Effect of retained trees on growth and structure of young Scots pine stands

1. Effect of retained trees on growth and structure of young Scots pine stands Juha Ruuska, Sauli Valkonen and Jouni Siipilehto Finnish Forest Research Institute, Vantaa, Finland

2. Maintain specific ecological processes (habitats) Create structurally complex stands Maintain aesthetic values Promote regeneration Green tree retention • An integral part of contemporary forestry in Europe Objectives

3. Problems • Poorly stocked gaps around retained trees (poor sites in N. Finland) • Loss of harvested volume • Logging damage, higher logging costs • Poor stability in solitarydistributions

4. In the vicinity of Retained Trees (RT), Seedling stand density is lower Seedling height and diameter growth is slower Influence on diameter growth is greaterthan on height growth,and seedlings are slimmer (height-diameter ratio) Seedlings have smaller and less branches Hypotheses • and • An aggregated RT distribution is less harmful than a dispersed distribution

5. 9 stands in Southern Finland Most Scots pine upland sites covered Seedling stand dominant height of 2-7 m Scots pines retained for 8-18 years Variable RT densities (32 - 117 ha-1) Variable but mostly scattered spatial distributions No significant tree removals or mortality Study stands • A sample of planted or naturally generated • Scots pine seedling stands with RT

6. 8 seedling sample plots per RT sample tree Plots systematically at 1, 3, 6 and 10 m distance from RT Seedlings measured for h, dbh, coordinates One sample seedling per plot per species selected among main crop seedlings (h, dbh, cr, cw, h growth, d growth, branch diameters etc.) One additional sample seedling per species to represent non-main crop seedlings Sampling • All RT mapped, measured for dbh, h • 10 sample RT per stand • measured for trunk and crown variables, t, dbh growth

7. Growth models For pine seedlings, RT Individual tree, spatially explicit Models for branching Max branch diameter and branch cross sectional areaof 3 whorls; for pine only Simulation Seedling diameter and height growth Branching variables Subject to site, density of the seedling stand, number and diameter of RT,and RT spatial distribution Approach

8. Light interception is not the critical factor Low interception rate with pine (15 % with 50 RT ha-1) Root competition is Water, nutrients Greatest on poor sandy soils Extent Roots up to >10 m Greatest near RT base Great variation in shape Overstory influence Root density and distance from a retained Scots pine tree (Kalela 1954)

9. A spatial competition index Incl. RT and seedlings Ecological field theory: resource availability(Wu et al. 1985) Index = f(diameter, distance) Max value = 1 (with max RT d in data and zero distance) Description of tree competition Competition effect of one tree by diameter and distance

10. Results1. RT effect on seedling stand density • Seedling stands were dense • 3,700-37,000 Scots pine seedlings ha-1 • Virtually no RT influence on pine density • Clearly less birch near RT • Greater RT effect in Northern Finland(Niemistö et al.1993)

11. 2. Seedling height growth • Cumulative growth = height • Clearly smaller pine seedlings near RT • Smaller effect than in Northern Finland (Niemistö et al.1993) Relative height of pine seedlings and RT distance and diameter

12. 3. Seedling diameter growth • Cumulative growth = diameter • RT effect negligible compared to that of stand density • Equal RT influence on height and diameter growth • Tree form not influenced by RT Diameter of 5 m high pine seedlings, RT distance and diameter, and seedling stand density

13. 4. Maximum branch diameter • Diameter of the thickest branch of a pine seedling • Slightly smaller near RT for given seedling height • Effect was minor compared to that of • Site index • Competition from other seedlings Influence of RT diameter and distance, site (H100), and seedling stand density on the maximum branch diameter of a 5 m high pin seedling

14. 5. Total branching • Sum cross-sectional area of branches in 3 whorls • Clearly less near RT • More pronounced than on maximum branch diameter • Site index and competition from other seedlings also had a strong effect Influence of RT diameter and distance, site (H100), and seedling stand density on the total branch Cross-sectional area of 3 whorls of a 5 m high pine seedling

15. With retention Without

16. 6. Spatial distribution of RT • Simulated alternatives • 16 to 48 RT ha-1 • Average RT dbh 25 cm and height 21 m • 15-year simulation period • Random, regular and clustered RT distribution • Results • RT spatial patterns had only a marginal effect on growth and branching of pine seedlings • 48 RT ha-1 reduced average seedling height 15%, diameter 11-16% and maximum branch diameter 9-10% compared to no retention

17. RT do not reduce seedling stand density in S Finland as much as in N Finland Can help control birch on problem sites Influence on diameter growth is not greaterthan on height growth, and height-diameter ratio is not affected much Seedlings have smaller and less branches, but the maximum branch diameter is not reduced much Reduction in branching is small compared to that achieved by higher density, site-species match An aggregated RT distribution is only little less harmful than a dispersed distribution Conclusions (I)

18. In that sense, the current practice with 5-10 trees ha-1seems not to have a great effect on wood production Growth effects are small No unstocked patches in the south Not effective in quality improvement It is a small improvement that groups are now placed near stand edges, or on special uncut sites Conclusions (II) Retention is practiced for ecological and aesthetical purposes. It is not mandated by law but is strongly suggested for example during a certification process.