Woody Net Primary Productivity Research Articles

Biometric-Based Net Primary Production (NPP) and the Effects of a Masting Event on Production Allocation in a Secondary Lucidophyllous Forest in Central Japan

Lucidophyllous (evergreen broad-leaved) forests are the dominant forests in human-dominated subtropical/warm-temperate regions in East Asia. Biometric-based estimates of net primary production (NPP) were conducted in a secondary lucidophyllous forest on Mt. Kinka (35°26′ N, 136°47′ E) near the northern limit of their distribution in central Japan for three years, including the masting event. The forest stand mainly consists of Castanopsis cuspidata (Thunb.) Schottky and Cleyera japonica Thunb. in the canopy and subtree layers, respectively. In 2018, the total NPP of the masting year was 14.53 ± 2.03 ton ha−1 yr−1, including woody NPP (above: 2.63 ± 0.35 ton ha−1 yr−1; below: 0.57 ± 0.08 ton ha−1 yr−1), foliage NPP (4.07 ± 0.23 ton ha−1 yr−1), reproductive NPP (4.81 ± 0.77 ton ha−1 yr−1), and fine root production (Pfr) (2.46 ± 1.84 ton ha−1 yr−1). Pfr and belowground production comprised 16.9% and 20.9%, respectively, of the total NPP. The nut production of C. cuspidata in 2018 (4.31 ± 0.75 ton ha−1 yr−1) was significantly higher than that in 2017 (0.77 ± 0.13 ton ha−1 yr−1) and 2019 (0.23 ± 0.06 ton ha−1 yr−1). No significant change was observed for the three years of foliage NPP and total NPP without Pfr. However, the woody NPP in 2018 (3.20 ± 0.43) was lower than in 2017 (5.37 ± 0.33 ton ha−1 yr−1) and 2019 (4.71 ± 0.38 ton ha−1 yr−1). This suggests that nut production in the masting years compensated by decreasing woody production in the Castanopsis forest.

Forest responses to simulated elevated CO2 under alternate hypotheses of size- and age-dependent mortality.

Elevated atmospheric carbon dioxide (eCO2 ) is predicted to increase growth rates of forest trees. The extent to which increased growth translates to changes in biomass is dependent on the turnover time of the carbon, and thus tree mortality rates. Size- or age-dependent mortality combined with increased growth rates could result in either decreased carbon turnover from a speeding up of tree life cycles, or increased biomass from trees reaching larger sizes, respectively. However, most vegetation models currently lack any representation of size- or age-dependent mortality and the effect of eCO2 on changes in biomass and carbon turnover times is thus a major source of uncertainty in predictions of future vegetation dynamics. Using a reduced-complexity form of the vegetation demographic model the Functionally Assembled Terrestrial Ecosystem Simulator to simulate an idealised tropical forest, we find increases in biomass despite reductions in carbon turnover time in both size- and age-dependent mortality scenarios in response to a hypothetical eCO2 -driven 25% increase in woody net primary productivity (wNPP). Carbon turnover times decreased by 9.6% in size-dependent mortality scenarios due to a speeding up of tree life cycles, but also by 2.0% when mortality was age-dependent, as larger crowns led to increased light competition. Increases in aboveground biomass (AGB) were much larger when mortality was age-dependent (24.3%) compared with size-dependent (13.4%) as trees reached larger sizes before death. In simulations with a constant background mortality rate, carbon turnover time decreased by 2.1% and AGB increased by 24.0%, however, absolute values of AGB and carbon turnover were higher than in either size- or age-dependent mortality scenario. The extent to which AGB increases and carbon turnover decreases will thus depend on the mechanisms of large tree mortality: if increased size itself results in elevated mortality rates, then this could reduce by about half the increase in AGB relative to the increase in wNPP.

Stand dynamics and aboveground net primary productivity of a mature subtropical mangrove forest on Ishigaki Island, south-western Japan

