Availability Of Suitable Microsites Research Articles

Recent Vegetation Change (1980–2013) in the Tundra Ecosystems of the Tuktoyaktuk Coastlands, NWT, Canada

ABSTRACTChange detection studies using remote sensing and plot-based sampling show that Arctic vegetation is changing. Most studies have focused on the proliferation of tall shrubs, but increased productivity in areas where shrub cover is low suggests that other functional groups may also be changing. To investigate vegetation change across the Tuktoyaktuk Coastlands we analyzed high-resolution repeat air photos from 1980 and 2013. Thirty-eight image pairs were used to estimate changes in the cover of six functional groups (tall shrub, dwarf shrub, non-tussock-forming sedge, tussock-forming sedge, moss, and lichen). The spatial extent of our airphotos allowed us to investigate changes across four terrain types (high-center polygonal terrain, low-center polygonal terrain, shrub tundra, and tussock tundra). Our analysis shows that all four terrain types experienced absolute increases in shrub cover (+7.71% to 11.98%), with the expansion of dwarf shrubs playing an especially important role in regional change. Significant declines in lichen cover were also observed. While the consistency of shrub encroachment across terrain types suggests that changes were facilitated by shifts in broad-scale processes like temperature or precipitation, our data also indicate that differences in the magnitude of change were mediated by community structure and the availability of suitable microsites.

Propagule pressure governs establishment of an invasive herb

The success of plant invasions may be limited by the availability of propagules and/or of suitable microsites, with microsite availability being affected by, for example, disturbance and interspecific competition. A mechanistic understanding of the contributions of propagule pressure and microsite limitation to plant invasions is therefore required to minimise future invasions. Here, we investigated the relative roles of propagule pressure, the availability of microsites, and their interaction on the establishment of an invasive herb, Lupinus polyphyllus, in two geographic regions representing different climate and growth conditions in Finland (a more productive southern region and a harsher central region). We carried out a field experiment in 14 L. polyphyllus populations, in which we manipulated both propagule pressure and disturbance. In a complementary greenhouse experiment, we manipulated propagule pressure and interspecific competition. Seedling establishment of L. polyphyllus was higher in the more productive southern region than in the harsher central region. The number of L. polyphyllus seedlings increased with increasing propagule pressure regardless of disturbance or interspecific competition. However, the number of L. polyphyllus seedlings per sown seed (relative establishment) tended to decrease with increasing propagule pressure, indicating that the positive effect of propagule pressure on early invasion is partially counteracted by density-dependent mortality at high seed densities. Our results highlight the dominant role of propagule pressure over disturbance and interspecific competition in the establishment of L. polyphyllus, suggesting that the early stage of invasion is limited by the availability of propagules rather than the availability of suitable microsites.

Survival and growth of dominant tree seedlings in seasonally tropical dry forests of Yucatan: site and fertilization effects

Aims Seasonally tropical dry forests of the Yucatan Peninsula are typically found in sites with nutrient-poor soils because of the recent geological origin of the region. The landscape is dominated by extensive karstic plates that shape environments where vegetation regeneration through seed germination may be limited by the availability of suitable microsites. In this study, we documented the survival and growth of seedlings from three dominant tree species (Bursera simaruba, Piscidia piscipula and Lysiloma latisiliquum) in seasonally tropical dry forests in Yucatan. Specifically, we evaluated the effect of nutrient addition (N and P, separately and in combination) on seedling survival and growth across three sites with differing levels of precipitation.

Integrating physiology, population dynamics and climate to make multi‐scale predictions for the spread of an invasive insect: the Argentine ant at Haleakala National Park, Hawaii

