Ecophysiological responses of Betula pendula, Pinus uncinata and Rhododendron ferrugineum in the Catalan Pyrenees to low summer rainfall

Climate change is producing modifications in the intensity and frequency of rainfall in some regions of the planet. According to predictions, annual rainfall distribution in Western Europe will result in an increase in episodes of drought, thereby negatively affecting nutrient availability. Since high mountain systems will be particularly vulnerable, the physiological and nutritional responses to changes in summer rainfall were monitored over the course of two consecutive summers on three species, which are representative of subalpine forests: birch (Betula pendula Roth.), rhododendron (Rhododendron ferrugineum L.) and mountain pine (Pinus uncinata Mill.). Birch was especially susceptible to scarce precipitation showing alterations in leaf morphology and a decline in net photosynthesis (A) due to stomatal closure, which led to photoinhibition and to early leaf senescence as shown by the photosynthetic nitrogen-use efficiency (PNUE), carbon/nitrogen (C/N) ratio, foliar N and ¹³C isotope discrimination (Δ¹³C) results. The Δ¹³C of the soluble fraction is a good estimator of intrinsic water-use efficiency in this species. Rhododendron and mountain pine had sclerophyllous leaves, as indicated by leaf mass per area, Δ¹³C, PNUE and C/N results. Rhododendron was particularly affected by short periods of scarce rainfall, which negatively affected gas exchange and photochemistry, and reduced the remobilization of leaf N and P. Mountain pine was the most tolerant species since alterations of gas exchange, photochemistry and Δ¹³C were not observed. Its highest investment of N in RuBisCo and highest potassium, iron and magnesium leaf concentration contributed to the highest A rates observed.

Plant-soil interactions in Mediterranean forest and shrublands: impacts of climatic change

BACKGROUND

In the Mediterranean climate, plants have evolved under conditions of low soil-water and nutrient availabilities and have acquired a series of adaptive traits that, in turn exert strong feedback on soil fertility, structure, and protection. As a result, plant-soil systems constitute complex interactive webs where these adaptive traits allow plants to maximize the use of scarce resources.

SCOPE

It is necessary to review the current bibliography to highlight the most know characteristic mechanisms underlying Mediterranean plant-soil feed-backs and identify the processes that merit further research in order to reach an understanding of the plant-soil feed-backs and its capacity to cope with future global change scenarios. In this review, we characterize the functional and structural plant-soil relationships and feedbacks in Mediterranean regions. We thereafter discuss the effects of global change drivers on these complex interactions between plants and soil.

CONCLUSIONS

The large plant diversity that characterizes Mediterranean ecosystems is associated to the success of coexisting species in avoiding competition for soil resources by differential exploitation in space (soil layers) and time (year and daily). Among plant and soil traits, high foliar nutrient re-translocation and large contents of recalcitrant compounds reduce nutrient cycling. Meanwhile increased allocation of resources to roots and soil enzymes help to protect against soil erosion and to improve soil fertility and capacity to retain water. The long-term evolutionary adaptation to drought of Mediterranean plants allows them to cope with moderate increases of drought without significant losses of production and survival in some species. However, other species have proved to be more sensitive decreasing their growth and increasing their mortality under moderate rising of drought. All these increases contribute to species composition shifts. Moreover, in more xeric sites, the desertification resulting from synergic interactions among some related process such as drought increases, torrential rainfall increases and human driven disturbances is an increasing concern. A research priority now is to discern the effects of long-term increases in atmospheric CO2 concentrations, warming, and drought on soil fertility and water availability and on the structure of soil communities (e.g. shifts from bacteria to fungi) and on patching vegetation and root-water uplift (from soil to plant and from soil deep layers to soil superficial layers) roles in desertification.