Metapopulation dynamics of Primula vulgaris

Primula vulgaris Huds. shows a patchy distribution within its forest habitat. Each patch is viewed as a local population within a metapopulation. Patches are associated with gaps in the canopy and the foundation and extinction of local populations are coupled with forest canopy dynamics. This study analyses the system formed by local populations and their forest environment with the aim of understanding the factors that affect P. vulgaris metapopulation dynamics. Data was collected on the demography of local populations in patches in different light conditions, and on the changes in the canopy brought about by the forest regeneration cycle. These were combined to model the effect of forest disturbance regime and seed dispersal on metapopulation dynamics and overall abundance of P. vulgaris in the forest. The demography of local populations varied spatially in forest patches under different light conditions. Population growth rate (ƛ) was higher in brighter patches than in closed canopy patches. Population under closed canopy conditions may show negative growth rates which may result in eventual extinction. The system of forest patches is a dynamic one: canopy gaps are constantly opening and closing. Canopy closure occurs at an exponential rate and the structure of the forest (i.e., proportion of patches with different light conditions) depends largely on the disturbance regime (i.e., rate of gap creation).Simulation results showed that higher disturbance regimes produced higher metapopulation growth rate and higher overall P. vulgaris abundance in the forest. Long-distance seed dispersal reduced overall P. vulgaris abundance because it implied an amount of seed loss; however, it increased metapopulation growth rate, as it allowed the colonisation of newly-opened gaps. Under low disturbance regimes, P. vulgaris may form non-equilibrium metapopulations. Whether there is a balance between colonisations and extinctions depends on the effectiveness of seed dispersal in allowing the occupation of gaps

Survival and early growth of two congeneric cacti that differ in their level of rarity

In this paper we analyze the growth and early survival of two congeneric columnar cacti: the rare Neobuxbaumia macrocephala (with low population densities and a narrow distribution range), and the relatively common Neobuxbaumia mezcalaensis (with dense populations and a broader distribution range). Seeds of both species were germinated and seedlings were subjected to different radiation, nutrient and watering treatments in greenhouse conditions for six months. By the end of the experiment seedling average dry mass was similar in both species. Recently emerged seedlings of N. macrocephala were smaller than those of N. mezcalaensis; however, N. macrocephala showed higher relative growth rates (average RGR = 0.0138 mg/mg/day) than N. mezcalaensis (average RGR = 0.0126 mg/mg/day). For both species RGR decreased in the shade. Root:shoot ratio (R/S) and K were higher in N. mezcalaensis than in N. macrocephala and showed an increasing trend as water availability decreased. Seedling survival was followed in natural conditions for nine months. The survival of N. mezcalaensis was significantly higher compared to N. macrocephala. In both species a slightly higher seedling survival was observed under the shade of nurse plants. The two species displayed different growth and survival responses, which accounts to some extent for their contrasting level of rarity

growth and vegetative spread of schizachyrium scoparium var. littoralis (poaceae) in sand dune microhabitats along a successional gradient

The architecture resulting from the iteration of modules during plant growth affects resource capture. Phalanx and guerrilla growth forms have been described as ends of a continuum regarding the spacing of modules in plants. In this study we investigated the growth form of the perennial, tussock-forming grass Schizachyrium scoparium Michx. var. littoralis (Nash) Hitchc. in three dune microhabitats at Fl Morro de la Mancha, southeastern Mexico: a mobile, a semi-mobile, and a stabilized site. We followed the growth of 15 genets at each site for two consecutive years and found that daughter-tussock formation was more frequent in the stabilized than in the semi-mobile or the mobile sites. Individual tussocks had a higher number of tillers in the mobile site than in the other two. Tiller production occurred within parental tussocks in the mobile site and in the form of daughter tussocks in the stabilized site. Reciprocal transplants suggested that phenotypic plasticity was responsible for the differences observed. Fertilization enhanced tiller production within parental tussocks but did not affect daughter tussock formation. Clearing experiments resulted in enhanced tiller production within tussocks. In these experiments, daughter-tussock production did not occur directionally towards nutrient-rich microsites. It appears that S. scoparium tillers are spaced at longer distances when resources are scarce and intraclonal competition is severe

Growth response of three globose cacti to radiation and soil moisture: an experimental test of the mechanism behind the nurse effect

Cactus seedlings often establish under nurse plants which modify environmental conditions by increasing moisture and decreasing solar radiation, which may cause beneficial and detrimental effects, respectively, on seedling growth. Three soil moisture treatments (5%, 25% and 60%) and two solar radiation levels (100% exposure=243 ?mol m?2 s?1, and 40%=102 ?mol m?2 s?1) were used in a factorial design to analyze seedling growth response of three rare cactus species (Mammillaria pectinifera, Obregonia denegrii and Coryphantha werdermannii). The variables evaluated were relative growth rate (RGR), root/shoot ratio (R/S), and K (RGRroots/RGRshoot), obtained from an initial seedling harvest (6-month-old seedlings) and a final harvest 6 months after treatment application. All three species had slow RGRs (0.002–0.012 g g?1 day?1). O. denegrii had the lowest RGR values, but was the only species for which R/S and K varied with soil moisture. While all seedlings responded markedly to soil moisture, no response was observed to radiation treatments. The latter might have been related to the relatively low solar radiation levels present in the greenhouse. Yet, our results suggest that the main benefit nurse plants offer to seedlings is the increase in soil moisture

germination response to temperature, salinity, light and depth of sowing of 10 tropical dune species

