Cembra Trees Research Articles

Rapid advance of climatic tree limits in the Eastern Alps explained by on-site temperatures

In the European Alps, mean temperature has risen by 2.5 K since the end of the nineteenth century. A 2 K warming of the growing season has taken place in the last 4 decades only. The 2.5 K warming should rise the position of the climatic treeline by about 400 m. Actual shifts in uppermost tree positions reported here for the Austrian Defereggen Valley and the Swiss Lower Engadine region of the Eastern Alps reach only around 140 m of elevation above the limit of old trees that date back to the nineteenth century. Uppermost Pinus cembra trees of > 2 m height currently occur at c. 2500 m, representing elevation records for the Eastern Alps. In situ temperature records for 2022–2023 revealed seasonal mean temperatures for uppermost trees that are 1–3 K higher than the equilibrium treeline isotherm of c. 6 °C in both regions (corrected for temperature anomalies from long-term records). The 2 K span reflects microhabitat differences and two ways to define the season. Thus, tree advances lag behind the upslope shift of the treeline isotherm, on average, by more than 200 m. The uppermost trees currently grow under quite warm conditions with annual shoot length increments frequently reaching 20 cm. Even without additional future warming, the new steady-state climatic treeline will exceed the Holocene maximum elevation in the Eastern Alps substantially.

Multiproxy tree ring reconstruction of glacier mass balance: insights from Pinus cembra trees growing near Silvretta Glacier (Swiss Alps)

Abstract. Glacier mass balance reconstructions provide a means of placing relatively short observational records into a longer-term context. Here, we use multiple proxies from Pinus cembra trees from God da Tamangur, combining tree ring anatomy and stable isotope chronologies to reconstruct seasonal glacier mass balance (i.e., winter, summer, and annual mass balance) for the nearby Silvretta Glacier over the last 2 centuries. The combination of tree ring width, radial diameter of earlywood cell lumina, and latewood radial cell wall thickness provides a highly significant reconstruction for summer mass balance, whereas for the winter mass balance, the correlation was less significant but still robust when radial cell lumina were combined with δ18O records. A combination of the reconstructed winter and summer mass balances allows the quantification of the annual mass balance of the Silvretta Glacier for which in situ measurements date back to 1919. Our reconstruction indicates a substantial increase in glacier mass during the first half of the 19th century and an abrupt termination of this phase after the end of the Little Ice Age. Since the 1860s, negative glacier mass balances have been dominant and mass losses accelerate as anthropogenic warming picks up in the Alps.

Tree-ring anatomy of Pinus cembra trees opens new avenues for climate reconstructions in the European Alps

Tree rings form the backbone of high-resolution palaeoclimatology and represent one of the most frequently used proxy to reconstruct climate variability of the Common Era. In the European Alps, reconstructions were often based on tree-ring width (TRW) and maximum latewood density (MXD) series, with a focus on European larch. By contrast, only a very limited number of dendroclimatic studies exists for long-lived, multi-centennial Pinus cembra, despite the widespread occurrence of the species at treeline sites across the European Alps. This lack of reconstructions can be ascribed to the difficulties encountered in past studies in extracting a robust climate signal from TRW and MXD chronologies. In this study, we tested various wood anatomical parameters from P. cembra as proxies for the reconstruction of past air temperatures. To this end, we measured anatomical cell parameters and TRW of old-growth trees from the God da Tamangur forest stand, known for being the highest pure, and continuous P. cembra forest in Europe. We demonstrate that several wood anatomical parameters allow robust reconstruction of past temperature variability at annual to multidecadal timescales. Best results are obtained with maximum latewood radial cell wall thickness (CWTrad) measured at 40 μm radial band width. Over the 1920–2017 period, the CWTrad chronology explains 62 % and >80 % of interannual and decadal variability of air temperatures during a time window corresponding roughly with the growing season. These values exceed those found in past work on P. cembra and even exceed the values reported for MXD chronologies built with L. decidua and hitherto considered the gold standard for dendroclimatic reconstructions in the European Alps. The wood anatomical analysis of P. cembra records therefore unveils a dormant potential and opens new avenues for a species that has been considered unsuitable for climate reconstructions so far.

