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1.
Low temperatures are associated high‐altitude treelines, but the functional mechanism of treeline formation remains controversial. The relative contributions of carbon limitation (source activity) and growth limitation (sink activity) require more tests across taxa and regions. We examined temporal variations of mobile carbon supply in different tissues of Abies fargesii across treeline ecotones on north‐ and south‐facing slopes of the Qinling Mountains, China. Non‐structural carbohydrate (NSC) concentrations in tissues along the altitudinal gradient on both slopes changed significantly in the early and late growing season, but not in the mid‐growing season, indicating the season‐dependent carbon supply status. Late in the growing season on both slopes, trees at the upper limits had the highest NSC concentrations and total soluble sugars and lowest starch concentrations compared to trees at the lower elevations. NSC concentrations tended to increase in needles and branches throughout the growing season with increasing elevation on both slopes, but declined in roots and stems. NSC concentrations across sampling dates also indicated increases in needles and branches, and decreases in roots and stem with increasing elevation. Overall altitudinal trends of NSC in A. fargesii revealed no depletion of mobile carbon reserves at upper elevation limits, suggesting limitation of sink activity dominates tree life across treeline ecotones in both north‐ and south‐facing slopes. Carbon reserves in storage tissues (especially roots) in the late growing season might also play an important role in winter survival and early growth in spring at upper elevations on both slopes, which define the uppermost limit of A. fargesii. 相似文献
2.
高山林线对环境变化具有高度的敏感性, 但林线形成机制仍然没有明确的结论。为了检验高山林线形成是由碳限制还是生长限制决定, 并探讨林线树种适应高山环境的生理生态机制, 选择山西省吕梁山脉北端芦芽山, 沿3个海拔梯度测定了林线树种白杄(Picea meyeri)各组织非结构性碳水化合物(NSC)及其组分含量。结果表明: 白杄总体及各组织NSC含量均随海拔升高而增加, 林线树木不存在碳限制; 白杄NSC源、汇均随海拔升高而增加, 源-汇比在3个海拔之间没有差异, 表明源-汇平衡关系对海拔的适应性, 林线树木碳源活动没有受到限制; 各组织中可溶性糖与淀粉的比值随海拔升高呈增大趋势, 说明树木生长的环境越寒冷, 树木组织中表现出越明显的保护策略, 也可能暗示林线区域的树木更多地受到生长限制。研究结果在一定程度上支持“生长限制”假说。 相似文献
3.
Bjartmar Sveinbjörnsson Matthew Smith Tumi Traustason Roger W. Ruess Patrick F. Sullivan 《Oecologia》2010,163(4):833-843
Two opposing hypotheses have been presented to explain reduced tree growth at the treeline, compared with growth in lower
elevation or lower latitude forests: the carbon source and sink limitation hypotheses. The former states that treeline trees
have an unfavorable carbon balance and cannot support growth of the magnitude observed at lower elevations or latitudes, while
the latter argues that treeline trees have an adequate carbon supply, but that cold temperatures directly limit growth. In
this study, we examined the relative importance of source and sink limitation in forest and treeline white spruce (Picea glauca) in three mountain ranges from southern to northern Alaska. We related seasonal changes in needle nonstructural carbohydrate
(NSC) content with branch extension growth, an approach we argue is more powerful than using needle NSC concentration. Branch
extension growth in the southernmost Chugach Mountains was much greater than in the White Mountains and the Brooks Range.
Trees in the Chugach Mountains showed a greater seasonal decline in needle NSC content than trees in the other mountain ranges,
and the seasonal change in NSC was correlated with site-level branch growth across mountain ranges. There was no evidence
of a consistent difference in branch growth between the forest and treeline sites, which differ in elevation by approximately
100 m. Our results point to a continuum between source and sink limitation of growth, with high-elevation trees in northern
and interior Alaska showing greater evidence of sink limitation, and those in southern Alaska showing greater potential for
source limitation. 相似文献
4.
