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An important strategy for meeting global landscape restoration goals is nursery production of high-quality seedlings. Growing seedlings with attributes that promote post-planting survival and growth can be dramatically influenced by the nursery container system. In many countries, nurseries produce seedlings in polybags filled with excavated soil. These seedlings often develop deformed roots with limited fibrosity which can lead to poor survival and growth after outplanting. Polybags are initially inexpensive but using these single-use plastic containers accrues expenses that are often untracked. Comparisons among nursery production systems must account for factors such as container longevity, labor efficiency, and seedling field performance. A more holistic approach to account for environmental, economic, social, logistic, and cultural elements in the cost–benefit equation that influences nursery production systems is needed. Converting to a modern container system requires concomitant adjustments in nursery scheduling and culturing matched to the new stock type. Doing so provides an opportunity to align nursery production techniques and resulting seedling attributes with anticipated field conditions. This article describes and discusses the advantages and disadvantages of nursery production systems and provides recommendations and case studies to aid nurseries in improving seedling quality toward meeting restoration goals in a cost-effective and timely manner.

Large-scale global reforestation goals have been proposed to help mitigate climate change and provide other ecosystem services. To explore reforestation potential in the United States, we used GIS analyses, surveys of nursery managers and foresters, and literature synthesis to assess the opportunities and challenges associated with meeting proposed reforestation goals. We considered a scenario where 26 million hectares (64 million acres) of natural and agricultural lands are reforested by 2040 with 30 billion trees at an estimated cost of $33 ($24–$53) billion USD. Cost per hectare will vary by region, site conditions, and other factors. This scenario would require increasing the number of tree seedlings produced each year by 1.7 billion, a 2.3-fold increase over current nursery production levels. Additional investment (not included in the reforestation cost estimate) will be needed to expand capacity for seed collection, seedling production, workforce development, and improvements in pre- and post-planting practices. Achieving this scenario will require public support for investing in these activities and incentives for landowners.

Seedlings are the foundation for many terrestrial ecosystems and are a critical consideration and investment for implementing global forest and landscape restoration programs. Global leaders have pledged to restore millions of hectares during the next decade, necessitating many millions of established plants. Although natural regeneration and direct seeding will likely meet a portion of that need, large quantities of high-quality, nursery-grown seedlings are also required. Insufficient plant quantities or poor-quality plants result in unsuccessful outplanting programs. Such failures have considerable economic and environmental consequences and will result in an inability to meet restoration goals. Nonetheless, the importance of restoration nurseries is often overlooked when making large-scale restoration commitments. Technology already exists to produce high-quality plants to meet a variety of goals. This technology cannot be applied, however, unless adequate resources and training are made available by overcoming political and socioeconomic barriers. In this article, we discuss the important role of nurseries to meet global restoration commitments and review three case studies where increased support to nursery programs resulted in improved restoration success.

Meeting forest restoration challenges relies on successful establishment of plant materials (e.g., seeds, cuttings, rooted cuttings, or seedlings, etc.; hereafter simply “seedlings”). The Target Plant Concept (TPC) provides a flexible framework that nursery managers and their clients can use to improve the survival and growth of these seedlings. The key tenets of the TPC are that (1) more emphasis is placed on how seedlings perform on the outplanting site rather than on nursery performance, (2) a partnership exists between the nursery manager and the client to determine the target plant based on site characteristics, and (3) that information gleaned from post-planting monitoring is used to improve subsequent plant materials. Through the nursery manager–client partnership, answers to a matrix of interrelated questions define a target plant to meet the reforestation or forest restoration objectives. These questions focus on project objectives; site characteristics, limiting factors, and possible mitigation efforts; species and genetic criteria; stocktype; outplanting tools and techniques; and outplanting window. We provide examples from the southeastern United States, Hawai‛i, and Lebanon on how the TPC process has improved performance of seedlings deployed for reforestation and forest restoration.