Ectomycorrhizal fungal communities of Swiss stone pine (Pinus cembra) depend on climate and tree age in natural forests of the Alps

Background and AimsPinus cembra represent a typical and important tree species growing in European subalpine and alpine habitats. The ectomycorrhizal (ECM) fungal communities associated to this tree under natural conditions are largely unknown.MethodsIn this study, we investigated the ECM fungal abundance and composition at four high-altitude sites (two northern-exposed and two southern-exposed habitats) in South Tyrol (Italy), and included also two different age classes of P. cembra. The ECM partners were characterized morphologically, and identified by rDNA ITS sequence analysis.ResultsThe degree of mycorrhization in adult P. cembra was typically 100% in these natural habitats, with a total species diversity of 20 ECM species. The four high-altitude sites were similar concerning their species richness and mycobiont diversity, but they differed significantly in ECM species composition. Young P. cembra had a mycorrhization degree of 100% and a total of 10 species were observed. All mycorrhizal partners of naturally regenerated young P. cembra were only detected in one specific location, with the exception of Cenococcum sp. and Amphinema sp. which were detected at two sites. Young trees harbour a distinct ectomycorrhizal fungal diversity, which is clearly lower than the diversity detected in adult P. cembra trees. The P. cembra bolete (Suillus plorans) is the most important symbiotic partner of P. cembra at Southern Tyrolean high-altitude sites and is known for its strict, species-specific host association.ConclusionsThe ectomycorrhizal fungal community composition strongly depends on geographic region and on the slope exposure (north or south) of the site.

Comparative anatomy of the conductive tissues of the scions of Pinus sibirica Du Tour, P. cembra L. and their rootstocks P. sylvest

To study the appearance of incompatibility in adult graft trees, we studied the anatomical characteristics ofconducting phloem and sapwood in 55-year-old scions of Pinus sibirica Du Tour and Pinus cembra L. and their rootstocks –Scots pine (Pinus sylvestris L.). It was found that in all variants the width of the sapwood in the rootstock is significantlyhigher than in the scion; however, when only the early tracheids are taken into account, the differences between the scionand the rootstock are not significant, and the total width of the early tracheids from the sapwood is some higher in theP. cembra trees. In the place of rootstock and scion union, the width of the rootstock sapwood decreases due to a decreasein the proportion of late tracheids, while the sapwood of the scion remains almost unchanged. The variability of the sapwood width in scions and rootstocks of the P. cembra is lower and it is less sensitive to the reduction of increments thanin the P. sibirica, it indicates interspecific differences. The average width of the conducting phloem for the growing seasonalso varies less in the P. cembra, and in individuals of the P. sibirica with small xylem increments it approaches the widthof the current xylem increment. The relationship between the width of the conducting phloem and the width of the sapwood is weak; the width of the phloem depends more on the current growth of the xylem. The quantitative differencesin the width of the sapwood and the conducting phloem make it possible to judge the preservation of their species-specificity during long-term coexistence of the scion and the rootstock, as well as the effect of the scion on to the rootstock.

Weak Apical Control of Swiss Stone Pine (Pinus cembra L.) May Serve as a Protection against Environmental Stress above Treeline in the Central European Alps

At the treeline in the Central European Alps, adverse climate conditions impair tree growth and cause krummholz formation of Swiss stone pine (Pinus cembra L.). Multi-stemmed trees (tree clusters) are frequently found in the treeline ecotone and are generally thought to originate from seed caches (multiple genets) of the European nutcracker (N. caryocatactes) or due to repeated damage of the leader shoot by browsing or mechanical stress (single genet). Additionally, lack of apical control can lead to upward bending of lateral branches, which may obscure single-genet origin if the lower branching points are overgrown by vegetation and the humus layer. The multi-stemmed growth form may serve as a means of protection against extreme environmental stress during winter, especially at wind-exposed sites, because leeward shoots are protected from, e.g., ice particle abrasion and winter desiccation. The aims of this study therefore were to analyze in an extensive field survey: (i) whether weak apical control may serve as a protection against winter stress; and (ii) to what extent the multi-stemmed growth form of P. cembra in the krummholz zone is originating from a single genet or multiple genets. To accomplish this, the growth habit of P. cembra saplings was determined in areas showing extensive needle damage caused by winter stress. Multi-stemmed saplings were assigned to single and multiple genets based on determination of existing branching points below the soil surface. The findings revealed that upward bending of lateral branches could protect saplings against winter stress factors, and, although multi-stemmed P. cembra trees were primarily found to originate from multiple genets (most likely seed caches), about 38% of tree clusters originated from upward bending of (partially) buried branches. The results suggest that weak apical control of P. cembra in the sapling stage might be an adaptation to increase survival rate under severe climate conditions prevailing above treeline during winter.

