Impact of climate change-induced drought on tree root hydraulic properties and competition belowground

Drought has been projected to increase as global warming progresses, putting forest health in jeopardy. Water availability already is a major factor limiting plant growth, especially in forests and orchards on shallow, sandy soils or in regions with low precipitation. Furthermore, salinisation of agricultural soils is an increasing problem, often caused by the increased water demand for irrigation in drier climates. Research on drought and salt resistance strategies of plants has classically focussed on the dynamics of stem and leaf water. However, it is in the soil where water uptake occurs and plants compete for water and nutrients. Although an increasing number of studies are addressing belowground traits, a better understanding of the adaptability of tree root systems to water limitation and belowground competition is needed to be able to predict the effects of climate change on mature forest stands and woody crops. This study on five tree species in temperate mixed forests and Mediterranean fruit tree orchards aims at answering important questions on the influence of reduced soil moisture and competitive interactions on structure and function of tree root systems. In particular, the aims of this study were to test whether (i) root hydraulic conductivity decreases in response to water shortage as does shoot hydraulic conductivity, (ii) fine roots act as ‘hydraulic fuses’ of the soil-plant-atmosphere continuum, (iii) the below-ground competitive ability is symmetric and directly linked to root system size, and (iv) belowground competitive ability differs between tree species and is not modified by resource availability. This study showed that: i) Quercus petraea (MATT.) LIEBL. generally responded to moderate or severe drought with an increase in root axial conductivity, while Fagus sylvatica L. mostly did not. The same pattern was found in salt-stressed Olea europaea L. roots, the conductivity of which increased in response to increasing salinity. Drought- and/or salt-adapted tree species such as Quercus petraea and Olea europea seem to be capable of partly compensating for water shortage-induced root biomass losses by increasing root axial conductivity, a mechanism rarely observed in aboveground organs. ii) Fine roots can act as ‘hydraulic fuses’ in the soil-plant-atmosphere continuum. In addition to most previous studies, which suggest root shedding as the underlying mechanism, this study indicates cavitation to be another, potentially reversible, ‘hydraulic fuse’ mechanism. This conclusion is based on the very high vulnerability of small diameter roots of both Quercus and Fagus to cavitation and the further increase of this vulnerability in drought-stressed Quercus roots. iii) This study revealed two lines of evidence for the existence of asymmetry in below-ground competition: fine root biomass of Quercus petraea was over-proportionally reduced in species-rich allospecific stand patches as compared to monospecific ones, and root growth rates and morphology depended on the competitor present but not on initial root system size differences. iv) Tree species may be ranked according to their belowground competitive ability. According to data on root biomass and root growth in experimentally altered neighbour-hoods, Fagus sylvatica seems to be a superior competitor belowground. However, a marked asymmetry in the outcome of root competition only developed when soil moisture in summer was sufficiently high. Resource limitation, i.e. drought, seems to affect the competitive ability of Fagus sylvatica and Quercus petraea roots in a similar manner with the consequence that species-specific differences in competitive ability are less conspicuous in more stressful environments. Therefore, the overall importance of direct biotic interactions belowground seems to be reduced by limited soil water availability.