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Ibanez I.,University of Michigan | Primack R.B.,Boston University | Miller-Rushing A.J.,United States National Phenology Network | Miller-Rushing A.J.,Wildlife Conservation Society | And 6 more authors.
Philosophical Transactions of the Royal Society B: Biological Sciences | Year: 2010

As a consequence of warming temperatures around the world, spring and autumn phenologies have been shifting, with corresponding changes in the length of the growing season. Our understanding of the spatial and interspecific variation of these changes, however, is limited. Not all species are responding similarly, and there is significant spatial variation in responses even within species. This spatial and interspecific variation complicates efforts to predict phenological responses to ongoing climate change, but must be incorporated in order to build reliable forecasts. Here, we use a long-term dataset (1953-2005) of plant phenological events in spring (flowering and leaf out) and autumn (leaf colouring and leaf fall) throughout Japan and South Korea to build forecasts that account for these sources of variability. Specifically, we used hierarchical models to incorporate the spatial variability in phenological responses to temperature to then forecast species' overall and site-specific responses to global warming. We found that for most species, spring phenology is advancing and autumn phenology is getting later, with the timing of events changing more quickly in autumn compared with the spring. Temporal trends and phenological responses to temperature in East Asia contrasted with results from comparable studies in Europe, where spring events are changing more rapidly than are autumn events. Our results emphasize the need to study multiple species at many sites to understand and forecast regional changes in phenology. © 2010 The Royal Society.

Crimmins T.M.,United States National Phenology Network | Crimmins T.M.,University of Arizona | Crimmins M.A.,University of Arizona | David Bertelsen C.,University of Arizona
American Journal of Botany | Year: 2013

Premise of the study: Community-level flowering patterns can be characterized by onset, duration, and end as well as constancy, the degree to which species commence, cease, and reinitiate flowering within a season. In the mountainous Sky Islands region of the southwestern United States, flowering onset is clearly influenced by elevation in the spring, but much less so in the summer season. We evaluated whether these flowering metrics reflect these dissimilar patterns between distinct spring and summer seasons regarding the influence of the elevation and moisture gradient. Methods: We characterized flowering onset, end, duration, and constancy by plant functional type and their relationships to climate variables in spring and summer. We also evaluated the influence of climate on seasonal flowering patterns. Key results: Gaps in seasonal flowering occur frequently in this system in both seasons and among all plant functional types. In both seasons, annual plants exhibit the shortest flowering durations and highest constancies, and plants at low elevations, inhabiting environments with variable moisture conditions, show a greater tendency for longer flowering durations and lower constancy than high-elevation plants. Spring flowering characteristics are most influenced by the total amount of October-March precipitation as well as temperatures in these months, whereas summer flowering characteristics are influenced by the timing of summer-season precipitation, and next by the total amount of summer precipitation. Conclusions: Flowering metrics, especially constancy and duration, show similar patterns in spring and summer and vary across elevation and moisture gradients. These patterns have substantial implications for plant and animal communities. © 2013 Botanical Society of America.

Crimmins T.M.,United States National Phenology Network | Crimmins T.M.,University of Arizona | Bertelsen C.D.,University of Arizona | Crimmins M.A.,University of Arizona
International Journal of Biometeorology | Year: 2014

Within-season breaks in flowering have been reported in a wide range of highly variable ecosystems including deserts, tropical forests and high-elevation meadows. A tendency for interruptions in flowering has also been documented in southwestern US "Sky Island" plant communities, which encompass xeric to mesic conditions. Seasonal breaks in flowering have implications for plant reproductive success, population structure, and gene flow as well as resource availability for pollinators and dependent animals. Most reports of multiple within-season flowering events describe only two distinct flowering episodes. In this study, we set out to better quantify distinct within-season flowering events in highly variable Sky Islands plant communities. Across a >1,200 m elevation gradient, we documented a strong tendency for multiple within-season flowering events. In both distinct spring and summer seasons, we observed greater than two distinct within-season flowering in more than 10 % of instances. Patterns were clearly mediated by the different climate factors at work in the two seasons. The spring season, which is influenced by both temperature and precipitation, showed a mixed response, with the greatest tendency for multiple flowering events occurring at mid-elevations and functional types varying in their responses across the gradient. In the summer season, during which flowering across the gradient is limited by localized precipitation, annual plants exhibited the fewest within-season flowering events and herbaceous perennial plants showed the greatest. Additionally, more distinct events occurred at lower elevations. The patterns documented here provide a baseline for comparison of system responses to changing climate conditions. © 2013 ISB.