LASY Z UDZIALEM LIMBY (Pinus cembra L.) W GORGANACH (KARPATY WSCHODNIE, UKRAINA)

The article present results of the research on the location and structure of forests, where the Pinus cembra trees grow. Surface structures were measured according to stand age classes and habitat categories. The total surface of plots, where old-age forests grow was summed up. The species composition of forest stands and plant communities on the whole is generalized. The values of forest plots areas in which the Pinus cembra trees constitute part of the volume of stands of 10%, 20% or more are summarized. To preserve the Pinus cembra population, it is recommended to expand the network of protected areas.

Long-term trends in leaf level gas exchange mirror tree-ring derived intrinsic water-use efficiency of Pinus cembra at treeline during the last century

The ability of treeline conifers in the Central European Alps to cope with recent climate warming and increasing CO2 concentration is still poorly understood. We determined basal area increment (BAI) and tree ring stable carbon isotope ratios (δ13C) of Pinus cembra trees from 1925 through 2013. Stable isotope ratios and BAI were compared with leaf level gas exchange measurements carried out in situ between 1934 and 2012, and thus, provided new insights into long-term trends of tree-ring derived intrinsic water-use efficiency (iWUE). Mature P. cembra trees at treeline responded to increasing Ca and air temperature with a parallel increase in maximum net CO2 uptake rate at ambient CO2 (Amax) and tree-ring-derived intercellular CO2 partial pressure (Ci). Amax tripled and was positively correlated to BAI and Ci. The latter increased in parallel with ambient CO2 concentration and stomatal conductance. In contrast to the instantaneous gas exchange parameters, δ13C derived iWUE informs about the long-term changes in the carbon water relations. These data showed three changes in the iWUE chronosequences, which could be identified with different long term gas exchange patterns: (1) from stomatal controlled functioning from 1925 to 1981, to a situation where (2) both net CO2 fixation (A) and leaf conductance for water vapour (gw), responded to the environment from 1982 to 1997, and (3) back to a stomata controlled pattern over iWUE from 1998 onwards. This temporal pattern was also mirrored in leaf level gas exchange assessments, suggesting a parallel increase of A and gw of P. cembra at treeline during the last nine decades.

Fine Root Abundance and Dynamics of Stone Pine (Pinus cembra) at the Alpine Treeline Is Not Impaired by Self-shading.

Low temperatures are crucial for the formation of the alpine treeline worldwide. Since soil temperature in the shade of tree canopies is lower than in open sites, it was assumed that self-shading may impair the trees’ root growth performance. While experiments with tree saplings demonstrate root growth impairment at soil temperatures below 5–7°C, field studies exploring the soil temperature – root growth relationship at the treeline are missing. We recorded soil temperature and fine root abundance and dynamics in shaded and sun-exposed areas under canopies of isolated Pinus cembra trees at the alpine treeline. In contrast to the mentioned assumption, we found more fine root biomass and higher fine root growth in colder than in warmer soil areas. Moreover, colder areas showed higher fine root turnover and thus lower root lifespan than warmer places. We conclude that P. cembra balances enhanced fine root mortality in cold soils with higher fine root activity and by maintaining higher fine root biomass, most likely as a response to shortage in soil resource supply. The results from our study highlight the importance of in situ measurements on mature trees to understand the fine root response and carbon allocation pattern to the thermal growth conditions at the alpine treeline.

Elevational trends in hydraulic efficiency and safety of Pinus cembra roots.

