Abstract
Stable carbon and oxygen isotope ratios of raw pollen sampled from nine abundant tree species growing in natural habitats of central and northern Europe were investigated to understand the intra- and inter-specific variability of pollen-isotope values. All species yielded specific δ13Cpollen and δ18Opollen values and patterns, which can be ascribed to their physiology and habitat preferences. Broad-leaved trees flowering early in the year before leaf proliferation (Alnus glutinosa and Corylus avellana) exhibited on average 2.6‰ lower δ13Cpollen and 3.1‰ lower δ18Opollen values than broad-leaved and coniferous trees flowering during mid and late spring (Acer pseudoplatanus, Betula pendula, Carpinus betulus, Fagus sylvatica, Picea abies, Pinus sylvestris and Quercus robur). Mean species-specific δ13Cpollen values did not change markedly over time, whereas δ18Opollen values of two consecutive years were often statistically distinct. An intra-annual analysis of B. pendula and P. sylvestris pollen revealed increasing δ18Opollen values during the final weeks of pollen development. However, the δ13Cpollen values remained consistent throughout the pollen-maturation process. Detailed intra-individual analysis yielded circumferential and height-dependent variations within carbon and oxygen pollen-isotopes and the sampling position on a tree accounted for differences of up to 3.5‰ for δ13Cpollen and 2.1‰ for δ18Opollen. A comparison of isotope ranges from different geographic settings revealed gradients between maritime and continental as well as between high and low altitudinal study sites. The results of stepwise regression analysis demonstrated, that carbon and oxygen pollen-isotopes also reflect local non-climate environmental conditions. A detailed understanding of isotope patterns and ranges in modern pollen is necessary to enhance the accuracy of palaeoclimate investigations on δ13C and δ18O of fossil pollen. Furthermore, pollen-isotope values are species-specific and the analysis of species growing during different phenophases may be valuable for palaeoweather reconstructions of different seasons.
Highlights
We address the species-specific natural variability of pollen-isotopes of nine abundant tree species across seven European sites ranging from Belgium to Poland and Finland to Italy, sampled during the years 2015 and 2016
Pinus sylvestris mean δ13Cpollen values range between -27.9‰ and -26.0‰ (1.9‰; Table 3, Fig 7) and the values are normally distributed with one exception at Muritz (2015; p = 0.02)
Mean B. pendula δ18Opollen values range from 23.5‰ to 24.8‰ (1.3‰) and are normally distributed with one exception (Forêt d’Anlier 2015, p = 0.004; Fig 5)
Summary
We sampled modern pollen from 658 individual trees of nine selected tree species growing in seven national and nature parks, which we consider natural habitats (Fig 1; Tables 1 and 2). None of the species sampled in this study is endangered or protected and sampling followed generally a non-invasive scheme of few inflorescences per individual tree. Pollen were collected during two consecutive vegetation periods (February to June of 2015 and 2016) within the species-specific flowering periods (Fig 2; Table 2). 20 flowering individuals were sampled per site and per species (Table 2). Bulk -samples of an individual tree were composed of several inflorescences of different branches from various heights. To investigate intra-tree isotope patterns, an additional 152 intra-tree sub-samples were taken at different heights and cardinal directions from 22 trees of eight species (Fig 3). Forest, montane to altimontane type, partly with fir and mm spruce. (Dystric Cambisol)
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