The role of competition in drought related tree dieback in Persian oak (Quercus brantii) forests, central Zagros, Iran

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1 INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 3, No 6, 2013 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN The role of competition in drought related tree dieback in Persian oak (Quercus brantii) forests, central Zagros, Ahmad Hosseini 1, Seyed Mohsen Hosseini 2, Ahmad Rahmani 3, Davoud Azadfar 4 1-Department of Forestry, Natural Resources and Marine Science Faculty of Tarbiat Modares University, Noor. Mazandaran,. 2-Corresponding Author, Department of Forestry, Natural Resources and Marine Sciences Faculty of Tarbiat Modares University, Noor. Mazandaran,. hosseini@modares.ac.ir 3- Department of Forestry, Research Institute of Forests and Rangelands, Peykanshahr, Tehran, 4- Department of Forest Ecology and Silviculture, Forestry and Wood Technology Faculty, Gorgan University of Agriculture Sciences and Natural Resources doi: /ijes ABSTRACT This research carried out to study the relationship between tree competition and Persian oak (Quercus brantii) dieback and mortality in different site conditions and stand structures in Shalam oak forest which located in north of Ilam province, western. We divided study area into 20 homogenize units based on topography using GIS. Then we established 6 transect with a length of 100 m in each landform unit and determined 5 random points with interval of 25m on each transect and marked the closest live and dead tree to them. To each focal tree we measured the distance, species, Diameter at breast height, crown diameter, height and crown dieback percent of neighbor trees. Competitive condition of focal live and dead trees was computed by Hegyi index. The results showed that the amount of tree competition varied among units. Also the results of correlation and Principal component analysis (PCA) showed the significant positive and direct relationship between competition and tree dieback amount. The lowest and highest amount of tree dieback was found respectively in units those had the lowest and highest amount of tree competition. Also the amount of tree competition had inverse relationship with crown diameter of focal tree and direct relationship with crown diameter and height of neighbor trees and crown dieback percent of focal trees. Key words: tree competition, tree mortality, Persian oak, Zagros. 1. Introduction Tree mortality is a complex phenomenon that often several factors including competition, borer beetles and disease outbreaks, environmental stress due to local climate and soil limitations are involved in its occurrence (Franklin et al, 1987; Waring, 1987). Identify the Multiple factors contributing to tree death and determining of their relative importance is very difficult (Franklin et al, 1987; Das et al, 2008). One of the most important factors that are involved in the process of tree dieback and mortality, but less considered is tree to tree competition. Competition is a factor that present in the natural life cycle of trees and its effect in critical junctures during the course of life and death of trees is more evident. Competition as a contributing factor in the phenomenon of tree mortality in temperate forests has been Received on May 2013 Published on July

2 known and several studies have confirmed the connection between the death of the tree and the competitive environment (Das et al, 2011). Some of the researchers (Bugmann, 2001; Moravie and Robert, 2003) introduced the competition as a primary factor predisposing and the others (Eid and Tuhus, 2001; Yang et al, 2003; Monserud et al, 2004; Temesgen and Mitchell, 2005; Bravo-Oviedo et al, 2006) as a contributing factor in the process of tree mortality. Research results of researchers have shown that although there is competition inherently in forest ecosystems, but its amount is varied in relation to site condition, stand structure, tree species, stand age, and ecosystem type. research of Olano et al, 2009 in Spain showed that the tree Mortality is affected by the specific nature of the neighboring trees. They concluded that the mortality rate for tree species is associated with their levels of tree competition. study of linares et al, 2010 in Spain showed that tree competition has predisposing role in death of trees. They concluded that intensity of competition associated to size and density of adjacent trees. Dwyer et al, 2010 in their research concluded that There is evidence in support of hypotheses "neighbour density" and "resource heterogeneity". they found strong support for an interaction between microsite quality and neighbourhood stem densities, and suggest that this interaction is driven by plant available water. Also Das et al, 2011 concluded that competition has a contributing role in tree death in softwood forests of California And also trees those dead or predisposed to die had more sever competitive environments than live trees. Zagros degraded forest ecosystems have been affected by widespread tree death in recent years. The importance of these forests and particularly oak species warrant a comprehensive study of the drought induced tree mortality phenomenon and identify of its factors. As respects, the density of Zagros forests also forests of study area- compared to dense forests such as Caspian forests in north of is more sparse, so some of researches have not believed to the role of tree competition in tree dieback in these forests and do not consider to it. Thus by doing this research we try to determine whether tree competition present in these forests or not and whether it contribute to drought related tree mortality or not? the goals of this paper are: 1) determine the variability of tree competition across the study area 2) determine the relationship between competition and site condition and stand structure characteristics 3) determine the relationship between competition and stand and tree level characteristics 4) determine the relationship of competition with tree dieback and mortality. 2. Material and methods 2.1 Study area For this research we selected Shalam oak forested area in north of Ilam province, central Zagros Mountains, western (Figure 1). The range of elevation in study area is 600m (from 1500 to 2000 m above sea level). This area has all of the main aspects and various slopes from low to high. Forest stands are generally formed of Quercus brantii and another tree and shrub species such as Pistacia atlantica, Acer cineraense, Crataegus azarolus, Amigdalus orientalis. Stand form in lower elevation is standard with coppice and in higher elevation is coppice with standard. Density of stands varies across the study area. 2.2 Research method First the study area was limited and determined on topographic map 1: then prepared layers of slope, aspect and elevation by Arcgis software. Aspect was divided into four Classes including North, South, East and West. Slope was divided into three classes (0-30, 30-60, 60<) and elevation was divided into two classes ( , m.s.l.). the study 2209

