Gross soil phosphorus fluxes remain constant along forest recovery trajectories in Central Africa

Detecting vulnerability of humid tropical forests to multiple stressors

QuestionsHuman disturbance is increasingly affecting forest dynamics across the tropics. Forests can recover via natural secondary succession to pre‐disturbance levels of biodiversity, species composition, and ecosystem carbon stocks. Central Africa will be subject to increasingly high shifting‐cultivation pressure in the next decades, but succession trajectories of these ecosystem properties are still poorly known for the Congo basin. We addressed two questions: (1) how does taxonomic and functional composition and diversity shift during secondary succession; and (2) how fast do above‐ground carbon stocks recover during secondary succession in tropical forests?LocationCentral Congo basin.MethodsWe conducted an inventory of trees (diameter at breast height [DBH] ≥ 10 cm), measured species traits and soil texture and carbon content in 18 plots, located along six secondary succession stages (i.e., from agricultural to old growth forest sites). We measured tree diameter, height for 20% of trees distributed across diameter classes, wood traits from all species, and leaf traits from species that contributed to 85% of the plot basal area.ResultsWe showed that secondary forests recover relatively fast in terms of tree species diversity, alpha functional diversity, and fine‐root carbon, with near‐old‐growth forest values after six decades past disturbance, while floristic composition exhibited slower recovery. Secondary forests only partially shifted from acquisitive to a conservative life history, with shifts in leaf traits being largely decoupled from wood traits. Only 43% of above‐ground carbon recovered after 60 years of forest regrowth, potentially through a slow recovery of the large‐sized tree stems that dominate carbon stocks of old‐growth forests.ConclusionsOur findings underline the capacity of Afrotropical forests to recover species and alpha functional diversity after clear‐cutting through shifting cultivation. Simultaneously, old‐growth forests harbor a particular floristic community and store a large quantity of carbon with much longer recovery trajectories, stressing the need for conservation of these forests in the Congo basin.

Post-disturbance recovery is a central element of forest resilience against intensifying disturbance regimes. Although recovery signals are strong across Central European forests, the relative roles of different factors contributing to recovery remain incompletely understood. As climate change increasingly challenges recovery, elucidating these processes is essential to adapt forest management to changing climate and disturbance regimes. We extended and applied a biologically grounded model of forest growth to remote sensing data to quantify how management shapes two key drivers of canopy recovery—disturbance legacies and post-disturbance height growth—across Bavaria, Germany. We combined 23,036 ha of quality-filtered photogrammetric canopy height model data with a Landsat-based disturbance map, a forest ownership map and environmental covariates in a Bayesian modelling framework. Post-disturbance growth rates were governed primarily by forest type and site conditions, whereas management strongly influenced disturbance legacies, i.e. the remaining post-disturbance vegetation height structure on site. Legacies varied widely across management types: Federal and set-aside forests retained the highest level of disturbance legacies, while private forests had the lowest legacy levels. Despite marginally lower growth rates, set-aside areas had recovery trajectories that were comparable to managed forests. The median recovery time to 5 m mean canopy height was 14.3 years over all forest and management types. Set-aside areas exhibited the greatest variation in recovery trajectories. We here show that (i) management affects disturbance legacies more strongly than post-disturbance tree growth, (ii) set-aside areas do not differ in recovery speed from managed areas, and (iii) legacies are diversifying forest recovery trajectories, with potential implications for future forest resilience. Our results underline that the post-disturbance reorganization window is a crucial period for management to influence long-term forest development. The framework presented here provides a scalable approach to monitor structural recovery and guide adaptive forest policy and management under increasing disturbance. • Forest management in Central Europe affects post-disturbance recovery more via legacies than tree growth rates. • Set-aside forests recover their canopy height equally fast as managed forests in Central Europe. • Homogenizing and removing disturbance legacies can reduce forest canopy variation across forest stand development. • We combined a biological growth model with remote sensing data to assess forest canopy recovery.

The time taken for forested tropical ecosystems to re-establish post-disturbance is of widespread interest. Yet to date there has been no comparative study across tropical biomes to determine rates of forest re-growth, and how they vary through space and time. Here we present results from a meta-analysis of palaeoecological records that use fossil pollen as a proxy for vegetation change over the past 20,000 years. A total of 283 forest disturbance and recovery events, reported in 71 studies, are identified across four tropical regions. Results indicate that forests in Central America and Africa generally recover faster from past disturbances than those in South America and Asia, as do forests exposed to natural large infrequent disturbances compared with post-climatic and human impacts. Results also demonstrate that increasing frequency of disturbance events at a site through time elevates recovery rates, indicating a degree of resilience in forests exposed to recurrent past disturbance.

