Performance of seedlings of Mediterranean woody species under experimental gradients of irradiance and water availability: trade-offs and evidence for niche differentiation

The aim of the study was to assess the potential importance for Mediterranean plants of trade-offs in the response to irradiance and water availability at the regeneration stage. Survival and growth patterns across an experimentally imposed irradiance gradient (1, 6, 20 and 100% sunlight) were studied in seedlings of eight Mediterranean woody species, together with the impact of a simulated summer drought. We found evidence of some of the trade-offs previously reported for non-Mediterranean plant communities, such as between survival in the shade and relative growth rate (RGR) at high light, but no evidence for others, such as between shade and drought tolerances. The impact of drought on survival and RGR was stronger in high light than in deep shade. The observed species-specific differences in performance provide a mechanistic basis for niche differentiation at the regeneration stage, contributing to possible explanations of species coexistence in Mediterranean ecosystems.     

The Effects of Drought and Shade on the Performance,Morphology and Physiology of Ghanaian Tree Species

In tropical forests light and water availability are the most important factors for seedling growth and survival but an increasing frequency of drought may affect tree regeneration. One central question is whether drought and shade have interactive effects on seedling growth and survival. Here, we present results of a greenhouse experiment, in which seedlings of 10 Ghanaian tree species were exposed to combinations of strong seasonal drought (continuous watering versus withholding water for nine weeks) and shade (5% irradiance versus 20% irradiance). We evaluated the effects of drought and shade on seedling survival and growth and plasticity of 11 underlying traits related to biomass allocation, morphology and physiology. Seedling survival under dry conditions was higher in shade than in high light, thus providing support for the “facilitation hypothesis” that shade enhances plant performance through improved microclimatic conditions, and rejecting the trade-off hypothesis that drought should have stronger impact in shade because of reduced root investment. Shaded plants had low biomass fraction in roots, in line with the trade-off hypothesis, but they compensated for this with a higher specific root length (i.e., root length per unit root mass), resulting in a similar root length per plant mass and, hence, similar water uptake capacity as high-light plants. The majority (60%) of traits studied responded independently to drought and shade, indicating that within species shade- and drought tolerances are not in trade-off, but largely uncoupled. When individual species responses were analysed, then for most of the traits only one to three species showed significant interactive effects between drought and shade. The uncoupled response of most species to drought and shade should provide ample opportunity for niche differentiation and species coexistence under a range of water and light conditions. Overall our greenhouse results suggest that, in the absence of root competition shaded tropical forest tree seedlings may be able to survive prolonged drought.     

Responses of temperate woody seedlings to shade and drought: do trade-offs limit potential niche differentiation?

Diversity in seedling responses to combined shade and drought can drive species niche differences, and thus natural forest and scrub establishment dynamics and diversity. However, inherent trade-offs between shade tolerance and drought tolerance, as hypothesized in the literature, would constrain potential niche differentiation. For thirteen species of European trees and shrubs the impacts were determined of moderate and extreme drought on the growth and survival of first-year seedlings in a given soil volume, in irradiances in the range typical for temperate forest and scrub understories (3% daylight) and large gaps (30% daylight). Pots were watered differentially each second day to equalize soil water content across species and irradiances. Comparisons within and across species supported independent tolerances of shade and drought rather than trade-offs. For all species, drought reduced relative growth rate in dry mass by the same proportion in 3% daylight and in 30% daylight. Consequently, drought generally reduced final dry mass significantly more strongly in 30% daylight than in 3% daylight. Extreme drought led to earlier mortality in 30% daylight than in 3% daylight for nine of the eleven species tested, to earlier mortality in 3% daylight for one species, and to equal mortality rate in both irradiances for one species. For each species, growth-based shade tolerance was quantified as the ratio of absolute growth in low irradiance to that in high irradiance, and growth-based drought tolerance as the ratio of absolute growth in low water supply to that in high water supply. Across species, growth-based shade tolerance varied ≈9-fold, and growth-based drought tolerance ≈2-fold; species’ tolerances correlated respectively with indices of field establishment in shade and drought. Growth-based drought tolerance correlated with survival time in extreme drought. Experimentally quantified shade and drought tolerances varied independently for the 13 species tested, indicating the potential for extensive species niche differentiation in combinations of irradiance and water supply.     

