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Department of Forestry, University College Dublin, Belfield, Dublin 4, Ireland. Summary. Birch in Ireland has ..... Scots pine, oak. –. –. –. Stand age (years). 34. 21.

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The growth potential of downy birch (Betula pubescens (Ehrh.)) in Ireland M. NIEUWENHUIS AND F. BARRETT Department of Forestry, University College Dublin, Belfield, Dublin 4, Ireland

Summary Birch in Ireland has long been regarded a timber species of minor importance, and it is noted more often for its invasiveness in young coniferous forests and clearfelled areas than for its potential as a commercial forestry species. While research on other native hardwood species such as oak or ash has been ongoing for a number of years, very little is known about the Irish birch resource. The lack of quantitative data relating to height growth, diameter growth and volume increment of birch in this country is of particular concern. The objective of this study was to examine the growth potential of birch in Ireland. Following a field survey, eight well-stocked, unthinned birch stands were selected for inclusion in the study. All of the selected stands were determined to be downy birch. Following analysis of sample tree disc sections, two stands were excluded (because of indistinct annual rings) and the study was restricted to the six remaining stands. A total of 100 sample trees were felled at the six sites. Tree ring data were collected from a total of 1333 sample tree disc sections. Using these ring data, the historic patterns of radial growth at breast height, height growth and volume growth of the six stands were reconstructed, examined and analysed in detail. The results showed that for well-stocked, unthinned, even-aged stands the period of maximum radial growth, and therefore diameter growth, occurred between the ages of 5 and 20 years. The fastest growing tree achieved a diameter of 25 cm in 32 years. It is suggested that for the stands included in this study, the lack of management, in particular the lack of adequate thinning, will have resulted in excessive crown competition and consequently reduced diameter growth. Maximum height increment occurred before the age of 20 years and fast growing trees achieved a height growth of >1 m per year during this period. The results showed that a well-stocked, unthinned downy birch stand can achieve a standing volume (under-bark) of 200 m3 ha–1 in 42 years. While some of the stands included in the study had not reached the age of maximum mean annual increment, comparison with the Forestry Commission yield models showed that stands of downy birch in Ireland can achieve a yield class of 8 and, given the correct thinning regime, total recovered volume production could possibly be raised to that equivalent with yield class 10.

Introduction Birch is one of the most common native woodland trees in Ireland. However, its potential for © Institute of Chartered Foresters, 2002

commercial timber production has been ignored because existing birch woodlands are often poorly stocked and generally composed of trees of poor form. Birch has a poor reputation Forestry, Vol. 75, No. 1, 2002

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amongst foresters, with the result that many existing birch stands have suffered neglect and mismanagement. However, interest in native tree species such as silver birch (Betula pendula (Roth)) and downy birch (Betula pubescens (Ehrh.)) is now growing. There has been increasing pressure to diversify the range of species in the national forest estate and in particular to increase the proportion of native hardwood tree species. A growing emphasis is also being placed on the need to preserve and extend existing semi-natural woodland of which birch is a major constituent. Aside from the problems of poor stem form, birch has many attributes considered advantageous in a forest tree species, e.g. rapid growth, ease of regeneration, ability to grow on a range of site types, self pruning and relatively short rotation lengths (Evans, 1984). The successes in breeding and research in Fennoscandia, where birch is the most important commercial hardwood species (Frivold and Mielikainen, 1991), have also highlighted the possibilities and potential for improving, managing and utilizing the birch resource in this country. To date, research on birch in Ireland has been relatively limited. Little is known about the abundance and distribution of the two species, the actual quality of the birch resource and the growth performance of existing stands. The lack of quantitative data relating to height growth, diameter growth and volume increment of birch in this country is of particular concern. To this end a study was initiated in 1998 with the aim of examining the growth potential of birch in Ireland. The historic patterns of radial growth at breast height, height growth and volume growth of selected birch stands were examined in detail using stem analysis.

