Reduction in plant size and tissue nutrient concentration is widely considered to increase 30 seedling drought resistance in dry and oligotrophic plantation sites. However, much evidence 31 indicates that increase in size and tissue nutrient concentration improves seedling survival in 32 Mediterranean forest plantations. This suggests that the ecophysiological processes and 33 functional attributes relevant for early seedling survival in Mediterranean climate must be 34 reconsidered. We propose a physiological conceptual model for seedling survival in 35 Mediterranean-climate plantations to provide a physiological explanation of the frequent 36 positive relationship between outplanting performance and seedling size and nutrient 37 concentration. The model considers the physiological processes outlined in the plantation 38 establishment model of Burdett (1990), but incorporates other physiological processes that 39 drive seedling survival, such as N remobilization, carbohydrate storage and plant hydraulics.40 The model considers that seedling survival in Mediterranean climates is linked to high growth 41 capacity during the wet season. The model is for container plants and is based on three main 42 principles, 1) Mediterranean climates are not dry the entire year but usually have two 43 seasons of contrasting water availability; 2) summer drought is the main cause of seedling 44 mortality; in this context deep and large roots is a key trait for avoiding lethal water stress; 3) 45 attainment of large root systems in the dry season is promoted when seedlings have high 46 growth during the wet season. High growth is achieved when seedlings can divert large 47 amount of resources to support new root and shoot growth. Functional traits that confer high 48 photosynthesis, nutrient remobilization capacity, and non-structural carbohydrate storage 49 promote high growth. Increases in seedling size and nutrient concentration, strongly affect 50 these physiological processes. Traits that confer high drought resistance are of low value 51 during the wet season because hinder growth capacity. We provide specific evidence to 52 support the model and finally we discuss its implications and the factors that may alter the 53 frequent increase in performance with increase in seedling size and tissue nutrient 54 concentration.

Plants store compounds that supplement external resources to maintain primary functions. We reviewed the role of stored non-structural carbohydrates (NSC) and nitrogen (N) in juvenile woody species for spring growth and cold and drought stress tolerance, which are crucial processes for early performance of forest plantations. Plant functional types differed in NSC and N partitioning and allocation to new growth. In general, however, new leaves/shoots were more enriched in remobilized resources than new fine roots. Conifers used less remobilized resources than broadleaf species for fine root growth. New shoots/leaves were mostly comprised of remobilized N ([60 %) in conifers and broadleaf deciduous species, while broadleaf evergreens relied more on soil N (\50 % remobilized N). In contrast, few differences among functional groups existed in the contribution of remobilized carbon (C) to new leaves/shoots, which comprised 28-45 % of stored C reflecting the importance of current photosynthesis and distinctions in C and N remobilization physiology. The amount of N remobilized by an organ was positively related to its contribution to seedling N content. However, leaves are priority N sources in evergreens, which remobilized more N than predicted by their contribution to seedling N content. In contrast, roots in broadleaf evergreens and conifers were poor contributors of remobilized N. Under low stress, spring growth has little effect on NSC reserves. However, prolonged and intense photosynthesis depression strongly reduces NSC. In contrast, N reserves usually decline after planting and their replenishment takes longer than for NSC reserves. Strong storage reduction can hinder seedling stress acclimation and survival capacity. Resource storage can be promoted in the nursery by arresting plant growth and Electronic supplementary material The online version of this article (supplying resources at a higher rate than seedling growth and maintenance rate. We conclude that the way in which woody plants manage stored resources drives their growth and stress tolerance. However, plant functional types differ in storage physiology, which should be considered in silvicultural management.

Seedlings of Mediterranean evergreen trees have distinct C and N storage physiologies, with relative growth rate driving the contribution of remobilized resources to new growth. These differences may reduce competition and facilitate species coexistence.

Nursery nitrogen (N) fertilization influences seedling N reserves, morphology, photosynthesis rate and stress tolerance and frequently enhances outplanting performance. Although mineral nutrition is a critical aspect of seedling quality, fertility targets of Mediterranean sclerophylous species have not been thoroughly quantified. We sought to define those fertility targets for seedlings of Quercus ilex, a key species in Mediterranean areas. Nine fertility treatments, ranging from 0 to 200 mg N seedling -1 applied under an exponential regime were tested in a greenhouse dose response trial in which phosphorus (P) and potassium (K) were increased in the same proportion as N (15N:5P:15K). Height and diameter growth were measured weekly, and biomass and nutritional status were analyzed at the end of culture (24 week). Plant growth and nutritional response to increased fertilization followed a curvilinear pattern depicting phases that ranged from deficiency to luxury consumption. Seedling dry mass production was maximized at 125 mg N seedling -1 (sufficiency level). N content and concentration increased with fertilization, reaching a maximum at 200 mg N seedling -1 (luxury consumption). P and K concentrations peaked at 75 and 25 mg N, respectively, suggesting a dilution effect of these nutrients. Root volume increased linearly up to 100 mg N and declined thereafter. The sufficiency level for Q. ilex (125 mg of applied N seedling -1 ) is notably higher than for other Quercus species from other biomes but intermediate to other Mediterranean Quercus species. No toxicity was observed at the highest treatment rate (200 mg N) suggesting that increased exponential N rates along with greater P and K proportions than those used in our experiment may further maximize nutrient storage.