The functional morphology of juvenile plants tolerant of strong summer drought in shaded forest understories in southern Spain

It has been hypothesized that plants cannot tolerate combined shade and drought, as a result of morphological trade-offs. However, numerous plant species are reportedly widespread in shaded forest understories that face drought, whether seasonal or occasional. We studied juveniles of six plant species that cope with strong summer drought in the understoreys of mixed Quercus forests in southern Spain: the tall-shrubs Phillyrea latifolia and Viburnum tinus, the perennial herb Rubia peregrina, the small shrub Ruscus aculeatus, and climbers Hedera helix and Smilax aspera. All of these species persist in evergreen shade (c. 3% daylight). Two other species were studied as comparators, Ruscus hypoglossum, less tolerant of drought, and Ceratonia siliqua, less tolerant of shade. Morphological and chemical variables relevant to shade and drought tolerance were measured for juveniles in a range of sizes, and also for the leaves of mature plants. The species converge in features that confer tolerance of shade plus drought by reducing demand for resources. Demand for water is reduced through a moderate to high below-ground mass fraction and low to moderate specific leaf area (respectively 0.22–0.52 and 112–172 cm² g^–1 at 1.00 g total dry mass). Demand for both irradiance and water is reduced through a low to moderate foliar nitrogen concentration and long-lived, physically protected leaves (2 yr). The species also converge in features that confer tolerance of either low irradiance or drought through specialized capture of resource, without precluding the other tolerance. These features include deep roots relative to shoot size, moderately higher specific leaf area in shade (1.2–2.0 × that in sun) and higher chlorophyll:nitrogen ratio in shade. Foliar chlorophyll per unit mass was higher in shade, but chlorophyll was not necessarily synthesized in greater amounts; rather, it was higher apparently due to shade effects on structural features linked with specific leaf area. In contrast, N per unit mass was higher in sun leaves independently of specific leaf area. Despite these convergences, the species diverge considerably in their root mass allocation and architecture, leaf saturated water content, density of stomata and guard cell size. No single narrowly defined functional type is needed for tolerance of shade plus drought.     

Effects of climate‐induced increases in summer drought on riparian plant species: a meta‐analysis

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Drought can be more critical in the shade than in the sun: a field study of carbon gain and photo-inhibition in a Californian shrub during a dry El Niño year

Diurnal courses of leaf water potential (Ψ_l), gas exchange and chlorophyll fluorescence were measured in natural sun and shade populations of Heteromeles arbutifolia throughout the seasons of an unusually dry El Niño year in Central California. The onset of drought resulted in decreased stomatal conductance and net photosynthesis in both sun and shade plants. However, the decline in Ψ_l was much greater and carbon gain was much more strongly limited by the development of drought stress in the shade than in the sun. Photorespiratory energy dissipation was significantly higher in the sun than in the shade in spring and autumn, but not during the summer. Pre‐dawn photochemical efficiency (F_v/F_m) was significantly higher in the shade than in the sun during the spring but the differences disappeared during the summer and autumn. The strong irradiance in the open field site studied led to a chronic but only mild reduction in F_v/F_m, with values around 0·79. Summer sunflecks led to a sustained photo‐inhibition in shade plants, which exhibited a significant reduction in pre‐dawn F_v/F_m of 10% with the onset of drought. Photo‐inhibition became relatively more important for carbon gain in the shade than in the sun due to the low photochemical efficiency under the low light that follows sunflecks. Sun plants of H. arbutifolia exhibited a rather efficient photoprotection against strong irradiance conferred by both the architecture of the crown and the physiology of the leaves. There is evidence that El Niño events and the associated droughts have become more frequent and severe. Counter‐intuitively, the effects on plant performance of such extreme droughts could be more critical in the shade than in the sun.     