Drought is one of the crucial factors limiting plant growth and is expected to become even more limiting in many regions as the effects of climate change increase. Knowledge of genetic adaptation to environmental factors can help develop strategies for successful reforestation in arid terrain. In this study, Austrian pine (Pinus nigra Arnold) seedlings grown from seed collected from 40 open-pollinated families from five Balkan Peninsula provenances were evaluated for survival and growth under imposed drought. Within each provenance, seed was collected from an extremely harsh habitat and from a favorable habitat. One-year-old seedlings, from each provenance, habitat type and open pollinated family were transplanted into three “stressbeds”, each designed to simulate different levels of water availability by filling with varying ratios of crushed stone and soil. Seedling height growth and survival were recorded every 20–30 days from April–September. In all provenances, seedlings grown from seed originating from the harsh habitats had lower survival regardless of stressbed treatment compared to those grown from seed collected from favorable habitats, with the exception of one population from the Visegrad provenance which had high survival rates throughout. Overall height growth was very low and did not follow consistent patterns among treatment groups. Cluster analyses of survival data showed that populations from a similar habitat generally grouped together, regardless of provenance. The lowest and highest survival values were found in seedlings from harsh habitats, while those from the favorable habitats clustered in the mid-range. These results suggest that higher seed quality from productive stands may be the most influential factor determining seedling vigor under drought stress and that genetic adaptation to drought was not detected in adjacent populations having different habitat conditions.

Simple, graphical techniques are available to plant professionals to evaluate morphological and physiological data. The most commonly used graphs show only one variable at a time. However, plotting two variables into a simple coordinate system and adding diagonal lines to show relationships between those two variables greatly enhances the interpretive possibilities available to the plant practitioner by facilitating simultaneous comparisons of multiple variables. This easy-to-use nomogram is useful for evaluating and comparing plant responses to treatments and environments, plant development over time, and stock quality among species, genotypes, and other factors. This article describes how to construct this type of graph, the kind of data for which this technique is most useful, and how it can be applied to operational and research activities.

Seed was collected from two habitat types (harsh/rocky and favorable/control) within five Balkan provenances. An experiment was conducted to examine water uptake (grams water per gram of fresh seed) during 48 h after exposure to water. A second experiment investigated the effect of simulated-drought conditions (4% sucrose solution) compared to non-drought conditions (distilled water) on subsequent germination percentage. Seed from rocky habitats had less water uptake than those from control habitats for three of the provenances. However, water uptake was not correlated with subsequent germination. Drought simulation decreased germination but patterns for seed within each provenance were similar to germination under non-drought conditions. With the exception of one provenance, seed from the rocky habitats had lower germination than seed from control habitats. The data suggests some degree of adaptation to environmental conditions although variability was very high within each provenance’s habitat types. This is of particular importance when considering potential impacts of climate change and the anticipated migration of species due to changes in habitat.

Reforestation on harsh, high-elevation sites near the crest of the Cascade Mountains in Washington can be challenging because of persistent snowpack and extreme climatic variation. The use of tree shelters was investigated with two species, Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and western larch (Larix occidentalis Nutt.), on two Yakama Nation sites across three fall planting dates. For both species, seedlings inside tree shelters had increased survival compared with nonsheltered seedlings after one growing season despite increased damage by crushing from snowpack. Volumetric soil moisture content on the western larch site increased from 10% to 17% between the first and second planting dates, resulting in no effect of planting date on western larch survival. On the Douglas-fir site, however, soil moisture content was below 12% for all fall planting dates, resulting in only 2% survival for those seedlings planted on the first planting date. First-season seedling growth was unaffected by planting date or tree shelter treatment for both species. These results indicate the critical importance of soil moisture at time of planting and onset of precipitation after planting. Use of tree shelters may improve seedling survival on a harsh, high-elevation site, but it increases potential seedling damage because of crushing by snowpack.

Nutrient‐release rates of controlled‐release fertilizer (CRF) with four different labeled release periods were evaluated. Samples (30 g) sealed with nylon mesh were buried at a clearcut forest site (Price soil series) in western Oregon, USA, in February 2000 and excavated every 7 weeks for 14 months to determine residual weight and composition. Cool, dry soil conditions apparently prolonged nutrient release beyond labeled rates; the fertilizer with the shortest release period (3–4 months) released approximately 72% of the fertilizer (by weight), whereas that with the longest release period (8–9 months) released 48%. Release varied among individual nutrients [nitrate (NO3)>ammonia (NH3)>potassium (K)>sulfur (S)>magnesium (Mg)>phosphorus (P)]. Minimal changes in micronutrient [iron (Fe), manganese (Mn), zinc (Zn), and molybdenum (Mo)] contents were attributed to the formation of insoluble compounds with P. Variable release among individual nutrients demonstrates a limitation toward delivering a ful...