Literature review Distribution of birch in Ireland Today stands of birch and small birch woods are common in many parts of Ireland on wet and dried-out bogs and clearfelled sites. In most cases they represent seral stages to oak or ash woodland and, except where they are on sites of former woodland, are characterized by a rather impoverished flora (Cross, 1987). Rackham (1980) noted that in the last 100 years in England, birch has become one of the most common trees both of ancient woodland and of secondary woods on the site of heaths, fens and in plantations. A similar pattern of birch colonization and development is also likely to have occurred in Ireland. Keogh (1987) indicated that there were some 5135 ha of birch in Ireland (Table 1). This represented 9 per cent of the total broadleaf resource. The extent of birch in Ireland has increased significantly in recent years through the widespread colonization of cut-away bog land in the Irish Midlands. Growth and development of birch Birch is a pioneer species and once established grows very rapidly for the first 20 years. Atkinson (1992) reported that its hardiness and rapid regeneration from seed may help the birches to establish before recolonization by other trees. On good sites trees can attain a height of 10 m in 10 years (Evans, 1984). Growth begins to slow, however, at pole stage. The life span of birch in Europe is often quoted as being around 60 years but it can be appreciably longer (Brown, 1984). In the central highlands of Scotland, healthy trees of silver birch up to 180 years old can be found (Mitchell, 1988). Ovington and Madgwick

Table 1: The birch resource in Ireland State (ha) Species Birch Other broadleaves Total broadleaves After Keogh (1987).

Private (ha)

Non-protected

Protected

Total (ha)

4 452 27 895 32 347

683 11 826 12 509

– 2 700 2 700

5 135 42 421 47 556

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(1959) reported that silver birch is the longerlived of the two species. Yield of birch In Britain, it is generally agreed that a yield class of 6 is above average for birch (Brown, 1991). Savill (1991) maintains that a maximum yield of 7 m3 ha–1 per year can be obtained on the best sites. Current annual volume increment culminates at around 15–30 years, while maximum mean annual increment is generally achieved at an age of 40–50 years. The highest growth rates in Europe appear to occur in Poland (Philip, 1978). Zaleski and Kantorowicz (1998) reported that on rich sites in Poland silver and downy birch can yield 7–11 m3 ha–1 average annual volume increment over 25 years. Recent Russian yield tables give maximum growth rates of only 4 m3 ha–1 per year (Philip, 1978). In England, potential production on good sites after 40 years (estimated to be close to the age of maximum mean annual increment at yield class (YC) 8 is ~170–200 m3 ha–1 (Cameron, 1996). In Finland, stands originating from genetically improved planting stock can reach the final target size after only 40 years and produce over 400 m3 ha–1 (equivalent to a YC of at least 10) (ViheraAarnio, 1994). Niemisto (1996) studied silver birch plantations growing in Finland and found that stands achieved a total yield of stem wood of 230 m3 ha–1 on average (nearly 400 m3 ha–1 maximum) at 30 years. Oikarinen (1983) noted that volume production in naturally regenerated stands was greater than in plantations. The growth of silver birch versus downy birch It is generally recognized in Europe that the growth and yield of silver birch is greater than that of downy birch. Seaman (1994) noted that downy birch is known to grow more slowly than silver birch, although the differences are difficult to quantify owing to interactions with site. Koski (1991) reported that downy birch is ecologically more flexible than silver birch, but is less productive. Worrell (1999) reported that silver birch in Scotland grows ~20–100 per cent faster than downy birch, depending on the site. In both Sweden and Finland, studies have shown that silver birch gives a 15–20 per cent greater yield by volume than downy birch (Frivold and Mielikainen, 1991). Braastad (1985) reported 1.5 m3 ha–1 per year higher potential yield of

