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Linking abiotic and biotic stress into the net outcome of plant interactions

Final Report Summary - BIOSTRESS (Linking abiotic and biotic stress into the net outcome of plant interactions)

Environmental and socio-economic changes are increasing the level of biotic and abiotic stress on numerous ecosystems. Many of these human-induced changes are planetary-scale (global) that affect Earth ecosystems. Land-use change and anthropogenic climate change are clear examples of new sources of stress for plants and ecosystems. However, species do not occur in splendid isolation and biodiversity is mostly a result of different interactions. The grand challenge of BIOSTRESS research project is to investigate the mechanisms underlying plant interactions at increasing levels of biotic and abiotic stress to improve our understanding of how plants and ecosystems respond to current global change. In addition, BIOSTRESS aims to provide sustainable management practices to decrease the actual biodiversity loss and to restore the increasing number of degraded ecosystems.

The general objective of BIOSTRESS is to combine both sources of stress (biotic and abiotic) at increasing levels to evaluate possible shifts in the sign and strength of plant-plant and plant-animal interactions. Water deficit (abiotic) and increase of herbivore pressure (biotic) are two main consequences of the current global change. We performed field and lab experiments in highly-diverse Mediterranean oak woodlands where both sources of stress are considered important drivers of tree regeneration failure and biodiversity loss. In two consecutive and contrasting meteorological years we monitored the survival, performance and physiological parameters of stressed oak plants in the lab and in the field. In addition, essential plant-animal interactions (seed dispersal and herbivory) were studied to evaluate possible changes in the sign (mutualistic vs. antagonistic) and the effectiveness of the interactions (quantity and quality of the interaction) within the oak regeneration cycle.

Our results show the crucial role played by certain nurse shrubs in protecting tree regeneration from both sources of stress and the applicability of using natural and artificial protectors to facilitate survival and growth of young oaks and, thus, enhance tree regeneration. Large unpalatable shrubs were found to provide, overall, 11-fold greater protection for seedlings in comparison to open microsites and clearly enhanced tree growth and performance. Interestingly, plants were able to compensate herbivory damage by significantly reducing evapotranspiration at moderate and high levels of water deficit and to respond by resprouting when water stress is low. Different ecophysiological variables (e.g. secondary metabolites related to herbivore defence, stomatal conductance, chlorophyll, nitrogen, etc.) were measured to understand the response of plants to simultaneous increase of both sources of stress. This has improved our understanding of the interacting effects of multiple stressors on tree growth and physiology and has provided useful and sustainable management practices to restore degraded oak systems, including woodlands, savannas and forests by identifying the main mortality agents, the microsite suitability for plant growth and survival, and the performance and success in advancing to reproductive stages. In addition, we characterized a complex network of seed-animal interactions at a community level based on dispersal effectiveness parameters and, hence, we were able to position each single animal species in a mutualism-antagonism continuum. We further expect to show the context-dependence of this network of interactions based on the interacting effects of different sources of stress. A final step of BIOSTRESS was to describe and predict tree population dynamics using the transition probabilities obtained in the field (from seed to adult trees), including plant-plant and plant-animal interactions and, thus, reveal the main demographic bottlenecks in foundation tree species as a function of multiple stressors. This novel and integrative approach helps to better understand how ecosystems response to global change in a long-term basis and to better manage our natural systems according to the most vulnerable interactions and demographic stages. Importantly, our results highlight a differential tree recruitment in California oak savannas where young individuals of deciduous oak species (Q. lobata and Q. douglasii) are being replaced by evergreen oaks (Q. agrifolia), anticipating a possible species turnover. These findings highlight that herbivory can strongly affect the abundance and distribution of regeneration niches in tree-dominated systems and should be taken into account in future species distribution models.
Overall, the results provided by BIOSTRESS encourage managers and policy-makers to promote tree regeneration through the use of natural protectors (unpalatable shrubs), the enhancement of effective seed dispersers (e.g. tree squirrels and jay populations), the control of large herbivore populations (e.g. deer populations) and the performance of crown pruning to advance tree regeneration and, thus, reduce the synergistic effects of biotic and abiotic stressors.

Contact details: Ramon Perea. Stanford University - Universidad Politécnica de Madrid. Email:
BIOSTRESS Webpage in Stanford:
Personal webpage in UPM with reference to BIOSTRESS project:

Figure 1 (attached photo)