Mechanistic models for predicting species’ distribution patterns present particular advantages and challenges relative to models developed from statistical correlations between distribution and climate. They can be especially useful for predicting the range of invasive species whose distribution has not yet reached equilibrium. Here, we illustrate how a physiological model of development for the invasive Argentine ant can be connected to differences in micro‐site suitability, population dynamics and climatic gradients; processes operating at quite different spatial scales. Our study is located in the subalpine shrubland of Haleakala National Park, Hawaii, where the spread of Argentine ants Linepithema humile has been documented for the past twenty‐five years. We report four main results. First, at a microsite level, the accumulation of degree‐days recorded in potential ant nest sites under bare ground or rocks was significantly greater than under a groundcover of grassy vegetation. Second, annual degree‐days measured where population boundaries have not expanded (456–521 degree‐days), were just above the developmental requirements identified from earlier laboratory studies (445 degree‐days above 15.9°C). Third, rates of population expansion showed a strong linear relationship with annual degree‐days. Finally, an empirical relationship between soil degree‐days and climate variables mapped at a broader scale predicts the potential for future range expansion of Argentine ants at Haleakala, particularly to the west of the lower colony and the east of the upper colony. Variation in the availability of suitable microsites, driven by changes in vegetation cover and ultimately climate, provide a hierarchical understanding of the distribution of Argentine ants close to their cold‐wet limit of climatic tolerances. We conclude that the integration of physiology, population dynamics and climate mapping holds much promise for making more robust predictions about the potential spread of invasive species.

Are Plant Populations Seed Limited? A Critique and Meta‐Analysis of Seed Addition Experiments

We examine the relative importance of processes that underlie plant population abundance and distribution. Two opposing views dominate the field. One posits that the ability to establish at a site is determined by the availability of suitable microsites (establishment limitation), while the second asserts that recruitment is limited by the availability of seeds (seed limitation). An underlying problem is that establishment and seed limitation are typically viewed as mutually exclusive. We conducted a meta-analysis of seed addition experiments to assess the relative strength of establishment and seed limitation to seedling recruitment. We asked (1) To what degree are populations seed and establishment limited? (2) Under what conditions (e.g., habitats and life-history traits) are species more or less limited by each? (3) How can seed addition studies be better designed to enhance our understanding of plant recruitment? We found that, in keeping with previous studies, most species are seed limited. However, the effects of seed addition are typically small, and most added seeds fail to recruit to the seedling stage. As a result, establishment limitation is stronger than seed limitation. Seed limitation was greater for large-seeded species, species in disturbed microsites, and species with relatively short-lived seed banks. Most seed addition experiments cannot assess the relationship between number of seeds added and number of subsequent recruits. This shortcoming can be overcome by increasing the number and range of seed addition treatments.

Are Plant Populations Seed Limited? A Critique and Meta-Analysis of Seed Addition Experiments

We examine the relative importance of processes that underlie plant population abundance and distribution. Two opposing views dominate the field. One posits that the ability to establish at a site is determined by the availability of suitable microsites (establishment limitation), while the second asserts that recruitment is limited by the availability of seeds (seed limitation). An underlying problem is that establishment and seed limitation are typically viewed as mutually exclusive. We conducted a meta‐analysis of seed addition experiments to assess the relative strength of establishment and seed limitation to seedling recruitment. We asked (1) To what degree are populations seed and establishment limited? (2) Under what conditions (e.g., habitats and life‐history traits) are species more or less limited by each? (3) How can seed addition studies be better designed to enhance our understanding of plant recruitment? We found that, in keeping with previous studies, most species are seed limited. However, the effects of seed addition are typically small, and most added seeds fail to recruit to the seedling stage. As a result, establishment limitation is stronger than seed limitation. Seed limitation was greater for large‐seeded species, species in disturbed microsites, and species with relatively short‐lived seed banks. Most seed addition experiments cannot assess the relationship between number of seeds added and number of subsequent recruits. This shortcoming can be overcome by increasing the number and range of seed addition treatments.

