Lichen Communities in Wetland Alder Swales
Introduction. Most studies on lichens and old-growth forests in B.C. have focused on coniferous forest stands. In mountain environments this has led to the examination of areas such as wet-toe slope positions, where topography and groundwater flow reduce the incidence of stand destroying fires. In contrast, forested landscapes in B.C.’s interior plateau are dominated by younger, often even-aged, coniferous forests; reflecting their past history of disturbance by forest harvesting, fires, and insect damage.
When we look closely at plateau landscapes in B.C.ís interior, however, we can find areas where the frequency of disturbance appears to be much lower. In particular alder swales along small first-order streams and in wet seepages areas (Figure 1) may represent quite stable plant communities. Fires often skip over these wet depressions and they are rarely targeted for forest harvesting (though they are sometimes disturbed by mechanical or herbicide treatments of adjacent coniferous stands). In a program of study initiated at UNBC in 2007 we have examined the possibility that alder swales may function as refugia for canopy lichen species (Figure 2).
Objectives. Our study had three main objectives:
- What proportion of interior plateau landscapes supports alder-swale features. Although the area covered by alder swales may be small, the linear nature of these features, which often run as elongate irregular forest galleries along small drainage channels (see Map 2), suggests that they may have a disproportionate influence within regional landscapes. This part of our work utilized queries of existing vegetation inventory maps to determine the spatial extent of alder swales.
- Second, we were trying to determine if there were features within alder swales that were important to the development of lichen communities. This part of our research addressed questions such as: What is the influence of tree age on lichen community development? How different are lichen communities in alder swales from those in surrounding (upland) coniferous forests? How does alder swale lichen diversity vary across the landscape? What are important predictor variables for that diversity? The influence of changes in bark texture that occur as alders age may be particularly important. When young alders possess relatively smooth bark (Figure 3). As they age, however, the bark develops lenticels, leading to the formation of textured bark on older stems (Figure 4).
- Third, we were attempting to document the contribution of lichens to nutrient cycling in alder swales. Canopy cyanolichens, which contain blue-green algal partners, are capable of fixing atmospheric nitrogen, in much the same way that legumes fix nitrogen in agricultural systems.
Methods. Field sampling identified representative alder swales in moist and cool, wet and cool, and very wet and cool climate variants of sub-boreal spruce landscapes in the Prince George Forest District (Map 1). Twenty-five alder swales were sampled in each climate zone for diversity and abundance of arboreal macrolichens. Lichen sampling was conducted along 100 meter transects placed through the long-axis of each site (parallel to small streams within each site).
Results and Discussion. Spatial analysis indicated that the average patch size for areas mapped as alder-lady fern dominated vegetation (which was used as a proxy for alder swale presence) was 5.7 ha. The mean width of the patches was 20.5 m and the mean length, following the contours of the patch, was 854 m. The total linear extent of the 3 158 patches present in the vegetation inventory maps was 1,360 km. This represents, on average, more than 1 km of alder swales per km2 of area in the 1 215 km2 area queried.
By sampling alder stems of different diameters we were able to assess the impact of diameter and bark texture on lichen diversity. Overall, larger stems were found to support greater diversity than smaller stems. Mean annual precipitation and mean annual temperature were the most important variables for predicting cyanolichen diversity. Greater cyanolichen diversity and abundance in wetter sites, especially in lower elevation (warmer) sites. Both abundance and diversity of foliose green-algal was higher in sites that contained many large alder stems (greater than 10 cm dbh). Alectorioid lichen diversity was relatively similar in all alder swale sites.
Cluster analysis (using PC-ORD software) indicated three main species groups in alder swales (Figure 5). The first cluster (outlined in blue in Figure 5) identified the most common lichens, these being the majority of the alectorioid lichens and several green-algal foliose lichens. The second cluster (outlined in red in Figure 5) included four cyanolichens and several green-algal foliose lichens which were present in many sites across the landscape. The third cluster (outlined in green in Figure 5) included the rarest lichens. Overall diversity present in the sampled sites was 43 species, with 6 additional genera not identified to species.
The temperature response of nitrogen fixation (measured as acetylene reduction) was calculated for six common cyanolichens. These species were Nephroma parile, Lobaria pulmonaria, Lobaria scrobiculata, Lobaria hallii, Pseudocyphellaria anomala, and Sticta fuliginosa. The highest rates were observed in S. fuliginosa and the lowest rates were observed in L. pulmonaria. Low-temperature acclimation of nitrogen fixation was observed in S. fuliginosaand L. scrobiculata, while L. scrobiculata and L. pulmonaria had higher temperature optima for nitrogen fixation.
In summary, though alder swales are limited in their aerial extent, they have disproportionate importance for lichen conservation biology due to their rich canopy lichen communities. We would hypothesize that alder swales along small streams play an important role as dispersal corridors for old-growth associated lichens. This conservation function may be particularly important in landscapes where much of the surrounding old-growth coniferous forest has been logged or attacked by mountain pine-beetles.
For details, see Doering and Coxson 2010.
Map 1. Location of the 25 study sites within each subzone of the sub-boreal spruce (SBS) biogeoclimatic zone of the Prince George Forest District. mk=moist and cool subzone, wk=wet and cool subzone, vk=very wet and cool subzone.
Map 2. Location of riparian (purple), lowland (green), and upland (yellow) decidous wetland swales in the Aleza Lake Research Forest, east of Prince George.