Seedling survival responses to irradiance are differentially influenced by low-water availability in four tree species of the Iberian cool temperate–Mediterranean ecotone

Inter-specific differences in seedling survival responses along a sun-shade gradient and the influence of low-water availability were examined for four Iberian tree species (Quercus robur L., Quercus pyrenaica Willd., Pinus sylvestris L. and Pinus pinaster Ait.) typical of the cool temperate–Mediterranean transition zone. Seedlings were grown under controlled conditions in a factorial experiment with four levels of irradiance (1%, 6%, 20% and 100% of full sunlight) and two levels of water availability. Five censuses (from late spring to autumn) leading to four regular intervals (T₀ → T_I; T_I → T_II; T_II → T_III; T_III → T_IV) were established. Statistical models of seedling survival as a function of irradiance were calibrated throughout the whole experiment (T₀ → T_IV) and also for each time interval and water availability level. Seedling survival responses among different species diverged both in the type of functional response to irradiance and in their response to water stress. Ranking of species according to shade tolerance (Q. pyrenaica > Q. robur > P. sylvestris > P. pinaster) contrasted with tolerance of high irradiance and conformed to a hypothetical sun-shade trade-off for survival (i.e. species having higher survival in low irradiance—oaks—had poorer survival at high irradiance and vice-versa). Low-water availability also differentially affected each species, with pines being more drought tolerant than oaks. At an intra-specific level, low-water availability decreased survival of Q. pyrenaica under both high and low irradiance. For Q. robur, however, low-water availability exerted a relatively stronger effect under low irradiance. Consequences of the interplay between irradiance and water availability for explaining segregation and coexistence of forest tree species at the ecotone between cool temperate and Mediterranean forests are discussed.     

The Importance of Soil Characteristics in Determining Survival of First‐Year Cottonwood Seedlings in Altered Riparian Habitats

Most major rivers in the southwestern United States have been hydrologically altered to meet human needs. Altered hydrological regimes have been associated with declines in native riparian forests. Today, many riparian areas have little or no regeneration of native riparian species and are now dominated by exotic Saltcedar (Tamarix chinensis Lour.). Success of riparian restoration efforts at least partially depends on the number of seedlings surviving the first growing season. Seedling survival is influenced by many abiotic and biotic factors including competition from other plants and available soil moisture, which is partially dependent on soil texture. In this study, we evaluated the relative importance of four soil categories (sandy loam, loam, silt, and clay), rate of soil moisture decline, salinity, beginning‐ and end‐season Saltcedar density, initial Cottonwood (Populus deltoides Marshall subsp. wislizenii (Wats.) Eckenw.) seedling density, percent vegetation cover by potential dominant competitors Pigweed (Amaranthus L.) and Barnyard grass (Echinochloa crusgalli L., Beauv.), and average total vegetation height to Cottonwood seedling survival. Factors influencing seedling survival differed among the four soil types. Rate of moisture decline was important in sandy soils, whereas vegetation height influenced seedling survival in loamy soils. Overall, models of seedling survival in all the four soil types indicated rate of moisture decline as the single most important variable influencing Cottonwood survival. High initial densities of Saltcedar were correlated to higher survival in Cottonwood seedlings. Therefore, it is important to identify soil texture and understand soil moisture decline rates when proposing riparian Cottonwood restoration.     

Acacia karroo invasion of grassland: environmental and biotic effects influencing seedling emergence and establishment

Acacia karroo Hayne is the most important woody invader of grassland in South Africa, and can greatly reduce the productivity of grassland. A field experiment was conducted to test the hypotheses that emergence, growth and the 1st year's survival of Acacia karroo would be enhanced by (1) defoliation of the grass sward, (2) increased irradiance, (3) increased moisture availability and (4) its germination within cattle dung pats. The study was conducted on one site above and one below the natural altitudinal treeline of this species in grassland of the eastern Cape, South Africa. Not one seedling emerged from dung pats. Neither location nor the other treatments affected the density of emerging seedlings, although only 40.4 seedlings m^–2 emerged of the 200 seeds m^–2 planted. Shading dramatically increased the density of surviving seedlings. In the open, only 3 and 1.5 seedlings m^–2 remained respectively at the end of the growing season or the beginning of the next, compared to 23.3 and 19.5 seedlings m^–2 under shading for these respective times. This was attributed to the effect of shade on moisture availability in a season which received only 54% of average rainfall. Seedling survival until the end of the growing season was enhanced (30%) by shade at both sites, but also by supplemental water (24%) and defoliation of the sward (7%) at the site above the treeline. Across sites and treatments, seedling survival was related to moisture availability, with no or poor survival for < 500 mm rainfall, indicating this species can only establish in certain rainfall years. Seedling survival over winter was not influenced by treatment, but was greater for larger seedlings. Treatments affected seedling size, in particular seedlings growing under shade and within a dense grass canopy were etiolated. A. karroo seedlings are capable of establishing and surviving within a dense grass sward for at least a year, tolerant of low irradiance and of interference, which, because most seeds do not persist for much longer than a year, suggests this species forms predominantly a seedling bank. This has implications for the invasion of grassland by woody species.     

