Variation in Leaf Morphological Traits of European Beech and Norway Spruce Over Two Decades in Switzerland

Abstract

Leaf morphological traits (LMTs) of forest trees have been observed to vary across space and species. However, long-term records of LMTs are scarce, due to a lack of measurements and systematic leaf archives. This leaves a large gap in our understanding of the temporal dynamics and drivers of LMT variations, which may help us understand tree acclimation strategies. In our study, we used long-term LMT measurements from foliar material collections of European beech (Fagus sylvatica) and Norway spruce (Picea abies), performed every second year from 1995 to 2019 on the same trees within the Swiss Long-term Forest Ecosystem Research Program LWF. The 11 study plots (6 beech, 4 spruce, and 1 mixed) are distributed along gradients of elevation (485–1,650 m a.s.l.), mean annual precipitation (935–2142 mm), and mean annual temperature (3.2–9.8°C). The investigated LMTs were (i) leaf or needle mass, (ii) leaf area or needle length, and (iii) leaf mass per area or needle mass per length. We combined this unique data set with plot variables and long-term data on potential temporal drivers of LMT variations, including meteorological and tree trait data. We used univariate linear regressions and linear mixed-effects models to identify the main spatial and temporal drivers of LMT variations, respectively. For beech LMTs, our temporal analysis revealed effects of mast year and crown defoliation, and legacy effects of vapor pressure deficit and temperature in summer and autumn of the preceding year, but no clear long-term trend was observed. In contrast, spruce LMTs were mainly driven by current-year spring conditions, and only needle mass per length showed a decreasing long-term trend over the study period. In temporal models, we observed that LMTs of both species were influenced by elevation and foliar nutrient concentrations, and this finding was partly confirmed by our spatial analyses. Our results demonstrate the importance of temporal analysis for determining less recognized drivers and legacy effects that influence LMTs, which are difficult to determine across space and species. The observed differences in the temporal drivers of beech and spruce LMTs suggest differences in the adaptation and acclimation potential of the two species.

Adjustment of hydraulic, anatomical and leaf traits in Fagus sylvatica (L.) towards drier conditions. A study in mature stands.

“…Hence, the on average nearly 40% greater mean LS in the 2018 data set as compared to the 2019 data was probably, at least partly, caused by mast fruiting, which reduces carbohydrate allocation to leaves. As LS was found to increase in beech with autumn temperature of the previous year (Zhu et al 2021), the greater LS in the 2018 data set could partly also be the result of the somewhat higher mean annual temperatures recorded at the sites sampled in 2018. However, repeated sampling of the same trees in 2018 and 2019 at the Unterlüss site also revealed significantly smaller leaves in 2019 than in 2018, and cross-regional surveys after the severe 2018 drought showed for many Central European beech forests particularly small leaves in 2019 as an after-effect Zhu et al, 2021).…”

“…As LS was found to increase in beech with autumn temperature of the previous year (Zhu et al 2021), the greater LS in the 2018 data set could partly also be the result of the somewhat higher mean annual temperatures recorded at the sites sampled in 2018. However, repeated sampling of the same trees in 2018 and 2019 at the Unterlüss site also revealed significantly smaller leaves in 2019 than in 2018, and cross-regional surveys after the severe 2018 drought showed for many Central European beech forests particularly small leaves in 2019 as an after-effect Zhu et al, 2021). From their analysis of a 25-year record of leaf morphological trait data from Swiss beech forests, Zhu et al (2021) conclude that higher vapour pressure deficit (VPD) results in reduced specific leaf area (SLA) in the subsequent year, as does a higher fruiting intensity.…”

“…Mean SLA of upper sun-crown leaves was less variable across the three sampling campaigns than LS, which might relate to the fact that cell expansion, the main determinant of leaf size, is more drought-susceptible than cell division (Hsiao et al, 1976). Generally, SLA has been found to decrease with higher radiation and increasing nutrient deficiency, but also with decreasing water availability in soil and atmosphere (Wright et al, 2002;Poorter et al, 2009;Zhu et al, 2021). In fact, the SLA of European beech trees has been found to be highly dependent on the light intensity in the crown (Curt et al, 2005;Čater & Diaci, 2017;Chelli et al, 2021).…”

“…However, repeated sampling of the same trees in 2018 and 2019 at the Unterlüss site also revealed significantly smaller leaves in 2019 than in 2018, and cross-regional surveys after the severe 2018 drought showed for many Central European beech forests particularly small leaves in 2019 as an after-effect Zhu et al, 2021). From their analysis of a 25-year record of leaf morphological trait data from Swiss beech forests, Zhu et al (2021) conclude that higher vapour pressure deficit (VPD) results in reduced specific leaf area (SLA) in the subsequent year, as does a higher fruiting intensity. Therefore, a combined effect of masting and atmospheric drought on leaf morphology in 2019 is likely.…”

“…These water deficits were not represented by our AWC data that only consider water storage capacity but not actual water reserves. Moreover, including the atmospheric saturation deficit as an explanatory variable has the potential to increase the explained variance, as Zhu et al (2021) could demonstrate a VPD effect on SLA in beech.…”