Juan de Dios Miranda

Variability in amount and frequency of water supply affects roots but not growth of arid shrubs

Rainfall and soil moisture variability have a strong effect on plant survival and seed germination in arid environments, yet very little is known about the effects on roots and growth of woody seedlings. Here we focused on the effects of variability in both amount and frequency of water supply on juvenile root and leaf functional traits and growth of seven Mediterranean shrub species occurring in arid SE Spain, Anthyllis cytisoides, Atriplex halimus, Ephedra fragilis, Genista umbellata, Lycium intricatum, Retama sphaerocarpa, and Salsola oppositifolia. In a 14-month greenhouse experiment we manipulated water supply expecting that reduced water amount and pulses of watering of different magnitude affected functional traits and seedling growth, even if the amount of water provided was the same. Different watering patterns altered soil drying dynamics, with reduced supply of water amount and frequent watering becoming the driest treatment. We found that roots of all species responded to alterations in water supply by changing biomass allocation patterns (i.e., higher root-to-shoot mass [R:S] ratio in droughted plants), and by altering fine roots diameter, measured in terms of specific root length. Indeed, differences in growth rate among species were significantly linked to fine roots diameter and biomass allocation, which relates to uptake capacity of roots. However, relative growth rate and leaf traits such as specific leaf area were insensitive, likely because prolonged droughts over longer periods of time seem necessary to constraint growth in all these arid shrubs.

Woody species of a semi-arid community are only moderately resistant to cavitation

Vulnerability to drought-induced cavitation and seasonal water relations of six shrub species with different functional traits (deep v. superficial roots; evergreen v. summer deciduous; leaves v. cladodes) were measured in a semi-arid plant community strongly limited by water availability. The underlying hypotheses were that species would differ in their hydraulic properties and resistance to drought, reflecting different adaptations to a common environment and that individual adaptations may involve tradeoffs that would cause hydraulic properties to co-vary. Species experiencing the lowest minimum leaf water potentials generally had lower stomatal conductance, but they were not more resistant to xylem embolism than species with higher leaf water potentials and stomatal conductance. Overall, the studied species were more vulnerable to xylem embolism than expected and experienced high rates of native embolism and percent of leafless branches during summer drought. However, recovery rates from leafless branches were also high. Xylem resistance to embolism varied between species but had no relationship with minimum leaf water potential, suggesting that (i) adaptation to arid environments does not necessarily imply high resistance to embolism; and (ii) the costs associated with embolism resistance can be compensated by other components of the hydraulic strategy of a given species.

The influence of environmental degradation processes on the arbuscular mycorrhizal fungal community associated with yew (Taxus baccata L.), an endangered tree species from Mediterranean ecosystems of Southeast Spain

AimsTo assess whether the yew roots, which are able to provide a very constant environment due to their long life-span, can maintain the original arbuscular mycorrhizal (AM) fungal community during yew population decline.MethodsThe diversity of AM fungi (AMF) colonizing the roots of yew was analyzed by selecting the small subunit ribosomal RNA genes to construct a database of the overall community of AMF in the experimental area. A terminal restriction fragment length polymorphism (TRFLP) approach was used to identify the AMF communities present in yew roots. Physiological and environmental variables related to topology and soil and plant characteristics were determined as markers of habitat degradation.ResultsThe AMF communities within yew roots were found to be dependent on soil, plant and topological variables indicative of habitat degradation surrounding the yew. The phylogenetic diversity of AMF associated to the yews was lower in habitats more exposed to degradation than in those better conserved.ConclusionsThe target yews can be grouped into two degradation levels. AMF communities were also affected by the degradation processes affecting their hosts. This finding rules out the role of these trees as refugia for their original AMF community, a fact that should be considered in plant reintroduction programs using AMF as bioenhancers.