Environmental tolerance of an invasive riparian tree and its potential for continued spread in the southwestern US

Questions: Exotic plant invasion may be aided by facilitation and broad tolerance of environmental conditions, yet these processes are poorly understood in species‐rich ecosystems such as riparian zones. In the southwestern United States (US) two plant species have invaded riparian zones: tamarisk (Tamarix ramosissima, T. chinensis, and their hybrids) and Russian olive (Elaeagnus angustifolia). We addressed the following questions: (1) is Russian olive able to tolerate drier and shadier conditions than cottonwood and tamarisk? (2) Can tamarisk and cottonwood facilitate Russian olive invasion? Location: Arid riparian zones, southwestern US. Methods: We analyzed riparian tree seedling requirements in a controlled experiment, performed empirical field studies, and analyzed stable oxygen isotopes to determine the water sources used by Russian olive. Results: Russian olive survival was significantly higher in dense shade and low moisture conditions than tamarisk and cottonwood. Field observations indicated Russian olive established where flooding cannot occur, and under dense canopies of tamarisk, cottonwood, and Russian olive. Tamarisk and native riparian plant species seedlings cannot establish in these dry, shaded habitats. Russian olive can rely on upper soil water until 15 years of age, before utilizing groundwater. Conclusions: We demonstrate that even though there is little evidence of facilitation by cottonwood and tamarisk, Russian olive is able to tolerate dense shade and low moisture conditions better than tamarisk and cottonwood. There is great potential for continued spread of Russian olive throughout the southwestern US because large areas of suitable habitat exist that are not yet inhabited by this species.     

Light Response of Seedlings of a Central African Timber Tree Species,Lophira alata (Ochnaceae), and the Definition of Light Requirements

Light is of primary importance in structuring tropical tree communities. Light exposure at seedling and adult stages has been used to characterize the ecological profile of tropical trees, with many implications in forest management and restoration ecology. Most shade‐tolerance classification systems have been proposed based on empirical observations in a specific area and thus result in contradictions among categories assigned to a given species. In this study, we aimed to quantify the light requirements for seedling growth of a Central African timber tree, Lophira alata (Ochnaceae), taking into account effects of population origin. In two controlled experiments: a light response experiment and a comparative population experiment, conducted in southwestern Cameroon, using seeds collected from four populations (three from Cameroon and one from Gabon), we examined the quantitative responses to irradiance of seedlings. After 2 years, mortality was very low (<3%), even in extremely low irradiance. Growth and biomass allocation patterns varied in response to light, with intermediate irradiance (24–43%) providing optimal conditions. Light response differed between populations. The Boumba population in the northeastern edge of the species' distribution exhibited the highest light requirements, suggesting a local adaptation. As a result of positive growth at low irradiance and maximum growth at intermediate irradiance, we concluded that L. alata exhibits characteristics of both non‐pioneer and pioneer species. Implications of our results to propose an objective way to assign the light requirement for tropical tree species are discussed.     

Non-linear effects of drought under shade: reconciling physiological and ecological models in plant communities