3 area was stratified and 20 homogenize units based on topography was obtained using GIS. Then we established 6 transect with a length of 100 m in each landform unit and determined 5 random points with interval of 25m on each transect and marked the closest live and dead trees (focal tree) to each of them. Figure 1: The study area and its location on the map of Ilam province To each focal tree we measured the distance, species, Diameter at breast height, crown diameter, height and crown dieback percent of neighbor trees. Competitive conditions of focal live and dead trees were computed by Hegyi competition index. The degree of competition experienced by the focal i tree was calculated as the sum of the quotients, for all j neighboring trees directly surrounding it, between the ratio cr j cr i and the distance between the i and the corresponding j trees (dist ij ): CI Hegyi = (cr j /cr i ) 1.3 /dist ij 0.4 Crown dieback classification of each tree was done according to kabrick et al, 2008: (safe= 5%>, low= 5-33%, moderate= 33-66%, sever=66%<). 2.3 Data analysis After the primary analysis of data, Statistical Summary of stand structure variables and tree dieback & mortality variables were computed. Also collected data to estimate the tree competition, separately analyzed in each unit and the obtained values of competition index were compared among units and between focal live and dead trees in each unit. We used from kolmogorov-smirnov test checks for 'goodness of fit' of a sample to a normal distribution and Levene's test used to assess Variance homogeneity, which is a precondition for parametric tests such as the t-test and ANOVA. Also we used from ANOVA, Duncan test and paired samples t-test respectively for analysis of variance, multiple and paired comparisons. Pearson's correlation coefficient (r) was used to measure of the strength of the association between the studied variables. Also we used from correlation analysis and ordination using Principal Component Analysis (PCA) to examine the relationship between parameters related to the tree competition with amount of tree dieback and mortality. 3. Results 3.1 Comparing the competitive values of trees among the units Results of analysis of variance of tree competition showed that competition index values varied across the whole units and they had significant differences among them (Table1). Result of Mean comparison showed that lowest amount of tree competition found in unit

4 (characteristics: aspect: West, slope: 0-30%, elevation: <1700m) and the highest amount of tree competition found in units 1 (characteristics: aspect: East, slope: 30-60% elevation: 1700m<) and 7 (characteristics: aspect: South, slope: 30-60%, elevation: 1700m<). Table 1: Analysis of variance of tree competition values among the homogeneous units Hegyi Sum of Squares df Mean Square F Sig. Between Groups Within Groups Total Comparison of the Competitive conditions of live and dead trees Results of non-paired T comparison showed that the competition value of focal dead trees was different from focal live trees and had higher values (Table2). Results showed that the dead trees Compared with live trees had smaller crown, lower height and closer distance to neighboring trees. Also the neighboring trees of focal dead trees had larger crown and more density. Analysis of variance of competition in the height classes showed that the amount of competition among the height classes is not equal (Table 3). According to the results of means comparison we determined that the lowest amount of competition was in tall class and the highest amount of competition was in short class (Figure 2). Table 2: Results of paired comparison of the competitive values of live and dead focal trees Levene's Test for Equality of Variances t-test for Equality of Means Hegyi competition index F Sig. t df Sig. (2-tailed) Equal variances assumed Equal variances not assumed Table 3: Analysis of variance of tree competition values in the height classes Hegyi competition index Sum of Squares df Mean Square F Sig. Between Groups Within Groups Total Table 4: Results of Pearson correlation of competition values of focal trees with individual characteristics of the neighbor trees 2211

