Stand structural attributes and canopy lichen diversity

BC’s wet-belt forests were historically dominated by old-growth forests, reflecting the long fire return intervals (average time between which a fire could be expected to occur in any given spot) that characterize windward slopes on BC’s interior mountain ranges (DeLong 1998, DeLong 2007 ). The introduction of commercial forest harvesting in this region, and particularly the widespread adoption of clearcut forest harvesting as the dominant silvicultural system, has now fragmented many of these old forest stands, creating a mosaic of different aged forest stands. This has raised major questions for forest managers:

  • How should conservation biology priorities be determined for remaining old forest stands within BC’s upper Fraser river watershed?
  • What steps should be taken to insure that biological values are maintained within remaining old forest stands?
  • What forestry practices will best maintain biodiversity values associated with old forest stands in regional landscapes?

In a program of research starting in 2004 at UNBC we have examined these questions, working within a 70 km2 area of cedar-hemlock and sub-boreal spruce forests of the upper Fraser River watershed (Map 1). Using canopy lichens as old-growth forest indicators we have identified a series of “biodiversity hotspots”, old-growth forest stands that support exceptional canopy lichen communities (both species diversity and abundance). Ecological characteristics associated with the development of these biodiversity hotspots were ranked using ordination and logistic regression approaches, allowing us to evaluate the importance of factors such as stand age, tree species composition, precipitation, temperature, and groundwater availability.

Right: The many small seepage areas and springs which characterize wet toe-slope positions in the inland rainforest are a key ecological factor in supporting canopy lichen biodiversity in old-growth western redcedar antique forest stands. In spring, these wet seepage areas are dominated by lush growth of skunk cabbages (Lysichitum americanum).

Assessments of lichen diversity and stand structural characteristics found greatly reduced canopy lichen diversity in submesic (drier) sites, such as hemlock forests on hillslopes. In marked contrast, old forest stands in wet toe-slope positions (hygric and sub-hygric sites) supported rich canopy lichen communities . Many of the lichen species found in old forests growing on these wet toe-slope positions were species that were very rare or largely absent elsewhere within the regional landscapes. Many of the individual lichen species show strong microsite preferences. The threatened canopy cyanolichen Nephroma occultum, for instance, was found mainly in wet cedar-dominated old forests, whereas another rare canopy lichen Sticta oroborealis, was found in a range of wet habitats, irregardless of forest composition. Logistic regression analysis of these distribution patterns was combined with GIS-based inventory data of stand ecological attributes (from Predictive Ecosystem Mapping and Vegetation Resource Inventory databases) to predict lichen biodiversity in wet-belt forests of the upper Fraser River valley (see Maps 2 and 3 below).

Overall, our analysis of canopy lichen diversity in regional landscapes of the upper Fraser River watershed suggests that old forest stands located in wet toe-slope positions represent biodiversity hotspots (see Map 2 below). We would hypothesize that site continuity (time between major disturbance vents, such as fire) in wet toe-slope positions (well in excess of a thousand years at many sites) fosters the accumulation of rare lichen species. Lichen growth in these sites is further enhanced by the nutrient rich status of these wet toe-slope positions, which receive groundwater flow (and hence nutrients) from higher elevations, and by the dominance of these sites by old western redcedars (often characterized as antique forest stands ), which provide nutrient enriched microsites for lichen establishment within forest canopies. Lichen populations found within these wet toe-slope positions likely sustain many of the smaller (and often ephemeral) lichen populations within surrounding landscapes, as lichen fragments disperse to surrounding lower quality habitats.

A major project recommendation is that remaining cedar-leading old forest stands in wet toe-slope positions of the upper Fraser river watershed (in the ICHvk2) be given immediate consideration for designation as protected areas or old growth management zones. Though old-growth stands in these wet toe-slope positions constitute only a small part of the landscape overall (less than 5% of the ICHvk2), they represent biodiversity "hotspots" that play a key role in sustaining canopy lichen biodiversity within broader regional landscapes. At the present time few old-growth forests in these wet toe-slope positions fall within designated protected areas or old-growth management zones. Given the disproportionate level of forest harvesting that has already occurred in wet toe-slope positions in the ICHvk2, we risk extirpation of an internationally significant assemblage of canopy lichens in the upper Fraser River watershed if further habitat loss occurs in these areas.

