Increasingly Important Role of Atmospheric Aridity on Tibetan Alpine Grasslands

et al. 2019

Ecology

Alpine biomes are climate change hotspots, and treeline dynamics in particular have received much attention as visible evidence of climate‐induced shifts in species distributions. Comparatively little is known, however, about the effects of climate change on alpine shrubline dynamics. Here, we reconstruct decadally resolved shrub recruitment history (age structure) through the combination of field surveys and dendroecology methods at the world's highest juniper (Juniperus pingii var. wilsonii) shrublines on the south‐central Tibetan Plateau. A total of 1,899 shrubs were surveyed at 12 plots located in four regions along an east‐to‐west declining precipitation gradient. We detected synchronous recruitment with 9 out of 12 plots showing a gradual increase from 1600 to 1900, a peak at 1900–1940, and a subsequent decrease from the 1930s onward. Shrub recruitment was significantly and positively correlated with reconstructed summer temperature from 1600 to 1940, whereas it was negatively associated with temperature in recent decades (1930–2000). Recruitment was also positively correlated with precipitation, except in the 1780–1830 period, when a trend toward wetter climate conditions began. Warming‐induced drought limitation has likely reduced the recruitment potential of alpine juniper shrubs in recent decades. Ongoing warming without a simultaneous increase in precipitation is expected to further impair recruitment at the world's highest juniper shrublines and alter the dynamics and competitive balance between woody plant species throughout these alpine biomes.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Past the climate optimum: Recruitment is declining at the world's highest juniper shrublines on the Tibetan Plateau

Liang

et al. 2019

Ecology

“…For the same reasoning, negative γ int should be found in dry season. In addition, lower stomatal conductance and hydraulic water stress during dry seasons limit temperature‐driven increase of transpiration (Gu et al., ), posing stress to photosynthesis due to larger vapor pressure deficit (Ding et al., ; Novick et al., ), which acts to make γ int negative for drier conditions. In current carbon cycle models, the limitations in reproducing the moisture regulation on γ int can either from biases in plant water uptake from the soil due to limitation in simulating plant root distribution (e.g., Fan, Miguez‐Macho, Jobbágy, Jackson, & Otero‐Casal, ) or from the limitation in simulating leaf temperature.…”

Section: Discussionmentioning

confidence: 99%

Divergent response of seasonally dry tropical vegetation to climatic variations in dry and wet seasons

Ciais

et al. 2018

Global Change Biology

Interannual variations of photosynthesis in tropical seasonally dry vegetation are one of the dominant drivers to interannual variations of atmospheric CO growth rate. Yet, the seasonal differences in the response of photosynthesis to climate variations in these ecosystems remain poorly understood. Here using Normalized Difference Vegetation Index (NDVI), we explored the response of photosynthesis of seasonally dry tropical vegetation to climatic variations in the dry and the wet seasons during the past three decades. We found significant (p < 0.01) differences between dry and wet seasons in the interannual response of photosynthesis to temperature (γ ) and to precipitation (δ ). γ is ~1% °C more negative and δ is ~8% 100 mm more positive in the dry season than in the wet season. Further analyses show that the seasonal difference in γ can be explained by background moisture and temperature conditions. Positive γ occurred in wet season where mean temperature is lower than 27°C and precipitation is at least 60 mm larger than potential evapotranspiration. Two widely used Gross Primary Productivity (GPP) estimates (empirical modeling by machine-learning algorithm applied to flux tower measurements, and nine process-based carbon cycle models) were examined for the GPP-climate relationship over wet and dry seasons. The GPP derived from empirical modeling can partly reproduce the divergence of γ , while most process models cannot. The overestimate by process models on negative impacts by warmer temperature during the wet season highlights the shortcomings of current carbon cycle models in representing interactive impacts of temperature and moisture on photosynthesis. Improving representations on soil water uptake, leaf temperature, nitrogen cycling, and soil moisture may help improve modeling skills in reproducing seasonal differences of photosynthesis-climate relationship and thus the projection for impacts of climate change on tropical carbon cycle.

“…Rising temperatures and decreasing precipitation could further accelerate drought stress in the future, especially in the highly sensitive, semi-arid trans-Himalayan region by increasing evapotranspiration and vapor pressure deficits (Wang et al 2013;Ding et al 2018). In addition, competition between nearby trees for soil moisture may further exacerbate moisture stress for tree growth (Liang et al 2016).…”

Section: Climate-growth Relationshipsmentioning

confidence: 99%

Growth response of Abies spectabilis to climate along an elevation gradient of the Manang valley in the central Himalayas

et al. 2019

The Himalayas are characterized by a broad gradient of bioclimatic zones along their elevation. However, less is known how forest growth responds to climatic change along elevation. In this study, four standard treering width chronologies of Himalayan fir (Abies spectabilis) were developed, spanning 142-649 years along an elevation gradient of 3076-3900 m a.s.l. Principal component analysis classified the four chronologies into two groups; the ones at lower elevations (M1 and M2) and higher elevations (M3 and M4) show two distinct growth trends. Radial growth is limited by summer (June-August) precipitation at M3, and by precipitation during spring (March-May) and summer at M4. It is limited by spring temperatures and winter precipitation (December-February) at M1. Tree-ring width chronologies also significantly correlate with winter and spring Palmer Drought Severity Index (PDSI) at M1, and with summer PDSI at M3 and M4. Thus, Himalayan fir growth at high elevations is mainly limited by moisture stress rather than by low temperatures. Furthermore, the occurrence of missing rings coincides with dry periods, providing additional evidence for moisture limitation of Himalayan fir growth.