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silver birch than downy birch in Norway. Finnish yield models constructed by Koivisto (1959) show the total yield of silver birch is over 100 m3 ha–1 greater than that of downy birch over a rotation of 60 years on similar site classes. Raulo (1977) measured the development of dominant trees in silver birch and downy birch plantations in Finland and determined that height and diameter increment in silver birch was more rapid than in downy birch. Problems with existing birch yield models There are few reliable records of the growth and yield of birch in Britain (Philip, 1978). Brown (1991) pointed to the fact that in Britain there are too few properly managed stands to provide an answer to the question of the potential growth and yield of birch. In addition to this lack of suitable data, the current birch yield models are defective in three important ways (Mann, 1977; Lorraine-Smith, 1991). In the first instance, only a single set of models is available for sycamore, ash and birch (an attempt to distinguish between the two birch species has not been made in the model). Secondly, the published models are based on one initial spacing only (i.e. 1.5 m  1.5 m). Furthermore, models have been designed for monocultures only, with no indication of how the species and respective volume yields may be affected by being grown in mixtures. Mann (1977) indicated that there may be additional problems arising out of the ‘bulking’ of data from regionally diverse plots in constructing the models. Attempts to convert Scandinavian yield models into a format similar to the Forestry Commission management tables have failed (Miller, 1998). Worrell (1999) reported that perhaps the best option for obtaining useful data on the growth of a birch stand is to produce a ‘local volume table’ describing the standing timber volumes and other stand parameters. Objective of the study The objective of the study, which was initiated in 1998, was to examine the growth potential of birch in Ireland. The growth patterns in terms of radial growth at breast height, height growth and volume growth of selected birch stands were examined in detail using stem analysis.

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Materials and methods

sections were then labelled according to the position (height) along the main stem.

Selection of stands Stands with a minimum area of 1 ha, having a stocking rate >80 per cent and with an age within the range of 15–60 years were selected as candidate study sites. In total 33 stands were visited. Initially eight stands were selected for inclusion in the study. No distinction was made between stands consisting of natural regeneration, planted stands and stands made up of mixtures of the two. Field measurements and sample tree selection A square 0.04 ha plot was measured out with surveying tapes in a randomly selected part of each of the eight stands. Care was taken to avoid locating the plot near the edge of the stands. A complete tally was made of all stems within the plot. The diameter at breast height (d.b.h.) of all living stems (≥7 cm) was recorded to the nearest millimetre for seven of the selected stands. The d.b.h. of trees >4 cm was recorded in the one remaining sample stand. This stand comprised young (~21-year-old) naturally regenerated downy birch. Trees with a d.b.h. >4 cm were tallied and included in this stand so as to obtain an insight into the height and volume increment of smaller diameter stems. The selection method of sample trees for tree ring analysis followed a procedure adopted by Borowski (1954) and Turnbull (1958). Trees were classified into 20 mm (2 cm) diameter classes. The average basal area in each diameter class was calculated. Two trees of average basal area from each diameter class were located and marked as sample trees within the plot. It was assumed that the selected sample trees had volume increments typical of their diameter class. When the average d.b.h. did not correspond to an actual tree diameter within a given diameter class, trees with a diameter nearest to the average diameter were selected. In instances where only one tree was recorded for a given diameter class, only this one tree was selected as a representative sample tree. Disc sections ~2–3 cm thick were cut, starting at the stump and progressing at 1 m intervals along the main stem from each sample tree to a height of 2–3 cm top diameter. A disc was also taken from each tree at breast height. Disc

Sample preparation and stem analysis procedures Sample tree discs were dried, sanded (first on a mechanical belt sander and subsequently by hand using 240 grade and, where necessary, 400 grade sandpaper) and prepared for stem analysis. Paraffin wax was added to the disc surface to make annual ring boundaries more visible. Discs were placed under a high power binocular microscope and annual rings delineated. The WinDENDRO system, comprising a specially designed software package and computer linked digital scanner, was the main tool used in capturing and calculating ring width measurements. Height/age curves for the felled sample trees were calculated using Carmean’s (1972) method of height estimation in stem analysis, including Newberry’s (1991) adjustment. Sample tree volume under bark (to tip) was calculated using the ring width and height data (calculated using Carmean’s algorithm) from discs taken at fixed heights (i.e. intervals of 1 m) along the bole of relevant sample trees. A full account of the sample preparation and stem analysis procedures and the methodology used in calculating sample tree height and volume growth is given in Barrett (2000). Output A height/age curve was constructed for each sample tree and the height curves for all sample trees taken from the same site were plotted on a single graph for comparison and analysis. For each sample tree, the distance from the pith to the bark (i.e. radial growth) was calculated at 5 year intervals from the ring width data collected from the 1.3 m disc. Under-bark radial growth curves were examined to determine the pattern and extent of under-bark radial growth at breast height across the range of diameter classes. Sample tree height calculations were used, along with ring counts and ring width data, to derive sample tree volume estimates. The historic patterns of under-bark volume production, mean annual increment (MAI) and mean periodic increment (MPI) were calculated and plotted for each sample tree. The historic patterns of under-bark

