303Famiani et al.—Effects of shelters on olive

F. FamianiP. ProiEttim. michElim. Bocoa. Standardi

dipartimento di Scienze agrarie e ambientali Università degli Studi di Perugia Borgo XX Giugno, 7406121 Perugia, italyemail: ffamiani@unipg.it

F. FErranti l. rEalE

dipartimento di Biologia Vegetale e Biotecnologie agroambientali e ZootecnicheUniversità degli Studi di Perugia Borgo XX Giugno, 7406121 Perugia, italy

Abstract a 3-year study was conducted to evaluate the effects of tree shelters on young olive (Olea europaea) tree growth and physiology. the trial was carried out in the apulia region in southern italy using 1-year-old plants of the cultivar ‘Coratina’. Four different types of polypropylene shelters were tested: 75-cm-high brown; 90-cm-high brown; 75-cm-high green; 120-cm-high light-green vented, with holes in their basal part. Both green shelters significantly increased the vertical growth of the trees, especially the 120-cm light-green vented ones. Shelters did not increase the amount of dry matter produced, but they modified its allocation within the tree, favouring vertical growth and the development of shoots in the apical parts of the trees.

Keywords leaf anatomy; establishment of new orchards; photosynthesis; pruning

Short communication

Effects of tree shelters on young olive (Olea europaea) tree growth and physiology

INTRODUCTION

tree shelters were originally developed in the United Kingdom to increase the survival of newly transplanted forest plants by protecting them from animal browsing. in their basic form they are individual nets or plastic protections that are placed around the trees at planting. they are generally constructed from plastic-based material (usually polypropylene) that may be translucent or coloured. the section shape can be either round, square, triangular, or polygonal. the height is usually in the range of 40–150 cm and width/diameter can vary from a few cm (wrapping shelters) to c. 15 cm; in some instances it can reach 30–35 cm (used for conifers). as a result of their use, it was soon observed that, other than merely protecting trees, shelters also enhanced the tree height of several species, with effects that appeared to be species-dependent (Frearson & Weiss 1987; Potter 1988; Burger et al. 1992; Buresti & Sestini 1994; West et al. 1999). For example, height increases of 60% and 600% were observed with cherry and oak seedlings, respectively (Frearson & Weiss 1987; Potter 1988). Shelters have subsequently been successfully used for nursery plant production and tree establishment in arboriculture for wood production, reforestation, landscape revegetation, and urban environments (Burger et al. 1992; Buresti & Sestini 1994; West et al. 1999). increases in temperature, relative humidity, and co2 concentration within the shelter have all been suggested as probable causes for the increased growth (tuley 1985; Frearson & Weiss 1987; Potter 1988, 1991; Burger et al. 1992). however, the nature of the relationship between these environmental characteristics and their effect on the physiology and growth of sheltered trees is not clear. a positive effect on growth could also be related to increased leaf area, chlorophyll retention promotion, and reduced water-use observed in sheltered plants (Burger et al. 1992; minter et al. 1992; Kjelgren 1994). in addition, shelters tend to prolong the net effective growing

H05123; Online publication date 11 August 2007 Received 17 October 2005; accepted 8 February 2007

New Zealand Journal of Crop and Horticultural Science, 2007, Vol. 35: 303–3120014–0671/07/3503–0303 © the royal Society of new Zealand 2007

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season, giving plants an extended period of more suitable temperatures, and reduce weed competition (Ponder 1994; dubois 2000). however, negative effects have also been ob-served in some studies. the taller plants produced with shelters often showed reduced stem calipers (tuley 1981; Smith 1983; lantagne et al. 1990; Kjelgren 1994; Burger et al. 1996). Sometimes a smaller root system was produced, causing potential stability problems for the plants after removing the shelter (Potter 1991; Burger et al. 1992; mayhead & Jenkins 1992; mayhead & Boothman 1997). Shelters reduce light intensity and can also be responsible for conditions leading to suboptimal air temperatures and decreased winter hardiness (Kozlowski 1979; mebrahtu & hanover 1991; Kjelgren 1994; Kjelgren & rupp 1997; Kjelgren et al. 1997; Swistock et al. 1999). Few data are available on the evaluation of the mechanism by which shelters determine their effects on plant physiology. Shelters were reported to reduce water use but not drought avoidance (Kjelgren 1994; Kielgren et al. 1997), to increase specific area of leaves growing inside them and to have an effect on photosynthesis that was dependent on the tree species (Kjelgren 1994; Kjelgren et al. 1997). data on the relationships or interactions between initial plant size and shelter dimension and colour are also sparse (Buresti & Sestini 1994; Kjelgren et al. 1997). the effects of shelters have also been evaluated on agriculturally important plant species, such as coffee (Kjelgren 1994), cherry rootstock (hammatt 1998) and, more recently, grape (lavezzi et al. 2001). in grape, shelters enhanced vine survival, shoot and root growth, disease avoidance, and improved weed control and pruning (lavezzi et al. 2001). these positive results have stimulated an interest in evaluating the use of shelters on other fruit species to accelerate initial growth and fruiting of new plantations as well as to simplify the application of common cultural practices. the two objectives of this work were: (1) to evaluate the effects of shelters on olive tree growing; and (2) to better understand the mechanism through which the shelters modify tree growth and assimilate availability.