Souza L.,University of Tennessee at Knoxville | Weltzin J.F.,United States National Phenology Network | Sanders N.J.,University of Tennessee at Knoxville
Journal of Plant Ecology | Year: 2011

Aims: In this study, we examined the effects of Solidago altissima (hereafter Solidago) and two species in the genus Verbesina, Verbesina virginica and Verbesina occidentalis (hereafter Verbesina), on the structure of an old-field plant community and establishment by an invasive plant species, Lespedeza cuneata (hereafter Lespedeza). MethodsWe removed Solidago, Verbesina and both Solidago and Verbesina from 4-m 2 plots in an intact old-field community during two growing seasons. We then quantified the effects of these removals on richness, evenness, diversity and composition of the subdominant plant community. We also measured the total aboveground biomass and the aboveground biomass of the subdominant community. To assess how these removals affected establishment by Lespedeza, we planted 20 seeds in each plot and tracked seedling emergence and survival for one growing season. Important FindingsSubdominant community evenness and Shannon diversity were higher in plots from which Solidago and Verbesina were removed relative to control plots. However, there were no effects of dominant species removal on species richness or composition of the subdominant community. Total aboveground biomass was not affected by dominant species removal, suggesting that the community of subdominant species exhibited compensation. In fact, subdominant community biomass was greater when Solidago, but not Verbesina, was removed. Light availability was also greater in plots where Solidago was removed relative to control plots throughout the growing season. In addition, removal of dominant species, in particular Solidago, indirectly reduced the emergence, but not survival, of Lespedeza seedlings by directly promoting subdominant community biomass. Taken together, our results suggest that dominant old-field plant species affect subdominant community structure and indirectly promote establishment by Lespedeza. © The Author 2010.

De Dios V.R.,University of Western Sydney | Weltzin J.F.,United States National Phenology Network | Weltzin J.F.,University of Tennessee at Knoxville | Sun W.,Northeast Normal University | And 3 more authors.
Journal of Vegetation Science | Year: 2014

Questions: Woody plant encroachment into former grasslands currently represents a major physiognomic shift globally. Seedling establishment is a critical demographic bottleneck and is considered to be alleviated by increases in water availability and negatively impacted by interactions with grasses, particularly when water stress increases. However, interactions with grasses that are not actively competing for resources ('passive interactions' when grasses are dead) has seldom been considered. Could the transition from a live to a dead grass (litter) canopy favour recruitment of woody seedlings in a semi-arid grassland of the American SW? How does the sign and intensity of grass-seedling interactions change across drastically different summer precipitation regimes with and without passive interactions? Location: Sonoran Desert shrub savanna at the Santa Rita Experimental Range, near Tucson, AZ, US. Methods: Four cohorts of Prosopis velutina seeds were planted annually (2002-2005) under rainout shelters that intercepted all incoming precipitation on a soil with sandy loam texture. Summer precipitation was manipulated to simulate either a 50% increase or decrease in the long-term mean, and cover was manipulated to simulate a grassland dominated by the C4 bunchgrass Heteropogon contortus or left unvegetated. Emergence and survival of P. velutina was monitored and compared across cover types, along with monitoring of soil water content and light interception. Results: Strong active competition was observed with live grasses, under both summer drought and also under ample summer water supply. However, the pattern was reversed and strong passive facilitation of P. velutina was observed over time when grass canopies died and remained in place. This passive facilitation under dry summers was so strong that recruitment under dead grass conditions was comparable to that observed when ample water supply removed the effects of competition on unvegetated plots. Conclusions: After significant mortality of standing grass canopies, which typically compete for limited soil moisture resources, rates of recruitment by shrubs may increase even with significant seasonal drought. This work extends our understanding of interactions among co-located organisms and their effects on plant community dynamics, and introduces a new hypothesis on how grass litter facilitates woody plant encroachment during seasonal droughts. Woody plant encroachment into semiarid grasslands is often limited by active competition from live grasses, which prevents seedling establishment. However, we observed strong passive facilitation on Prosopis velutina seedlings by grass canopies that died and remained in place (became litter). This passive facilitation occurred during drought, and led to establishment levels comparable to those during wet periods and no competition. © 2014 International Association for Vegetation Science.

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