Some differences between the vascular and non-vascular floras of ancient and secondary forests result from changes inflicted on long-established communities that have considerable amounts of dead wood. In the virgin Norway spruce forests of Sweden five liverworts growing on logs (Calypogeia suecica, Lophozia incisa, Odontoschisma denudatum, Riccardia palmata and Scapania umbrosa), together with Herzogiella seligeri and Splachnum rubrum, did not survive initial logging and transition to a managed forest. Other secondary forests develop from the planting of formerly cultivated land (whose soils often have a high nutrient content), grasslands or heaths. Ruderals (weeds) are initially abundant on former farm soils, while characteristic grassland and heathland species may persist for a very long period, competing with the young trees until suppressed by shade. In North America secondary forest very often adjoins old forest, as it does in the Harvard Forest, Massachusetts. Conditions for the rapid migration of old woodland indicator species into such secondary woodlands seem straightforward, yet it does not seem to be occurring. A number of studies have followed the migration rates of plants into new woodlands that have been planted against old woodlands. The rates have been measured at 0-1.25 m per year in deciduous woodland in south Sweden (Brunet and von Oheimb, 1998), 0-2.5 m y_1 in eastern North American hardwood forests (Matlack, 1994) and as slow as 0.18-0.38 m y_1 from an oak-hornbeam woodland into a pine plantation in southern Poland (Dzwonko, 2001). But there can be surprises; wood anemone Anemone nemorosa can migrate at more than 20 m per year when seeds are carried by deer and wild boar. The usual slow migration seems to be linked to slow rates of seed dispersal and the harsh woodland conditions of low light and high humidity (fostering fungal diseases). Matlack (1994) observed that ingested seeds moved quickest, followed by adhesive seeds clinging to fur and feathers, and then wind-dispersed seeds, while the slowest were those moved by ants.
6.4.3 Small-scale diversity and variation: cryptogamic soil crusts and mats
Trees include the largest organisms known, but some of the greatest sources of diversity in forests are the small organisms that live on their outer surfaces as epiphytes or are present in or on the soil. Cryptogamic (or microbiotic) soil crusts are delicate symbioses of cyanobacteria, lichens and mosses that exist on the surface of arid and semi-arid soils. They increase the amounts of organic matter and available phosphorus in the soil, assist soil stability and rates of water infiltration, form favourable sites for the establishment of vascular plants including trees and assist in nitrogen fixation. They are also extremely fragile and on a world basis their area has been greatly reduced by trampling, particularly by livestock.
In contrast to cryptogamic soil crusts, the cryptogamic mat of lower plants developed over bare rock in many northern forests is a different and sturdier community; its most important components are mosses, liverworts and lichens, with larger vascular plants invading once a largely organic substrate has accumulated. Lichens are symbiotic associations between fungi and the photosynthetic algae, which are sometimes unicellular, that inhabit them. There are at least 14000 lichens on the world list; many develop on bare rock or soil, while others are epiphytic on trees and shrubs. Their beauty and diversity is illustrated by the photographic plates in Moberg and Holmasen (1990), which also show their distributions in Scandinavia.
A very brief description of a cryptogamic mat has been given earlier; indeed Fig. 2.8 demonstrates how its existence assists the establishment of tree seedlings. On the other hand, erosion of the cryptogamic mat on the path along Goat ride, Fiby urskog, Sweden, illustrates its vulnerability to damage caused by visitor pressure, trampling by moose, roe deer and hare, animal faeces, sliding movements of mat units, tree fall, and naturally initiated fires. Investigations of the cryptogamic mat developed over the granite in Fiby urskog (Fig. 6.7), have demonstrated both its complexity and the patterns of cyclic succession involved. Change, in the form of cyclic succession (Section 9.2.2), results in a mosaic structure within the vegetation concerned and, because conditions differ in various parts of the mosaic, it promotes biodiversity. This is true at many different scales. In boreal forests patch size can vary from the gaps caused by fires or the fall of one or a group of trees due to gales or some other cause, to the much smaller scale involved here. In this instance the scale involved is so small that the species present were recorded in
Figure 6.7 Heights of lichen and bryophyte species commonly occurring in the cryptogamic mat, together with depths of substrate resting on the underlying granite in Fiby urskog, Sweden. The main horizontal line represents the top of the substrate of soil and organic matter. The heights and depths of the columns represent mean values. Vertical lines show maxima and minima; standard errors are indicated by x, apart from the four species where less than the standard 10 observations were made. Cladonia floerkeana rested directly on the granite. (Unpublished data of Moberg, Hytteborn and Packham.)
six hundred 0.01m2 quadrats which together formed an area 3 x 1m. The resulting map of the cryptogamic mat on a small hillock away from the main paths in the Lichen Hill area, Fiby urskog (Area 1 in Fig. 2.7) was divided into nine zones. Cladonia stellaris, C. rangiferina and C. arbuscula were prominent amongst the lichens here; there was also quite a high proportion of bare rock, mosses, litter and heather Calluna vulgaris.
