Environmental variables during growing seasons 2007 and 2008
Climate in 2007 was characterized by the occurrence of exceptionally mild temperatures in spring (, ). An almost continuous drought period was recorded from 20 March to 6 May 2007, when total monthly precipitation in April reached < 2 mm (LTM 39 mm) and mean monthly air temperature was 13.9 °C, i.e. c. 6 °C above LTM (). In contrast to 2007 climate at the beginning of the growing season in 2008 was cool and wet, whereby air temperature in April corresponded to LTM and precipitation exceeded LTM by almost 50 % (, ). While mean daily air temperatures recorded in summer 2008 were similar to temperature records of 2007, total summer precipitation was c. 25 % lower than in 2007 ().
Fig. 1 Climate variables and soil water content recorded during the growing seasons 2007 and 2008 within the study area. (a) Mean daily air (grey line) and soil temperature (b) Daily precipitation sum (bars) and soil water content. Study sites are denoted by (more ...)
Table 2 Monthly and summer (June-August) mean daily air temperature, precipitation sum and mean soil water content during 2007 and 2008 growing seasons recorded within the study area. Long-term mean values (LTM, 1911 - 2006) of air temperature and precipitation (more ...)
In April 2007, soil temperatures reached 15.6 and 10.9 °C at the xeric and dry-mesic site, respectively, which exceeded soil temperature records in April 2008 by c. 5 °C (). Soil temperatures in mid July 2007 and end of June 2008 reached maximum values of c. 25 and c. 17 °C at the xeric and dry-mesic site, respectively. During the drought period in spring 2007 soil water content dropped to 10.8 Vol. % at the dry-mesic and 6.3 Vol. % at the xeric site (, ). Starting with rainfall events in May 2007, soil moisture at the dry-mesic site reached 30 Vol. % in June. During a drought in mid-July, which lasted c. 10-days, soil moisture sharply decreased but was replenished and stayed at high values until early October, when missing precipitation caused increasing dryness of the soil. In contrast to 2007, frequent rainfall events in spring 2008 caused high soil moisture at the start of the growing season in April and May. Throughout both growing seasons studied, records of mean soil water content were predominantly c. 10 – 15 % lower at the xeric compared to dry-mesic site (, ).
Dynamics of tree ring growth and relationship with xylogenesis
Typical diurnal cycles of stem shrinking and swelling during a drought period in 2008 are depicted in . At the dry mesic site stem radius reached maximum and minimum values in the morning (between 6 and 9 a.m.) and afternoon (between 4 and 6 p.m.), respectively, and were closely related to relative air humidity. At the xeric site, maximum and minimum values were reached c. 150 and 30 min earlier, respectively, than at the dry-mesic site. Mean amplitude of diurnal fluctuations in stem radius during the drought period varied between c. 150 μm at the xeric and c. 75 μm at the dry-mesic site (), whereby mean standard deviation in diurnal amplitudes between trees at the same site was 59 and 28 μm at the xeric and dry-mesic site, respectively. As depicted in , daily radius change showed highest direct correlations calculated over the whole measurement period with relative air humidity (r = 0.548, p < 0.001, and r = 0.593, p < 0.001, at the dry-mesic and xeric site, respectively) and precipitation (Kendall τ = 0.417, p < 0.001, and τ = 0.448, p < 0.001, at the dry-mesic and xeric site, respectively). Significant indirect relationships were observed between daily radius change and air temperature at both study plots (r = −0.134, p < 0.01, and r = −0.224, p < 0.01, at the dry-mesic and xeric site, respectively). Low but statistically significant coefficients were also found with soil water content (r = 0.095, p < 0.05, and r = 0.201, p < 0.01, at the dry-mesic and xeric site, respectively), when considering a time lag of one day (). Without accounting for a time lag, correlation coefficients amounted to −0.007 (p = 0.884) at the dry-mesic and 0.131 (p = 0.071) at the xeric site (data not shown).
Fig. 2 Diurnal cycles of stem radius variation during a dry period in 2008 compared to relative air humidity (solid line) and precipitation (bars). Study sites are denoted by open and filled circles for the xeric and dry-mesic site, respectively. Dotted lines (more ...)
Fig. 3 Correlations between daily radius change and climate parameters (relative air humidity, precipitation, air temperature) and soil water content at the dry-mesic (a, filled circles) and xeric site (b, open circles) from April through October 2007 and 2008 (more ...)
Relationships between stem radial increments extracted from DMR and environmental variables (climate variables and soil water content) at the study plots are depicted in . During the growing period 2007 and 2008 stem radial increments at the dry-mesic site were directly related to relative air humidity (r = 0.306, p < 0.05) and precipitation (Kendall τ = 0.184, p < 0.05). At the xeric site, the corresponding correlation coefficient for air humidity was 0.290 (p < 0.05) and Kendall’s τ coefficient for precipitation was 0.234 (p < 0.01). No statistically significant relationships were found between radial stem increments and air temperature and soil water content at either study plot (). Correlation coefficients decreased or showed only minor changes when time lags in growth response from one to three days were considered (data not shown).
Fig. 4 Correlations between radial stem increment and climate parameters (relative air humidity, precipitation, air temperature) and soil water content at the dry-mesic (a, filled circles) and xeric site (b, open circles). Period of radial stem increase used (more ...)
In intra-annual radial growth in 2007 and 2008 is compared between continuous DMR (daily means) and XCD that allowed measurement of increase in ring width over 1-2 week periods. As a baseline for the comparison, dendrometer traces were set to zero at the day of the year, when first row of enlarging cells were detected (for details see Materials and Methods). Exceptionally mild spring temperatures in spring 2007 caused early bud break by the end of March (85 d), whereas in 2008 bud break in the upper crown occurred two weeks later (99 d). Mean onset of radial enlargement at the xeric site in 2007 and 2008 occurred on 12 and 30 April (102 and 120 d of the year, respectively). At the dry-mesic site start of radial cell enlargement was delayed in 2007 and 2008 by c. 1 week.
Fig. 5 Comparison between mean radial increment determined by dendrometers (n = 3 trees/site) and measurement of the developing xylem (including enlarging, wall thickening and mature xylem cells; n = 5 trees/site) in 2007 (a, c) and 2008 (d, f). Mean standard (more ...)
Relative air humidity was closely related not only to daily radius change (cf. ), but also to prolonged radial stem shrinkage during drought periods (). At the xeric site, changes in stem radius detected by DMR were more pronounced than at the dry-mesic site and amounted to c. 50 % of total annual increment. Records of intra-annual radial growth determined by DMR and XCD were quite similar except for the dry-mesic site in 2007 (), where the first row of enlarging tracheids was detected in mid-April when drought caused pronounced stem shrinkage.
In 2007 maximum daily radial growth determined by DMR and XCD peaked around 18th May (138 d of the yr) at both study plots. Compared to 2007, maximum growth in 2008 was delayed at the dry-mesic site by about 10 d (). Enlarging cells assumed to indicate radial stem growth were detected at the xeric site until 195 (14th July) and 205 d of the year (24th August) in 2007 and 2008, respectively, and until 215 (2nd August) and 218 d of the year (5th August), respectively, at the dry-mesic site ().
Table 3 Parameters of the Gompertz function for intra-annual radial growth in 2007 and 2008 (see ) based on dendrometer records (DMR) and analysis of xylem cell development (XCD) at study plots and R2 of the model (A = upper asymptote, Ip = inflection point, (more ...)