Increasing tree species diversity in Mediterranean forests could reduce drought-induced hydraulic impairments through improved microclimate and reduced competition for water. However, it remains unclear if and how species diversity modulates tree hydraulic functions and how impacts may shift during the growing season. Using unmanaged Mediterranean forest stands composed of one (i.e., monospecific) or four (i.e., multispecific) tree species, we examined the seasonal dynamics of in-situ hydraulic traits (predawn and midday leaf water potential - Ψpd and Ψmd, xylem- and leaf-specific hydraulic conductivity - KS and KL, percentage loss of conductivity - PLC, specific leaf area - SLA, and Huber value - HV) in four co-existing Pinus and Quercus species over two years. We mainly observed adverse impacts of species diversity with lower Ψpd, Ψmd, KS, KL, and higher PLC in multispecific compared to monospecific stands, especially for the two pines. These impacts were observed all along the growing season but were stronger during the driest periods of the summer. Beneficial impacts of diversity were rare and only occured for oaks (Q. faginea) after prolonged and intense water stress. Our findings reveal that mixing oaks and pines could mainly enhance hydraulic impairments for all species during the dry season, suggesting a potential decline in mixed Mediterranean forests under future climate.
This study investigated the effect of mixing volatile organic compounds (VOC) emitted by host and non-host plants on the orientation of key pests of Brassicaceae. The study aimed to understand how these mixed VOCs influence pest behaviour, which could help in tailoring pest management strategies. The orientations of flea beetles, Phyllotreta spp., and the diamondback moth (DBM), Plutella xylostella, towards cabbage VOCs mixed with faba bean VOCs were assessed using Y-tube olfactometry. The pests' preferences were measured to determine if the presence of faba bean alongside cabbage altered their olfactory orientation compared to cabbage alone. Flea beetles showed a preference for cabbage VOCs alone over the cabbage-faba bean VOC mix. For DBM, no significant preference was observed between cabbage alone and the cabbage-faba bean mix. Previous findings indicated that faba bean attracts DBM, and in this study the mixture of cabbage and faba bean appeared to be more attractive than cabbage alone. The results indicate that faba bean VOCs can deter flea beetles from cabbage, potentially offering a pest management strategy. However, the effect on DBM was inconclusive, with no clear preference observed. This suggests that while faba bean VOCs may influence pest orientation, their effectiveness varies among different pest species. Additionally, herbivore damage to cabbage leaves did not appear to influence the odour-guided orientation of either pest, irrespective of the presence or absence of faba bean.
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