ECOPHYSIOLOGICAL PROPERTIES OF WHITE AND CANADIAN POPLAR IN HABITATS WITH VARIOUS WATER REGIMES

In this paper are presented the analyzes of some morphometric properties, leaves area stomata density and the intensity of leaves transpiration of white poplar, Populus alba L. and canadian poplar, Populus x canadensis Moench ‘I 214’ in populations at (1) flooded (wet) and (2) unflooded (dry) habitats on the Ada Ciganlija river island during the growing season. Six trees of each species from both habitats were selected, with similar height, age and physiological condition. The morphometric analysis of the analysed poplar species show that the type of habitat did not influence tree height and trunk as well as the leaves area. However, crown width and trunk diameter were greater in the wetter habitat. It was found that the intensity of transpiration of Populus x canadensis ‘I 214’ trees had higher average values (0,73 gH2Odmˉ2hˉ1) than white poplar (0,68 gH2Odmˉ2hˉ1). Transpiration of both taxa was more intensive on the drier than on the flooded habitats. By analyzing the number and size of stomata on leaves of the investigated Populus x canadensis ‘I 214’ trees, it was found that stomata are more numerous but of smaller size on the leaves from drier habitats. It could be concluded that both species could have impact on microclimate conditions by increasing air moisture content and decreasing air temperature but Canadian comparing to white poplar, slightly greater. UDK: 581.11+581.522.5]:582.681.81 (497.11 Ada Ciganlija) UDK: 630*161.1+630*181.31]:630*176.818.1 (497.11 Ada Ciganlija) Оригинални научни рад DOI: 10.2298/GSF1410073D


INTRODUCTION
Predictions of future changes in climate are uncertain, but it is very likely that global warming will be accelerated by continued and increasing emissions of CO 2. Widespread changes in extreme temperatures have been observed over the last 50 years.Cold days, cold nights and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent.
Policy makers are seriously concerned with the fact that human-induced climate change is causing increasing risks of floods and droughts, and they need to be able to identify areas where these will occur.The risk of landslides and fires is also influenced by the type of vegetation present in any particular area (IPCC, 2007).Increases in atmospheric carbon dioxide influence higher photosynthetic rates and increased plant water-use efficiency (N o b e l , 2005).Some authors (Wu l l s c h l e g e r and N o r b y 2001, Wu l l s c h l e g e r et al. 2002) warn that this could have very bad consequences on stand-level water relations.
The influence of vegetation on microclimate, especially in urban areas, is avery important factor in alleviating the consequences of this warming.The most important physiological plant process, transpiration, is a major consumer of solar energy on the Earth.Plants through transpiration mitigate summer air temperature indirectly, by increasing its relative humidity.If it is known that the surface of leaves of woody plants can be an average of several tens of times larger than the area of land on which they grow, it is clear that the air in the surrounding area has higher moisture content than the areas with no vegetation.A single isolated tree can lose 200-400 liters of water per day.For every kg of tree dry matter production several hundred kilograms of water are necessary (K o z l o w s k i and P a l l a r d y, 1997) .
Transpiration is the process of releasing the water from living plants and can be conducted through open stoma, cuticle, and periderm lenticels.The driving force behind this process is lower water potential of plants in relation to land and higher water potential in relation to the atmosphere.Plants use this gradient to transport water from the soil through the plant and then pay to the atmosphere without their own energy consumption.Plants have an efficient system for regulating the water regime that is strongly influenced by external factors, especially the intensity of solar radiation.Specifically, photosynthesis uses only 0.5-2% of energy that reaches the leaves, and 96-97% is spent on the process of transpiration and heat transfer depending on the type and structure of leaves.
Stomata density varies according to plant age and is directly influenced by environmental conditions.However, such characteristic was positively related to CO 2 assimilation (A b r a m s et al., 1994), due to the positive relation between stomata density, gas exchanges (A r a u s , 1986) and greater stomata conductance (B o a r d m a n , 1977).
Stomata are among the most important physiological features of the body of a tree (G r a v a n o et al., 2003).Their characteristics can help determine the nature of physiological processes, water regime and photosynthesis in a specific plant.Plants with more stomata per unit area have more transpiration intensity and photosynthetic production.Generally, a tree with few stomata per unit of leaf surface tends to have large stomata.K o z l o w s k i and P a l l a r d y (1997) reported the variations in the stomata size and frequency among species and genotypes.Populus maximowiczii A.Henry x P. nigra L, Acer negundo L. and A. saccharium L. had many small stomata and Populus deltoides W.Bartram, P. nigra L., and Ginkgo biloba L. had few but large ones.
Populus alba L. inhabits a habitat rich in moisture and light, most commonly by rivers, lakes and canals.It is tolerant to slightly acidic or salty soils.It has a very rapid growth and can reach a height of 40 m and a crown width of 12-15 m.Formed leaves are simple and dimorphic, on the long shoots they are ovoid and lobular (3-5 lobes) whereas thez are ovate on short shoots.The length of the leaves is from 4 -7 cm, and on the edges they are coarse and blunt toothed.The underside of the leaf is covered with white hair and the face is dark green, which makes it an effective ornamental tree.
Populus x canadensis 'I 214' was developed by crossing European and American black poplars.The trunk is quite right, well established and growing in whorls.It usually has better technical characteristics than the parents and it is therefore cultivated.The crown of the poplar is narrow and half-pyramidal.The leaves are dark green, slightly leathery and glossy, serrated.It is favorable for planting in urban areas because of its resistance to harmful gases.The best-known varieties and clones are : 'Serotina', 'Robusta','Regenerata','Marilandica','I 214',and 'I 154'.Trees of Canadian and white poplar are very numerous in the area of Ada Ciganlija river island.The share of white poplar is 57,8%, while clone I 214 participates with 18.2% in the total covered surface (Posebne osnove gazdovanja šumama za G.J. ,,Ada Ciganlijа̕ ', 2004.This paper analyzes the intensity of leaves transpiration, leaves area and stomata density of Populus alba and Populus x canadensis 'I 214' in populations of ( 1) flooded (wet) and ( 2) unflooded (dry) habitats on the river island Ada Ciganlija in the Belgrade area during the growing season 2011.This kind of investigation can be useful in future plans of tree planting because poplars are the most efficient in water absorbing and they influence the air temperature and microclimate conditions.

