Mark G. Tjoelker

The worldwide leaf economics spectrum

Bringing together leaf trait data spanning 2,548 species and 175 sites we describe, for the first time at global scale, a universal spectrum of leaf economics consisting of key chemical, structural and physiological properties. The spectrum runs from quick to slow return on investments of nutrients and dry mass in leaves, and operates largely independently of growth form, plant functional type or biome. Categories along the spectrum would, in general, describe leaf economic variation at the global scale better than plant functional types, because functional types overlap substantially in their leaf traits. Overall, modulation of leaf traits and trait relationships by climate is surprisingly modest, although some striking and significant patterns can be seen. Reliable quantification of the leaf economics spectrum and its interaction with climate will prove valuable for modelling nutrient fluxes and vegetation boundaries under changing land-use and climate.

Thermal acclimation and the dynamic response of plant respiration to temperature

Temperature-mediated changes in plant respiration (R) are now accepted as an important component of the biospheres response to global climate change. Here we discuss the underlying mechanisms responsible for the dynamic response of plant respiration to short and long-term temperature changes. The Q(10) is often assumed to be 2.0 (i.e. R doubles per 10 degrees C rise in temperature); however, the Q(10) is not constant (e.g. it declines near-linearly with increasing temperature). The temperature dependence of Q(10) is linked to shifts in the control exerted by maximum enzyme activity at low temperature and substrate limitations at high temperature. In the long term, acclimation of R to temperature is common, in effect reducing the temperature sensitivity of R to changes in thermal environment, with the temperature during plant development setting the maximal thermal acclimation of R.

Universal scaling of respiratory metabolism, size and nitrogen in plants

The scaling of respiratory metabolism to body size in animals is considered to be a fundamental law of nature, and there is substantial evidence for an approximate ¾-power relation. Studies suggest that plant respiratory metabolism also scales as the ¾-power of mass, and that higher plant and animal scaling follow similar rules owing to the predominance of fractal-like transport networks and associated allometric scaling. Here, however, using data obtained from about 500 laboratory and field-grown plants from 43 species and four experiments, we show that whole-plant respiration rate scales approximately isometrically (scaling exponent ≈ 1) with total plant mass in individual experiments and has no common relation across all data. Moreover, consistent with theories about biochemically based physiological scaling, isometric scaling of whole-plant respiration rate to total nitrogen content is observed within and across all data sets, with a single relation common to all data. This isometric scaling is unaffected by growth conditions including variation in light, nitrogen availability, temperature and atmospheric CO2 concentration, and is similar within or among species or functional groups. These findings suggest that plants and animals follow different metabolic scaling relations, driven by distinct mechanisms.

The hot and the cold: unravelling the variable response of plant respiration to temperature

When predicting the effects of climate change, global carbon circulation models that include a positive feedback effect of climate warming on the carbon cycle often assume that (1) plant respiration increases exponentially with temperature (with a constant Q10) and (2) that there is no acclimation of respiration to long-term changes in temperature. In this review, we show that these two assumptions are incorrect. While Q10 does not respond systematically to elevated atmospheric CO2 concentrations, other factors such as temperature, light, and water availability all have the potential to influence the temperature sensitivity of respiratory CO2 efflux. Roots and leaves can also differ in their Q10 values, as can upper and lower canopy leaves. The consequences of such variable Q10 values need to be fully explored in carbon modelling. Here, we consider the extent of variability in the degree of thermal acclimation of respiration, and discuss in detail the biochemical mechanisms underpinning this variability; the response of respiration to long-term changes in temperature is highly dependent on the effect of temperature on plant development, and on interactive effects of temperature and other abiotic factors (e.g. irradiance, drought and nutrient availability). Rather than acclimating to the daily mean temperature, recent studies suggest that other components of the daily temperature regime can be important (e.g. daily minimum and / or night temperature). In some cases, acclimation may simply reflect a passive response to changes in respiratory substrate availability, whereas in others acclimation may be critical in helping plants grow and survive at contrasting temperatures. We also consider the impact of acclimation on the balance between respiration and photosynthesis; although environmental factors such as water availability can alter the balance between these two processes, the available data suggests that temperature-mediated differences in dark leaf respiration are closely linked to concomitant differences in leaf photosynthesis. We conclude by highlighting the need for a greater process-based understanding of thermal acclimation of respiration if we are to successfully predict future ecosystem CO2 fluxes and potential feedbacks on atmospheric CO2 concentrations.

