Ecological and methodological drivers of non-stationarity in tree growth response to climate

Radial tree growth is sensitive to environmental conditions, making observed growth increments an important indicator of climate change effects on forest growth. However, unprecedented climate variability could lead to non-stationarity, that is, a decoupling of tree growth responses from climate over time, potentially inducing biases in climate reconstructions and forest growth projections. Little is known about whether and to what extent environmental conditions, species, and model type and resolution affect the occurrence and magnitude of non-stationarity. To systematically assess potential drivers of non-stationarity, we compiled tree-ring width chronologies of two conifer species, Picea abies and Pinus sylvestris, distributed across cold, dry, and mixed climates. We analyzed 147 sites across the Europe including the distribution margins of these species as well as moderate sites. We calibrated four numerical models (linear vs. non-linear, daily vs. monthly resolution) to simulate growth chronologies based on temperature and soil moisture data. Climate–growth models were tested in independent verification periods to quantify their non-stationarity, which was assessed based on bootstrapped transfer function stability tests. The degree of non-stationarity varied between species, site climatic conditions, and models. Chronologies of P. sylvestris showed stronger non-stationarity compared with Picea abies stands with a high degree of stationarity. Sites with mixed climatic signals were most affected by non-stationarity compared with sites sampled at cold and dry species distribution margins. Moreover, linear models with daily resolution exhibited greater non-stationarity compared with monthly-resolved non-linear models. We conclude that non-stationarity in climate–growth responses is a multifactorial phenomenon driven by the interaction of site climatic conditions, tree species, and methodological features of the modeling approach. Given the existence of multiple drivers and the frequent occurrence of non-stationarity, we recommend that temporal non-stationarity rather than stationarity should be considered as the baseline model of climate–growth response for temperate forests.     

Mosses modify effects of warmer and wetter conditions on tree seedlings at the alpine treeline

Climate warming enables tree seedling establishment beyond the current alpine treeline, but to achieve this, seedlings have to establish within existing tundra vegetation. In tundra, mosses are a prominent feature, known to regulate soil temperature and moisture through their physical structure and associated water retention capacity. Moss presence and species identity might therefore modify the impact of increases in temperature and precipitation on tree seedling establishment at the arctic‐alpine treeline. We followed Betula pubescens and Pinus sylvestris seedling survival and growth during three growing seasons in the field. Tree seedlings were transplanted along a natural precipitation gradient at the subarctic‐alpine treeline in northern Sweden, into plots dominated by each of three common moss species and exposed to combinations of moss removal and experimental warming by open‐top chambers (OTCs). Independent of climate, the presence of feather moss, but not Sphagnum, strongly supressed survival of both tree species. Positive effects of warming and precipitation on survival and growth of B. pubescens seedlings occurred in the absence of mosses and as expected, this was partly dependent on moss species. P. sylvestris survival was greatest at high precipitation, and this effect was more pronounced in Sphagnum than in feather moss plots irrespective of whether the mosses had been removed or not. Moss presence did not reduce the effects of OTCs on soil temperature. Mosses therefore modified seedling response to climate through other mechanisms, such as altered competition or nutrient availability. We conclude that both moss presence and species identity pose a strong control on seedling establishment at the alpine treeline, and that in some cases mosses weaken climate‐change effects on seedling establishment. Changes in moss abundance and species composition therefore have the potential to hamper treeline expansion induced by climate warming.     

长白山红松和鱼鳞云杉在分布上限的径向生长对气候变暖的不同响应

用树木年代学方法研究了近50年来气候变化对长白山自然保护区两种广泛分布的重要乔木树种红松(Pinus koraiensis)和鱼鳞云杉(Picea jezoensis var. komarovii)分布上限树木径向生长的影响, 发现红松年轮宽度具有与温度升高相一致的趋势, 而鱼鳞云杉年轮宽度则出现随温度升高而下降的“分离现象”。对水热条件的正响应是分布上限红松年表与温度保持一致的关键: 生长季的温度和降水的增加对上限红松的生长有促进作用, 且二者对树木生长的有利效应有相互促进的现象; 生长季的延长也有利于红松的生长。升温导致的水分胁迫是造成上限分布的鱼鳞云杉年轮宽度与温度变化趋势相反的重要因素: 分布上限的鱼鳞云杉年表与大多数温度指标均呈负相关关系; 随着温度升高, 年表与年降水量尤其是春季降水量的相关性逐渐由负转正; 各月的高温以及生长季中后期的少雨是形成上限鱼鳞云杉窄轮的主要气候因素, 而较低的各月温度以及生长季后期充足的降水则有利于上限鱼鳞云杉的生长; 此外, 生长季长度没有变化也可能是造成鱼鳞云杉年表序列对温度变化敏感性下降的重要因素。     

大兴安岭山地樟子松径向生长对气候变暖的响应——以满归地区为例

基于树木年代学的理论和方法,建立了大兴安岭满归地区樟子松年轮宽度年表,分析了标准化年表与该区主要气候因子(温度和降水)之间的相关关系,揭示了气候因子对树木径向生长的影响.结果表明:当年4-8月的各月平均温度是研究区樟子松径向生长的主要限制因子;4-8月各月温度的不断升高对樟子松径向生长产生了不利影响.利用4-8月平均温度的变化模拟樟子松径向生长变异(1958 -2008年),发现随着区域气候暖干化趋势的加强,该区樟子松生长将呈现出衰退的特征.     

