The mechanistic basis underpinning forest succession is the gap-phase paradigm in which overstory distur- bance interacts with seedling and sapling shade tolerance to determine successional trajectories. The theory, and ensu- ing simulation models, typically assume that understory plants have little impact on the advance regeneration layer's composition. We challenge that assumption by reviewing over 125 papers on 38 species worldwide that form dense and persistent understory canopies. Once established, this layer strongly diminishes tree regeneration, thus altering the rate and direction of forest succession. We term these dense strata recalcitrant understory layers. Over half of the cases re- viewed were linked to increases in canopy disturbance and either altered herbivory or fire regimes. Nearly 75% of the studies declared that competition and allelopathy were the likely interference mechanisms decreasing tree regeneration, yet only 25% of the studies used manipulative field experiments to test these putative mechanisms. We present a con- ceptual model that links the factors predisposing the formation of recalcitrant understory layers with their interference mechanisms and subsequent impacts on succession. We propose that their presence constricts floristic diversity and ar- gue for their explicit inclusion in forest dynamics theory and models. Finally, we offer management suggestions to limit their establishment and mitigate their impacts.

https://www.fs.fed.us/pnw/pubs/gtr802/Vol2/pnw_gtr802vol2_royo.pdf

On the formation of dense understory layers in forests worldwide: consequences and implications for forest dynamics, biodiversity, and succession

Afforestation, the conversion of non-forested lands to forest plantations, can sequester atmospheric carbon dioxide, but the rapid growth and harvesting of biomass may deplete nutrients and degrade soils if managed improperly. The goal of this study is to evaluate how afforestation affects mineral soil quality, including pH, sodium, exchangeable cations, organic carbon, and nitrogen, and to examine the magnitude of these changes regionally where afforestation rates are high. We also examine potential mechanisms to reduce the impacts of afforestation on soils and to maintain long-term productivity. Across diverse plantation types (153 sites) to a depth of 30 cm of mineral soil, we observed significant decreases in nutrient cations (Ca, K, Mg), increases in sodium (Na), or both with afforestation. Across the data set, afforestation reduced soil concentrations of the macronutrient Ca by 29% on average (P , 0.05). Afforestation by Pinus alone decreased soil K by 23% (P , 0.05). Overall, plantations of all genera also led to a mean 71% increase of soil Na (P , 0.05). Mean pH decreased 0.3 units (P , 0.05) with afforestation. Afforestation caused a 6.7% and 15% (P , 0.05) decrease in soil C and N content respectively, though the effect was driven principally by Pinus plantations (15% and 20% decrease, P , 0.05). Carbon to nitrogen ratios in soils under plantations were 5.7-11.6% higher (P , 0.05). In several regions with high rates of afforestation, cumulative losses of N, Ca, and Mg are likely in the range of tens of millions of metric tons. The decreases indicate that trees take up considerable amounts of nutrients from soils; harvesting this biomass repeatedly could impair long-term soil fertility and productivity in some locations. Based on this study and a review of other literature, we suggest that proper site preparation and sustainable harvest practices, such as avoiding the removal or burning of harvest residue, could minimize the impact of afforestation on soils. These sustainable practices would in turn slow soil compaction, erosion, and organic matter loss, maintaining soil fertility to the greatest extent possible.

/pdf/a-global-meta-analysis-of-soil-exchangeable-cations-ph-4jtmtopit9.pdf

A global meta-analysis of soil exchangeable cations, pH, carbon, and nitrogen with afforestation.

https://www.alces.ca/references/download/33/Scenario%20Analysis%20in%20Environmental%20Impact%20Assessment.pdf

Scenario analysis in environmental impact assessment: Improving explorations of the future

https://www.archipel.uqam.ca/1563/1/2003_064_Coates.pdf

Use of a spatially explicit individual-tree model (SORTIE/BC) to explore the implications of patchiness in structurally complex forests

Afforestation, the conversion of unforested lands to forests, is a tool for sequestering anthropogenic carbon dioxide into plant biomass. However, in addition to altering biomass, afforestation can have substantial effects on soil organic carbon (SOC) pools, some of which have much longer turnover times than plant biomass. An increasing body of evidence suggests that the effect of afforestation on SOC may depend on mean annual precipitation (MAP). The goal of this study was to test how labile and bulk pools of SOC and total soil nitrogen (TN) change with afforestation across a rainfall gradient of 600-1500 mm in the Rio de la Plata grasslands of Argentina and Uruguay. The sites were all former grasslands planted with Eucalyptus spp. Overall, we found that afforestation increased (up to 1012 kg Cha � 1 � yr � 1 ) or decreased (as much as 1294 kg Cha � 1 � yr � 1 ) SOC pools in this region and that these changes were significantly related to MAP. Drier sites gained, and wetter sites lost, SOC and TN (r 2 ¼ 0.59, P¼ 0.003; and r 2 ¼ 0.57, P ¼0.004, respectively). Labile C and N in microbial biomass and extractable soil pools followed similar patterns to bulk SOC and TN. Interestingly, drier sites gained more SOC and TN as plantations aged, while losses reversed as plantations aged in wet sites, suggesting that plantation age in addition to precipitation is a critical driver of changes in soil organic matter with afforestation. This new evidence implies that longer intervals between harvests for plantations could improve SOC storage, ameliorating the negative trends found in humid sites. Our results suggest that the value of afforestation as a carbon sequestration tool should be considered in the context of precipitation and age of the forest stand.

/pdf/soil-c-and-n-changes-with-afforestation-of-grasslands-across-24ctyffa4q.pdf

Soil C and N changes with afforestation of grasslands across gradients of precipitation and plantation age.

A variety of competing hypotheses have been described to explain yield decline in Chinese-fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations. The difficulty in implementing field experiments s...

Yield decline in Chinese-fir plantations: a simulation investigation with implications for model complexity

The ability of the forest ecosystem management model FORECAST to project a 29-year record of stand response to factorial thinning and fertilization treatments in a Douglas-fir (Pseudotsuga menziesi...

Testing the performance of a forest ecosystem model (FORECAST) against 29 years of field data in a Pseudotsuga menziesii plantation

Modelling tools to assess the sustainability of forest management scenarios.

An ecosystem management model was used to project total aboveground and belowground production in hybrid poplar plantations for two sites in Saskatchewan that were previously in agricultural produc...

C. Welham

Papers

Yield decline in Chinese-fir plantations: a simulation investigation with implications for model complexity

Testing the performance of a forest ecosystem model (FORECAST) against 29 years of field data in a Pseudotsuga menziesii plantation

Modelling tools to assess the sustainability of forest management scenarios.

Projected long-term productivity in Saskatchewan hybrid poplar plantations: weed competition and fertilizer effects

Defining stand-level sustainability exploring stand-level stewardship