Forests strongly aect climate through the exchange of large amounts of atmospheric CO2 (ref. 1). The main drivers of spatial variability in net ecosystem production (NEP) on a global scale are, however, poorly known. As increasing nutrient availability increases the production of biomass per unit of photosynthesis2 and reduces heterotrophic3 respiration in forests, we expected nutrients to determine carbon sequestration in forests. Our synthesis study of 92 forests in dierent climate zones revealed that nutrient availability indeed plays a crucial role in determining NEP and ecosystem carbon-use eciency (CUEe; that is, the ratio of NEP to gross primary production (GPP)). Forests with high GPP exhibited high NEP only in nutrient-rich forests (CUEe D 33 4%; mean s.e.m.). In nutrient-poor forests, a much larger proportion of GPP was released through ecosystem respiration, resulting in lower CUEe (6 4%). Our finding that nutrient availability exerts a stronger control on NEP than on carbon input (GPP) conflicts with assumptions of nearly all global coupled carbon cycle–climate models, which assume that carbon inputs through photosynthesis drive biomass production and carbon sequestration. An improved global understanding of nutrient availability would therefore greatly improve carbon cycle modelling and should become a critical focus for future research
Trees with sufficient nutrition are known to allocate carbon preferentially to aboveground plant parts. Our global study of 49 forests revealed an even more fundamental carbon allocation response to nutrient availability: forests with high-nutrient availability use 58 ± 3% (mean ± SE; 17 forests) of their photosynthates for plant biomass production (BP), while forests with low-nutrient availability only convert 42 ± 2% (mean ± SE; 19 forests) of annual photosynthates to biomass. This nutrient effect largely overshadows previously observed differences in carbon allocation patterns among climate zones, forest types and age classes. If forests with low-nutrient availability use 16 ± 4% less of their photosynthates for plant growth, what are these used for? Current knowledge suggests that lower BP per unit photosynthesis in forests with low- versus forests with high-nutrient availability reflects not merely an increase in plant respiration, but likely results from reduced carbon allocation to unaccounted components of net primary production, particularly root symbionts.
Recent temperature increases have elicited strong phenological shifts in temperate tree species, with subsequent effects on photosynthesis. Here, we assess the impact of advanced leaf flushing in a winter warming experiment on the current year's senescence and next year's leaf flushing dates in two common tree species: Quercus robur L. and Fagus sylvatica L. Results suggest that earlier leaf flushing translated into earlier senescence, thereby partially offsetting the lengthening of the growing season. Moreover, saplings that were warmed in winter-spring 2009-2010 still exhibited earlier leaf flushing in 2011, even though the saplings had been exposed to similar ambient conditions for almost 1 y. Interestingly, for both species similar trends were found in mature trees using a long-term series of phenological records gathered from various locations in Europe. We hypothesize that this longterm legacy effect is related to an advancement of the endormancy phase (chilling phase) in response to the earlier autumnal senescence. Given the importance of phenology in plant and ecosystem functioning, and the prediction of more frequent extremely warm winters, our observations and postulated underlying mechanisms should be tested in other species.climate change | tree phenology | spring flushing | leaf senescence L eaf phenology of temperate trees has recently received particular attention because of its sensitivity to the ongoing climate change (1-3), and because of its crucial role in the forest ecosystem, water and carbon balances, and species distribution (4-6).A wide variety of methods, such as long-term phenological records (7), indirect measurements of ecosystem greening by remote sensing using satellites or webcam digital images (8-10), and modeling approaches (11-13), have been applied to monitor and study phenological changes. These different approaches, conducted at different spatial scales (from individual plants to biomes), have documented a clear advancement of leaf flushing in temperate climate zones and, to a lesser extent, a delay in leaf senescence (14,15). Furthermore, various temperature manipulation experiments have simulated the impact of future winter warming on leaf phenology and confirmed an advancement in the timing of leaf flushing in response to warming (16-18). However, the response of leaf flushing to climate warming is highly nonlinear (16,19,20), because trees also depend on cold temperatures to break bud dormancy (21-23). This chilling requirement may not (fully) be met in a warming climate, especially at the southern edges of species distribution ranges (5,24,25).Most previous phenological studies have focused on specific phenophases, but how a phenological change (e.g., advanced leaf flushing) affects subsequent phenological events is rarely investigated. Nonetheless, the annual growth cycle of boreal and temperate trees forms an integrated system, where one phenophase in the cycle can affect the subsequent phases (26, 27). Such carryover effects have already been detected in fruit and nu...
