ALBERT

Description: Nocturnal water flux has been observed in trees under a variety of environmental conditions and can be a significant contributor to diel canopy water flux. Elevated atmospheric CO 2 (elevated [CO 2 ]) can have an important effect on day-time plant water fluxes, but it is not known whether it also affects nocturnal water fluxes. We examined the effects of elevated [CO 2 ] on nocturnal water flux of field-grown Eucalyptus saligna trees using sap flux through the tree stem expressed on a sapwood area ( J s ) and leaf area ( E t ) basis. After 19 months growth under well-watered conditions, drought was imposed by withholding water for 5 months in the summer, ending with a rain event that restored soil moisture. Reductions in J s and E t were observed during the severe drought period in the dry treatment under elevated [CO 2 ], but not during moderate- and post-drought periods. Elevated [CO 2 ] affected night-time sap flux density which included the stem recharge period, called ‘total night flux’ (19:00 to 05:00, J s,r ), but not during the post-recharge period, which primarily consisted of canopy transpiration (23:00 to 05:00 , J s,c ). Elevated [CO 2 ] wet (EW) trees exhibited higher J s,r than ambient [CO 2 ] wet trees (AW) indicating greater water flux in elevated [CO 2 ] under well-watered conditions. However, under drought conditions, elevated [CO 2 ] dry (ED) trees exhibited significantly lower J s,r than ambient [CO 2 ] dry trees (AD), indicating less water flux during stem recharge under elevated [CO 2 ]. J s,c did not differ between ambient and elevated [CO 2 ]. Vapour pressure deficit ( D ) was clearly the major influence on night-time sap flux. D was positively correlated with J s,r and had its greatest impact on J s,r at high D in ambient [CO 2 ]. Our results suggest that elevated [CO 2 ] may reduce night-time water flux in E. saligna when soil water content is low and D is high. While elevated [CO 2 ] affected J s,r , it did not affect day-time water flux in wet soil, suggesting that the responses of J s,r to environmental factors cannot be directly inferred from day-time patterns. Changes in J s,r are likely to influence pre-dawn leaf water potential, and plant responses to water stress. Nocturnal fluxes are clearly important for predicting effects of climate change on forest physiology and hydrology.

Description: Although partial harvesting has been proposed as a measure for maintaining compositional and structural characteristics of natural old-growth stands, it has not been rigorously evaluated. The first objective of this study was to compare forest composition and structural characteristics after killing a portion of canopy trees and after partial cuts in boreal mixedwoods of eastern Canada by natural disturbances. The second objective was to evaluate the effectiveness of the partial harvest treatments in meeting the silvicultural objectives of increased growth of residual stems and advances in regeneration. Higher densities of shade-intolerant species were found after partial cuts when comparedwith naturally disturbed stands. Similar diameter distributions of residual living trees and densities of shade-tolerant species were found following all disturbances. Increase in growth was similar for all height classes (〈1 m, 1–2 m, 2–4 mand 〉 4 m) of balsam fir ( Abies balsamea ) advance regeneration in naturally disturbed and partially cut stands. In the overstory, trembling aspen ( Populus tremuloides ), white birch ( Betula papyrifera ), white spruce ( Picea glauca ) and balsam fir had growth increases after partial cuts, whereas black spruce ( Picea mariana ) did not. This study provides evidence that partial cutting in these mixedwood stands may emulate the effects of natural mortality resulting from natural partial disturbances by retaining key compositional and structural attributes and inducing increased growth rates in residual trees.

Description: Pulse-labelling of trees with stable or radioactive carbon (C) isotopes offers the unique opportunity to trace the fate of labelled CO 2 into the tree and its release to the soil and the atmosphere. Thus, pulse-labelling enables the quantification of C partitioning in forests and the assessment of the role of partitioning in tree growth, resource acquisition and C sequestration. However, this is associated with challenges as regards the choice of a tracer, the methods of tracing labelled C in tree and soil compartments and the quantitative analysis of C dynamics. Based on data from 47 studies, the rate of transfer differs between broadleaved and coniferous species and decreases as temperature and soil water content decrease. Labelled C is rapidly transferred belowground—within a few days or less—and this transfer is slowed down by drought. Half-lives of labelled C in phloem sap (transfer pool) and in mature leaves (source organs) are short, while those of sink organs (growing tissues, seasonal storage) are longer. 13 C measurements in respiratory efflux at high temporal resolution provide the best estimate of the mean residence times of C in respiratory substrate pools, and the best basis for compartmental modelling. Seasonal C dynamics and allocation patterns indicate that sink strength variations are important drivers for C fluxes. We propose a conceptual model for temperate and boreal trees, which considers the use of recently assimilated C versus stored C. We recommend best practices for designing and analysing pulse-labelling experiments, and identify several topics which we consider of prime importance for future research on C allocation in trees: (i) whole-tree C source–sink relations, (ii) C allocation to secondary metabolism, (iii) responses to environmental change, (iv) effects of seasonality versus phenology in and across biomes, and (v) carbon–nitrogen interactions. Substantial progress is expected from emerging technologies, but the largest challenge remains to carry out in situ whole-tree labelling experiments on mature trees to improve our understanding of the environmental and physiological controls on C allocation.

