The stress gradient hypothesis : plant facilitation at the forest-tundra transition (Mealy Mountains, Labrador, Canada)

Facilitative interactions (Stress Gradient Hypothesis) at the Forest-Tundra (F-T) Transition zone around the globe have yet to be evaluated. As climate warms, the balance between facilitation and competition shifts, allowing species to expand their ranges. This study investigates the rote of nurse shrubs ( Betula glandulosa [dwarf birch], and Vaccinium uliginosum [bilberry]) in facilitating recruitment of Picea mariana and P. glauca (black and white spruce) in the boreal F-T Transition zone. Treatments were designed to disentangle the roles of nurse height, shade, and nurse canopy removal by herbivores in determining recruitment success. Seeds germinated at temperatures 5°C below the minimum required for germination in the open and at significantly greater levels (20% ± 6.12 of P. mariana and 12% ± 2.8 of P. Glauca ) compared with the non-shrub reference site (0.5%) and previous studies. Treatments generally had negligible effects on growth, but nurse litter increased levels of soil calcium, suggesting a weak net positive association. If viable seed is available, shrubs will facilitate recruitment. The balance between competition and facilitation is highly site dependent, and cannot be generalized over larger spatial scales. Understanding how species interactions are affected by regional climate change is critical for predicting range expansion of boreal trees

What makes a good neighbour? Drivers of facilitation in alpine cushion plant communities

Species interactions, whether facilitative or competitive, play key roles in structuring plant communities. Research into these associations has focused on competitive interactions, however recently, facilitation research has increased in popularity. Using cushion plants as a model, the objective of this thesis is to determine the effect of abiotic drivers on species interactions as well as on the morphology and reproduction of a potential facilitator. Results from a New Zealand cushion species, Donatia novae-zelandiae found in mosaic alpine environments and oceanic sea-level sites were compared to those from a widespread Northern Hemisphere cushion species, Silene acaulis. Alpine (~1000m) D. novae-zelandiae cushions produced three times as many flowers and seven times as many seeds per capsule than at sea-level, but leaves were larger at sea-level. Cushion compactness was greatest at alpine sites. After two seasons of artificially warming 1.8°C, significant decreases in seed production (35%), leaf length (5%), and width (13%) were observed in the cushions. Donatia novae-zelandiae modestly increased species richness at the sea-level sites (1.4 ± 0.5 more species), but no species was specifically limited to growing within the cushions. Most species showed no significant association, although Dracophyllum longifolium, D. prostratum, Phyllachne colensoi, Rhacomitrium pruinosum, and Coprosma cheesemanii showed significant negative associations with Donatia novae-zelandiae at the alpine sites. Donatia novae-zelandiae may marginally increase species diversity locally, but diversity is not affected at the community level. At alpine sites, D. novae-zelandiae decreased species richness (2.5 ± 0.8 fewer species) compared to open areas. Removing neighbours growing within the D. novae-zelandiae resulted in significant heat stress to the cushions, reducing seed production, compactness, and leaf size. Donatia novae-zelandiae, unlike other cushion species, does not appear to be acting as key a facilitator regardless of whether they grow in an alpine community, or in a more mesic, low altitude habitat. D. novae-zelandiae may in fact benefit from their association with neighbouring plants. Unlike D. novae-zelandiae, Silene acaulis fitness at a high elevation site (2560 m) was reduced compared to the lower site (2317 m); female flower production decreased by 40%, seeds per fruit by 11.6%, and leaf size by 24% at the high site. Strong facilitators, hermaphroditic Silene acaulis individuals supported a greater number of plant species than females (hermaphrodites: 4.2 ± 0.3, females: 3.5 ± 0.2). Facilitative effects also significantly increased with elevation (2560 m: 2.1 ± 1.6, 2317 m: 3.2 ± 1.8). The typical ‘cushion model’ so often reported in the facilitation literature does not accurately represent the cushion plant functional type as a whole. D. novae-zelandiae is not a facilitator, but may rather be acting as a competitor, particularly in alpine cushion mosaic communities. This is likely driven by the peat substrate, typical of cushion mosaic communities. In comparison, cushions growing on more skeletal substrates, such as Silene acaulis are important facilitators. Results of this thesis stress the importance of studying cushion facilitation over a broader spectrum of cushion life forms and habitats

Bryophyte community diversities and expected change under a warming climate in contrasting habitats of the Torngat Mountains, Labrador

In this study we provide the first record of bryophyte community diversities in contrasting habitats of the Torngat Mountains National Park (Labrador, Canada), a coastal mountain subarctic ITEX (International Tundra Experiment) study site. The point-intercept method was used to describe and track changes in the bryophyte community diversity (cover, species richness, diversity indices) at dry vs. wet habitats, in 2008 and 2010. The majority of species observed are common and abundant to the Canadian polar region. Bryophyte community composition differed between wet and dry habitats; Racomitrium lanuginosum, Polytrichum piliferum and P. juniperinum dominated dry and exposed habitats, whereas Drepanocladus spp., Aulacomnium spp. and Sphagnum spp. dominated wet habitats. Wet habitats were more diverse (4.8 6 2.9 vs. 1.6 6 1.2 species m–2) and had higher bryophytes cover (66.1 6 35.4% vs. 12.6 6 9.9%) compared to dry habitats. A survey of the tundra change literature, suggests that a warming climate will decrease bryophyte cover as shrub cover increases, and that the cover of certain species (e.g., Polytrichum piliferum) will likely increase in dry habitats where shrub cover is not expected to increase to the same degree

Complex Changes in Plant Communities across a Subarctic Alpine Tree Line in Labrador, Canada + Supplementary Appendix Table (See Article Tools)