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volume production were also calculated, examined and analysed for each diameter class. Finally, weighted curves describing the underbark volume production and volume increment (MAI and MPI) for each of the 0.04 ha study plots were constructed. These weighted curves were derived by multiplying the number of trees in a given diameter class in the relevant study plot by the average volume production values obtained for the two representative trees selected in that diameter class. The under-bark volumes for all diameter classes were summed to determine plot volume at 5 year intervals and MAI and MPI for the study plot were then calculated. The weighted under-bark volume and volume increment curves were used to summarize the historic pattern of under-bark volume production and volume increment in each 0.04 ha study plot throughout the life of the stand. This article focuses on the comparison of total volume production, height growth and yield class among the six sites. An article presenting the periodic volume production patterns for the six sites is being published in Irish Forestry (Nieuwenhuis and Barrett, 2001).

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examination of discs taken from two of the sites (i.e. Ballyroan, Co. Laois and Ballyross, Co. Wicklow) that full stem analysis of many of the selected sample trees would not be possible. The majority of sample trees from these two sites had annual rings so indistinct (particularly in the smaller diameter classes) that it proved impossible to count and measure them. For this reason the study was restricted to the remaining six stands (Table 2). A total of 100 sample trees were felled on the six sites. Tree ring data were collected from a total of 1333 sample tree discs. All six stands included in this study contained trees with morphological characteristics commonly associated with downy birch. A chemical test devised by Lundgren et al. (1995) was used to come to a final determination of sample tree species. No precipitate of platyphylloside was formed for any of the sample trees tested. It was concluded therefore that all stands included in this study consisted of downy birch trees. Four stands originated from planting, one from natural regeneration and one consisted of a mixture of both. Basic stand data

Results Stand quality A total of 33 stands were actually visited during preliminary fieldwork. The stocking of the stands visited was highly variable, while the form of the stands could be considered very poor. Trees were crooked, forked and in many instances had fluted stems. High numbers of epicormic clusters and shoots were common on the main stem of trees in older stands (i.e. stands >30 years of age). Many stands, particularly those found on peat sites, were composed of scrub birch. The vigour and form of the trees on the peat sites was generally very poor in comparison with stands and trees found on mineral soil. Stands of young (0–20 years), dense, naturally regenerated birch generally had superior form. This was for instance true of the trees in the stand examined at Colt, Co. Laois (site 2). Study sites/stands Initially eight stands were selected for inclusion in this study. However, it became obvious upon

Estimates for age, stocking, basal area, mean diameter, standing volume (over bark) and top heights for the six stands were obtained from the inventory data (Table 3). The high stocking levels were the most obvious sign of the lack of thinning and overall management in all stands. Comparison of the sites Comparison of diameter growth Diameter growth rates for five of the stands examined in this study ranged from 0.36 cm per year for site 1 to 0.42 cm per year for site 5 (Table 3). The stand at site 6 (a fen peat), had a much lower diameter growth rate (0.22 cm per year) than the other five sites. Of the sample trees examined in this study, radial growth was greatest for the largest diameter tree at site 5, which achieved a diameter of 25.6 cm after 32 years, equivalent to a mean annual diameter increment of 0.80 cm. Comparison of under-bark volume production Curves of under-bark volume production at 5year intervals for the six sites were determined

Site 4

Croneybyrne Togher – Portlaoise Wicklow Laois SW NW Sheltered Sheltered Grey-brown podzolic Gley 2.0 2.9 Downy birch Downy birch Natural reg. Planted 81 100 Scots pine, oak – 49–52 41–43 150 60 20 18 286 271

Site 3

Note. Age or age range determined from sample tree stump ring counts.