通常认为, 随着林木不断接近其海拔分布极限, 光合作用产量不断下降, 导致碳水化合物供应不足(碳供应限制), 或者低温限制了碳投资(生长限制)。植物组织内非结构性碳水化合物(Nonstructural carbohydrates, NSC)的含量反映了植物碳供应与碳吸收的平衡。为了检验“碳供应限制”和“生长抑制”假说, 我们对长白山海拔1 700~ 2 050 m的自然生境下生长的岳桦(Betula ermanii)的叶片和枝条组织的NSC含量进行了比较。结果表明: 岳桦叶片的NSC含量随海拔升高变化不显著, 枝条的NSC含量随海拔升高显著增加; 叶片和枝条中淀粉含量与可溶性总糖含量的比值均随海拔的升高而减小; 林线附近的岳桦林木不存在碳水化合物供应不足的问题, 这在一定程度上表明生长限制导致长白山岳桦林线的形成。 相似文献
5.
As trees and shrubs approach the high elevation tree limit, it is often assumed that they fall short in photosynthate (source limitation). Alternatively, low temperature may restrict carbon investment (growth, sink limitation). The content of mobile non-structural carbohydrates (NSC) in tissues is considered a measure of the carbon source–sink balance. To test the source vs. sink limitation hypothesis, we compared late-season NSC concentrations of various woody taxa across altitudinal gradients from the subalpine forest to the treeline at the eastern edge of the Tibetan Plateau. Since we were interested in the generality of trends, we present “community” trends across four taxa, namely Quercus aquifolioides, Abies faxoniana, Rhododendron fabri subsp. prattii and Sorbus rufopilosa. NSC concentrations increased significantly with altitude in branch wood, current-year and last-year leaves, while there were no significant trends in stem sapwood and root xylem. The sugar to starch ratio was roughly 1:1 in branches and evergreen leaves, while stems and roots showed a higher starch fraction. Analyses of total nitrogen in leaves and wood tissues indicated no change in the trees’ nitrogen supply with elevation. The overall altitudinal trends of NSC in this group of woody plant species revealed no depletion of carbon reserves near the tree limit, suggesting that sink limitation predominates woody plant life across this treeline ecotone community. 相似文献
6.
Carbon dynamics in the deciduous broadleaf tree Erman's birch (Betula ermanii) at the subalpine treeline on Changbai Mountain,Northeast China
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Qing‐Wei Wang Lin Qi Wangming Zhou Cheng‐Gang Liu Dapao Yu Limin Dai 《American journal of botany》2018,105(1):42-49
Premise of the Study
The growth limitation hypothesis (GLH) and carbon limitation hypothesis (CLH) are two dominant explanations for treeline formation. The GLH proposes that low temperature drives the treeline through constraining C sinks more than C sources, and it predicts that non‐structural carbohydrate (NSC) levels are static or increase with elevation. Although the GLH has received strong support globally for evergreen treelines, there is still no consensus for deciduous treelines, which experience great asynchrony between supply and demand throughout the year.Methods
We investigated growth and the growing‐season C dynamics in a common deciduous species, Erman's birch (Betula ermanii), along an elevational gradient from the closed forest to the treeline on Changbai Mountain, Northeast China. Samples were collected from developing organs (leaves and twigs) and main storage organs (stems and roots) for NSC analysis.Key Results
Tree growth decreased with increasing elevation, and NSC concentrations differed significantly among elevations, organs, and sampling times. In particular, NSC levels varied slightly during the growing season in leaves, peaked in the middle of the growing season in twigs and stems, and increased continuously throughout the growing season in roots. NSCs also tended to increase or vary slightly in developing organs but decreased significantly in mature organs with increasing elevation.Conclusions
The decrease in NSCs with elevation in main storage organs indicates support for the CLH, while the increasing or static trends in new developing organs indicate support for the GLH. Our results suggest that the growth limitation theory may be less applicable to deciduous species' growth than to that of evergreen species. 相似文献7.
