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The effects of treeshelters on the growth of Quercus coccifera L. seedlings in a semiarid environment J. BELLOT, J.M. ORTIZ DE URBINA, A. BONET AND J.R. SÁNCHEZ Departamento de Ecología, Universidad de Alicante, PO Box 99, Alicante 03080, Spain

Summary In a field trial, the effects of five different types of treeshelter on the microclimatic conditions, survival, aerial relative growth rate (RGR) and root growth of Quercus coccifera L. seedlings were compared with the results obtained from seedlings of the same species grown without treeshelters. The tested treeshelter characteristics were: ventilation, height and material of which they were made. During the experiment the microclimatic conditions were monitored in order to control the factors affecting the seedlings’ development. Results showed that a brown plastic protector of only 30 cm in height appears to be the most beneficial for biomass growth, both above and below ground. The more extensive development of the root system facilitates a faster growth rate as accessibility to soil water is increased. The benefit gained from such a shelter is probably due to the reduction of radiation inside the protector to around the optimum photosynthetic levels for the species, and also due to a small increase in the temperature compared with values detected in taller protectors. The shelter ventilation did not show a significant effect on root and biomass growth compared with unventilated shelters.

Introduction The land restoration of degraded areas, wildfiredamaged forests and shrubland, as well as the revegetation of old fields (EC, 1999), is a priority in semiarid ecosystems in order to prevent the process of desertification. In the driest and most wildfire-affected areas, the regeneration of a layer of vegetation that guarantees the protection of the soil during the periods of intense rainfall is extremely difficult (Abad et al., 1997). The restoration programmes of the Spanish Forestry Services frequently report very low survival and growth rates of the native and non-native introduced species (Vallejo and Alloza, 1999). Such a © Institute of Chartered Foresters, 2002

situation demands specific research in order to improve the survival of the plantations, introducing techniques of seedling treatments in the nursery, and improving the traditionally used methods of afforestation. Among the available techniques (Domínguez et al., 1999), the advantages gained by the use of any type of shelters arise both from physical protection from wind and animal browsing (Potter, 1991; Mayhead and Jenkins, 1992; Balandier et al., 1995), and from changes in the micrometeorological conditions inside the shelters surrounding the seedlings. These micrometeorological improvements, which mean increases in temperature, air humidity and reduction of Forestry, Vol. 75, No. 1, 2002

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radiation, along with the mechanical protection against wind, is a special benefit for seedlings grown in temperate and cold environments (Tuley, 1984; Potter, 1991; Ponder, 1995; Sharpe et al., 1999). Potter (1991) and Burger et al. (1996) stated that the beneficial effects of the shelters are limited to the time when the tree seedlings grow inside them. Negative impacts on diameter growth and aboveground biomass were evident in many sites and species (Burger et al., 1996). In consequence, some authors discourage the use of treeshelters or recommend the short ones (maximum 0.6 m tall) to avoid the reduction of the above-ground biomass (Burger et al., 1996; Dupraz, 1997). The use of shelters has been increased in Spain during the last decade, mainly in agricultural lands (Navarro et al., 1998), in spite of installation difficulties and the added cost in comparison with afforestation without shelters. In many parts of Spain (without arid and semiarid climates), their use is justified economically and ecologically by their positive effects on the survival and growth of planted seedlings, as animal predation is reduced (Navarro et al., 1998). However, from a practical point of view, their possible re-use in new afforestations is not applied due to the labour costs involved in removing them from the field. In semiarid regions of Spain their use is less widespread and questioned due to their added cost (i.e. 25 per cent of plantation cost for shelters and labour), and mainly because the beneficial effect on plants is unclear (Burger et al., 1997). However, their use is recommended for reducing post-transplanting shock in the restoration of degraded areas (Bainbridge et al., 1993; Vallejo, 1997). In dry and hot environments, where the mortality of seedlings is frequently high during periods of drought, the use of treeshelters has improved the survival and increased the growth rates of Mediterranean species such as Pinus halepensis Miller, Quercus ilex L. subsp. ballota (Desf.) Samp., Ceratonia siliqua L., Olea europaea L. and others (Potter, 1988; Burger et al., 1996; Costello et al., 1996; Svihra et al., 1996; Bergez and Dupraz, 1997; Navarro et al., 1998; Domínguez et al., 1999). However, there is a wide variation in the effects of the treeshelters on growth and survival rates. Such studies indicate that both the characteristics of the shelters and the species may determine the observed results, and demonstrate that certain treeshelters are not always compatible with these hot, dry

