Scientific and Research Projects
Who will win the competition? Beech and sessile oak under the influence of changing climate conditions
Description of the project:
The question of climate sensitivity of beech and oak provides disparate answers from different forest science disciplines. Research with a focus on site conditions and/or vegetation analysis, emphasizes the wide range of beech and conclude on a high adaptability of this tree species in a changing climate while studies based on physiological processes frequently show that beech has a higher drought sensitivity than oak.
The presented work highlights the issue of dependency on weather conditions and drought stress tolerance of the two species in the context of retrospective growth analysis. To this end, along a water gradient in Baden-Württemberg eight mixed stands with beech and oak were selected in two different age groups (± 60 years old and ± 120 years old) within two different regions (western part of the Black Forest representing an abundant water-supply due to high average precipitation totals and Stromberg where temporary water deficits are common due to low average precipitation totals). Only stands on free draining soils were selected. In every stand five predominant tree pairs were felled, five beeches (Fagus sylvatica L.) and five sessile oaks (Quercus petraea (Matt.) Liebl.). The height and diameter growth of these trees was investigated using stem analysis. The study focused on the years 1976 and 2003 regarding the relationship between height/diameter growth and drought stress. With reference to the relationships between radial growth and weather, the period 1971 to 2010 was analyzed. Data were taken from a soil water balance model (LWF-BROOK-90) for the latter period. This model was parameterized with soil data collected from the field and subsequently analyzed in the laboratory coupled with reconstructed high spatial resolution daily weather data.
The height increment after the drought years of 1976 and 2003 were analyzed in the four 60 year old stands. The annual height increment of the beech trees differs on the sites with higher water supply, in some cases significantly from the height growth of sessile oak. Beech shows an above-average growth in the drought year and a pronounced stress response in the following one to two years, no clear reaction due to the drought year can be determined from the annual height growth of sessile oak. At the driest site the height growth curves of the two tree species show large similarities, especially after the drought year of 1976. Based on the comparison of individually modeled and observed tree growth curves only at this location a distinct drought reaction, which persists over several years, can be shown. Nevertheless, the differences between the two species are low.
The correlations and regressions with monthly and daily weather and soil water data show that the diameter growth of beech depends more on precipitation and soil moisture during the period of annual ring formation than that of sessile oak. Along the water gradient no directional trends were observed. For the oaks, only the southern slopes in the Black Forest (i.e. not the driest investigated sites) show a significant and stable correlation between radial growth and precipitation/soil moisture during the summer of the same year. The radial growth of beech is also affected significantly negatively by high temperatures in the summer of the same year (young stands) or of the previous year (old stands). For sessile oak, temperature dependence is marginal.
The analysis of drought stress response based on drought stress indicators exhibits further interesting differences between the two species. Drought stress indicators facilitate the comparison of a tree species’ radial growth reaction in different drought years. They provide information about the strength of the growth depression (resistance), the recovery and the resilience (post-drought level divided by pre-drought level). On the one hand the radial growth of beech, in the drought years 1976 and 2003 has a significantly lower resistance than that of sessile oak, while on the other, the young beech trees also recover significantly faster than sessile oak. This results for the examined 60 year old stands after both drought years (1976 and 2003) in a higher resilience of beech when compared with sessile oak, although the difference between the two species is not significant. Resistance and recovery are quite different in the old stands. In these stands sessile oak shows a higher resilience than beech (in 1976 significant, in 2003 not significant). While the old beech in 1976 shows a similar recovery as oak, recovery after 2003 is significantly different. Sessile oak shows an increase in radial growth compared to the drought year. This increase does not occur for the old beech trees. These differences between the two species become even more obvious when the drought stress indicators are analyzed on the basis of the radial growth at higher stem heights. The absence of recovery of the old beech trees after 2003 cannot be interpreted as a decreasing vitality within the old stands, but must to be viewed in the context of low precipitation in these years.