There is a scattered field layer of heather and cowberry Vaccinium vitis-idaea rooted in the cryptogamic mat at the top of Lichen Hill; other field-layer species here include wavy hair-grass Deschampsia flexuosa, bilberry Vaccinium myrtillus, narrow buckler-fern Dryopteris carthusiana and juniper Juniperus communis.
Apart from relatively small amounts of mineral nutrients delivered in rain and weathered from the acid granite, and nitrogen fixed from the air, nutrients available to plants on the granite ridges are of organic origin, particularly from plant litter. The small amount of mineral soil is confined to depressions in the rocks and rock joints. Commencement of the invasion of the bare rock surface is of particular interest with the root-like rhizinae of the lichens and the rhizoids of the bryophytes being fastened directly to the granite. The commonest bryophytes growing directly on the bedrock are Andraea rupestris, Dicranum scoparium and Racomitrium microcarpon. True mat-forming vascular species grow on a rather thin layer of humus and decomposing plant litter, which is only truly anchored where roots penetrate rock joints. Parts of the mat resting on sloping rocks occasionally move, leaving totally bare patches from which even algae and crustose lichens are absent. These gliding movements tend to occur after heavy rain in areas of the mat that have exceeded a certain critical thickness.
The heights reached by the various components of the mat vary considerably, as Fig. 6.7 shows. Substrate depths were determined by probing with a metal pin. The three species of Cladonia formerly placed in the genus Cladina, including C. rangiferina the so-called reindeer 'moss' and C. arbuscula, are the tallest plants present, while the much-branched mildly calcifuge (calcium-hating = acid-loving) liverwort Ptilidium ciliare and the more markedly calcifuge moss Pohlia nutans are the shortest. The latter is, in Britain, common on heaths and moors, and to some extent in woodlands, being an almost constant member of the limited moss flora of cut peat surfaces in the north-west. Of the three Dicranum species shown, D. polysetum is the tallest, slightly over-topping another conspicuous acrocarpous (upright) moss Polytrichum juniperinum. The red-stemmed and strongly calcifuge moss Pleurozium schreberi, is in contrast pleurocarpous so its shoots are horizontal and much-branched. Of the remaining lichens, the eight species of Cladonia vary greatly in height. Cetraria islandica is present throughout Scandinavia.
Broadly speaking the bryophytes dominate the slopes with lichens towards the top and the most exposed places. The dominating lichen species are, in order of height, Cladonia stellaris, C. arbuscula and C. uncialis. These and the mat-forming bryophytes are not generally fastened to the bedrock, the mat being stabilized primarily by its own weight and its fit against the uneven rock surface.
When trees and large shrubs establish in the mat they reach mature size only if able to root in the rock joints, obtaining additional supplies of water and nutrients, as well as effective anchorage. Tree falls tend to pull up areas of cryptogamic mat leaving bare areas of granite, thus initiating another sequence of cyclic change, in which crustose lichens such as Rhizocarpon geographicum are amongst the first to re-invade.
6.4.4 Biodiversity in relation to current ecoclimatic stability: the use of remote sensing
One of the major aims of large-scale studies of biodiversity is to try both to locate and understand the positions of biodiversity 'hotspots'. Fjeldsa et al. (1997) used long-term GAC (Global Area Coverage) remote sensing data provided by satellite images in an attempt to determine whether such hotspots were related to how stable the ecoclimate (equivalent to microclimate: the immediate climate of individual organisms) has been over recent years. This subject is too complex fully to explain here, but some of the major points are of interest. The first is that this method gives an entirely new viewpoint concerning environmental variation. Light areas on maps based on daily scenes photographed from meteorological satellites for the period 1981-1991 indicate high inter-annual differences in surface conditions; the darker ones are more stable. 'Hotspots' of bird diversity and aggregates of endemic plants (see Section 6.3.2) are superimposed on the map.
Regions with the most stable climate (characterized by low ecoclimatic variability between years) are predominantly occupied by old species of forest birds. Those whose ecoclimatic variability is complex are dominated by birds of recent origin. Interestingly, however, the 'hotspots' themselves, which have the peak concentrations of endemic species, either have a local reduction in climatic variability, or are situated on the boundary of a stable region. Endemic plant species in these lowland tropical forests are usually associated with special soil conditions (such as being rich in heavy-metals or calcium). But even with these there seems to be a link with ecoclimate since endemics in the tropics are strongly associated with mist zones, and so presumably the more constant the mist zones are from year to year, the greater the number of endemics. Though this very broadbrush approach to biodiversity and the problems of endemic species has its limitations, it is clearly useful as a preliminary to much more detailed investigations of the impact of climatic variations on biodiversity and endemic species in tropical forests.
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