Indicators for life forms and microhabitat complexity

According to Schaefer (2003), life forms are defined as organisms which show similar responses to specific environmental conditions (for instance in their morphological structures, developmental stages or behavior) and “having similar effects on the dominant ecosystem processes” (Díaz and Cabido 1997), such as hydrophytes, helophytes or xerophytes which are plants being adapted to different water contents of their habitat. Further kinds of life forms exhibit a specific adaption to nutrition, for instance parasites, hemiparasites, saprophytes and carnivorous plants.

Previous studies about Baltic amber inclusions show that the ‘Baltic amber forest’ harboured various types of life forms, raising the complexity of this palaeoecosystem. The recent discovery of two Baltic amber leaf inclusions proved the presence of carnivorous plants within the Baltic amber flora (Sadowski et al.

2015). Both leaves, belonging to the family Roridulaceae, show the same specific morphology as their extant relatives Roridula dentata (Fig. 5D, I) and R. gorgonias (Fig. 5E-H). Both extant species excrete a terpenoid trapping glue through their glandular tentacles which trap all kinds of arthropods very effectively (Simoneit et al.

2008). But the trapping glue lacks specific enzymes and thus, the plant itself cannot digest the trapped prey. To solve this problem, extant Roridulaceae show a peculiar ecology: they live in a digestive mutualism with endemic hemipterans, which are able to walk on the tentacled leaves without getting trapped (Fig. 5D) (Anderson and Midgley 2003). These hemipterans feed on the entangled prey and defecate on the leaves of Roridula (Ellis and Midgley 1996). Their leaf surfaces possess nano-sized gaps to take up the hemipteran faeces compounds and the nutrients therein, ensuring the survival in a nutrient-poor habitat (Ellis and Midgley 1996, Anderson and Midgley 2002, Anderson 2005). Following the definition of plant carnivory, Roridulaceae fulfil all criteria: attraction and retention of the prey, prey digestion and nutrient uptake (Givnish et al. 1984, Adamec 1997, Anderson and Midgley 2003, Adamec 2013).

The question arises whether roridulid plants from Baltic amber were carnivorous as well or even had this digestive mutualism. Sadowski et al. (2015) argued that several morphological features of the inclusions allow the conclusion of a carnivorous nature. First of all, the morphology of the tentacles show signs for excretion, such as the singular pore at the glandular head of the tentacles (Fig. 5C).

Fagaceous trichomes which are attached to the tentacles further indicate that the leaf surface was very suitable for entangling or even catching things (Sadowski et al.

2015). Moreover, the trap organization of extant Roridula is also present in the leaf inclusions: both show a hierarchical organization of the trap with different size classes of tentacles (long ones for the first contact and entanglement of prey; medium ones for the slowdown of prey, and short ones for final immobilization; Fig. 5B, F;

(Voigt et al. 2009, Sadowski et al. 2015). Sadowski et al. (2015) concluded that the signs for excretion, entangled plant material, as well as the functional units of prey capture are good indicators for a carnivorous nature of the roridulid plants from Baltic amber. However, there was no evidence for a digestive mutualism so far (Sadowski et al. 2015).

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Another life form reported from Baltic amber is indicated by inclusions of aerial parasites or mistletoes which are defined as hemiparasitic plants depending on a host plant, but also serving as important resource for various organisms (Calder 1983, Aukema 2003). Sadowski et al. (2017b [6]) described six species of dwarf mistletoes (Arceuthobium spp., Viscaceae) from Baltic amber. Their extant relatives (Fig. 8) exclusively parasitize conifers of the Pinaceae and Cupressaceae, causing extensive damage to the host tree, such as a decreased growth rate and reduced survival (Mathiasen 1996, Geils and Hawksworth 2002). Having numerous representatives of Arceuthobium in the ‘Baltic amber forest’ raises the question whether these ancient species were already parasitic and if so, how they influenced their palaeoenvironment on a micro- or even macrohabitat scale.

The morphology of the inclusions assigned to Arceuthobium is, except for a few features, in congruence with their extant relatives (Fig. 8D-G). Sadowski et al.

(2017b [6]) particularly highlight the presence of squamate bracts, the characteristics of the fruits and the overall reduced morphology of the fossils which is interpreted as an adaptation to a parasitic life form. Also, phylogenetic analyses of the Santalales (sandalwood order; including Viscaceae) showed that parasitism developed within the Santalales (Nickrent 2011). Thus, except for three basal clades, all sandalwood families are parasitic (Nickrent 2011), supporting the assumption that the Baltic amber Arceuthobium species were parasitic. As shown by Sadowski et al. (2017a [5]), the ‘Baltic amber forest’ encompassed numerous conifer species of Pinaceae and Cupressaceae which could have served as potential dwarf mistletoe hosts.

Interestingly, one of the Arceuthobium inclusions, A. groehnii, had clumps of pinaceous pollen attached to its base, indicating proximity to a tree of the Pinaceae (Sadowski et al. 2017b [6]).

