Uncoupled changes in tree cover and field layer vegetation at two Pyrenean treeline ecotones over 11 years

Background: The alpine treeline ecotone is regarded as a sensor of the effects of global change on alpine plant communities. However, little is known about how treeline dynamics influence the diversity and composition of alpine plant communities. Such information is necessary to forecast how ascending montane forests may affect the composition of alpine flora.

Aims: We analysed the temporal variations in tree cover, plant diversity and composition, and the effect of tree cover dynamics on field layer vegetation over a period of 11 years, at two alpine treeline ecotones in the central Pyrenees, Spain.

Methods: Tree and field layer vegetation was sampled in permanent transects in 1998 and 2009, using the point-intercept method. Temporal changes in tree cover, plant species richness and abundance were characterised along the ecotone by using a randomisation approach, rarefaction curves, and a non-parametric multivariate test, respectively.

Results: Tree cover increased significantly at one of the sites, whereas plant species richness only increased at the other site where tree cover had not changed. Vegetation composition changed significantly at both sites, but it was not spatially coupled with changes in tree cover along the ecotone.

Conclusions: A change of tree cover does not necessarily trigger changes in the ground flora at the treeline over relatively short periods (decade scale). The results challenge our ability to infer short-term biodiversity impacts from upslope advance of forests. Integrated tree and field layer monitoring approaches are necessary to produce a better understanding of the impact of ongoing global change on treeline ecotones.     

Shrub–tree interactions and environmental changes drive treeline dynamics in the Subarctic

Treelines have drawn persistent research interest as they can respond markedly to climate. However, the mechanisms that determine tree seedling recruitment and the response of the forest‐tundra ecotone to environmental changes remain poorly understood. We hypothesise that treeline tree seedling performance depends on the interplay between climatic and soil nutritional changes and facilitative and competitive interactions between trees and shrubs. We conducted a seedling transplantation experiment with Betula pubescens at a subarctic treeline, in northern Sweden, which followed a full factorial design with four treatment factors relating to environmental regimes of stress and resource availability: site (forest vs treeline); temperature (+/? passive warming); shrub presence (+/?Vaccinium myrtillus removal); and nutrient availability (+/? NPK addition). During three growing seasons we assessed the establishment and performance of Betula. The experimental manipulations caused highly significant effects on seedling performance. Although Vaccinium enhanced seedling survival and reduced the effects of excessive solar radiation and insect herbivory, the seedlings growing with the shrub had a poorer performance by the end of the experimental period. Also, seedlings in the forest had a poorer performance than those at the treeline. Betula seedlings showed a very pronounced and positive response to passive warming and to nutrient addition, but such effects were more evident at the treeline site and often interacted with the presence of Vaccinium. This experiment shows that shrub–tree interactions are important drivers of subarctic treeline dynamics and that they vary with time and space. Facilitation, competition, herbivory and environmental changes at the tree seedling stage act as important filters in structuring the forest–tundra ecotone. We demonstrate that changes in this ecotone cannot be simply predicted from changing temperature patterns alone, and that complex interactions need to be considered, not only between shrubs and trees, but also with herbivores and between warming and soil nutrient availability.     

Changes in plant taxonomic and functional diversity patterns following treeline advances in the South Urals

Background: Treeline ecotones represent environmental boundaries that fluctuate in space and time and thus induce changes in plant taxonomic and functional diversity.

Aims: To study changes through time in taxonomic and functional plant diversity patterns along the treeline ecotone.

Methods: In 2002, vegetation was sampled along a gradient from upper montane forest to the treeline–alpine transition in the South Ural Mountains, Russia. In 2014, vegetation was resampled and plant functional traits were collected. We studied spatial and temporal changes in plant species composition, functional composition and functional diversity.

Results: Species composition and diversity changed along the elevational gradient. The functional composition in height, leaf area, specific leaf area and leaf nitrogen content decreased with elevation, whereas functional composition of leaf carbon content increased. We found a temporal shift towards shorter plants with smaller leaves in treeline sites. Functional richness varied in several traits along the elevational gradient, while functional dispersion showed a trend towards increased functional dispersion in height, specific leaf area and leaf nitrogen in the treeline–tundra transition.

