Paper birch
Betula papyrifera Marsh

Introduction

Paper birch is a deciduous broad-leaved tree reaching up to 40 m in height. It is shade-intolerant and grows either in even-aged, pure stands or in mixed stands with shade-tolerant conifers. It is mostly a pioneer species, but it is also present in secondary stages of succession. Its wood is used for several purposes, and although it was formerly considered a weed, its commercial potential has now been recognized (Peterson 1997). The species is under active management and some breeding efforts are being made (Carter et al. 1988). It is suitable for short-rotation intensive forestry because of its productivity, easy regeneration, and the fact that it has few serious damaging agents, with the exception of the bronze birch borer (Klinka et al. 2000). Fires, though uncommon, may cause severe damage to the thin, flammable bark. The first peoples stripped its bark and made these waterproof sheets into baskets and canoes (Turner 1979).

Paper birch has an incomplete trans-continental North-American distribution. In British Columbia the species reaches the western and southern limits of its natural range. Paper birch grows in most of British Columbia, except parts of the coast and the islands. It is more common in the Cordilleran than in the Pacific region (Little 1971).

Distribution and Protected Areas – from Hamann et.al. 2005

In situ Conservation Status Summary – from Chourmouzis et.al. 2009

No in situ conservation concerns were identified for this species.

Inter situ collections

A range-wide provenance test was planted in New Brunswick in 1977 (Carter 1982).

Reproduction

Paper birch regularly produces heavy crops of wind-dispersed seed, and regenerates well on mineral soil, especially after fire. The interval between good seed crops is two years (Banerjee et al. 2001). It also reproduces vegetatively from stump sprouts. Paper birch occurs in many BEC zones, but most trees grow in the BWBS and SBS zones, where the species is abundant. Substantial numbers of trees also grow in the CWH, IDF and ICH zones. The populations of the SBS and IDF zones are under-represented in current protected areas. Those of the PP and BG zones appear not to be protected at all.

Genetic structure

Six varieties of paper birch are known, which hybridize extensively with each other and with other birch species (Klinka et al. 2000). Chromosome number varies considerably within the species and even within varieties. Paper birch may have 70, 84 or, more rarely, 56 chromosomes (Brittain and Grant 1966; Grant and Thompson 1975). Li et al. (1996) found that the increased drought tolerance of polyploids was a consequence of both structural and physiological characteristics: polyploids had fewer stomates, a thicker epidermis and more pubescence. They had a lower osmotic potential at saturation. Photosynthesis of diploids was more sensitive to water deficit. Bevington (1986) examined the effects of light and temperature on germination. Sources at the northern limits of the range germinated faster than those at the southern limits. Light permits germination at lower temperatures. Chilling increased dark germination and sensitivity to light. Wang et al. (1998) evaluated the physiological responses of four populations from British Columbia to different water and nutrient regimes. Observed traits included photosynthesis, stomatal conductance, water use efficiency and nitrogen use efficiency. The responses were complex and sometimes surprising. Populations had different strategies for dealing with environmental limitations. A few practical implications were apparent, though. Populations from areas with growing season drought should not be transferred to areas without drought, since it would result in a loss of productivity. The reverse transfer may be possible if drought is not extreme and if there are no other limiting factors (such as winter temperature). Benowicz et al. (2000) found geographical trends for physiological characteristics of 18 birch populations from climatically diverse locations in British Columbia. Photosynthetic capacity increased with increasing climate coldness and was inversely correlated with fall frost hardiness (which is related to leaf longevity). Benowicz et al. (2001) evaluated the same 18 populations for germination parameters, fall and winter frost hardiness, biomass accumulation and gas exchange parameters. They found large population differences in all traits except gas exchange parameters, which often have large error variances. Climatic variables could more easily be related to population differences than geographic parameters. The southern interior zone showed large differentiation among populations and more intensive sampling for gene conservation purposes may be needed there. Resistance to the bronze birch borer is partially genetic (Miller et al. 1991), although environmental factors, such as drought stress, are known to play a role as well. Borer resistance varies among birch species and hybrids. A better understanding of the defense mechanism may provide scope for breeding for resistance in the future. Water birch and paper birch both possess some degree of resistance that could be used in breeding and selection programs (Santamour 1999).

Resource management and seed transfer

Eighteen seed sources from five regions in British Columbia were evaluated on six sites in the field with the aim to select fast-growing sources and develop seed-transfer guidelines (Carlson et al. 2000). Regions differ little in height growth, but seed sources within regions vary more. Northern sources, when transferred south, grow almost as well as southern sources. However, for frost damage the majority of genetic variation is found among regions, suggesting adaptive differentiation, with small differences between families within regions. Simpson and others (2000) examined the effects of seed origin on bud flushing, growth cessation and cold acclimation. Latitude and elevation explained most of bud flushing variation as well as freezing tolerance. Northerly sources needed a larger heat sum to flush than more southerly sources. Freezing tolerance was highest in central seed sources. In 1998, the first progeny test of paper birch in British Columbia was planted. It consists of 195 open-pollinated families representing phenotypically selected trees from 19 stands across the Kamloops and Nelson forest regions (Carlson et al. 2000).