Containerized coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were grown with conventional soluble fertilizer and supplemented with one of four slow-release fertilizers (SRF treatments) or unsupplemented (control treatment). Seedlings were outplanted to two sites in 1998. At the time of outplanting, SRF incorporated into the growing medium resulted in larger seedlings with higher foliar nutrient concentrations as compared to conventionally fertilized seedlings. After four growing seasons, SRF-amended seedlings at both sites had significantly greater height, basal stem diameter, and stem volume, with increases up to 19, 21, and 73%, respectively, as compared to conventionally fertilized seedlings. Additionally, three stock sizes were compared at one of the sites. Increasing stock size resulted in increased growth during the first two seasons, enabling larger stock to maintain their size advantage. Seedling responses to SRF are attributed to larger initial size, increased internal nutrient reserves at planting, and continued fertilization after planting.

Detailed root and shoot development of bareroot and container Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedling stocktypes were compared during the first growing season after outplanting. The study was installed in raised beds with ideal environmental conditions and at a field reforestation site. Survival at both sites was 98% and did not differ between stocktypes. Seedlings were excavated in spring (5, 8, 12, and 16 weeks after planting) and in fall (35, 40, and 45 weeks after planting). In spring, container seedlings had more numbers of new roots and greater new root and shoot biomass than bareroot seedlings at both sites. In fall, bareroot seedlings consistently averaged more new root growth (though nonsignificant) than container seedlings suggesting that stocktype differences may not continue long-term. Container seedlings had significantly greater water percent than bareroot seedlings at the field site (all sample dates) and the raised bed site (weeks 5, 8, and 40 only). Regardless of environmental conditions or season, seedlings at both sites maintained water percent between 60 and 70% of fresh weight. Seedlings grown in the raised beds had much greater growth than those grown in the field. However, relative growth patterns for the two stocktypes were very similar on each site. The data generated establish baseline differences between stocktypes for root initiation, growth, and allometry during the first year after planting. Challenges associated with root development research are discussed.

Polymer-coated fertilizer (PCF) provides a potential means to optimize nutrient delivery for plant uptake, while minimizing leaching. Coating technology varies by manufacturer, which may alter patterns of nutrient release over time among comparable products. Three months following sowing, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were transplanted into containers with Osmocote Plus (OS) (15-9-12) and Apex (AP) (16-5-9) PCF (each with 5–6 month longevity) applied at four rates (0, 600, 1200, and 1800 mg total N per seedling) as a supplement to periodic addition of conventional water-soluble fertilizer. Seedlings fertilized with OS had 11% greater stem diameter growth during the first 4 months after transplant than those fertilized with AP, but differences were negligible after 9 months. After 4 months, foliar concentrations were 11% higher for N and 10% higher for P in seedlings fertilized with AP, although contents were the same. These results suggest that different PCF types have variable temporal patterns of nutrient release, which may affect seedling development over time. Differences among products must be understood by growers to help formulate fertilizer prescriptions that optimize plant response. Seedling response varied marginally among PCF rates, suggesting little benefit to PCF when used in conjunction with water-soluble fertilizer. West. J. Appl. For. 20(1):58–63.

This experiment evaluated the influence of manure, peat, and vermiculite incorporated at low and high rates (0.0118 and 0.0236 m3/m2) and under two soil moisture regimes on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedling (1+0 for 1+1) xylem water potential (Ψxylem), whole-plant growth, root architectural development, and subsequent field performance under fertilized and non-fertilized conditions. Trends in soil moisture retention were observed (high manure > high peat > control) but there were no differences in Ψxylem. Root length in the wetter soil moisture experiment was initially (three months) greatest for seedlings in high vermiculite and least in high manure but there were no differences among treatments at lifting (eight months). Mean height was greatest for seedlings grown in vermiculite and peat (wetter nursery experiment) after two field seasons. Field fertilization (35 g/seedling) with controlled-release fertilizer in the planting hole stimulated height growth initially, but decreased height and diameter growth during the second growing season. Dramatic improvements associated with the use of nursery soil amendments were not realized, but the failure to identify negative effects, a potential reduction in disease incidence, and improvement of nursery soil physical and chemical properties may justify their use.