Formation mechanisms of the alpine Erman’s birch (Betula ermanii) treeline on Changbai Mountain in Northeast China 总被引:1,自引:0,他引:1
Dapao Yu Qingwei Wang Jiaqing Liu Wangming Zhou Lin Qi Xiaoyu Wang Li Zhou Limin Dai 《Trees - Structure and Function》2014,28(3):935-947
Key message
The treeline on Changbai Mountain controlled by low temperature and water stress, has not reached the position most commonly expected.Abstract
Treeline pattern is an important consideration in exploring the general mechanisms controlling the response of treelines to climatic change. However, most of the present conclusions were derived from evergreen and/or conifer treeline, it is still not clear about the deciduous treeline. This study analyzed concentrations of non-structural carbohydrates (NSC) and their components (total soluble sugars and starch) in tree tissues of the deciduous species Erman’s birch (Betula ermanii) at four points along an elevational gradient ranging from 1,908–2,058 m a.s.l at the end of the growing season on Changbai Mountain in Northeast China. The mean 10-cm soil temperature of 8.2 °C under trees across the 129-day growing season at the treeline in this region was higher than that of the average threshold temperature found at treeline positions in the global and China’s climate studies. However, altitudinal trends of NSC concentrations increased significantly in all tissue types along the altitudinal gradients, revealing no depletion of carbon reserves at the treeline on Changbai Mountain. At the same time, the pronounced variation of δ13C in leaves and aged branches suggested that low temperature and water stress may simultaneously be operating at high altitudes to restrict the growth and NSC accumulation in trees above the treeline. In light of the above, we conclude that treeline formation on Changbai Mountain is no carbon depletion at the end of growing season, and most likely the result of sink limitation reflecting the combined effects of low temperature and water stress that determined the actual position of the treeline. 相似文献8.
贡嘎山地区不同海拔冷杉比叶质量和非结构性碳水化合物含量变化 总被引:4,自引:0,他引:4
以贡嘎山地区磨西山谷东北坡和康定山谷西南坡为样地,研究了高山林线和低海拔冷杉休眠期和生长旺盛期针叶比叶质量和组织非结构性碳水化合物含量.结果表明:生长在温暖湿润的磨西山谷东北坡的冷杉针叶比叶质量和组织非结构性碳水化合物含量均高于炎热干燥的康定山谷西南坡;高山林线冷杉比叶质量大于低海拔冷杉;高山林线冷杉组织非结构性碳水化合物含量总体高于低海拔冷杉,且生长期比休眠期更为显著.研究结果不支持"碳供应不足导致高山林线形成"的假说. 相似文献
9.
Carbon balance of conifer seedlings at timberline: relative changes in uptake,storage, and utilization 总被引:2,自引:0,他引:2
Low rates of growth for conifers at high elevations may relate to problems in acquiring or utilizing carbon. A traditional hypothesis for growth limits of trees at alpine treeline is that low photosynthesis (A) leads to insufficient supply of carbon for growth. Alternatively, the sink-limitation hypothesis questions the importance of low A, and suggests that trees at treeline have abundant carbon for growth as a result of greater decreases in carbon use [respiration (R) and growth] than A at higher elevations. Concentrations of carbon intermediates (e.g., nonstructural carbohydrates, NSC) have been used to support the sink-limitation hypothesis, with the assumption that NSC reflects the ratio of carbon source to sinks. Our objective was to determine elevation effects on carbon balance (whole-plant uptake, storage, and efflux of carbon) of tree seedlings during their critical establishment phase at timberline. Changes in A and R (of whole crowns), NSC (starches and simple sugars), and growth were measured in seedlings of a treeline (Abies lasiocarpa) and nontreeline species (Pseudotsuga menziesii). Seedlings were outplanted at the lower (2,450 m) and upper (3,000 m) edges of the timberline zone in the Rocky Mountains, USA. At the upper compared with lower elevation, both species had 10-20% less root and needle growth, 13-15% less A, 35-39% less R, and up to 50% greater NSC. A. lasiocarpa allocated more biomass to needles and had greater A but less NSC than P. menziesii. The greater effects of elevation on R compared with A led to a 1.3-fold increase in A:R at the upper elevation, and a corresponding increase in starch (r2 = 0.42)-a pattern consistent with the predictions of the sink-limitation hypothesis. Nevertheless, A and dry mass gain were positively correlated (r2 = 0.42), indicating that variation in photosynthesis was related to growth of seedlings at timberline. 相似文献
10.
