Original scientific paper
Spatial Analysis of Pedunculate Oak Mortallity Rate for Adaptive Forest Ecosystem Management in Croatia
Ivica Tikvić
; Šumarski fakultet Sveučilišta u Zagrebu, Zavod za ekologiju i uzgajanje šuma, Svetošimunska 25, HR-10 000 Zagreb, HRVATSKA
Damir Ugarković
; Šumarski fakultet Sveučilišta u Zagrebu, Zavod za ekologiju i uzgajanje šuma, Svetošimunska 25, HR-10 000 Zagreb, HRVATSKA
Josip Gašpar
; »Hrvatske šume« d.o.o. Zagreb, Uprava šuma podružnica Nova Gradiška, J. J. Strossmayera 11, HR-35 400 Nova Gradiška, HRVATSKA
Abstract
Pedunculate oak (Qercus robur L.) is one of the most important tree species in Croatia. As a dominant species, it covers about 10% of the forested area (about 210 000 ha), which accounts for about 12% of the total growing stock. This tree species is distributed in the lowland and floodplain area of the rivers Drava, Sava and Kupa and their tributaries. Forests of pedunculate oak are managed according to close-to-nature principles and with rotations of over 140 years. In the past 100 years, the management of pedunculate oak forests has been hindered by a serious management and ecological problem: the occurrence of tree dieback and decline. From the management point of view, the problem entails the increased quantity of dead or severely damaged trees, increased management costs and a disturbance in natural forest regeneration. From the ecological point of view, the problem centers around changed site conditions, which is unfavorable for both present and future growth of pedunculate oak trees. In the period from 1995 to 2007, an amount of 35% of the current pedunculate oak volume declined in Croatia, with mortality rising to 40% if the past several years are taken into account. Tree dieback incurs grave financial losses, which reach up to 40% of the potential market value of undamaged and healthy wood mass. The losses are further aggravated by increased costs of managing such forests and reduced non-timber forest functions. The mortality rate of pedunculate oak trees is an indicator of the instability of lowland forest ecosystems, especially when dieback occurs in optimal site conditions for the pedunculate oak (micro-elevations and micro-depressions). Individual, intensive and catastrophic tree dieback has been recorded. Salvage cuts are applied in the attempt to mitigate the effects of disturbances caused by tree dieback. These cuts are undertaken continuously, depending on tree dieback intensity, and are based on the assessment of tree condition. Tree condition is assessed on the basis of tree damage criterion, which comprises crown defoliation and leaf discoloration. These are indicators of lessened tree vitality, but have so far not proved adequate for the reduction of pedunculate oak tree dieback. In order to advance the current condition of forest ecosystems, additional indicators of decreased tree and stand vitality are being sought, and so are the indicators of their disturbed stability. Mortality rates that take into account the volume of dead tree mass are used for this purpose. The objective of this work is to establish spatial distribution of different mortality rates of pedunculate oak trees and assess their correlation with certain ecological factors.
The research was carried out in three management units (MU) in the Forest Administration of Nova Gradiška. These include MU »Međustrugovi« in Stara Gradiška Forest Office, MU »Zelenika« in Novska Forest Office, and MU »Grede–Kamare« in Jasenovac Forest Office. »Međustrugovi« is situated in the area between the rivulets of Veliki and Mali Strug, the tributaries of the River Sava, »Zelenika« is situated along the Zagreb – Lipovac motorway and north of the rivulet Veliki Strug, and »Grede Kamare« is southeast of Novska between the Strug and the Sava.
The data gathered involved marked damaged and dead pedunculate oak trees by compartments for the period from 1998–2007. The wood volume of these trees was calculated on the basis of tree diameter and local tariffs. Dead wood mass of every individual compartment was divided with the compartment area to obtain tree dieback per surface unit in the monitoring period (m³/ha). Based on the growing stock of pedunculate oak by compartments and wood volume of dead pedunculate oak trees per surface unit, the percentage share of dead wood mass was calculated.
The wood volume of dead wood mass by compartments (m³/ha) was then divided by 10 to obtain the annual volume of dead wood mass of pedunculate oak per hectare. These values were juxtaposed with the current annual increment of pedunculate oak by compartments to obtain the percentage share of dead wood mass in relation to the current annual increment of the compartment.
Based on the obtained values, every compartment was assigned a degree of wood mass mortality and a degree of increment decline. Wood mass mortality was classified into four degrees: I (0.1–10%, low mortality), II (10.1–25%, moderate mortality), III (25.1–50%, intensive mortality) and IV (50.1–100%, extreme mortality). Increment decline was also classified into four degrees, but with different percentages: I (0.1–25%, low), II (25.1–50%, moderate), III (51.1–100%, intensive) and IV (>100%, extreme).