MATERIALS AND METHODS

the trial was carried out in the apulia region in southern italy. the apulia region is the area of high-est olive production in italy, representing 35–40%

of total italian olive production. it is a relatively flat and low-lying region characterised by mild temperatures during winter (average minimum and maximum temperatures of c. 4.0°c and 13.0°c, respectively), high temperatures during summer (average minimum and maximum temperatures of c. 19.0°c and 31.0°c, respectively), and an annual rainfall of c. 600 mm. The experiment was carried out in low-lying flat land where the soil is sandy-clay like (c. 40% sand and c. 30% clay). in February 2002, a new olive orchard was established using 1-year-old grafted plants of ‘Coratina’, one of the main olive cultivars in the apulia region. Four different types of shelters (tubex, aberaman, United Kingdom) were tested: 75-cm-high brown; 90-cm-high brown; 75-cm-high green; 120-cm-high light-green vented. The first three were “Standard treeshelters”. the fourth one included an integral ventilation system consisting of a series of holes at the base of the tube that created a “chimney effect”. trees without shelters were used as controls. the polymer used to make the shelters was an ethylene polypropylene block copolymer with low flow properties, high impact resistance and high stiffness. the polymer was UV stabilised with a hindered amine light stabiliser (halS) to provide a long and predictable useful life. the two brown shelters were coloured with the same brown pigment concentration (mainly composed of ferric oxide). the two other tree shelters were coloured with the same green inorganic pigment, but the concentration of green pigment in the 120 cm vented shelters was less, to increase the light transmission properties of the shelter. For purposes of clarity, this product (tubex Ventex, EU Patent no. 0558386) will be described as 120-cm light-green vented shelter throughout the text. Forty plants were used per treatment. they were randomly distributed as rows in groups of five, with a spacing of 3 × 3 m. this is not the normal spacing used in olive production, but it was used in this experiment which only lasted 3 years. immediately after planting, shelters were placed over the plants and the base was inserted 5 cm into the ground. Shelters were then firmly attached to supporting stakes placed on the north side of the tube. For the first 2 years of the study, temperature (t), relative humidity (rh), and photosynthetically active radiation (Par) inside and outside the shelters (control plants) were measured on 30 trees per treatment, once for each season, using a thermohygrometer (hi 8064, hanna instruments, Padova, italy) for t and rh and a quantum sensor

