Forest annual growth under influence of beaver-made inundation in low land territories

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M. Gackis
Latvia University of Agriculture, Forest Faculty, Jelgava, Latvia, maris. gackis@gmail. com
Eurasian beaver Castor fiber L. population has increased since its reintroduction start in year 1927 (Hailey, Rosell, 2002- Ozolins, Baumanis, 2001- Nolet, Rosell, 1998- BanogMC, 1990) and now stands as one of biggest (~82 000) wild — huntable animal populations in Latvia (Bebru skaita dinamika, 2012). In some researches (BanoflMC, 1990) maximal ecological and economical capacity of nature of Latvian for beaver population was estimated 50 000 animals. It was predicted that reaching this number could cause high negative influence to silvicultural and agricultural industries mainly through damaged drainage systems and road infrastructure. Nowaday beaver population size and forest damage statistics (Boja gajusas audzes, 2011- Izsniegtie san-itarie atzinumi, 2011) show situation that in period from year 1991 to 2010 every year in average 250 ha of forest stands were damaged to critical level by beaver actions — dam building and inundation creation. The influence of beaver actions on nearby forests mainly appear through inundation formation that decreases tree root aeration and results in tree vitality (Rosell et al., 2005- Liepa, 1996- Dams et al., 1995- BanogMC, 1990). Not always large open-surface ground above-water is needed to create influence on tree vitality. High level of groundwater even in not the most active increment formation period (july, august) is enough to leave influence (ZalTtis, 2006).
Visible forest damages are easy to find and calculate, but unvisible ones' mainly stay hidden from forest owner or manager and come to light some years later. Negative changes in tree increment formation in longer period can result in reduced stand productivity and financial income from forest management. Authors previous researches show mainly negative inundation effect on coniferous tree growth (Gackis, 2010- Gackis, 2009) at exact inundation side, but doesn’t reveal stand parameters that influence the average amont and direction of increment changes.
Material and methods
Research field data was collected in central part of Latvia on october 2010 in drained pine Pinus sylvestris,
spruce Picea abies and birch Betula pendula stands. Some of these stands should be under impact of beaver-made inundation in nearby drainage system for at least last 5 years.
Data collected in total 15 research (with influence of inundation) stands — 5 in each tree species and 9 control (without influence of inundation) stands — 3 in each tree species. All stands selected with similar age, density, growing conditions and percentage of main tree species in stand. At each research stand taxation parameters and radial growth samples were collected on 3 straight, 50 m long and 1 m wide transects, which went perpendicular to flooded drainage system. First transect started at beaver-made dam, second transect 25 m up-stream from dam, third transect 50 m up-stream from dam. Radial increment samples taken with M. Presler increment borer from last 16 annual rings from trees which were straight on transect lines. Taxation measurements done on transect line trees and closest 4 trees to calculate tree average diameter and stand density. Data in control stands were collected by the same transect layout in most typical part of stand.
Annual radial increment measured with LinTAB 5 measurement station microscope in resolution of ± 0,001 mm. Measured last 16 annual rings of each sample tree — last 6 rings (counting from bark) represent tree growth in inundation influence period, oldest 10 — tree growth before inundation period. Calculations of additional increments made by method of prof. I. Liepa (Liepa, 1996) where the main result indicators are: 1) additional increment per unit of stand basal area and 2) cumulative additional increment per unit of stand basal area (both in m3ha-1). Additional increment is calculated from real and predicted radila increments, average tree diameter, particular stand density and constant coeficients of particular tree specie trunk form (Liepa, 1996).
Results and discussion
All beaver-made inundations with it water surface at sampling stands didn’t stand over the edges of drainage system and this situation shows that inundation affect nearby forest stands mainly by rising groundwater level over tree root system.
In total annual increment measurements were collected: in pine stands from 129 influenced and 75 control trees, in spruce stands from 144 influenced and 71 control trees, in birch stands form 168 influenced and 67 control trees. Pine stands were 81 — 85 years old, spruce and birch stands accordingly 65 — 67 and 66 — 71 year old. For an illustration of tree annual radial increment average values of annual increment of sampled pine stands are shown
Figure 1. Radial increment average values of sample pine stands
In nearly all cases visually comparing influenced and control stand radial increment trend lines it is visible that inundations are with negative influence on coniferous tree annual growth. The increment drop is quite slight to both coniferous tree species and confirms fast reaction of trees beside inundation in previous studies (Gackis, 2009- Harkonen, 1999- Балодис, 1990- Дьяков, 1975).
For further calculations an adequate control from all control trees was done — with correlation analysis relating average radial increment values to all control tree radial increments. The goal was to choose trees with most similar increment formation frequency. The forecasted increment (how trees would normally grow) of influenced stands was calculated basing on regression equation from retrospection period average values of adequate control and influenced stand tree radial increments. Forecasted increment is used to estimate the drop of radial increment and calculate additional increments.
From average increment values it is clear that pine and spruce stands show fast and similar drop in radial increments under effect of inundation. In the end of 5 year period of inundation influence the radial increment in both coniferous tree species stands has decreased by about 40%. In birch stands this process stays variable.
In calculations with influenced tree division in distance zones 1 — 15, 16 — 30 and 31 — 50 meters from inundation edge shows more clear view on tree response reaction. Estimations of cumulative additional increment in influenced stands done depending on measured tree distance from inundation edge and represents inundation influence of 5 year period. In this case cumulative additional increment indicates negative additional increment which «works» against natural increment by slowing down total volume formation on certain area. For example, data shows
3 1
about 5.4 m ha volume increment loss at the end of 5 year period of pine trees growing in distance of 1 -15 m distance from inundation (Table 1).
In spruce stands this result is about 2 times bigger (about 13 m3ha-1). Cumulative additional increment changes are influenced by tree annual natural increment in the same direction (-/+) and proportion. Birch stands react unlike coniferous ones and show the higher cumulative additional increment (-17.5 m3ha-1) more further from inundation edge (15. 130 m). Cumulative additional increment can change also under changes of proper stand density.
Beaver-made inundations create nagative effect on pine and spruce annual growth, which in 5 year period can result in not-formed 5.5 m3ha-1 of wood in pine and 13 m3ha-1 in spruce stands till 15 m distance from inundation side. After 15 m distance from inundation side the negative effect decreases and more sharply in pine than spruce stands (cumulative additional increment accordingly -2.5 and -9.5 m3ha-1). Negative cumulative additional increment has tendency to decrease with sampling tree withdraw from inundation side but holds higher in spruce stands. Birch stands are more affected in distance of 15. 1−30 m from inundation side and represent growth decrease as 17.5 m3ha-1 in last 5 year period.
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Table 1
Cumulative additional increment in influenced coniferous stands in last year depending on tree distance from inunda-_______________________________________________________tion edge_____________________________________________
Pine sample stands
Spruce sample stands
Birch sample satnds
Sample tree distance from inundation _ edge, m _
1 — 15 | 15.1 — 30 | 30.1 — 50 | 1 — 15 | 15.1 — 30 | 30.1 — 50 | 1 — 15 | 15.1 — 30 | 30.1 — 50
Cumulative additional increment,
-5. 41 -2. 36
-2. 53 -12. 78 -10. 73
-9. 55 -11.6 -17. 5
-4. 8

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