Evidence for a trade-off between growth rate and xylem cavitation resistance in Callitris rhomboidea

Substantial capacitance found in the roots of 2 contrasting conifer species

High rates of photosynthesis require abundant water delivered to the canopy to replace water lost to transpiration. In addition to water drawn immediately from the soil, stem capacitance has been identified as an additional water source, particularly during transient transpiration states. However, little information is available about the potential of roots to contribute to plant capacitance because methodological constraints have made it challenging to quantify root capacitance. In this study, we present a method to measure the water storage capacity of the root system and assess its contribution to daytime transpiration. We used an optical dendrometer to obtain in situ measurements of water potential and transpiration in 2 contrasting conifer species, Oyster Bay pine (Callitris rhomboidea) and Monterey pine (Pinus radiata), allowing us to quantify diurnal changes in plant water deficit. We employed a modified flow meter to gauge the rehydration kinetics of the below-ground and above-ground systems separately. We observed that root capacitance is a major supplier to the water demands during transient changes in transpiration for both species. Notably, the total below-ground capacitance exceeded the above-ground capacitance in C. rhomboidea, while the 2 capacitances were similar in P. radiata. Our findings highlight the importance of measuring and including below-ground capacitance in hydraulic models to accurately predict diurnal plant water status and stomatal behavior.

Adaptation in Wood Anatomical Traits to Temperature and Precipitation-A Common Garden Study

Indisputably, temperature and precipitation are key environmental variables driving plant trait variation and shaping plant ecological strategies. However, it is challenging to ascertain their relative influences because site temperature and precipitation are often correlated. Here, using Eucalyptus as a model system representing woody evergreen species more broadly, we sought to disentangle their influence on wood anatomical traits underpinning plant hydraulics. From a common garden we sampled 29 pairs of closely-related Eucalyptus species, each species-pair representing either a contrast in site temperature or precipitation, but never both. Very clearly, and both in phylogenetic and non-phylogenetic analyses, species from lower-rainfall and from colder regions had thicker vessel walls, likely an adaptation to drought and freezing, enabling water transport at more negative water potentials with reduced risk of cavitation or vessel implosion. On average, species from warmer regions had smaller vessels, but theoretical hydraulic conductivity remained stable across site temperatures due to increased vessel density compensating for reduced diameters. These trends being observed for adult plants grown under common conditions suggests that key hydraulic anatomy traits are "hard-wired", and gene × environment interactions are relatively weak. This is a key insight for understanding the trait-basis of plant ecological strategies related to site climate.

Contrasts in hydraulics underlie the divergent performances of Populus and native tree species in water-limited sandy land environments

Populus tree species are commonly used for creating shelter forests in vast areas of northern China, at least partially due to their fast growth. However, they are facing severe problems of decline and mortality caused by drought. In contrast, tree species native to water-limited environments usually have slow growth and are currently not commonly used in afforestation, while these species are gaining more attention in forestry for their greater resilience to drought. In Horqin Sandy Land, we conducted a comparative analysis of xylem hydraulics and associated physiological traits between six Populus tree species and six tree species native to drought-prone areas. Compared to the native species, the Populus species exhibited significantly higher stem hydraulic conductivity but lower resistance to drought-induced xylem embolism than the native tree species. The observed interspecific variations and contrasts in xylem hydraulics between the two species groups were predominantly attributed to xylem anatomical characteristics at the pit level rather than at the tissue level. In line with the divergences in hydraulics, we found significantly lower intrinsic water use efficiency (WUEi) in Populus than in the native species, suggesting that the two groups adopted relatively acquisitive and conservative water use strategies, respectively. The trade-off between hydraulic efficiency and safety, as well as that between hydraulic efficiency and WUEi, underlies the contrasts in performance between Populus species and the native tree species, that is, fast growth of Populus species but high risk of hydraulic dysfunction when facing drought, and vice versa.

Weak link or strong foundation? Vulnerability of fine root networks and stems to xylem embolism

Resolving the position of roots in the whole-plant hierarchy of drought-induced xylem embolism resistance is fundamental for predicting when species become isolated from soil water resources. Published research generally suggests that roots are the most vulnerable organ of the plant vascular system, although estimates vary significantly. However, our knowledge of root embolism excludes the fine roots (< 2 mm diameter) that form the bulk of total absorptive surface area of the root network for water and nutrient uptake. We measured fine root and stem xylem vulnerability in 10 vascular plant species from the major land plant clades (five angiosperms, three conifers, a fern and lycophyte), using standardised in situ methods (Optical Methods and MicroCT). Mean fine root embolism resistance across the network matched or exceeded stems in all study species. In six of these species (one fern, one lycophyte, three conifers and one angiosperm), fine roots were significantly more embolism resistant than stems. No clear relationship was found between root xylem conduit diameter and vulnerability. These results provide insight into the resistance of the plant hydraulic pathway at the site of water and nutrient uptake, and challenge the long-standing assumption that fine roots are more vulnerable than stems.