H. Shi Q. Zhou X. Liu F. Xie T. Li Q. Zhang H. Dang 《Plant biology (Stuttgart, Germany)》2019,21(1):64-70
- Cold‐adapted trees display acclimation in both carbon source and carbon sink capacity to low‐temperature stress at their upper elevational range limits. Hence a balanced carbon source–sink capacity might be required for their persistence and survival at the elevational tree limits.
- The present study examined the spatial dynamics of carbon source–sink relationship in subalpine fir (Abies fargesii) trees along elevational gradients in the northern slope of the temperate region and in the southern slope of the subtropics in terms of climate in the Qinling Mountain range, north‐central China.
- The results showed that non‐structural carbohydrate (NSC) concentrations in both the source and sink tissues increased with the increase in elevation. The ratio of carbon source–sink displayed a consistent decreasing trend with the increase in elevation and during growing season, showing that it was lowest at a ratio of 2.93 in the northern slope and at a ratio of 2.61 in the southern slope at the upper distribution elevations in the late growing season. Such variations of carbon source–sink ratio might be attributable to the balance between carbon source and sink activities, which changed seasonally across the elevational distribution range.
- We concluded that a ratio of carbon source–sink of at least 2.6 might be essential for subalpine fir trees to persist at their upper range limits. Therefore, a sufficient source–sink ratio and a balanced source–sink relationship might be required for subalpine fir trees to survive and develop at their upper elevational distribution limits.
11.
Many studies have tried to explain the physiological mechanisms of the alpine treeline phenomenon, but the debate on the alpine treeline formation remains controversial due to opposite results from different studies. The present study explored the carbon-physiology of an alpine shrub species (Quercus aquifolioides) grown at its upper elevational limit compared to lower elevations, to test whether the elevational limit of alpine shrubs (<3 m in height) are determined by carbon limitation or growth limitation. We studied the seasonal variations in non-structural carbohydrate (NSC) and its pool size in Q. aquifolioides grown at 3000 m, 3500 m, and at its elevational limit of 3950 m above sea level (a.s.l.) on Zheduo Mt., SW China. The tissue NSC concentrations along the elevational gradient varied significantly with season, reflecting the season-dependent carbon balance. The NSC levels in tissues were lowest at the beginning of the growing season, indicating that plants used the winter reserve storage for re-growth in the early spring. During the growing season, plants grown at the elevational limit did not show lower NSC concentrations compared to plants at lower elevations, but during the winter season, storage tissues, especially roots, had significantly lower NSC concentrations in plants at the elevational limit compared to lower elevations. The present results suggest the significance of winter reserve in storage tissues, which may determine the winter survival and early-spring re-growth of Q. aquifolioides shrubs at high elevation, leading to the formation of the uppermost distribution limit. This result is consistent with a recent hypothesis for the alpine treeline formation. 相似文献
12.
The decrease in temperature with increasing elevation may determine the altitudinal tree distribution in different ways: affecting survival through freezing temperatures, by a negative carbon balance produced by lower photosynthetic rates, or by limiting growth activity. Here we assessed the relative importance of these direct and indirect effects of altitudinal decrease in temperature in determining the treeline in central Chile (33°S) dominated by Kageneckia angustifolia. We selected two altitudes (2000 and 2200 m a.s.l.) along the treeline ecotone. At each elevation, leaf non-structural carbohydrates (NSC) and gas exchange parameters were measured on five individuals during the growing season. We also determined the cold resistance of K.␣angustifolia, by measuring temperatures that cause 50% seedling mortality (LT50) and ice nucleation (IN). No differences in net photosynthesis were found between altitudes. Although no differences were detected on NSC concentration on a dry matter basis between 2000 and 2200 m, when NSC concentration was expressed on a leaf area basis, higher contents were found at the higher elevation. Thus, carbon sink limitations may occur at the K. angustifolia’s upper altitudinal limit. For seedlings derived from seeds collected at the 2200 m, LT50 of cold-acclimated and non-acclimated plants were −9.5 and −7 °C, respectively. However, temperatures as low as −10 °C can frequently occur at this altitude during the end of winter. Therefore, low temperature injury of seedlings seems also be involved in the treeline formation in this species. Hence, a confluence of global (carbon sink limitation) and regional (freezing tolerance) mechanisms explains the treeline formation in the Mediterranean-type climate zone of central Chile. 相似文献
13.
