Regeneration dynamics in fragmented landscapes at the leading edge of distribution: Quercus suber woodlands as a study case

Aims We studied the regeneration dynamics of woodlands and abandoned old fields in a landscape dominated by Quercus suber in its lower limits of rainfall and temperature. Two hypotheses were established: (1) regeneration of Quercus species is strongly favored by the presence of tree cover; and (2) growth of Q. suber is driven by the climatic variables that represent the lower ecological limit of its leading distribution edge. Methods We selected woodlands and old fields with and without tree remnants (n = 3 per type), and analyzed stand structure, soil parameters and tree growth. Results Succession was arrested in old fields without tree remnants. By contrast, remnant trees were accelerators of forest recovery in old fields. Tree cover played a fundamental role in Quercus recruitment throughout seed dispersal and facilitation that mitigate the effects of summer drought on seedlings. Also, tree cover improved soil parameters (e.g., organic matter) that are important factors for understanding differences in regeneration. Winter/spring precipitation exerted a positive effect on tree growth, as well as temperatures during winter/spring and September. Conclusions Regeneration dynamics are modeled by the density of tree cover in the cold and dry edge of the distribution area of Q. suber where Q. ilex is increasing in abundance. Although temperature has a positive effect on the tree growth of Q. suber, when demographic processes are considered, decreases in water availability likely play a critical role in Q. ilex recruitment. This in turn changes dominance hierarchies, especially in abandoned areas with little or no tree cover.

Regeneration Dynamics in Fragmented Landscapes at the Leading Edge of Distribution: Quercus Suber L. as A Study Case

AimsWe studied the regeneration dynamics of woodlands and abandoned old fields in a landscape dominated by Quercus suber in its lower limits for rainfall and temperature. Two hypotheses were established: (1) recruitment of Q. suber is restricted more by abiotic variations than other species adapted to more extreme Mediterranean conditions; and (2) decreases in precipitation reduce growth, but temperature positively affects growth in the leading cold edge of this species distribution area.MethodsWe selected nine sites containing forest stands and old fields with and without tree remnants, and analyzed stand structure, soil parameters and tree growth.ResultsSuccession was arrested in plots without tree remnants after cultivation abandonment. By contrast, remnant trees were accelerators of forest recovery. Tree cover played a fundamental role in Quercus recruitment throughout seed dispersal and facilitation effects that ameliorate summer drought. However, soil variables also significantly explained much of the variance observed and are important for understanding differences in regeneration. Winter and spring precipitation exerted a positive effect on tree growth, as well as temperatures during winter/spring and September.ConclusionsRegeneration dynamics are modeled by the density of tree cover in the cold and dry edge of the distribution area of Q. suber where Q. ilex is increasing in abundance. Although temperature has a positive effect on the tree growth of Q. suber, when demographic processes are considered, decreases in water availability likely play a critical role in Q. ilex recruitment. This in turn changes dominance hierarchies, especially in abandoned areas with little or no tree cover.

Shade is the most important factor limiting growth of a woody range expander

The expansion of woody plants into grasslands and old fields is often ascribed to fire suppression and heavy grazing, especially by domestic livestock. However, it is also recognized that nutrient availability and interspecific competition with grasses and other woody plants play a role in certain habitats. I examined potential factors causing range- and niche expansion by the eastern redcedar Juniperus virginiana, the most widespread conifer in the eastern United States, in multifactorial experiments in a greenhouse. Historical records suggest that the eastern redcedar is a pioneer forest species, and may be replaced as the forest increases in tree density due to shading. Another possible factor that affects its distribution may be nutrient availability, which is higher in old fields and other disturbed lands than in undisturbed habitats. In its historic range, eastern redcedars are particularly abundant on limestone outcrops, often termed ‘cedar barrens’. However, the higher abundance on limestone could be due to reduced interspecific competition rather than a preference for high pH substrates. I manipulated shade, fertilization, lime, and interspecific competition with a common dominant tree, the post oak Quercus stellata. In a separate experiment, I manipulated fire and grass competition. I measured growth rates (height and diameter) and above- and belowground biomass at the end of both experiments. I also measured total non-structural carbohydrates and nitrogen in these plants. Shade was the most important factor limiting the growth rates and biomass of eastern redcedars. I also found that there were significant declines in nitrogen and non-structural carbohydrates when shaded. These results are consistent with the notion that the eastern redcedar is a pioneer forest species, and that shade is the reason that these redcedars are replaced by other tree species. In the second experiment, I found that a single fire had a negative effect on young trees. There was no significant effect of competition with grass, perhaps because the competitive effect was shading by grasses and not nutrient depletion. Overall, the effects of shade were far more apparent than the effects of fire.

