Contrasting nurse plants and nurse rocks: The spatial distribution of seedlings of two sub-Antarctic species

Effects of post oak (Quercus stellata) and smooth brome (Bromus inermis) competition on water uptake and root partitioning of eastern redcedar (Juniperus virginiana)

Eastern redcedar Juniperus virginiana is encroaching into new habitats, which will affect native ecosystems as this species competes with other plants for available resources, including water. We designed a greenhouse experiment to investigate changes in soil moisture content and rooting depths of two-year-old J. virginiana saplings growing with or without competition. We had four competition treatments: 1) none, 2) with a native tree (Quercus stellata), 3) with an invasive grass (Bromus inermis), and 4) with both Q. stellata and B. inermis. We measured soil moisture content over two years as well as root length, total biomass, relative water content, midday water potential, and mortality at the end of the experiment. When J. virginiana and B. inermis grew together, water depletion occurred at both 30-40 cm and 10-20 cm. Combined with root length results, we can infer that J. virginiana most likely took up water from the deeper layers whereas B. inermis used water from the top layers. We found a similar pattern of water depletion and uptake when J. virginiana grew with Q. stellata, indicating that J. virginiana took up water from the deeper layers and Q. stellata used water mostly from the top soil layers. When the three species grew together, we found root overlap between J. virginiana and Q. stellata. Despite the root overlap, our relative water content and water potential indicate that J. virginiana was not water stressed in any of the plant combinations. Regardless, J. virginiana saplings had less total biomass in treatments with B. inermis and we recorded a significantly higher mortality when J. virginiana grew with both competitors. Root overlap and partitioning can affect how J. virginiana perform and adapt to new competitors and can allow their co-existence with grasses and other woody species, which can facilitate J. virginiana encroachment into grasslands and woodlands. Our data also show that competition with both Q. stellata and B. inermis could limit establishment, regardless of water availability.

Spatial patterns and interspecific relationships of two dominant cushion plants at three elevations on the Kunlun Mountain, China

One of the most important ecological processes is the formation of interspecific relationships in relation to spatial patterns among alpine cushion plants in extreme environmental habitats. However, such relationships remain poorly understood. Here, we examined the spatial patterns of alpine cushion plants along an altitudinal gradient of environmental severity and the interspecific relationship between two cushion species (Thylacospermum caespitosum and Androsace tangulashanensis) on the eastern Kunlun Mountain of China. Our results showed that the two species were highly aggregated within a distance of 2.5-5 m at the mid (S2) altitude, whereas they were randomly distributed at the low (S1) and high (S3) altitudes. A positive spatial interaction between the two species was observed over shorter distances at the mid (S2) altitude, and the spatial patterns were related to the size of individuals of the two species. Moreover, the impact of A. tangulashanensis on T. caespitosum (RII_{T. caespitosum}) was negative in all the study plots, and a positive impact of T. caespitosum on A. tangulashanensis (RII_{A. tangulashanensis}) was only observed at the mid (S2) altitude. Together, these results demonstrated that the spatial patterns of these two cushions varied with environmental severity, since the outcome of the interactions were different, to some extent, at the three altitudes. Plant size is the main factor affecting the spatial correlation and interspecific relationship between two cushions. Therefore, its potential influence should be considered when discussing interspecific relationships among cushions and their community construction at small scales in alpine ecosystems.

The influence of landscape, climate and history on spatial genetic patterns in keystone plants (Azorella) on sub-Antarctic islands

The distribution of genetic variation in species is governed by factors that act differently across spatial scales. To tease apart the contribution of different processes, especially at intermediate spatial scales, it is useful to study simple ecosystems such as those on sub-Antarctic oceanic islands. In this study, we characterize spatial genetic patterns of two keystone plant species, Azorella selago on sub-Antarctic Marion Island and Azorella macquariensis on sub-Antarctic Macquarie Island. Although both islands experience a similar climate and have a similar vegetation structure, they differ significantly in topography and geological history. We genotyped six microsatellites for 1,149 individuals from 123 sites across Marion Island and 372 individuals from 42 sites across Macquarie Island. We tested for spatial patterns in genetic diversity, including correlation with elevation and vegetation type, and clines in different directional bearings. We also examined genetic differentiation within islands, isolation-by-distance with and without accounting for direction, and signals of demographic change. Marion Island was found to have a distinct northwest-southeast divide, with lower genetic diversity and more sites with a signal of population expansion in the northwest. We attribute this to asymmetric seed dispersal by the dominant northwesterly winds, and to population persistence in a southwestern refugium during the Last Glacial Maximum. No apparent spatial pattern, but greater genetic diversity and differentiation between sites, was found on Macquarie Island, which may be due to the narrow length of the island in the direction of the dominant winds and longer population persistence permitted by the lack of extensive glaciation on the island. Together, our results clearly illustrate the implications of island shape and geography, and the importance of direction-dependent drivers, in shaping spatial genetic structure.

