Within-crown plasticity of hydraulic properties influence branch dieback patterns of two woody plants under experimental drought conditions.

In recent year, widespread declines of Populus bolleana Lauche trees (P. bolleana, which dieback from the top down) and Haloxylon ammodendron shrubs (H. ammodendron, which dieback starting from their outer canopy) have occurred. To investigate how both intra-canopy hydraulic changes and plasticity in hydraulic properties create differences in vulnerability between these two species, we conducted a drought simulation field experiment. We analyzed branch hydraulic vulnerability, leaf water potential (Ψ), photosynthesis (A), stomatal conductance (gs), non-structural carbohydrate (NSCs) contents and morphological traits of the plants as the plants underwent a partial canopy dieback. Our results showed that: (1) the hydraulic architecture was very different between the two life forms; (2) H. ammodendron exhibited a drought tolerance response with weak stomatal control, and thus a sharp decline in Ψ while P. bolleana showed a drought avoidance response with tighter stomatal control that maintained a relatively stable Ψ; (3) the Ψ of H. ammodendron showed relative consistent symptoms of drought stress with increasing plant stature, but the Ψ of P. bolleana showed greater drought stress in higher portions of the crown; (4) prolonged drought caused P. bolleana to consume and H. ammodendron to accumulate NSCs in the branches of their upper canopy. Thus, the prolonged drought caused the shoots of the upper canopy of P. bolleana to experience greater vulnerability leading to dieback of the upper branches first, while all the twigs of the outer canopy of H. ammodendron experienced nearly identical degrees of vulnerability, and thus dieback occurred uniformly. Our results indicate that intra-canopy hydraulic change and their plasticity under drought was the main cause of the observed canopy dieback patterns in both species. However, more work is needed to further establish that hydraulic limitation as a function of plant stature was the sole mechanism for causing the divergent canopy dieback patterns.

[Effects of groundwater depth on functional traits of young Haloxylon ammodendron.] / 地下水埋深对幼龄梭梭功能性状的影响.

Groundwater is an important water source for phreatophytic shrubs in arid desert areas. In order to understand the impacts of groundwater depth on functional traits of phreatophytic shrubs, two groups of groundwater levels (2 and 3.5 m) were set up using lysimeter with automatic water replenishing instrument. We measured hydraulic traits, gas exchange characteristics, and root morphological parameters of young Haloxylon ammodendron during the growing season. The results showed that predawn assimilating branch water potential, osmotic potential at full turgor, and root length ratio of young H. ammodendron in the groundwater depth of 3.5 m were lower by 48.2%, 41.5% and 56.7% than that under groundwater depth of 2 m, respectively, while maximum net photosynthetic rate of late growing season, root volume, specific root length and specific root area of fine root were 75.7%, 41.0%, 273.7% and 67.7% higher, respectively. Midday water potential and water content of assimilating branch tended to decrease first in the early growing season and then increase in the late growing season. Root distribution of young H. ammodendron along soil profile showed a significant positive correlation between the average root diameter and soil depth, while the proportion of fine root surface area showed a significant negative correlation with soil depth at both groundwater levels. There was synergy of aboveground assimilating branch hydraulic traits and photosynthetic capacity with belowground root morphological traits in young H. ammodendron. Under the condition of increasing groundwater depth, young H. ammodendron adopted the ecological strategies of reducing predawn assimilating branch water potential and osmotic potential at full turgor, and increasing root diameter and length to enhance water deficit tolerance and expanding the area of water uptake to sustain their survival.