Cultivation Using Coir Substrate and P or K Enriched Fertilizer Provides Higher Resistance to Drought in Ecologically Diverse Quercus Species.

Nursery cultivation practices can be modified to increase resistance to water stress in forest seedlings following field establishment, which may be increasingly important under climate change. We evaluated the morphological (survival, growth) and physiological (chlorophyll fluorescence, leaf water potential) responses to water stress for three ecologically diverse Quercus species (Q. robur, Q. pubescens, and Q. ilex) with varying traits resulting from the combination of growing media (peat, coir) and fertilization (standard, P-enriched, K-enriched). For all species under water stress, seedlings grown in coir had generally higher growth than those grown in peat. Seedlings fertilized with P performed better, particularly for survival; conversely, K fertilization resulted in inconsistent findings. Such results could be explained by a combination of factors. P fertilization resulted in higher P accumulation in seedlings, while no K accumulation was observed in K fertilized seedlings. As expected, the more drought-sensitive species, Q. robur, showed the worst response, while Q. pubescens had a drought resistance equal or better to Q. ilex despite being classified as intermediate in drought resistance in Mediterranean environments.

Assessing growth, frost tolerance, and acclimation of pine seedlings with contrasted dormancy strategies as influenced by organic nitrogen supply.

Freezing stress is a critical environmental factor affecting survival, distribution, and evolution of plants. Although there is evidence that nitrogen (N) affects frost tolerance of juvenile conifers, the magnitude and direction of such effect can diverge among species. The influence of the N source on frost tolerance has been barely studied. Particularly, how organic N sources could affect the cold acclimation dynamics of seedlings is poorly understood. We studied morpho-physiological responses to organic N supply (amino acids) in comparison to inorganic N in seedlings of two Mediterranean pine species: Pinus halepensis and P. sylvestris. Fertilization was applied at low and high N doses (30 and 130 mg N seedling-1 ) in the first growing season. Then, tolerance of seedlings to freezing stress was evaluated through the cold season. This study confirmed that organic N supply promotes growth of both species as effectively as inorganic N sources. At low N availability, seedlings had acute phosphorus deficiencies when grown with inorganic N, but not with organic N. Likewise, high organic-N availability improved chlorophylls concentration. Both species increased their frost tolerance through time, especially during late autumn. Although organic N supply did not show clear benefits on frost tolerance, it seemed to enhance cold acclimation via increases of compatible solutes, such as soluble sugars and proline, particularly in P. halepensis. Thus, the effects of organic N supply could depend on the extent that such osmolytes contribute to the dormancy strategy of the species. Other species-specific mechanisms to cope with freezing stress are further discussed.

Climate and species stress resistance modulate the higher survival of large seedlings in forest restorations worldwide.

Seedling planting plays a key role in active forest restoration and regeneration of managed stands. Plant attributes at outplanting can determine tree seedling survival and consequently early success of forest plantations. Although many studies show that large seedlings of the same age within a species have higher survival than small ones, others report the opposite. This may be due to differences in environmental conditions at the planting site and in the inherent functional characteristics of species. Here, we conducted a global-scale meta-analysis to evaluate the effect of seedling size on early outplanting survival. Our meta-analysis covered 86 tree species and 142 planting locations distributed worldwide. We also assessed whether planting site aridity and key plant functional traits related to abiotic and biotic stress resistance and growth capacity, namely specific leaf area and wood density, modulate this effect. Planting large seedlings within a species consistently increases survival in forest plantations worldwide. Species' functional traits modulate the magnitude of the positive seedling size-outplanting survival relationship, showing contrasting effects due to aridity and between angiosperms and gymnosperms. For angiosperms planted in arid/semiarid sites and gymnosperms in subhumid/humid sites the magnitude of the positive effect of seedling size on survival was maximized in species with low specific leaf area and high wood density, characteristics linked to high stress resistance and slow growth. By contrast, high specific leaf area and low wood density maximized the positive effect of seedling size on survival for angiosperms planted in subhumid/humid sites. Results have key implications for implementing forest plantations globally, especially for adjusting nursery cultivation to species' functional characteristics and planting site aridity. Nursery cultivation should promote large seedlings, especially for stress sensitive angiosperms planted in humid sites and for stress-resistant species planted in dry sites.

Normalization criteria determine the interpretation of nitrogen effects on the root hydraulics of pine seedlings.