Extant dwarf mistletoes are of great ecological significance and thus, are termed “ecological keystones”, since they have a disproportionately large influence on their environment compared to their relative abundance (Power et al. 1996). One reason for being termed an ecological keystone is that dwarf mistletoes increase the structural complexity of a forest by inducing malformations in their host trees. Dwarf mistletoe infected branches first show specific swellings (Fig. 8D) and then, excessively ramify into numerous distorted branches, forming dense clumps (=

witches brooms) in the tree canopy (Fig. 8A-C) (Geils and Hawksworth 2002). These witches brooms change the canopy shape, decrease the crown density or even result into canopy gaps in case of host tree mortality (Mathiasen 1996). Besides structural impact, extant dwarf mistletoes increase the ecological complexity as well. Witches brooms serve as microhabitats that offer shelter and forage areas for many kind of arthropods, increasing the arthropod diversity of the forest (Hawksworth and Geils 1996, Halaj et al. 2000). Also, the avian and mammal diversity is positively influenced by dwarf mistletoes and their witches brooms: the densely branched witches brooms are a suitable nesting side for birds (Fig. 8H) and small mammals, while the dwarf mistletoe itself represents a food resource, especially during the

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Fig. 8: Extant dwarf mistletoe species (Arceuthobium spp., Viscaceae) from the United States (A-C, G, H: Crater Lake National Park, southern Oregon; D, E, Siskiyou Mountains, Oregon-Californian border). (A, B) Stands of Pinus albicaulis and P. monticola with witches’ brooms in the forest canopy (arrowheads). (C) Witches broom of Pinus contorta subsp. latifolia. (D) Male inflorescences of Arceuthobium monticola on P. lambertiana; note the swelling of the branch (white arrowhead). (E) Fruiting inflorescences of A. campylopodum, infecting P.

ponderosa. (F) Fruiting inflorescence of A. monticola on P. monticola. (G) Fruiting inflorescence of A.

americanum on P. contorta subsp. latifolia. (H) Bird nest in a witches broom of P. contorta subsp. latifolia. All photos E. M. Sadowski.

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winter season when resources are generally scarce (Parks et al. 1999, Watson 2001, Hedwall and Mathiasen 2006, Watson and Herring 2012). Applying knowledge of extant Arceuthobium to ancient dwarf mistletoes of the ‘Baltic amber forest’, it is likely that they influenced the palaeoecosystem in a similar way as in extant forests (Sadowski et al. 2017b [6]). Arceuthobium from Baltic amber probably increased the structural complexity of the amber source forests by changing the canopy shape or affecting the tree survival, thus influencing the forest heterogeneity and habitat patchiness. According to Sadowski et al. (2017b [6]) potential interactions of the ancient dwarf mistletoes with the Baltic amber fauna are difficult to reconstruct, but however, should not be entirely ruled out, considering syninclusions of insects (e.g.

Diptera, Aphids) closely located to the dwarf mistletoe inclusions. Last but not least, Sadowski et al. (2017b [6]) point out that extant Arceuthobium are known to induce high resin release in their hosts as a reaction to the infection or due to heavy witches brooms that may break off (Geils and Hawksworth 2002). Therefore, Baltic amber dwarf mistletoes should also be taken into account when discussing reasons for resin release in connection to the formation of the Baltic amber deposit (Sadowski et al.

2017b [6]).

Numerous Baltic amber inclusions of different liverwort and bryophyte species indicate the presence of epiphytic life forms within the ‘Baltic amber forest’.

In their comprehensive study, Grolle and Meister (2004) identified about 22 liverwort species, and more species were discovered and revised in the following years (e.g. Heinrichs et al. 2015a, Heinrichs et al. 2016). According to Heinrichs et al. (2015b), these liverworts were likely epiphytic, growing in close proximity or even on trunks of resin-releasing trees within the ‘Baltic amber forest’. Also, Feldberg et al. (2014) points out that “the humidity maintained in forests is the most probable factor controlling the assembly of epiphytic liverwort diversity”, meaning that the highly diverse liverwort community is a good humidity indicator in the Baltic amber source area, at least at a microhabitat scale.

The same holds true for moss inclusions from Baltic amber, of which approximately 60 species have been described so far (Frahm 2010). Frahm (2010) mentions that extant analogues of Baltic amber moss species are epiphytes, occurring on trunks in oak-pine forests of mainly eastern and southern Asia. Besides epiphytic mosses, also terrestrial ones are known from Baltic amber. Heinrichs et al. (2014) reported a moss community, enclosed in a single piece of amber. Extant analogous species of these mosses are terrestrial, inhabiting shaded microhabitats, such as rocks and degraded wood. A syninclusion of a chilopod, a typical component of soil faunas, further supported the assumption of close vicinity to the forest floor and a likely terrestrial habitat of the mosses (Heinrichs et al. 2014). Besides mosses, also ferns of the Mationaceae were components of the terrestrial microhabitats within the

‘Baltic amber forest’, likely inhabiting rocks or the forest floor (Schmidt and Dörfelt 2007).