There are insufficient data regarding forest productivity of mature mangroves near the northern limit of mangrove distribution in subtropical East Asia. We conducted the present study to determine the stand dynamics and net primary production (NPP) of a mature mangrove forest on Ishigaki Island in south-western Japan over three successive years, using a large permanent plot. The two mangrove species present were Bruguiera gymnorrhiza and Rhizophora stylosa. The aboveground biomass in the plot was high, despite the high latitude (24 ∘29′N) of the site, gradually increasing from 158.4 Mg ha−1 to 164.6 Mg ha−1 over the three-year study period. This increase was attributable to an increase in the biomass of B. gymnorrhiza. The biomass of R. stylosa, however, decreased during the same period because of a threefold-higher mortality rate and a lower relative growth rate of this species compared with B. gymnorrhiza. The mangrove forest was in the late successional stage, following a pioneer R. stylosa forest that was widely distributed throughout the study plot as standing dead trees and logs. The aboveground NPP of the mangrove forest was 10.66 ± 1.46 Mg ha−1 y−1, partitioned into 3.10 ± 0.51 Mg ha−1 y−1 as woody NPP (net increase in aboveground woody parts, SI), and 7.56 ± 0.99 Mg ha−1 y−1as foliage NPP (litter production, including foliage and reproductive organs, Ln). The mature mangrove forest had a relatively low SI/Ln ratio (0.41 ± 0.03), although litter fall production was within the range previously recorded for mangroves. The lower woody NPP in the mature mangroves was due to the exclusion of R. stylosa once the stand was in a late successional stage dominated by B. gymnorrhiza.

A generic pixel-to-point comparison for simulated large-scale ecosystem properties and ground-based observations: an example from the Amazon region

Abstract. Comparing model output and observed data is an important step for assessing model performance and quality of simulation results. However, such comparisons are often hampered by differences in spatial scales between local point observations and large-scale simulations of grid cells or pixels. In this study, we propose a generic approach for a pixel-to-point comparison and provide statistical measures accounting for the uncertainty resulting from landscape variability and measurement errors in ecosystem variables. The basic concept of our approach is to determine the statistical properties of small-scale (within-pixel) variability and observational errors, and to use this information to correct for their effect when large-scale area averages (pixel) are compared to small-scale point estimates. We demonstrate our approach by comparing simulated values of aboveground biomass, woody productivity (woody net primary productivity, NPP) and residence time of woody biomass from four dynamic global vegetation models (DGVMs) with measured inventory data from permanent plots in the Amazon rainforest, a region with the typical problem of low data availability, potential scale mismatch and thus high model uncertainty. We find that the DGVMs under- and overestimate aboveground biomass by 25 % and up to 60 %, respectively. Our comparison metrics provide a quantitative measure for model–data agreement and show moderate to good agreement with the region-wide spatial biomass pattern detected by plot observations. However, all four DGVMs overestimate woody productivity and underestimate residence time of woody biomass even when accounting for the large uncertainty range of the observational data. This is because DGVMs do not represent the relation between productivity and residence time of woody biomass correctly. Thus, the DGVMs may simulate the correct large-scale patterns of biomass but for the wrong reasons. We conclude that more information about the underlying processes driving biomass distribution are necessary to improve DGVMs. Our approach provides robust statistical measures for any pixel-to-point comparison, which is applicable for evaluation of models and remote-sensing products.

Logging disturbance shifts net primary productivity and its allocation in Bornean tropical forests.

Tropical forests play a major role in the carbon cycle of the terrestrial biosphere. Recent field studies have provided detailed descriptions of the carbon cycle of mature tropical forests, but logged or secondary forests have received much less attention. Here, we report the first measures of total net primary productivity (NPP) and its allocation along a disturbance gradient from old-growth forests to moderately and heavily logged forests in Malaysian Borneo. We measured the main NPP components (woody, fine root and canopy NPP) in old-growth (n=6) and logged (n=5) 1ha forest plots. Overall, the total NPP did not differ between old-growth and logged forest (13.5±0.5 and 15.7±1.5 Mg C ha-1 year-1 respectively). However, logged forests allocated significantly higher fraction into woody NPP at the expense of the canopy NPP (42% and 48% into woody and canopy NPP, respectively, in old-growth forest vs 66% and 23% in logged forest). When controlling for local stand structure, NPP in logged forest stands was 41% higher, and woody NPP was 150% higher than in old-growth stands with similar basal area, but this was offset by structure effects (higher gap frequency and absence of large trees in logged forest). This pattern was not driven by species turnover: the average woody NPP of all species groups within logged forest (pioneers, nonpioneers, species unique to logged plots and species shared with old-growth plots) was similar. Hence, below a threshold of very heavy disturbance, logged forests can exhibit higher NPP and higher allocation to wood; such shifts in carbon cycling persist for decades after the logging event. Given that the majority of tropical forest biome has experienced some degree of logging, our results demonstrate that logging can cause substantial shifts in carbon production and allocation in tropical forests.

