonlinear multifactorial model considering daily and cumulative climatic effects together with inter-and intra-specific competition indices.Our results provide evidence of a spatiotemporal niche separation between species. Compared with other species, Q. ilex subsp. ballota showed a wider thermal amplitude, a higher mean optimum temperature for growth, and a milder effect of drought on its secondary growth.Under current climate change projections, simulations based on our model predict a secondary growth reduction for P. pinea and a growth enhancement for Q. ilex subsp. ballota in these Mediterranean mixtures.

Monitoring cambial phenology and intra-annual growth dynamics is a useful approach for characterizing the tree growth response to climate change. However, there have been few reports concerning intra-annual wood formation in lowland temperate forests with high time resolution, especially for the comparison between deciduous and coniferous species. The main objective of this study was to determine how the timing, duration and rate of radial growth change between species as related to leaf phenology and the dynamics of non-structural carbohydrates (NSC) under the same climatic conditions. We studied two deciduous species, Fagus sylvatica L. and Quercus petraea (Matt.) Liebl., and an evergreen conifer, Pinus sylvestris L. During the 2009 growing season, we weekly monitored (i) the stem radial increment using dendrometers, (ii) the xylem growth using microcoring and (iii) the leaf phenology from direct observations of the tree crowns. The NSC content was also measured in the eight last rings of the stem cores in April, June and August 2009. The leaf phenology, NSC storage and intra-annual growth were clearly different between species, highlighting their contrasting carbon allocation. Beech growth began just after budburst, with a maximal growth rate when the leaves were mature and variations in the NSC content were low. Thus, beech radial growth seemed highly dependent on leaf photosynthesis. For oak, earlywood quickly developed before budburst, which probably led to the starch decrease quantified in the stem from April to June. For pine, growth began before the needles unfolding and the lack of NSC decrease during the growing season suggested that the substrates for radial growth were new assimilates of the needles from the previous year. Only for oak, the pattern determined from the intra-annual growth measured using microcoring differed from the pattern determined from dendrometer data. For all species, the ring width was significantly influenced by growth duration and not by growth rate, which differs from previous studies. The observed between-species difference at the intra-annual scale is key information for anticipating suitability of future species in temperate forests.

In addition to global warming, the frequency and the intensity of droughts will probably increase in central and southern Europe. Resulting climate changes and soil water deficits could alter tree growth, according to sensitivity of each species. The aim of this study was to compare the growth response of three European species (Fagus sylvatica, Quercus petraea and Pinus sylvestris) to climatic variations and soil water deficits in the same temperate forest. Three mature stands per species were sampled to obtain earlywood, latewood and total ring chronologies from 1960 to 2007. Climate-growth relationships were established by bootstrapped correlations and response function analysis. Monthly bioclimatic regressors were simulated by a physiological water balance model that used daily climatic data and stand parameters to estimate soil water deficits. Our results highlighted a common sensitivity to precipitation from May to July for the dominant tree growth of the three species but also differences in the species vulnerability to climate and soil water deficits. Beech was the most sensitive species to the climatic conditions of the current growing season. Beech growth was positively correlated with precipitation from May to July and negatively with maximal temperatures in June and July. Oak growth was negatively correlated with minimal temperatures in the previous August and September and positively with precipitation in the previous October and December during pointer years. This led to long-term consequences for growth, probably due to carbon reserve depletion. Pine growth was positively influenced by warm December but was also vulnerable to maximal temperatures and soil water deficits from June to August. The climate in August only influenced the pine growth probably because the growing season of pine was longer than that of the deciduous species. For both oak and pine, latewood was the component that was most sensitive to climatic variations and soil water deficits. According to the study findings, an increase in the frequency and the intensity of droughts could affect the three species. Maximum summer temperatures could have negative impacts for beech and pine growth. Dry and warm autumns could lead to long-term consequences and decrease the oak growth. (C) 2011 Elsevier B.V. All rights reserved.

We investigated whether timing and rate of growth are related to the life strategies and fitness of three conifer species. Intraannual dynamics of wood formation, shoot elongation and needle phenology were monitored over 3 years in five Norway spruces (Picea abies (L.) Karst.), five Scots pines ( Pinus sylvestris L.) and five silver firs ( Abies alba Mill.) grown intermixed. For the three species, the growing season (delimited by cambial activity onset and cessation) lasted about 4 months, while the whole process of wood formation lasted 5–6 months. Needle unfolding and shoot elongation followed the onset of cambial activity and lasted only one-third of the season. Pines exhibited an ‘extensive strategy’ of cambial activity, with long durations but low growth rates, while firs and spruces adopted an ‘intensive strategy’ with shorter durations but higher growth rates. We estimated that about 75% of the annual radial increment variability was attributable to the rate of cell production, and only 25% to its duration. Cambial activity rates culminated at the same time for the three species, whereas shoot elongation reached its maximal rate earlier in pines. Results show that species-specific life strategies are recognizable through func tional traits of intra-annual growth dynamics. The opposition between Scots pine extensive strategy and silver fir and Norway spruce intensive strategy supports the theory that pioneer species are greater resource expenders and develop riskier life strategies to capture resources, while shade-tolerant species utilize resources more efficiently and develop safer life strate gies. Despite different strategies, synchronicity of the maximal rates of cambial activity suggests a strong functional convergence between co-existing conifer species, resulting in head-on competition for resources.

Summary Amplification profiles of the M13 minisatellite core sequence from 47 isolates of Heterobasidion annosum s.l., collected mostly from Abies alba in different European countries, were analysed in comparison with profiles of four isolates of H. annosum s.s. and three isolates of H. parviporum. Genetic variation within and among groups of populations was studied by analysis of molecular variance. A dendrogram constructed with the Neighbor-Joining method differentiated the three species from each other. Isolates of H. abietinum from Balkan peninsula, Italian peninsula and French Pyrenees each separated into clusters according to geographical origin. Such clear geographic clustering was not detected among isolates from central Europe. The variations between H. abietinum isolates in relation to the migration history of Abies are discussed.

The taxonomic status and relationship of Tricholoma sulphureum and the similar T. bufonium were investigated using different sets of characters. These included morphological data on fruit bodies, ecological and chorological data, and analysis of the sequence data obtained for the ITS of basidiomes of different ecological and geographic origin. Moreover, the ectomycorrhizas formed by T. bufonium on Abies alba and Quercus sp. were characterised, and anatomical features compared with those of T. sulphureum mycorrhizas on coniferous and broad-leaved host trees. Our results revealed extensive ITS variation in members of the T. sulphureum group, but this variation was not correlated with morphology, ecology, or geographical distribution. We conclude that T. bufonium cannot be maintained as an autonomous taxon and should be treated as an infraspecific variant of T. sulphureum.