Further microhabitat constituents were lichens of which a high number of inclusions have just been reported recently (Kaasalainen et al. 2017). Lichens represent “stable mutualistic associations in which photoautotrophic algae and/or

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cyanobacteria provide carbohydrates for heterotrophic fungi” (Kaasalainen et al.

2015). Those symbionts were even detected in several Baltic amber inclusions which exhibited sufficient preservation to differentiate between the lichen tissues, including the photobiont layer (Hartl et al. 2015, Kaasalainen et al. 2017). Lichen inclusions also serve as indicator taxa to estimate microhabitat conditions of the ‘Baltic amber forest’. For instance, Rikkinen and Poinar (2002) described a lichen inclusion as Anzia electra, with strong affinities to extant Anzia species of East Asia and eastern North America. According to the extant ecological preferences of Anzia, the fossil indicates a humid microclimate and a well-lit microhabitat, such as branches in an open canopy or sun-exposed trunks and rocks (Rikkinen and Poinar 2002). Similar microhabitat conditions (humid environment and illuminated areas) are as well estimated based on the presence of calicioid lichen inclusions which show affinities to extant Calicium and Chaenotheca (Rikkinen 2003). The most comprehensive work about lichens from Baltic amber was recently published by Kaasalainen et al.

(2017) who discovered numerous, morphologically diverse lichen inclusions.

Besides crustose and squamulose lichens, the authors highlighted the high amount of foliose and fructicose lichens, indicating that the majority of Baltic amber lichens were epiphytic. The morphological adaptations of the Baltic amber lichens gave insight into their ancient microenvironment which was “a humid and moderately well-illuminated temperate forest” (Kaasalaien et al. 2017).

Another important component of microhabitat communities of the ‘Baltic amber forest’ are fungi, such as Metacapnodium succinum (Ascomycota), an epiphytic sooty mould. It is a mat-forming fungus which abundantly occurs on Baltic amber plant inclusions, for instance on Cupressaceae twigs, oak leaves, but also on a foliose lichen (Schmidt et al. 2014). Another epiphytic fungus is Casparytorula which is another abundant constituent of Baltic amber microhabitats. According to Kettunen et al. (2015 [2]), inclusions of Casparytorula show that this fungus grew close or even on freshly excreted resin and thus, was likely epiphytic on the amber bearing tree (Kettunen et al. 2015 [2]). This is supported by syninclusions of flowers, spider webs and epiphytic lichens which indicate proximity to more elevated forests layers. Casparytorula was also reported to grow on a coniferous leaf with affinities to Pinaceae, ‘Taxodiaceae’ and Taxaceae (Kettunen et al. 2015 [2]). However, a more recent study of conifer leaves from Baltic amber (including this particular specimen), showed that this leaf is actually from an angiosperm with yet unclear affinities (Dicotylophyllum var. sp.; Sadowski et al. 2017a [5]).

Besides epiphytic fungi, parasitic fungi were also reported from Baltic amber, such as Gonatobotryum. Dörfelt and Schmidt (2007) found an inclusion of a coniferous seedling (possibly related to Picea) which was infected by Gonatobotryum. The well preserved nuclleus remnant and cotyledons of the seedling indicated that it was still alive when the fungus infected it; thus, the authors supposed that the fungi attack caused the seedling’s death (Dörfelt and Schmidt 2007). A further fungus with affinities to either Gonatobotryum or to the related Gonatobotrys was found on a dwarf mistletoe inclusion (Arceuthobium viscoides; Sadowski et al.

2017b [6]). The dwarf mistletoe is partly entangled in a spider web and shows signs

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of degradation, such as a shrunken surface (Sadowski et al. 2017b [6]). However, the dwarf mistletoe inclusion did not exhibit a clear morphological response to a parasitic attack, such as epidermal cells that block the fungal growth. Therefore, it is likely that in this case the fungus was an opportunistic saprotroph, starting to attack the dwarf mistletoe when it already had broken off the main plant and fallen into a spider web (pers. comm. Elina Kettunen, Helsinki). A very specific fungus in terms of nutrient supply is Chaenothecopsis, a further fungal taxon from Baltic amber.

Many extant Chaenothecopsis species are resinicolous which means that they are able to grow on and even digest fresh or semisolidified coniferous resin (Tuovila et al. 2013). The same holds true for the ancient Chaenothecopsis from Baltic amber which shows similar morphological adaptations to the resinous habitat as its extant representative (Tuovila et al. 2013).

In summary, microhabitat communities of the ‘Baltic amber forest’ are very diverse in their taxonomical composition but also in the presence of different life forms, including saprophytes, parasites, symbionts, carnivorous plants and highly specialized resinicolous fungi. They also give insight into the environmental conditions of microhabitats, indicating that they were well-lit to shaded and humid.

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