Past and future radial growth and water-use efficiency of Fagus sylvatica and Quercus robur in a long-term climate refugium

The low-latitudinal range margins of many temperate and boreal tree species consist of scattered populations that persist locally in climate refugia. Recent studies have shown that such populations can be remarkably resilient, yet their past resilience does not imply that they are immune to threats from future climate change. The functioning of refugial tree populations therefore needs to be better understood if we are to anticipate their prospects correctly. We performed a detailed study of tree radial growth and vigor in a long-term climate refugial population of beech (Fagus sylvatica), comparing the observed trends with those of co-occurring pedunculate oak (Quercus robur). Annual growth rates (basal area increment, BAI) for both species were similar to those observed in range-core populations, but natural lifespan was half that in the mountains. The master chronologies spanning 1870–2015 revealed 22% (Fagus) and 20% (Quercus) increases in BAI until the 1980s and a smaller decrease (−6% for Fagus, −9% for Quercus) since then. Stable carbon isotope measurements (δ¹³C) revealed no effect of cambial age and an increase in water-use efficiency (iWUE) from 1870–2015 of about 50% for Fagus and 20% for Quercus. The trend continued until 2015 in Fagus, whereas Quercus reached its maximum in the 1980s. A detailed analysis of the relationship between climate and annual growth based on a 118-year meteorological record revealed a major role of water availability in the current and previous year. We used the observed climatic relationships to model future growth trends until 2100 for the IPCC scenarios RCP4.5 and RCP8.5. Most projections revealed no change in current growth rates, suggesting that this climate refugium will be able to provide suitable conditions for the persistence of Fagus and Quercus over the coming decades even under warmer and drier regional climate conditions. Overall, our study provides valuable insight into the precise climatic and biological mechanisms enhancing the persistence of refugial tree populations under ongoing climate change.     

Tree growth and climate sensitivity in open and closed forests of the southeastern Tibetan Plateau

The creation of forest openings is a frequently observed phenomenon in many types of forests. On the southeastern Tibetan Plateau, where the average elevation is greater than 4000 m above sea level, differences in tree growth between forest stands with openings and completely closed stands are poorly characterized. Here, we presented a dendrochronological study of Tibetan juniper (Juniperus tibetica Kom.) and Sikkim spruce (Picea spinulosa (Griff.) Beissn.) in an open and a closed stand, near Qamdo of eastern Tibet. We found that the growth of juniper responded to climate in a similar way in the open and closed stands, and was positively correlated with temperature from October to January and with the Palmer Drought Severity Index (PDSI) from September to June. In contrast, the growth of spruce responded to climate differently in the open and closed stands: growth was positively correlated with the PDSI from September to May in the open stand, whereas it was positively correlated to November and December temperatures (of the prior year) and current June temperature in the closed stand. Interannual variation in, and standard deviations among, juniper tree ring widths were similar in both stands for the past four centuries, whereas they differed in spruce over the past two centuries, particularly in the 1900s. These results suggest that juniper tree ring growth is less sensitive to stand structure than that of spruce, thus providing more reliable climate signals. The data obtained from our study will help forest managers understand the ecology of juniper and spruce in open and closed stands and are therefore useful for management planning.     

Estimating the relative nutrient uptake from different soil depths in Quercus robur, Fagus sylvatica and Picea abies

The distribution of fine roots and external ectomycorrhizal mycelium of three species of trees was determined down to a soil depth of 55 cm to estimate the relative nutrient uptake capacity of the trees from different soil layers. In addition, a root bioassay was performed to estimate the nutrient uptake capacity of Rb⁺ and NH₄⁺ by these fine roots under standardized conditions in the laboratory. The study was performed in monocultures of oak (Quercus robur L.), European beech (Fagus sylvatica L.) and Norway spruce [Picea abies (L.) Karst.] on sandy soil in a tree species trial in Denmark. The distribution of spruce roots was found to be more concentrated to the top layer (0–11 cm) than that of oak and beech roots, and the amount of external ectomycorrhizal mycelia was correlated to the distribution of the roots. The uptake rate of [⁸⁶Rb⁺] by oak roots declined with soil depth, while that of beech or spruce roots was not influenced by soil depth. In modelling the nutrient sustainability of forest soils, the utilization of nutrient resources in deep soil layers has been found to be a key factor. The present study shows that the more shallow-rooted spruce can have a similar capacity to take up nutrients from deeper soil layers than the more deeply rooted oak. The distribution of roots and mycelia may therefore not be a reliable parameter for describing nutrient uptake capacity by tree roots at different soil depths.     

