Biomass and Species Diversity of Different Alpine Plant Communities Respond Differently to Nitrogen Deposition and Experimental Warming

The ability of fragile ecosystems of alpine regions to adapt and thrive under warming and nitrogen deposition is a pressing conservation concern. The lack of information on how these ecosystems respond to the combined impacts of elevated levels of nitrogen and a warming climate limits the sustainable management approaches of alpine grasslands. In this study, we experimented using a completely random blocked design to examine the effects of warming and nitrogen deposition on the aboveground biomass and diversity of alpine grassland plant communities. The experiment was carried out from 2015 to 2018 in four vegetation types, e.g., alpine desert, alpine desert steppe, alpine marsh, and alpine salinised meadow, in the Aerjin Mountain Nature Reserve (AMNR) on the Qinghai–Tibetan Plateau (QTP). We found that W (warming) and WN (warming plus N deposition) treatment significantly increased the aboveground biomass of all the vegetation types (p < 0.05) in 2018. However, W and WN treatment only significantly increased the Shannon diversity of salinised meadows in 2018 and had no significant effect on the Shannon diversity of other vegetation types. Such results suggested that long-term nitrogen deposition and warming can consistently stimulate biomass accumulation of the alpine plant communities. Compared with other vegetation types, the diversity of alpine salinised meadows are generally more susceptible to long-term warming and warming combined with N deposition. Warming accounts many of such variabilities, while short-term N deposition alone may not significantly have an evident effect on the productivity and diversity of alpine grasslands. Our findings suggested that the effects of short-term (≤4 years) N deposition on alpine vegetation productivity and diversity were minimal, while long-term warming (>4 years) will be much more favourable for alpine vegetation.

Characterising the response of plant-mycorrhizal networks to a changing alpine environment

<p>Anthropogenic climate change effects are particularly acute in alpine ecosystems. New Zealand’s alpine regions are experiencing climatic changes at higher than global mean rates, particularly warming and drying. These communities are also facing increasing rates of invasion by exotic plant species. Notably, multiple drivers of change, such as warming and invasion, have been evidenced to interact and facilitate greater ecosystem change. This is of particular concern as New Zealand's alpine plant communities are unique globally and represent national hotspots of biodiversity. Therefore there is a pressing need to understand how they may be affected by the independent and interactive drivers of global environmental change. Alpine plant species form ubiquitous and obligate symbiotic associations with mutualistic mycorrhizal fungi. Plant-mycorrhiza networks are foundational interactions that underpin the diversity and function of terrestrial communities. Plant-mycorrhiza networks are also particularly sensitive to temperature shifts and plant invasions. In this thesis, I investigate the independent and interactive effects of warming and the presence of an invasive species (Common Heather, Calluna vulgaris) on the fungal community composition and the network of mycorrhiza interactions of alpine plants in Tongariro National Park, New Zealand. I sampled the roots of plant species within the Warming and species Removal in Mountains (WaRM) experiment, a factorial combination of warming and Calluna vulgaris removals (n = 8 per treatment) established in TNP in 2015. The plant community at the site consists of plant species that form either arbuscular mycorrhizas or ericoid mycorrhizas. I selected the three most abundant plant species of each mycorrhizal type at the site scale for sampling in each of the 32 plots. DNA was extracted from plant roots, and the internal transcribed spacer of the fungal rRNA gene was amplified by PCR and sequenced on the Illumina Mi-seq platform. Sequence data was demultiplexed and fungal OTUs were identified using the PIPITS pipeline, referencing the UNITE fungal database. In my second chapter, I consider plant species and treatment effects on the diversity and community composition of mycorrhizal fungi. I found WaRM treatments were significant determinants of mycorrhizal compositions in host plant species. Warming simultaneously increased the mycorrhizal fungal diversity and richness of invasive Calluna vulgaris and reduced that of the native host plant species. In chapter 3, using network analyses from the bipartite package of R, I constructed 32 plant mycorrhizal networks of the plot sampled and calculated metrics pertaining to properties of network structure and robustness at the whole network, trophic-level and species/mycorrhizal fungal OTU scales. I then examined the responses of these metrics to the WaRM treatments. I found that warming significantly reduced the robustness of native plant-mycorrhizal networks and increased the strength of the interaction network associated with invasive C. vulgaris. The removal of C. vulgaris had a secondary effect on how mycorrhizal fungal compositions and interaction networks responded to warming. As a generalist C. vulgaris was critical for the ongoing diversity of ericoid mycorrhizal fungi, particularly under warming. However, C. vulgaris simultaneously suppressed the mycorrhizal interaction- networks of native plant species, which further fragmented under warming. I conclude that warming and the presence of invasive C. vulgaris synergistically reduced and decentralised the native plant-mycorrhizal interactions within the network. In summary, my thesis demonstrates the below-ground interactions of alpine plant communities are destabilising under multiple interacting drivers of global environmental change.</p>