Functionally distinct assembly of vascular plants colonizing alpine cushions suggests their vulnerability to climate change

BACKGROUND AND AIMS

Alpine cushion plants can initially facilitate other species during ecological succession, but later on can be negatively affected by their development, especially when beneficiaries possess traits allowing them to overrun their host. This can be reinforced by accelerated warming favouring competitively strong species over cold-adapted cushion specialists. However, little empirical research has addressed the trait-based mechanisms of these interactions. The ecological strategies of plants colonizing the cushion plant Thylacospermum caespitosum (Caryophyllaceae), a dominant pioneer of subnival zones, were studied in the Western Himalayas.

METHODS

To assess whether the cushion colonizers are phylogenetically and functionally distinct, 1668 vegetation samples were collected, both in open ground outside the cushions and inside their live and dead canopies, in two mountain ranges, Karakoram and Little Tibet. More than 50 plant traits related to growth, biomass allocation and resource acquisition were measured for target species, and the phylogenetic relationships of these species were studied [or determined].

KEY RESULTS

Species-based trait-environment analysis with phylogenetic correction showed that in both mountain ranges Thylacospermum colonizers are phylogenetically diverse but functionally similar and are functionally different from species preferring bare soil outside cushions. Successful colonizers are fast-growing, clonal graminoids and forbs, penetrating the cushion by rhizomes and stolons. They have higher root-to-shoot ratios, leaf nitrogen and phosphorus concentrations, and soil moisture and nutrient demands, sharing the syndrome of competitive species with broad elevation ranges typical of the late stages of primary succession. In contrast, the species from open ground have traits typical of stress-tolerant specialists from high and dry environments.

CONCLUSION

Species colonizing tight cushions of T. caespitosum are competitively strong graminoids and herbaceous perennials from alpine grasslands. Since climate change in the Himalayas favours these species, highly specialized subnival cushion plants may face intense competition and a greater risk of decline in the future.

Maintaining distances with the engineer: patterns of coexistence in plant communities beyond the patch-bare dichotomy

Two-phase plant communities with an engineer conforming conspicuous patches and affecting the performance and patterns of coexisting species are the norm under stressful conditions. To unveil the mechanisms governing coexistence in these communities at multiple spatial scales, we have developed a new point-raster approach of spatial pattern analysis, which was applied to a Mediterranean high mountain grassland to show how Festuca curvifolia patches affect the local distribution of coexisting species. We recorded 22 111 individuals of 17 plant perennial species. Most coexisting species were negatively associated with F. curvifolia clumps. Nevertheless, bivariate nearest-neighbor analyses revealed that the majority of coexisting species were confined at relatively short distances from F. curvifolia borders (between 0-2 cm and up to 8 cm in some cases). Our study suggests the existence of a fine-scale effect of F. curvifolia for most species promoting coexistence through a mechanism we call 'facilitation in the halo'. Most coexisting species are displaced to an interphase area between patches, where two opposite forces reach equilibrium: attenuated severe conditions by proximity to the F. curvifolia canopy (nutrient-rich islands) and competitive exclusion mitigated by avoiding direct contact with F. curvifolia.