Consideration should also be given to implementing a strategy of placing buffer zones around remaining cedar-leading old forest stands in wet toe-slope positions. Forest harvesting in these buffer zones should be restricted to partial-cutting or variable retention harvesting designs, which have been shown to reduce edge effects on adjacent canopy lichen communities (see Influence of forest edges on canopy lichens ).

Map 1. Location of study plots by forest type within the upper Fraser River watershed.

Map 2. The distribution of 'wet' and 'dry' site series conditions within regional landscapes is an important predictive variable for understanding the distribution of canopy lichen communities. This map portrays the distribution of 'wet' and 'dry' sites in old-growth western red cedar and western hemlock forests (140 years and older) in the wettest variant of the Interior Cedar Hemlock (ICH) biogeoclimatic zone of the upper Fraser River watershed (ICHvk2). The dataset for this map comes from the provincial governments' Vegetation Resource Inventory (VRI) (for age and leading species) and the Predictive Ecosystem Map (PEM), which identifies site series from ground moisture, aspect, and slope. Noteworthy features of this map include the location of: 1) 'Dry Cedar leading' (yellow) forests on south facing and mid-slope positions; 2) 'Wet Cedar leading' (light green) forests on primarily north facing and toe slope positions; 3) the sparseness of 'Wet Hemlock' forests (Dark Green); and 4) the increasing abundance of 'Dry Hemlock' on ridge locations and in drier absolute precipitation conditions to the west (towards Hungary Creek). Preliminary results from this study identifies the usefulness of dividing (or further refining) the 'mesic' site series (from the PEM database) into two categories: 'Mesic wet' and 'Mesic dry'. An assignment of 'Mesic dry' was given to all mesic polygons within PEM where the secondary site series contained 10% or greater submesic conditions. This refinement will: 1. Increase our ability to predict more accurately the presence/absence of arboreal macrolichens across the landscape and; 2. Investigate ecological processes (i.e. fire frequency) and return intervals between these differing mesic site conditions.

Map 3. The probability of occurrence of the rare canopy cyanolichen Nephroma occultum in old-growth western red cedar and western hemlock forests (140 years and older) in the wettest variant of the Interior Cedar Hemlock (ICH) biogeoclimatic zone of the upper Fraser River watershed (ICHvk2). Nephroma occultum is listed as a species of concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) and is widely regarded as an indicator species of old-growth forest stands. Probability classes (very low, low, medium, and high probability of occurrence) were derived from standard deviations of beta predictions generated using logistic regression analysis on presence/absence data for Nephroma occultum against major environmental variables such as moisture, leading tree species, temperature, and precipitation.

Radies, D.N., D.S. Coxson, C.J. Johnson, and K. Konwicki. 2009.

Predicting canopy macrolichen diversity and abundance within old-growth inland temperate rainforests. Forest Ecology and Management 259: 86-97.
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Canopy cyanolichen diversity is low in dry (submesic) old-growth hemlock dominated stands.

The rare cyanolichen, Nephoma occultum, is typically limited to old-growth wet cedar-leading stands, particularly in wet toe-slope positions. The predicted probability of occurrence of this species is depicted in Map 2 below.

Cyanolichen diversity is generally much higher inwet (mesic to subhygric) old-growth cedar-leading stands, shown here on the Ancient Forest trail.

Groundwater flow in wet toe-slope positions, evidenced here by the location of a beaver dam across the face of a previously logged toe-slope position in Sugar Bowl Grizzly Den Provincial Park, is a major ecological factor which sustains old forests and associated canopy lichen communities.

The University of Northern British Columbia