environmental conditions (Burger et al., 1996; Costello et al., 1996; Kjelgren et al., 1997; Sharpe et al., 1999). Often the effect leads to a large increase in air temperature inside the shelters along with a reduction in radiation levels (Sorensen et al., 1993). The increase in temperature inside the shelters is due to the high radiation levels in the region, or to the colours or shelter opacity; fewer opportunities to lose energy have a negative impact on tree growth (Tuley, 1985). In this context, ventilation appears to be beneficial in stimulating tree growth (Bergez and Dupraz, 2000). Conversely, the air humidity inside the shelters shows a slight or great increase (Sharpe et al., 1999; Swistock et al., 1999). Therefore, the water vapour pressure deficit (VPD) inside the shelters shows a pattern that rises until midday and then falls, which means a variation in the seedling transpiration levels during the day (Bergez and Dupraz, 1997). Such variations are fundamental in the success of the seedlings, due to the fact that water deficit is the main factor that produces stress and mortality in seedlings in dry environments (Costello et al., 1996; Navarro et al., 1998). Moreover, the treeshelters’ effect on roots is not well known (Ponder, 1995; Svihra et al., 1996; Navarro et al., 1998), and needs to be evaluated to quantify the development of the root system, which increases the seedling’s capacity to extract water from the soil. To evaluate the treeshelters’ usefulness in hot, dry environments, and the most appropriate shelter type, we measured the microclimatic conditions inside five commercial types of treeshelters used frequently in afforestation, and compared them with the conditions outside. We analysed the effect of treeshelter type (ventilated, unventilated, tall or short, brown translucent plastic or opaque grass leaves), on the biomass, length and structure of the root system and on the crown projection of the seedling development of the Mediterranean shrub Quercus coccifera L.

Material and methods Experimental site The experiment was carried out in an experimental area within the Campus of the University of Alicante (coordinates 38º21N, 0º30W, 100 m

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a.s.l.), during the period January 1997 to August 1999. The average annual temperature is 18.23ºC and precipitation 358.4 mm (INM, 1994), though precipitation does vary throughout the year and severe drought is common in the summer. No watering was done during the trial in order to reproduce field conditions. A homogeneous loam substrate (the most abundant type of soil in the region), of ~40 cm deep was spread over a surface area of 250 m2 on top of the original loam–clay soil in order to reduce spatial soil variability and guarantee homogeneous soil characteristics for root development. Although this is not a usual practice in afforestation, it was done in the experiment to avoid the spatial heterogeneity of the untreated soil normally caused by the presence of stones, other roots and discontinuities in physical structure. Experimental design A set of 60 Q. coccifera seedlings were distributed for plantation, allocated between six treatments (five types of treeshelter and a control without any type of treeshelter), with 10 replicates for each type. All seedlings were planted in January 1997 in a rectangular plot (25 10 m) following a systematic spatial pattern distribution (6 columns 10 rows). The different seedlings were randomly distributed in the vertices of a 2.25 1.5 m grid, estimated in previous experiments in semiarid conditions (Villagrosa et al., 1997), at a great enough distance to ensure that different treatments did not affect each other for the first 3 years, avoiding possible neighbour effects. The characteristics of each different type of shelter utilized in the experiment is described in Table 1. In all cases except for the EGT treatment, the treeshelters were staked to the ground with a single rod, EGT was staked with two rods. The treeshelters were placed on the surface of the soil and not inserted into the soil. All of them, except the EGT shelter, are translucent treeshelters made from UV stabilized polypropylene. The EGT is made by covering a plastic mesh with a network of Stipa tenacissima L. (esparto grass) leaves. Plant material and growth analysis In January 1997 we planted 16-month-old seedlings from a private nursery, supplied in