The combined effects of shade and drought on plant performance and the implications for species interactions are highly debated in plant ecology. Empirical evidence for positive and negative effects of shade on the performance of plants under dry conditions supports two contrasting theoretical models about the role of shade under dry conditions: the trade-off and the facilitation hypotheses. We performed a meta-analysis of field and greenhouse studies evaluating the effects of drought at two or more irradiance levels on nine response variables describing plant physiological condition, growth, and survival. We explored differences in plant response across plant functional types, ecosystem types and methodological approaches. The data were best fit using quadratic models indicating a humped-back shape response to drought along an irradiance gradient for survival, whole plant biomass, maximum photosynthetic capacity, stomatal conductance and maximal photochemical efficiency. Drought effects were ameliorated at intermediate irradiance, becoming more severe at higher or lower light levels. This general pattern was maintained when controlling for potential variations in the strength of the drought treatment among light levels. Our quantitative meta-analysis indicates that dense shade ameliorates drought especially among drought-intolerant and shade-tolerant species. Wet tropical species showed larger negative effects of drought with increasing irradiance than semiarid and cold temperate species. Non-linear responses to irradiance were stronger under field conditions than under controlled greenhouse conditions. Non-linear responses to drought along the irradiance gradient reconciliate opposing views in plant ecology, indicating that facilitation is more likely within certain range of environmental conditions, fading under deep shade, especially for drought-tolerant species.     

Abiotic constraints on the establishment of Quercus seedlings in grassland

High evaporative demand and periodic drought characterize the growing season in midwestern grasslands relative to deciduous forests of the eastern US, and predicted climatic changes suggest that these climatic extremes may be exacerbated. Despite this less than optimal environment for tree seedling establishment, deciduous trees have expanded into adjacent tallgrass prairie within the last century leading to a dramatic land cover change. In order to determine the role of light and temperature on seedling establishment, we assessed carbon and water relations and aboveground growth of first‐year Quercus macrocarpa seedlings exposed to one of three conditions: (1) intact tallgrass prairie communities (control), (2) aboveground herbaceous biomass removed (grass removal), and (3) shade plus biomass removal to reduce light (PFD) to levels typical of the grassland‐forest ecotone (shade). In the 2000 growing season, precipitation was 35% below the long‐term average, which had a significant negative effect on oak seedling carbon gain at midseason (photosynthesis declined to 10% of maximum rates). However, net photosynthesis and stomatal conductance in the shade treatment was ca. 2.5 and 1.5 times greater, respectively, than in control treatment seedlings during this drought. During this period, leaf and air temperatures in control seedlings were similar whereas leaf temperatures in the shade treatment remained below air temperature. A late‐season recovery period, coincident with decreased air temperatures, resulted in increased net photosynthesis for all seedlings. Increased photosynthetic rates and water relations in shaded seedlings compared to seedlings in full sun suggest that, at least in dry years, high light and temperature may negatively impact oak seedling performance. However, high survival rates for all seedlings indicate that Q. macrocarpa seedlings are capable of tolerating both present‐day and future climatic extremes. Unless historic fire regimes are restored, forest expansion and land cover change are likely to continue.     

Heritable variation in the survival of seedlings from Patagonian cypress marginal xeric populations coping with drought and extreme cold

The rear edges of tree species have begun to be perceived as highly valuable for genetic resources conservation and management. In view of expected climatic changes, the responses of trees at their xeric limits may largely be determined by their capacity to cope with augmented environmental variance. We assess the heritability of early survival of Patagonian cypress in two common-garden field tests with contrasting summer water deficits, comprising 140 and 163 open-pollinated families from 10 marginal xeric populations. The first experiment underwent less rigorous conditions than the average mesic, Mediterranean climatic conditions, which were sufficient to reveal additive genetic effects of summer drought on seedling survival. The second trial suffered strong summer water-deficit stress and a winter extreme cold event. In this harsher environment, the heritabilities of survival under summer water-deficit stress were high in all the populations (h ²?=?0.84 on average), while the heritabilities of seasonal, extreme cold survival were moderate or even nil (h ²?=?0.28 on average). We did not find evidence of genetic differentiation among populations in their capabilities to survive droughts and cold extremes. Our results indicate that even when climatic changes were strong enough to cause the extinction of the most threatened populations, heritable variation for traits underlying drought and cold tolerances may allow the marginal xeric edge of cypress to persist under augmented environmental variance, without losing overall genetic diversity.     