305Famiani et al.—Effects of shelters on olive

(li-185, licor, lincoln, nebraska, United States) apparatus for Par. tree height and basal stem diameter 10 cm above the collar were measured and recorded at planting time and periodically thereafter (see Fig. 3 and Fig. 5 for measurement dates) on 30 trees per treatment. the number of nodes and the length of internodes on the stem were recorded at planting and after c. 1 year (January 2003) on 10 control plants and on 10 plants with light-green 120-cm vented shelters. in January 2003, the area of leaves located inside and outside the shelters was determined by using a leaf area meter (hayashi denkoh co., model aam-7, Tokyo, Japan) on 4 leaves per tree. Specific leaf weight (dry weight (dW) per unit of leaf surface – mg dW cm–2 of leaf) was then calculated after drying them to a constant mass (dry weight) in a forced-air oven at 90°c. in January 2003 and February 2004, on three plants per treatment, the proportions of shoots and leaves located inside and outside the shelters were estimated by weighing. two years after planting (February 2004), before pruning, three plants per treatment were carefully removed from the soil using a spade and, after washing the roots to remove any soil, dry weight of roots, trunk, branches, shoots, and leaves was determined by drying all of the different parts to a constant dry weight in a forced-air oven at 90°c. in February of each year pruning was performed and trees were trained according to a free vase system with a free trunk of 1–1.2 m height. Some vigorous shoots were also eliminated in summer. Weight of all pruned material was recorded and summed up. in general, pruning was applied by limiting the elimination of shoots to a minimum. cutting regarded only very vigorous shoots located in inappropriate positions. Bending and pinching off of shoots were also used as an alternative to cutting. Periodically, every 1–2 months, visual obser-vations were made of the overall appearance of the trees (leaves, shoots, buds, branches, and trunk), to evaluate possible changes induced by shelters on the susceptibility of plants to biotic and abiotic adversities. net leaf photosynthesis was determined in June, July, and october 2002. Photosynthetic rate measurements were taken on cloudless days, from 0900 to 1100 h in the morning, on three different kinds of leaves—fully-expanded leaves present on the stem at the time of planting (“old” leaves); fully-expanded leaves grown inside the shelters; fully-expanded leaves grown outside the shelters. all measurements were made using a portable gas

exchange analyser (lca-2, analytical development co., hoddesdon, herts, United Kingdom) and a type Plc(n) Parkinson leaf chamber. according to previous studies detached leaves can be used for gas exchange measurements if immediately put in the chamber and exposed perpendicularly to sunrays (incoming PPFd 1400–1600 µmol m–2 s–1) (Proietti & Famiani 2002). depending on the treatment and the kind of leaf used, measurements were taken in full light or after a piece of shelter was placed over the chamber (cut from the respective type of shelter used for the tree considered) so that light levels and quality were similar to those inside the shelter. all measurements were taken under steady-state conditions and after c. 1 min, necessary to reach equilibrium. Four replicates per treatment were used for all net leaf photosynthesis measurements. to determine leaf lamina and palisade thickness, leaves grown inside and outside the shelters and leaves of the control trees were collected, fixed in glutaraldehyde (5% v/v) for 12 h at room temperature and post-fixed in osmium tetroxide (1% w/v) for 4 h (both in 0.075M cacodylate buffer, ph 7.2). after dehydration with a graded ethanol series and propylene oxide, the samples were embedded in resin (Epon 812 resin, 2-dodecenylsuccinic anhydride and methylnadic anhydrid mixture; Sigma-aldrich, St.louis, United States) (loreto et al. 2001). transversal and longitudinal sections (1.5 µm thick) from each sample were stained with toluidine blue and mounted in Eukitt medium (Sigma-aldrich, St. louis, United States) for light microscopy observation. Photomicrographs were taken using a photomicroscope (dmr hc, leica, Wetzlar, Germany). leaf thickness and palisade parenchyma height were measured with an image analysis software (im1000, leica, cambridge, United Kingdom). For all the anatomical leaf observations 5 replicates per treatment were used. T, RH, and PAR data are reported in the figures as means ± standard error. all other data were statistically analysed by anoVa, according to a randomised block design, and averages were compared by the Student-newman-Keuls test.

RESULTS

in summer, temperatures measured inside the shelters were higher than in the control (Fig. 1a). in the other seasons differences were smaller or the values were similar to the control, particularly the light-green 120-cm vented shelter. in general, the

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relative humidity within the shelters was higher than in the control, with the exception of the light-green 120-cm vented shelter which showed values lower or similar to the control (Fig. 1B). the shelters reduced the light intensity inside them. the green 120-cm

Fig. 1 Effects of different types of shelters on: A, air temperature; and B, relative humidity around the trees. Each value is the mean of 30 replicates taken for each season in the first 2 years of the experiment ± standard error.

Fig. 2 Effects of different types of shelters on light intensity in shelters with plants inside. Each value is the mean of 30 replicates taken in spring and summer of the first year of field growth ± stand-ard error.

vented shelter allowed penetration of a proportion of light greater than the others (Fig. 2; data not shown for the second year of growth). In general, green shelters significantly increased the height of the trees (Fig. 3). the greatest increase

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Fig. 3 Effects of different types of shelters on tree-height growth. means within the same date accompanied by the same letter are not significantly different at P ≤ 0.05. Each value is the mean of 30 replicates.