In alpine regions, elevational gradients in environmental parameters are reflected by structural and functional changes in plant traits. Elevational changes in plant water relations have also been demonstrated, but comparable information on root hydraulics is generally lacking. We analyzed the hydraulic efficiency (specific hydraulic conductivity ks, entire root system conductance KR) and vulnerability to drought-induced embolism (water potential at 50 % loss of conductivity Ψ50) of the roots of Pinus cembra trees growing along an elevational transect of 600 m. Hydraulic parameters of the roots were compared with those of the stem and related to anatomical traits {mean conduit diameter (d), wall reinforcement [(t/b)2]}. We hypothesized that temperature-related restrictions in root function would cause a progressive limitation of hydraulic efficiency and safety with increasing elevation. We found that both root ks and KR decreased from low (1600 m a.s.l.: ks 5.6 ± 0.7 kg m−1 s−1 MPa−1, KR 0.049 ± 0.005 kg m−2 s −1 MPa−1) to high elevation (2100 m a.s.l.: ks 4.2 ± 0.6 kg m−1 s−1 MPa−1, KR 0.035 ± 0.006 kg m−2 s−1 MPa−1), with small trees showing higher KR than large trees. ks was higher in roots than in stems (0.5 ± 0.05 kg m−1s−1MPa−1). Ψ50 values were similar across elevations and overall less negative in roots (Ψ50 −3.6 ± 0.1 MPa) than in stems (Ψ50 −3.9 ± 0.1 MPa). In roots, large-diameter tracheids were lacking at high elevation and (t/b)2 increased, while d did not change. The elevational decrease in root hydraulic efficiency reflects a limitation in timberline tree hydraulics. In contrast, hydraulic safety was similar across elevations, indicating that avoidance of hydraulic failure is important for timberline trees. As hydraulic patterns can only partly be explained by the anatomical parameters studied, limitations and/or adaptations at the pit level are likely.

Sap flow characteristics and whole-tree water use of Pinus cembra across the treeline ecotone of the central Tyrolean Alps

Whole-tree water use of nine Pinus cembra trees was estimated in the treeline ecotone of the Central Austrian Alps. Sap flow density using Granier-type thermal dissipation probes and environmental parameters was monitored along an elevational gradient from the forest limit up to treeline and finally mediating the krummholz limit throughout two growing seasons. Normalized sap flow density (Q s) was significantly correlated with solar radiation (R s) and vapor pressure deficit (D) throughout the treeline ecotone. Multiple regression analysis indicated that at the forest limit and at treeline, D had a similar effect on Q s than R s. At the krummholz limit by contrast, D had a greater effect on Q s than R s due to partially stomatal closure and wind-induced clustering of the needles, which impaired their response to available irradiance. Whole-tree water use scaled to crown surface area estimated for an entire growing (172 days) declined from 449 mm at the forest limit to 274 mm at treeline and was 251 mm at the krummholz limit, which is within the values estimated for other European forest ecosystems. Nevertheless, the observation above the forest limit in the central Tyrolean Alps tree transpiration scaled to crown surface area is comparable to the water use of adjacent low-stature vegetation, which should also be taken into account when forecasting potential effects of global change on the water balance of the treeline ecotone.

Snow Fungi—Induced Mortality of Pinus cembra at the Alpine Treeline: Evidence from Plantations

Identifying the factors controlling tree mortality is key to understanding the effects of ongoing global change on treeline movement and species composition. One potentially significant but little studied factor that impacts the formation of treelines is mortality caused by snow fungi. We studied the mortality patterns of Pinus cembra, a typical treeline species of the Central Alps of Switzerland, in two plantations located on opposite slopes of the Dischma valley (Northeast [NE] and Southwest [SW]) above the current treeline. In 1975, 33,000 P. cembra trees were planted at the NE site and 550 trees at the smaller SW site. All trees have been periodically monitored for survival since then. After 30 years, only ca. 5% of all P. cembra trees survived in both plantations. Two species of pathogenic snow fungus, Gremmeniella abietina and Phacidium. infestans, were major mortality agents for saplings. High rates of infection by Gremmeniella corresponded to late snowmelt and high ratios of rainfall to temperature. High rates of infection by Phacidium corresponded to earlier snowmelt and were spatially associated with P. cembra mature trees that had been present in the area before the time of planting. We provide experimental evidence that snow fungi are a primary cause of sapling tree mortality at treeline. Although additional evidence from different geographic regions and more natural treeline distributions is needed, our results suggest that the prevalence of snow fungi may prevent establishment of trees above the current treeline at present, and under future climate scenarios.

