Xylem form and function under extreme nutrient limitation: an example from California's pygmy forest

Xylem anatomy and function have large implications for plant growth as well as survival during drought, but the impact of nutrient limitation on xylem is not fully understood. This study examines the pygmy forest in California, a plant community that experiences negligible water stress but is severely stunted by low-nutrient and acidic soil, to investigate how nutrient limitation affects xylem function. Thirteen key anatomical and hydraulic traits of stems of four species were compared between pygmy forest plants and nearby conspecifics growing on richer soil. Resistance to cavitation (P₅₀ ), a critical trait for predicting survival during drought, had highly species-specific responses: in one species, pygmy plants had a 26% decrease in cavitation resistance compared to higher-nutrient conspecifics, while in another species, pygmy plants had a 56% increase in cavitation resistance. Other traits responded to nutrient limitation more consistently: pygmy plants had smaller xylem conduits and higher leaf-specific conductivity (K_L ) than conspecific controls. Edaphic stress, even in the absence of water stress, altered xylem structure and thus had substantial impacts on water transport. Importantly, nutrient limitation shifted cavitation resistance in a species-specific and unpredictable manner; this finding has implications for the assessment of cavitation resistance in other natural systems.

Current-year shoot hydraulic structure in two boreal conifers-implications of growth habit on water potential

Metabolic scaling theory allows us to link plant hydraulic structure with metabolic rates in a quantitative framework. In this theoretical framework, we considered the hydraulic structure of current-year shoots in Pinus sylvestris and Picea abies, focusing on two properties unaccounted for by metabolic scaling theories: conifer needles are attached to the entire length of shoots, and the shoot as a terminal element does not display invariant properties. We measured shoot length and diameter as well as conduit diameter and density in two locations of 14 current-year non-leader shoots of pine and spruce saplings, and calculated conductivities of shoots from measured conduit properties. We evaluated scaling exponents for the hydraulic structure of shoots at the end of the water transport pathway from the data and applied the results to simulate water potential of shoots in the crown. Shoot shape was intermediate between cylindrical and paraboloid. Contrary to previous findings, we found that conduit diameter scaled with relative, not absolute, distance from the apex and absolute under-bark shoot diameter independently of species within the first-year shoots. Shoot hydraulic conductivity scaled with shoot diameter and hydraulic diameter. Larger shoots had higher hydraulic conductance. We further demonstrate by novel model calculations that ignoring foliage distribution along the hydraulic pathway overestimates water potential loss in shoots and branches and therefore overestimates related water stress effects. Scaling of hydraulic properties with shoot size enhances apical dominance and may contribute to the decline of whole-tree conductance in large trees.

Hydraulic architecture and vulnerability to drought-induced embolism in southern boreal tree species of Inner Asia

The branch xylem of six important Inner Asian southern boreal forest trees was studied for wood-anatomical and hydraulic traits in order to infer the species' drought tolerance from embolism resistance, potential hydraulic conductivity, mean conduit diameters and conduit density. The only studied angiosperm tree, Betula pendula Roth, was much more sensitive to cavitation than all five conifers (evergreen or summer-green), even when using 88% loss of conductivity (P88) in birch, but 50% (P50) in the conifers as critical thresholds. This suggests that pioneer birch forests, which have widely replaced the conifer climax forests after anthropogenic disturbance (e.g., logging, man-made fire), are more vulnerable to climate warming-induced drought than the original conifer forests. In contrast to expectation, the generally more drought-exposed light taiga species (Larix sibirica Ledeb., Pinus sylvestris L.) did not have consistently lower P50 and P88 values than the dark taiga conifers, suggesting that other drought survival traits are equally important. Among the dark-taiga species, only Pinus sibirica Du Tour, but not Abies sibirica Ledeb. and Picea obovata Ledeb., had relatively high P50 values indicating higher vulnerability. In the light-taiga forest, P. sylvestris revealed lower embolism resistance than L. sibirica. In the face of rapid climate warming and drying in Inner Asia, the drought survival strategies of southern boreal tree species deserve further intensive study, which should include other drought survival traits.

Is it getting hot in here? Adjustment of hydraulic parameters in six boreal and temperate tree species after 5 years of warming

Global temperatures (T) are rising, and for many plant species, their physiological response to this change has not been well characterized. In particular, how hydraulic parameters may change has only been examined experimentally for a few species. To address this, we measured characteristics of the hydraulic architecture of six species growing in ambient T and ambient +3.4 °C T plots in two experimentally warmed forest sites in Minnesota. These sites are at the temperate-boreal ecotone, and we measured three species from each forest type. We hypothesized that relative to boreal species, temperate species near their northern range border would increase xylem conduit diameters when grown under elevated T. We also predicted a continuum of responses among wood types, with conduit diameter increases correlating with increases in the complexity of wood structure. Finally, we predicted that increases in conduit diameter and specific hydraulic conductivity would positively affect photosynthetic rates and growth. Our results generally supported our hypotheses, and conduit diameter increased under elevated T across all species, although this pattern was driven predominantly by three species. Two of these species were temperate angiosperms, but one was a boreal conifer, contrary to predictions. We observed positive relationships between the change in specific hydraulic conductivity and both photosynthetic rate (P = 0.080) and growth (P = 0.012). Our results indicate that species differ in their ability to adjust hydraulically to increases in T. Specifically, species with more complex xylem anatomy, particularly those individuals growing near the cooler edge of their range, appeared to be better able to increase conduit diameters and specific hydraulic conductivity, which permitted increases in photosynthesis and growth. Our data support results that indicate individual's ability to physiologically adjust is related to their location within their species range, and highlight that some wood types may adjust more easily than others.

Light requirements of Australian tropical vs. cool-temperate rainforest tree species show different relationships with seedling growth and functional traits

BACKGROUND AND AIMS

A trade-off between shade tolerance and growth in high light is thought to underlie the temporal dynamics of humid forests. On the other hand, it has been suggested that tree species sorting on temperature gradients involves a trade-off between growth rate and cold resistance. Little is known about how these two major trade-offs interact.

METHODS

Seedlings of Australian tropical and cool-temperate rainforest trees were grown in glasshouse environments to compare growth versus shade-tolerance trade-offs in these two assemblages. Biomass distribution, photosynthetic capacity and vessel diameters were measured in order to examine the functional correlates of species differences in light requirements and growth rate. Species light requirements were assessed by field estimation of the light compensation point for stem growth.

RESULTS

Light-demanding and shade-tolerant tropical species differed markedly in relative growth rates (RGR), but this trend was less evident among temperate species. This pattern was paralleled by biomass distribution data: specific leaf area (SLA) and leaf area ratio (LAR) of tropical species were significantly positively correlated with compensation points, but not those of cool-temperate species. The relatively slow growth and small SLA and LAR of Tasmanian light-demanders were associated with narrow vessels and low potential sapwood conductivity.

CONCLUSIONS

The conservative xylem traits, small LAR and modest RGR of Tasmanian light-demanders are consistent with selection for resistance to freeze-thaw embolism, at the expense of growth rate. Whereas competition for light favours rapid growth in light-demanding trees native to environments with warm, frost-free growing seasons, frost resistance may be an equally important determinant of the fitness of light-demanders in cool-temperate rainforest, as seedlings establishing in large openings are exposed to sub-zero temperatures that can occur throughout most of the year.