5 Hegyi index The role of competition in drought related tree dieback in Persian oak (Quercus brantii) forests, central Zagros, Hegyi competition index Crown width of focal trees(m) Height of focaltrees(m) Crown width of neighbor trees(m) Height of neighbor trees(m) Neighboring distance(m) * **.685 **.771 **.782 ** c a b tall medium short tree height(m) Figure 2: Means comparison of tree competition values in the height classes 3.3 Relationship of tree competition with tree dieback and mortality Analysis of variance of competition in the crown dieback classes showed that the amount of competition among the crown dieback classes is not equal (Table5). According to the results of means comparison we determined that the lowest amount of competition was in safe class (0-5%) and the highest amount of competition was in severe (> 66%) class (Figure 3). Also competition had a significant positive correlation with amount of crown dieback and mortality (Table 4). The results of ordination by Principal component analysis (PCA) showed that the vector amount of mortality with all of the competition variables was aligned. Namely it showed a significant relationship of tree competition with tree dieback and mortality (Figure 4). Table 5: Analysis of variance of competition values in the dieback classes Hegyi competition index Sum of Squares df Mean Square F Sig. Between Groups Within Groups Total

6 Hegyi index The role of competition in drought related tree dieback in Persian oak (Quercus brantii) forests, central Zagros, c d a b - - Dieback classes - Figure 3: means Comparison of tree competition values in the dieback classes Figure 4: Multivariate analysis of tree mortality amount associated with tree competitive parameters (hegyi deadtrees= CI of dead trees, hegyi live trees= CI of live trees, dead trees.ha= amount of tree mortality per hectare, hegyi 5-33%= CI of trees in dieback class of 5-33%, hegyi 33-66%= CI of trees in dieback class of 33-66%, hegyi %= CI of trees in dieback class of %, hegyi 1+2= CI of trees in dieback classes of 0-5% and 5-33%, hegyi 3+4= CI of trees in dieback classes of 33-66% and %, hegyi total= CI of trees in whole dieback classes). 4. Discussion Results showed that tree competition is an important contributor factor in the occurrence of tree dieback and mortality phenomenon in the study area. The results determined that tree competition is affected by stand and tree level characteristics and thereupon caused various and different competitive conditions and environments, as effects trees with different intensity that contributed to dieback and dying of predisposed trees in critical conditions. This result is consistent with the results of Dwyer et al, 2010 and Das et al, More over variability in physiographic factors and site conditions can effect on variability of Competitive conditions and caused the various competitive environments with different 2213

7 Competitive intensity. Mean comparison results of competition index values among units showed that amount of competition across the whole units is not equal, so that the max value of it is belongs to unit 1 with site characteristic; aspect: East, slope: 30-60%, and elevation: 1700m< and unit 7 with site characteristic; aspect: Southern, slope: 30-60%, elevation: More than 1700m above sea level and min value of it is belongs to unit 37 with site characteristic; aspect: Western, slope: 0-30%, and elevation: Less than 1700m above sea level and other units With differences have located between these two ends. Based on these results we found that physiography has significant effect on amount of tree competition. Also in some of stands with denser than the others amount of competition were more because of proximity of trees to each other. In critical conditions such as drought occurred in recent years amount of soil moisture have decreased and caused the sever drought stress. In denser stands, tree to tree competition for obtaining the soil water would be more sever due to the proximity of trees to each other that as a result the drought stress would be more sever in these situations. Thus the trees with lower competitive power have been defeated and the trees with the vigorous root system and higher competitive power would be overcome. So the weaker trees those can not obtain the Water requirements, inevitably affected by crown dieback and eventually die because of gradually reduction of soil moisture and increasing the competition. Comparison of live and dead trees showed that these two groups have very different competitive conditions and environment. This result is agreed with results of Linares et al, 2010 and Das et al, The focal dead trees have smaller crowns and neighboring trees around them located with the closer spacing, so that their density is high. So in such situation focal trees are under more crown and root competitive pressure from neighboring trees and are most affected. Also the crown size of neighbors of dead trees was larger. This group of adjacent trees have a stronger root system due to having the larger crown and thus their crown and root competitive force on focal trees are more and can predisposed them to die. But in contrast the focal live trees had a larger crown and commensurate with it have a larger root system that can successfully obtain the water requirement. Their neighbors had less density and smaller crown width and located in further distance from focal trees. Thus competitive space of the focal live trees was larger and less affected by neighbors. At a result the focal live trees less affected by drought stress and they do not die. However in sever drought conditions- similar to the occurred drought in Zagros forests and also in study area- they are suffering from mild to moderate dieback. This result is agreed with results of Linares et al, 2010 and Das et al, It is interesting that in units and in stands with higher amount of tree dieback and mortality, the amount of tree competition was higher too. Also in units and in stands with lower amount of tree dieback and mortality, the amount of tree competition was lower too. Results of Pearson correlation and PCA ordination confirm this claim. This result is consistent with the results of Olano et al, 2009 and Dwyer et al, Totally we conclude that although competition is of predisposing factors effective on tree mortality, but its effect and performance in the presence and combined with other environmental factors such as topography and stand structure has found concept and its effect has varied due to variation of them. So we can suggest that for studying the phenomenon of tree dieback and mortality must be see all of the effective factors together and have an ecosystem perspective, because all of the environmental factors forming the forest ecosystem 2214