Plant hydraulics is key for plant survival and growth because it is linked to gas exchange and drought resistance. Although the environment influences plant hydraulics, there is no clear consensus on the effect of nitrogen (N) supply, which may be, in part, due to different hydraulic conductance normalization criteria and studied species. The objective of this study was to compare the variation of root hydraulic properties using several normalization criteria in four pine species in response to three contrasting N fertilization regimes. We studied four closely related, yet ecologically distinct species: Pinus nigra J.F. Arnold, Pinus pinaster Ait., Pinus pinea L. and Pinus halepensis Mill. Root hydraulic conductance (Kh) was measured with a high-pressure flow meter, and values were normalized by total leaf area (leaf specific conductance, Kl), xylem cross-section area (xylem specific conductance, Ks), total root area (root specific conductance, Kr) and the area of fine roots (fine root specific conductance, Kfr). Controlling for organ size differences allowed comparison of the hydraulic efficiency of roots to supply or absorb water among fertilization treatments and species. The effect of N on the root hydraulic efficiency depended on the normalization criteria. Increasing N availability reduced Kl and Ks, but increased Kh, Kr and especially Kfr. The positive effect of N on Kr and Kfr was positively related to seedling relative growth rate and was also consistent with published results at the interspecific level, whereby plant hydraulics is positively linked to photosynthesis and transpiration rate and fast growth. In contrast, normalization by leaf area and xylem cross-sectional area (Kl and Ks) reflected opposite responses to Kr and Kfr. This indicates that the normalization criteria determine the interpretation of the effect of N on plant hydraulics, which can limit species and treatment comparisons.

Drought tolerance and acclimation in Pinus ponderosa seedlings: the influence of nitrogen form.

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.

Species ecology determines the role of nitrogen nutrition in the frost tolerance of pine seedlings.

Frost determines the evolution and distribution of plants in temperate and cold regions. Several environmental factors can influence frost acclimation of woody plants but the magnitude and direction of the effect of nitrogen (N) availability is controversial. We studied the effect of N availability on root and shoot frost tolerance in mid-fall and in winter in seedlings of four pines of contrasting ecology: Pinus nigra J.F. Arnold, P. pinaster Ait., P. pinea L. and P. halepensis Mill.. Organ N and soluble sugar concentration, and timing of cessation of shoot elongation were measured to assess the physiological mechanisms underlying frost acclimation. Nitrogen was supplied at high and low rates only during the pre-hardening period and at a moderate N rate during hardening in the fall. Shoot frost tolerance increased over winter while root frost tolerance did not change in any species. Pre-hardening N availability affected the frost tolerance of both roots and shoots, although the effect was species-specific: high N reduced the overall root and shoot frost tolerance in P. pinea and P. halepensis, and increased the frost tolerance in P. nigra, but had no effect in P. pinaster. Nitrogen supply in the fall consistently increased frost tolerance in all species. Differences in frost tolerance among species and N treatments were not explained by variations in organ N or soluble carbohydrate concentration, nor by timing of cessation of shoot elongation; however, the most frost tolerant species ceased elongation earlier than the least frost tolerant species. Despite the close phylogenetic relatedness of the studied species, the effect of N availability on seedling frost tolerance differed among species, indicating that species ecology (especially frost acclimation physiology) and timing of N supply drives the effect of N availability on frost tolerance of pine species.

Drought tolerance and transplanting performance of holm oak (Quercus ilex) seedlings after drought hardening in the nursery.

Drought stress is the main cause of mortality of holm oak (Quercus ilex L.) seedlings in forest plantations. We therefore assessed if drought hardening, applied in the nursery at the end of the growing season, enhanced the drought tolerance and transplanting performance of holm oak seedlings. Seedlings were subjected to three drought hardening intensities (low, moderate and severe) for 2.5 and 3.5 months, and compared with control seedlings. At the end of the hardening period, water relations, gas exchange and morphological attributes were determined, and survival and growth under mesic and xeric transplanting conditions were assessed. Drought hardening increased drought tolerance primarily by affecting physiological traits, with no effect on shoot/root ratio or specific leaf mass. Drought hardening reduced osmotic potential at saturation and at the turgor loss point, stomatal conductance, residual transpiration (RT) and new root growth capacity (RGC), but enhanced cell membrane stability. Among treated seedlings, the largest response occurred in seedlings subjected to moderate hardening. Severe hardening reduced shoot soluble sugar concentration and increased shoot starch concentration. Increasing the duration of hardening had no effect on water relations but reduced shoot mineral and starch concentrations. Variation in cell membrane stability, RT and RGC were negatively related to osmotic adjustment. Despite differences in drought tolerance, no differences in mortality and relative growth rate were observed between hardening treatments when the seedlings were transplanted under either mesic or xeric conditions.