Scaling of respiration to nitrogen in leaves, stems and roots of higher land plants

Using a database of 2510 measurements from 287 species, we assessed whether general relationships exist between mass-based dark respiration rate and nitrogen concentration for stems and roots, and if they do, whether they are similar to those for leaves. The results demonstrate strong respiration-nitrogen scaling relationships for all observations and for data averaged by species; for roots, stems and leaves examined separately; and for life-forms (woody, herbaceous plants) and phylogenetic groups (angiosperms, gymnosperms) considered separately. No consistent differences in the slopes of these log-log scaling relations were observed among organs or among plant groups, but respiration rates at any common nitrogen concentration were consistently lower on average in leaves than in stems or roots, indicating that organ-specific relationships should be used in models that simulate respiration based on tissue nitrogen concentrations. The results demonstrate both common and divergent aspects of tissue-level respiration-nitrogen scaling for leaves, stems and roots across higher land plants, which are important in their own right and for their utility in modelling carbon fluxes at local to global scales.

Needle respiration and nitrogen concentration in Scots Pine populations from a broad latitudinal range : a common garden test with field-grown trees

Models of tree function and forest ecosystem carbon budgets often assume that potential global changes in temperature and/or other factors may alter tissue nitrogen (N) and dark respiration rates (R d ). However, little is known of patterns of co-variation in tissue N and R d among intraspecific populations originating along climatic gradients, and of whether an N-based model of R d can link these two variables. To address these issues, we studied N and R d in fully expanded needles of 10-year-old trees of 14 Scots Pine (Pinus sylvestris) populations of wide-ranging origin (43 ° to 60 °N), grown under common garden conditions. For 11 lowland populations (elevation < 200 m) from the contiguous part of the species range (48 ° to 60°N) grown at a field site in Kornik, western Poland (52 °N), there were greater needle %N in populations from increasing latitude of origin or decreasing mean annual temperature (r≥0.93, P<0.01). Similar %N and latitude of origin correlations were observed in another year at this site and in retrospective analyses of published data for different sets of Scots Pine populations grown in common gardens at 48°, 52 °C and 62 °N latitudes. Needle R d rates of the 11 lowland populations growing at Kornik and measured at a common temperature (20 °C) were greater, by as much as 50%, for more northerly than southerly populations. Mean R d rates were positively correlated to latitude of origin and to mean annual temperature (P < 0.05, r = 0.7 to 0 8). R d and needle %N were positively correlated (P<0.01, r=0.75), with one relationship fitting all data. Across the entire range from 1.15 to to 1.55 needle %N, R d increased from 4.5 to to 6.9 nmol g -1 s -1 . Mean needle %N and R d values for two montane southern populations (43 ° and 44 °N, elevation ≥ 885 m) growing in the same common garden at Kornik were consistent with the relationships between mean annual temperature, needle %N and R d observed for the more northerly populations but did not fit the latitudinal patterns. This suggests that temperature and/or associated climate variables are likely the driving force for observed genetic variation in Scots Pine needle %N and R d across latitudinal and altitudinal gradients. Results of these common garden studies support the idea of a general relationship between needle dark respiration and N concentration, and indicate that there is intraspecific genetic variation in physiology that is selected by climate that persists in a common environment, resulting in higher needle %N and respiration in plants originating from colder habitats. Such patterns need to be better understood and quantified, and merit consideration in modelling of current and potential global change effects on plant function and global carbon cycles.

Isoprene emission from terrestrial ecosystems in response to global change: minding the gap between models and observations

Coupled surface–atmosphere models are being used with increased frequency to make predictions of tropospheric chemistry on a ‘future’ earth characterized by a warmer climate and elevated atmospheric CO2 concentration. One of the key inputs to these models is the emission of isoprene from forest ecosystems. Most models in current use rely on a scheme by which global change is coupled to changes in terrestrial net primary productivity (NPP) which, in turn, is coupled to changes in the magnitude of isoprene emissions. In this study, we conducted measurements of isoprene emissions at three prominent global change experiments in the United States. Our results showed that growth in an atmosphere of elevated CO2 inhibited the emission of isoprene at levels that completely compensate for possible increases in emission due to increases in aboveground NPP. Exposure to a prolonged drought caused leaves to increase their isoprene emissions despite reductions in photosynthesis, and presumably NPP. Thus, the current generation of models intended to predict the response of isoprene emission to future global change probably contain large errors. A framework is offered as a foundation for constructing new isoprene emission models based on the responses of leaf biochemistry to future climate change and elevated atmospheric CO2 concentrations.