Interactions of drought and shade effects on seedlings of four Quercus species: physiological and structural leaf responses

Here, we investigated the physiological and structural leaf responses of seedlings of two evergreen and two deciduous Quercus species, grown in a glasshouse and subjected to contrasted conditions of light (low, medium and high irradiance) and water (continuous watering vs 2-months drought). The impact of drought on photosynthetic rate was strongest in high irradiance, while the impact of shade on photosynthetic rate was strongest with high water supply, contradicting the hypothesis of allocation trade-off. Multivariate causal models were evaluated using d-sep method. The model that best fitted the dataset proposed that the variation in specific leaf area affects photosynthetic rate and leaf nitrogen concentration, and this trait determines stomatal conductance, which also affects photosynthetic rate. Shade conditions seemed to ameliorate, or at least not aggravate, the drought impact on oak seedlings, therefore, the drought response on leaf performance depended on the light environment.     

Rapid root extension during water pulses enhances establishment of shrub seedlings in the Atacama Desert

Questions: (1) What is the water threshold for shrub seedling establishment in arid scrubland? (2) How do seedling root growth morphological traits affect the water threshold required for seedling establishment? Location: Arid scrubland, Atacama Desert, north‐central Chile. Methods: We conducted a field experiment with nine native shrub species under a gradient of simulated rainfall to test if species with deep root architecture have higher seedling survival rates and establish more successfully during water pulses. Results: Seedling survival rate was very low; only 2% of the 12 150 planted seedlings survived the first summer drought. Seedling survival required at least 206 mm of water, which is twice the average rainfall and roughly equivalent to the precipitation during an ENSO (El Niño Southern Oscillation) event in this region. Seedling survival at the onset of the summer drought was best explained by leaf habit, root length and initial seedling size. However, only Senna cumingii seedlings survived through the first year. S. cumingii seedlings had distinctively longer roots than the other shrub species, enabling them to reach moister soil layers. Conclusions: Water conditions resembling rainy years enhance shrub seedling establishment in the Atacama Desert, but the effects of higher water availability are strongly dependent on the shrub species. Rapid and deep rooting appears to be a very important functional trait for successful first‐year survival in this arid system where water availability is highly episodic.     

Irradiance and temperature affect the competitive interference of blackberry on the physiology of European beech seedlings

The potential negative influence of competition from early successional species like blackberry (Rubus fruticosus) may be decisive for the natural regeneration success of drought-sensitive beech (Fagus sylvatica), especially in the light of climate change. With a split plot glasshouse experiment, we investigated the influence of two air temperature and irradiance levels on the competitive interference of blackberry on the water, nitrogen (N) and carbon (C) balance of beech seedlings under moderate drought. When increased temperature was accompanied by low irradiance the biomass, root-to-shoot ratio, N uptake and assimilation rates of blackberry were lower compared with beech, either grown alone or with blackberry. By contrast, when elevated temperature and high irradiance were combined, the root-to-shoot ratio and specific N uptake rate of blackberry were substantially increased, while the N acquisition of beech was impaired. Under lower temperature, with either full light or shade, the presence of blackberry had no significant effects on beech, for almost all tested parameters. Under elevated air temperature beech was impaired by the presence of blackberry at high irradiance. These findings emphasize the interacting effects between environmental factors and competition on the establishment of beech regeneration, which should be considered for future forest management in the frame of climate change.     

Effects of position, understorey vegetation and coarse woody debris on tree regeneration in two environmentally contrasting forests of north-western Patagonia: a manipulative approach

