Mixing oak and pine trees in Mediterranean forests increases aboveground hydraulic dysfunctions

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.

提升地中海森林（Mediterranean forests）的树种多样性，可通过改善微气候（microclimate）、降低水分竞争（competition for water），减少干旱诱导的水力损伤（drought-induced hydraulic impairments）。然而，目前尚不清楚树种多样性是否、如何调控树木水力功能，以及其影响在生长季（growing season）内会如何变化。本研究以由单树种林分（monospecific stands）或四树种林分（multispecific stands）组成的未受管理地中海林分为研究对象，在为期两年的监测中，对4种共存的松属（Pinus）和栎属（Quercus）物种的原位水力性状（in-situ hydraulic traits）开展季节动态监测，所测性状包括：黎明前叶水势（predawn leaf water potential，Ψpd）、正午叶水势（midday leaf water potential，Ψmd）、木质部比导水率（xylem-specific hydraulic conductivity，KS）、叶片比导水率（leaf-specific hydraulic conductivity，KL）、导水率损失百分比（percentage loss of conductivity，PLC）、比叶面积（specific leaf area，SLA）以及休伯值（Huber value，HV）。研究主要观察到，相较于单树种林分，多树种林分中的树木Ψpd、Ψmd、KS、KL更低，PLC更高，即树种多样性带来了不利影响，这一现象在两种松树中尤为显著。此类不利影响贯穿整个生长季，但在夏季最干旱的时段表现得更为突出。而树种多样性的有益影响则较为罕见，仅在经历长期高强度水分胁迫（prolonged and intense water stress）后的Q. faginea中出现。本研究结果表明，将栎属与松属树种混交，在旱季可能会加剧所有物种的水力损伤，这意味着在未来气候变化背景下，地中海混交林存在潜在衰退风险。