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Fajardo A, Piper FI (2021) How to cope with drought and not die trying: drought acclimation across tree species with contrasting niche breadth. Functional Ecology, 35(9), 1903-1913. 

The intensity and frequency of drought events are increasing worldwide. These events have had a direct negative effect on the performance of trees, with massive tree mortality being the worst end result. Major research efforts have been made to elucidate whether all species are at the same risk. With most cases of tree mortality occurring in northern hemisphere conifers, we still do not know if drought-induced negative effects can be translated to other species elsewhere. Benefiting from the occurrence in 2015-2016 of the most severe El Niño drought event in the last 70 years in Patagonia, we studied how a bunch of broad- leaved species react to an unprecedent 90% precipitation deficit during the growing season. How to measure drought-induced effects in trees? In the first place, we determined stem radial growth, assuming that unhealthy trees will grow poorly. Second, we measured carbon reserves in branches to see whether trees were starving of carbon. Third, we also determined carbon and oxygen isotopes as these two together act as a proxy of how plants “sense” drought, i.e., whether plants tend to close their stomata as a first reaction to drought. We finally measured the diameter of xylem conduits (vessels that conduct water from roots to leaves) because they might acclimate to drought by narrowing their diameter, thus reducing the risk of xylem cavitation (the blockage of vessels with air bubbles, commonly observed under high tension due to drought). Across species, and contrary to previous results reporting drought responses indicative of drastic reductions in water use, our results show that all the species tended to maintain or even increase their water use (growth and vessel diameter were not reduced, and isotopes pointed to higher stomatal conductance under drought). All species also exhibited drought-induced acclimation based on starch conversion into soluble sugars, which helps plants to face drought by keeping cells and tissues with an adequate water balance. To conclude, broad-leaved species of southern Patagonia responded positively to a severe drought event, suggesting that these forests will be highly resilient to climate change.

 

Fajardo A, Martínez-Pérez C, Cervantes-Alcayde MA, & Olson, M.E. (2019) Stem length, not climate, controls vessel diameter in two tree species across a sharp precipitation gradient. New Phytologist, in press, doi:10.1111/nph.16287

In this last publication, we assessed whether climate, or stem length (tree height), is the ultimate driver of vessel diameter in plants.  To distinguish between both potential causes we benefited from the Patagonian flora; there are here two tree species, Embothrium coccineum (see picture) and Nothofagus antarctica, that have a rare wide niche breadth, particularly they prevail across a sharp precipitation gradient.

It has been traditionally thought that climate controls vessel diameter but recent evidence points to stem length as the ultimate driver of vessel diameter.  In fact, we found that for both species vessel diameter did not differ among locations with contrasting precipitation regimes.  We did find that stem length was by far the variable responsible for vessel diameter variation.

This study focusing on within-species variation (thus, avoiding noise associated with the great morphological variation across species) showed unequivocally that plant size, not climate, is the main driver of variation in vessel diameter.  Therefore, to the extent that climate selects for differing vessel diameters, it will inevitably also affect plant height.

Finally, this study was financed by my previous Fondecyt project 1160329.

 

Fajardo A, Piper FI (2019) Cluster-root bearing Proteaceae species show a competitive advantage over non-cluster root species. Annals of Botany 124:1121-1131

Historically, underground plant–plant interactions have been understood through the lens of competition for limited resources. Driven by the promising hypothesis that Proteaceae species, in relation to cluster-root (CR) nutrient-uptake facilitation, induce a positive effect on the fitness of non-cluster root-bearing (NCR) species, we established a plant interaction experiment which disproved the hypothesis. We found that Nothofagus (NCR, Nothofagus betuloides and N. pumilio) species did not improve their survival and growth in the presence of Proteaceae species (Embothrium coccineum and Gevuina avellana); in fact, they only increased their leaf Mn concentrations, a sign of nutrient-uptake facilitation that, however, did not enhance their survival and growth.

Thus, we assert that, at least for southern South American Proteaceae species, it appears all but certain that CRs do not mediate a facilitative role in community structuring. In contrast to our facilitative hypothesis, we observed that Proteaceae species improved their growth more when planted with any neighbour (i.e. any species) than when planted alone. Bolstering this result, we evidenced a significant accretion in N and P in the plant tissue of Proteaceae when planted with Nothofagus or with a conspecific, something that was mirrored in the values of nutrient concentrations found in the substrate. This increase in N and P concentrations in the substrate together with the positive effect of growing with other seedlings for Proteaceae may be explained by the rapid mineralization of the organic matter provided by the two seedlings, which, in most cases, would double that mineralized in pots with only one seedling (as all seedlings were originally of similar size). That Proteaceae species had significantly higher survival and growth when planted with conspecifics than when planted alone is congruent with other experimental evidence in the region that points to infraspecific facilitation, and is applicable to developing seedling planting methods, distinct from those currently used (i.e. seedlings planted alone instead than in clusters), for restoration efforts.