The recovery potential of beech and sessile oak in the Stromberg region decreases along with a decreasing water gradient (i.e. increasing aridity) to a similar extent. The drought stress intensity, defined as the relative decline of the matrix potential in relation to the long-term average at each location, affects the recovery and resilience of the two tree species in the Stromberg region. This result could be interpreted as a more pronounced effect of relative drought stress intensity than of the general aridity of the site on the drought stress reaction of the two tree species. For both gradual changes (recovery and resilience) along with the drought stress intensity (of the sites), no significant differences between beech and sessile oak were found. For the plots in the Black Forest drought stress responses showed no significant correlations, neither with the relative water supply (drought stress intensity) nor with the general water supply of the sites (aridity).
The comparison of the radial growth curves of both tree species in the common overlap period 1962-2010 shows a strikingly synchronous course. Contrasting growth responses of the two species, for example increasing radial growth of beech and decreasing radial growth of sessile oak, are clearly observed only in the years 1965/66 (both regions) and 1979/80 (Stromberg only). In these two years there is no weather anomaly when comparing with the long-term average monthly values. However, the two preceding years (1965 and 1979, Stromberg only) are characterized by uncharacteristic dry periods in late summer/autumn. In addition, the analysis of the weather conditions during the particularly superior growth years for beech (1987 and 1988), show that this species - more than sessile oak - is able to turn expressed rainfalls during the main growing season (especially in June) directly into radial growth.
The results are discussed with respect to the hypotheses:
The height growth of young beech trees has not suffered significantly more in and after the drought years compared to the young sessile oak trees. On the contrary, in both drought years beech shows even stronger height growth than sessile oak. After the drought years only in exceptional cases, significant differences between beech and sessile oak exist. At the study sites after the drought years the height growth curves of two tree species do not deviate significantly from their "normal" curves. Therefore, it was not possible to show with the results of this study that young sessile oaks are overgrown by neighboring beeches earlier because of exceptional, extreme drought events. This may be different when drought years occur more frequently and on sites that are not penetrated in the same way by the rooting system of both tree species. Signs for a possibly greater effect of drought on the height growth of beech can be found at all four investigated sites after the drought in 2003 (Figure 35b).
At all studied sites, there are distinctly higher positive correlations between radial growth and rainfall or soil moisture in the vegetation period of the same year for beech than for sessile oak. High temperatures in the summer of the same or the previous year however have a stronger negative effect on the radial growth of beech, than of sessile oak. A delayed growth reaction of the old stands compared with the young stands is observed only regarding temperature in beech stands. High precipitation and abundant soil moisture had a similar and immediate effect on both beech age groups.
In the young stands sessile oak showed in both drought years a significant higher resistance than beech. In these stands beech also showed a significantly faster recovery reaction. Therefore, in the young stands no significant difference in resilience between two tree species exists. In the old stands sessile oak has compared with beech: a higher resistance (1976 significant), a faster recovery (2003 significant) and thus a higher resilience (1976 significant).
No differences between beech and sessile oak in their drought stress response could be observed along the water gradient. For both tree species the drought stress intensity is more critical than the general water supply (aridity) of the site.
These results must be interpreted considering the following background information:
The investigated sites in Baden-Württemberg do not reach the limits of natural tree species distribution for beech and sessile oak. In a changing climate many sites covered with beech and oak in Baden-Württemberg are forecasted to approach these limits. Under such conditions the drought stress reaction of the two species may possibly appear different.
The selected trees were located on free drained soils which are penetrated by both species in more or less the same manner. Temporarily water logged sites with a higher portion of clay in the subsoil are common in Baden-Württemberg. On these sites the differences in the drought stress reaction between beech and sessile oak due to their different rooting system and rooting power will be more pronounced.
The presented distinctions between beech and sessile oak are based on retrospective growth analysis and should be considered in relation to future climate change and spatial significance as "minimal scenario" in Baden-Württemberg.
Additional information: http://freidok.uni-freiburg.de/data/10058
contact person: Arno Mattes
Phone: +49 761 203-3737
Start of project: 2010
End of project: 2014
Dr. Arno Mattes, Prof. Dr. Dr. h.c. Heinrich Spiecker
- Forst BW, Sonstiges
- Institut für Waldwachstum, Sonstiges