The vertical distribution of N and K uptake in relation to root distribution and root uptake capacity in mature Quercus robur, Fagus sylvatica and Picea abies stands

We have measured the uptake capacity of nitrogen (N) and potassium (K) from different soil depths by injecting ¹⁵N and caesium (Cs; as an analogue to K) at 5 and 50 cm soil depth and analysing the recovery of these markers in foliage and buds. The study was performed in monocultures of 40-year-old pedunculate oak (Quercus robur), European beech (Fagus sylvatica) and Norway spruce (Picea abies (L.) Karst.) located at an experimental site in Palsgård, Denmark. The markers were injected as a solution through plastic tubes around 20 trees of each species at either 5 or 50 cm soil depth in June 2003. After 65 days foliage and buds were harvested and the concentrations of ¹⁵N and Cs analysed. The recovery of ¹⁵N in the foliage and buds tended to be higher from 5 than 50 cm soil depth in oak whereas they where similar in spruce and beech after compensation for differences in immobilization of ¹⁵N in the soil. In oak more Cs was recovered from 5 than from 50 cm soil depth whereas in beech and spruce no difference could be detected. Out of the three investigated tree species, oak was found to have the lowest capacity to take up Cs at 50 cm soil depth compared to 5 cm soil depth also after compensating for differences in discrimination against Cs by the roots. The uptake capacity from 50 cm soil depth compared with 5 cm was higher than expected from the root distribution except for K in oak, which can probably be explained by a considerable overlap of the uptake zones around the roots and mycorrhizal hyphae in the topsoil. The study also shows that fine roots at different soil depths with different physiological properties can influence the nutrient uptake of trees. Estimates of fine root distribution alone may thus not reflect the nutrient uptake capacity of trees with sufficient accuracy. Our study shows that deep-rooted trees such as oak may have lower nutrient uptake capacity at deeper soil layers than more shallow-rooted trees such as spruce, as we found no evidence that deep-rooted trees obtained proportionally more nutrients from deeper soil layers. This has implications for models of nutrient cycling in forest ecosystems that use the distribution of roots as the sole criterion for predicting uptake of nutrients from different soil depths.     

Phenology and growth of Fagus sylvatica and Quercus robur seedlings in response to temperature variation in the parental versus offspring generation

• Plants are known to respond to warming temperatures. Few studies, however, have included the temperature experienced by the parent plant in the experimental design, in spite of the importance of this factor for population dynamics.
• We investigated the phenological and growth responses of seedlings of two key temperate tree species (Fagus sylvatica and Quercus robur) to spatiotemporal temperature variation during the reproductive period (parental generation) and experimental warming of the offspring. To this end, we sampled oak and beech seedlings of different ages (1–5 years) from isolated mother trees and planted the seedlings in a common garden.
• Warming of the seedlings advanced bud burst in both species. In oak seedlings, higher temperatures experienced by mother trees during the reproductive period delayed bud burst in control conditions, but advanced bud burst in heated seedlings. In beech seedlings, bud burst timing advanced both with increasing temperatures during the reproductive period of the parents and with experimental warming of the seedlings. Relative diameter growth was enhanced in control oak seedlings but decreased with warming when the mother plant experienced higher temperatures during the reproductive period.
• Overall, oak displayed more plastic responses to temperatures than beech. Our results emphasise that temperature during the reproductive period can be a potential determinant of tree responses to climate change.

     

Age estimation of Norway spruce using incomplete increment cores: Testing new and improved methods

Information on tree age is often vital for dendrochronological studies, especially when the Regional Curve Standardisation technique is used. Several linear and non-linear methods of tree age estimation using partial increment cores (without the presence of pith) were evaluated and modified to provide more accurate estimations than are currently used. To achieve the objective, core samples from 142 Norway spruce trees (Picea abies [L.] Karst.) were collected from an altitudinal gradient in the Western Carpathians. The samples in which the pith was included were then used for further analyses. Several known age estimation approaches were adjusted to combine the advantages of direct increment- and indirect age-diameter-based methods. Inverse differential forms of non-linear growth functions were tested and proposed as a new advanced approach for age estimation. The results show that most of the modified linear methods achieved a mean square error of less than 10% when the length of the partial core exceeded 90% of the stem radius and less than 20% when the length of the core was at least 60% of the stem radius. Using an appropriate differential form of the non-linear growth functions, a mean square error of less than 20% was reached, even when the core length was shorter than 60% of the radius. The results show that current linear methods for age estimation can only be used if the missing part of the core sample is very short, with only a few rings absent. In the case of a large number of missing rings, a differential form of non-linear functions should preferably be used instead.     