Abstract. Since 70 % of global forests are managed and forests impact the global carbon cycle and the energy exchange with the overlying atmosphere, forest management has the potential to mitigate climate change. Yet, none of the land-surface models used in Earth system models, and therefore none of today's predictions of future climate, accounts for the interactions between climate and forest management. We addressed this gap in modelling capability by developing and parametrising a version of the ORCHIDEE land-surface model to simulate the biogeochemical and biophysical effects of forest management. The most significant changes between the new branch called ORCHIDEE-CAN (SVN r2290) and the trunk version of ORCHIDEE (SVN r2243) are the allometric-based allocation of carbon to leaf, root, wood, fruit and reserve pools; the transmittance, absorbance and reflectance of radiation within the canopy; and the vertical discretisation of the energy budget calculations. In addition, conceptual changes were introduced towards a better process representation for the interaction of radiation with snow, the hydraulic architecture of plants, the representation of forest management and a numerical solution for the photosynthesis formalism of Farquhar, von Caemmerer and Berry. For consistency reasons, these changes were extensively linked throughout the code. Parametrisation was revisited after introducing 12 new parameter sets that represent specific tree species or genera rather than a group of often distantly related or even unrelated species, as is the case in widely used plant functional types. Performance of the new model was compared against the trunk and validated against independent spatially explicit data for basal area, tree height, canopy structure, gross primary production (GPP), albedo and evapotranspiration over Europe. For all tested variables, Published by Copernicus Publications on behalf of the European Geosciences Union. K. Naudts et al.: A vertically discretised canopy description for ORCHIDEEORCHIDEE-CAN outperformed the trunk regarding its ability to reproduce large-scale spatial patterns as well as their inter-annual variability over Europe. Depending on the data stream, ORCHIDEE-CAN had a 67 to 92 % chance to reproduce the spatial and temporal variability of the validation data.
Summary paragraph 34Plants acquire carbon through photosynthesis to sustain biomass production, autotrophic 35 respiration, and production of non-structural compounds for multiple purposes 1 . The fraction 36 of photosynthetic production used for biomass production, the biomass production 37 efficiency 2 , is a key determinant of the conversion of solar energy to biomass. In forest 38 ecosystems, biomass production efficiency was suggested to be related to site fertility 2 . Here 39 we present a global database of biomass production efficiency from 131 sites compiled from 40 individual studies using harvest, biometric, eddy covariance, or process-based model 41 estimates of production -dominated, however, by data from Europe and North America. We 42show that instead of site fertility, ecosystem management is the key factor that controls 43 biomass production efficiency in terrestrial ecosystems. In addition, in natural forests, 44 grasslands, tundra, boreal peatlands and marshes biomass production efficiency is 45 independent of vegetation, environmental and climatic drivers. This similarity of biomass 46 production efficiency across natural ecosystem types suggests that the ratio of biomass 47 production to gross primary productivity is constant across natural ecosystems. We suggest 48 that plant adaptation results in similar growth efficiency in high and low fertility natural 49 systems, but that nutrient influxes under managed conditions favour a shift to carbon 50 investment from the belowground flux of non-structural compounds to aboveground biomass. 51 52 53 Main text 54The fraction of gross primary production (GPP) used for biomass production (BP) of 55 terrestrial ecosystems has recently been coined biomass production efficiency (BPE) 2 . BPE is 56 typically used as a proxy for the carbon-use efficiency or NPP-to-GPP ratio, where NPP refers 57 to net primary production i.e. BP plus the production of non-structural organic compounds 1 . 58 4 Current knowledge about BPE is mainly derived from research on forests. Earlier work 59 reported BPE to be conservative across forests 3 , whereas more recent syntheses suggest high 60 inter-site variability 2,4 . The variation in BPE was first attributed to vegetation properties 61 (forest age) and climate variables 4 . More recently, it was shown that forest BPE in a range of 62 natural and managed sites was correlated with site fertility, with management as a secondary 63 BPE driver 2 . 64Fertility and management are strongly correlated as management enhances 65 productivity by increasing plant-available resources, including nutrients. For instance, 66 fertilization of grasslands directly increases the ecosystem nutrient stock, whereas forest 67 thinning indirectly increases nutrient availability at the tree level by reducing plant-plant 68 competition. In addition, fertile sites are more likely than infertile sites to be managed. 69Atmospheric deposition of nutrients, especially nitrogen (N), might further complicate the 70 relationship between BPE, fertility and ...
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