Contrasting survival strategies for seedlings of two northern conifer species to extreme droughts and floods

Lowland northern white-cedar (Thuja occidentalis L.) forests are increasingly exposed to extreme droughts and floods that cause tree mortality. However, it is not clear the extent to which these events may differentially affect regeneration of cedar and its increasingly common associate, balsam fir (Abies balsamea (L.) Mill.). To test this, we measured how seedlings of cedar and fir were able to avoid, resist and recover from experimental drought and flood treatments of different lengths (8 to 66 days). Overall, we found that cedar exhibited a strategy of stress resistance and growth recovery (resilience) from moderate drought and flood stress. Fir, on the other hand, appears to be adapted to avoid drought and flood stress and exhibited overall lower growth resilience. In drought treatments, we found evidence of different stomatal behaviors. Cedar used available water quickly and therefore experienced more drought stress than fir, but cedar was able to survive at water potentials > 3 MPa below key hydraulic thresholds. On the other hand, fir employed a more conservative water-use strategy and therefore avoided extremely low water potential. In response to flood treatments, cedar survival was higher and only reached 50% if exposed to 23.1 days of flooding in contrast to only 7.4 days to reach 50% mortality for fir. In both droughts and floods, many stressed cedar were able to maintain partially brown canopies and often survived the stress, albeit with reduced growth, suggesting a strategy of resistance and resilience. In contrast, fir that experienced drought or flood stress had a threshold-type responses and they either had full live canopies with little effect on growth or they died suggesting reliance on a strategy of drought avoidance. Combined with increasingly variable precipitation regimes, seasonal flooding and complex microtopography that can provide safe sites in these forests, these results inform conservation and management of lowland cedar stands.

Gradients in embolism resistance within stems driven by secondary growth in herbs

The stems of some herbaceous species can undergo basal secondary growth, leading to a continuum in the degree of woodiness along the stem. Whether the formation of secondary growth in the stem base results in differences in embolism resistance between the base and the upper portions of stems is unknown. We assessed the embolism resistance of leaves and the basal and upper portions of stems simultaneously within the same individuals of two divergent herbaceous species that undergo secondary growth in the mature stem bases. The species were Solanum lycopersicum (tomato) and Senecio minimus (fireweed). Basal stem in mature plants of both species displayed advanced secondary growth and greater resistance to embolism than the upper stem. This also resulted in significant vulnerability segmentation between the basal stem and the leaves in both species. Greater embolism resistance in the woodier stem base was found alongside decreases in the pith-to-xylem ratio, increases in the proportion of secondary xylem, and increases in lignin content. We show that there can be considerable variation in embolism resistance across the stem in herbs and that this variation is linked to the degree of secondary growth present. A gradient in embolism resistance across the stem in herbaceous plants could be an adaptation to ensure reproduction or basal resprouting during episodes of drought late in the lifecycle.

Variation in xylem vulnerability to cavitation shapes the photosynthetic legacy of drought

Increased drought conditions impact tree health, negatively disrupting plant water transport which, in turn, affects plant growth and survival. Persistent drought legacy effects have been documented in many diverse ecosystems, yet we still lack a mechanistic understanding of the physiological processes limiting tree recovery after drought. Tackling this question, we exposed saplings of a common Australian evergreen tree (Eucalyptus viminalis) to a cycle of drought and rewatering, seeking evidence for a link between the spread of xylem cavitation within the crown and the degree of photosynthetic recovery postdrought. Individual leaves experiencing >35% vein cavitation quickly died but this did not translate to a rapid overall canopy damage. Rather, whole canopies showed a gradual decline in mean postdrought gas exchange rates as water stress increased. This gradual loss of canopy function postdrought was due to a significant variation in cavitation vulnerability of leaves within canopies leading to diversity in the capacity of leaves within a single crown to recover function after drought. These results from the evergreen E. viminalis emphasise the importance of within-crown variation in xylem vulnerability as a central character regulating the dynamics of canopy death and the severity of drought legacy through time.