Tree,sex and size: Ecological determinants of male vs. female fecundity in three Fagus sylvatica stands

Interindividual variation in fecundities has major consequences on population evolutionary potential, through genetic drift and selection. Using two spatially explicit mating models that analyse the genotypes of seeds and seedlings, we investigated the variation of male and female fecundities within and among three European beech (Fagus sylvatica) stands situated along an altitudinal gradient. Female and male individual fecundity distributions were both skewed in this monoecious species, and we found a higher variance in female as compared to male fecundities. Both female and male fecundities increased with tree size and decreased with density and competition in the neighbourhood, the details of these effects suggesting sex‐specific strategies to deal with the impact of limited resource on fecundity. The studied populations were functionally male‐biased. Among‐individual variations in functional gender were not driven by tree size but by density and competition in the neighbourhood. Femaleness decreased under limited resource availability, an expected consequence of the higher cost of female reproduction. Considering the variation of gene flow and genetic drift across elevation, our results suggest that the adaptive potential could be enhanced by low genetic drift at low elevation, and by high pollen‐mediated gene flow at high elevation. Finally, this study predicts a more efficient response to selection for traits related to male vs. female fitness, for a given selection intensity.     

Variation in length and age at sexual maturity of Atlantic groundfish: a reply

Synopsis The assertion has been made by Halliday (1987) that trends in size and age at maturity of Atlantic groundfish published by Beacham (1983a, b, c, d, e, f) are artifacts induced by errors in determining the sex of an individual, distinguishing between immature and mature fish, sampling fish outside of the regular spawning season, and by nonrandom sampling of the population. In particular, Halliday asserts that for the Atlantic argentine,Argentina silus analysis, the conclusions of Beacham (1983a) that: (1) median length at sexual maturity declined over time; and (2) males matured at older ages than did females are invalid owing to biases in both sampling and analysis. In fact, if some of the biases indicated by Halliday were significant, then the decline in median length at sexual maturity is enhanced and the conclusions of Beacham (1983a) reinforced. Size and age at sexual maturity are dynamic characters in many vertebrate populations, and the fact that they should change for Atlantic groundfish should not be surprising given variable exploitation patterns in the fisheries since 1960.     

Regeneration dynamics and life history differences in southern Chilean Nothofagus forests: a synthesis

Large, infrequent natural disturbances have been proposed as a key component in determining the distribution and abundance of Nothofagus in southern Andean temperate forests. In this study, a comparison of influences of small-to-intermediate-sized natural disturbances and effects of selective logging on the establishment and growth of Nothofagus forests in south-central Chile are synthesized with the results of other forest studies to develop general hypotheses on the regeneration dynamics of southern Chilean Nothofagus alpina forests. A synthesis is given for regeneration pattern in the abundance of 18 temperate rain forests in the Andean Range, Chile. The study aimed to determine the role of life history differences in promoting coexistence of the five main tree species (N. alpina, N. dombeyi, N. pumilio, N. macrocarpa and Laurelia philippiana) in N. alpina-dominated forests. Age data reported for N. alpina, a shade mid-tolerant emergent tree in the temperate rain forests of southern Chile, indicate maximum lifespans > 650 years, figures unprecedented for N. alpina. In low elevation stands, N. alpina coexists with broad-leaved evergreen tree species, such as L. philippiana, L. sempervirens, Persea lingue, and in these stands an intermittent establishment of Nothofagus occurred and appeared to be most dependent on small-to-intermediate disturbances. In high-elevation stands, in contrast, Nothofagus establishment was less dependent on disturbance, regeneration being much more continuous even in the absence of canopy openings. The forests studied provide another example of the general pattern of increasing dependence of Nothofagus on disturbance towards the more productive end of the environmental gradients. As a long-lived pioneer and despite its dependence on disturbance in lowland sites, N. alpina has been subject to selection for complementary growth, adult survivorship, and mid-tolerance to shade. Thus, interspecific differences in juvenile and adult life history characteristics of N. alpina and its competitors may be sufficient to maintain its persistence in the landscape. In conclusion, this study of population structures and replacement patterns provides a comprehensive picture on our understanding of regeneration dynamics and trait differentiation in southern temperate forests by recognizing both the influences of environmental gradients (i.e., altitude) on competing species and the disturbance regimes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Seasonal and diel activity of Ixodes ricinus (Acari: Ixodidae) subpopulations in Denmark. Aspects of size, physiological age, and malate dehydrogenase genotype in a forest site without any undergrowth