Stand Dynamics and Biomass Increment in a Lucidophyllous Forest over a 28-Year Period in Central Japan

Secondary lucidophyllous forest is one of the dominant forests in human-dominated subtropical/warm-temperate regions in East Asia. There were few direct monitoring techniques to elucidate the following hypotheses: (a) self-thinning may govern the stand development process and (b) wood production decline can be observed during secondary succession in a lucidophyllous forest. We conducted a long-term study at a permanent plot in central Japan, since 1989. The forest consists mainly of Castanopsis cuspidata in a canopy layer, Cleyera japonica, and Eurya japonica in a subtree layer. During the 28-year period, the basal area of the stand significantly increased due to the growth of C. cuspidata, from 29.18 ± 1.84 (87.8% of total) to 38.71 ± 2.22 m2 ha−1 (91.9%), while the stem density of C. cuspidata significantly decreased from 666 ± 13 to 404 ± 10 stems ha−1 in proportion to accumulating biomass (117.8 to 166.6 ton ha−1). The annual woody net primary production ranged from 2.40 ± 0.13 to 3.93 ± 0.33 ton ha−1 year−1 as a nearly 70-year-old forest. There was no age-related decline of woody net primary production (NPP) was found during secondary succession, and the growth of individual tree still increased when the self-thinning process governed the stand.

Variation in stem mortality rates determines patterns of above-ground biomass in Amazonian forests: implications for dynamic global vegetation models.

Understanding the processes that determine above‐ground biomass (AGB) in Amazonian forests is important for predicting the sensitivity of these ecosystems to environmental change and for designing and evaluating dynamic global vegetation models (DGVMs). AGB is determined by inputs from woody productivity [woody net primary productivity (NPP)] and the rate at which carbon is lost through tree mortality. Here, we test whether two direct metrics of tree mortality (the absolute rate of woody biomass loss and the rate of stem mortality) and/or woody NPP, control variation in AGB among 167 plots in intact forest across Amazonia. We then compare these relationships and the observed variation in AGB and woody NPP with the predictions of four DGVMs. The observations show that stem mortality rates, rather than absolute rates of woody biomass loss, are the most important predictor of AGB, which is consistent with the importance of stand size structure for determining spatial variation in AGB. The relationship between stem mortality rates and AGB varies among different regions of Amazonia, indicating that variation in wood density and height/diameter relationships also influences AGB. In contrast to previous findings, we find that woody NPP is not correlated with stem mortality rates and is weakly positively correlated with AGB. Across the four models, basin‐wide average AGB is similar to the mean of the observations. However, the models consistently overestimate woody NPP and poorly represent the spatial patterns of both AGB and woody NPP estimated using plot data. In marked contrast to the observations, DGVMs typically show strong positive relationships between woody NPP and AGB. Resolving these differences will require incorporating forest size structure, mechanistic models of stem mortality and variation in functional composition in DGVMs.

Long-term Wood Production in Water-Limited Forests: Evaluating Potential CO2 Fertilization Along with Historical Confounding Factors

Increased aridity may have severe effects on productivity of dry forests. However, it remains unclear to what degree the positive effects of elevated CO2 (both increased carboxylation rates and enhanced water-use efficiency) may offset the negative effects of drought and climate warming. In forest ecosystems, it is particularly challenging to evaluate CO2 effects on productivity because the impacts of climate variability, competition, and management, combine to have longlasting effects on stand-level productivity. Here we address this problem using a unique long-term database containing repeated inventories of wood biomass for every decade from 1912 to 2002 in a pine forest (Pinus pinaster Ait.) in central Spain (≈7,500 ha.). The approach is based upon a combination of statistical analyses of long-term historical management data and mechanistic modeling which allows us to evaluate the effects of potential CO2 fertilization, climate, and stand structure on woody net primary production (W-NPP). We found a significant negative effect of drought on W-NPP during the first half of the twentieth century that diminishes at the turn of the century. Simulations with a process-based ecosystem model, ORCHIDEE, suggest that wood production under conditions that included CO2 fertilization produced a more highly correlated long-term W-NPP than simulations keeping CO2 values in preindustrial levels. Interestingly, however, the CO2 effect was only apparent when accounting for confounding factors such as competition and management legacies. Identifying CO2 fertilization on forest growth is a critical issue, and requires partitioning CO2 effects from confounding factors that have jointly shaped stand dynamics and carbon balance during the twentieth century.