Ontogenetic shifts in plant interactions vary with environmental severity and affect population structure

Environmental conditions and plant size may both alter the outcome of inter-specific plant-plant interactions, with seedlings generally facilitated more strongly than larger individuals in stressful habitats. However, the combined impact of plant size and environmental severity on interactions is poorly understood. Here, we tested explicitly for the first time the hypothesis that ontogenetic shifts in interactions are delayed under increasingly severe conditions by examining the interaction between a grass, Agrostis magellanica, and a cushion plant, Azorella selago, along two severity gradients. The impact of A. selago on A. magellanica abundance, but not reproductive effort, was related to A. magellanica size, with a trend for delayed shifts towards more negative interactions under greater environmental severity. Intermediate-sized individuals were most strongly facilitated, leading to differences in the size-class distribution of A. magellanica on the soil and on A. selago. The A. magellanica size-class distribution was more strongly affected by A. selago than by environmental severity, demonstrating that the plant-plant interaction impacts A. magellanica population structure more strongly than habitat conditions. As ontogenetic shifts in plant-plant interactions cannot be assumed to be constant across severity gradients and may impact species population structure, studies examining the outcome of interactions need to consider the potential for size- or age-related variation in competition and facilitation.

Testing the stress-gradient hypothesis at the roof of the world: effects of the cushion plant Thylacospermum caespitosum on species assemblages

Many cushion plants ameliorate the harsh environment they inhabit in alpine ecosystems and act as nurse plants, with significantly more species growing within their canopy than outside. These facilitative interactions seem to increase with the abiotic stress, thus supporting the stress-gradient hypothesis. We tested this prediction by exploring the association pattern of vascular plants with the dominant cushion plant Thylacospermum caespitosum (Caryophyllaceae) in the arid Trans-Himalaya, where vascular plants occur at one of the highest worldwide elevational limits. We compared plant composition between 1112 pair-plots placed both inside cushions and in surrounding open areas, in communities from cold steppes to subnival zones along two elevational gradients (East Karakoram: 4850–5250 m and Little Tibet: 5350–5850 m). We used PERMANOVA to assess differences in species composition, Friedman-based permutation tests to determine individual species habitat preferences, species-area curves to assess whether interactions are size-dependent and competitive intensity and importance indices to evaluate plant-plant interactions. No indications for net facilitation were found along the elevation gradients. The open areas were not only richer in species, but not a single species preferred to grow exclusively inside cushions, while 39–60% of 56 species detected had a significant preference for the habitat outside cushions. Across the entire elevation range of T. caespitosum, the number and abundance of species were greater outside cushions, suggesting that competitive rather than facilitative interactions prevail. This was supported by lower soil nutrient contents inside cushions, indicating a resource preemption, and little thermal amelioration at the extreme end of the elevational gradient. We attribute the negative associations to competition for limited resources, a strong environmental filter in arid high-mountain environment selecting the stress-tolerant species that do not rely on help from other plants during their life cycle and to the fact the cushions do not provide a better microhabitat to grow in.

Cushions of Thylacospermum caespitosum (Caryophyllaceae) do not facilitate other plants under extreme altitude and dry conditions in the north-west Himalayas

BACKGROUND

Cushion plants are commonly considered as keystone nurse species that ameliorate the harsh conditions they inhabit in alpine ecosystems, thus facilitating other species and increasing alpine plant biodiversity. A literature search resulted in 25 key studies showing overwhelming facilitative effects of different cushion plants and hypothesizing greater facilitation with increased environmental severity (i.e. higher altitude and/or lower rainfall). At the same time, emerging ecological theory alongside the cushion-specific literature suggests that facilitation might not always occur under extreme environmental conditions, and especially under high altitude and dryness.

METHODS

To assess these hypotheses, possible nursing effects of Thylacospermum caespitosum (Caryophyllaceae) were examined at extremely high altitude (5900 m a.s.l.) and in dry conditions (precipitation <100 mm year(-1)) in Eastern Ladakh, Trans-Himalaya. This is, by far, the highest site, and the second driest, at which the effects of cushions have been studied so far.

KEY RESULTS

In accordance with the theoretical predictions, no nursing effects of T. caespitosum on other alpine plants were detected. The number and abundance of species were greater outside cushions than within and on the edge of cushions. None of the 13 species detected was positively associated with cushions, while nine of them were negatively associated. Plant diversity increased with the size of the area sampled outside cushions, but no species-area relationship was found within cushions.

CONCLUSIONS

The results support the emerging theoretical prediction of restricted facilitative effects under extreme combinations of cold and dryness, integrating these ideas in the context of the ecology of cushion plants. This evidence suggests that cases of missing strong facilitation are likely to be found in other extreme alpine conditions.