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containers (250 cm3) and using peat as substrate. The average size (height and basal diameter) measured just after planting were 10.28 ± 0.52 cm and 2.29 ± 0.1 mm, respectively, with no significant differences between groups of treatments. At the same time, crown projection and number of leaves and branches were counted in order to test the effects of the shelters’ on them. The relative growth rates (RGRs) in height, diameter and crown projection (Pooter and Garnier, 1999) were calculated at the same time, and survival of all seedlings was seasonally measured on eight occasions (May, July and November 1997, February, April and July 1998, March and August 1999). We calculated the crown projection using the ellipse model and measuring two diameters: maximum projection axis of the seedling and its perpendicular axis. We selected this measure because it takes into account the fact that the main objective of any land restoration in semiarid areas is to cover the soil with vegetation in order to reduce the risk of erosion (Generalitat Valenciana, 1995). We also wanted to test whether some shelter types (tall and rigid) reduce the branch extension during the first seedling years, thus affecting the soil cover potential of introduced plants. Root system analysis At the end of the study (August 1999), the root systems of 15 individual seedlings belonging to the three groups of treatment (ST, EGT and Control) that exhibited the greatest difference in aerial structure were removed and measured. This measurement was carried out by lifting the entire root system from the ground and making a digital analysis of the roots, i.e. their total length, average diameter and length of every diametrical class (every 0.5 mm), using the WinRhizoV3.6 program (Regent Instruments, Canada). The roots belonging to the root ball (developed inside the container during the nursery period) and the root growth outside the root ball since planting in the experimental plot were measured separately. The weight of the different fractions of above and below ground biomass (leaves, shoots, trunk, root ball and roots out of ball) of the 15 seedlings were also measured after drying at 80ºC for 5 days. The shoots and leaves of the removed seedlings were used to analyse biomass, specific

Key LT

VLT4

VLT8

ST EGT

Large treeshelter

Ventilated large treeshelter with moderate ventilation

Ventilated large treeshelter with maximum ventilation

Short treeshelter

Esparto grass treeshelter

UV stabilized polypropylene tube 10 cm diameter Biodegradable grass tube over a PVC mesh

UV stabilized polypropylene tube 10 cm diameter

Ventilation through the broad mesh holes

Unventilated

Ventilated along the length

Ventilated along the length

Unventilated

Ventilation type

Material opaque, the sunlight passing through the broad mesh holes

Translucent brown with eight holes of 2 cm diameter Translucent brown

Translucent brown with four holes of 2 cm diameter

Translucent brown

Colour

60

30

60

60

60

Height (cm)

92



Material

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Shelter type

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Table 1: Characteristics of the different shelter types used in the experiments

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leaf weight and surface area, and plant water potential. The total leaf area of each seedling and the average area per leaf were calculated using a scanner and computer controlled area meter system (WinRhizoV3.6 software). The specific leaf weight (SLW) was determined after removing 40 discs of 0.225 cm2 of different leaves from each treatment, which were dried at 60ºC for 1 day and then weighed. Before being lifted from the ground, the pre-dawn leaf water potential (PWP) of the 15 removed seedlings was measured using a pressure chamber system Soil Moisture 2005 (Santa Barbara, Calif.) as described by Ritchie and Hinckley (1975). The soil water content was measured at four points around the seedling at the same time as the PWP using the Time Domain Reflectometry system (Tektronic 1502C, Holland) as described by Topp and Davis (1985), with probes of 30 cm depth for the measurements. Microclimatic measurements The microclimatic conditions inside the shelters were monitored (in two shelters per treatment) by measuring air temperature, relative humidity (RH) and total radiation, which were then compared with measurements made on the control treatment. The measurement period was from January 1997 to November 1999, through nine seasonal periods each of 6–7 consecutive days, with automatic records taken at 15 min intervals. A total of 12 automatic registers of air temperature and relative humidity (Hobo H8; Onset Computer Corporation, Cape Cod, Mass.), and 12 radiation sensors (silica photocells) calibrated with a pyranometer (SP1110; Skye Instruments, Skye, UK) were distributed among the five treeshelter treatments and the control treatment. The temperature of the thermo-couple and radiation patterns were monitored automatically with a data-logger (Hobo H8; Onset); the data were scanned at 1-min intervals and recorded as 15min averages. The sensors for total radiation were placed immediately above the seedling (at 25 cm height) and held in place by a steel rod. The automatic air temperature and relative humidity sensors were placed at mid-height and mid-diameter, close to the seedling leaves, next to the trunk but without coming into contact with it, at a height of 15 cm inside the treeshelters or directly

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exposed in the control conditions. These parameters allowed the vapour pressure deficit (VPD) to be calculated. Statistical analysis Microclimate and growth variables were subjected to a one-way analysis of variance (ANOVA), and comparisons between treatment means were made using Tukey’s multiple range test and the Kruskal-Wallis test (Steel and Torrie, 1980; Fowler et al., 1998). Effects of treatments on measured variables were tested for significance at the 0.05 level of confidence. A fitted curve model based on the MichaelisMenten approach was applied to calculate the relationships between the global radiation (W m–2) within the short treeshelter (ST), esparto grass treeshelter (EGT) and large treeshelter (LT) treatments, with the direct radiation monitored in the control (C) treatment. The fitted function corresponded to the Michaelis-Menten model (Rin = (RADmax Rext)/(Km + Rext)), where Rin = radiation inside shelters and Rext = radiation outside shelters, as measured variables, and RADmax = maximum inside radiation and Km = outside radiation value, which produce the half of the RADmax inside, as the fitted parameters.