Conclusions: Tree encroachment across the treeline ecotone has resulted in a shift in plant species relative abundances and functional diversity, possibly affecting plant community assembly patterns.     

Spatial patterns of Smith fir alpine treelines on the south-eastern Tibetan Plateau support that contingent local conditions drive recent treeline patterns

Background: Recent work has shown little change in the position of the Smith fir treeline on the south-eastern Tibetan Plateau in response to global warming. However, the relationship between tree distribution patterns within the treeline ecotone and low responsiveness is unknown, and additional constraints than climate might be major drivers of these patterns (e.g. microsite availability for regeneration).

Aims: To characterise the spatial patterns of Smith fir alpine treelines and to infer the underlying processes driving their dynamics.

Methods: We investigated spatial patterns of Smith fir trees across two treeline ecotones in the Sygera Mountains, south-eastern Tibetan Plateau. The O(r)-ring statistic was used to analyse the univariate and bivariate spatial point patterns of three size classes (adults, juveniles and seedlings).

Results: Mature trees presented random spatial patterns. Clusters of juveniles and seedlings colonised areas not occupied by mature trees. Seedlings were clustered and established preferentially near juvenile firs, Rhododendron mats and over moss–lichen and organic matter substrates, indicating the importance of microsite availability for successful Smith fir recruitment.

Conclusions: Local factors such as microsite availability may play a major role in driving recent Smith fir treeline patterns and determine the lack of significant warming-induced upward shifts of these ecotones.     

植物群落类型和围栏封育对高山林线岷江冷杉幼苗成活的影响

林线树种幼苗的生长紧密关系着高山林线的形成。通过室内发芽实验测定了岷江冷杉种子萌发能力。2015年春,在川西巴郎山岷江冷杉林线之上的高山林线交错带,通过播种处理(播种和不播种),研究了不同植物群落类型(灌木和草地群落)和围栏处理(围栏和不围栏)对岷江冷杉幼苗成活率的影响,以期为进一步探讨高山林线形成机制提供科学参考。结果表明:(1)岷江冷杉室内发芽率31.4%,成活率随时间下降。(2)灌木群落的全年日平均空气温湿度显著高于草本群落,而两群落的全年日平均土壤温度无显著差异。(3)未播种样地没有发现岷江冷杉幼苗,说明林线交错带缺乏岷江冷杉种子。(4)与不围栏样地相比,围栏封育分别显著提高了岷江冷杉幼苗成活率2.0%(2015年)和2.2%(2016年);与灌木群落相比,草本群落中岷江冷杉幼苗成活率显著提高0.8%(2015年)和1.2%(2016年),说明灌木对林线交错带幼苗更新具有更强的竞争作用。围栏处理下,草本群落中岷江冷杉幼苗成活率显著高于灌木群落;不围栏处理下,草本群落中岷江冷杉幼苗成活率显著低于灌木群落,2017年在不围栏样地没有幼苗存活,说明动物干扰对林线交错带森林更新有阻碍作用。研究表明,高山林线交错带森林更新,受种子、植物竞争和动物干扰(例如,放牧)等多方面因素的共同影响,可以通过播种、植物剔除(去除竞争)和围栏等促进幼苗更新和成活。     

The forest–alpine ecotone: a multi-scale approach to spatial and temporal dynamics of treeline change at Niwot Ridge

Background: Current understanding of treeline or forest-alpine ecotone (FAE) dynamics does not fully explain past and present FAE patterns and their underlying processes, nor allow prediction of their response to climate change.

Aims: We address the overarching hypothesis that the FAE is a mosaic of distinct landscape units of vegetation and landforms that result in differential responses to climate change. We focus on climate-related, landscape and vegetation characteristics, but also consider the effect of landscape heterogeneity on biogeochemistry and overall resilience of the FAE to climate change.

Results: There are three distinct FAE land units at Niwot Ridge, generated by different interactions of climate with vegetation, landforms and topography. Within these FAEs, a process of self-organisation takes place from organism to patch to landscape scales, and is modulated by positive and negative feedback loops along an elevation gradient. The underlying controls cannot be attributed solely to temperature, but to a combination of interactions along a physical/biotic gradient.