This study describes the germination responses of ten tropical dune species to several factors to which their seeds are exposed in the field. Species studied were: three sand dune endemics (Amaranthus greggii, Palafoxia lindenii, and Trachypogon gouini), three pantropical coastal species (Sesuvium portulacastrum, Sporobolus virginicus and Ipomoea stolonifera) and four cosmopolitan grasses also found inland (Panicum repens, Panicum maximum, Pappophorum vaginatum and Andropogon glomeratus). Six species germinated when. exposed to different constant temperatures. Four required temperature fluctuation (S. portulacastrum, P. lindenii, S. virginicus, P. repens). Light promoted germination of three species (S. Portulacastrum, P. vaginatum, P. lindenii). Species varied in their degree of sensitivity to salinity. Seeds of T. gouini, L stolonifera, S. portulacastrum, P. repens, P. maximum and P. vaginatum were able to germinate under some of the salinity concentrations. Not all species were able to recover after being transferred to distilled water. Seedling emergence was inhibited when seeds were buried. This response was related to depth and to seed size. S. portulacastrum and S. virginicus were the most affected. Nitrates only affected germination response of two species. Seed age promoted germination under a wider range of conditions. P. lindenii showed very heterogeneous responses depending on seed cohort. No dormancy mechanisms were found, other than a thermoperiod and/or light requirement for some of the species. Cosmopolitan grasses tolerated both sand burial and salinity, although the endemics were the most successful in emerging from sand buria

Variation in the population dynamics of the palm Sabal yapa in a landscape shaped by shifting cultivation in the Yucatan Peninsula, Mexico

To understand the population dynamics of a species, it is necessary to document the way in which its demographic behaviour varies through space and time. Anthropogenic disturbance, such as shifting cultivation, is an important factor causing demographic variation in many tropical non-timber forest products. The leaves of the palm Sabal yapa are an important non-timber forest product used for thatching by Mayan peoples. The demography of Sabal yapa was studied in three habitats (mature forest, successional forest and crop fields), representing successional phases along the slash-and-burn agricultural cycle in the Yucatan Peninsula. Matrix population models, along with elasticity analyses and life-table-response experiments were employed. Population growth rate differed between patches (MF: ? = 1.043; SF: ? = 1.027; CF: ? = 0.959). Only the ? value of the mature forest was significantly higher than unity. Fecundity and seedling survival were lowest in the crop fields and highest in the mature forest. The elasticity analyses and life-table-response experiments showed that entries with a high positive contribution to ? also showed high elasticity values, while those with a negative contribution to ? showed low elasticity. Thus, both analyses are crucial to understand the demography of a species and to aid in conservation and management practices

Demography of Bursera glabrifolia, a tropical tree used for folk woodcrafting in southern Mexico: an evaluation of its management plan

Tropical dry forest in Mesoamerica has long been disturbed by the extraction of different forest products by local inhabitants. Bursera glabrifolia (white “copalillo”) is a dry tropical forest tree extensively used in several communities of the State of Oaxaca, Mexico, for the elaboration of folk-art woodcarvings called “alebrijes”. Historically, the extraction of B. glabrifolia had not been regulated; however, in October 2002, the first management plan for this species was implemented in San Juan Bautista Jayacatlán, based on abundance, distribution and dasonomic data. However, no demographic information is available on which to base a sounder management plan that would guarantee the ecological sustainability of this extractive activity. Thus, we studied the population dynamics of this species over a 2-year period in Jayacatlán, where no tree extraction had taken place since 1998. We used stem expansion rates and observations on survival and fecundity to build size-based population projection matrices. The projected population growth rate values (?) were 1.14 and 1.04 for the 2001–2002 and the 2002–2003 periods, respectively. These values suggest that the B. glabrifolia population at Jayacatlán shows a growing trend. Several harvesting scenarios were simulated to theoretically assess the impact on population dynamics of harvesting whole trees and, in particular, to ascertain whether the extraction regime recommended by the management plan (8 trees/ha/yr) is sustainable. For this purpose, we used an average population matrix where specific entries were modified to simulate different extraction levels. The results suggest that the harvesting regimes established in the management plan for B. glabrifolia are sustainable. However, resource managers should not exceed the amounts recommended by the management plan and should avoid extrapolating our results to other regions of the country where no demographic studies have been carried out. The need for long-term demographic studies and their incorporation in ‘adaptive management plans’ is discussed