Fig. 4 Effects of green 120 cm vented shelter on: A, number of nodes; and B, internode elongation evaluated at the end of the first year of field growth. Means within the same date in a and means in B accompanied by the same letter are not significantly different at P ≤ 0.05. Each value is the mean of 10 replicates.

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in height was obtained with the 120-cm light-green vented shelters, where c. 1 year after planting (January 2003) the trees were taller than control trees 2 years after planting (February 2004). Both types of green shelters induced greater tree-height increments in the first year and these differences were maintained in the second and third year. one year after planting, the number of nodes along the stems of control plants and plants in the 120-cm light-green vented shelters was similar (Fig. 4a), whereas the sheltered plants showed longer internodes in the stem portion grown inside the shelters (Fig. 4B). during the first 2 years of growth, trees with brown shelters showed a smaller trunk cross-section than the control (Fig. 5). at the end of 2004 no significant differences in trunk cross-section were detected among the different treatments. after 2 years, dry matter in the roots, trunk, branches, shoots, and leaves of trees grown in both types of green shelters was similar to that of the control, whereas values were lower in trees grown in the brown shelters (Fig. 6). After 1 year of growth, a significant proportion of shoots and leaves were still located inside the shelters (38%, 55%, 24%, and 65% of total shoots in weight and 18%, 29%, 23%, and 48% of total leaves in weight, respectively, for brown 75-cm, brown 90-cm, green 75-cm, and 120-cm light-green vented shelters), especially in the 120-cm light-green vented shelters. after 2 years of growth, the proportion of leaves inside the shelters decreased substantially as a result of the growth of the trees outside the

Fig. 5 Effects of different types of shelters on trunk cross-section growth. means within the same date accompanied by the same let-ter are not significantly different at P ≤ 0.05. Each value is the mean of 30 replicates.

shelters (8%, 11%, 7%, and 5% of total shoots in weight and 7%, 7%, 6%, and 6% of total leaves in weight, respectively). At the end of the first year the leaves inside the shelters generally had a larger average surface area (4.53 ± 0.47 cm2 leaf–1) and a lower average specific-weight (17.43 ± 1.96 mg dW cm–2 of leaf) than the leaves outside the shelters (on average +7% and –21%, respectively). leaves grown inside the shelters had lower lamina thickness and palisade height than leaves in corresponding positions on the control (table 1). the shelters did not affect the anatomy of leaves grown outside them compared with leaves in corresponding positions on control trees (leaf thickness and palisade parenchyma height were, respectively, 403 ± 5 µm and 160 ± 8 µm for the control and 416 ± 9 µm and 145 ± 8 µm

for the green 120-cm vented shelter). all leaves inside the shelters had lower net leaf photosynthesis values than the control (Fig. 7). leaves inside the 120-cm light-green vented shelters had higher values than those in the other shelters. at the times considered, net phtosynthesis of “old” leaves and leaves grown inside the shelters was higher if exposed to full light, but, in general, values as high as those of leaves from control plants were not reached (data not shown). no differences in net photosynthesis between leaves of control and those grown outside the shelters of sheltered trees were observed in the first (Fig. 7) and in the second year (data not shown) of growth. no damage as a result of high temperatures was observed during the experimental period. no

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significant effects of shelters on biotic adversities were observed. Shelters prevented damage caused by harrowing during soil tillage, whereas 6–7% of control trees showed some damage. occasionally, weeds grew inside the shelters in spring but, in general, they died during the summer. in sheltered trees, pruning was easily performed by eliminating, pinching off, or bending shoots that were in an inappropriate position. Shoots which had grown vertically inside the shelters were bent or pinched off, depending upon their particular location on the trunk. control trees required more severe pruning than sheltered ones (Fig. 8), because they had more vigorous branches located at the basal portion of the trunk.

DISCUSSION AND CONCLUSIONS

in general, the profiles of temperature, relative humidity, and light observed in the shelters confirmed what has been already reported in other studies, that is the tendency to increase temperature and relative humidity and reduce light intensity (Burger et al. 1992; Kjelgren 1994; Kjelgren et al. 1997; Sharpe et al. 1999; lavezzi et al. 2001; Johansson 2004). in the present study, the 120-cm light-green vented shelters reduced light intensity less than the other shelters. its integral ventilation system determined a lower increase of temperature and relative humidity values which were lower than or similar to those of the control, showing an opposite behaviour compared with the other shelters that increased relative humidity. the extent of increase in tree height was related to the type of tree shelter that was used. the greatest increases in height were obtained with the green shelters, especially the 120-cm light-green vented

Fig. 6 Effects of different types of shelters on dry matter allocation in the different organs of the trees at the end of the second year of field growth. Means accompanied by the same letter are not significantly different at P ≤ 0.05. Each value is the mean of 3 replicates.