Tombrick Colt Bunclody Portlaoise Wexford Laois SE NW Exposed Moderately exposed Brown podzolic Raised peat 2.3 1.0 Downy birch Downy birch Planted Mixture 100 97 – – 34 21–22 180 120 14 14 181 162

Site 2 Falsk Kinnitty Offaly S Moderately exposed Raised peat 2.3 Downy birch Planted 100 – 32–38 50 17 223

Site 5

Cloonagh Lough Owel Westmeath W Moderately exposed Fen peat 4.8 Downy birch Planted 100 – 58–68 80 17 210

Site 6

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Property Forest County Aspect Exposure Soil Stand area (ha) Species Origin % Birch Other species Stand age (years) Elevation (m) No. sample trees No. sample discs

Site 1

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Table 2: Study sites – location, stand characteristics and sample tree data

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Table 3: Estimated age, stocking, basal area, mean diameter, total standing volume (under-bark, u.b.) and top height for the six stands Parameter

Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Stand age % Birch Birch stocking (trees ha–1) Basal area (m2 ha–1) Basal area increment (m2 ha–1 a–1) Mean diameter (cm) Diameter increment (cm a–1) Total volume u.b. (m3 ha–1) Volume u.b. increment (m3 ha–1 a–1) Top height (m) Height increment (m a–1)

34 100 2750 33 0.97 12.4 0.36 155 4.56 14.03 0.41

21–22 97 3263 19 0.88 8.6 0.40 79 3.67 12.83 0.60

49–52 81 975 29 0.57 19.4 0.38 143 2.83 17.08 0.34

41–43 100 1513 32 0.76 16.3 0.39 200 4.76 16.40 0.39

32–38 100 1763 31 0.89 14.8 0.42 140 4.00 14.86 0.43

58–68 100 1413 22 0.35 14.0 0.22 86 1.37 14.63 0.23

based on the tree ring analysis data (Figure 1). Under-bark volume production per plot was calculated by multiplying the number of trees in a given diameter class in the relevant study plot by the volume values obtained for that diameter class and summing these values for all diameter classes. Only trees ≥7 cm were included in the calculations. The results showed that a well-stocked, unthinned stand of downy birch could achieve a standing volume (under-bark) of 200 m3 ha–1 in 42 years (Table 3, site 4). The level of under-bark volume production was lowest at site 2 at 79 m3 ha–1 at 21 years. Annualized volume production in an unthinned birch stand ranged from 1.37 m3 ha–1 in a 63-year-old stand (site 6) to 4.76 m3 ha–1 in a 42-year-old stand (site 4). The patterns of under-bark volume production in the study plots at sites 1, 2, 4 and 5 were very similar (Figure 1). The rate of volume production increased at site 1 to 30 years, at site 2 to 20 years, at site 3 to 35 years, at site 4 to 25 years and at site 6 to 55 years. After this time the rate of volume production began to decrease. The rate of volume production at site 5 was still increasing after 32 years of stand growth. The highest rate of volume growth occurred at site 5 between the ages of 30 and 32 years. Comparison of top height Top height/age curves (Figure 2) were constructed for each site by calculating the average height growth of the

four largest diameter trees sampled in each study plot. With the exception of sites 5 and 6, the largest diameter trees sampled were also the largest diameter trees within the plot. Inclusion of trees that were not the largest diameter plot trees in the calculation of plot top height increment (for sites 5 and 6) is considered to have introduced only a slight error. The most rapid top height growth occurred at site 2 (Table 3). The maximum top height increment in any one year (i.e. 0.94 m) occurred at site 5 in year 7. Top height growth at site 6 was the slowest of the six sites examined in this study. Top height at the age of 64 years at site 6 was 14.63 m. The pattern of top height growth at sites 4 and 5 was almost identical. There was a notable reduction in top height growth at sites 4 and 5 after 15 years. The tallest trees in this study were recorded at site 3 where sample trees 13 and 20 achieved heights of 17.90 m and 18.40 m, respectively. Top height increment at sites 1, 2, 3, 4 and 5 was greatest between the ages of 10 and 15 years and at site 6 between the ages of 20 and 25 years. Maximum annual top height increment occurred before the age of 15 years at all of the sites examined in the study. Yield class comparison A comparison of top height/age curves with the general FC YC curves for sycamore, ash and birch (SAB) indicated that the YC for the sites examined ranged from