The holly in the area of the Bavarian Pre-Alps: Occurrence and genetics

From the described state of knowledge, the following questions arise:

1. Current distribution in the Bavarian foothills of the Alps

• Are there still occurrences at the historical sites?
• Are they indigenous populations or plantations?
• What habitats or forest communities are involved?
• What dimensions does holly reach?
• Does it come to flowering and fructification?
• Is there natural regeneration?
• Is the regeneration affected by game browsing?
• Is there a discernible tendency to spread?

2. Genetic independence of the Bavarian populations

• How independent is the pre-alpine population compared to the nearest natural occurrences in Germany?
• Are there detectable genetic differences between wild and planted specimens in the foothills of the Alps?
• If so, where is the geographical origin of the planted individuals?

3. Recommendations for practice

• Hotspots of holly in forests of the Pre-Alps
• Possible protection and maintenance measures
• Is a progeny of native holly recommended for plantings in forests and gardens?
• What are the possible uses in silviculture? Are genetic differences between wild and planted specimens detectable in the foothills of the Alps?
• If so, where is the geographical origin of the planted individuals?

To address the research questions listed in 1. above, 267 holly occurrences were studied in five subregions and intersected with environmental variables from the BaSIS site information system. The extent to which holly has moved to higher elevations over the last century was investigated by comparing current distribution (inventories and observations by citizen scientists) with historical finds by Sendtner (1854) from the Little Ice Age. Different sampling designs and sequencing techniques were chosen to answer the questions on genetic distinctness mentioned in 2. In the Alpine region, 50 trees from gardens and semi-natural areas were sampled. This was combined with the analysis of herbarium specimens from the entire range of the species from Morocco to Denmark. From these samples, Sanger sequencing was used to sequence variable regions in the plastid and nuclear genomes, as well as three highly variable microsatellite regions from the nuclear genome. Illumina sequencing was then used to read the complete genomes of 21 more samples.

How common is Ilex in the subregions of the Bavarian Alps?

Holly is widespread from Lake Constance to the Berchtesgadener Land and beyond in the Austrian peripheral Alps to the Wienerwald. A more or less closed area of scattered, locally individual-rich occurrences in the Alps (limestone Alps, flysch zone, molasse foothills) is opposed by isolated outposts in the Alpine foothills. Natural occurrences north of the young moraine area are not known in Bavaria, from parks and gardens, however, it also occurs in the extra-Alpine Bavaria to feral.

Which locations are preferentially colonized?

In the Bavarian Pre-Alps, Ilex is a companion tree species of the montane mixed mountain forest (600-1100 m a.s.l.) with outposts in the submontane beech forest (min. 400 m a.s.l.) with preference for moderately inclined to steep, sun-exposed slopes. The amplitude of mean temperatures of Ilex sites in the Bavarian Prealps ranges from 4-9 °C (growing season 11-17 °C) with preference for low- to mid-montane sites with 6-8 °C (growing season 14-16 °C). While January means are below the critical 0° isotherm throughout the area, lethal barefrost events <-22 °C have been recorded sporadically at Hohenpeißenberg since records began, and not at all at stations in the Alps in 2012 since 1961. At the growing sites, between 900 and 2500 mm of precipitation fall annually (preference for 1400-1700 mm), more than half of it in the growing season, more than one third in the three summer months. Preference is given to silty rendzines with high base saturation, widespread on forested slopes of the foothills of the Alps, besides acid brown soils on flysch and molasse sandstones, while loamy-clayey backwater soils and bogs are avoided.

What stand densities and dimensions are reached?

Holly was found predominantly in larger or smaller collectives (groups of understory shrubs and small trees under the loose canopy of beech, spruce, fir, and sycamore), but only in a quarter of the cases as single individuals at forest edges or in open areas. The largest hollies had modest dimensions, with heights of 10 m, breast height diameters of 23 cm, and crown diameters of 7 m. One explanation for the capping of tree size is the secular frost events of 1929 and 1956 with area-wide dieback of larger hollies, after which regeneration likely occurred from ground-level cane and root outcrops.

Has the distribution expanded to higher elevations in the last century?

Because of their tight relationship to sea level (0.48 °C per 100 m), mean temperatures can be readily regionalized from terrain models. Over the past 170 years, annual mean temperatures at the Hohenpeißenberg mountain weather station have increased by about 1.8 °C, suggesting an increase in elevation limits of up to 375 m in the Bavarian Alps on suitable soils. Comparison of the two precisely relocatable historical sites with nearby present-day elevation maxima yields increases of 219 m (corresponding to about 1.1 °C) and 474 m (about 2.3 °C), respectively. The absolute rise of the observed altitude maximum in the Bavarian Alps (currently 1300 m in the Mangfall Mountains, 44 km southeast of the historical find at Blomberg, 907 m) amounts to 393 m (ca. 1.9 °C). As an endozoochoric spreading tree species with broad site amplitude, holly demonstrates a remarkable dispersal ability. Climate change has increased the extent of thermally suitable growing sites for holly in the Bavarian Alps by 2.4 times since 1854, from 90,000 ha to nearly 220,000 ha. With a further warming of 1 °C, the site niche increases to 310,000 ha, at 2 °C to 370,000 ha. Further spread of holly to ever higher elevations can therefore be expected. It also seems reasonable to use holly up to altitudes of about 1300 m.

Are the hollies of the Bavarian Alps genetically differentiated from other European occurrences? Are hollies of alien origin used in gardens? Is the use of native seed and planting material recommended?

The genetic data show only minimal differences in the usually variable regions in the plastid and nuclear genome. Even the microsatellites have relatively little variability and do not show a clear geographic distribution that would allow genetic characterization of alpine holly. The whole genome analysis gave a better resolution due to the large amount of data. However, it also does not show a reliable separation from the French genotype. This suggests immigration from the west in the recent past, but it must be taken into account that only one French holly was sampled. It would make sense to sample France extensively, from the western Alps to the Atlantic and Mediterranean, to get a more accurate picture of the genetic structure of holly populations here. There was only isolated evidence of planting of alien holly in gardens of the study area in the Alpine region. The majority of the planted hollies were obviously taken from the forests in the immediate vicinity. Accordingly, there are no clear signs of potentially problematic genetic admixture. Despite the relatively small genetic differences of hollies throughout Central Europe, planting Dutch or Danish nursery stock in the Alpine region should nevertheless be avoided. To what extent a genetic adaptation process to low temperatures and frost events has already taken place in alpine hollies remains open. Although the individuals, some of which were found at very high altitudes, suggest this, the very short time span since recolonization after the last ice age rather argues against it.

The entire final report is published in the Forstliche Forschungsberichte München (No. 223 / 2022) and can be obtained from the Förderverein Zentrum Wald Forst Holz Weihenstephan e.V..