8 are contributor in development and changing of forest condition and variation of tree mortality amount. In this case we can identify the occurring factors of this phenomenon and provide the guidelines to its control. For example whenever we find that amount of tree competition has direct relationship with stand level characteristics such as stand density and canopy density and also with tree level characteristics such as crown size and indirect relationship with distance of neighbors from focal trees, certainly we can suggest that thinning of dense stands can help to reduce the competition between trees to obtain the water requirements and decrease the drought stress and so Improve the health of trees. Or by pruning and Canopy volume reduction of large trees we can decrease their water requirement amounts, improve the health of them and so decrease their competitive pressure on smaller trees. Certainly understand the performance of competition factor and importance of proposed strategies to control its effects seem to matter when we know that the studied forests similar to most of zagros forests are pure and have a little mix, thus their intra-species competitive force is high. Because trees often formed from oak species that have similar root systems which use from similar soil depth and have high root competition. 5. Conclusion There is tree competition with varying intensities across the forests of our study area and Affect on the incidence of tree dieback. However, the amount of tree competition changed due to stand and tree characteristics, but the amount of soil moisture available in different topographic positions affects on the intensity of tree competition in the competitive environments. There is no doubt that in terms of stress, such as drought occurred in these forests, lack of soil moisture increases the amount of tree competition and more dieback of trees. So the competition factor as an indicative the real situation of forest, can help us in practical silviculture and protective management of forests. 6. References 1. Bravo-Oviedo A., Sterba H., del Rio M., Bravo F., (2006), Competition-induced mortality for Mediterranean Pinus pinaster Ait. and P-sylvestris L. Forest Ecology and Management. 222, pp Bugmann H., (2001), A review of forest gap models, Climate Change 51, pp Das, A.J., Battles, J.J., van Mantgem, P.J., Stephenson, N.L., (2008), Spatial elements of mortality risk in old-growth forests. Ecology 89, Das, A., Battles, J., Stephenson, N.L., van Mantgem, P.J., (2011), The contribution of competition to tree mortality in old-growth coniferous forests, Forest Ecology and Management, 261, pp Dwyer, J.M., Fensham, R.J., Fairfax, R.J. and Buckley, Y.M., (2010), Neighbourhood effects influence drought-induced mortality of savanna trees In Australia, Journal of Vegetation Science, 21(3), pp Eid, T., Tuhus, E., (2001), Models for individual tree mortality in Norway. Forest Ecology and Management 154, pp

9 7. Franklin, J.F., Shugart, H.H., Harmon, M.E., (1987), Tree death as an ecological process, Bioscience 37, Olano, J. M., Laskurain, N.A., Escudero, A., De La Cruz, M., (2009), Why and where do adult trees die in a young secondary temperate forest? The role of neighbourhood, Annals of Forest Science, 66, pp Kabrick, J.M., Dey, D.C., Jensen, R. G. and Wallendorf, M., (2008), The role of environmental factors in oak decline and mortality in the Ozark Highlands, Forest Ecology and Management, 255(5-6), pp Linares, J.C., Camarero, J. J., Bowker, M.A., Ochoa, V., Carreira, J.A., (2010), Stand structural effects on Heterobasidion abietinum-related mortality following drought events in Abies pinsapo, Oecologia, 164, pp Linares, J.C., Camarero, J.J., Carreira, J.A., (2010), Competition modulates the adaptation capacity of forests to climatic stress: insights from recent growth decline and death in relict stands of the Mediterranean fir Abies pinsapo, Journal of Ecology, 98, pp Monserud, R.A., Ledermann, T., Sterba, H., (2004), Are self-thinning constraints needed in a tree-specific mortality model? Forest Science, 50, pp Moravie, M.A., Robert, A., (2003), A model to assess relationships between forest dynamics and spatial structure, Journal of Vegetation Science, 14, pp Temesgen, H., Mitchell, S.J., (2005), An individual-tree mortality model for complex stands of southeastern British Columbia. West, Journal of Applied Forestry, 20, pp Waring, R.H., (1987), Characteristics of trees predisposed to die. Bioscience 37, pp Yang, Y.Q., Titus, S.J., Huang, S.M., (2003) Modeling individual tree mortality for white spruce in Alberta, Ecol. Model. 163, pp