Results Microclimatic characteristics Measurements on climate inside and outside the shelters were performed throughout the different seasons; to characterize the pattern of two typical sunny day records, we chose a representative hot day in summer (24 July 1998) and a cool day in autumn (4 November 1998) in Mediterranean semiarid areas. Figure 1 shows the average daily course of air temperature (ºC) and total radiation (W m–2) inside the five types of shelter, compared with the control. Maximum levels of total radiation were similar in both periods (close to 900 W m–2 on a summer day, and ∼700 W m–2 in autumn), but the total radiation received was higher in summer due to the difference in number of sunny hours. Large treeshelters (LT and VLT8) reduce total radiation by ∼70–80 per cent of the control levels in summer and autumn. Only the

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Figure 1. Average hourly temperature (top), and solar radiation inside and outside the treeshelters (middle and bottom), on two representative sunny days in 1998 (24 July and 4 November), in the semiarid Mediterranean climate. Figure keys compare each treatment with the control.

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Figure 1. continued

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short shelter (ST) in summer changes this pattern, with less reduction at midday. In general, a strong reduction in radiation levels inside the treeshelters was observed. The fitted curve of the Michaelis-Menten model between total radiation outside and inside the shelters using all data from the studied period shows that this reduction tends to be greater in the large treeshelters (ventilated and unventilated), always below 200 W m–2, even when environmental radiation reaches 1200 W m–2 (Figure 2). This reduction is lower in the ST treatment (~50 per cent of the environmental radiation), while EGT remains at an intermediate position. The goodness of the fitted curves (R2), and the parameters of the functions are shown in Figure 2. Regarding air temperature, the daily pattern shows some differences in both experimental periods (Figure 1). Outside the shelters, summer days rise to 32ºC at midday, with a daily pattern similar to the autumn day, when the maximum air temperature is 29ºC also at midday, and with minimum values that rise to 6ºC outside the

shelter during the autumn night, but do not drop below 22ºC in the summer night. There were large differences between inside and outside the shelters, reflecting the differences in incoming radiation, in agreement with other work (Potter, 1988; Kjelgren et al., 1997). In both seasons, the temperature range outside the shelters was higher than inside the shelters, in agreement with previous results (Kjelgren and Rupp, 1997; Kjelgren et al., 1997; Swistock et al., 1999). On a summer day, the results show that inside all the large shelters (ventilated or unventilated), air temperatures were 7–10ºC higher than the control. Ventilation in shelters (VTL4 and VTL8) reduces the air temperature by an average of 5ºC in comparison with the LT treeshelter, which we assume is a positive effect for the plant. Temperature conditions are improved inside the STs, which increase the air temperature only a little in comparison with the control. Only the EGTs reduce the air temperature throughout the morning, but not during the afternoon. These patterns were different from those observed in the autumn days, when the morning temperatures outside the

600

Radiation inside treeshelter (watt m - 2 )

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Y = ((RADmax * X)/(km + X))

500 RADmax = km = Rsquare =

ST AT LT 584.6 2764.4 274.2 1023.7 9297.9 634.9 0.899 0.824 0.869

400 1/1 300

ST 200

AT LT

100

0 0

200

400

600

800

1000

1200

1400

Radiation outside treeshelter (watt m- 2 )

Figure 2. Fitted line of the global radiation (W m–2) within the short treeshelter (ST), esparto grass treeshelter (EGT) and large treeshelter (LT) compared with the direct radiation monitored in the Control treatment. The fitted functions correspond to the Michaelis-Menten model (Rin = (RADmax Rext)/(Km + Rext)), where Rin = radiation inside shelters, Rext = radiation outside shelters, RADmax = maximum inside radiation, Km = outside radiation value which produces half of the RADmax inside.