Boreal Shield forest disturbance and recovery trends using Landsat time series

In order to understand the characteristics of soil nitrogen and phosphorus loss under different land use patterns in the small watershed of the Three Gorges Reservoir area and provide a scientific basis for the prevention and control of agricultural non-point source pollution, a field test method was used to study the paddy fields and drought in the small Shipanqiu Watershed in the Three Gorges Reservoir area. The characteristics of different runoff concentrations and the fluxes of nitrogen and phosphorus in surface runoff under the five land use schemes of paddy filed, slope land, woodlands, citrus orchards, and vegetable land. The results show that the annual total nitrogen loss followed the order of paddy field[17.73 kg·(hm2·a)-1] > citrus orchards[4.86 kg·(hm2·a)-1] > dry slope land[4.33 kg·(hm2·a)-1] > vegetable field[4.00 kg·(hm2·a)-1] > woodland[2.41 kg·(hm2·a)-1]. The annual total phosphorous loss followed the order of vegetable fields[4.97 kg·(hm2·a)-1] > Citrus orchards[1.87 kg·(hm2·a)-1] > paddy fields[0.93 kg·(hm2·a)-1] > woodlands[0.27 kg·(hm2·a)-1] > dry slope land[0.19 kg·(hm2·a)-1]. The nitrogen and phosphorus losses under the five land use methods were mainly concentrated from April to May with frequent rainfall events, accounting for 53.80%-96.52% and 56.03%-87.78% of the total annual nitrogen and phosphorus losses. Nitrogen loss was mainly in the form of nitrate nitrogen (16.16%-52.70%), and the total nitrogen loss flux and runoff showed a significant positive correlation (R2=0.9826). Particulate phosphorus was the main form of phosphorus loss in vegetable fields (83.30%), but in other land use schemes it is not significant. There were significant differences in the loss of different forms of nitrogen and phosphorus under the different land use schemes. Among them, measures should be taken in vegetable fields to deal with the problem of particulate phosphorus loss under conditions of heavy rainfall. Fertilization should be avoided in paddy fields during periods of concentrated rainfall. Scientific fertilization and reasonable land use configurations are important ways to control agricultural non-point source pollution in small watersheds.

The effects of long-term biochar application on soil phosphorus (P) flux across the root-soil interface and its availability in the rhizosphere of rice (Oryza sativa L) remain unclear. We used diffusive gradients in thin films (DGT), laser ablation-inductively coupled plasma mass spectrometry, and planar optode sensor techniques to characterize, in-situ, the 2D heterogeneity and dynamics of rhizosphere soil P, iron (Fe), sulfur (S) and trace element fluxes, dissolved oxygen and pH in paddy soil, after 10 years of biochar application. Compared to the control (no biochar applied), biochar applied at 4.5, 22.5 and 45.0 Mg ha−1 yr−1 decreased rhizospheric P fluxes by 11.6%, 63.4% and 79.0%, respectively. This decrease under biochar treatments was attributed to changed redox status of Fe and S caused by the lower dissolved oxygen in rhizosphere soil and increased soil pH induced precipitating of soluble inorganic P into insoluble P forms, such as calcium-bound and residual P that are unavailable for crop uptake. Higher application rate of biochar resulted in lower As and Pb fluxes in rice rhizosphere and their availabilities for crop uptake. The in-situ observation results in rice rhizosphere at μm-scale after 10 years of biochar addition directly showed the complex effects of long-term biochar and rhizosphere heterogeneity on P transformation process.Graphical