Description: Elevated atmospheric [CO 2 ] (e C a ) often decreases stomatal conductance, which may delay the start of drought, as well as alleviate the effect of dry soil on plant water use and carbon uptake. We studied the interaction between drought and e C a in a whole-tree chamber experiment with Eucalyptus saligna . Trees were grown for 18 months in their C a treatments before a 4-month dry-down. Trees grown in e C a were smaller than those grown in ambient C a (a C a ) due to an early growth setback that was maintained throughout the duration of the experiment. Pre-dawn leaf water potentials were not different between C a treatments, but were lower in the drought treatment than the irrigated control. Counter to expectations, the drought treatment caused a larger reduction in canopy-average transpiration rates for trees in the e C a treatment compared with a C a . Total tree transpiration over the dry-down was positively correlated with the decrease in soil water storage, measured in the top 1.5 m, over the drying cycle; however, we could not close the water budget especially for the larger trees, suggesting soil water uptake below 1.5 m depth. Using neutron probe soil water measurements, we estimated fractional water uptake to a depth of 4.5 m and found that larger trees were able to extract more water from deep soil layers. These results highlight the interaction between rooting depth and response of tree water use to drought. The responses of tree water use to e C a involve interactions between tree size, root distribution and soil moisture availability that may override the expected direct effects of e C a . It is essential that these interactions be considered when interpreting experimental results.

Description: Nearly 30 years ago, Whitehead and Jarvis and Whitehead et al. postulated an elegant mechanistic explanation for the observed relationship between tree hydraulic structure and function, hypothesizing that structural adjustments promote physiological homeostasis. To date, this framework has been nearly completely overlooked with regard to varying atmospheric carbon dioxide ([CO 2 ]). Here, we evaluated Whitehead's hypothesis of leaf water potential ( l ) homeostasis in faster-growing ( Eucalyptus saligna ) and slower-growing ( Eucalyptus sideroxylon ) tree saplings grown under three [CO 2 ] (pre-industrial, current and future) and two temperature (ambient and ambient + 4 °C) treatments. We tested for relationships between physiological (stomatal conductance and l ) and structural (leaf and sapwood areas ( A l , A s ), height ( h ), xylem conductivity ( k s )) plant variables as a function of the [CO 2 ] and temperature treatments to assess whether structural variables adjusted to maintain physiological homeostasis. Structural components ( A l , A s , h ) generally increased with [CO 2 ] or temperature, while g s was negatively correlated with [CO 2 ]. Contrary to Whitehead's hypothesis, l did not exhibit homeostasis in either species; elevated temperatures were associated with more negative l in faster-growing E. saligna , and less negative l in slower-growing E. sideroxylon . Moreover, individual structural variables were generally uncorrelated with l . However, across both species, the integrated hydraulic property of leaf specific hydraulic conductance ( K l ) was positively correlated with an independent calculation of K l determined exclusively from leaf physiological variables. These results suggest that physiological homeostasis may not apply to saplings exposed to global change drivers including [CO 2 ] and temperature. Nevertheless, Whitehead et al.'s formulation identified K l as a sensitive measure of plant structural–physiological co-variation across species.

Description: Dwarf mistletoes, obligate, parasitic plants with diminutive aerial shoots, have long-term effects on host tree water relations, hydraulic architecture and photosynthetic gas exchange and can eventually induce tree death. To investigate the long-term (1886–2010) impacts of dwarf mistletoe on the growth and gas exchange characteristics of host western hemlock, we compared the diameter growth and tree-ring cellulose stable carbon (C) and oxygen (O) isotope ratios ( 13 C cell , 18 O cell ) of heavily infected and uninfected trees. The relative basal area growth of infected trees was significantly greater than that of uninfected trees in 1886–90, but declined more rapidly in infected than uninfected trees through time and became significantly lower in infected than uninfected trees in 2006–10. Infected trees had significantly lower 13 C cell and 18 O cell than uninfected trees. Differences in 18 O cell between infected and uninfected trees were unexpected given that stomatal conductance and environmental variables that were expected to influence the 18 O values of leaf water were similar for both groups. However, estimates of mesophyll conductance ( g m ) were significantly lower and estimates of effective path length for water movement ( L ) were significantly higher in leaves of infected trees, consistent with their lower values of 18 O cell . This study reconstructs the long-term physiological responses of western hemlock to dwarf mistletoe infection. The long-term diameter growth and 13 C cell trajectories suggested that infected trees were growing faster than uninfected trees prior to becoming infected and subsequently declined in growth and leaf-level photosynthetic capacity compared with uninfected trees as the dwarf mistletoe infection became severe. This study further points to limitations of the dual-isotope approach for identifying sources of variation in 13 C cell and indicates that changes in leaf internal properties such as g m and L that affect 18 O cell must be considered.

Description: Since 1950, the creation of open woodlands has increased in Canada's northeastern continuous boreal forest and recent studies have demonstrated that the mechanisms underlying their creation are similar to those found in the lichen woodland zone. Since no natural re-densification of open woodlands has been observed to date, afforestation is necessary to counteract an increase of these types of stands in the continuous boreal forest. The aim of this study was to test the operational feasibility and success of afforestation efforts in open woodlands, 10 years after planting. The experimental design included different containerized seedling stocks and site preparation approaches, such as patch and disk scarification, and covered most of the geographical range of accessible continuous boreal forest in Québec. In open woodlands, regardless of black spruce stock size, disk scarification increased planted seedling survival and growth compared with patch scarification and direct planting, possibly due to a beneficial effect on seedling nutrition. However, even if seedling growth in open woodlands and feathermoss stands submitted to disk scarification was comparable, growth was higher in the latter stands. Nonetheless, we conclude that disk scarification followed by planting is an appropriate method to afforest boreal open woodlands.