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shelters were similar to that inside. Throughout the afternoon all treeshelters increased their inside air temperature by 4–6ºC compared with the control. Ventilation in large shelters (VLT4 and VLT8) reduced air temperature by 4ºC in comparison with unventilated LTs. However, the best conditions (less warming) were achieved inside the ST and EGT shelters, the latter being the most temperate at midday, because the ambient air temperature decreased by 3ºC. In all cases, the translucent brown polypropylene shelters showed higher air temperatures than the esparto grass treeshelters, probably due to their shade effect and the easiest air circulation through the latter. The average values of air relative humidity (RH) remained quite similar in each treatment, the range of the data being narrower in the control conditions (Figure 3). Relative air humidity (%) was much higher inside the treeshelters than outside, and was statistically significant between control and the treatments (KruskalWallis test 2 = 71.977, n = 32, for P < 0.001). The highest differences between the percentage relative humidity inside and outside treeshelters was 15 per cent in the afternoon for the unventilated LT, showing other shelter values more similar to the ambient levels, probably because of ventilation holes (VLT8, VLT4) or the esparto grass net (EGT). Vapour pressure deficit (VPD) was calculated from RH and air temperature measurements, and is shown for a representative autumn day in Figure 3. The average water VPD was also similar in each analysed treatment, being the daily range of variation, but less for the esparto grass shelter (0–3.2 kPa). The VPD at noon was much higher inside the treeshelters than outside, and statistically significant between the control and the treatments (Kruskal-Wallis test 2 = 91.093, n = 32, P < 0.001). The unventilated treeshelters experienced high temperatures and VPD owing to the high temperature and the confinement of tree transpiration as already reported by Bergez and Dupraz (2000). VPD in the shelters was generally 1 kPa lower than ambient from morning through to afternoon, which is similar to previous results (Kjelgren, 1994; Kjelgren et al., 1997). The maximum estimated values of 3.2 kPa in the morning outside the shelters, were reduced to 2.3 kPa inside the EGT. In the late evening and through

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the night the VPD values converged and the pattern shows this inverse relationship. Shoot development Figure 4 shows the survival of the seedlings in July 1997, August 1998 and August 1999. In July 1997, 6 months after planting, the survival rate ranged between 100 per cent for the EGT and VLT8 treatments, and 80 per cent for ST and LT. The survival rate fell progressively until it reached values between 70 per cent in the ST treatment and 60 per cent in the VLT8, EGT and control treatments by the end of 1999. At the end of the study (30 months after planting), the survival percentages were similar (60 and 70 per cent) among treatments, without significant statistical differences, as a one-way ANOVA indicates (arcsin transformation, F = 0.035; p = 0.999). The pattern of the time-course of height, diameter and crown projection of the seedlings subjected to the different treatments is shown in Figure 5a, b and c, respectively. The increment in height was greatest in the ST treatment and lowest in the control, although such differences were only significant (P ≤ 0.05) in August 1997. The growth in terms of height was highest during the first 6 months, falling progressively thereafter. There were no significant differences in diameter growth (Figure 5b). As Figure 5c shows, crown projection was greatest in the EGT treatment, the difference being significant compared with the rest of treatments for most measurements. These results are corroborated by the relative growth rate (RGR) calculated for the whole study period (30 months). Table 2 shows the RGR for height, basal diameter, shape factor index (height/basal diameter ratio) and crown projection. Only differences for crown projection area were significant. Concerning the characteristics of leaves, number of leaves and branches per seedling (Table 3), there only appeared to be significant differences during July 1997 and January 1998, with all measurements being lower in the control treatment. The total leaf area of seedlings after 30 months of growth (in August 1999) was not significantly different from the analysed treatments (Control, ST, EGT), although it was greater in the ST treatment (Table 3). However, the average area per leaf was significantly greater in

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Figure 3. Average hourly relative air humidity (RH) and vapour pressure deficit (VPD) inside and outside treeshelters, on a representative sunny autumn day in the semiarid Mediterranean climate.


F O R E S T RY

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100

99

July 1997 August 1998 August 1999

Survival (%)

80

60

40

20

0 C

ST

LT

VLT4

VLT8

AT

Treatment

Figure 4. Survival rate (%) of seedlings belonging to the different treatments during the summers of 1997, 1998 and 1999.

the seedlings belonging to the EGT treatment (2.15 cm2) compared with 1.04 cm2 in the control treatment, while this value in the ST treatment remained in an intermediate position with 1.24 cm2. Also, the specific leaf weight (SLW) was significantly lower (P = 0.021) in the EGT treatment (14.81 mg cm–2) compared with the control treatment and ST (16.21 and 17.26 mg cm–2, respectively), there being no significant difference between both variables (Table 3). Root system Figure 6 shows the differences in length and average diameter of the roots between the control, ST and EGT treatments, distinguishing between roots in the root ball and roots growing outside the ball. There were no significant differences between either total length or in average diameter in roots in the root ball. On the other hand, the roots outside the root ball in the ST treatment were significantly longer (P ≤ 0.05) and wider in diameter (P ≤ 0.01) than in the control