The underlying population dynamics and the behavioural patterns of the vectors are key issues in understanding the transmission of vector borne pathogens. For the tick Ixodes ricinus both seasonal and diel activity have been described as bimodal patterns, which in seasonal aspect has been interpreted as representing two cohorts. However, recent studies have shown that this interpretation may be incorrect. The aim of this study was to obtain more detailed information on nymph host seeking by studying subpopulations of ticks during the day and season. The study was designed to allow for comparisons of the diel variation and seasonal variation in their dependency in a number of tick characteristics. The study took place in a forest with planted beech trees without any undergrowth. Ticks were collected by flagging the dead leaves on the forest floor. For each nymph, a number of visual observations were made. The size and physiological age was observed and the nymphs were genotyped in the malate dehydrogenase locus (MDH, E.C. 1.1.1.37). Briefly the main results can be given as: (i) There were significant differences in the composition of size classes during the season, but only limited trends in time. (ii) The proportion of the small nymphs was highly variable, with a variation from 3% to 24% in October and September, respectively. (iii) The diel variation in MDH genotypes was significant in May and August. (iv) Nymph size classes and physiological age appeared to interact. The non-random interaction was caused by a relatively even distribution of small nymphs in all four age classes, while large nymphs tended to fall into age class 2 and 3. The length by age interaction for the individual months was noted to be significant in May, July, August and September, but not in June. Similarly the interaction was significant in the morning and afternoon, but not at midday. The overall results describe the seasonal and diel activity patterns as changing systematically for several characteristics under the influence of weather condition and population dynamics. In conclusion: The observations are best interpreted as being produced of a single cohort of ticks, but the revealed complexity of the host seeking activity suggest that measures of activity × abundance should be interpreted very cautiously in relation to population dynamics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comment arising from a paper by Wittmer et al.: hypothesis testing for top-down and bottom-up effects in woodland caribou population dynamics

Conservation strategies for populations of woodland caribou Rangifer tarandus caribou frequently emphasize the importance of predator–prey relationships and the availability of lichen-rich late seral forests, yet the importance of summer diet and forage availability to woodland caribou survival is poorly understood. In a recent article, Wittmer et al. (Can J Zool 83:407–418, 2005b) concluded that woodland caribou in British Columbia were declining as a consequence of increased predation that was facilitated by habitat alteration. Their conclusion is consistent with the findings of other authors who have suggested that predation is the most important proximal factor limiting woodland caribou populations (Bergerud and Elliot in Can J Zool 64:1515–1529, 1986; Edmonds in Can J Zool 66:817–826, 1988; Rettie and Messier in Can J Zool 76:251–259, 1998; Hayes et al. in Wildl Monogr 152:1–35, 2003). Wittmer et al. (Can J Zool 83:407–418, 2005b) presented three alternative, contrasting hypotheses for caribou decline that differed in terms of predicted differences in instantaneous rates of increase, pregnancy rates, causes of mortality, and seasonal vulnerability to mortality (Table 1, p 258). These authors rejected the hypotheses that food or an interaction between food and predation was responsible for observed declines in caribou populations; however, the use of pregnancy rate, mortality season and cause of mortality to contrast the alternative hypotheses is problematic. We argue here that the data employed in their study were insufficient to properly evaluate a predation-sensitive foraging hypothesis for caribou decline. Empirical data on seasonal forage availability and quality and plane of nutrition of caribou would be required to test the competing hypotheses. We suggest that methodological limitations in studies of woodland caribou population dynamics prohibit proper evaluation of the mechanism of caribou population declines and fail to elucidate potential interactions between top-down and bottom-up effects on populations. An erratum to this article can be found at