Zbigniew Borowski Forest Research Institute Poland
Beaver population in Poland like in many other countries during last 40 years increased rapidly from 270 to over 80 000 ind. This unexpected success in species reintroduction was partly caused by special program for reintroduction (Program for Active Protection of the European Beaver). However, we still do not know what mechanisms stay behind this success. The return of beaver, although beneficial from a wildlife standpoint, has created problems for many landowners. Damage caused by beaver usually consists of flooded timber and agricultural land, girdling and cutting valuable trees and interference with drainage systems. Whereas locally damages caused by beavers are very important there is no information about their intensity on the country level.
The objective of the study was to answer to the following questions: What is the pattern of distribution of beaver population in Poland and its density? What is the spatial pattern of damages caused by beavers in Polish forests and their intensity? When damages caused by beavers are the most numerous and what tree species are the most preferred? How important is beaver s impact on forest economy?
The Eurasian beaver (Castor fiber) is the continent s largest rodent characterised by: A semi-aquatic life-style, inhabitant of all kinds of freshwater systems Strictly herbivorous Monogamous species living in small family groups Produce 1-3 young per year, lifespan 7-8 years
Beavers are agents of landscape change, altering the structure and species composition of vegetative communities thought herbivory and water impoundment Beaver s status in Poland: Until 1994 strictly protected species From 1994 protected with some exceptions
Number of beavers [ind.] 90 000 80 000 70 000 The level of compensation: 2011-2,5 mln EUR 2008 1 mln EUR 68993 80 000 60 000 50 000 40 000 43499 30 000 20 000 24464 10 000 7600 0 0 130 270 1000 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 Years
over 14 000 ind. 8 001-14 000 ind. 4 001-8 000 ind. 1 001-4 000 ind. < 1 000 ind.
First record
Number of beaver colonies
Number of forest districts Mean percent of damages Tree species damaged by beaver 50 40 mean percentage of damages per tree species 30 20 10 0 300 250 Poplar Willow Lime Larch Ash Elm Fir Birch Beech Maple Alder Scotch Pine number of forest districts [out of n=4] where damages of consequitive tree species were recorded Oak Norway Spruce 200 150 100 50 0 Poplar Alder Willow Oak Birch Scotch Pine Beech Ash Norway Spruce Larch Maple Lime Elm Fir
Tree numbers Seasonal dynamics of tree damages caused by beavers 300 250 Cutting Debarking 200 150 100 50 0
100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Birch Oak Elm Rowan Poplar Bird cherry Alder Scotch Pine Norway Spruce Cutted Debarked
%of cutted trees 100 r = -0,73 80 60 40 20 0 0 5 10 15 20 Mean diametes of tree species [cm]
Jacobs' electivity index 2,5 2 1,5 Cutted Debarked 1 0,5 0-0,5 <6 6-10 10-14 14-18 18-22 22-26 26-30 >30-1 -1,5-2 -2,5 Diameter [cm]
Jacobs' electivity index Jacobs electivity index -4 1,5 1 Beaver preferences in relation to tree species Napiwodzko-Ramuckie Forests 0,5 0-0,5-1 -1,5-2 -2,5-3 2 Knyszyńska Forest 1 0-1 -2-3
Distance from water to tree species damaged by beavers in two locations: Knyszyńska Forest and Napiwodzko-Ramuckie Forests 100 Knyszyńska Forest 100 Napiwodzko-Ramuckie Forests Distance from the water [m] 80 60 40 20 80 60 40 20 0 Birch Bird cherry Norway Spruce Oak Scotch Pine Rowan Willow Hazel Poplar 0 Alder Hazel Birch HornbeamWillow Larch Scotch Pine Oak
Diameter [cm] 18 17 16 15 14 13 12 11 10 9 8 7 6 cutted debarked Birch Spring Summer Autumn - Winter Seasons 18 Bird cherry 16 14 Diameter [cm] 12 10 8 6 4 2 wiosna lato jesień-zima Sezony
Diameter [cm] Mean (±SD) diameter of felling trees which were taken or left by beavers in Napiwodzko-Ramuckie Forests ***-p<0.001, *-p<0.05. 35 30 *** Taken Left *** 25 20 *** *** *** 15 * 10 5 0 Birch Oak Hornbeam Alder Scotchpine Willow
Estimated number of beaver population in Poland amounted to 80 000 animals In the local scale beaver s impact on forest stands was small; in places of beavers forage damages averaged 28% (Knyszyńska Forest) and 31% (Napiwodzko-Ramuckie Forests) of available trees Tree species the most preferred by beavers were: poplar, oak, willow, birch whereas the less preferred were: alder, Scotch pine and lime
The distance of beaver operation was not bigger than 40 m from water (except poplar 62m). Autumn and Winter (from October to March) are the periods with the highest intensity of beavers damages. Trees with lower diameter most often were cut whereas bigger trees were debarked. Moreover, among felling trees beavers were taking thinner trees. Scale and intensity of beaver s damages was low, therefore in the majority of cases there was no significant conflict with forest economy.
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