Environmental conditions and vegetation
The Sava River undoubtedly had the most influence on the emergence and evolution of the Ada Ciganlija lake island: the regime change and the annual flooding of the level of maximum and minimum water level is a crucial influence on the properties of geological materials and profiles, the origin and evolution of ground and relief formations, the maintenance of specific micro-climate and the evolution, structure and properties of vegetation.
The geological structure of the soil are powerful tertiary layers of clay which are deposited in Quaternary sand, gravel, and their different mixtures are shifting toward approaching the surface of the muddy sands, loess and clay.Soil type is in a wide range of very wet alluvial pararendzine and its transition to clay soils over moderately moist alluvial pararendzine.
The area is characterized by temperate continental climate with the mean temperature in the three summer months that is higher than 20ºC, mean duration of the frost-free period is from 180 to 215 days, average rainfall is 691 mm, the minimum is in February and September, a maximum in May and in June, while during the growing season (March-September) 434 mm falls, which is 60% of annual precipitation.(Posebne osnove gazdovanja šumama za G.J. Ada Ciganlijа, 2004).
The natural vegetation of Ada Ciganlija is quite diverse.At the lowest positions occurs the Saliceto Populetum Raj.community, which is close to the shore of the river, providing the stability of the surface compared to the negative effects of flooding.At a higher altitude there are communities Populetum nigrae Knapp.Community Populetum albo -nigrae Slav.covers the most areas where there are populations of gray poplar trees.A significant part of the depression is in habitats with elm and ash.The dominant species is the white poplar, which participates with 57.8% of the total volume.It builds pure and mixed populations.A significant portion of these stands is of good quality, high productivity and secondary health conditions.Populations of white poplar trees are about 62 years old.The volumetric representation, the Canadian poplar clone I 214 with 18.3% of the total volume management unit, elm with 6.1%, ash 7.9% and oak 3.5%.Other species, such as white willow, silver linden, aspen, birch, white ash, maple tree, and the tree of heaven are present together with 16.4% (Posebne osnove gazdovanja šumama za G.J. Ada Ciganlijа, 2004).

Plant material
Leaf samples were collected from twelve poplar trees of both poplar species, approximate age, height, diameter and physiological vitality, three times during the growing season (May, June, and September).Tree height was measured by laser altimeter Tru Pulse and trunk diameter by millimeter diameter.
Out of six white poplar trees and six Canadian poplar clones I-214, three are on the moister habitat, often subject to flooding, while the other three are located in a more dry and drain position.The leaves that were used for this survey were taken from the lower third of the crown and from north side position.The research was carried out during the vegetative growth period when leaves were completely developed.