Of climate and its resulting tree growth: Simulating the productivity of temperate forests

We parametrize the maintenance respiration of a single tree depending on reference climate parameters (light, temperature and precipitation) and the observed stem diameter increase resulting from that climate. The simulated biomass increment results from photosynthesis under the given climate scenario and is then reduced by maintenance and growth respiration. We incorporate this new carbon allocation algorithm into the established individual based gap model FORMIND to reproduce the biomass development of typical central European forest stands.Yield tables for northern Germany recorded over the last century are used for our parametrizations, along with the climate of the area at the time of recording. The model simulates eight tree species based on data on pine, spruce, beech, oak, ash, poplar, birch and robinia. The model dynamics emerge from tree competition, growth and mortality. These processes are calculated on an annual scale. The climate variables (global radiation, air temperature and precipitation) are entered into the model in daily resolution. This new version of FORMIND version reproduces the forest biomass development represented in the yield tables for northern Germany as well as those for western France. The modeled annual fluxes of gross primary production, woody net primary production and autotrophic respiration correspond with results from eddy flux measurements. Therefore, this version of FORMIND with the new carbon allocation is a suitable tool to investigate the carbon flux, biomass development and potential yield of forests at the individual tree level in the temperate climate zone.

Improving simulated Amazon forest biomass and productivity by including spatial variation in biophysical parameters

Abstract. Dynamic vegetation models forced with spatially homogeneous biophysical parameters are capable of producing average productivity and biomass values for the Amazon basin forest biome that are close to the observed estimates, but these models are unable to reproduce observed spatial variability. Recent observational studies have shown substantial regional spatial variability of above-ground productivity and biomass across the Amazon basin, which is believed to be primarily driven by a combination of soil physical and chemical properties. In this study, spatial heterogeneity of vegetation properties is added to the Integrated Biosphere Simulator (IBIS) land surface model, and the simulated productivity and biomass of the Amazon basin are compared to observations from undisturbed forest. The maximum RuBiCo carboxylation capacity (Vcmax) and the woody biomass residence time (τw) were found to be the most important properties determining the modeled spatial variation of above-ground woody net primary productivity and biomass, respectively. Spatial heterogeneity of these properties may lead to simulated spatial variability of 1.8 times in the woody net primary productivity (NPPw) and 2.8 times in the woody above-ground biomass (AGBw). The coefficient of correlation between the modeled and observed woody productivity improved from 0.10 with homogeneous parameters to 0.73 with spatially heterogeneous parameters, while the coefficient of correlation between the simulated and observed woody above-ground biomass improved from 0.33 to 0.88. The results from our analyses with the IBIS dynamic vegetation model demonstrated that using single values for key ecological parameters in the tropical forest biome severely limits simulation accuracy. Clearer understanding of the biophysical mechanisms that drive the spatial variability of carbon allocation, τw and Vcmax is necessary to achieve further improvements to simulation accuracy.

Comparison of modeling approaches for carbon partitioning: Impact on estimates of global net primary production and equilibrium biomass of woody vegetation from MODIS GPP

Partitioning of gross primary production (GPP) to aboveground versus belowground, to growth versus respiration, and to short versus long‐lived tissues exerts a strong influence on ecosystem structure and function, with potentially large implications for the global carbon budget. A recent meta‐analysis of forest ecosystems suggests that carbon partitioning to leaves, stems, and roots varies consistently with GPP and that the ratio of net primary production (NPP) to GPP is conservative across environmental gradients. To examine influences of carbon partitioning schemes employed by global ecosystem models, we used this meta‐analysis‐based model and a satellite‐based (MODIS) terrestrial GPP data set to estimate global woody NPP and equilibrium biomass, and then compared it to two process‐based ecosystem models (Biome‐BGC and VISIT) using the same GPP data set. We hypothesized that different carbon partitioning schemes would result in large differences in global estimates of woody NPP and equilibrium biomass. Woody NPP estimated by Biome‐BGC and VISIT was 25% and 29% higher than the meta‐analysis‐based model for boreal forests, with smaller differences in temperate and tropics. Global equilibrium woody biomass, calculated from model‐specific NPP estimates and a single set of tissue turnover rates, was 48 and 226 Pg C higher for Biome‐BGC and VISIT compared to the meta‐analysis‐based model, reflecting differences in carbon partitioning to structural versus metabolically active tissues. In summary, we found that different carbon partitioning schemes resulted in large variations in estimates of global woody carbon flux and storage, indicating that stand‐level controls on carbon partitioning are not yet accurately represented in ecosystem models.