Conclusions: FAE dynamics result from interactions among mechanisms and processes at the microsite, patch and landscape scales: (1) tree persistence; (2) forest patch establishment; (3) drivers of patch forest configurations and (4) resilience, increasing along a gradient of biotic control.     

高山林线生态交错区木本植物幼苗分布特征、更新机制及其对气候变化的响应

木本植物幼苗是高山林线生态交错区的重要组成部分,其更新对气候变化背景下树线的移动至关重要.本研究通过对近几十年来全球范围内林线生态交错区的木本植物幼苗分布特征、更新机制及其对气候变化响应的研究总结得出:林线生态交错区木本植物幼苗的空间分布类型主要为渐变型和聚集型,且不同分布类型对树线动态的指示意义各异.在全球尺度上,其分布的海拔高限通常与生长季长度、均温和物种特性等有关,而在区域尺度上则多受降水影响.在幼苗更新初期,种源在很大程度上决定了种子的萌发及分布位置,之后微环境的促进作用为幼苗的定植提供庇护,提高其存活率,而在更新后期多种生物和非生物因素及其相互作用则非常关键.气候变暖促使林线生态交错区气温升高、降水充沛,有利于幼苗生长,使其向高海拔区域扩张而成为树线上移的先兆,但部分物种受遗传特性或适应策略影响,仅表现为密度增加,使树线保持相对稳定.未来应借助树轮、14C等精确定年技术,通过长期的野外定位观测和室内模拟,加强多时空尺度下林线幼苗的空间分布特征和更新机制研究,分析不同类型林线内木本植物幼苗的适应策略,预测气候变化背景下的树线动态,为山地生态系统恢复及保护提供科学依据.     

Gradual expansion of moisture sensitive Abies spectabilis forest in the Trans-Himalayan zone of central Nepal associated with climate change

Population structure and tree recruitment dynamics in the natural treeline ecotone of high mountains are strong indicators of vegetation responses to climate. Here, we examined recruitment dynamics of Abies spectabilis across the treeline ecotone (3439–3638 m asl) of Chimang Lekh of Annapurna Conservation Area in the Trans-Himalayan zone of central Nepal. Dendrochronological techniques were used to establish stand age structure by ring counts of adults, and by terminal bud scar count for seedlings and saplings. The results showed abundant seedling recruitment, higher regenerative inertia and colonization with a consistent range shift of the A. spectabilis treeline. The upward expansion of this sub-alpine treeline was found to be driven by a strong dependence of seedling recruitment and radial growth on snowmelt and precipitation as temperatures rise. The radial growth of A. spectabilis at the alpine timberline ecotone (ATE) and closed timberline forest (CTF) showed sensitivity to spring season (March–May) climate. Tree ring indices of CTF showed a strong positive correlation with spring and annual precipitation, and a significant negative correlation with spring and annual temperature, however, moisture sensitivity was less strong at ATE than CTF.     

Vegetation patterns at the alpine treeline ecotone: the influence of tree cover on abrupt change in species composition of alpine communities