Aim To investigate the differential effects of position within gaps, coarse woody debris and understorey cover on tree seedling survival in canopy gaps in two old‐growth Nothofagus pumilio (Poepp. & Endl.) Krasser forests and the response of this species to gaps in two forests located at opposite extremes of a steep rainfall gradient. Location Nahuel Huapi National Park, at 41° S in north‐western Patagonia, Argentina. Methods In both study sites, seedlings were transplanted to experimental plots in gaps in three different positions, with two types of substrate (coarse woody debris or forest floor), and with and without removal of understorey vegetation. Survival of seedlings was monitored during two growing seasons. Soil moisture and direct solar radiation were measured once in mid‐summer. Seedling aerial biomass was estimated at the end of the experiment. Results Mid‐summer soil water potential was lowest in the centre of gaps, in plots where the understorey had been removed, and highest at the northern edges of gaps. Direct incoming radiation was highest in gap centres and southern edges, and lowest at northern edges. Seedling mortality was highest in gap centres, in both sites. Coarse woody debris had a positive effect on seedling survival during summer in the mesic forest and during winter in the xeric forest. The removal of understorey cover had negative effects in gap centres during summer. Seedling final aerial biomass was positively affected by understorey removal and by soil substrate in both sites. In the dry forest gaps, seedling growth was highest in northern edges, whereas it was highest in gap centres in the mesic forest. Overall growth was positively related to survival in the xeric forest, and negatively related in the mesic forest. Main conclusions Survival and growth were facilitated by the shade of gap‐surrounding trees only in the xeric forest. Understorey vegetation of both forests facilitated seedling survival in exposed microsites but competed with seedling growth. Nurse logs were an important substrate for seedling establishment in both forests; however, causes of this pattern differed between forests. Water availability positively controls seedling survival and growth in the xeric forest while in the mesic forest, survival and growth are differentially controlled by water and light availability, respectively. These two contrasting old‐growth forests, separated by a relatively short distance along a steep rainfall gradient, had different yet unexpected microenvironmental controls on N. pumilio seedling survival and growth. These results underscore the importance of defining microscale limiting factors of tree recruitment in the context of large‐scale spatial variation in resources.     

Is hemiepiphytism an adaptation to high irradiance? Testing seedling responses to light levels and drought in hemiepiphytic and non‐hemiepiphytic Ficus

The epiphytic growth habit in many Ficus species during their juvenile stages has commonly been hypothesized to be an adaptation for avoiding deep shade in the forest understory, but this has never been tested experimentally. We examined growth and ecophysiology in seedlings of three hemiepiphytic (Hs) and three non‐hemiepiphytic (NHs) Ficus species grown under different irradiance levels. Both Hs and NHs exhibited characteristics of high light requiring species, such as high plasticity to growth irradiance and relatively high maximum photosynthetic assimilation rates. Diurnal measurements of leaf gas exchange showed that Hs have much shorter active photosynthetic periods than NHs; moreover, leaves of Hs have lower xylem hydraulic conductivity but stronger drought tolerance as indicated by much lower rates of leaf diebacks during the drought treatment. Seedlings of NHs had 3.3‐ and 13.3‐fold greater height and biomass than those of Hs species after growing in the nursery for 5 months, indicating a trade‐off between growth and drought tolerance due to the conflicting requirements for xylem conductivity and cavitation resistance. This study does not support the shade‐avoidance hypothesis; rather, it suggests that the canopy regeneration in Hs is an adaptation to avoid alternative terrestrial growth‐related risks imposed to tiny Ficus seedlings. The NHs with terrestrial regeneration reduce these risks by having an initial burst of growth to rapidly gain relatively large seedling sizes, while in Hs seedlings more conservative water use and greater drought tolerance for surviving the canopy environment are intrinsically associated with slow growth.     

Simulated herbivory does not constrain phenotypic plasticity to shade through ontogeny in a relict tree

Ecological limits to phenotypic plasticity (PP), induced by simultaneous biotic and abiotic factors, can prevent organisms from exhibiting optimal plasticity, and in turn lead to decreased fitness. Herbivory is an important biotic stressor and may limit plant functional responses to challenging environmental conditions such as shading. In this study we investigated whether plant functional responses and PP to shade are constrained by herbivory, and whether such constraints are due to direct effects based on resource limitation by considering ontogeny. We used as a model system the relict tree Prunus lusitanica and implemented an indoor experiment to quantify the response of saplings of different ages to shade and herbivory. We measured five functional traits and quantitatively calculated PP. Results showed that herbivory did not constrain functional responses or PP to shade except for shoot:root ratio (SR), which, despite showing a high PP in damaged saplings, decreased under shade instead of increasing. Damaged saplings of older age did not exhibit reduced constraints on functional responses to shade and generally presented a lower PP than damaged saplings of younger age. Our findings suggest that herbivory‐mediated constraints on plant plasticity to shade may not be as widespread as previously thought. Nonetheless, the negative effect of herbivory on SR plastic expression to shade could be detrimental for plant fitness. Finally, our results suggest a secondary role of direct effects (resource‐based) on P. lusitanica plasticity limitation. Further studies should quantify plant resources in order to gain a better understanding of this seldom‐explored subject.