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Vegetation processes are fundamentally limited by nutrient and water availability, the uptake of which is mediated by plant roots in terrestrial ecosystems. While tropical forests play a central role in global water, carbon, and nutrient cycling, we know very little about tradeoffs and synergies in root traits that respond to resource scarcity. Tropical trees face a unique set of resource limitations, with rock-derived nutrients and moisture seasonality governing many ecosystem functions, and nutrient versus water availability often separated spatially and temporally. Root traits that characterize biomass, depth distributions, production and phenology, morphology, physiology, chemistry, and symbiotic relationships can be predictive of plants’ capacities to access and acquire nutrients and water, with links to aboveground processes like transpiration, wood productivity, and leaf phenology. In this review, we identify an emerging trend in the literature that tropical fine root biomass and production in surface soils are greatest in infertile or sufficiently moist soils. We also identify interesting paradoxes in tropical forest root responses to changing resources that merit further exploration. For example, specific root length, which typically increases under resource scarcity to expand the volume of soil explored, instead can increase with greater base cation availability, both across natural tropical forest gradients and in fertilization experiments. Also, nutrient additions, rather than reducing mycorrhizal colonization of fine roots as might be expected, increased colonization rates under scenarios of water scarcity in some forests. Efforts to include fine root traits and functions in vegetation models have grown more sophisticated over time, yet there is a disconnect between the emphasis in models characterizing nutrient and water uptake rates and carbon costs versus the emphasis in field experiments on measuring root biomass, production, and morphology in response to changes in resource availability. Closer integration of field and modeling efforts could connect mechanistic investigation of fine-root dynamics to ecosystem-scale understanding of nutrient and water cycling, allowing us to better predict tropical forest-climate feedbacks.

The boreal landscape stores an estimated 40% of the earth's carbon (C) found in terrestrial vegetation and soils, with a large portion collected in thick organic soil layers. These ground stores are subject to substantial removals due to the centurial return of wildfire, which has strong impacts on the soil microbial community and nutrient cycling, which in turn can control ecosystem recovery patterns and process rates, such as C turnover. Currently, predictive knowledge used in assessing fire impacts is largely focused on ecosystems that experience only superficial burning and few robust observations exist regarding the effect that smoldering combustion in deeper active soil layers has on post-fire soil activity. This study provided a highly replicated and regionally extensive survey of wildfire impact on microbial community structure (using fatty acid biomarkers) and nutrient cycling (using in situ ionic resin capsules) across broad gradients of climate, forest properties and fire conditions within 50 separate burn scars and 50 additional matched unburnt boreal forest soils. The results suggest a strong metabolic shift in burnt soils due to heat impact on their structure and a decoupling from aboveground processes, releasing ecosystem N limitation and increasing mobilization of N, P, K, and S as excess in conjunction with an altered, C-starved microbial community structure and reduced root uptake due to vegetation mortality. An additional observed climatic control over burnt soil properties has implications for altered boreal forest function in future climate and fire regimes deserving of further attention.

Forest information is requested at many levels and for many purposes. Samplingbasednational forest inventories (NFIs) can provide reliable estimates on national andregional levels. By combining expensive field plot data with different sources of remotelysensed information, from airplanes and/or satellite platforms, the precision in estimatorsof forest variables can be improved. This paper focuses on the design-based modelassistedapproach to using NFI data together with remotely sensed data to estimateforest variables for small areas, where the variables studied are total growing stockvolume, volume of Norway spruce (Picea abies), and volume of broad-leaved trees.Remote sensing variables may be highly correlated with one another and some mayhave poor predictive ability for target forest variables, and therefore model selectionand/or coefficient shrinkage may be appropriate to improve the efficiency of modelassistedestimators of forest variables. For this purpose, one can use modern shrinkageestimators based on lasso, ridge, and elastic net regression methods. In a simulationstudy using real NFI data, Sentinel 2 remote-sensing data, and a national airborne laserscanning (ALS) campaign, we show that shrinkage estimators offer advantages overthe (weighted) ordinary least-squares (OLS) estimator in a model-assisted setting. Forexample, for a sample size n of about 900 and with 72 auxiliary variables, the RMSE wasup to 41% larger when based on OLS. We propose a data-driven method for findingsuitable transformations of auxiliary variables, and show that it can improve estimatorsof forest variables. For example, when estimating volume of Norway spruce, using asmaller expert selection of auxiliary variables, transformations reduced the RMSE byup to 10%. The overall best results in terms of RMSE were obtained using shrinkageestimators and a larger set of 72 auxiliary variables. However, for this larger set ofvariables, the use of transformations yielded at most small improvements of RMSE,and at worst large increases of RMSE, except in combination with ridge and elasticnet regression.

Introduction: Survival and growth of tree seedlings are key processes of regeneration in forest ecosystems. However, little is known about how climate warming modulates seedling performance either directly or in interaction with understory vegetation and post-fire successional stages.