Aims: The upper elevation limit of forest vegetation in mountain ranges (the alpine treeline ecotone) is expected to be highly sensitive to global change. Treeline shifts and/or ecotone afforestation could cause fragmentation and loss of alpine habitat, and are expected to trigger considerable alterations in alpine vegetation. We performed an analysis of vegetation structure at the treeline ecotone to evaluate whether distribution of the tree population determines the spatial pattern of vegetation (species composition and diversity) across the transition from subalpine forest to alpine vegetation. Location: Iberian eastern range of the Pyrenees. Methods: We studied 12 alpine Pinus uncinata treeline ecotones. Rectangular plots ranging from 940 to 1900 m² were placed along the forest‐alpine vegetation transition, from closed forest to the treeless alpine area. To determine community structure and species distribution in the treeline ecotone, species variation along the forest‐alpine vegetation transition was sampled using relevés of 0.5 m² set every 2 m along the length of each plot. Fuzzy C‐means clustering was performed to assess the transitional status of the relevés in terms of species composition. The relation of P. uncinata canopy cover to spatial pattern of vegetation was evaluated using continuous wavelet transform analysis. Results: Vegetation analyses revealed a large degree of uniformity of the subalpine forest between all treeline ecotone areas studied. In contrast, the vegetation mosaic found upslope displayed great variation between sites and was characterized by abrupt changes in plant community across the treeline ecotone. Plant richness and diversity significantly increased across the ecotone, but tree cover and diversity boundaries were not spatially coincident. Conclusions: Our results revealed that no intermediate communities, in terms of species composition, are present in the treeline ecotone. Ecotone vegetation reflected both bedrock type and fine‐scale heterogeneity at ground level, thereby reinforcing the importance of microenvironmental conditions for alpine community composition. Tree cover did not appear to be the principal driver of alpine community changes across the treeline ecotone. Microenvironmental heterogeneity, together with effects of past climatic and land‐use changes on ecotone vegetation, may weaken the expected correlation between species distribution and vegetation structure.     

Tree recruitment at the treeline across the Continental Divide in the Northern Rocky Mountains,USA: the role of spring snow and autumn climate

ABSTRACT

Background: Topoclimate can influence tree establishment within treeline ecotones. Yet much less is known about how regional topography, such as the Continental Divide, Rocky Mountains, mediates the role of climate in governing treeline dynamics.

Aims: To utilise the Continental Divide to test whether contrasts in growing-season moisture regimes to the west (summer-dry) and east (summer-wet) impact the spatio-temporal patterns of tree establishment and rates of treeline advance in the Northern Rocky Mountains.

Methods: We sampled trees at sites on north- and south-facing slopes, west and east of the Continental Divide. We used dendroecological techniques to reconstruct patterns of tree establishment. Age-structure data were quantitatively compared with climate to evaluate possible mechanistic linkages.

Results: Across all sites, 96% of trees established after 1950. There was a treeline advance (range = 39–140 m) accompanied by increases in tree density. Significantly more trees established during wet springs on both sides of the Divide.

Conclusions: Overall, snow duration in spring and autumn temperatures appear to influence patterns of tree recruitment at the treeline. Continued warming will likely amplify the role of autumn climate in regulating tree establishment throughout treeline ecotones in the Northern Rocky Mountains, particularly west of the Divide where summer-dry conditions persist.     

贡嘎山雅家埂峨眉冷杉林线种群的时空动态

通过对贡嘎山雅家埂峨眉冷杉种群林线附近6个3000 m2样地(阴阳坡各3个)中峨眉冷杉(Abies fabri Craib)种群的定位调查,分析了过去100a间该区峨眉冷杉种群的时间-空间动态。结果表明:1)雅家埂林线附近峨眉冷杉种群密度在过去100 a(主要是近50 a)有显著的升高,但树线的海拔位置并无明显的爬升;2)阴阳坡林线格局存在显著的坡向分异:阴坡林线和树线的海拔高度显著高于阳坡(分别比阳坡高152.5 m和135.8 m),阳坡林线附近峨眉冷杉早期的生长速率在大于阴坡,但后期的生长速率却低于阴坡;3)热量(温度)控制假说不能完全解释雅家埂目前的树线格局,除气候因素之外,其它因素也限制了雅家梗地区树线位置的变化。     

Climate‐driven speedup of alpine treeline forest growth in the Tianshan Mountains,Northwestern China

Forest growth is sensitive to interannual climatic change in the alpine treeline ecotone (ATE). Whether the alpine treeline ecotone shares a similar pattern of forest growth with lower elevational closed forest belt (CFB) under changing climate remains unclear. Here, we reported an unprecedented acceleration of Picea schrenkiana forest growth since 1960s in the ATE of Tianshan Mountains, northwestern China by a stand‐total sampling along six altitudinal transects with three plots in each transect: one from the ATE between the treeline and the forest line, and the other two from the CFB. All the sampled P. schrenkiana forest patches show a higher growth speed after 1960 and, comparatively, forest growth in the CFB has sped up much slower than that in the ATE. The speedup of forest growth at the ATE is mainly accounted for by climate factors, with increasing temperature suggested to be the primary driver. Stronger water deficit as well as more competition within the CFB might have restricted forest growth there more than that within the ATE, implying biotic factors were also significant for the accelerated forest growth in the ATE, which should be excluded from simulations and predictions of warming‐induced treeline dynamics.     

