Effect of growing density on biomass and stem volume growth of downy birch stands on peatland in Western and Northern Finland

Abstract

The objective of the study was to ascertain the relationship of thinning intensity of downy birch (Betula pubescens Ehrh.) stands with height, crown, and diameter development as well as pulp- wood,�stemvolume, �andbioma ssincrementusinglong-term �(20−30 �years) �fieldexperiments. � Diametergrowt hofbi rchesi ncreasedwi tht hinningi ntensityduri ngt hefirst �15 �ye arsfromt hinning� in all development phases, though after that it did so only for the youngest stands. The thinning responsewaslow . �Thinningintensityhadnoinfluenceonincreaseinheight. �Intermsofstem � volume with bark, the mortality in unthinned stands during the study period was 30-45 m 3 ha -1 . The mean stem number in unthinned birch thickets fell from 25 000 ha -1 at a dominant height of 7 m to 3000 ha -1 �at �18 �m. �Thestemvolumeincre mentoverthefirst �15 �yearswashighest �(5-6 � m 3 ha -1 a -1 )�ontheverylightlythinnedorunthinnedplots, �butlatertherewasnosignificantdif- ferencebetweeninitialthinningintensities. �Themaximumabove-groundleaflessbiomass �(over � 100 Mg ha -1 ) was achieved on very lightly thinned plots. Also, the total production (including thinning removal) of biomass or stem volume or even the production of pulpwood increased with stand density, with these values being greatest for very lightly thinned or unthinned plots. During 50-yearrotation , �thehighestleaflessabove-groundbiomassproductionwas �2.5 �Mgha -1 a -1 as a meanvaluefromtheexperiments. �Thehighestmeanannualproductionofpulpwood �(d�> �6.5 �cm) � was 3.2 m 3 ha -1 a -1 , and, in practice, no saw timber or veneer timber was produced, because of the small size and low quality of the stems. A thinning in downy birch stands increased slightly thesizeofstemstoberemovedinfuturecuttings, �butwithexceptionforverylightthinningit � decreased the production of biomass and merchantable wood.