Transpiration intensity, leaf area and stomata density determination
The transpiration intensity, mass transpired water per unit leaf area per unit time (gH 2 Odmˉ²hˉ¹) was determined in the laboratory by the gravimetric method.Samples of cut off leaf tops were immersed in water with paraffin on the surface to prevent evaporation.Leaf area was determined by the gravimetric method.Transpiration intensity was calculated and expressed in g H 2 Odmˉ²hˉ¹.
Stomata density was analyzed on the upper side (abaxial epidermis) and lower side (adaxial epidermis) of the leaf in six selected trees of Canadian poplar from both sites (dry and wet).The analysis was done on the fully developed, vital leaves.Samples of epidermal tissue were taken from the middle part of the plant and the middle part of leave blade near the main vein.Three samples of every leaf were taken for making permanent preparations.Determination of the number of stomata per mm² was estimated using light microscopy with ocular scale and stage micrometer scale.The mean value of these three calculations was considered mean value for a plant and mean value for 6 plants, as a mean value for a population.

Statistics
All data were processed in the statistical analysis of data STATGRAPHIC.The conclusions were made on the basis of the analysis of variance (ANOVA) and LSD test was used to determine significant differences among the mean values of the treatments (p<0.05).The analysis of white poplar and Canadian poplar trees in both habitats indicates that trees in the dry habitat are higher (16.33 m; 19.67 m, respectively), but not at a significant level.The larger diameter at breast height was noticed in both poplar species in wet sites (2.83 m; 2.93 m, respectively), but a significantly lower value was found in white poplar in the dry site (1.73 m).Trunk height clear of branches did not show significant differences between the species in both sites.Crown width was greater in the wet site for both species (Table 1).Crown heigt was calculated as the total tree height -trunk height clear of branches.The presence of rot was observed on the open trunk and thicker branches of the most of trees as well as phytopathological disease and entomological damage.There are some dry branches in lower parts of the crowns.The presence of semi-parasite plant Viscum album L was also noticeable.

Leaf area
Data on Graph 1. show that site type does not significantly influence the leaf area in both poplar species.White poplar trees have lower value (average on both sites 33,62 cm 2 ) than Canadian poplar trees (average on both sites 52.47 cm 2 ) .

Transpiration intensity
The intensity of transpiration in Canadian poplar on the wet habitat was from 0.28 to 1.20, while on dry habitat from 0.52 to 1.93 in gH 2 Odmˉ²hˉ¹.The intensity of transpiration in white poplar on the wet habitat ranges from 0.15 to 1 g H 2 Odmˉ²hˉ¹ and on the dry habitat from 0.50 to 1.96 gH 2 Odmˉ²hˉ¹ (Graph.2).
It can be concluded that the average values of transpiration intensity in Canadian poplar and white poplar trees on dry sites were significantly (p<0.05)higher (0.96; 0.93 g H 2 Odmˉ²hˉ¹, respectively) than in the wet habitat (0.50; 0.43 g H 2 Odmˉ²hˉ¹, respectively).
The average intensity of transpiration in Canadian poplar is higher than in the white poplar, but not statistically significant.The lowest transpiration rates were found in white poplar in the wet site (0.43 g H 2 Odmˉ²hˉ¹) (Graph.2).

Stomata density in Canadian poplar
Table 2 shows the number of stomata on the upper and lower leaf epidermis in the Canadian poplar tree from two habitats.It a significantly (p<0.05)higher number of stomata was observed in the dry site (52; 42.67) than in the wet site (40.33; 27).