Village relocations from protected areas provide natural experiments for understanding forest recovery dynamics. We investigated vegetation recovery across 14 villages relocated from Panna Tiger Reserve, India, spanning 8-30 years post-relocation, to test whether recovery follows classical successional trajectories or diverges into alternative stable states. Using 198 plots, we surveyed trees, seedlings, saplings, shrubs, herbs, grasses, and invasive species, and compared communities with adjacent buffer forest. Contrary to succession theory predictions, time since relocation had no significant effect on woody vegetation recovery (p = 0.607). Instead, landscape position determined recovery trajectory: all riverine villages (n = 7) achieved forest state while all interior villages (n = 7) remained grassland, regardless of time elapsed. Riverine sites supported 4.7 times higher seedling abundance and 2.5 times higher sapling abundance than interior sites. Beta diversity partitioning revealed turnover-dominated differentiation (>92%) rather than nestedness, indicating species replacement between states rather than progressive accumulation. NMDS ordination showed discrete forest-grassland clusters, and indicator species analysis identified state-specific assemblages: fire-adapted Themeda-Heteropogon grasses dominated grassland while shade-tolerant Dichanthium-Oplismenus characterized forest state. Critically, seedling-to-tree ratios were identical between states (10.1 vs 10.3), demonstrating that recruitment limitation occurs post-germination rather than at seed dispersal. Invasive species declined autonomously (10.4%/year, p = 0.008), suggesting competitive exclusion by native grasses. These findings demonstrate that grass-fire feedbacks maintain alternative stable states, with landscape position determining initial trajectory. Passive restoration is insufficient for interior sites; active intervention breaking grass-fire feedbacks is required. Village relocation alone does not guarantee forest recovery as outcome depends fundamentally on landscape context.

Eutrophication (the overenrichment of aquatic ecosystems with nutrients leading to algal blooms and anoxic events) is a persistent condition of surface waters and a widespread environmental problem. Some lakes have recovered after sources of nutrients were reduced. In others, recycling of phosphorus from sediments enriched by years of high nutrient inputs causes lakes to remain eutrophic even after external inputs of phosphorus are decreased. Slow flux of phosphorus from overfertilized soils may be even more important for maintaining eutrophication of lakes in agricultural regions. This type of eutrophication is not reversible unless there are substantial changes in soil management. Technologies for rapidly reducing phosphorus content of overenriched soils, or reducing erosion rates, are needed to improve water quality.

Ecological restoration is widely recognized as an essential technique for addressing soil degradation, biomass decline, and biodiversity loss. Improving and maintaining soil quality is critical to ensuring environmental sustainability and successful forest recovery. This systematic review aimed to assess the impact of ecological forest restoration efforts on soil quality in humid regions, as well as to compare the effectiveness of various ecological restoration strategies on soil quality indicators. Subsequently, a systematic search on various databases (e.g., Scopus and Google Scholar) yielded 696 records, of which 28 primary studies met the inclusion criteria. The results emphasized that chemical and physical soil properties are the key indicators for assessing ecosystem performance during forest restoration. The most commonly measured parameters were soil carbon, nitrogen, phosphorus, pH, bulk density, and soil porosity. It was shown that the restoration process required a longer duration to reach a comparable level of recovery as seen in mature forests, particularly in terms of fully restoring soil quality. Additionally, it has been noted that prior land use influences the length of time needed for soil quality recovery. In planted sites, soil quality may keep improving as the site ages, though it tends to stabilize after a certain period.

Wildfires constitute the most important natural disturbance of Mediterranean forests, driving vegetation dynamics. Although Mediterranean species have developed ecological post-fire recovery strategies, the impacts of climate change and changes in fire regimes may endanger their resilience capacity. This study aims at assessing post-fire recovery dynamics at different stages in two large fires that occurred in Mediterranean pine forests (Spain) using temporal segmentation of the Landsat time series (1994–2018). Landsat-based detection of Trends in Disturbance and Recovery (LandTrendr) was used to derive trajectory metrics from Tasseled Cap Wetness (TCW), sensitive to canopy moisture and structure, and Tasseled Cap Angle (TCA), related to vegetation cover gradients. Different groups of post-fire trajectories were identified through K-means clustering of the Recovery Ratios (RR) from fitted trajectories: continuous recovery, continuous recovery with slope changes, continuous recovery stabilized and non-continuous recovery. The influence of pre-fire conditions, fire severity, topographic variables and post-fire climate on recovery rates for each recovery category at successional stages was analyzed through Geographically Weighted Regression (GWR). The modeling results indicated that pine forest recovery rates were highly sensitive to post-fire climate in the mid and long-term and to fire severity in the short-term, but less influenced by topographic conditions (adjusted R-squared ranged from 0.58 to 0.88 and from 0.54 to 0.93 for TCA and TCW, respectively). Recovery estimation was assessed through orthophotos, showing a high accuracy (Dice Coefficient ranged from 0.81 to 0.97 and from 0.74 to 0.96 for TCA and TCW, respectively). This study provides new insights into the post-fire recovery dynamics at successional stages and driving factors. The proposed method could be an approach to model the recovery for the Mediterranean areas and help managers in determining which areas may not be able to recover naturally.