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

Abstract

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.

Keywords: Dendroecology; Drought; Earlywood; Latewood; Pinus teocote; Xylogenesis.

Fig. 1

Map of the study area in north-western Mexico showing the location of the study site and the climatological station (El Salto)

Fig. 2

Images showing the major phases of xylogenesis in Pinus teocote during 2018: (a) radial enlargement of tracheids (late April), (b) wall thickening (early June), and (c) tracheid maturation (early December). The scale bars measure 0.1 mm. The bark is situated to the right of the images

Fig. 3

Number of xylem cells according to their development stage observed in Pinus teocote. In the lowermost graph, the numbers of mature earlywood and latewood tracheids are presented, and empty and filled bars correspond to the rates of production (right y axis) of earlywood and latewood mature cells, respectively. The lowermost x axis shows the day of the year (DOY) from 16 April 2018 to 13 April 2019 and the uppermost x axis shows months. Values are means ± SE

Fig. 4

Chronologies of earlywood (EW) and adjusted latewood (LW_a) width of Pinus teocote considering the best replicated period 1968–2017. Bars represent annual NDVI for the period 1999–2017 (right y axis)

Fig. 5

Climate- and NDVI-growth associations. Pearson correlation coefficients were calculated by relating earlywood (EW) and adjusted latewood (LWadj) indexed series with precipitation (a), mean maximum temperature (b) and mean minimum temperature (c). The NDVI data were also related to these climatic variables (Tmax, mean maximum temperature; Tmin, mean minimum temperature; PP, precipitation) (d). The horizontal dashed lines indicate the 0.05 significance levels. The prior and current year months are abbreviated by lowercase and uppercase letters, respectively. Seasons are abbreviated as follows: winter, dJF; spring, MAM; summer, JJA; and autumn, SON

Fig. 6

Growth-SPEI associations. Pearson correlation coefficients calculated between the SPEI drought index and earlywood (a) and adjusted latewood (b) chronologies. Correlation values are significant (p < 0.05) at r > 0.22

Fig. 7

Common variability between the earlywood (EW) chronology and April soil moisture