Mother trees, altruistic fungi, and the perils of plant personification.

There are growing doubts about the true role of the common mycorrhizal networks (CMN or wood wide web) connecting the roots of trees in forests. We question the claims of a substantial carbon transfer from 'mother trees' to their offspring and nearby seedlings through the CMN. Recent reviews show that evidence for the 'mother tree concept' is inconclusive or absent. The origin of this concept seems to stem from a desire to humanize plant life but can lead to misunderstandings and false interpretations and may eventually harm rather than help the commendable cause of preserving forests. Two recent books serve as examples: The Hidden Life of Trees and Finding the Mother Tree.

Forest stand productivity derived from site conditions: an assessment of old Douglas-fir stands (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) in Central Europe.

KEY MESSAGE: Douglas-fir growth correlates with the climate, the soil moisture regime, and the soil nutrient status, reflecting a broad physiological amplitude. Even though planting this non-native tree species is suggested as a viable strategy to improve adaptiveness of European forests to a more extreme climate and to assure future productivity, the expected temperature increase may induce a decline in forest stand productivity for Douglas-fir in already warm and dry regions. CONTEXT: Tree species selection is one of the most important forest management decisions to enhance forest productivity and stand stability on a given site. Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii), a non-native species from north-western America, is seen as an important additional species option for adapting Central European forests to a changing climate. AIMS: This study assesses Douglas-fir forest productivity derived from site conditions. We investigate climatic and physico-chemical soil characteristics and productivity of 28 mature Douglas-fir stands growing on siliceous, as well as carbonate bedrock material in southern Germany and north-eastern Austria. METHODS: The importance of climatic and physico-chemical soil characteristics was analyzed with the machine learning method Random Forests. RESULTS: The results show that Douglas-fir growth correlates with climate, soil moisture, and soil nutrient availability derived from ten climatic and physico-chemical soil parameters. CONCLUSION: The broad pH optimum between 4.5 and 7.2 reflects the broad physiological amplitude of Douglas-fir, and no significant differences were detectable between carbonate and siliceous bedrock. We also conclude that climate change may induce a forest stand productivity decline, because lower productivity with the highest mean summer temperature across our study range was observed at the warmest sites in Eastern Austria.

North American Douglas-fir (P. menziesii) in Europe: establishment and reproduction within new geographic space without consequences for its genetic diversity.

Genetic admixture and plasticity along with propagule pressure, large seed dispersal distances and fast adaptation support successful establishment and spread of introduced species outside their native range. Consequently, introductions may display climatic niche shifts in the introduced range. Douglas-fir, a controversial forest and ornamental conifer represented by two ecologically different and hybridising varieties, was transferred multiple times outside the native range in North America. Here, we compare climatic and genetic patterns of 38 native populations from North America with six old Pseudotsuga menziesii populations with natural regeneration in the introduced range in Central Europe. Following variety and geographic origin assessment of introduced populations, genotypic and climatic data were examined for signatures of inter-varietal gene flow, reduced genetic diversity, presence of fine-scale spatial genetic structure (SGS), dispersal patterns, and climate similarities between native and introduced range. In the introduced range, dominating coastal variety originated from a restricted area in the US, whereas the interior variety, with limited presence in the European sites, displayed wider geographic origin. Variety hybrids with contributing coastal, but not the interior parent were identified. Differences in genetic diversity between both ranges, but also among the parent and their respective offspring populations in Europe were not found. Old populations in general lacked any SGS, whereas natural regeneration revealed different patterns of SGS. Distances of propagule dispersal ranged between 2.5 and 92 m. The climate of the studied European introduced range was most similar to the climate of the coastal variety from the western Cascade range from which the majority of the analysed coastal European Douglas-fir, but not the European interior variety, was assigned to originate. The results we present here shed not only light on dynamics of invasive species in the introduced range in general, but also allow for refinement of climatic niche modeling when using lower than species level.

Climate variability drives recent tree mortality in Europe.

Tree mortality is an important process in forest ecosystems, frequently hypothesized to be highly climate sensitive. Yet, tree death remains one of the least understood processes of forest dynamics. Recently, changes in tree mortality have been observed in forests around the globe, which could profoundly affect ecosystem functioning and services provisioning to society. We describe continental-scale patterns of recent tree mortality from the only consistent pan-European forest monitoring network, identifying recent mortality hotspots in southern and northern Europe. Analyzing 925,462 annual observations of 235,895 trees between 2000 and 2012, we determine the influence of climate variability and tree age on interannual variation in tree mortality using Cox proportional hazard models. Warm summers as well as high seasonal variability in precipitation increased the likelihood of tree death. However, our data also suggest that reduced cold-induced mortality could compensate increased mortality related to peak temperatures in a warming climate. Besides climate variability, age was an important driver of tree mortality, with individual mortality probability decreasing with age over the first century of a trees life. A considerable portion of the observed variation in tree mortality could be explained by satellite-derived net primary productivity, suggesting that widely available remote sensing products can be used as an early warning indicator of widespread tree mortality. Our findings advance the understanding of patterns of large-scale tree mortality by demonstrating the influence of seasonal and diurnal climate variation, and highlight the potential of state-of-the-art remote sensing to anticipate an increased likelihood of tree mortality in space and time.