DISCUSSION
Morphometric parameters (Table 1) show that Canadian poplar trees have a higher volume of crown (average height 14.01 m and width 15.12 m) than Populus alba (average height 11.47 m and width 14.72 m).
Leaf area and transpiration influence on higher photosynthetic efficiency and biomass production (D j u k i ć et D j u n i s i j e v i ć , 2002).
According to the comparative analysis of the intensity of transpiration of Canadian and white poplar in both sites, it can be concluded that the values of transpiration intensity of Canadian poplar are higher than the values of white poplar but not at a significant level.The reason for the higher transpiration intensity of Canadian poplar might be a greater leaf area, longer leaf petal, great stomata density and absence of trichoma on leaves.
In our research, the leaf area in both poplar species did not differ significantly in the dry and wet sites, which means that the environmental factors did not influence the leaf area significantly.White poplar trees have lower values of leaves area than Canadian poplar trees.
Leaf area index and stomata conductance determine the canopy conductance.However, in high radiation shorter plants and trees with large leaves are less well coupled to the atmosphere because aerodynamic conductance is small.In these communities, if transpiration did not continue at an adequate rate, high radiation loads would quickly overheat the foliage, unless the amount of radiation intercepted can be reduced by wilting or leaf curling (B e e r l i n g et al. 2001).Leaf area and transpiration can be increased by irrigation and fertilization.It was reported that in Populus deltoids W.Bartram trees irrigation and fertilization increased the transpiration by 66% and 90%, respectively and also the leaf area index from an average 1.16 in control stands to 1.45 (S a m u e l s o n et al., 2007).
Stomata analysis can help a better understanding of physiological processes, water regime and photosynthesis.The number, size, shape and arrangement of stomata are influenced by environmental factors such as the humidity of habitats, mineral nutrition, temperature, light, and other.Stomata are significant because they represent the openings through which the plant receives carbon dioxide, which is necessary for carrying out the process of photosynthesis, releases water and oxygen (D j u k i c et al. 2011).Thus, the analysis of stomata of beech on Mt.Kopaonik determined that when the stoma are at a higher altitude, the more arid habitats are smaller and higher density than those in the forest community at a lower altitude (D j u k i c et D j u n i s i j e v i ć , 2002).
Stomata density varies according to plant age and is directly influenced by environmental conditions (J u s t o , 2005).However, such characteristic was positively related to CO 2 assimilation (A b r a m s et al., 1994), due to the positive relation between stomata density and gas exchanges (A r a u s , 1986) and greater stomata conductance (B o a r d m a n , 1977).
On the basis of the footprints of stomata on the leaves of Canadian poplar, it was found that trees in the wet habitat have lower stomata density than in the dry site.This confirms data that the trees in drier positions transpire more, because stomata sizes are greater and also these trees probably have much more developed root systems.The role of roots proved to be crucial for the intensity of transpiration.It is assumed that the roots of poplars in the wet habitat are probably less developed due to large amounts of water in the shallow zones of the soil (P a l l a r d y, 2007).Poplars in the drier habitat develop a stronger and deeper root in order to provide sufficient moisture from deeper soil layers.The larger the root in relation to leaf area, transpiration intensity is increased (P a l l a r d y, 2007).
Various types of vegetation and their differences in sun energy use, aerodynamic conductance and water use efficiency interact with the local climate and mitigate the negative consequences.
This research could contribute to the identification of physiological principles that can be applied to attributes of local vegetation.
The results emphasize the need for further investigation which is significant for poplar selection under the conditions of the changed climate and water regimen in the soil.

CONCLUSION
The morphometric analysis of both poplar tree taxa species has shown that the type of habitat did not influence the tree and trunk heights and the leaves area.However, crown width and trunk diameter were greater in the wet habitat.
The transpiration of both species was more intensive in the drier than in the flooded habitat.By analyzing the number and size of stomata on the leaves of the investigated Canadian poplar trees, it was observed that stomata are more numerous but of smaller size on the leaves from drier habitats.
It was found that the intensity of transpiration of Canadian poplar trees had higher, but not significantly higher, average values (0,730 gH 2 Odmˉ²hˉ¹) than white poplar (0,385 gH 2 Odmˉ²hˉ¹).
It can be concluded that Canadian poplar and white poplar, may have an impact on microclimate conditions by absorbing ground water and increasing air moisture content.Canadian poplar can be recommended for air temperature reduction because of higher crown volume, greater leaf area, longer leaf petal, great stomata density and greater transpiration intensity in both sites.
The results emphasize the need for further investigation which is significant for poplar selection under the conditions of changed water regimen in the soil.2002

Diagram 2 .
Тranspiration intensity (g H 2 Odmˉ²hˉ¹) mean values of white poplar and Canadian poplar in wet and dry sites on the Ada Ciganlija locality.Means followed by the same letters are not significantly different at p<0.05 Графикон 2. Интензитет транспирације беле и канадске тополе (g H 2 Odmˉ²hˉ¹ ) на влажном и сувом станишту на Ади Циганлији (средње вредности).Вредности означене истим словима се сигнификантно не разликују на нивоу p<0.05 P a l l a r d y S. G. (2007): Physiology of woody plants.Academic Press, San Diego G r a v a n o E, G i u l i e t t i V, D e s o t g i u R, B u s s o t t i F, G r o s s o n i P, G e r o s a G, Ta n i C (2003).Foliar response of an Ailanthus altissima clone in two sites with different levels of ozone pollution.Environ.Pollut.121 (137-146) N o b e l P. S. (2005): Physicochemical and environmental plant physiology, Burlington, MA: Elsevier/Academic Press.Posebne osnove gazdovanja šumama za G.J.,,Ada Ciganlijа̕ ̕ ̕ » 2004, Knjiga I, Beograd.S a m u e l s o n L.J, S t o k e s T.A, C o l e m a n M.D. (2007): Influence of irrigation and fertilization on transpiration and hydraulic properties of Populus deltoids.Tree Physiol.27 (5) (765-774) Wu l l s c h l e g e r S. D., R. J. N o r b y. (2001): Sap velocity and canopy transpiration for a 12-yearold sweetgum stand exposed to free-air CO 2 enrichment.New Phytologist 150 (489-498) Wu l l s c h l e g e r S. D, G u n d e r s o n C. A., H a n s o n P. J, W i l s o n K. B, N o r b y R. J. (