and EGT treatments. The root systems of seedlings growing inside large treeshelters (ventilated or not) were excluded from our root study due to the poor development of their shoots (Table 3, Figure 5). The extension of the seedling root systems is expressed as accumulated root length, range from 2 m to 18 m. We calculated a close linear relationship (R2 = 0.659, n = 12), between root length (m) and the pre-dawn leaf water potential (MPa) of the seedlings from the three treatments (Figure 7), which suggests that the root length is positively related to the ability of the seedling to seek and consume the soil water. The longer the root, the less the seedling suffers from water stress, even in a homogeneous dry-hot soil. A possible variability in plant water status due to differences in the soil water availability can be discounted because, while the soil water content remains constant (between 4 and 6 per cent) at the extraction time, the pre-dawn leaf water potential of the seedlings ranges between –1.5 and –7.5 MPa (i.e. between low and very high water stress).

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C LT VLT4 VLT8 ST AT

Height (cm)

30

25

20

*

15

10

5

Diameter (mm)

6

5

4

3

2

*

120

Crown Projection (cm 2)

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*

100

*

* 80 60

*

40 20 0 Apr

Jul

1997

Oct

Jan

Apr

Jul

1998

Oct

Jan

Apr

1999

Figure 5. Time-course pattern of the mean values and standard error (SE) of three structural characteristics (height, basal diameter and crown projection) of the seedlings, according to the type of treeshelter used, for each measurement date. Asterisk (*) shows significant differences between treatments at P ≤ 0.05 (ANOVA, Tukey test).

172.25 245.7 167.2 226.4 215.0 136.0 n.s.

CONTROL ST LT VLT4 VLT8 EGT Significance

4.97 5.35 4.76 4.49 5.19 4.35 n.s.

Basal diameter (mm) BD 34.70 45.92 35.12 50.42 41.42 31.26 n.s.

77.3 145.7 66.0 119.6 111.4 26.1 n.s.

59.33 43.98 48.55 39.24 39.79 107.05 n.s.

Crown projection (cm2) 33.51 ab 20.65 b 21.30 b 7.45 b 15.53 b 80.14 a P ≤ 0.05

Crown projection increment (cm2)

32.0 ± 23.3 57.6 ± 07.7 73.7 ± 14.1 3.1 ± 0.9 3.0 ± 0.9 6.3 ± 1.0 254.7 ± 42.5 1.24 ± 0.5 17.26 ± 0.59

24.1 ± 4.2 29.9 ± 07.1 46.6 ± 10.8 3.9 ± 0.7 3.1 ± 0.8 9.9 ± 2.3 124.6 ± 35.2 1.04 ± 0.28 16.21 ± 0.63

ST

4.1 ± 0.6 5.8 ± 0.8 8.1 ± 1.1 202.7 ± 44.7 2.15 ± 0.82 14.89 ± 0.19

41.4 ± 30.4 58.6 ± 08.1 51.4 ± 10.6

EGT

3.7 ± 0.9 4.6 ± 1.0 7.7 ± 1.5 – – –

35.5 ± 26.8 53.2 ± 06.8 62.9 ± 10.7

LT

Asterisks (**) show significant differences between each group at P ≤ 0.05 (ANOVA and Tukey test).

Leaves July 1997** January 1998 March 1999 Branches July 1997 January 1998** March 1999 Total leaf area (cm2) Mean leaf area** (cm2) SLW** ( mg cm2)

C

2.8 ± 0.5 4.0 ± 0.7 6.9 ± 1.0 – – –

27.9 ± 21.7 49.9 ± 07.3 49.2 ± 09.3

VLT4

3.6 ± 0.7 7.3 ± 1.5 8.6 ± 1.6 – – –

27.5 ± 15.6 53.7 ± 10.1 47.0 ± 10.4

VLT8

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Table 3: Average number of leaves and branches in seedlings according to type of treatment (mean ± standard error), and the characteristics of leaves according to total area; average area and specific leaf weight of the Control (C), short treeshelter (ST) and esparto grass treeshelter (EGT)

4.7 5.1 4.5 4.3 5.0 4.1 n.s.

Shape factor Height increment* BD increment* index (H/BD) in 30 months in 30 months

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Note. Different letters in bold indicate a statistical difference between shelter types. *The increments are in relation to their initial values (RGR).

Height (mm) H

Shelter type and control

Table 2: Average morphological characteristics of the Quercus coccifera L. seedlings as affected by different kinds of treeshelters, 2 years after planting.