Whole-tree water use by Pinus cembra at the treeline in the Central Tyrolean Alps

Background and Aims: In contrast to the wealth of information on sap flow characteristics of forest trees at low-elevation sites, knowledge is scarce for trees within the treeline ecotone. Thus, our aim was to identify environmental controls on water loss in P. cembra trees at the treeline. Methods: Three isolated individuals were equipped with thermal dissipation probes for monitoring sap flow density (QS ). A Jarvis-type model and multiple linear regression, with irradiance and vapour pressure deficit as explanatory variables, were applied to compare predicted and measured sap flow density. Results: Both models successfully predicted measured QS . The Jarvis model tended to underestimate QS values by 25% on average, as compared with an underestimate of 8% by multiple linear regression analysis. The multiple linear regression analysis also allowed discrimination between effects of irradiance (RS ) and vapour pressure deficit (D) on QS , indicating that RS (ß-coefficient = 0.70) had a greater effect on QS than D (ß-coefficient = 0.25). Whole-tree water loss scaled to ground surface area (1.9 ± 0.5 mm d−1) was within the range reported for adjacent low-stature vegetation. Conclusions: The results suggest that the multiple linear regression approach is superior to the non-linear Jarvis-type model, as this approach allows discrimination between the effects of RS and the effects of D on QS .

Spatial and seasonal variations in mobile carbohydrates in Pinus cembra in the timberline ecotone of the Central Austrian Alps

To test whether the altitudinal limit of tree growth is determined by carbons shortage or by a limitation in growth we investigated non structural carbohydrates and their components starch and total soluble sugars in Pinus cembra trees along an elevational gradient in the timberline ecotone of the Central Austrian Alps. NSC contents in needles, branches, stems, and coarse roots were measured throughout an entire growing season. At the tissue level NSC contents were not significantly more abundant in treeline trees as compared to trees at lower elevations. Along our 425 m elevational transect from the closed forest to the treeline we failed to find a stable elevational trend in the total NSC pool of entire trees and observed within season increases in the tree's NSC pool that can be attributed to an altitudinal increase in leaf mass as needles contained the largest NSC fraction of the whole tree NSC pool. Furthermore, whole tree NSC contents were positively correlated with net photosynthetic capacity. Although our observed NSC characteristics do not support the hypothesis that tree life at their upper elevational limit is determined by an insufficient carbon balance we found no consistent confirmation for the sink limitation hypothesis.

Climate signal age effects—Evidence from young and old trees in the Swiss Engadin

A potential limitation of tree-ring based climate reconstructions is related to climate signal age effects (CSAE). CSAE may arise if the climatic response of young tree-rings differs from that of old tree-rings. This could mean that climatic signals become stronger (or weaker) with tree aging, or that the seasonality of signals or the sensitivity to a specific element (e.g., temperature, precipitation) changes over time. Such changes would affect the interpretation of dendroclimatic reconstructions, as the tree-rings included in these records are generally oldest at the end of a record (e.g., 21st century)—which is the time period generally used for calibration with instrumental data. We here addressed this concern by analyzing young and old Pinus cembra trees from three high elevation sites in the central European Alps. Core and disc samples were collected in pre-defined plots to allow for a representative analysis of tree ages with tree-ring width (TRW) measurement series categorized into age classes (i) >1880, (ii) 1880–1939, and (iii) 1940–2002. Notably we report on the signal of the very young category (iii) not yet described in literature, and thus allow estimation of climate response and signal strength characteristics during the first years of the trees’ lifespans. Comparison of age classes (i)–(iii) revealed differences in TRW coherence and size, but little change in climatic signal. CSAE are in the order of the differences recorded among high elevation sites—a conclusion that holds for inter-annual to decadal scale TRW variations at near-treeline Swiss stone pine. Such data are typically included in regional and larger-scale temperature reconstructions; thus, our results add confidence to long-term climate estimates integrating a range of tree-ring age classes. Other findings, such as the reaction wood in juvenile tree-rings, and sensitivity of the climate signal to sample replication, suggest that comparisons of young and old age classes, and separate calibration of these categories against instrumental climate data might further the estimation of long-term uncertainty changes in tree-ring based climate reconstructions.