Arracacia xanthorrhiza (arracacha).

A. xanthorriza is a biennial to perennial herb, which is mainly reported from cultivation or as persistent in old fields and nearby areas. It is mostly confined to South American Andean regions and is a major commercial crop in Brazil. It is used by an estimated 30 million people in the Andes and 30 million people elsewhere, mainly in Brazil (Heywood, 2014). It is not considered as an invasive species and wild populations are scarce (Hermann, 1997). Outside of South America, it is reported as commercially cultivated in Puerto Rico and Costa Rica (Hodge, 1954). It is not reported as an invasive species in any of the places where is cultivated. It is listed as a species that has naturalised in Cuba with the tendency to spread in some localities (Oviedo Prieto et al., 2012). Its introduction has been unsuccessful in some countries due to its photoperiod and temperature requirements, long growing cycle, its susceptibility to pests and diseases and the short shelf life of the roots (FAO, 2016).

Patterns of vegetation succession in abandoned fields in semi-arid conditions

Successions in old fields were studied on the example of the Starobilski steppes within Luhansk Oblast (Ukraine), region located in the basin of the left tributaries of the Siversky Donets, in the southern spurs of the Central Russian Upland. Stationary surveys were conducted in the Striltsivsky Steppe affiliate of the Luhansk Nature Reserve, with 269 ha of old fields (26% of the area) in its territory. Geobotanical releves were performed in 26 plots of abandoned fields, constant monitoring surveys were carried out in 15–27 year-old abandoned fields in the territory of the Reserve and 5–15 year-old abandoned fields in the territory of its buffer zone. In total, the itinerary and detailed surveys were conducted for about 50 plots of abandoned fields. The restoration of the steppe communities was seen only in the plots where grazing and/or mowing took place, which prevented the formation of dense litter and distribution of woody species. The long domination of rhizome grasses (Elytrigia repens) were characteristic in conditions of moderate-level disturbance (in the reserve), transition to the domination of bunchgrasses (Festuca valesiaca) was seen at the age of around 25 years, during the transitional period (20–25 years) communities with unstable structure formed, comprising E. repens, F. valesiaca and forbs (Achillea pannonica and Fragaria viridis). Duration of the stage of segetal and ruderal species in the sites under low effect was 10 years, the communities of unstable structure formed at the age of 5–10 years. In the conditions of stable moderate and high effect of grazing (high disturbance), we observed formation of bunchgrass communities at the age of 15 years. In the unused plots, we saw formation of phytocoenoses of shrubs at different stages of succession, starting from 10 years. The most abundant group of such kind comprised thickets of Ulmus pumila (with E. repens and Poa angustifolia in the herbaceous layer) and Prunus stepposa. The peculiarity of contemporary processes in the abandoned fields related to exacerbation of the vegetation’s anthropogenic transformation is distribution of alien tree species from plantations, the most aggressive of which are Ulmus pumila, Fraxinus lanceolata, Acer negundo, Elaeagnus angustifolia. The peculiarity of the succession processes in the region of studies was the domination of species of broad ecological spectrum, characteristic for abandoned fields of northern forest-steppe regions (Bromopsis inermis, Calamagrostis epigeios, Poa angustifolia, Fragaria viridis) and southern steppe regions (Bromus sguarrosus, Anisantha tectorum, Achillea pannonica, Artemisia austriaca), certain communities form dependent on the climatic conditions and type of land use in the period of succession in particular plots.