Does shrub invasion indirectly limit grass establishment via seedling herbivory? A test at grassland‐shrubland ecotones

Question: Does shrub invasion at ecotones indirectly limit grass establishment by increasing mammalian seedling herbivory? Location: Chihuahuan Desert, New Mexico, USA. Methods: We tested the hypothesis that herbivore‐related mortality of seedlings of the dominant perennial grass Bouteloua eriopoda would be highest in shrub‐dominated portions of grassland‐shrubland ecotones. We tested the hypothesis in two Chihuahuan Desert sites featuring similar shrub encroachment patterns but different shrub species, grass cover, and different abundances of small mammals. Within each site we transplanted B. eriopoda seedlings to grass‐dominated, middle, and shrub‐dominated positions of replicate ecotones during the time of year (mid‐summer) when they would naturally appear and monitored seedling fates. We estimated population size/activity of putative small mammal herbivores. Results: Seedlings were killed by mammals in greater numbers in shrubland than in grassland or middle ecotone positions at the site with large herbivore numbers. At the site with low herbivore numbers, most seedlings were killed in middle ecotone positions. The abundance patterns of herbivores did not parallel patterns of seedling herbivory across the ecotones or between sites. Conclusions: Seedling herbivory is an important process and is related to vegetation composition, but the mechanisms underlying the relationship are not clear. We speculate that variation in small mammal foraging behavior may contribute to seedling herbivory patterns. Restoration strategies in the Chihuahuan Desert need to account for the abundance and/or behavior of native herbivores.     

Conifer seedling recruitment across a gradient from forest to alpine tundra: effects of species,provenance, and site

Background: Seedling germination and survival is a critical control on forest ecosystem boundaries, such as at the alpine–treeline ecotone. In addition, while it is known that species respond individualistically to the same suite of environmental drivers, the potential additional effect of local adaptation on seedling success has not been evaluated.

Aims: To determine whether local adaptation may influence the position and movement of forest ecosystem boundaries, we quantified conifer seedling recruitment in common gardens across a subalpine forest to alpine tundra gradient at Niwot Ridge, Colorado, USA.

Methods: We studied Pinus flexilis and Picea engelmannii grown from seed collected locally at High (3400 m a.s.l.) and Low (3060 m a.s.l.) elevations. We monitored emergence and survival of seeds sown directly into plots and survival of seedlings germinated indoors and transplanted after snowmelt.

Results: Emergence and survival through the first growing season was greater for P. flexilis than P. engelmannii and for Low compared with High provenances. Yet survival through the second growing season was similar for both species and provenances. Seedling emergence and survival tended to be greatest in the subalpine forest and lowest in the alpine tundra. Survival was greater for transplants than for field-germinated seedlings.

Conclusions: These results suggest that survival through the first few weeks is critical to the establishment of natural germinants. In addition, even small distances between seed sources can have a significant effect on early demographic performance – a factor that has rarely been considered in previous studies of tree recruitment and species range shifts.     

Growth and biomass allocation of shrub and grass seedlings in response to predicted changes in precipitation seasonality

