Generally in arctic region, north of the boreal treeline, the dendrochronological-based research is limited and hindered by the absence of erect tree species (Callaghan et al., 1989; Johnstone & Henry, 1997; Rayback & Henry, 2005). However, knowledge of the relationship between the growth of plants and their populations and climate is important in this area for many reasons. Studies have unveiled that several species of prostrate shrubs possessing distinguishable annual growth increments reflecting the marked seasonality of the Arctic environments are found throughout the Arctic, however, short life-span, extremely small ring widths and numerous growth anomalies characteristic of some of these species restricts their usefulness for long-term retrospective growth analysis and dendroecological analysis (Callaghan et al., 1989; Johnstone & Henry, 1997).
In theory, any species may be amenable to dendroecological analysis, provided that it possess a few essential characteristics: (1) often clear and distinguishable innate markers of annual increments due to marked seasonality in growth caused by prolonged winter dormancy, (2) Annual growth rings or other features that can be cross-dated, (3) the attainment of sufficient age to provide adequate time control for a given investigation, and (4) often slow decomposition rates so that records of past growth may remain for many years (Callaghan et al., 1989; Johnstone & Henry, 1997).
One relatively long-lived arctic tundra species, Cassiope sp., has potential for use in retrospective growth analysis in tundra environments. Cassiope sp. is a long-lived evergreen, dwarf-shrub with a circumpolar distribution. The species is a component of, and often dominant in low shrub-heath, dwarf shrub-heath and mixed heath communities (Bliss & Matveyeva, 1992 cited in Rayback & Henry, 2005). It is also found in alpine ecosystems of Mountainous regions. Cassiope tetragona for example is the widely studies species (Rozema et al., 2009; Rayback & Henry, 2005; Callaghan et al., 1989; Johnstone & Henry, 1997). In Nepal Himalaya also, the ericaceous (Rhododendron family) dwarf species are available. Cassiope fastigiata, the Himalayan heather (Phallu in Vernacular), is a small long lived evergreen dwarf tufted shrub found in Himalaya in the altitudinal range of 2800-4500 m. This caespitose and fastigiate plant generally attains height up to 30 cm. Stems are initially decumbent, ultimately much branched, stiff, and white pubescent. Stem bears many lance-like thick leaves overlapping with each other (densely imbricate) so as to fully cover the stem (stem hugging) giving the stem the green chain like look. Leaf blade is ovate, triangular (4-6 mm long and 1.5-2 mm wide) leathery, abaxially deeply furrowed with hyaline margin. 6-9 mm long outward curving pendulous nutant solitary flowers have 3-7 mm densely crisped-tomentose pedicel, white and broadly campanulate petals and brown sepals (Fig. 1 & 2). Generally, the shrub flowers in June – August.
The plant produces two alternating sets of opposite leaves (generally retaining for 20-40 years) along the stems, forming four distinct rows (Figure 2). Patterns in the positioning of nodes in adjacent leaf rows appear to be analogous to patterns in leaf length (Fig. 2). Small leaves produced in the spring and fall of each year frame larger leaves formed during the summer (Warming, 1908 cited in Rayback & Henry, 2005). The smaller leaves delimit an annual growth increment (AGI). Annual growth increments are measured as the sum of the internode lengths with the terminus of each year’s growth delimited by the shortest internode length at the end of each wave-series (Johnstone, 1995). Since the visible leaf node scars are retained over the full length of a shoot, the pattern can be used to measure and date annual growth increments and to generate longer chronologies. Moreover, chronologies for the annual production of leaves and flowers can also be generated (Johnstone & Henry, 1997). These chronologies have been used to generate hypotheses concerning the growth dynamics of the species, as well as predictive models of the sensitivity of growth parameters to variations in climate (Callaghan et al., 1989) i.e. Cassiope has good potential for use in retrospective growth analyses similar to those applied in dendrochronology.According to Johsntone and Henry (1997), growth measurements of Cassiope consist of internode lengths i.e. the distance between adjacent leaf scars (Fig. 2) performed under a dissecting microscope using a manually operated caliper system. The measuring device consists of a digital caliper and location marker mounted on a solid base. Samples are placed in narrow glass tubes (1 cm diameter) that can be precisely positioned relative to the location marker with distances from a reference point recorded on the digital caliper. Internode lengths are measured from the base to the tip of the shoot. The positions of auxiliary branches, flower buds and flower peduncles are also recorded during these measurements.
The patterns in internode length are helpful to delimit the annual growth along the stem. The shortest internode length within each wave series is used to indicate the end of each year’s growth. In this analysis, stem elongation, leaf production and flower production are measured to represent the annual growth and reproduction for the Cassiope population. Annual stem elongation values are calculated as the sum of internode lengths within the year’s growth. Leaf production can be measured by counting the total number of leaf nodes produced in each year within the two rows of leaves measured on each shoot whereas, flower production is generally estimated by counting the number of peduncle scars within each section of annual growth.
There are some of the interesting parallels with tree-ring-based research and recent developments in the use of dendrochronological techniques on Cassiope. Data series for each shoot are cross-dated by using skeleton plots of internode lengths and flower production. Each chronology is then standardized to remove low frequency variation such as growth trends. The trends in the leaf and flowering chronologies are estimated using a linear fit and then can be standardized as described by Fritts (1976). Following standardization and cross-dating, individual shoot chronologies within each plant are averaged together to form a mean whole plant chronology. The resulting plant chronologies are then averaged to form a master chronology for each of the measured variables.
Various studies demonstrate that retrospective analysis of growth and reproduction of Cassiope tetragona provides new insights about species biology and responses to environmental fluctuations. Correlation and regression analyses link climate with three growth and reproductive parameters showing that this shrub has significant potential for generating proxy climate data for arctic environments, where such records do not exist or must be inferred from data collected at considerable distance from the site of interest. Cassiope -based climate reconstructions may also be used in conjunction with other arctic proxy data (e.g. tree rings, ice cores, lake varves, freshwater diatoms), where available, to reconstruct the paleoclimatological and paleoecological histories of northern sites (Rayback and Henry 2006). Several studies have shown that the principles and techniques developed in dendrochronology can be effectively utilized in patterns of past growth and reproduction of Cassiope spp. Clearly, future work using dendrochronological techniques on Cassiope spp. plants from sites across the circumpolar north holds great promise, and offers new research directions for investigating the past climatic and environmental history of arctic (and alpine) sites beyond treeline.(Rayback & Henry, 2005).
Moreover, quite a few species of Cassiope are found in Nepal Himalaya; however the detail study is still to be carried out.
Furthermore, studies on this shrub especially in relation to climatic variability have not been reported. Cassiope fastigiata is one of the species having a wide distribution in Nepal Himalaya. So, along with the tree-ring chronology, retrospective analysis of growth and reproduction of Cassiope spp. might also offer significance in climate change study in Nepal Himalaya, where the climate change symptoms are paramount.