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.     

Seedling recruitment, survival and facilitation in alpine Pinus uncinata tree line ecotones. Implications and potential responses to climate warming

Aims Alpine tree line ecotones are harsh environments where low temperatures constrain tree regeneration and growth. However, the expected upward shift of tree line ecotones in response to climate warming has not been ubiquitous. The lack of coupling between tree line dynamics and climate warming might be explained by factors other than climate variation that determine seedling recruitment in these ecotones. We want to assess how the availability of suitable habitat for establishment and the effects of facilitation on seedling survival and growth affect tree recruitment within tree line ecotones and modulate their responses to climate. Location We evaluate the relevance of these factors for Pinus uncinata tree line ecotones in the Catalan Pyrenees (north‐east Spain) and Andorra. Methods We analysed the microhabitat of naturally established seedlings in rectangular plots at the tree line ecotone, assessing the habitat type and the proximity to potentially protective elements that may improve microsite conditions. We tested whether krummholz individuals influence regeneration at the tree line by performing a transplantation field experiment to evaluate the extent of facilitation on seedling survival and growth in height. A total of 820 seedlings were transplanted at different distances and orientations (resulting in 12 positions) from krummholz mats and monitored over 2 years. Results Safe sites for P. uncinata recruits consisted of sparse vegetation covering bare soil, gravel or litter, and close to protective elements that may ameliorate microsite conditions. The field experiment showed that directional positive interactions enhance seedling survival and growth, altering the spatial patterns of recruit survivorship, especially during harsh winter conditions (shallow and irregular snowpack). Main conclusions Our results suggest that scarce availability of safe sites and uneven facilitation by krummholz control seedling recruitment patterns within alpine tree line ecotones. Such constraints may distort or counter the response of tree line ecotones to climate warming at local and regional scales.     

Did tree‐Betula,Pinus and Picea survive the last glaciation along the west coast of Norway? A review of the evidence,in light of Kullman (2002)

Aim We discuss the hypotheses proposed by Kullman [Geo‐Öko 21 (2000) 141; Nordic Journal of Botany 21 (2001) 39; Journal of Biogeography 29 (2002) 1117] on the basis of radiocarbon‐dated megafossils of late‐glacial age from the central Swedish mountains that boreal trees survived the glaciation along the south‐west coast of Norway and subsequently migrated eastward early in the late‐glacial to early deglaciated parts of the central Swedish Scandes mountains. Methods We assess these hypotheses on the basis of glacial geological evidence and four lines of palaeoecological evidence, namely macrofossil records of the tree species, vegetation and climate reconstructions from plant evidence, independent climate reconstructions from other proxies for the late‐glacial environment of south‐west Norway, and the patterns of post‐glacial spread of the tree species. Location South and west Norway, central Swedish Scandes mountains (Jämtland). Results and conclusions South‐west Norway and the adjacent continental shelf were under ice at the last‐glacial maximum (LGM). The late‐glacial vegetation of south‐west Norway was treeless and summer temperatures were below the thermal limits for Betula pubescens Ehrh., Pinus sylvestris L. and Picea abies (L.) Karst. Instead of spreading immediately after the onset of Holocene warming, as might have been expected if local populations were surviving, B. pubescens showed a lag of local arrival of 600 to > 1000 years, Pinus lagged by 1500 to > 2000 years, and Picea only reached southern Norway c. 1500 years ago and has not colonized most of south‐west Norway west of the watershed. Glacial geological evidence shows the presence of an ice sheet in the Scandes at the LGM and in the Younger Dryas, which was cold‐based near or at the area where the late‐glacial‐dated megafossils were recovered by Kullman. We conclude that the samples dated by Kullman (2002) should be evaluated carefully for possible sources of contamination. All the available evidence shows that the biogeographical hypotheses, based on these radiocarbon dates taken at face value, of late‐glacial tree survival at the Norwegian coast and subsequent eastwards spread to the mountains, are unsupportable.     