Characterising the response of plant-mycorrhizal networks to a changing alpine environment

<p>Anthropogenic climate change effects are particularly acute in alpine ecosystems. New Zealand’s alpine regions are experiencing climatic changes at higher than global mean rates, particularly warming and drying. These communities are also facing increasing rates of invasion by exotic plant species. Notably, multiple drivers of change, such as warming and invasion, have been evidenced to interact and facilitate greater ecosystem change. This is of particular concern as New Zealand's alpine plant communities are unique globally and represent national hotspots of biodiversity. Therefore there is a pressing need to understand how they may be affected by the independent and interactive drivers of global environmental change. Alpine plant species form ubiquitous and obligate symbiotic associations with mutualistic mycorrhizal fungi. Plant-mycorrhiza networks are foundational interactions that underpin the diversity and function of terrestrial communities. Plant-mycorrhiza networks are also particularly sensitive to temperature shifts and plant invasions. In this thesis, I investigate the independent and interactive effects of warming and the presence of an invasive species (Common Heather, Calluna vulgaris) on the fungal community composition and the network of mycorrhiza interactions of alpine plants in Tongariro National Park, New Zealand. I sampled the roots of plant species within the Warming and species Removal in Mountains (WaRM) experiment, a factorial combination of warming and Calluna vulgaris removals (n = 8 per treatment) established in TNP in 2015. The plant community at the site consists of plant species that form either arbuscular mycorrhizas or ericoid mycorrhizas. I selected the three most abundant plant species of each mycorrhizal type at the site scale for sampling in each of the 32 plots. DNA was extracted from plant roots, and the internal transcribed spacer of the fungal rRNA gene was amplified by PCR and sequenced on the Illumina Mi-seq platform. Sequence data was demultiplexed and fungal OTUs were identified using the PIPITS pipeline, referencing the UNITE fungal database. In my second chapter, I consider plant species and treatment effects on the diversity and community composition of mycorrhizal fungi. I found WaRM treatments were significant determinants of mycorrhizal compositions in host plant species. Warming simultaneously increased the mycorrhizal fungal diversity and richness of invasive Calluna vulgaris and reduced that of the native host plant species. In chapter 3, using network analyses from the bipartite package of R, I constructed 32 plant mycorrhizal networks of the plot sampled and calculated metrics pertaining to properties of network structure and robustness at the whole network, trophic-level and species/mycorrhizal fungal OTU scales. I then examined the responses of these metrics to the WaRM treatments. I found that warming significantly reduced the robustness of native plant-mycorrhizal networks and increased the strength of the interaction network associated with invasive C. vulgaris. The removal of C. vulgaris had a secondary effect on how mycorrhizal fungal compositions and interaction networks responded to warming. As a generalist C. vulgaris was critical for the ongoing diversity of ericoid mycorrhizal fungi, particularly under warming. However, C. vulgaris simultaneously suppressed the mycorrhizal interaction- networks of native plant species, which further fragmented under warming. I conclude that warming and the presence of invasive C. vulgaris synergistically reduced and decentralised the native plant-mycorrhizal interactions within the network. In summary, my thesis demonstrates the below-ground interactions of alpine plant communities are destabilising under multiple interacting drivers of global environmental change.</p>

Plant Phenology Dynamics and Pollination Networks in Summits of the High Tropical Andes: A Baseline for Monitoring Climate Change Impacts

