DOI: 10.5061/dryad.w9ghx3frk
title: Alpine and Arctic tundra shrub populations show similar ontogenetic growth trends but differing absolute growth rates and lifespan

description: The majority of terrestrial primary production is performed by plants, the
 ontogenetic growth trends of which greatly influence biomass and carbon
 dynamics. Here, we study ontogenetic trends of primary (apical) and
 secondary (stem thickening) growth in Arctic (Svalbard, Norway) and alpine
 (Krkonoše, Czechia) populations of black crowberry (Empetrum nigrum), the
 dominant plant species of certain tundra communities. The environmental
 conditions in alpine areas are more favourable for plant growth than those
 in the High Arctic, where temperatures are lower, there is less
 precipitation and soils are shallower, among other differences. These
 differences were reflected in significant differences in absolute growth
 rates and shrub age between the populations under study. However, we found
 almost no differences in ontogenetic growth trends between the
 populations. In both populations, primary growth and secondary (stem base)
 growth decrease throughout ontogeny whereas secondary (stem top) growth
 and basal area increment increase. No significant differences in the slope
 of the trends were found in either primary or secondary (stem base)
 growth. Trends in the ratio between basal area increment and primary
 growth revealed neither absolute nor relative differences between the
 populations. Ontogenetic trends in the shrubs analysed were surprisingly
 stable despite the prominently different environmental conditions.
 Empetrum plants have adapted to the different environments by altering
 their absolute growth rate only. This adaptation has probably also
 resulted in the different longevity of plants constituting the study
 populations, confirming the theory that slower-growing plants live longer.
 Primary growth and secondary (diameter) growth at the stem base seem to be
 more basic characteristics of plant growth compared to basal area
 increment and secondary (diameter) growth at the apex because the latter
 two seem to be dependent on the absolute growth rate.
description: <strong>Study sites and sampling</strong>
 Arctic and alpine populations of the black crowberry
 (<em>Empetrum nigrum </em>L.), be it the nominal subspecies or
 <em>E. nigrum</em> subsp. <em>hermaphroditum
 </em>(Hagerup) Böcher, is exposed to different climatic, soil, and
 vegetation conditions. We sampled forty individuals from an Arctic
 population at the end of August 2018 in the Colesdalen valley (78°6′ N;
 15°5′ E), Svalbard, Norway. The average annual precipitation in the valley
 is 190 mm, mostly in the form of snow. From 1969 to 1990, the average
 annual temperature was −6.7°C, the coldest month was February, with an
 average temperature of −16.2°C, and the warmest month was July, with an
 average temperature of 5.9°C (Longyearbyen Airport station, 18 km north of
 the site; Norwegian Meteorological Institute, 2010). The onset of the
 growing season spans between days number 165 and 174 (12–23 June) in the
 Svalbard Islands. Samples were taken at elevations between 40 to 80 m
 a.s.l. on a south-facing slope with an inclination of 10–20%. Shallow and
 poor soils were present on the site. There were no other shrubs or herbs
 growing in the vicinity of the individuals sampled. The
 temperate alpine population was represented by 41 individuals collected in
 July and August 2019 in the Krkonoše Mountains, Czechia. Both co-occurring
 subspecies of <em>E. </em><em>nigrum</em> (subsp.
 <em>nigrum</em> and subsp.
 <em>hermaphroditum</em>) were sampled because they are
 impossible to identify in the field. Individuals were sampled at three
 localities: Zadní Planina (50°42′49″ N; 15°40′35″ E), Pančavská louka
 (50°45′56″  N; 15°32′38″ E), and Labská louka (50°46′12″ N; 15° 32′20″ E).