大兴安岭兴安落叶松和樟子松径向生长对气候变化的响应差异

大兴安岭是我国气候变化最为显著的地区之一,兴安落叶松和樟子松是该地区最为重要的树种,研究它们径向生长对气候变化的响应差异,可以为预测气候变化下我国北方森林动态提供科学依据。在大兴安岭地区选择6个样点共采集兴安落叶松树轮和樟子松树轮样芯451个,建立了12个标准年表。比较了1900年以来树木径向生长趋势,利用Pearson相关分析法分析各样点兴安落叶松和樟子松生长对气候因子的响应,运用线性混合模型探讨温度和降水对兴安落叶松和樟子松年径向生长的影响,通过滑动相关对比两个树种生长-气候关系的时间稳定性。结果表明: 兴安落叶松径向生长与3月平均温度呈负相关,与上一年冬季和当年7月降水呈正相关。樟子松径向生长与当年8月温度呈正相关,与当年生长季(5—9月)降水呈正相关。冬季降雪对兴安落叶松径向生长起到重要的促进作用,夏季过多降水对樟子松径向生长起到显著的限制作用。兴安落叶松和樟子松生长对气候变化的响应存在明显差异,因此,气候变化可能会影响北方森林生态系统的树木生长、物种组成以及空间分布等。     

Physical Sheltering and Liming Improve Survival and Performance of Mountain Birch Seedlings: A 5-Year Study in a Heavily Polluted Industrial Barren

Denuded landscapes adjacent to big polluters represent an extremely harsh environment for plants due to a unique combination of soil toxicity and physical stress. In a 5‐year experiment we tested whether survival and performance of seedlings of Mountain birch (Betula pubescens ssp. czerepanovii) planted in two barren sites around the nickel–copper smelter at Monchegorsk, northwestern Russia, can be enhanced by physical sheltering and how large the supposed benefits to survival and performance are in relation to liming. Seedling performance was assessed by measuring growth parameters and chlorophyll fluorescence. Physical sheltering was found to be beneficial in some conditions: when soil characteristics were not so harsh as to cause 100% mortality, sheltering increased both survival and performance of birch seedlings. Although the benefits of liming on seedling performance and survival were stronger than the benefits of sheltering, sheltering may still have its uses in restoration when large‐scale liming is not applicable, for example, when the ecological side effects of liming are to be avoided.     

Different ways to die in a changing world: Consequences of climate change for tree species performance and survival through an ecophysiological perspective

Anthropogenic activities such as uncontrolled deforestation and increasing greenhouse gas emissions are responsible for triggering a series of environmental imbalances that affect the Earth's complex climate dynamics. As a consequence of these changes, several climate models forecast an intensification of extreme weather events over the upcoming decades, including heat waves and increasingly severe drought and flood episodes. The occurrence of such extreme weather will prompt profound changes in several plant communities, resulting in massive forest dieback events that can trigger a massive loss of biodiversity in several biomes worldwide. Despite the gravity of the situation, our knowledge regarding how extreme weather events can undermine the performance, survival, and distribution of forest species remains very fragmented. Therefore, the present review aimed to provide a broad and integrated perspective of the main biochemical, physiological, and morpho‐anatomical disorders that may compromise the performance and survival of forest species exposed to climate change factors, particularly drought, flooding, and global warming. In addition, we also discuss the controversial effects of high CO₂ concentrations in enhancing plant growth and reducing the deleterious effects of some extreme climatic events. We conclude with a discussion about the possible effects that the factors associated with the climate change might have on species distribution and forest composition.     

Distinguishing local from global climate influences in the variation of carbon status with altitude in a tree line species

Aim Two alternative hypotheses attempt to explain the upper elevation limit of tree lines world‐wide, the carbon‐limitation hypothesis (CLH) and the growth‐limitation hypothesis (GLH); the altitudinal decrease of temperature is considered the driver constraining either carbon gain or growth. Using a widely distributed tree line species (Nothofagus pumilio) we tested whether tree line altitude is explained by the CLH or the GLH, distinguishing local from global effects. We elaborated expectations based on most probable trends of carbon charging with altitude according to both hypotheses, considering the alternative effects of drought. Location Two climatically contrasting tree line ecotones in the southern Andes of Chile: Mediterranean (36°54′ S) and Patagonia (46°04′ S). Methods At both locations, 35–50 trees of different ages were selected at each of four altitudes (including tree line), and stem and root sapwood tissues were collected to determine non‐structural carbohydrate (NSC) concentrations. NSC accumulates whenever growth is more limited than photosynthesis. An altitudinal increase in NSCs means support for the GLH, while the opposite trend supports the CLH. We also determined stable carbon isotope ratios (δ¹³C) to examine drought constraints on carbon gain. Results NSC concentrations were positively correlated with altitude for stem tissue at the Mediterranean and root sapwood tissue at the Patagonia site. No depletion of NSC was found at either site in either tissue type. For both tissues, mean NSC concentrations were higher for the Patagonia site than for the Mediterranean site. Mean root sapwood NSC concentration values were five times higher than those of the corresponding stem sapwood at all altitudes. Values for δ¹³C were positively correlated with altitude in the Mediterranean site only. Main conclusions We found support for the GLH at the site without drought effects (Patagonia) and no support for the CLH at either site. It is suggested that drought moderated the effects of low temperature by masking the expected trend of the GLH at the Mediterranean site.