Three months following sowing, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were transplanted into pots with controlled-release fertilizer (CRF) applied at rates of 0, 8, 16, and 24 g/2200 cm3 soil as a single uniform layer beneath the root system. Seedlings were destructively harvested periodically, and roots were divided into vertical segments above (S1), within (S2), and below (S3) the fertilizer layer. Two months following transplant, the number of active root tips was positively correlated with CRF rate in S1 and negatively correlated with rate in S2 and S3. At 6 months, root penetration into S3 was severely restricted at 16 and 24 g. This was attributed to detrimental changes in soil osmotic potential in S2. Fertilizer improved seedling growth at 8 g after 6 months compared with controls but was inhibitory at 24 g. Photochemical quantum yield was higher in all CRF treatments compared with controls 3 months following transplant, which corresponded with rapid initial CRF nutrient release. Despite improvements in nutrient release technology with CRF, high application rates may result in excessive concentrations of fertilizer nutrients in media, which can restrict root penetration and negatively affect seedling growth. Conservative application rates and improvements in CRF technology will help reduce the potential for adverse effects on seedling development.

Two-year-old coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings from two seed lots were exposed to controlled freezing temperatures every 4 weeks from October 1993 through April 1994. Freezing effects were assessed by measuring chlorophyll fluorescence emissions 1 day after freezing and by evaluating damage to the bud, cambium, and needle tissues 7 days after freezing. Differences between the seed lots as well as changes in cold hardiness among the bud, cambium, and needle tissues were evident throughout the duration of the study. Tissue damage was higher with increased freezing stress. Severity of damage to each of the tissues varied seasonally. Chlorophyll fluorescence emissions were lower with higher freezing stress (except during November and December, when test temperatures were not low enough to significantly damage the seedlings) and showed a strong relationship with morphological assessments of freezing stress. The slope of the slow kinetics phase of the chlorophyll fluorescence curve tended to be less steep (i.e., quenching was reduced) with higher freezing stress. Nonfrozen chlorophyll fluorescence measurements showed no obvious relationship with LT50 for either seed lot. However, chlorophyll fluorescence measurements are useful for determining cold hardiness and resistance to stress, because they provide a rapid assessment of seedling vigor following exposure to freezing.

Coastal Douglas-fir (Pseudotsuga menziesii) 1+1 seedlings were fertilized with two fertilizers [NH4NO3+K2SO4 and (NH4)2SO4+KCl] at four rates (0, 80, 160, 320 kg N and K/ha) split over three application dates (September 19, October 13, November 1, 1996). Fertilizer type did not affect total Kjeldahl nitrogen (TKN) levels on any of the sampling dates. By January 10, TKN concentrations had increased 16, 30, and 34%, and chloride concentrations had increased 57, 77, and 112% relative to the unfertilized seedlings for the 80, 160, and 320 kg N+K/ha treatments, respectively. Nitrate levels increased briefly after the first application of NH4NO3+K2SO4. Potassium levels remained relatively unchanged. Levels of most other nutrients, as well as foliar dry weight, increased between September 16 and January 10, but these increases were generally unrelated to the fertilizer treatments. Root growth potential and cold hardiness did not differ among treatments. Seedlings that received 160 or 320 kg N/ha broke bud an average of 3 days earlier than did the unfertilized seedlings. Chlorophyll fluorescence (Fv/Fm) of fertilized seedlings was consistently higher than that of unfertilized seedlings on November 13 and December 30. These treatment differences were not reflected in seedling outplanting performance after one growing season. West. J. Appl. For. 16(2):71–79.

