We studied the hydraulic architecture and water relations of nine co-occurring woody species in a Spanish evergreen oak forest over the course of a dry season. Our main objectives were to: (1) test the existence of a trade-off between hydraulic conductivity and security in the xylem, and (2) establish the safety margins at which the species operated in relation to hydraulic failure, and compare these safety margins between species and tissues (roots vs. stems). Our results showed that the relationship between specific hydraulic conductivity (K s) and resistance to cavitation followed a power function with exponent ≈-2, consistent with the existence of a trade-off between conductivity and security in the xylem, and also consistent with a linear relationship between vessel diameter and the size of inter-vessel pores. The diameter of xylem conduits, K s and vulnerability to xylem embolism were always higher in roots than in stems of the same species. Safety margins from hydraulic failure were narrower in roots than in stems. Among species, the water potential (Ψ) at which 50% of conductivity was lost due to embolism ranged between -0.9 and <-8 MPa for roots and between -2.0 and <-8 MPa for stems. Vulnerability to xylem embolism followed a pattern of: Quercus ilex=Acer monspessulanum=Arbutus unedo=Sorbus torminalis=Cistus laurifolius>Cistus albidus=Ilex aquifolium>Phillyrea latifolia>Juniperus oxycedrus. Gas exchange and seasonal Ψ minima were in general correlated with resistance to xylem embolism. Hydraulic safety margins differed markedly among species, with some of them (J. oxycedrus, I. aquifolium, P. latifolia) showing a xylem overly resistant to cavitation. We hypothesize that this overly resistant xylem may be related to the shape of the relationship between K s and security we have found.

Woody encroachment is occurring in all tropical savannas of the world. However, in the Brazilian savanna (the Cerrado), the extent of this phenomenon is still poorly documented. Here woody encroachment was quantified throughout the Cerrado biome and transitional ecotones using a trend analysis of the annual maximum of enhanced vegetation index obtained from the Moderate Resolution Imaging Spectroradiometer. The associations with potential local drivers, such as fire and land use regime, were assessed using satellite data of land cover and fire regime. We found that 19% of the remaining native vegetation showed significant evidence of woody encroachment in the last 15 years and 7% exhibited degradation processes. The local factors that favored woody expansion in 19% of the biome were a decrease of fire (34%) and land use abandonment (26%). Our study highlights that local human‐associated drivers are playing a major role in woody encroachment and savanna degradation.

We assessed data from 11 experiments examining the effects of the timing and/or frequency of fire on tropical forest and/or savanna vegetation structure over one decade or more. The initial 'control treatment' in many such cases consisted of previously cleared land. This is as opposed to natural vegetation subject to some sort of endogenous fire regime before the imposition of fire treatments. Effects of fire on fractional foliar cover are up to 10-fold greater when clearing pre-treatments are imposed. Moreover, because many of the 'classic' fire trials were initialised with applied management questions in mind, most have also used burning regimes much more frequent and/or severe than those occurring in the absence of human activity. Once these factors are taken into account, our modelling analysis shows that nonanthropogenic fire regimes serve to reduce canopy vegetative cover to a much lower extent than has previously been argued to be the case. These results call into question the notion that fire effects on tropical vegetation can be of a sufficient magnitude to maintain open-type savanna ecosystems under climatic/soil regimes otherwise sufficient to give rise to a more luxurious forest-type vegetation cover.