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Background and aims: Plants differ in their ability to use different nitrogen (N) forms and these differences can be related to their ecology and drive community structure. The capacity to uptake intact organic N has been observed in plants of several ecosystems. However, soil organic N uptake by Mediterranean plants is unknown despite organic N being abundant in Mediterranean ecosystems. We compare the uptake of different N forms in two widespread coexisting Mediterranean forest trees with contrasting ecophysiological characteristics: Quercus ilex and Pinus halepensis.Methods: To estimate root uptake rate of each N form we used equimolar solutions (1 mM N) of 15 NO3 -, 15 NH4 + and 15 N-13 C glycine.Results: NH4 + and glycine were taken up at a similar rate, but faster than NO3in both species. Intact dual labeled glycine was found in both species, demonstrating that both species can absorb intact organic N.Conclusions: Despite their ecological differences, both species had similar preference for N forms suggesting no niche complementarity for N uptake. The higher preference for NH4 + and glycine over NO3possibly reflects adaptation to the differing proportions of N forms in Mediterranean soils.

Most studies examining inorganic N form effects on growth and nutrition of forest trees have been conducted on single species from boreal or temperate environments, while comparative studies with species from other biomes are scarce. We evaluated the response of two Mediterranean trees of contrasting ecology, Quercus ilex L. and Pinus halepensis Mill., to cultivation with distinct inorganic N forms. Seedlings were fertilized with different NH4 + / NO3proportion at either 1 or 10 mM N. In both species N forms had small effects at low N concentration, but at high N concentration they markedly affected plant performance. A greater proportion of NH4 + in the fertilizer at high N caused toxicity as it reduced growth and caused seedling death, with the effect being greater in Q. ilex than in P. halepensis. An increase in the proportion of NO3at high N strongly enhanced growth relative to low N plants in P. halepensis but had minor effects in Q. ilex. Relatively more NH4 + in the fertilizer enhanced plant P concentration but reduced K concentration in both species, while the opposite effect occurred with NO3 -, and these effects were enhanced under high N concentration. We conclude that species responses to inorganic N forms were related to their ecology. P. halepensis, a pioneer tree, had improved performance with NO3at high N concentration and showed strong plasticity to changes in N supply. Q. ilex, a late successional tree, had low responsiveness to N form or concentration.

Drought is a limiting factor to forest regeneration and restoration, which is likely to increase in intensity and duration under future climates. Nitrogen (N) nutrition is related to drought-resistance mechanisms in trees. However, the influence of chemical N form (inorganic and organic N) on physiological traits related to drought resistance has been sparsely studied in conifer seedlings. We investigated the effect of N forms on morpho-physiological traits of Pinus ponderosa Dougl. ex Laws. seedlings and subsequent influences in drought tolerance and acclimation. One-year-old seedlings were fertilized during 10 weeks at 9 mM N with different N forms [either NH4+, NO3− or organic N (amino acids mixture)] in their second year of growth. After fertilization, we measured traits associated with intrinsic drought tolerance (shoot water relations, osmotic regulation, photosynthesis and cell membrane stability). Seedlings were then subjected to an 8-week drought period at varying drought intensities to evaluate plant acclimation mechanisms. We demonstrated that P. ponderosa seedlings could efficiently use amino acids as a primary N source, showing similar performance to those grown with inorganic N forms. Nitrogen form influenced mainly drought-acclimation mechanisms rather than intrinsic drought tolerance. Osmotic potential at saturation (Ψπsat) was marginally affected by N form, and a significant relationship between proline concentration in needles and Ψπsat was found. During acclimation, seedlings fertilized with organic N minimized needle senescence, retained more nutrients in the oldest needles, had maximum increments in proline concentration and hastened the development of water-use efficiency mechanisms compared with those fertilized with inorganic N sources. Our results suggest an improved physiological drought acclimation of organic N-fertilized seedlings.