No benefits from warming even for subnival vegetation in the central Himalayas

An investigation into the potential for upward range expansion in high-montane species on the roof of the world

Climate warming is occurring in high-mountain areas at a faster rate than the global average. To escape the increasing temperatures, alpine species may shift in distribution upwards, threatening cold-adapted nival plant specialists. However, little is known about the success of seedling emergence and establishment at high altitudes outside the current range, particularly in the highest mountain areas of the Himalayas. We selected four native alpine species occurring around 4000 m a.s.l. and sowed seeds at the natural growing site (GS), at a high elevation site (HS; 5000 m a.s.l.) and at high elevation with soil from the growing site (HS-S) in the Khumbu Valley, north-eastern Nepal. We monitored seedling emergence and establishment for two consecutive years. Seedling emergence and establishment varied between species. Emergence was similar between GS and HS and improved at HS-S. Establishment was low at high elevations with all but one species having high mortality after winter. Seedling emergence of low elevation plants is possible at high elevations in the Everest region, indicating species may be able to shift their distribution range upwards. However, successful establishment may be limited by the soil and high winter mortality at high elevations, although not in all species. Climate warming will potentially lead to upward migration of some Himalayan plant species, leading to altered community composition in high-mountain areas.

Warming-induced phenological mismatch between trees and shrubs explains high-elevation forest expansion

Despite the importance of species interaction in modulating the range shifts of plants, little is known about the responses of coexisting life forms to a warmer climate. Here, we combine long-term monitoring of cambial phenology in sympatric trees and shrubs at two treelines of the Tibetan Plateau, with a meta-analysis of ring-width series from 344 shrubs and 575 trees paired across 11 alpine treelines in the Northern Hemisphere. Under a spring warming of +1°C, xylem resumption advances by 2-4 days in trees, but delays by 3-8 days in shrubs. The divergent phenological response to warming was due to shrubs being 3.2 times more sensitive than trees to chilling accumulation. Warmer winters increased the thermal requirement for cambial reactivation in shrubs, leading to a delayed response to warmer springs. Our meta-analysis confirmed such a mechanism across continental scales. The warming-induced phenological mismatch may give a competitive advantage to trees over shrubs, which would provide a new explanation for increasing alpine treeline shifts under the context of climate change.

Shrub-mediated effects on soil nitrogen determines shrub-herbaceous interactions in drylands of the Tibetan Plateau

INTRODUCTION

Shrub promotes the survival, growth and reproduction of understory species by buffering the environmental extremes and improving limited resources (i.e., facilitation effect) in arid and semiarid regions. However, the importance of soil water and nutrient availability on shrub facilitation, and its trend along a drought gradient have been relatively less addressed in water-limited systems.

METHODS

We investigated species richness, plant size, soil total nitrogen and dominant grass leaf δ¹³C within and outside the dominant leguminous cushion-like shrub Caragana versicolor along a water deficit gradient in drylands of Tibetan Plateau.

RESULTS

We found that C. versicolor increased grass species richness but had a negative effect on annual and perennial forbs. Along the water deficit gradient, plant interaction assessed by species richness (RII_species) showed a unimodal pattern with shift from increase to decrease, while plant interaction assessed by plant size (RII_size) did not vary significantly. The effect of C. versicolor on soil nitrogen, rather than water availability, determined its overall effect on understory species richness. Neither the effect of C. versicolor on soil nitrogen nor water availability affected plant size.

DISCUSSION

Our study suggests that the drying tendency in association with the recent warming trends observed in drylands of Tibetan Plateau, will likely hinder the facilitation effect of nurse leguminous shrub on understories if moisture availability crosses a critical minimum threshold.

Transient Effects of Snow Cover Duration on Primary Growth and Leaf Traits in a Tundra Shrub

With the recent climate warming, tundra ecotones are facing a progressive acceleration of spring snowpack melting and extension of the growing season, with evident consequences to vegetation. Along with summer temperature, winter precipitation has been recently recognised as a crucial factor for tundra shrub growth and physiology. However, gaps of knowledge still exist on long-living plant responses to different snowpack duration, especially on how intra-specific and year-to-year variability together with multiple functional trait adjustments could influence the long-term responses. To fill this gap, we conducted a 3 years snow manipulation experiment above the Alpine treeline on the typical tundra species Juniperus communis, the conifer with the widest distributional range in the north emisphere. We tested shoot elongation, leaf area, stomatal density, leaf dry weight and leaf non-structural carbohydrate content of plants subjected to anticipated, natural and postponed snowpack duration. Anticipated snowpack melting enhanced new shoot elongation and increased stomatal density. However, plants under prolonged snow cover seemed to compensate for the shorter growing period, likely increasing carbon allocation to growth. In fact, these latter showed larger needles and low starch content at the beginning of the growing season. Variability between treatments slightly decreased over time, suggesting a progressive acclimation of juniper to new conditions. In the context of future warming scenarios, our results support the hypothesis of shrub biomass increase within the tundra biome. Yet, the picture is still far from being complete and further research should focus on transient and fading effects of changing conditions in the long term.