Analyzing plant phenology and plant–animal interaction networks can provide sensitive mechanistic indicators to understand the response of alpine plant communities to climate change. However, monitoring data to analyze these processes is scarce in alpine ecosystems, particularly in the highland tropics. The Andean páramos constitute the coldest biodiversity hotspot on Earth, and their species and ecosystems are among the most exposed and vulnerable to the effects of climate change. Here, we analyze for the first time baseline data for monitoring plant phenological dynamics and plant–pollinator networks along an elevation gradient between 4,200 and 4,600 m asl in three mountain summits of the Venezuelan Andes, which are part of the GLORIA monitoring network. We estimated the presence and density of plants with flowers in all the summits and in permanent plots, every month for 1 year. Additionally, we identified pollinators. We calculated a phenological overlap index between species. We summarized the plant–pollinator interactions as a bipartite matrix and represented a quantitative plant–pollinator network, calculating structural properties (grade, connectance, nestedness, and specialization). We also evaluated whether the overall network structure was influenced by differences in sampling effort, changes in species composition between summits, and phenology of the plant species. Finally, we characterized the pollination syndrome of all species. Flowering showed a marked seasonality, with a peak toward the end of the wet season. The overall phenological overlap index was low (0.32), suggesting little synchrony in flowering among species. Species richness of both plants and pollinators decreased along the elevation gradient. Flies, bumblebees, and hummingbirds were the most frequent pollinators in the network, while entomophily and anemophily were the prevailing pollination syndromes. The interaction network in all summits showed high connectance values, significant specialization (H2), and low nestedness. We did not find a significant effect of sampling effort, summit plant species composition, or plant phenology on network structure. Our results indicate that these high tropical alpine plant communities and their plant-pollination networks could be particularly vulnerable to the loss of species in climate change scenarios, given their low species richness and functional redundancy coupled with a high degree of specialization and endemism.

Network motifs involving both competition and facilitation predict biodiversity in alpine plant communities

Biological diversity depends on multiple, cooccurring ecological interactions. However, most studies focus on one interaction type at a time, leaving community ecologists unsure of how positive and negative associations among species combine to influence biodiversity patterns. Using surveys of plant populations in alpine communities worldwide, we explore patterns of positive and negative associations among triads of species (modules) and their relationship to local biodiversity. Three modules, each incorporating both positive and negative associations, were overrepresented, thus acting as "network motifs." Furthermore, the overrepresentation of these network motifs is positively linked to species diversity globally. A theoretical model illustrates that these network motifs, based on competition between facilitated species or facilitation between inferior competitors, increase local persistence. Our findings suggest that the interplay of competition and facilitation is crucial for maintaining biodiversity.

Diversification in the Arctic: Biogeography and Systematics of the North American Micranthes (Saxifragaceae)

Abstract—Flora endemic to the cold habitats of the Northern Hemisphere provide important models for investigating diversification and disjunctions, given both the intense climatic fluctuations of these areas in the recent past and the fascinating biogeographic patterns of today’s Arctic-alpine plant communities. Micranthes Haw. (Saxifragaceae), a clade of small-flowered herbaceous flowering plants comprising ∼80 species, is an ideal group for investigating the evolution and diversification of plants in montane and Arctic ecosystems. Micranthes has proven to be a particularly challenging clade to unravel taxonomically due in part to rampant auto- and allopolyploidy, hybridization, and cryptic speciation. With the goal of providing an updated conspectus for this group, we build upon a recent large phylogenomic analysis to help elucidate the evolution of Micranthes. Here, we present new downstream analyses including diversification analyses, biogeographical reconstructions, and a comparison of methods for dating phylogenomic data sets. To complement these new analyses, we also synthesize chromosomal variation, new observations regarding morphology and species identification, comprehensive field studies, and an extensive review of the literature for Saxifragaceae and Micranthes. A new perspective on the systematics and taxonomy of Micranthes is provided.

Changes in plant composition and diversity in an Alpine heath and meadow after 18 years of experimental warming

Background and aim Global warming is expected to have large impacts on high alpine and Arctic ecosystems in future. Here we report the effects of 18 years of experimental warming on two contrasting high alpine plant communities in subarctic Sweden.Methods Using open-top chambers (OTCs), we analysed the effects of long-term passive experimental warming on two high alpine plant communities, a species- and nutrient-poor heath and a more nutrient- and species-rich mesic meadow. We determined the impact on species composition, species diversity (at the level of rare, frequent and dominant species in each community), and phylogenetic and functional diversity.Key results Long-term warming drove differentiation in the species composition in both heath and meadow vegetation, with the warmed plots having distinctly different species composition in 2013 compared with 1995. In addition, variability in species composition increased in the meadow, while it decreased in the heath. The long-term warming had a significant negative effect on the three orders of phylogenetic Hill diversity in the meadow. There was a similar tendency in the heath, but only the phylogenetic diversity of dominant species was significantly affected. Long-term warming caused a reduction in graminoids in the heath, while deciduous shrubs increased. In the meadow, cushion-forming plants showed an increase in abundance from 2001 to 2013 in the warmed plots. Conclusions Responses in species and phylogenetic diversity to experimental warming varied over both time (medium vs long-term responses) and space (i.e. between the two neighbouring plant communities heath and meadow). The meadow community was more negatively affected in terms of species and phylogenetic diversity than the heath community. A potential driver for the changes in the meadow may be decreased soil moisture caused by the long-term warming.