Competition for microsites during recruitment in semiarid annual plant communities.

The concept of microsites for recruitment is central to plant ecology, but it is unclear whether these sites are abstract constructs or real entities. I hypothesize that, in generally microsite-limited communities, microsites comprise a limiting physical resource for which different species compete. I tested this hypothesis on winter-annual communities on biocrust in the semiarid Northern Negev of Israel, in which most species are microsite-limited, while the dominant grass (Stipa capensis) has overcome this limitation by efficient microsite acquisition and a lack of secondary seed dormancy. I tested whether the dominant suppresses the subordinate species, collectively, during recruitment, rather than during growth. To this end, I conducted a field experiment with three blocks of six plots (6 m × 6 m) with two treatments - mowing in spring 2006 (intershrub, intershrub + shrub patches, and none) and shrub-patch removal (0% or 50% of the patches). I collected data from four seed traps per plot before spring 2007 and from five plant samples per plot at the end of spring. Mowing significantly reduced both seed and plant density of the dominant species, reflecting seed-limited recruitment, and increased subordinate plant density by competitive release. Multiple regressions of per-plant and per-gram effects and responses showed that competition was a direct effect of the dominant's density. Total and per-group biomass was proportional to density, implying density-independent per capita growth. Subordinate species number also increased with their density, due to the sample-size effect. These findings indicate that the seed-limited dominant diffusely suppresses the subordinates during recruitment, supporting the microsite competition hypothesis. The shift from growth resources to microsites extends the role of inter-specific competition along productivity and disturbance gradients, and highlights the asymmetric relationship between the two kinds of competition, as microsite competition is only observable if initial abundances are not overshadowed by density-dependent growth and mortality. The findings also demonstrate that (1) lacking secondary seed dormancy is an evolutionarily stable strategy in dryland annuals, alongside seed dormancy in microsite-limited species, and (2) biomass removal (e.g., by herbivory) increases small-scale biodiversity, enhancing the sustainability of dryland grazing, but without compensatory growth.

Limited prospects for future alpine treeline advance in the Canadian Rocky Mountains.

Treeline advance has occurred throughout the twentieth century in mountainous regions around the world; however, local variation and temporal lags in responses to climate warming indicate that the upper limits of some treelines are not necessarily in climatic equilibrium. These observations suggest that factors other than climate are constraining tree establishment beyond existing treelines. Using a seed addition experiment, we tested the effects of seed availability, predation and microsite limitation on the establishment of two subalpine tree species (Picea engelmannii and Abies lasiocarpa) across four treelines in the Canadian Rocky Mountains. The effect of vegetation removal on seedling growth was also determined, and microclimate conditions were monitored. Establishment limitations observed in the field were placed in context with the effects of soil properties observed in a parallel experiment. The seed addition experiment revealed reduced establishment with increasing elevation, suggesting that although establishment within the treeline ecotone is at least partially seed limited, other constraints are more important beyond the current treeline. The effects of herbivory and microsite availability significantly reduced seedling establishment but were less influential beyond the treeline. Microclimate monitoring revealed that establishment was negatively related to growing season temperatures and positively related to the duration of winter snow cover, counter to the conventional expectation that establishment is limited by low temperatures. Overall, it appears that seedling establishment beyond treeline is predominantly constrained by a combination of high soil surface temperatures during the growing season, reduced winter snowpack and unfavourable soil properties. Our study supports the assertion that seedling establishment in alpine treeline ecotones is simultaneously limited by various climatic and nonclimatic drivers. Together, these factors may limit future treeline advance in the Canadian Rocky Mountains and should be considered when assessing the potential for treeline advance in alpine systems elsewhere.

Factors limiting rosette recruitment in scarlet gilia, Ipomopsis aggregata: seed and disturbance limitation.

We performed a factorial combination of seed additions, surface soil disturbance, and protection from potential seed predation in experimental plots to elucidate the relative importance of each factor in the recruitment of scarlet gilia seedlings. We detected a significant interaction between seed addition and surface soil disturbance on seedling recruitment over 2 years of establishment. Plots that received both supplemental seed addition and disturbance established 10.5 times as many rosettes as control plots. We did not detect main or interactive effects of caging on seedling establishment. We explored the importance of density-dependent recruitment by investigating the number of rosettes per experimental plot through time. In addition, we used planned contrasts to compare the absolute and percent changes in control and treatment plot rosette densities between the study years. We found significant downward concave curvature to the seedling recruitment curve, suggesting negative density-dependent recruitment across the experimental range of rosette densities. We found a significant difference between the control plots and the seed addition-disturbance plots in terms of absolute changes in plot density from 1997 to 1998. Greater net mortality occurred in plots that received both seed addition and disturbance. However, we found no differences among treatments in the percent change in rosette density across the study years. Our study demonstrates that seedling recruitment in natural plant populations may be limited by the interaction of seed and microsite availability, and that seedling recruitment in scarlet gilia may be negatively density dependent. Our results suggest that the fecundity effects of particular plant-animal interactions (e.g., pollination, herbivory) may affect scarlet gilia population dynamics, particularly under conditions of high soil disturbance.

Seedling establishment in an epiphytic orchid: an experimental study of seed limitation.

Will increased fruit and seed production in a severely pollination-limited orchid stimulate population growth? We tested whether safe sites for germination and seedling establishment are limiting for the twig epiphyte, Tolumnia variegata, by manipulating fruit set and monitoring subsequent seedling establishment for two seasons (1991-1992, 1992-1993). In the Cambalache Forest Reserve of Puerto Rico, we established 36 plots along a transect. Each plot consisted of nine trees. A center tree was designated as the site for attaching Tolumnia and manipulating fruit set. The other eight potential host trees were 1-3 and 3-5 m from the center tree in each of the cardinal directions. A 1-m length of stem 1 m from the ground was monitored for recruits on each of the nine trees of 24 fruit-enhanced plots and 12 controls (23 and 13, respectively for the 1992-1993 season). Fruit enhancement plots were divided among two treatments: one-fruit and five-fruit additions for the 1st year and one to five and more than five fruits for the 2nd year. Availability of suitable host species was not limiting. T. variegata showed little specificity for host tree species, good host trees and shrubs were common, and there was no evidence that the orchid had a preference for small branches, despite possessing the entire suite of characteristics thought to respresent "obligate" twig epiphytes. Fruit enhancement increased seed rain and seedling establishment consistently in only the high-fruit treatment plots. Most recruitment occurred near fruiting plants. Over the 2-year period, mortality was 18% for adults and 85.5% for the 1991-1992 cohort of recruits. Net recruitment was positive for both the treatment (average = 1.74) and control plots (average = 0.67). Seedling establishment at our study site was not microsite-limited. If selection for increased pollinator attraction occurs, then an increase in seed output should result in population growth.