高山林线形成机理研究进展 总被引:1,自引:0,他引:1
高山林线是郁闭森林与高山植被之间的分布界限,作为重要的生态过渡带,对全球和区域性气候变化的反应极为敏感,被认为是气候变化的理想监测器.高山林线研究从最初的形态描述到林线成因假说都是为了寻找高山林线形成的原因.迄今出现的高山林线成因假说都能够在局地尺度解释高山林线成因,但仍然缺乏可以普遍解释全球高山林线现象的假说.温度是林线分布的限制因子,低温限制了林线树种的生存及生长,但是低温影响了哪一个生化过程仍不明确,其影响机理还需进一步探讨.本文对高山林线形成机理,特别是对低温对高山林线植物光合特性、养分特征、非结构性碳水化合物和抗氧化系统的影响等研究进展进行综述,并提出了未来林线研究应该关注的问题. 相似文献
14.
Aim Treelines occur globally within a narrow range of mean growing season temperatures, suggesting that low‐temperature growth limitation determines the position of the treeline. However, treelines also exhibit features that indicate that other mechanisms, such as biomass loss not resulting in mortality (dieback) and mortality, determine treeline position and dynamics. Debate regarding the mechanisms controlling treeline position and dynamics may be resolved by identifying the mechanisms controlling prominent treeline spatial patterns (or ‘form’) such as the spatial structure of the transition from closed forest to the tree limit. Recent treeline studies world‐wide have confirmed a close link between form and dynamics. Location The concepts presented refer to alpine treelines globally. Methods In this review, we describe how varying dominance of three general ‘first‐level’ mechanisms (tree performance: growth limitation, seedling mortality and dieback) result in different treeline forms, what ‘second‐level’ mechanisms (stresses, e.g. freezing damage, photoinhibition) may underlie these general mechanisms, and how they are modulated by interactions with neighbours (‘third‐level’ mechanisms). This hierarchy of mechanisms should facilitate discussions about treeline formation and dynamics. Results We distinguish four primary treeline forms: diffuse, abrupt, island and krummholz. Growth limitation is dominant only at the diffuse treeline, which is the form that has most frequently responded as expected to growing‐season warming, whereas the other forms are controlled by dieback and seedling mortality and are relatively unresponsive. Main conclusions Treeline form provides a means for explaining the current variability in treeline position and dynamics and for exploring the general mechanisms controlling the responses of treelines to climatic change. Form indicates the relative dependence of tree performance on various aspects of the external climate (especially summer warmth versus winter stressors) and on internal feedbacks, thus allowing inferences on the type as well as strength of climate‐change responses. 相似文献
15.
Sanna Susiluoto Martti Permki Eero Nikinmaa Frank Berninger 《Environmental and Experimental Botany》2007,60(3):334-339
In this paper, we tested if sink removal in treeline trees induces downscaling of the stomatal conductance in treeline trees. Körner [Körner, C., 1998. A re-assessment of high elevation treeline positions and their explanation. Oecologia115, 445–459.] hypothesized that treeline trees are particularly sink limited due to low temperatures during the growing season. This implies that they would have lower sapflow rates and their sapflow rates would be less sensitive to variations in temperature than the sapflow rates of control trees. We removed about 75% per year or all buds during 2 subsequent years. We did not find any differences in sapflow rates and their sensitivity to daily mean temperatures in our data. Therefore, our data indicate that the photosynthesis of trees at the treeline might not be sink limited. Furthermore, trees increased their branch extension growth in response to debudding and it seems probable that trees can compensate for a sink removal by increasing the growth rates of remaining meristems. 相似文献
16.