In response to environmental concerns and the need for peatland conservation, alternative growing media for conifer seedling production must be investigated. We grew Douglas-fir seedlings in 6 media; components included peat moss, peat moss amended with sawdust, and 2 sources of coir (coconut fiber) mixed with and without peat moss. Coir had higher pH, P, K, and Na and lower Ca and N than peat moss and a peat moss-sawdust mixture. Bulk densities of coir and coir-based media were lower than those in peat moss and a peat moss-sawdust mixture. After 21 wk, seedlings grown in coir-based media were significantly smaller and had lower foliar N and Ca than those grown in peat moss. Because of coir’s many favorable qualities, we suggest further research using culturing regimes specific to the substrate’s nutrient properties.

Four species of Pacific Northwestconifer seedlings (ponderosa pine [Pinusponderosa Dougl. ex Laws.], Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], westernredcedar [Thuja plicata Donn ex D. Donn], andwestern hemlock [Tsuga heterophylla (Raf.)Sarg.]) were planted in individual pots and grownunder shade-cloth shelters that provided four levelsof shade (0% [full sunlight], 35%, 55%, and 75%)for approximately 30 weeks. Height growth wasrecorded every 2 weeks. Initial and final seedlingmorphology and chlorophyll concentrations weremeasured. Chlorophyll fluorescence was measuredmonthly. All species responded similarly to shade. Although height growth was greatest under 75% shadeand least in 0% shade, total biomass production wassignificantly lower and shoot:root ratio significantlyhigher in 75% shade than in 0% shade. As thetreatment shade level increased from 0% to 75%,Fv/Fm was significantly lower whilechlorophyll concentrations were significantly higher. These results indicate that photochemical efficiencyof all four species was lower under higher shade. Morphological and physiological responses differedgreatly among species and corresponded with theirdegree of shade tolerance.

This is the final summary of two studies on the relationship between root volume and seedling growth; early results were published previously. Survival, growth, and stem volume were determined for 2+0 ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) seedlings after 8 growing seasons. For each species, seedlings from three seedlots were assigned to one of three root-volume categories [7 cm3 (RV3) for ponderosa pine; 13 cm3 (RV3) for Douglas-fir]. On a dry harsh ponderosa pine site on the eastern slopes of Mt. Hood in Oregon, where gopher and cattle damage decreased the number of seedlings, more seedlings in the highest root-volume category survived (70%) than in the smaller root-volume categories (62% and 50%). Douglas-fir on a good site in the Coast Range of Oregon showed significantly greater height and stem volume for the largest root-volume category, whereas annual shoot growth and survival did not differ. Root volume is one of several potentially useful criteria for predicting long-term growth and survival after outplanting. West. J. Appl. For. 12(3):69-73.

Containerized Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) seedlings were subjected to six moisture-stress treatments (ranging from 7 to 65% soil water content by volume) for 12 weeks. At the end of this period, there were significant differences in phenological, physiological, and morphological responses among the seedlings in the various moisture-stress treatments. In general, seedlings grown under very high or very low soil moisture conditions were adversely affected, while those grown under moderate conditions (29 to 53% soil water content) exhibited optimum growth, bud development, and nutrient and starch reserves. The use of vector analysis was found to be helpful in data interpretation. The results indicate the importance of closely monitoring nursery moisture regimes in order to achieve the best seedling quality.

Two-year-old bareroot Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco.) seedlings were graded on the basis of four root-volume categories and transplanted to four moisture-stress treatments (6, 12, 18, and 24% soil water content) in pots. Macronutrient concentrations and contents of both old and new foliar tissue were determined. Decreasing soil water content resulted in higher concentrations of phosphorus, potassium, and particularly nitrogen in both old and new foliar tissue. This can be attributed to reduced growth, translocation, metabolic activity, and nutrient requirement in response to moisture stress. Seedlings with relatively higher root volumes exhibited higher nutrient concentrations and contents, as well as increased growth. Thus, increased total root biomass per unit of soil area with increasing seedling root volume may have resulted in greater nutrient use, supply, uptake, and storage.