Mortality as a key driver of the spatial distribution of aboveground biomass in Amazonian forest: results from a dynamic vegetation model

Abstract. Dynamic Vegetation Models (DVMs) simulate energy, water and carbon fluxes between the ecosystem and the atmosphere, between the vegetation and the soil, and between plant organs. They also estimate the potential biomass of a forest in equilibrium having grown under a given climate and atmospheric CO2 level. In this study, we evaluate the Above Ground Woody Biomass (AGWB) and the above ground woody Net Primary Productivity (NPPAGW) simulated by the DVM ORCHIDEE across Amazonian forests, by comparing the simulation results to a large set of ground measurements (220 sites for biomass, 104 sites for NPPAGW). We found that the NPPAGW is on average overestimated by 63%. We also found that the fraction of biomass that is lost through mortality is 85% too high. These model biases nearly compensate each other to give an average simulated AGWB close to the ground measurement average. Nevertheless, the simulated AGWB spatial distribution differs significantly from the observations. Then, we analyse the discrepancies in biomass with regards to discrepancies in NPPAGW and those in the rate of mortality. When we correct for the error in NPPAGW, the errors on the spatial variations in AGWB are exacerbated, showing clearly that a large part of the misrepresentation of biomass comes from a wrong modelling of mortality processes. Previous studies showed that Amazonian forests with high productivity have a higher mortality rate than forests with lower productivity. We introduce this relationship, which results in strongly improved modelling of biomass and of its spatial variations. We discuss the possibility of modifying the mortality modelling in ORCHIDEE, and the opportunity to improve forest productivity modelling through the integration of biomass measurements, in particular from remote sensing.

Beyond annual budgets: carbon flux at different temporal scales in fire-prone Siberian Scots pine forests

Along four chronosequences of fire-prone Siberian Scots pine forests we compared net primary production (NPP) and two different mass-based estimates of net ecosystem productivity (NEPC and NEPS). NEPC quantifies changes in carbon pools along the chronosequences, whereas NEPS estimates the short-term stand-level carbon balance in intervals between fires. The chronosequences differed in the mean return interval of surface fires (unburned or moderately burned, 40 yr; heavily burned, 25 yr) and site quality (lichen versus Vaccinium type). Of the Vaccinium type (higher site quality) only a moderately burned chronosequence was studied.NEPC was derived from the rate of changes of two major carbon pools along the chronosequence time axes: (1) decomposition of old coarse woody debris (CWD) left from the previous generation after stand-replacing fire, and (2) accumulation of new carbon in biomass, CWD and soil organic layer by the regenerating stand. Young stands of all chronosequences were losing carbon at rates of−4 to −19 mol C m−2 yr−1(−48 to −228 g C m−2 yr−1). Depending on initial CWD pools and site-specific accumulation rates the stands became net carbon sinks after 12 yr (Vaccinium type) to 24 yr (lichen type) following the stand-replacing fire and offset initial carbon losses after 27 and 70 yr, respectively. Highest NEPC was reached in the unburned chronosequence (10.8 mol C m−2 yr−1 or 130 g C m−2 yr−1). Maximum NEPC in the burned chronosequences ranged from 1.8 to 5.1 mol C m−2 yr−1 (22 to 61 g C m−2 yr−1) depending on site quality and fire regime. Around a stand age of 200 yr NEPC was 1.6 ± 0.6 mol C m−2 yr−1 (19 ± 7 g C m−2 yr−1) across all chronosequences. NEPS represents the current stand-level carbon accumulation in intervals between recurring surface fires and can be viewed as a mass-based analogue of net ecosystem exchange measured with flux towers. It was estimated based on measurements of current woody NPP, modelled decomposition of measured CWD pools and organic layer accumulation as a function of time since the last surface fire, but ignores carbon dynamics in the mineral soil. In burned mature lichen type stands, NEPS was 6.2 ± 2.6 mol C m−2 yr−1 (74 ± 31 g C m−2 yr−1) and thus five times higher than NEPC at the respective age (1.2 ± 0.6 mol C m−2 yr−1 or 14 ± 7 g C m−2 yr−1). Comparing NEPS and NEPC of mature stands, we estimate that 48% of NPP are consumed by heterotrophic respiration and additional 35% are consumed by recurrent surface fires. As expected, in unburned stands NEPC and NEPS were of similar magnitude. Exploring a site specific model of CWD production and decomposition we estimated that fire reduces the carbon pool of newly produced CWD by 70%. Direct observation revealed that surface fire events consume 50% of the soil organic layer carbon pool (excluding CWD). We conclude that surface fires strongly reduced NEPC. In ecosystems with frequent fire events direct flux measurements using eddy covariance are likely to record high rates of carbon uptake, since they describe the behaviour of ecosystems recovering from fire without capturing the sporadic but substantial fire-related carbon losses.