Table 1 characteristics of leaves of olive (Olea europaea) trees grown inside four different types of polypropylene shelters at the end of the first growing season. in each column, means accompanied by the same letter are not significantly different at P ≤ 0.05. Each value is the mean of 5 replicates.

leaf thickness Palisade (µm) parenchyma height (µm)

no shelter (control) 415 b 150 bBrown 75 cm 355 a 118 aBrown 90 cm 352 a 115 aGreen 75 cm 375 a 123 alight-green 120 cm 380 a 125 a

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Fig. 7 Effects of different types of shelters on net leaf photosynthesis during the first year of field growth. Means within the same date accompanied by the same letter are not significantly different at P ≤ 0.05. Each value is the mean of 4 replicates.

Fig. 8 Effects of different types of shelters on the cu-mulated amount of pruned material in the first 3 years of field growth. Means accompanied by the same letter are not significantly different at P ≤ 0.05. Each value is the mean of 30 replicates.

ones. this is in contrast with a study conducted on Quercus robur in which brown shelters gave better results than green ones (Buresti & Sestini 1994). during the current experiment, it can be estimated that young trees (1-year-old trees) gained 1–1.5 years of growth in height with the green shelters. the greatest increase in height of sheltered trees was the result of a greater elongation of the internodes grown inside the shelters. this growth advantage persisted in successive years. the shelters substantially affected the internode length of the stem portion grown inside them, but the influence of the shelter on height decreased once the trees had grown beyond the height of the shelter. at the beginning of the experiment, brown shelters reduced the diametrical growth of the trunk compared with the control, but after 2–3 years of growth, brown sheltered trees had recovered. the examination of the amount of dry matter produced per tree showed that the increased height of green sheltered trees was not the result of an increase in the dry matter production, but of a different allocation of assimilates which favoured height growth and development of shoots in the distal part of the trees. Similarly in other species, no substantial differences

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were found in biomass production between sheltered and unsheltered plants (West et al. 1999; lavezzi et al. 2001). Photosynthesis of leaves inside the shelters was negatively affected by the lower levels of light. however, the higher amount of light that was transmitted by the 120-cm light-green vented shelters compared with the other shelters allowed higher photosynthesis values and this could have contributed to the increase in the amount of assimilate available to the trees. limitations on photosynthesis were only observed on leaves grown inside the shelters; therefore, the shelters did not present indirect or detrimental effects on the photosynthesis of leaves grown outside them. the larger surface area and lower lamina and palisade tissue thickness of leaves grown inside the shelters represent a form of shade acclimatisation (Bjorkman 1981; Proietti et al. 1988). considering the overall outcome of the study regarding plant growth and the implications for the use of shelters on cultural practices, it is possible to draw some practical indications concerning the potential impact of using tree shelters for the establishment of new olive orchards. the use of green shelters (75 cm and 120 cm vented ones) with young plants (1-year-old, 50–60 cm high at planting) provides an interesting possibility to increase the initial rapid growth of young trees and, at the same time, allows a simple training pruning. that is, trees could be easily trained to a free vase system with a trunk 100–120 cm high which is essential for mechanical harvesting with trunk shakers. in all instances shelters prevent the damage to trees during soil tillage, facilitate the eventual application of herbicides, protect the trees from the eventual browsing of animals and do not seem to increase the susceptibility of the trees to biotic or abiotic adversities. the economic incentive to use the shelters has to be evaluated taking into consideration the low price of young plants, and the indirect advantages in terms of pruning, soil management, etc., versus the cost of purchasing and installing the shelter. no cost for removing the shelters used in this work has to be considered, since such protections open automatically along a laser-generated perforation forming a line of weakness in case the trunks fill the shelters and photodegrade naturally in some years time.

ACKNOWLEDGMENTS

We are grateful for the financial support provided by Tubex limited for this project, and for the valuable technical expertise and facilities provided by Professors Gerardo

Fornelli and Riccardo Liso of the “Istituto Tecnico Agrario-Umberto i”, andria (Ba), italy.

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