Treeline advance has occurred throughout the twentieth century in mountainous regions around the world; however, local variation and temporal lags in responses to climate warming indicate that the upper limits of some treelines are not necessarily in climatic equilibrium. These observations suggest that factors other than climate are constraining tree establishment beyond existing treelines. Using a seed addition experiment, we tested the effects of seed availability, predation and microsite limitation on the establishment of two subalpine tree species (Picea engelmannii and Abies lasiocarpa) across four treelines in the Canadian Rocky Mountains. The effect of vegetation removal on seedling growth was also determined, and microclimate conditions were monitored. Establishment limitations observed in the field were placed in context with the effects of soil properties observed in a parallel experiment. The seed addition experiment revealed reduced establishment with increasing elevation, suggesting that although establishment within the treeline ecotone is at least partially seed limited, other constraints are more important beyond the current treeline. The effects of herbivory and microsite availability significantly reduced seedling establishment but were less influential beyond the treeline. Microclimate monitoring revealed that establishment was negatively related to growing season temperatures and positively related to the duration of winter snow cover, counter to the conventional expectation that establishment is limited by low temperatures. Overall, it appears that seedling establishment beyond treeline is predominantly constrained by a combination of high soil surface temperatures during the growing season, reduced winter snowpack and unfavourable soil properties. Our study supports the assertion that seedling establishment in alpine treeline ecotones is simultaneously limited by various climatic and nonclimatic drivers. Together, these factors may limit future treeline advance in the Canadian Rocky Mountains and should be considered when assessing the potential for treeline advance in alpine systems elsewhere 相似文献
17.
《Plant Ecology & Diversity》2013,6(3):233-241
Background and Aims: High elevation treelines occur worldwide at similar mean growing season temperatures. Does this result from direct impact of low temperature on growth or carbon metabolism, or does nutrient limitation, induced by low soil temperature, play a role? Similar treeline elevations at contrasting soil fertility argue against the latter, but the actual nutritional status of treeline trees (here addressed as foliage nitrogen concentration) has never been assessed systematically. Although needle nitrogen (N) concentration does not necessarily indicate growth limitation by N, the relative abundance of N would indicate obvious depletions at the treeline. Methods: A central problem with any foliage nutrient assay is that the units for describing the element concentration are dependent on elevation themselves. Here we separate changes in N per unit tissue from changes in reference units. Results: Needles of Pinus cembra and Picea abies in the Alps do not show elevational differences in N concentration per dry weight, water content, area or volume, thus, there is no N depletion near the elevational tree limit. Hence, nutrient supply is either unaltered, or growth is adjusted so that nutrient depletion in needle tissue does not occur. Conclusions: Chronic N shortage at needle level is not an explanation for low tree vigour at the treeline. 相似文献
18.
Recent climate warming forces contrasting growth responses of white spruce at treeline in Alaska through temperature thresholds 总被引:12,自引:0,他引:12
Martin Wilmking Glenn P. Juday Valerie A. Barber Harold S. J. Zald† 《Global Change Biology》2004,10(10):1724-1736
Northern and high‐latitude alpine treelines are generally thought to be limited by available warmth. Most studies of tree‐growth–climate interaction at treeline as well as climate reconstructions using dendrochronology report positive growth response of treeline trees to warmer temperatures. However, population‐wide responses of treeline trees to climate remain largely unexamined. We systematically sampled 1558 white spruce at 13 treeline sites in the Brooks Range and Alaska Range. Our findings of both positive and negative growth responses to climate warming at treeline challenge the widespread assumption that arctic treeline trees grow better with warming climate. High mean temperatures in July decreased the growth of 40% of white spruce at treeline areas in Alaska, whereas warm springs enhance growth of additional 36% of trees and 24% show no significant correlation with climate. Even though these opposing growth responses are present in all sampled sites, their relative proportion varies between sites and there is no overall clear relationship between growth response and landscape position within a site. Growth increases and decreases appear in our sample above specific temperature index values (temperature thresholds), which occurred more frequently in the late 20th century. Contrary to previous findings, temperature explained more variability in radial growth after 1950. Without accounting for these opposite responses and temperature thresholds, climate reconstructions based on ring width will miscalibrate past climate, and biogeochemical and dynamic vegetation models will overestimate carbon uptake and treeline advance under future warming scenarios. 相似文献
19.