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Length Diameter

Roots inside ball

5

a

4

6

a

3

a 4

A

Length (m)

2

A

Diameter (mm)

2 1

A 0

0 1

2

B

4

2 3

6

8

b

b

B

4

Roots outside ball

5

A

a

6

10

Control

ST

AT

Figure 6. Length and diameter of the root of the seedlings belonging to the treatments Control (C), short treeshelter (ST) and esparto grass treeshelter (EGT). The upper figure shows roots within the root ball, while the lower figure shows the characteristics of the roots outside the root ball. Different letters indicate significant differences between treatments at P ≤ 0.05 (ANOVA, Tukey test). 20 b[0] = 18.64 b[1] = 2.33 r2 = 0.659

18

AT ST

16

ST

14 Roots length (m)

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C

ST ST

10

ST C

C

8

C

6 4

AT

AT AT

2 0 –8

–7

–6

–5

–4

–3

–2

–1

PWP (MPa)

Figure 7. Calculated relationship between the total root length (m) and the pre-dawn leaf water potential (PWP: MPa) during August 1999, just before lifting the seedlings from the ST and EGT treatments and Control (C).

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Total

b

a

103

b

Aboveground biomass (g)

12 10 8 6

a Leaves ab

b

4 2 0

Shoots

ab

a

b

ab

a

b

Roots

b

a

Total

ab

Shoot

2 Underground biomass (g)

4 6

b

8 10 12 14

Control

a ST

b AT

Figure 8. Different fractions of above- and below-ground biomass belonging to seedlings of the control (C), short treeshelter (ST) and esparto grass treeshelter (EGT) treatments. Different letters indicate significant differences between the treatments for each analysed fraction at P ≤ 0.05 (ANOVA, Tukey test).

Figure 8 shows the different fractions of aboveground biomass (leaves and shoots + trunk) and underground biomass (buried trunk and roots) of the control, ST and EGT treatments. The total above-ground biomass was higher in the ST treatment, both for leaves and for shoots, though the differences between the shoots of C and ST and between the leaves of EGT and ST are not significant. The underground biomass of the ST treatment was also higher than for the control and EGT. In general the roots show significantly greater development in the ST group compared with the others, while the buried trunk biomass was highest in ST, lowest in EGT and in an intermediate position in control. All these differences were statistically significant at P ≤ 0.05.

Discussion The use of treeshelters as a complementary tool in land restoration with native shrubs like

Quercus coccifera in semiarid dry-hot environments, has promoted certain growth patterns with seedlings, which do not appear in the control seedlings, and is dependent on the type of shelters used. In terms of above-ground development these are a greater height growth in ST shelters, although not statistically significant, a significantly greater crown projection in EGT shelters, and finally, a non-significant difference in the basal diameter of the seedlings in all shelter types and control. There was significantly greater accumulative above-ground biomass in ST, which contrasts with the results of Burger et al. (1996), who reported a reduction in the total biomass of different tree species included inside shelters. These authors attributed the effect to a reduction in the photosynthetic activity caused by CO2 limitations (Dupraz and Bergez, 1999) or poor light or high temperatures (Kjelgren et al., 1997; Sharpe et al., 1999). The EGT treatment produced short seedlings which had poor shoot and trunk development, probably due to the buds

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getting tangled in the shelter’s mesh. The effects of large treeshelters (ventilated or unventilated) were considered the least positive in our study, due to the poor development of their shoots. Despite Burger et al. (1996) suggesting this height (60 cm) as the most convenient in other areas, we considered that for semiarid environments the short shelters (30 cm tall), and the esparto grass shelters are more appropriate. However, the latter (EGT) seem to reduce the seedlings’ capacity to grow in height. The esparto grass leaves lost consistency and verticality, getting tangled and falling down over the seedlings. The total length of the root system, and in particular the roots outside the ball coming from the container, is one of the main factors for the survival and growth rates of seedlings (Bellot, 2000), but the effect of the treeshelters on this is poorly understood (Svihra et al., 1996). Our results have shown that the root system also has greater development in ST, in total root biomass, and in root length and average root diameter. Significant differences were not apparent between the structural characteristics of the roots belonging to EGT and control, though average biomass, length and diameter were always higher in the control treatment. Such positive or insignificant effects of treeshelters on root development of Q. coccifera contrast with the inhibitory effect detected by Svihra et al. (1996) in Sequoia sempervirens L. On the other hand, they agree with the results obtained by Ponder (1995) with red oak (Quercus rubra L.). Survival rates did not significantly differ among the various treatments. The rain pattern during the dry season was probably the most important factor on which survival depends. Survival rates were between 100 and 80 per cent during 1997, which coincided with a relatively wet summer (70.7 mm in 3 months: June, July and August), compared with an average of 36 mm for the period 1940–1994. The years 1998 and 1999, with severe dry summer periods having precipitation of 3.2 and 0 mm, respectively, led to a sharp reduction in survival rates to ~60 per cent in all treatments. Of course the rain pattern is of major importance, but the greater development of the root system may be an equally important factor in enhancing survival capability during periods of drought as is seen in the ST treatment where growth rates are increased. A close relationship has been noted between the pre-dawn water status and root length, which in this homogeneous soil

