Inter and intra-annual links between climate, tree growth and NDVI: improving the resolution of drought proxies in conifer forests

The inter- and intra-annual variability in radial growth reflects responses to climatic variability and water shortage, especially in areas subjected to seasonal drought. However, it is unknown how this variability is related to forest productivity, which can be assessed by measuring changes in canopy greenness and cover through remote sensing products as the Normalized Difference Vegetation Index (NDVI). We combine xylogenesis with measurements of inter-annual changes in seasonal wood production (earlywood width, adjusted latewood width) and NDVI to improve the understanding of climate and drought impacts on growth and forest productivity in a Pinus teocote stand located in northern Mexico. Cambial dynamics accelerated in March and a high production of radially enlarging and thickening tracheids were observed from April to October and from June to October, respectively. Tracheid maturation was very active in October when latewood production peaked. Wet conditions in winter-spring and summer-autumn enhanced earlywood and latewood production, respectively. Earlywood and latewood were constrained by long (4–10 months) and short (2–3 months) droughts, respectively. The earlywood production depended on April soil moisture, which agrees with the peak of radially enlarging tracheid production found during that month. Aligning drought proxies at inter- and intra-annual scales by using growth and productivity measures improves our understanding of conifer forest responses to water shortage.

Comparison of dendroclimatic relationships using multiple tree-ring indicators (tree-ring width and δ 13 C) from Masson pine

This study addressed the effects of climate drivers on the tree-ring width (TRW) parameters (total ring width (TR), earlywood width (EW) and latewood width (LW)) and the total ring δ 13 C series of different wood components (whole wood, α-cellulose and holocelluose) from Masson pine in subtropical China. Pairwise correlation coefficients between three ring width parameters were statistically significant. EW and LW did not reveal much stronger climate sensitivity rather than TR. This indicated that the use of intra-annual ring width has little benefit in extracting more climate information. The mean δ 13 C series of the three components of the total ring had the strongest climate response to the July–September relative humidity ( r = −0.792 (whole wood), −0.758 (holocellulose) and −0.769 (α-cellulose)). There are no significant differences in the dendroclimatic relationships of the δ 13 C series of different wood components. Through both stationary temporal and spatial-statistical perspectives, the moisture drivers (summer/autumn) had a significant impact on three ring width parameters and three components of Masson pine. Overall, the radial growth and the δ 13 C series showed different responses to the same climate drivers during the same period. Moreover, the R-squared values of the strongest climate-proxy correlation coefficients were smaller than 50% for TRW. Consequently, the δ 13 C series of Masson pine may be a more representative climate proxy than TRW parameters for dendroclimatology in subtropical China.

Dendroindication of ecoclimatic condition in forest remediation area within Northern Steppe of Ukraine

We analyzed ring width, latewood width and earlywood width of Pinus sylvestris trees under normal and flood condition in Dnipropetrovsk region, within Northern Steppe of Ukraine. Precipitation from February to August seems to be the most stable climatic factor which influenced Scots pine growth rate and caused the difference between maximum and minimum ring width in normal conditions. Meteorological conditions were mainly associated with general ring values and earlywood width, and were less associated with latewood width values. Assessment of the effect of climatic signals on tree rings’ growth process in living and dead trees and in the normal and flood condition by analyses of correlation and response function was conducted. Average annual temperatures affected the tree growth negatively in normal conditions and tree increment positively in flood conditions. Annual precipitation was correlated positively with ring width, earlywood width series in normal conditions, but negatively with these series in flood conditions.

Early summer hydroclimatic signals are captured well by tree-ring earlywood width in the eastern Qinling Mountains, central China

In the humid and semi-humid regions of China, tree-ring-width (TRW) chronologies offer limited moisture-related climatic information. To gather additional climatic information, it would be interesting to explore the potential of the intra-annul tree-ring-width indices (i.e., the earlywood width, EWW, and latewood width, LWW). To achieve this purpose, TRW, EWW, and LWW were measured from the tree-ring samples of Pinus tabuliformis originating from the semi-humid eastern Qinling Mountains, central China. Standard (STD) and signal-free (SSF) chronologies of all parameters were created using these detrending methods including (1) negative exponential functions combined with linear regression with negative (or zero) slope (NELR), (2) cubic smoothing splines with a 50 % frequency cutoff at 67 % of the series length (SP67), and (3) age-dependent splines with an initial stiffness of 50 years (SPA50). The results showed that EWW chronologies were significantly negatively correlated with temperature but positively correlated with precipitation and soil moisture conditions during the current early-growing season. By contrast, LWW and TRW chronologies had weaker relationships with these climatic factors. The strongest climatic signal was detected for the EWW STD chronology detrended with the NELR method, explaining 50 % of the variance in the May–July self-calibrated Palmer Drought Severity Index (MJJ scPDSI) during the instrumental period 1953–2005. Based on this relationship, the MJJ scPDSI was reconstructed back to 1868 using a linear regression function. The reconstruction was validated by comparison with other hydroclimatic reconstructions and historical document records from adjacent regions. Our results highlight the potential of intra-annual tree-ring indices for reconstructing seasonal hydroclimatic variations in humid and semi-humid regions of China. Furthermore, our reconstruction exhibits a strong in-phase relationship with a newly proposed East Asian summer monsoon index (EASMI) before the 1940s on the decadal and longer timescales, which may be due to the positive response of the local precipitation to EASMI. Nonetheless, the cause for the weakened relationship after the 1940s is complex, and cannot be solely attributed to the changing impacts of precipitation and temperature.