Anthropogenic emissions contribute to an annual 0.5% increase in atmospheric CO₂. As global CO₂ levels increase, regional precipitation patterns will likely be altered. Our primary objective was to determine whether a reduction in summer precipitation or an increase in winter/spring precipitation, predicted by global climate change models, will favor the establishment of C₄ grasses or C₃ shrubs in southern savannas. Our secondary objective was to determine how defoliation and microsite light availability interact with altered precipitation regimes to influence grass and shrub seedling growth and biomass allocation patterns. Seedlings of 3 shrub species (Prosopis glandulosa var. glandulosa, Acacia berlandieri, and A. greggii var. wrightii) and 3 grass species (Aristida purpurea var. wrightii, Setaria texana, and Stipa leucotricha) were watered based on probable changes in precipitation in a CO₂ enriched atmosphere (0.6, 0.8, and 1.0 current ambient summer precipitation and 1.0, 1.15, and 1.30 current winter/spring precipitation). Seedlings were defoliated at 3 levels (non-defoliated, single defoliation, and repeated defoliation) within 2 levels of microsite light availability (100 and 50% ambient). Defoliation significantly reduced total shrub and grass seedling biomass. Reducing light availability decreased shrub seedling root:shoot ratio, but total biomass was not significantly affected. Grass seedling biomass and root:shoot ratio decreased when light availability was reduced. Changing the seasonality of precipitation by reducing summer rainfall or increasing winter/spring rainfall did not significantly influence growth or biomass allocation of grass and shrub seedlings in a semiarid savanna. Microsite variations in defoliation intensity and light availability influence seedling growth and biomass allocation more than changing seasonality of precipitation. Shrub and grass seedling establishment and growth on semiarid rangelands are already limited by summer precipitation, so a further reduction as proposed by climate change models will have a limited impact on seedling dynamics.     

Limited prospects for future alpine treeline advance in the Canadian Rocky Mountains

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     

Treeline dynamics in relation to climatic variability in the central Tianshan Mountains, northwestern China

Aim Climate variability may be an important mediating agent of ecosystem dynamics in cold, arid regions such as the central Tianshan Mountains, north‐western China. Tree‐ring chronologies and the age structure of a Schrenk spruce (Picea schrenkiana) forest were developed to examine treeline dynamics in recent decades in relation to climatic variability. Of particular interest was whether tree‐ring growth and population recruitment patterns responded similarly to climate warming. Location The study was conducted in eight stands that ranged from 2500 m to 2750 m a.s.l. near the treeline in the Tianchi Nature Reserve (43°45′?43°59′ N, 88°00′?88°20′ E) in the central Xinjiang Uygur Autonomous Region, northwestern China. Methods Tree‐ring cores were collected and used to develop tree‐ring chronologies. The age of sampled trees was determined from basal cores sampled as close as possible to the ground. Population age structure and recruitment information were obtained using an age–d.b.h. (diameter at breast height) regression from the sampled cores and the d.b.h. measured on all trees in the plots. Ring‐width chronologies and tree age structure were both used to investigate the relationship between treeline dynamics and climate change. Results Comparisons with the climatic records showed that both the radial growth of trees and tree recruitment were influenced positively by temperature and precipitation in the cold high treeline zone, but the patterns of their responses differed. The annual variation in tree rings could be explained largely by the average monthly minimum temperatures during February and August of the current year and by the monthly precipitation of the previous August and January, which had a significant and positive effect on tree radial growth. P. schrenkiana recruitment was influenced mainly by consecutive years of high minimum summer temperatures and high precipitation during spring. Over the last several decades, the treeline did not show an obvious upward shift and new recruitment was rare. Some trees had established at the treeline at least 200 years ago. Recruitment increased until the early 20th century (1910s) but then decreased with poor recruitment over the past several decades (1950–2000). Main conclusions There were strong associations between climatic change and ring‐width patterns, and with recruitments in Schrenk spruce. Average minimum temperatures in February and August, and total precipitation in the previous August and January, had a positive effect on tree‐ring width, and several consecutive years of high minimum summer temperature and spring precipitation was a main factor favouring the establishment of P. schrenkiana following germination within the treeline ecotone. Both dendroclimatology and recruitment analysis were useful and compatible to understand and reconstruct treeline dynamics in the central Tianshan Mountains.     