can be directly related to soil volume explored. Direct observations verified when roots were dug out confirm the claim that root length is a major factor in survival. Thus, the ST treatment may be the most beneficial of the treatments in the trial, although 2 years after planting, the survival rate was only marginally higher than in the other treatments. Another possible factor that has been cited in the literature as a potential generator of the increase in the water plant availability is the decrease in VPD inside shelters, which leads to reduced transpiration losses (Bergez and Dupraz, 1997; Kjelgren and Rupp, 1997), but this can be discounted as no difference in VPD has been observed so far during the trial. Similar results were reported by Sorensen et al. (1993), rejecting their preliminary hypothesis that treeshelters reduce the water stress in plants. Among the possible reasons to explain the benefits to the seedlings derived from the ST treatment is the reduction in the highest radiation levels, which were monitored in the open environmental conditions. When coinciding with high temperatures and water deficits, these three factors tend to generate respiration losses (Pearcy, 1999; Werner et al., 1999; MartínezFerri et al., 2000). Under such conditions of radiation, above 900 W m–2, which are common at noon, the ST shelter reduces it to 500 W m–2, while levels remain around 200–300 W m–2 inside the LT and EGT shelters. Such levels of high radiation in open environmental conditions involve photoinhibition processes (Werner et al., 1999; Martínez-Ferri et al., 2000; Maestre et al., 2001) that could imply a certain loss of the carbon gain in the whole plant (Werner et al., 1998). However, the conditions inside the ST shelter remained close to the optimum photosynthetic point for this species in the Mediterranean region (Martínez-Ferri, 1999), while high levels of radiation are avoided. On the other hand, radiation levels inside the rest of the shelters were too low, and the seedlings could suffer large periods of shade. The different levels of radiation in the different treatments are probably the stimulus for differences detected in the average area per leaf cited previously. In this sense, the higher levels of radiation in the control conditions promoted smaller leaf sizes, while the shadier conditions of the EGT treatment generated both the largest leaves and the lowest specific leaf weight, as the results show.

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Moreover, the maximum temperature monitored inside the ST shelters remains only slightly higher than under open conditions (∼2ºC) contrasting with EGT and LT treatments, which surpass the air temperatures in open conditions by up to 10ºC. The reduced height of the short treeshelters (ST) at 30 cm probably facilitates air circulation inside them, and thus avoids the overwarming observed in taller shelters.

Conclusions The trial results show that the ST treatment is the most favourable for the planting of Q. coccifera in semiarid environments. The development of the seedlings both above and below ground was greater in ST than in the other treatments. The EGT treatment appears to be the second most appropriate for planting Q. coccifera seedlings in semiarid environments. This assertion is backed in spite of the observed lack of development of its root system (lower even than that observed in the control treatment) and the deformities produced in the aerial structure, due to buds catching on the mesh of the shelter. However, they present advantages over the polypropylene shelters, since they are made of biodegradable materials. Large treeshelters (LT, VLT8 and VLT4) appear to be the most inappropriate for planting in semiarid environments. Because of the conditions in which the trials were carried out, the findings may be of use in wide areas of Spain. Average afforestation areas from 1993 to 1999 were raised by 45 000 ha per year through new afforestations or post-wildfire restoration, and by 60 000 ha per year by afforestation of abandoned crops within the framework of the Common Agricultural Policy (CAP), which is available for reforestation to avoid problems of soil degradation and erosion. However, we must emphasize that because, according to our findings, apparently similar protectors generate quite different results in the growth and structure of a single species, we suggest that more specific research is needed with different species, shelters and conditions before final guidelines can be drawn up. Finally, we conclude that shelters could provide significant benefits for plant establishment in arid regions, but over longer time periods their effects could be detrimental.

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Acknowledgements This research was supported by the RESEL programme of the Ministerio Medio Ambiente (Spanish Government), the CICYT project HID97-1014, and by funds from different programmes of the Valencian Autonomous Government, through CEAM and the GV97RN-14–4 project.

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Received 20 November 2000