Plant species and soil nutrient interactions along primary successions: the role of ecological stoichiometry

Abstract

A long-standing question in plant ecology remains about what underlying mechanisms are responsible for the distribution of plant species across environmental gradients. Most plant ecologists agree that the answer lies within the concept of ‘niche’ and thus in the set of driving ‘key-resources’ for plant growth under specific environmental conditions. Here I examine two key soil resources: nitrogen (N) and phosphorus (P). I ask whether changes in the availability of N and P in soils may predict changes in critical plant element ratios (i.e. C:N:P stoichiometry). Very few studies have used an ‘ecological stoichiometric’ approach to address whether changes in soil nutrient availability and content could explain changes in plant C:N:P tissue stoichiometry along strong soil development gradients. I address this question using four ecological successions distributed across Europe where I measured C:N:P stoichiometry of roots, stems and leaves of 72 plant species. If plant nutrient stoichiometry is determined by N and P uptake in response to local growth conditions (i.e. pioneer vs. advanced soil development stages), I would expect soil and plant N:P ratios positively related along the soil chronosequence. Overall my results show that soil N:P ratios strongly increased along the gradient of soil development from pioneer to advanced successional stages. However, I found that neither changes in soil nutrient availability, nor total soil nutrient content or soil N:P stoichiometry were significantly related to changes in plant nutrient stoichiometry. Instead I found that plant functional group identity (i.e. legumes vs. grasses vs. forbs) has a strong control over changes in plant N:P ratios regardless of the temporal stage of the ecological succession. I have also found that carbon:nutrient ratios (either C:N or C:P) along primary successions are quite variable and are partly related to changes in soil nutrient status, plant functional group identity and plant compartment (e.g. roots vs. leaves). The lack of any correlation between soil and plant stoichiometry could be attributed to (1) the presence of internal mechanisms of plant nutrient and carbon use efficiency, (2) belowground eco-physiological processes (e.g. root exudation), and (3) plant-microbial interactions (e.g. symbiosis) all which can affect plant nutrients uptake and nutrient reallocation within compartments. Finally, my study shows that wild-plant species seem to have a high-degree of internal nutrient regulation (i.e. high stoichiometric homeostasis), whose underlying mechanisms need to be better understood because may be key to better understand plant distribution along environmental gradients.

Date of Award	May 2014
Original language	English
Sponsors	VCRS
Supervisor	Dario Fornara (Supervisor) & Paul McKenzie (Supervisor)