Intervarietal and intravarietal genetic structure in Douglas-fir: nuclear SSRs bring novel insights into past population demographic processes, phylogeography, and intervarietal hybridization.

Douglas-fir (Pseudotsuga menziesii) is one of numerous wide-range forest tree species represented by subspecies/varieties, which hybridize in contact zones. This study examined the genetic structure of this North American conifer and its two hybridizing varieties, coastal and Rocky Mountain, at intervarietal and intravarietal level. The genetic structure was subsequently associated with the Pleistocene refugial history, postglacial migration and intervarietal hybridization/introgression. Thirty-eight populations from the USA and Canada were genotyped for 13 nuclear SSRs and analyzed with simulations and traditional population genetic structuring methods. Eight genetic clusters were identified. The coastal clusters embodied five refugial populations originating from five distinct refugia. Four coastal refugial populations, three from California and one from western Canada, diverged during the Pleistocene (56.9-40.1 ka). The three Rocky Mountain clusters reflected distinct refugial populations of three glacial refugia. For Canada, ice covered during the Last Glacial Maximum, we present the following three findings. (1) One refugial population of each variety was revealed in the north of the distribution range. Additional research including paleodata is required to support and determine whether both northern populations originated from cryptic refugia situated south or north of the ice-covered area. (2) An interplay between intravarietal gene flow of different refugial populations and intervarietal gene flow by hybridization and introgression was identified. (3) The Canadian hybrid zone displayed predominantly introgressants of the Rocky Mountain into the coastal variety. This study provides new insights into the complex Quaternary dynamics of this conifer essential for understanding its evolution (outside and inside the native range), adaptation to future climates and for forest management.

Assessing the impacts of climate change and nitrogen deposition on Norway spruce (Picea abies L. Karst) growth in Austria with BIOME-BGC.

The aim of this paper is to determine whether a detectable impact of climate change is apparent in Austrian forests. In regions of complex terrain such as most of Austria, climatic trends over the past 50 years show marked geographic variability. As climate is one of the key drivers of forest growth, a comparison of growth characteristics between regions with different trends in temperature and precipitation can give insights into the impact of climatic change on forests. This study uses data from several hundred climate recording stations, interpolated to measurement sites of the Austrian National Forest Inventory (NFI). Austria as a whole shows a warming trend over the past 50 years and little overall change in precipitation. The warming trends, however, vary considerably across certain regions and regional precipitation trends vary widely in both directions, which cancel out on the national scale These differences allow the delineation of 'climatic change zones' with internally consistent climatic trends that differ from other zones. This study applies the species-specific adaptation of the biogeochemical model BIOME-BGC to Norway spruce (Picea abies (L.) Karst) across a range of Austrian climatic change zones, using input data from a number of national databases. The relative influence of extant climate change on forest growth is quantified, and compared with the far greater impact of non-climatic factors. At the national scale, climate change is found to have negligible effect on Norway spruce productivity, due in part to opposing effects at the regional level. The magnitudes of the modeled non-climatic influences on aboveground woody biomass increment increases are consistent with previously reported values of 20-40 kg of added stem carbon sequestration per kilogram of additional nitrogen deposition, while climate responses are of a magnitude difficult to detect in NFI data.

Modeling effects of hydrological changes on the carbon and nitrogen balance of oak in floodplains.

We extended the applicability of the ecosystem model BIOME-BGC to floodplain ecosystems to study effects of hydrological changes on Quercus robur L. stands. The extended model assesses floodplain peculiarities, i.e., seasonal flooding and water infiltration from the groundwater table. Our interest was the tradeoff between (a). maintaining regional applicability with respect to available model input information, (b). incorporating the necessary mechanistic detail and (c). keeping the computational effort at an acceptable level. An evaluation based on observed transpiration, timber volume, soil carbon and soil nitrogen content showed that the extended model produced unbiased results. We also investigated the impact of hydrological changes on our oak stands as a result of the completion of an artificial canal network in 1971, which has stopped regular springtime flooding. A comparison of the 11 years before versus the 11 years after 1971 demonstrated that the hydrological changes affected mainly the annual variation across years in leaf area index (LAI) and soil carbon and nitrogen sequestration, leading to stagnation of carbon and nitrogen stocks, but to an increase in the variance across years. However, carbon sequestration to timber was unaffected and exhibited no significant change in cross-year variation. Finally, we investigated how drawdown of the water table, a general problem in the region, affects modeled ecosystem behavior. We found a further amplification of cross-year LAI fluctuations, but the variance in soil carbon and nitrogen stocks decreased. Volume increment was unaffected, suggesting a stabilization of the ecosystem two decades after implementation of water management measures.