Responses of white spruce (Picea glauca) to experimental warming at a subarctic alpine treeline

From 2001 to 2004 we experimentally warmed 40 large, naturally established, white spruce [Picea glauca (Moench) Voss] seedlings at alpine treeline in southwest Yukon, Canada, using passive open‐top chambers (OTCs) distributed equally between opposing north and south‐facing slopes. Our goal was to test the hypothesis that an increase in temperature consistent with global climate warming would elicit a positive growth response. OTCs increased growing season air temperatures by 1.8°C and annual growing degree‐days by one‐third. In response, warmed seedlings grew significantly taller and had higher photosynthetic rates compared with control seedlings. On the south aspect, soil temperatures averaged 1.0°C warmer and the snow‐free period was nearly 1 month longer. These seedlings grew longer branches and wider annual rings than seedlings on the north aspect, but had reduced Photosystem‐II efficiency and experienced higher winter needle mortality. The presence of OTCs tended to reduce winter dieback over the course of the experiment. These results indicate that climate warming will enhance vertical growth rates of young conifers, with implications for future changes to the structure and elevation of treeline contingent upon exposure‐related differences. Our results suggest that the growth of seedlings on north‐facing slopes is limited by low soil temperature in the presence of permafrost, while growth on south‐facing slopes appears limited by winter desiccation and cold‐induced photoinhibition.     

An experimental test of limits to tree establishment in Arctic tundra

1 Five treeline species had low seed germination rates and low survivorship and growth of seedlings when transplanted into Alaskan tundra. Seed germination of all species increased with experimental warming, suggesting that the present treeline may in part result from unsuccessful recruitment under cold conditions.
2 Growth, biomass and survivorship of seedlings of treeline species transplanted into tundra were largely unaffected by experimental warming. However, transplanted seedlings of three species ( Betula papyrifera , Picea glauca and Populus tremuloides ) grew more when below‐ground competition with the extant community was reduced. All three measures of transplant performance were greater in shrub tundra than in the less productive tussock or heath tundra. Establishment of trees in tundra may thus be prevented by low resource availability and competition.
3 Two species ( Alnus crispa and Populus balsamifera ) had low seed germination and survivorship of germinated seeds; transplants of these species did not respond to the manipulations and lost biomass following transplanting into tundra. Isolated populations of these two species north of the present treeline in arctic Alaska probably became established during mid‐Holocene warming rather than in recent times.
4 Of all the species studied here, Picea glauca was the most likely to invade intact upland tundra. Its seeds had the highest germination rates and it was the only species whose seedlings survived subsequently. Furthermore, transplanted seedlings of Picea glauca had relatively high survivorship and positive growth in tundra, especially in treatments that increased air temperature or nutrient availability, two factors likely to increase with climate warming.     

Climate influences on growth and reproduction of Pinus banksiana (Pinaceae) at the limit of the species distribution in eastern North America

The presence of conflicts in the allocation of resources among the different functions of an organism is a fundamental postulate of modern ecology. It is assumed that reproduction occurs at a cost because it monopolizes resources that could be used for other functions (e.g., growth). These conflicts may be particularly evident under stressful conditions, such as under low water or nutrient availability, or under severe climatic conditions. There we may expect to find strong negative relationships between an organism's growth and reproduction. We studied a population of Pinus banksiana (Pinaceae) at the northern limit of the species distribution, in subarctic Québec (Canada) where Pinus banksiana occupies nutrient-poor, sandy terraces along the Great Whale river. Serotinous cones of Pinus banksiana produced between 1969 and 1992 were sampled to estimate interannual variations in several variables representing reproduction, and to relate these to climate and tree growth. Climate appears to influence each developmental stage involved in the production of viable seeds, from the time of cone initiation to that of seed maturation. In general, reproductive variables are positively related to high temperatures during the three growing seasons required for seed production; growth is also positively correlated to summer temperatures. Consequently, investment in maturing seeds is positively associated with growth. Thus, both reproduction and growth covary with climate: during relatively warm and long growing seasons, resource allocation to both functions increases. Under these conditions, no trade-off is apparent.     

云南小中甸地区丽江云杉径向生长对气候变化的响应

丽江云杉原始林是云南省香格里拉县小中甸地区的主要森林类型.本文采用树轮年代学的方法分析了丽江云杉年轮宽度对气候变化的响应;选取相对保守的结果负指数曲线或线性回归拟合生长趋势建立年表,进行了不同时间尺度的气候因素与差值年表(RES)序列的相关及响应函数分析,并利用特征年分析了产生宽窄年轮的原因.结果表明:研究区丽江云杉的径向生长与温度升高在1990-2008年存在一定的“分离现象”;上一生长季的水热状况是限制丽江云杉当年生长的主要气候因子,特别是上年7月的气温对当年径向生长具有负反馈作用,而上年7月的充足降水则促进当年的径向生长;上年生长季温度与降水变化的相反趋势是导致宽窄年轮形成的主要原因;丽江云杉的生长对帕尔默干旱指数(PDSI)的变化不敏感.     