Biomass distribution of different-aged needles in young and old Pinus cembra trees at highland and lowland sites

Conifer needles of different ages perform differently in ecophysiology. However, no study has investigated the biomass distribution of different-aged needles in a tree crown or/and a stand canopy. We carried out a study on young (∼50 years old) and old (∼200 years) Pinus cembra L. trees at highland (2100–2300 m a.s.l.) and lowland (570 m) sites in Switzerland. We found that both the young and the old trees living in the highlands had more needle biomass per tree than the same-aged trees of the same species living in the lowlands. This is mainly due to the greater longevity of needles in highland trees. It reflects the strategic responses of trees to low resource availability or high abiotic stress level. Having older needles increases the time that nutrients are resident in trees in less favorable environments, and compensates for shorter growing period in cold temperatures.

Evidence for potential impacts of ozone on Pinus cembra L. at mountain sites in Europe: An overview

We summarize what is known about the impact of ozone (O 3) on Pinus cembra in the timberline ecotone of the central European Alps and the Carpathian Mountains. In the central European Alps exposure to ambient and two-fold ambient O 3 throughout one growing season did neither cause any visible injury nor affect the photosynthetic machinery and biochemical parameters in current to 1-year-old needles. By contrast, in the southern French Alps and in the Carpathians 1-year-old needles of Pinus cembra trees showed visual symptoms similar to those observed in O 3 stressed pine stands in southern California. For the southern French Alps the observed symptoms could clearly be attributed O 3 and differences in O 3 uptake seems to be the likely key factor for explaining the observed decline. For the Carpathians however, other reasons such as drought may not be excluded in eliciting the observed symptoms. Thus, the action of O 3 has always to be evaluated in concert with other environmental impacts, determining the tree's sensitivity to stress.

AGE-DEPENDENT TREE-RING GROWTH RESPONSES TO CLIMATE IN LARIX DECIDUA AND PINUS CEMBRA

Dendrochronology generally operates under the assumption that climate–growth relationships are age independent, once growth trends and/or disturbance pulses have been accounted for. However, several studies have demonstrated that tree physiology undergoes changes with age. This may cause growth-related climate signals to vary over time. Using chronology statistics and response functions, we tested the consistency of climate–growth responses in tree-ring series from Larix decidua and Pinus cembra trees of four age classes. Tree-ring statistics (mean sensitivity, standard deviation, correlation between trees, and first principal component) did not change significantly with age in P. cembra, whereas in L. decidua they appeared to be correlated with age classes. Response function analysis indicated that climate accounts for a high amount of variance in tree-ring widths in both species. The older the trees are, the higher the variance explained by climate, the significance of the models, and the percentage of ...

Physiological study of declining Pinus cembra (L.) trees in southern France

Pinus cembra (L.) trees growing at an elevation of 1,800 m in the Mercantour Alps National Park in southern France have shown various degrees of damage (yellowing and needle loss) since the early 1980s. Two stands were selected, one populated by healthy and the other by damaged trees, and 1-year-old needles were analysed. The needles of the declining trees displayed chlorotic mottled patterns associated with a significantly lower level of total chlorophyll compared to healthy trees. Measurements of gas exchange showed a lower photosynthetic rate linked to a lower activity of ribulose bisphosphate carboxylase and a higher stomatal conductance in needles of declining trees compared to healthy trees. In contrast, phosphoenolpyruvate carboxylase activity was found to be higher in declining tree needles, reaching twice the level measured in healthy tree needles. A significant oxidative stress was also observed in the needles which presented a 68% higher malondialdehyde content than healthy tree needles. The atmospheric concentration of ozone was quite high (40 nl·l–1, monthly mean in summer) in both stands but preliminary results based on stomatal conductance and ozone concentration measurements suggested a higher ozone flux in needles of declining trees. Ozone is thus suspected as a contributing factor in the observed decline of Pinus cembra trees.