Response of Plant Respiration to Changes in Temperature: Mechanisms and Consequences of Variations in Q 10 Values and Acclimation

The effects of short- and long-term changes in temperature on plant respiration (R) are reviewed. We discuss the methods available for quantifying the short- and long-term temperature-dependence of R. The extent to which the Q10 (the proportional change in R with a 10 °C increase in temperature) and the degree of thermal acclimation (change in the temperature-response curve of R following a long-term change in growth temperature) vary within and amongst plant species are assessed. We show that Q10 values are highly variable (e.g., being affected by measuring and growth temperature, irradiance and drought), but most plant species exhibit similar Q10 values (in darkness) when grown and measured under identical conditions (i.e. little evidence of inherent differences in the Q10 of plant R). The possible mechanisms responsible for variability in the Q10 are discussed; high Q10 values occur in tissues where respiratory flux is substrate saturated (i.e. capacity limited). This is illustrated using plots of reduced ubiquinone versus O2 uptake in isolated mitochondria. The degree of acclimation is also highly variable amongst plant species. This variability is due, in part, to some studies exposing pre-existing roots/leaves to a new growth temperature, whereas others compare roots/leaves that develop at different temperature. In most cases, maximal acclimation requires that new leaves and/or roots be developed following a change in growth temperature. In addition to its link with development, acclimation is also often associated with changes in the Q10, particularly in pre-existing leaves/roots transferred from one environment to another. The importance of acclimation in determining annual rates of R as a component of net primary productivity and net ecosystem CO2 exchange is discussed. The importance of acclimation for future atmospheric CO2 concentrations is highlighted, including a positive feedback effect of climate warming on the carbon cycle. This review shows that the assumptions of coupled global circulation models (that Q10 values are constant and that R does not acclimate to long-term changes in temperature) are incorrect, and this may lead to overestimation of the effects of climate warming on respiratory CO2 flux.

Acclimation of respiratory temperature responses in northern and southern populations of Pinus banksiana

Temperature acclimation of respiration may contribute to climatic adaptation and thus differ among populations from contrasting climates. Short-term temperature responses of foliar dark respiration were measured in 33-yr-old trees of jack pine (Pinus banksiana) in eight populations of wide-ranging origin (44-55 degrees N) grown in a common garden at 46.7 degrees N. It was tested whether seasonal adjustments in respiration and population differences in this regard resulted from changes in base respiration rate at 5 degrees C (R(5)) or Q(10) (temperature sensitivity) and covaried with nitrogen and soluble sugars. In all populations, acclimation was manifest primarily through shifts in R(5) rather than altered Q(10). R(5) was higher in cooler periods in late autumn and winter and lower in spring and summer, inversely tracking variation in ambient air temperature. Overall, R(5) covaried with sugars and not with nitrogen. Although acclimation was comparable among all populations, the observed seasonal ranges in R(5) and Q(10) were greater in populations originating from warmer than from colder sites. Population differences in respiratory traits appeared associated with autumnal cold hardening. Common patterns of respiratory temperature acclimation among biogeographically diverse populations provide a basis for predicting respiratory carbon fluxes in a wide-ranging species.

DIFFERENTIAL ABOVE- AND BELOW-GROUND BIOMASS ACCUMULATION OF EUROPEAN PINUS SYLVESTRIS POPULATIONS IN A 12-YEAR-OLD PROVENANCE EXPERIMENT

Abstract Growth and the distribution of biomass among above- and below-ground components were measured in 12-year-old Scots pine (Pinus sylvestris L.) from 19 populations grown in a provenance experiment in central Poland (52° N). The populations originated from the northern (>55°N in Russia, Sweden and Latvia), central (54–47° N in Poland, Germany, Belgium, France, Slovakia, Hungary), and southern (<45° N in Bosnia, Montenegro and Turkey) European range of Scots pine. Height, diameter and biomass were all significantly related to latitude of origin. For populations of northern, central and southern origin, above-ground biomass averaged 3.1, 4.7 and 3.3 kg tree−1 and 25, 43 and 12 Mg ha−1. Total root biomass accounted for 22, 19 and 28% of total stand biomass for northern, central and southern populations, respectively. These differences were primarily the result of proportionally higher fine root biomass in the slower-growing northern and southern than central populations. Since the allometric regression...