 All samples were taken at elevations between 1,300 and 1,400 m a.s.l. The
 locality at Zadní Planina was on a 20% steep south-eastern slope and the
 other two localities were flat. At the elevation of 1,603 m a.s.l., the
 average annual temperature is 0.2°C, the warmest month being July (8.3°C)
 and the coldest January (−7.0°C). The average annual precipitation reaches
 1,400 mm. The onset of cellular division (i.e. the growing season) in
 co-occurring <em>Pinus mugo </em>is between days 138 and 145
 (18–25 May) in the Krkonoše Mts. Region is botanically relatively well
 described and Krkonoše Mts show high plant endemism from the central
 European perspective. Sampled shrubs occurred in closed vegetation with
 <em>Calluna vulgaris</em>, <em>Vaccinium
 myrtillus</em>, <em>Vaccinium uliginosum</em>,<em>
 Pinus mugo, </em>and several grass species
 (<em>Poaceae</em>). The sampling consisted
 of the extraction of complete individuals (in clonal alpine individuals
 only the relevant part), including lignified roots (to allow the
 laboratory assessment of radial growth and plant age from the root collar)
 and whole aboveground organs. Root samples were cleaned, placed in a
 marked plastic bag, and treated with 40% alcohol to prevent
 mold. <strong>Growth
 measurements</strong> The method of serial
 sectioning was used in order to determine the rate of primary growth. It
 consisted of tree-ring measurements of cross-sections at different heights
 and cross-dating the obtained series at the intra-individual level. The
 first cross-section was always made at the root collar or the bottom part
 of the stem and was followed by one to ten cross-sections evenly spaced
 across the shrub body so that the distances between the cross-sections
 were approximately 10 cm. All branches longer than 10 cm were included to
 account for the species’ greater growth complexity, as it branches
 heavily. The exact distances between all the cross-sections were measured
 for subsequent primary growth analysis. Slices no
 thicker than 20 µm were extracted from all cross-sections using a sledge
 microtome. The slices were stained using a 1:1 Safranin/Astra blue
 solution and embedded in Canada balsam. The entire area of each slice
 (i.e. whole cross-section) was photographed at 100× magnification in order
 to measure annual rings along multiple axes as well as to detect partially
 missing annual rings and other growth defects. Measurements were obtained
 from the images using NIS-Elements software for two radii per
 cross-section. The first measurement was always performed for the longest
 radius whereas the second measurement, aimed to reveal partially missing
 rings, was made at an angle of at least 90° to the first radius.
 Cross-dating was done mostly visually using PAST5 software, first at the
 cross-sectional level by comparing radii measurements and, secondly, at
 the level of the whole individual (including branches) by comparing mean
 cross-sectional series. In total, we were able to use a series from 30
 individuals from Svalbard (92 missing and 232 partially missing annual
 rings detected) and 33 individuals from the Krkonoše Mts (23 missing and
 126 partially missing annual rings detected). The remaining samples were
 not cross-dated successfully and were excluded from further
 analyses. We calculated the primary growth rate between
 cross-sections using the difference in the number of tree rings and stem
 length between two consecutive cross-sections for each considered branch.
 For the corresponding set of tree rings, we also calculated the rate of
 secondary growth at the shrub base (using measurements from the basal
 cross-section) and at the shrub top (using the measurements at the lower
 cross-section of each pair). We also calculated secondary growth expressed
 as basal area increment (measured in mm<sup>2</sup>) and the
 ratio between basal area increment and primary growth (measured in mm).
 Basal area increment, in contrast to simple ring width, includes the
 circumference of the stem on which the ring is produced and is, therefore,
 assumed to be more representative of stem conducting capacity and stem
 biomass produced.
 <strong>Statistics</strong> To
 test for relationships between the growth variables (primary growth,
 secondary growth at the base and top, basal area increment, and growth
 ratio), shrub age, and population age structure, we used multiple linear
 and polynomial regression models. These models were based on average
 growth rates within shrub sections. We also used a subset of the data for
 shrubs younger than ten years to avoid bias caused by large differences in
 shrub age between the two study regions. Similarly, to remove the age
 effect from the relationship, the residuals of growth vs age models were
 used to test for a relationship between primary and secondary growth.
 Finally, we calculated the age distribution of each sample population and
 tested for differences in these distributions between the populations. All
 the analyses were performed in R (R Core Team 2020).
description: # Alpine and Arctic tundra shrub populations show similar ontogenetic
 growth trends but differing absolute growth rates and lifespan
 [https://doi.org/10.5061/dryad.w9ghx3frk](https://doi.org/10.5061/dryad.w9ghx3frk) ## Description of the data and file structure ### Files and variables #### Files: empetrum\_data.txt and empetrum\_data.csv ##### Variables * Age = mean age of the shrub section compared to the stem base * Primgr = Primary (shoot) growth rate for each section (mm) * Secgr_base = the rate of secondary (diameter) growth at the shrub base (using the measurements at the basal cross-section) for the corresponding set of tree rings (mm) * Secgr_top = the rate of secondary (diameter) growth at the shrub top (using the measurements at the basal cross-section) for the corresponding set of tree rings (mm) * Secgr_bai = secondary (diameter) growth expressed as the basal area increment at the shrub base (using the measurements at the basal cross-section) for the corresponding set of tree rings (mm2) * Grration = the growth ratio between bai and the primary growth (mm2/mm) * ShrubID = id of each shrub * ShrubAge = total age of the shrub compared to the stem base * ShrubLength = total length of the shrub compared to the stem base * Population = name of the population, alpine or arctic (Krkonose/Svalbard) * Locality = name of the locality, where samples were taken in Krkonose