Effects of experimental and real land use on seedling recruitment of six fen species

Summary Seedling emergence and early establishment of six fen species differing in seed mass and growth form were investigated under experimental land use with changed vegetation structure and under real land use in a calcareous fen. Seeds of all six species were sown in plots with different experimental land-use treatments: summer and autumn mowing with or without litter removal, trampling and abandonment. Additionally, emergence and survival of experimentally sown seeds was investigated under real land use on adjacent sites managed by mowing, grazing, intense trampling or abandonment. On abandoned plots and on plots without litter removal of the land-use experiment, emergence rates of all species were negatively affected either by high litter and moss cover or by tall canopy. No differences were found between autumn and summer mowing. Gap creation by experimental trampling did not increase germination rate. Under real land use, establishment of seedlings of most species was positively affected by litter cover and tall canopy. Trampling, in contrast, had a severe negative effect on seedling survival. The investigated species differed in their germination ability which was tested in the germination chamber and in their response to land use. Succisa pratensis with the highest seed mass germinated well in the chamber and in the field more or less regardless of land use. The low germination rate of Parnassia palustris in the germination chamber indicated a limitation of viable seeds. In the field, however, seedling emergence was additionally limited by microsite availability. Seeds of Serratula tinctoria and Primula farinosa germinated well in the germination chamber, but seedling recruitment in the field was hampered in the presence of a high litter or moss cover. Seeds of Tofieldia calyculata and Pinguicula vulgaris were strongly dependent on the availability of suitable microsites in the field. They hardly germinated under natural conditions, in spite of a high number of germinable seeds in the germination chamber.

EXPERIMENTAL TESTS OF THE SPATIOTEMPORAL SCALE OF SEED PREDATION IN MAST-FRUITING DIPTEROCARPACEAE

Mast-fruiting Dipterocarpaceae exhibit highly synchronous, interspecific seedfall at irregular, multiyear intervals. To investigate how the temporal pattern of seedfall affects dipterocarp seed and seedling survival, in both a logged and a primary lowland tropical forest, we planted Shorea stenoptera Burck seeds in the last three weeks of a 12-wk synchronous dipterocarp seedfall during a major community mast-fruiting event in West Kalimantan, Indonesia. As a result of commercial timber harvest of dipterocarp individuals eight years before, total dipterocarp seed production in the logged site was only 23% of that in the primary forest. At both sites, an average of 35 kg of seed was sown across a large area (≥1 km2) to examine the spatial pattern of seed destruction. During the period in which “natural” community mast seed was available to predators, 92% and 99% of experimentally sown seed escaped predators in logged and in primary forest, respectively. After regional seed resources were exhausted, nomadic vertebrates (primarily the bearded pig, Sus barbatus) were observed in both forest areas, and all ungerminated seed was destroyed. Seed predators arrived earlier in the logged area, before most experimentally sown seed had germinated, and the logged site experienced greater seed loss to vertebrates than did the primary forest. Because nomadic seed predators were absent during peak fruit fall of naturally occurring communities at both study sites, there was no evidence of local predator satiation. Rather, experimentally sown seed escaped predation because of rapid germination before predator arrival, as opposed to being ignored by satiated predators. Seed escape was more dependent on the late arrival of pigs than on the amount of local seed production. There was no significant spatial autocorrelation of seed predation. All remaining seed at the scale of the experiment (>1 km2) was destroyed by predators. These findings suggest that satiation of nomadic predators occurs at the landscape scale. Postdispersal seed predators caused significantly greater seed destruction in the experimentally sown seed populations than in naturally dispersed, mast-fruiting communities at both sites. In both logged and primary forests, there was significantly greater loss of experimentally sown seed to predation than was found in the entire natural mast-fruiting Shorea community combined (21 spp.). Moreover, a naturally occurring, but late-fruiting, Shorea species also exhibited greater seed losses to predation than did all other species within each mast-fruiting community, and these proportional losses were similar to those observed in the experimentally sown seeds. Seeds that escaped predation and vertebrate herbivory on post-establishment seedlings displayed high survival, indicating that the availability of suitable microsites did not limit recruitment. In the primary forest, 65% of the germinated experimental seed that survived early causes of mortality was alive 40 mo post-planting, which coincided with the next mast-fruiting event. The spatial distribution of these seedlings was modified primarily by the foraging behavior of vertebrate seed predators in the first two weeks post-planting. The influence of vertebrate predation on seed and seedling survival suggests that foraging behavior by terrestrial vertebrate seed predators may cause directional and/or stabilizing selection for synchronous, interspecific supra-annual dipterocarp seed production across forest regions in Kalimantan.