Retrospective Analysis of Wood Anatomical Traits Reveals a Recent Extension in Tree Cambial Activity in Two High-Elevation Conifers

Abstract

The study of xylogenesis or wood formation is a powerful, yet labor intensive monitoring approach to investigate intra-annual tree growth responses to environmental factors. However, it seldom covers more than a few growing seasons, so is in contrast to the much longer lifespan of woody plants and the time scale of many environmental processes. Here we applied a novel retrospective approach to test the long-term (1926-2012) consistency in the timing of onset and ending of cambial activity, and in the maximum cambial cell division rate in two conifer species, European larch and Norway spruce at high-elevation in the Alps. We correlated daily temperature with time series of cell number and lumen area partitioned into intra-annual sectors. For both species, we found a good correspondence (1-10 days offset) between the periods when anatomical traits had significant correlations with temperature in recent decades (1969-2012) and available xylogenesis data (1996-2005), previously collected at the same site. Yet, results for the 1926-1968 period indicate a later onset and earlier ending of the cambial activity by 6-30 days. Conversely, the peak in the correlation between annual cell number and temperature, which should correspond to the peak in secondary growth rate, was quite stable over time, with just a minor advance of 4-5 days in the recent decades. Our analyses on time series of wood anatomical traits proved useful to infer on past long-term changes in xylogenetic phases. Combined with intensive continuous monitoring, our approach will improve the understanding of tree responses to climate variability in both the short- and long-term context.

Keywords: Larix decidua Mill.; Picea abies (L.) Karst.; cambial activity; tree-ring anatomy; treeline; xylem phenology.

FIGURE 1

Tree-ring partitioning and profiles of cell-lumen area within the rings. (A) Graphical representation of tree-ring partitioning using 10 sectors. (B) Dimension of the mean lumen area by sector for each year between 1926 and 2012 for both species. Red lines represent the overall mean values.

FIGURE 2

Principal component analysis (PCA) applied to the chronologies of wood anatomical parameters. Scatter plots of weighting coefficients for PC1 and PC2 calculated on chronologies built with tree-ring width (TRW), cell number (CN) and with mean cell-lumen area (MCA) for each of the 10 tree-ring sectors. Numbers in the scatter plots refer to sectors ranging from earlywood (1, 2, 3,...) to latewood (..., 8, 9, 10) (See Figure 1 for comparison). Axis labels report the percentage of variance expressed by each component.

FIGURE 3

Temperature-growth correlations (1926–2012), for the two species, computed between anatomical parameters (cell number and sectors chronologies) and ring-width chronologies, and mean temperature expressed as a 15-day moving window. Correlation coefficients are represented for each time window shifted at daily steps and are coded according to the color scale on the right. Gray cases are not significant, colored boxes are significant (p ≤ 0.05) with the standard approach (i.e., considering each test as independent). The arrows at the top of each plot correspond to the onset and ending dates detected by intensive monitoring with dendrometers or following xylem phenology (Rossi et al., 2006, 2008).

FIGURE 4

Individual-tree temperature-growth correlations for the two species. Correlations (1926–2012) have been computed at tree level between cell-number time series and mean temperature expressed as a 15-day moving window. The representation, color coding and significance of the correlation coefficients are the same as in Figure 3. The green bars and arrows at the top of each plot correspond to the maximum growth rate detected by intensive monitoring with dendrometers and xylem phenology (Rossi et al., 2006, 2008).

FIGURE 5

Temperature-growth correlations for the two species split in two sub-periods (1926–1968 and 1969–2012). Temperature-growth correlations computed between mean temperature expressed as a 15-day moving window and cell-lumen chronologies of the first (growth onset) and last (growth end) sector, and cell number (maximum growth rate). Correlation coefficients are computed and the color coding assigned as in Figures 3, 4. White circles with vertical flag represent the first (for the 1st sector; onset) or last (10th sector; ending) day of continuous significant correlations, and the peak of correlation values for cell number/maximum growth rate, respectively. Significant (p < 0.05) correlation values are higher than |0.30| .

FIGURE 6

Temperature-growth correlations for the two species in the extreme years. Correlations have been computed between the same anatomical, tree-ring and temperature parameters as in Figure 3 but selecting just the 30 coldest and warmest years (based on mean April-September temperature) within the period 1926–2012. The representation and color coding are the same as Figure 3, however, in this case the threshold for significant (p < 0.05) correlations is set at |0.35| according to the fewer years considered than in the previous analysis.