Based on the degree of wood mass mortality and degree of increment decline of each single compartment, maps of wood mass mortality rate and of increment decline rate were constructed. In the Management Unit »Međustrugovi«, the mortality rate of wood mass by compartments was relatively low. A total of 85 dieback-affected compartments over an area of 1302 ha were analyzed. In 98% of the compartments, low to moderate wood mass mortality rate was recorded (Fig. 2), but no intensive or extreme dieback was found. Rates of increment decline by compartments were unfavorable. Extreme decline was found in 6 compartments over an area of about 30 ha, intensive increment decline was recorded in 12 compartments, i.e., significant reduction in the increment was recorded in over 20% of the compartments (Fig. 3). Intensive and extreme increment decline was found in an area exceeding 260 ha. Compartments with the highest values of wood mass mortality did not manifest the most extreme values of increment decline. The map does not show any significant participation of the compartments with extreme increment decline, although the grouping of the compartments with intensive and extreme decline was observed, and so was their higher participation in the southern part of the management unit along the Strug watercourse.
In relation to the three study areas, the lowest mortality of wood mass was recorded in the Management Unit of »Zelenika«. The research included 50 compartments over an area of 1065 ha. Moderate wood mass mortality was found in 6 compartments and slight mortality in the remaining compartments. Intensive and extreme increment decline was recorded in 10% of the compartments comprising an area of over 100 ha (Fig. 6). The grouping of compartments with a certain degree of increment decline is observed. This refers particularly to degrees II and III of increment decline, which were recorded in the compartments situated in the eastern part of the Management Unit of »Zelenika« (Fig. 7).
The highest participation of compartments and sub-compartments with intensive and extreme rates of wood mass mortality was recorded in the Management Unit of »Grede–Kamare«. Of a total of 102 dieback-affected compartments and sub-compartments in an area of 1474 ha, wood mass mortality was extreme in 12 compartments and intensive in 30 compartments, in an area of over 570 ha. Moderate and low mortality of wood mass was found in 57% of the compartments (Fig. 8).
An exceptionally unfavorable condition was recorded in relation to the degrees of increment decline. Extreme increment decline was found in over 55% of the compartments; in other words, every second compartment or sub-compartment in the Management Unit of »Grede–Kamare« was afflicted by disturbed stand growth and increment in an area of over 800 ha. Other degrees of increment decline had similar participation by compartments and sub-compartments (Fig. 9). The largest number of compartments and sub-compartments with extreme and intensive increment decline was recorded in the southeastern part of the management unit. They form larger units and are grouped in one part of the management unit in the map (Fig. 10).
A comparison of increment decline rates by management units reveals the occurrence of intensive increment decline in a smaller number of compartments (10–15% of the compartments), and of extreme decline in fewer than 10% of the compartments in two management units. In one of the management units, extreme increment decline affected more than 50% of the compartments.
Changes in the sites of pedunculate oak in the last 100 years have affected the stability of these forests and the vitality of trees, which has decreased considerably in recent times. Pedunculate oak trees of lessened vitality are permanently exposed to impact of a variety of adverse factors. This consequently leads to a gradual decline of trees, of which some die before the end of the rotation. Dieback of trees aged between 80 and 120 recorded in the Spačva forest area is an important indicator of disturbed growth and development of the most valuable forests in Croatia and a sign of general disturbances in the stability of lowland forests. Intensive tree mortality continues despite management procedures aimed at regulating the structure and quality of the stands. This phenomenon is particularly characterized by an uneven spatial distribution and intensity. In some areas of the pedunculate oak range in Croatia, dieback is the biggest management and ecological forestry problem, since management largely involves damaged and dead trees of pedunculate oak. As a result, the quality of the stands and total wood mass production are severely reduced. Since dead pedunculate oak trees are less exploitable and achieve lower market values, the consequences of tree dieback reflect unfavorably on the entire forestry sector.
Monitoring the intensity and dynamics of tree dieback in stands is one of the ways to determine certain patterns of this phenomenon and improve the management with forests of disturbed stability. This requires more intensive and regular marking of damaged and dead trees. The most common indicators used for the marking of these trees are morphological features of the crown and biotic damage of the stem. However, these indicators are insufficient for achieving better stand vitality and reducing tree decline and dieback. Hence, spatial distribution of tree dieback by compartments and sub-compartments is one of the possible indicators that can be used in planning the management with stands of lessened vitality.
Our research found chronic and continuous dieback of pedunculate oak trees, although dieback rates vary by years. Significant spatial differences were also found in the rates of wood mass mortality and increment decline, both among the management units and among their compartments and sub-compartments. Spatial grouping of compartments with intensive and extreme dieback shows correlation with site factors, which are considered the principal causes of dieback. Management with pedunculate oak forests with disturbed stability should be adjusted to the condition of individual ecosystems and to the trends that indicate their development. Presentations of spatial distribution of tree dieback can significantly contribute to this goal.
Keywords
dieback of trees; pedunculate oak; adaptive management; vitality of trees; forest ecosystems
Hrčak ID:
68006
URI
Publication date:
8.4.2011.
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