Warm-season hydroclimate variability in Central China since 1866 AD and its relations with the East Asian Summer Monsoon: evidence from tree-ring earlywood width

Historical hydroclimate records derived from tree-ring parameters are scarce in the core region of East Asian Summer Monsoon (EASM) in China, limiting our understanding of the inter-decadal hydroclimate variability of this region and its possible connections with the EASM. In this study, standard chronologies of total tree-ring width (TRW), earlywood width (EWW), and latewood width (LWW) were created using tree-ring samples of Pinus tabulaeformis in the eastern Qinling Mountains, Central China. The strongest growth-climate relationship was found between EWW and May–July self-calibrated Palmer Drought Severity Index (MJJ scPDSI). Therefore, a linear regression model, which explained 50.3 % of the variance in MJJ scPDSI (1951–2005), was developed to estimate the past MJJ scPDSI variations using EWW. The time series of MJJ scPDSI was extended back to the year 1866, and validated by independent hydroclimate series from nearby regions. Before the mid-1950s, the variations of MJJ scPDSI were in-phase with those of EASM intensity on decadal and longer timescales, suggesting that wet conditions would occur in the eastern Qinling Mountains when EASM was strengthened. Since the mid-1950s, however, the relationship has been out-of-phase. This phase change may be associated with an intensified dipole pattern of EASM precipitation.

Climate sensitivity of radial growth in Norway spruce (Picea abies (L.) Karst.) under different CO2 concentrations

This investigation examined the effects of two different carbon dioxide concentrations ([CO2]): Ambient (A, 385 μmol (CO2) mol−1) and elevated (E, A+385 μmol (CO2) mol−1)) on the tree-ring width and early to latewood proportion in Norway spruce for seven years (2006-2012). Further, to improve our understanding of the influence of climatic variables, we assessed the effects of precipitation and temperature. Our observations showed that spruce trees growing under elevated CO2 (EC) formed less early (p > 0.05) and latewood (p < 0.05) and hence smaller annual increments (p > 0.05) than trees in ambient CO2 (AC). Early to latewood proportion was nearly 73% and 75% in AC and EC, respectively. In both CO2 concentrations, the largest tree-rings and earlywood width was observed during 2009 and 2010, which is coincident with the highest precipitation in May (2010) and the highest air temperature in April (2009). Moreover, to determine the association between the latewood formation and air temperature during the second half of the growing season, and correlation between the earlywood formation and precipitation during the first half of the growing season we run Spearman’s correlation test, the determination coefficient values for latewood formation were r = 0.45 (AC) and r = 0.68 (EC), and for earlywood formation were r = 0.53 (AC) and r = 0.42 (EC), although coefficient values were not statistically significant (p > 0.05). Also, our study indicated that temperature had stonger influence than precipitation in EC, but in AC precipitation had the strongest effect on radial growth.

Climate impacts on radial growth and vegetation activity of two co-existing Mediterranean pine species

Improved knowledge of the time scales at which drought stress mostly influences tree growth is crucial for the early detection of forest dieback. This study aimed to evaluate the impact of climate (temperature and precipitation) on vegetation activity (normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI)) of Pinus halepensis Mill. and Pinus pinea L. mixed forest located in western Albania and to assess the drought impact (standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI)) calculated at different time scales (1–12 months) on radial growth (earlywood width, latewood width, and tree-ring width) and vegetation activity of these species. Both vegetation indices showed a negative response to August temperatures, and the EVI responded positively to September precipitation. NDVI and EVI were significantly affected by the SPI in spring and late summer. All tree-ring features in P. halepensis were positively related with EVI in August, whereas P. pinea latewood width showed a significant and positive relationship with NDVI in September. Radial growth of P. halepensis responded significantly to both drought indices in late summer and early autumn, particularly the latewood width. Contrastingly, in P. pinea, only earlywood width showed vulnerability, mostly to the summer SPEI drought indices. These results are relevant to understand the impacts of increased drought intensity and frequency on tree radial growth and vegetation activity in a region that is vulnerable to climate variability.

Effects of climate on earlywood vessel formation of Quercus robur and Q. pyrenaica at a site in the northwestern Iberian Peninsula

We analysed climate–growth relationships for two ring-porous oak species (Quercus robur L. and Q. pyrenaica Willd.) at one site in northwestern Spain. Increment cores from 12 living trees per species were taken for ring and earlywood vessel measurements, considering three width variables (for earlywood, latewood, and the whole ring) and six anatomical variables (based on vessel area distribution, number of vessels, and conductivity) in the earlywood. We used standard dendrochronological procedures and compared each mean series with meteorological records of temperature, precipitation, and estimates of soil water content. Earlywood width was negatively related to precipitation in the previous late season for Q. robur, but it was controlled by the previous spring temperature for Q. pyrenaica. These responses were highly dependent on vessel number and probably related to carbohydrate storage dynamics but differed between species. Vessel size showed a stronger relationship to climate and was mainly coupled to water excess during the winter for Q. robur but dependent on quiescence temperature for Q. pyrenaica; also, climatic signal in vessel size was maximized by the largest vessels for Q. pyrenaica but only increased slightly for Q. robur. Consequently, vessel parameters showed a much stronger climate signal than ring width and were successful at highlighting the differences between both oak species.