Con el fin de determinar los efectos combinados del volumen radicular y la tasa de fertilizacion sobre el comportamiento en terreno de plantas de pino oregon producidas a traves del metodo 1+1, plantas producidas operacionalmente en el vivero de la empresa Weyerhaeuser ubicado en Aurora, OR, Estados Unidos, fueron clasificadas en cuatro categorias de acuerdo a su volumen radicular (8-13, 14-17, 18-22, y 23-35 cm3), y establecidas en terreno durante el invierno del 2000, utilizando un diseno de bloques aleatorios con cinco replicas en un arreglo factorial. Una mezcla de fertilizantes de entrega controlada (Simplot) que contenia iguales proporciones de tres fertilizantes con diferentes efectos residuales (3-4, 5-6, y 8-9 meses) y similares concentraciones de NPK (19-6-12), fue aplicada en el fondo del hoyo de plantacion en cinco dosis: 0, 15, 30, 45 y 60 g por planta. Despues de dos temporadas de crecimiento en terreno, no existio interaccion entre el volumen radicular y la dosis de fertilizacion para la supervivencia y crecimiento inicial de las plantas. Aquellas plantas con mayor volumen radicular sobrevivieron y se desarrollaron mejor que aquellas con menor volumen radicular. El aumento de la tasa de fertilizacion redujo significativamente la supervivencia y el crecimiento de las plantas.

Los fertilizantes de entrega controlada (FEC) presentan un gran potencial para aumentar la competitividad de las plantas forestales para una variedad de sitios de reforestación. Sin embargo, normalmente existe un pobre conocimiento sobre ellos. Con el fin de usar los FEC en forma exitosa se deben considerar su formulación, el patrón de entrega y sus interacciones ambientales. Este trabajo sintetiza los distintos tipos de FEC (orgánicos, productos con nitrógeno de lenta solubilidad y productos recubiertos) y diversas investigaciones asociadas a su utilización. Además, revisa las ventajas y desventajas asociadas a los FEC, se realizan ciertas recomendaciones sobre su utilización y menciona algunos de los desafíos para futuras investigaciones destinadas a mejorar la comprensión en las formas de aplicar los FEC en situaciones forestales.

International audience; Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings (1 + 1 bareroot) were divided into 8-13 cm3 and 23-35 cm3 initial root-volume categories and either not fertilized or fertilized with 60 g Apex® in the planting hole at a drought-prone clearcut site in the Oregon Coast Range, USA. Regardless of initial root volume, fertilized seedlings consistently reached lower predawn xylem water potential ($\Psi$xylem) values than non-fertilized seedlings during summer. Roots did not proliferate relative to fertilizer placement and mean values for lateral-root length, root tips, and root biomass were less for fertilized seedlings in each of four vertical root zones. Poor root system development of fertilized seedlings may have been associated with the rapid release of high fertilizer salt concentrations into the rhizosphere. Root-volume growth was positively correlated with $\Psi$xylem, emphasizing the importance of new root growth following planting for drought avoidance. Criteria for successful field fertilization must consider the anticipated drought level of the site.; La fertilisation à la plantation perturbe le développement du système radiculaire et réduit la résistance à la sécheresse des plants de Douglas (Pseudotsuga menziesii). On a utilisé deux catégories de plants à racines nues (1 + 1) de Douglas (Pseudotsuga menziesii (Mirb.) Franco) dont le volume initial des racines était respectivement de 8 à 13 cm3 et de 23 à 35 cm3. Ces plants furent installés dans une station de clairière plutôt sèche, située dans la Chaîne côtière de l'Oregon aux USA, et soumis à deux traitements : pas de fertilisation ou fertilisation avec 60 g d'Apex® introduits dans le trou de plantation. Quelque soit le volume racinaire initial, les plants fertilisés révèlent, au cours de l'été, des valeurs de potentiel hydrique du xylème en fin de nuit ($\Psi$xylem) toujours inférieures à celles obtenues sur plants non fertilisés. Les racines ne se sont pas développées dans les zones en contact avec l'engrais, et les valeurs moyennes de longueur de racines latérales, de nombre de pointes racinaires, et de biomasse racinaire sont inférieures pour les plants fertilisés, dans les quatre plans verticaux. Ce mauvais développement du système racinaire des plants fertilisés peut être associé à la libération rapide d'engrais à forte concentration minérale dans la rhizosphère. La croissance racinaire en volume était corrélée positivement avec le $\Psi$xylem ce qui permet de souligner l'importance de la croissance des racines après plantation pour limiter les effets de la sécheresse. Les critères à adopter pour réussir une opération de fertilisation de terrain doivent donc prendre en considération l'éventuel degré de sécheresse de la station.