Irradiance and soil nitrogen effects on growth, net photosynthesis and radiation use efficiency (RUE) of Brachiaria decumbens were investigated in fertilized and non-fertilized stands. Three levels of photosynthetic photon flux (PPF: S0 = 100%, S1 = 50% and S2 = 30%) and two N supplies, with (N+) and without (NÀ), were used. Forage biomass and nutrient accumulation, specific leaf area (SLA), leaf area index (LAI), fractional intercepted photosynthetic photon flux (fPPF), leaf photosynthetic response to light and efficiency of radiation use at leaf (A/Q) and canopy (RUE) levels were measured. Shade effects were mostly independent of soil N. Final yield was decreased by 34% (S1) and 57% (S2). Shade increased SLA (25-46%), so maximum LAI (2Á4-3Á3) was similar among light regimes. In NÀ stands, reductions in leaf biomass (14%), SLA (17%) and LAI (27%) were recorded, although forage yield was similar between soil N conditions. Under shade, peaks of A were comparable to those at full light, so A/Q was higher around midday. Derived parameters of the A-PPF curves were similar between S0 and S2. A maximum fPPF = 0Á8 (S0N+, S1N+) was recorded at LAI = 3-4. Under limited sunlight, relatively high RUE (1Á6-2Á8 g MJ À1 ) were observed over both soil N conditions. We concluded that B. decumbens had a high plasticity to shade, thus explaining its success under silvopastoral systems.

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Sustainable silvopastoral systems rely on the capacity of forage species to withstand shade, competition for soil resources and grazing. On a field‐plot experiment, we studied the effects of artificial shade on the performance of two tropical forage legumes: Centrosema molle (CIAT 15160) and C. macrocarpum (CIAT 5713). Acclimation to shade in these two species was investigated at individual leaf and whole canopies, to explain differences in the efficiency of radiation interception and use, and thus forage yield. Under 30% sunlight, midday leaf net photosynthesis (A) was reduced by 32%–44%, although instantaneous light use efficiency increased up to 44%–140%. Almost 50% increase in leaf specific area was accompanied by 61% in total chlorophyll content. From analysis of the light‐response curves, net photosynthesis at light saturation was only reduced (by 36%) in C. macrocarpum. Efficiency of chloroplast photosystem II (PSII) was unaffected by shade. Canopy structure for light interception was not modified by shade, although conversion into biomass and final yield were significantly reduced (40%–43%) in C. macrocarpum. Results are discussed in terms of factors affecting potential acclimation to shade and the agronomic implications of using both forage legumes in mixed pastures under tree canopies.

Xylem network structure and function have been characterized for many woody plants, but less is known about fern xylem, particularly in species endemic to climates where water is a limiting resource for months at a time. We characterized seasonal variability in soil moisture and frond water status in a common perennial fern in the redwood understory of a costal California, and then investigated the consequences of drought-induced embolism on vascular function. Seasonal variability in air temperature and soil water content was minimal, and frond water potential declined slowly over the observational period. Our data show that Polystichum munitum was protected from significant drought-induced hydraulic dysfunction during this growing season because of a combination of cavitation resistant conduits (Air-seeding threshold (ASP) = -1.53 MPa; xylem pressure inducing 50% loss of hydraulic conductivity (P50 ) = -3.02 MPa) and a soil with low moisture variability. High resolution micro-computed tomography (MicroCT) imaging revealed patterns of embolism formation in vivo for the first time in ferns providing insight into the functional status of the xylem network under drought conditions. Together with stomatal conductance measurements, these data suggest that P. munitum is adapted to tolerate drier conditions than what was observed during the growing season.

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Stylosanthes Sw. is considered one of the economically most important tropical forage legumes. The genus is essentially Neotropical, and Brazil, Venezuela and Mesoamerica are the main centres of species diversification. In view of its importance, an extensive review of Venezuelan Stylosanthes genetic resources was carried out. Firstly, the known natural distributions of all 11 species based on herbarium specimens were georeferenced and mapped and, by comparing them with information of germplasm (=seed) collection site distributions, for each species regions were identified which in terms of collecting and safeguarding genetic resources of Stylosanthes had not been explored yet. Secondly, the actual status of the Stylosanthes collection in Venezuela in terms of collected seed samples and available germplasm is discussed, as is the need for further explorations, including targeting ecologically important locations. Finally, the different Stylosanthes species in Venezuela are grouped according to their perceived agronomic value and potential as forage for animal production systems, and recommendations are given regarding research, collection and preservation needs. Overall, this study compiled baseline information for future considerations of an important genetic resource in Venezuela. The endangerment of the three endemic species, Stylosanthes falconensis Calles et SchultzeKr., Stylosanthes sericeiceps S.F. Blake and Stylosanthes venezuelensis Calles et Schultze-Kr. should be a major concern and warrants priority considerations for preservation.

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