Shade shelters increase survival and photosynthetic performance of oak transplants at abandoned fields in semi-arid climates

Forest restorations conducted in semiarid, seasonally dry climates must deal with the intense drought stress that affects tree seedlings during the dry season. Although this water deficit is the most commonly invoked source of mortality for seedlings, several other environmental factors may also preclude survival of transplants. For instance, it has been widely reported that excessive light reduces the efficiency of the photosynthetic apparatus, hence decreasing plant survival, but most seedling transplants in deforested areas are conducted under these light conditions. This study is focused in determining whether excessive light affects the photosynthetic performance and survival of Quercus coccolobifolia, a Mexican oak species, when their seedlings are transplanted in semiarid deforested areas. Further, this study tests the possibility of using artificial shade shelters to improve the ecophysiological performance and survival of seedlings. Oak seedlings were transplanted under full sunlight conditions and beneath artificial shade shelters of two different colors： white and black. To reduce water stress, and hence isolate the effects of light treatments, a drip irrigation system was implemented at each experimental plot. Seedling survival was monitored weekly for 128 days and photosynthetic performance was assessed by measuring chlorophyll fluorescence at three opportunities during the experiment. Sun-exposed seedlings showed lower photosynthetic performance and survival rates than those beneath shelters of both colors. These results suggest that sunlight damage can reduce seedling survival when they are transplanted in exposed sites, and that shade shelters can improve the success of forest restoration programs in semiarid climates.

Experimental approaches to select tree species for forest restoration:effects of light,water availability and interspecific competition in degraded areas

Knowing what native trees can recr uit on degraded areas allows selecting the best species to restore these sites.However,as this information is not often available,experimentation is required before large-scale planting.This study used ex situ experiments to make these decisions on recruitment.Competition with r-strategist plants,excessive solar radiation and water shortage commonly impair tree recruitment in open habitats.The experiments focused on the interactions among these factors and were conducted with three pioneer species from seasonally dry forests of northwest Argentina,Anadenanthera colubrina,Ceiba chodatii and Jacaranda mimosifolia.Seeds of each species were sown at two light levels(sunlight/shade),two rainfall levels(full/reduced)and two levels of interspecific competition(with/without competitor)in a tree nursery.Seedling emergence and survival were monitored over a year and the results indicate that species differentially respond to varying levels of light and water.Seedlings of A.colubrina tolerated water shortages under elevated solar radiation,which are desirable features for forest restoration.Seedlings of C.chodatii tolerated shade and drought,suggesting that they require shading for establishing in open areas.However,J.mimosifolia seedlings neither tolerated full sunlight nor water shortages,suggesting that this species requires shading and regular watering if used in reforestation.Regardless of the effects of light and water,the survival of all species was reduced by interspecific competition.These results highlight the importance of experimentation for selecting the best species for forest restoration and can enhance the cost/benefit ratios of these actions.

Facilitation by nurse plants contributes to vegetation recovery in human-disturbed desert ecosystems

Aims Facilitation by nurse plants is a common interaction in harsh environments and this positive plant-plant interaction may promote vegetation recovery in ecosystems affected by human activities.Determining the relevance of this process,however,requires assessing how nurse plants influence the establishment of other species,as well as the proportion of species in the regional species pool that would benefit from the presence of nurse plants in human-disturbed areas.Further,since vegetation recovery is a time-dependent process,the community-level consequences of facilitation are likely to vary among landscapes with different disturbance history.Thus,an integrative perspective of the relevance of nurse plants for vegetation recovery could be obtained by measuring their effects across different human-disturbed landscapes of the target region.This study focuses on these issues and uses a regional-scale approach to assess the community-level effects of a widespread nurse plant of American deserts,the creosotebush(Larrea tridentata).Methods This study was conducted in the southernmost portion of Chihuahuan Desert because most floodplain valleys of this region have been affected by human activities during the past centuries.For this study,we selected 10 floodplain valleys differing in their age(i.e.the time elapsed after human activities were ceased).At each landscape,we measured the cover of creosotebushes and the proportion of plant species positively associated with them,as well as the density of seeds in the soil beneath creosotebush canopies.All these data were regressed against the age of the landscapes.Further,to assess whether positive association patterns were due to facilitation or other processes,we conducted field experiments and measured the ecophysiological performance of plant species established beneath and outside creosotebush canopies.Important Findings Most plant species from the target region were positively associated to creosotebushes,and our field experiments and ecophysiological measures indicated that these distribution patterns can be attributed to facilitative interactions.In most landscapes,the density of seeds was higher beneath creosotebushes than in the surrounding habitats,suggesting that these shrubs may also act as seed traps.The community-level effects of creosotebushes increased with landscape age and creosotebush cover,indicating that magnitude of these effects depends on the disturbance history of each site.These results highlight the relevance of performing large-scale assessments for identifying the consequences of facilitation on vegetation recovery across space and time.We then propose that this kind of large-scale approach should be taken into account in the development of conservation programs aimed at the recovery and preservation of plant biodiversity in harsh environments.

