ALBERT

Description: Non-structural carbohydrates (NSC) in plant tissue are frequently quantified to make inferences about plant responses to environmental conditions. Laboratories publishing estimates of NSC of woody plants use many different methods to evaluate NSC. We asked whether NSC estimates in the recent literature could be quantitatively compared among studies. We also asked whether any differences among laboratories were related to the extraction and quantification methods used to determine starch and sugar concentrations. These questions were addressed by sending sub-samples collected from five woody plant tissues, which varied in NSC content and chemical composition, to 29 laboratories. Each laboratory analyzed the samples with their laboratory-specific protocols, based on recent publications, to determine concentrations of soluble sugars, starch and their sum, total NSC. Laboratory estimates differed substantially for all samples. For example, estimates for Eucalyptus globulus leaves (EGL) varied from 23 to 116 (mean = 56) mg g –1 for soluble sugars, 6–533 (mean = 94) mg g –1 for starch and 53–649 (mean = 153) mg g –1 for total NSC. Mixed model analysis of variance showed that much of the variability among laboratories was unrelated to the categories we used for extraction and quantification methods (method category R 2 = 0.05–0.12 for soluble sugars, 0.10–0.33 for starch and 0.01–0.09 for total NSC). For EGL, the difference between the highest and lowest least squares means for categories in the mixed model analysis was 33 mg g –1 for total NSC, compared with the range of laboratory estimates of 596 mg g –1 . Laboratories were reasonably consistent in their ranks of estimates among tissues for starch ( r = 0.41–0.91), but less so for total NSC ( r = 0.45–0.84) and soluble sugars ( r = 0.11–0.83). Our results show that NSC estimates for woody plant tissues cannot be compared among laboratories. The relative changes in NSC between treatments measured within a laboratory may be comparable within and between laboratories, especially for starch. To obtain comparable NSC estimates, we suggest that users can either adopt the reference method given in this publication, or report estimates for a portion of samples using the reference method, and report estimates for a standard reference material. Researchers interested in NSC estimates should work to identify and adopt standard methods.

Description: Research on the early stages of stand dynamics in uneven-aged forests often favours regeneration over recruitment of trees into forest stands. We contrasted both regeneration (i.e. seedlings and saplings existing in a stand) and recruitment (i.e. the number of trees annually crossing the threshold of 10 cm dbh) in two main stand types of uneven-aged forests (plenter and group selection). Data from 1710 permanent plots across the Dinaric Mountains in Slovenia were used to study recruitment; on 165 plots, regeneration was additionally analysed. The zero-inflated negative binomial modelling procedure was applied to identify factors influencing regeneration and recruitment. Total regeneration (30 212 ha –1 ) and that of light-demanding species (14 879 ha –1 ) were abundant. The latter regenerated more successfully in group selection stands compared with plenter stands. A large reduction in regeneration density was determined during its growth, which was more dramatic for light-demanding species (e.g. Acer pseudoplatanus ) than for shade-tolerant species (e.g. Fagus sylvatica , Abies alba ). The number of recruited trees (5.83 ha –1 y –1 ) seemed to be sufficient to maintain the uneven structure but was less promising for light-demanding species (0.13 ha –1 y –1 ). However, light-demanding species have the potential to establish and recruit into uneven-aged stands with a limited target proportion in the growing stock. Both indicators—regeneration and recruitment—are indispensable for understanding patterns of stand dynamics in uneven-aged forests.

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: The causes of the decline of silver fir across Europe in the 1960s and 1970s and its subsequent recovery remain open to debate. We studied the radial growth of silver fir and possible influential factors on growth decline in uneven-aged silver fir–Norway spruce–European beech forests in the Dinaric Mountains of Croatia (180 000 ha) since 1900 using dendrochronological methods. Data on the radial growth of large firs (diameter at breast height 〉 50 cm) sampled in 2002 across the region (151 plots, 491 m 2 each), regional climatic data (temperature, precipitation and drought) and SO 2 emissions were used in both statistical and dynamic factor analyses. Two main growth patterns of fir trees were identified: the first features a sharp decline in radial growth in the period 1966–1983, followed by a dramatic recovery, while the second is characterized by a relatively constant increase in radial growth. The most influential climatic factors were mean monthly temperatures in February, July and August; mean precipitation in May and September; and summer drought. However, SO 2 emissions explained a larger part of the total variability of radial growth than climatic factors (46–60 per cent and 0–42 per cent, respectively). The growth recovery of silver fir supports further application of uneven-aged silviculture in the region.

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.