In this new post, Thomas Caignard, post-doc at the University of Bordeaux, presents his latest paper ‘Counter-gradient variation of reproductive effort in a widely distributed temperate oak’, discusses the relevance of the rarely found ‘counter-gradients’ and talks about the multi-disciplinary approach is currently using.
About the paper
Our paper aims to study the phenotypic and genetic variability of one specific life history traits in trees: reproduction. Unlike growth or survival related traits, there is a lack of information about the genetic and environmental determinisms of tree reproduction. In 2014, during my first year of PhD, we started monitoring seed production and seed mass of a temperate oak species (Quercus petraea) along a gradient of elevation in the French Pyrenees with the aim of measuring tree reproductive effort. This gradient is divided in two valleys with 5 populations each distributed at different elevations: from 100 m to 1600 m above sea level. This experimental design is a long standing experiment where in addition to the gradient in situ, seeds from every population of the gradient were collected, grown and planted within a lowland common environment located near Bordeaux in 2006, also called common garden. When we started monitoring reproduction along the gradient of elevation in situ, seedlings from the common garden started to produce seeds. Hence, we had the opportunity to investigate, at the same time, the phenotypic variability of oak reproduction along the gradient in situ and the genetic variability in the common garden. In the meantime, we wanted to compare this variability with other life history traits and monitor growth related traits: height and diameter increment within the same experimental design.
We show that with increasing elevation, trees produce less and smaller acorns, showing strong environmental effects likely due to temperature. On the contrary, in the common garden, populations from high elevation produce more and bigger acorns than the trees from low elevation. What we observe here is an opposition of signs between the phenotypic and genetic pattern observed along the gradient also called counter-gradient. Counter-gradients are rarely observed in plant species. Usually the phenotypic and genetic patterns display the same sign as observed for growth traits here, indicating that the value of the traits selected followed the genetic response driven by adaptation.
About the research
Our paper brings new insight about the different life history strategies between populations of a same species and encourages to investigate further reproductive traits of forest tree species in relation to other life history traits. These results will be useful for forest modellers and beyond and will help understand and predict how forest will respond to climate change.
Measuring the total reproductive effort of forest tree species is difficult. This is due to the long generation times of such organisms and to the masting pattern i.e. the synchronized and intermittent production of large amounts of seeds, characterizing several tree species such as oaks. This specific characteristic was a difficulty for us as it implied that a long term monitoring was necessary to assess the reproductive effort of an individual. This study emphasises the use of long-standing monitoring experiment in ecology and the necessity to accumulate information through time in order to better understand the complexity of forest ecosystems. I am very thankful to my co-workers and co-authors for the tremendous efforts they made, from the Pyrenees to the common garden of Toulenne, to set up and maintain this experimental design. Besides our last work published in Functional Ecology, the multiple significant findings already published thanks to the elevation gradient, show the value of such an experiment. Today, we continue our monitoring survey of tree reproduction along the gradient and in the common garden. Our objective now is to investigate the determinisms of oak masting along the gradient to better understand this phenomenon.
About The Author
I finished my PhD in 2018 and I am currently doing a post-doc at the University of Bordeaux where I try to elucidate the mechanisms underlying tree masting. Although major advances have been done during the last decade on masting ecology, this phenomenon remains a mystery on several aspects. I use approaches related to ecophysiology, functional ecology and quantitative genetics to test different hypotheses underlying this pattern. A lot is currently going on in that field and it’s exciting to be a part of this research community.
During this post-doc I have had the chance to investigate further the multiple questions developed during my PhD. I feel very lucky and proud to be part of this long term project where I can develop new research ideas thanks to unique facilities developed by my lab in Bordeaux and our research partners in Lyon, Paris and Montpelier.
Functional Ecologists 