Variation of mobile carbon reserves in trees at the alpine treeline ecotone is under environmental control 总被引:1,自引:0,他引:1
? In low temperature-adapted plants, including treeline trees, light-saturated photosynthesis is considerably less sensitive to temperature than growth. As a consequence, all plants tested so far show increased nonstructural carbohydrate (NSC) tissue concentrations when exposed to low temperatures. Reduced carbon supply is thus an unlikely cause for low temperature range limits of plants. For altitudinal treeline trees there is, however, a possibility that high NSC genotypes have been selected. ? Here, we explored this possibility using afforestations with single-provenance conifers along elevational gradients in the Southern Chilean Andes and the Swiss Alps. ? Tree growth was measured at each of four approximately equidistant elevations at and below the treeline. Additionally, at the same elevations, needle, branch and stem sapwood tissues were collected to determine NSC concentrations. ? Overall, growth decreased and NSC concentrations increased with elevation. Along with previous empirical and experimental studies, the findings of this study provide no indication of NSC reduction at the treeline; NSC increased in most species (each represented by one common population) towards their upper climatic limit. The disparity between carbon acquisition and structural carbon investment at low temperature (accumulation of NSC) thus does occur even among genotypes not adapted to treeline environments. 相似文献
20.
Aims Several mechanistic hypotheses have been developed to explain the existence of alpine treelines worldwide. The reproduction limitation hypothesis (RLH) postulates that reproductive processes such as seed production and dispersal are restricted in treeline environments, thereby limiting the establishment of new individuals in advance of extant treelines. Despite its popularity, no study has tested this hypothesis in a comprehensive fashion. In this experiment, we attempted to answer whether there are enough viable seeds being dispersed beyond treeline for sexually mediated treeline advance, and what the implications of climate change might be on these processes.Methods We established 30 plots across two aspects (north vs. south) and three elevational habitats (forest, treeline and tundra) in a white spruce (Picea glauca) boreal forest-alpine tundra ecotone in southwest Yukon, Canada. In each plot, tree characteristics, seed production and predispersal damage were measured. Additionally, eight dispersal trays were positioned in each plot to measure seed rain, and germination trials with and without predation exclosures were constructed in a subset of plots to quantify dispersal and germination success.Important findings Results were highly variable both temporally and spatially. In 2014, a mast year, 69% of adult trees produced cones compared to 0.4% in the following year. Higher density of trees in forest plots compared to treeline and tundra resulted in greater seed production at lower elevations. Across all plots, 88% of seeds were damaged before dispersal or were not viable. Treeline plots had significantly greater predispersal damage. Seed rain was greater in south-facing plots than north-facing plots. Less than 2% of seeds produced on the landscape were dispersed into Tundra plots, located 50 m above treeline. There was a net movement of seeds from the north-facing slope to the south at our study site, likely due to prevailing winds during the dispersal period. Germination counts were more than double on north-facing slopes and one-third higher inside exclosures. Cumulatively, the results provide some evidence for the RLH. Collectively, the high amount of predispersal damage and non-viable seeds, variability associated with dispersal and significant seed predation can functionally influence treeline dynamics. These findings suggest that global treeline distribution models, which rely largely on temperature, may not be entirely accurate for predicting treeline advance—at least at finer temporal scales. Many stochastic factors need to align temporally for successful advance, which is likely to result in a lag of many decades between the period of temperature amelioration and an increased number of trees beyond extant treelines. 相似文献