Positive interactions by cushion plants in high mountains:fact or artifact?

Positive interactions are defined as non-trophic interactions where at least one of the interacting species is benefited in terms of fitness and the other remains unaffected.Nevertheless,the bidirectional feedbacks between species may be positive,neutral or negative.Thus,if facilitated species induce negative effects on their‘nurses’,the assumed definition of positive interactions could be reconsidered.Methods We assessed if ecological interactions between cushions of Azorella madreporica and their facilitated species are positive.Specifically,we tested if cover of facilitated species has any costs for cushion plants from an ecophysiological perspective,and if these costs increase with the amount of cover of facilitated species.In addition,through pathway analysis and correlations,we assessed if cover and richness of facilitated species have a direct and/or indirect effect on the fitness of cushion plants.Important Findings We found that facilitated plant species induced a significant cost for their nurses(cushion plants),and this cost increases with cover of the facilitated species.Additionally,the facilitated species exert a strong direct negative effect on the cushion’s fitness and a moderate indirect negative cost evident through the nutrient status and physiological performance of cushion plants.We thus contribute evidence that positive interactions between high mountain cushion plants of central Chile and their‘facilitated’species may be an artifact more than a fact,especially when bidirectional effects are considered;contrasting with the majority of studies that document only one side of the interaction.

Effect of light on seed germination and seedling shape of succulent species from mexico

Aims Light requirements for cactus seed germination have been considered to be associated with their adult plant height and seed mass,but this has not been thoroughly studied for other succulent species.In order to understand seed photosensitivity from desert species belonging to Asparagaceae(subfamily Agavoideae)and Cactaceae,we performed a germination experiment with and without light for 12 species and 2 varieties from 1 species from the Southern Chihuahuan Desert.We also determined if adult growth is totally determined by seedling‘growth form’in cacti.Methods We performed a germination experiment using light and darkness for 13 species from Southern Chihuahuan Desert:10 rosette species(Asparagaceae),as well as 1 globose,1 columnar and 2 varieties from 1 depressed-globose species(Cactaceae).The response variables were seed germination percentage and relative light germination(RLG).In addition,in order to determine if adult-globose cacti could have cylindrical seedlings,we calculated the shape index(height/width ratio)for Coryphanta clavata and Mammillaria compressa.Important Findings All species were considered neutral photoblastic.Eleven species had similar seed germination in both light and dark conditions,and three taxa(M.compressa and the two varieties of Ferocactus latispinus)showed higher germination with light than without it.Agave salmiana,M.compressa and the two varieties of F.latispinus had higher RLG than the other species.Seed mass was an important factor because with higher seed mass there was lower dependence to light.These findings support the hypothesis that small seed mass and light requirements have coevolved as an adaptation to ensure germination.One adult-globose cactus species,M.compressa,and one adult-columnar species,C.clavata,had small seeds and neutral fotoblasticism.Seedlings from these two species exposed to light were cylindrical and those under darkness conditions were columnar.Perhaps seeds from this species are able to germinate in the dark because they produce columnar seedlings with the ability to emerge from greater soil depths where sunlight cannot penetrate.