Investigating Masking Effects of Age Trends on the Correlations among Tree Ring Proxies

The influence of hydrometeorological factors on tree growth in mountainous watersheds of the Qilian mountains in China

To examine the influence of hydrometeorological factors on forest ecosystems, this study focused on the growth response of the Qinghai spruce (Picea crassifolia Kom.) to hydrometeorological factors, such as soil moisture, relative humidity, vapor pressure deficit, temperature, precipitation and wind speed, in a mountainous watershed. The Dayekou watershed, which is situated in the Qilian Mountains, was used to study the increase in stem diameter based on the daily-monthly fluctuations, cumulative growth, and stem diameter expansion in response to hydrometeorological parameters. The stem diameters of six dominant trees (categorized in three classes) were recorded using the Dendrometer DRL26 tree stem diameter growth monitor and combined with hydrometeorological monitoring data. The influence of hydrometeorological factors on stem diameter growth was statistically analyzed. The results indicated that the daily fluctuation of stem diameter growth of Qinghai spruce exhibited a parabolic pattern, which could be divided into three stages: contraction (from 10:00 to 21:00), expanding (from 21:00 to 5:00 the following day), and growth (from 5:00 to 11:00 the following day). The monthly stem diameter growth also exhibited a trend, which could be divided into three stages: initial growth (May), rapid expansion (June-July) and slow growth (August-October). At a 40 cm depth, soil water content, air humidity, and atmospheric pressure all showed positive correlations with stem diameter growth (P < 0.01), while saturated water pressure differential, wind speed, and photosynthetically active radiation showed negative correlations (P < 0.01). Our results demonstrated that relative air humidity, soil moisture, air temperature, and atmospheric pressure at a 40 cm depth had the highest impact on the Qinghai spruce's growth in stem diameter. Changes in these hydrometeorological factors due to potential climate change will affect forest growth in the future.

Climate impacts on tree-ring stable isotopes across the Northern Hemispheric boreal zone

Boreal regions are changing rapidly with anthropogenic global warming. In order to assess risks and impacts of this process, it is crucial to put these observed changes into a long-term perspective. Summer air temperature variability can be well reconstructed from conifer tree rings. While the application of stable isotopes can potentially provide complementary climatic information over different seasons. In this study, we developed new triple stable isotope chronologies in tree-ring cellulose (δ¹³C_trc, δ¹⁸O_trc, δ²H_trc) from a study site in Canada. Additionally, we performed regional aggregated analysis of available stable isotope chronologies from 6 conifers' tree species across high-latitudinal (HL) and - altitudinal (HA) as well as Siberian (SIB) transects of the Northern Hemispheric boreal zone. Our results show that summer air temperature still plays an important role in determining tree-ring isotope variability at 11 out of 24 sites for δ¹³C_trc, 6 out of 18 sites for δ¹⁸O_trc and 1 out of 6 sites for δ²H_trc. Precipitation, relative humidity and vapor pressure deficit are significantly and consistently recorded in both δ¹³C_trc and δ¹⁸O_trc along HL. Summer sunshine duration is captured by all isotopes, mainly for HL and HA transects, indicating an indirect link with an increase in air and leaf temperature. A mixed temperature-precipitation signal is preserved in δ¹³C_trc and δ¹⁸O_trc along SIB transect. The δ²H_trc data obtained for HL-transect provide information not only about growing seasonal moisture and temperature, but also capture autumn, winter and spring sunshine duration signals. We conclude that a combination of triple stable isotopes in tree-ring studies can provide a comprehensive description of climate variability across the boreal forest zone and improve ecohydrological reconstructions.

Towards the Third Millennium Changes in Siberian Triple Tree-Ring Stable Isotopes

Significant air temperature and precipitation changes have occurred since the 2000s in vulnerable Siberian subarctic regions and urged updates of available chronologies towards the third millennium. It is important to better understand recent climatic changes compared to the past decades, centuries and even millennia. In this study, we present the first annually resolved triple tree-ring isotope dataset (δ13C, δ18O and δ2H) for the eastern part of the Taimyr Peninsula (TAY) and northeastern Yakutia (YAK) from 1900 to 2021. We found that the novel and largely unexplored δ2H of larch tree-ring cellulose was linked significantly with δ18O for the YAK site, which was affected by averaged April–June air temperatures and evaporation. Simulated by the Land Surface Processes and Exchanges (LPX-Bern 1.0) model, the water fraction per year for soil depths at 0–20 and 20–30 cm was significantly linked with the new eco-hydrological tree-ring δ2H data. Our results suggest increasing evapotranspiration and response of trees’ water relation to rising thaw water uptake from lower (20–30 cm) soil depth. A positive effect of July air temperature on tree-ring δ18O and a negative impact of July precipitation were found, indicating dry conditions. The δ13C in larch tree-ring cellulose for both sites showed negative correlations with July precipitation and relative humidity, confirming dry environmental conditions towards the third millennium.