The interacting effects of temperature,ground disturbance,and herbivory on seedling establishment: implications for treeline advance with climate warming

Thermal control of treeline position is mediated by local environmental and ecological factors, making trends in treeline migration difficult to extrapolate geographically. We investigated the ecological dynamics of conifer establishment at treeline in the Mealy Mountains (Labrador, Canada) and the potential for its expansion with climate warming. Available seedbed and tree seedling emergence in the treeline ecotone were monitored, and seeds and seedlings of Picea mariana were planted along an elevational gradient from open-canopy forest through tree islands to alpine tundra. Experimental treatments included passive warming of daytime air, ground disturbance, and vertebrate herbivore exclosures. Responses in seed germination and seedling growth, damage, and mortality were monitored over two growing seasons, and re-surveyed after 5 years. While no tree seedlings were observed growing naturally above the treeline, planted seeds were able to germinate, develop and overwinter, and persist for 4 years in all habitats examined. Disturbance of the seedbed was important for seedling emergence in the forest and tree islands. While temperature enhancement alone had little impact on emergence, even moderate temperature increases had significantly disproportionate effects on emergence of seedlings in the alpine habitat when combined with soil disturbance, indicating that future climate warming could lead to treeline advance if viable seed and suitable substrate for recruitment are available. The positive effect of excluding herbivores suggests that herbivory may be an important filter modifying future species distribution. While seedbed conditions and herbivory would control the rate of individual species advance, the results indicate potential upslope migration of the treeline in the Mealy Mountains, with consequent loss of alpine ecosystems.     

Living in a world of fire: the population dynamics of Mulinum spinosum in Northwestern Patagonia grasslands

Questions

How do fire frequency and fire size affect the long-term population dynamics of Mulinum spinosum? Which demographic parameters contribute most to the overall effect of fire on population growth? What is the relative importance of resprouting in sustaining population increase?

Location

Grass-shrub northwestern Patagonian steppe, Argentina.

Methods

We monitored five permanent plots excluded from grazing for 6 years. We measured shrub abundance, dimensions, reproductive status, seedling emergence, and the size structure of M. spinosum, a resprouting native shrub inhabiting the northwest Patagonian steppe. Data were used to parametrize a stochastic matrix model developed to explore the influence of fire frequency and fire size on long-term population dynamics. We analyzed hypothetical scenarios that included fire frequencies ranging from one per year up to exclusion.

Results

Except for annual fires, projections show growing populations regardless of fire frequency. For fire return intervals greater than 50 years, the population becomes independent of fires, with an annual population growth rate of 5.6%. The results suggest two relevant aspects of the population dynamics of this species: M. spinosum is well adapted to the current fire frequency and its resprouting capability will allow M. spinosum to survive and persist in the community, even under frequent fires.

Conclusions

Climate change models forecast an increase in summer temperature in NW Patagonia and, consequently, an enhanced fire frequency. Fire is a driver of M. spinosum encroachment that gets worse in overgrazed grasslands. Mulinum spinosum encroachment derivate in a relative replacement of palatable grasses by shrubs changes the ecosystem functionality and reduces productivity. Controlling this process is highly difficult and we suggest a change in the land use for the areas already deteriorated.

     

Tree recruitment above the treeline and potential for climate‐driven treeline change

Questions: How do population structure and recruitment characteristics of Betula saplings beyond the treeline vary among climatic regions, and what is the potential for development into tree‐sized individuals with interacting grazing pressure? Location: Scandes Mountains. Methods: Sapling characteristics of Betula pubescens subsp. tortuosa, their topographic position above the treeline, growth habitat and evidence of recent grazing was investigated in three areas with a long continuous grazing history, along a latitudinal gradient (62‐69°N). Results: Saplings were common up to 100 m above the treeline in all areas. The northern areas were characterised by small (<30 cm) and young (mean 14 years old) saplings in exposed micro‐topographic locations unfavourable to long‐term survival. In the southern area, broad height (2‐183 cm) and age (4‐95 years; mean 32 years) distributions were found in sheltered locations. Age declined with altitude in all areas. Sapling growth rate varied within and between areas, and the age × height interaction was significant only in the southern area. Growth rates decreased from south to north and indicated a considerable time required to reach tree size under prevailing conditions. Conclusions: Regional differences can be attributed to climatic differences, however, interacting biotic and abiotic factors such as micro‐topography, climate and herbivory, mutually affect the characteristics of birch saplings. In view of the long time needed to reach tree size, the generally expected evident and fast treeline advance in response to climate warming may not be a likely short‐term scenario. The sapling pool in the southern region possesses strongest potential for treeline advance.