Unraveling the drivers of intensifying forest disturbance regimes in Europe

Natural disturbances like wildfire, windthrow and insect outbreaks are critical drivers of composition, structure and functioning of forest ecosystems. They are strongly climate‐sensitive, and are thus likely to be distinctly affected by climatic changes. Observations across Europe show that in recent decades, forest disturbance regimes have intensified markedly, resulting in a strong increase in damage from wind, bark beetles and wildfires. Climate change is frequently hypothesized as the main driving force behind this intensification, but changes in forest structure and composition associated with management activities such as promoting conifers and increasing standing timber volume (i.e. ‘forest change’) also strongly influence susceptibility to disturbances. Here, we show that from 1958 to 2001, forest change contributed in the same order of magnitude as climate change to the increase in disturbance damage in Europe's forests. Climate change was the main driver of the increase in area burnt, while changes in forest extent, structure and composition particularly affected the variation in wind and bark beetle damage. For all three disturbance agents, damage was most severe when conducive weather conditions and increased forest susceptibility coincided. We conclude that a continuing trend towards more disturbance‐prone conditions is likely for large parts of Europe's forests, and can have strong detrimental effects on forest carbon storage and other ecosystem services. Understanding the interacting drivers of natural disturbance regimes is thus a prerequisite for climate change mitigation and adaptation in forest ecosystem management.     

Radial growth response of four dominant boreal tree species to climate along a latitudinal gradient in the eastern Canadian boreal forest

To address the central question of how climate change influences tree growth within the context of global warming, we used dendroclimatological analysis to understand the reactions of four major boreal tree species –Populus tremuloides, Betula papyrifera, Picea mariana, and Pinus banksiana– to climatic variations along a broad latitudinal gradient from 46 to 54°N in the eastern Canadian boreal forest. Tree‐ring chronologies from 34 forested stands distributed at a 1° interval were built, transformed into principal components (PCs), and analyzed through bootstrapped correlation analysis over the period 1950–2003 to identify climate factors limiting the radial growth and the detailed radial growth–climate association along the gradient. All species taken together, previous summer temperature (negative influences), and current January and March–April temperatures (positive influences) showed the most consistent relationships with radial growth across the gradient. Combined with the identified species/site‐specific climate factors, our study suggested that moisture conditions during the year before radial growth played a dominant role in positively regulating P. tremuloides growth, whereas January temperature and growing season moisture conditions positively impacted growth of B. papyrifera. Both P. mariana and P. banksiana were positively affected by the current‐year winter and spring or whole growing season temperatures over the entire range of our corridor. Owing to the impacts of different climate factors on growth, these boreal species showed inconsistent responsiveness to recent warming at the transition zone, where B. papyrifera, P. mariana, and P. banksiana would be the most responsive species, whereas P. tremuloides might be the least. Under continued warming, B. papyrifera stands located north of 49°N, P. tremuloides at northern latitudes, and P. mariana and P. banksiana stands located north of 47°N might benefit from warming winter and spring temperatures to enhance their radial growth in the coming decades, whereas other southern stands might be decreasing in radial growth.     

The heterotrophic eubacterial and archaeal co-inhabitants of the halophilic Dunaliella salina in solar salterns fed by Bay of Bengal along south eastern coast of India

Halophilic microbes are studied to understand the metabolic pathways adopted by organisms in such extreme environment and for their biotechnological exploitation. In thallosohaline environments worldwide, the autotrophic alga Dunaliella salina Teodoresco is omnipresent, but it is being recently realised that the heterotrophic components vary in different regions. The unexplored eastern coastline of India abutted by Bay of Bengal was investigated for the heterotrophic halophilic microbes in this region. The waters in the salterns – replicas of natural hyper-saline water bodies of that region, were collected at four sites along 650 km of the coastal belt. In cultures set up from these waters, green and pink colonies were observed. The green colonies were found to be those of D. salina while the pink colonies were of heterotrophs. To identify the heterotrophic microbes, light microscopy, 16S rRNA typing and pigment profiling through spectrophotometry and HPLC were done. The cells in pink colonies were rod shaped. 16S rRNA typing of cells in these colonies detected the presence of Halomonas sp. – a eubacterium. The pigment profile of cells in pink cultures matched that of the archaea – Halobacterium; bacterioruberin derivatives were found. Thus, it was concluded that Halomonas and Halobacterium spp. are among the co-inhabitant heterotrophs of D. salina. Cultures of D. salina established from these salterns showed the typical three colours seen in the ponds of different sub-plots of salterns. They were green until 30 days, turning dark orange by 60 days and pink when 90 day old. In the 90 day old cultures, innumerable rod shaped cells were found. These cells were similar to the cells of the waters from the ponds of pink sub-plots of salterns and the pink colonies established from saltern waters in the laboratory. In the old (90 days) laboratory cultures of D. salina, the glycerol and proteins released from degenerating cells and the increase in salt concentration to super saturation levels due to evaporation of water in the medium led to the gregarious appearance of the heterotrophs – the co-inhabitants in natural environment.