Geology topics

Jean Knops

Publications and source records attributed to Jean Knops.

3 recordsLinked to original sources

USGS2026

Flowering and flower maturation in valley oak Quercus lobata

We investigated factors driving flowering and flower maturation and their effects on acorn production at both the annual and individual trees levels in a population of valley oak ( Quercus lobata ) in central coastal California, USA. Among years, female flower production was negatively correlated with rainfall, while warm spring temperatures and a large acorn crop the previous year decreased the proportion of flowers maturing into acorns. Among trees, individuals with greater access to ground water growing in warmer microclimates produced more catkins and more female flowers. In addition, male and female flower abundance were positively correlated even after controlling for tree size and resources, countering the hypothesis of a life-history tradeoff between investment in male and female flowering. Factors limiting pollen availability include weather-driven effects (both the pollination Moran effect and environmental vetos), phenological synchrony, and pollen coupling, factors that were difficult to distinguish. All three flower indices explained a significant proportion of variance in acorn production at both the annual and individual tree levels. Most important, however, was variation in female flower maturation, while both female and male flower abundance explained between one-quarter and one-third of variance in acorn production. Quercus lobata is not only a flower maturation masting species, but also a female flower and male flower masting species. The underlying drivers of flower initiation, fertilization, and flower maturation yield important insights into the factors influencing masting behavior in perennial plants and represent an area that can benefit from more empirical study.

California↗

USGS2023

Masting is uncommon in trees that depend on mutualist dispersers in the context of global climate and fertility gradients

The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on mutualist dispersers. These effects play out in the context of variable climate and site fertility among species that vary widely in nutrient demand. Meta-analyses of published data have focused on variation at the population scale, thus omitting periodicity within trees and synchronicity between trees. From raw data on 12 million tree-years worldwide, we quantified three components of masting that have not previously been analysed together: (i) volatility, defined as the frequency-weighted year-to-year variation; (ii) periodicity, representing the lag between high-seed years; and (iii) synchronicity, indicating the tree-to-tree correlation. Results show that mast avoidance (low volatility and low synchronicity) by species dependent on mutualist dispersers explains more variation than any other effect. Nutrient-demanding species have low volatility, and species that are most common on nutrient-rich and warm/wet sites exhibit short periods. The prevalence of masting in cold/dry sites coincides with climatic conditions where dependence on vertebrate dispersers is less common than in the wet tropics. Mutualist dispersers neutralize the benefits of masting for predator satiation, further balancing the effects of climate, site fertility and nutrient demands.

Nature Plants↗

USGS2020

Population ecology and spatial synchrony in abundance within and among populations of valley oak (Quercus lobata) leaf gall wasps

What factors drive population variability through space and time? Here we assess patterns of abundance of seven species of gall wasps in three genera occurring on the leaves of valley oaks ( Quercus lobata ) at 10 sites throughout this species' statewide range in California, from 2000 to 2006. Our primary goals were to understand the factors driving variability in gall abundance and to assess the extent of spatial synchrony in gall wasp communities at both large and small geographic scales. On the large, statewide scale, there was significant site‐to‐site variation in gall abundance, driven in all cases primarily by differences in mean maximum seasonal temperatures, and lesser year‐to‐year variation. In contrast, on the small, local scale, differences were more pronounced from year to year than from tree to tree, and were to some extent correlated with differences in acorn production, suggesting an interaction with the reproductive effort of hosts. Significant spatial synchrony was detected, particularly at the statewide scale, but in no case did synchrony decline significantly with distance, despite sites being up to 741 km apart. Variation in spatial synchrony was correlated with a number of exogenous factors, including seasonal weather conditions, the acorn crop at the statewide scale and soil phosphorus availability at the local scale; however, most variation in spatial synchrony in our analyses remained unexplained.

California↗