Methods: We measured survival (over 3 years) and growth of seedlings of three tree species (Betula pubescens, Pinus sylvestris, and Picea abies) in a full-factorial field experiment with passive warming and removal of two plant functional groups (feather moss and/or ericaceous shrubs) along a post-fire chronosequence in an unmanaged boreal forest.

Results: Warming had no effect on seedling survival over time or on relative biomass growth. Meanwhile, moss removal greatly increased seedling survival overall, while shrub removal canceled this effect for B. pubescens seedlings. In addition, B. pubescens and P. sylvestris survival benefitted most from moss removal in old forests (>260 years since last fire disturbance). In contrast to survival, seedling growth was promoted by shrub removal for two out of three species, i.e., P. sylvestris and P. abies, meaning that seedling survival and growth are governed by different understory functional groups affecting seedling performance through different mechanism and modes of action.

Discussion: Our findings highlight that understory vegetation and to a lesser extent post-fire successional stage are important drivers of seedling performance while the direct effect of climate warming is not. This suggests that tree regeneration in future forests may be more responsive to changes in understory vegetation or fire regime, e.g., indirectly caused by warming, than to direct or interactive effects of rising temperatures.

Ecosystem responses of carbon and nitrogen (N) biogeochemistry to N deposition have a high variation across sites. Phosphorus (P), which can interact strongly with N, can be the cause of some of this variation. We quantified plant N and P concentrations and estimated P stocks in aboveground foliage, and soil O-horizon P concentrations and stocks after 18 years in a long-term stand-scale (0.1 ha) N addition experiment [12.5 kg (N1) and 50 kg (N2) N ha(-1) year(-1)] in a c. 100-years-old boreal spruce [Picea abies (L.) Karst] forest. Basal area growth had increased by 65% in the N2 treatment compared to control, along with a higher leaf area index, and lower litter decomposition rates. The higher tree growth occurred during the initial c. 10-years period thereafter resuming to control rates. We hypothesized that increased plant demand for P together with decreased recycling of organic matter in this initially N limited system may have decreased plant-available P, with possible consequences for longer-term biogeochemistry and ecosystem production. However, resin-extractable P did not differ between the three treatments (0.32 kg P ha(-1)), and plant NP ratios and P concentrations and O-horizon P characteristics were similar in the N1 and control treatments. The N2 treatment doubled total P in the O-horizon (100 vs. 54 kg P ha(-1)), explained by an increase in organic P The N concentration, NP ratio, and spruce needle biomass were higher in N2, while the P stock in current year needles was similar as in the control due to a lower P concentration. In addition to P dilution, increased light competition and/or premature aging may have caused the reduction of N-stimulated growth of the trees. For the dominant understory shrub [Vaccinium myrtillus (L.)] no changes in growth was apparent in N2 despite a significantly higher NP ratio compared to control (15 vs. 9, respectively). We therefore conclude that increased NP ratio of vegetation cannot be used as a sole indicator of P limitation. The vegetation and O-horizon changes in N2 were still large enough to merit further studies addressing whether such high N loads may alter ecosystem biogeochemistry toward P limitation. For the lower N addition rate, relevant from an anthropogenic N deposition perspective, we suggest it had no such effect.

Drought and fire reduce productivity and increase tree mortality in tropical forests. Fires also produce pyrogenic carbon (PyC), which persists in situ for centuries to millennia, and represents a legacy of past fires, potentially improving soil fertility and water holding capacity and selecting for the survival and recruitment of certain tree life-history (or successional) strategies. We investigated whether PyC is correlated with physicochemical soil properties, wood density, aboveground carbon (AGC) dynamics and forest resistance to severe drought. To achieve our aim, we used an Amazon-wide, long-term plot network, in forests without known recent fires, integrating site-specific measures of forest dynamics, soil properties and a unique soil PyC concentration database. We found that forests with higher concentrations of soil PyC had both higher soil fertility and lower wood density. Soil PyC was not associated with AGC dynamics in non-drought years. However, during extreme drought events (10% driest years), forests with higher concentrations of soil PyC experienced lower reductions in AGC gains (woody growth and recruitment), with this drought-immunizing effect increasing with drought severity. Forests with a legacy of ancient fires are therefore more likely to continue to grow and recruit under increased drought severity. Forests with high soil PyC concentrations (third quartile) had 3.8% greater AGC gains under mean drought, but 33.7% greater under the most extreme drought than forests with low soil PyC concentrations (first quartile), offsetting losses of up to 0.68 Mg C ha^–1yr^–1 of AGC under extreme drought events. This suggests that ancient fires have legacy effects on current forest dynamics, by altering soil fertility and favoring tree species capable of continued growth and recruitment during droughts. Therefore, mature forest that experienced fires centuries or millennia ago may have greater resistance to current short-term droughts.