As climate warms, abiotic factors, as well as plant community and biodiversity structure, may constrain or promote the movement of ecotone boundaries. Our study sought to examine how plant communities change across the tree-line ecotone of the Mealy Mountains in Labrador, Canada. We established eight transects (50–100 m in length) along an elevation gradient in three tree-line zones (forest, forest-tundra, and alpine-tundra) and recorded all species and cover of vegetation in contiguous 1 × 1 m quadrats. Companion abiotic parameters of nutrients and soil temperature were also measured. The absence of abrupt changes in important soil nutrients and growing season temperatures suggests that these factors do not limit tree species establishment beyond the current tree line. Vegetation cover and richness, however, were highly variable and in some cases changed non-linearly across the tree-line ecotone. Tree cover and species density generally decreased with elevation, while some field layer species (< 25 cm in height) increased; the latter change seems to be influenced by ground shrubs rather than herbaceous species. As expected, transects separated by the greatest difference in elevation were the least similar (higher beta diversity/species turnover); however, species turnover between the forest and forest-tundra transects was higher than it was between forest-tundra and alpine-tundra transects, even though the latter were separated by a greater elevation. Community structure and species turnover vary greatly across a tree line with the greatest differences between the forest and the forest-tundra, suggesting a biotic or abiotic barrier. While our ability to predict how the tree line will respond to continued climate change is complicated by these patterns in plant communities, the potential barriers investigated and others identified will be a useful focus for future studies. Au fur et à mesure que le climat se réchauffe, les facteurs abiotiques de même que la structure des communautés végétales et de la biodiversité pourraient contraindre ou promouvoir le mouvement des délimitations des écotones. Notre étude cherche à examiner comment les communautés de plantes changent dans l’écotone de la limite forestière des montagnes Mealy au Labrador, Canada. Nous avons établi huit transects (de 50 à 100 m de longueur) le long d’un gradient d’altitude chevauchant trois zones de la limite forestière (forêt, toundra forestière et toundra alpine), puis répertorié toutes les espèces et la couverture végétale se trouvant dans des quadrats contigus d’un mètre sur un mètre. Nous avons également mesuré les paramètres abiotiques des éléments nutritifs et des températures du sol connexes. L’absence de changements abrupts en ce qui a trait aux importants éléments nutritifs du sol et aux températures de la période de croissance suggère que ces facteurs ne limitent pas l’établissement des espèces d’arbres au-delà de la limite forestière actuelle. Cependant, la couverture végétale et sa richesse variaient beaucoup et, dans certains cas, changeaient de façon non linéaire dans l’écotone de la limite forestière. De manière générale, le couvert arboré et la densité des espèces diminuaient avec l’altitude, tandis que certaines espèces de la couverture vivante au-dessus du sol (< 25 cm de hauteur) augmentaient. Ce dernier changement semble influencé par le couvert arbustif et non pas par les espèces herbacées. Comme nous nous y attendions, les transects qui étaient séparés par la plus grande différence d’altitude étaient les moins semblables (plus grande diversité bêta et plus grand renouvellement des espèces). Cependant, le renouvellement des espèces entre le transect de la forêt et le transect de la toundra forestière était plus grand qu’entre les transects de la toundra forestière et de la toundra alpine, même si ces derniers étaient séparés par de plus grandes élévations. La structure des communautés et le renouvellement des espèces varient beaucoup au sein d’une limite forestière, les plus grandes différences étant enregistrées entre la forêt et la toundra forestière, ce qui suggère une barrière biotique ou abiotique. Bien que notre aptitude à prévoir comment la limite forestière réagira au changement climatique continu soit embrouillée par les modèles de communautés végétales, les barrières potentielles faisant l’objet d’études et d’autres barrières joueront un rôle important dans des études ultérieures

The effects of foundation species on community assembly: a global study on alpine cushion plant communities

Foundation species can change plant community structure by modulating important ecological processes such as community assembly, yet this topic is poorly understood. In alpine systems, cushion plants commonly act as foundation species by ameliorating local conditions. Here, we analyze diversity patterns of species’ assembly within cushions and in adjacent surrounding open substrates (83 sites across five continents) calculating floristic dissimilarity between replicate plots, and using linear models to analyze relationships between microhabitats and species diversity. Floristic dissimilarity did not change across biogeographic regions, but was consistently lower in the cushions than in the open microhabitat. Cushion plants appear to enable recruitment of many relatively stress-intolerant species that otherwise would not establish in these communities, yet the niche space constructed by cushion plants supports a more homogeneous composition of species than the niche space beyond the cushion’s influence. As a result, cushion plants support higher a- diversity and a larger species pool, but harbor assemblies with lower b-diversity than open microhabitats. We conclude that habitats with and without dominant foundation species can strongly differ in the processes that drive species recruitment, and thus the relationship between local and regional species diversity

Facilitative plant interactions and climate simultaneously drive alpine plant diversity

Interactions among species determine local-scale diversity, but local interactions are thought to have minor effects at larger scales. However, quantitative comparisons of the importance of biotic interactions relative to other drivers are rarely made at larger scales. Using a data set spanning 78 sites and five continents, we assessed the relative importance of biotic interactions and climate in determining plant diversity in alpine ecosystems dominated by nurse-plant cushion species. Climate variables related with water balance showed the highest correlation with richness at the global scale. Strikingly, although the effect of cushion species on diversity was lower than that of climate, its contribution was still substantial. In particular, cushion species enhanced species richness more in systems with inherently impoverished local diversity. Nurse species appear to act as a safety net' sustaining diversity under harsh conditions, demonstrating that climate and species interactions should be integrated when predicting future biodiversity effects of climate change