Der Einfluß der Photoperiode auf das Austreiben, die Frosthärte und die Hitzeresistenz von Zirben und Alpenrosen

From September, 1965 to January, 1967, phenological observations of new growth initiation and measurements of frost and heat resistance of Pinus cembra and Rhododendron ferrugineum were carried out monthly. Samples were taken from plants growing near timberline (on the Patscherkofel Mountain near Innsbruck, Austria at 2000 m. elev.) and from plants which were growing in a greenhouse with a constant temperature of 15 °C with the natural photoperiod or under various constant daylengths (8, 12 or 16 hr). 1. The plants in the greenhouse initiated new growth much earlier than the plants in nature, especially under long day conditions. Short day or 12-hour photoperiods inhibited the early initiation of new growth rather strongly. Also, under the natural photoperiod in the greenhouse, not all the buds broke, early; this was particularly true for P. cembra . These plants apparently were exhibiting a vernalization effect (Table 1). 2. The frost resistance (the lowest temperature at which the leaves or needles could withstand a 3-hour exposure without evident damage) changed considerably during the course of the year, particularly for plants in nature. The maximum resistance values in winter were − 43 °0 ( P. cembra tree), − 40 °C (P. cembra seedling), and −18 °C (R. ferruginum ). The minimum values in summer were − 6 °C, − 4 °C, and − 4 °C, respectively. The annual variation in frost resistance was reasonably synchronous with the prevailing temperature but even more closely correlated with the changing photoperiod during the year. The dominant influence of photoperiod on the variation in frost resistance for both species was evident from the fact that plants in the greenhouse with constant temperature behaved almost the same as plants in nature when under natural light conditions. However, when a constant photoperiod was applied, the annual variation in frost resistance resistance was greatly damped. Under a continuously long day photoperiod, the frost resistance dropped and remained at a low value. Under a short day regime, the plants achieved and maintained a high resistance against low temperatures throughout the course of these experiments. In the second year of the experiments, all annual variation in frost resistance essentially disappeared. This suggests that the endogenous rhythm, which together with temperature and photoperiod controlled the annual variation in frost resistance, has slowly faded out during the course of this first year under constant temperature and photoperiod. After the endogenous rhythm is dissipated because it is no longer regulated by environmental conditions, the frost resistance variation also deviates from this natural rhythm; it then oscillates more quickly (especially for P. cembra under an 8-hour day length). 3. The heat resistance (the highest temperature at which the leaves or needles could withstand a 30-minute exposure without apparent damage) also displayed an annual variation whose amplitnde, however, was much smaller than that of the variation in frost resistance. The amplitude was 6 °C ( P. cembra trees), 7 °C ( P. cembra seedlings) and 8 °C ( Rhododendron ferrugineum ) for these plants in nature. This variation in heat resistance closely paralleled that of frost resistance; i. e., the maximum and also the minimum values of frost and heat resistance occurred at the same time, An increase in heat resistance in summer was not demonstrable for these two species; apparently the natural habitat of these two species is too cool. Plants in the greenhause also exhibited an annual variation in heat resistance. Although, when p, cembra was placed under short day conditions, the heat resistance remained high, the effect of long days or a light interruption of the night did not seem to decrease the heat I esistance of this species, Rhododendron ferrugineum , on the other hand, reacted more clearly to long day conditions; the heat resistance was permanently decreased. Under natural day lengths or with short day conditions, the heat resistance variation was almost as pronounced as in nature, even during the second year. Therefore, it appears that the endogenous rhythm for the control of heat resistance is of greater significance than that for the control of frost resistance. This basic difference suggests that the internal processes and conditions of these two types of temperature resistance cannot be exactly the same. 4. These investigations show clearly for P. cembra and Rhododendron ferrugineum that not only temperature and endogenous rhythms are important in determining the annual course of variation in temperature resistance, but that photoperiod also plays a very significant role.