Felisa Covelo

Differences in thallus chemistry are related to species‐specific effects of biocrust‐forming lichens on soil nutrients and microbial communities

Summary 1. It is well-known that vascular plants have species-specific effects on soil properties. However, little is known on how individual species forming biocrusts, communities dominated by lichens, mosses and cyanobacteria that are prevalent in many ecosystems world-wide, affect microbial communities and soil variables related to nutrient cycling. 2. We evaluated the relationship of six biocrust-forming lichens (Buellia epipolia, Diploschistes diacapsis, Fulgensia subbracteata, Psora decipiens, Squamarina cartilaginea and Squamarina lentigera) with microbial abundance and multiple variables associated with soil nitrogen (N), carbon (C) and phosphorus (P) cycling and storage. We also evaluated whether the composition of lichen tissues (contents in C, N, P and polyphenols) is related to the C, N, P availability and microbial abundance in soils. Finally, we assessed what lichen species positively and negatively relate to soil fertility compared to bare ground areas without biocrusts. 3. We found contrasted C, N, P availability and soil microbial abundance under the different biocrust-forming lichens. Interestingly, inorganic P and amino acids were the most important factors differentiating lichen microsites. These differences in nutrient availability seem to be related to the C, N and P composition of the lichen tissues. For example, soils under D. diacapsis and P. decipiens, which had the lowest and highest C, N and P contents in their tissues, respectively, had the lowest and highest nutrient availability, respectively. We also found contrasted soil microbes abundance under the different soil lichens. For instance, F. subbracteata and D. diacapsis were negatively related to the abundance of bacteria compared to bare ground areas. 4. Our results support the idea that, as found with vascular plants, biocrust-forming lichens have species-specific effects on soil microbial communities and C, N and P cycling. Thus, continuing considering biocrusts as a unique entity will only add confusion to our knowledge of how they control nutrient availability and microbial abundance in the ecosystems where this key community is prevalent.

Spatial pattern and scale of leaf N and P resorption efficiency and proficiency in a Quercus robur population

Leaf nutrient resorption allows plants to lower their dependence on current soil nutrients, thereby influencing ecosystem-level processes such as litter decomposition and soil nutrient availability. Among different factors controlling nutrient resorption are the availability of plant resources such as nutrients, water or light. The heterogeneous spatial distribution of these resources in natural environments may influence the spatial pattern of nutrient resorption. The spatial variability of leaf N and P resorption efficiency and proficiency and their relationship to the spatial pattern of soil and light resources were evaluated by using descriptive statistics and geostatistics in a Quercus robur L. (pedunculate oak) population. Resorption efficiency and resorption proficiency were significantly higher for P than for N. Levels of N resorption proficiency indicated incomplete resorption in all individuals. However, 80% of individuals exhibited intermediate or complete levels of P resorption. Resorption efficiency and proficiency of leaf N and P showed spatial dependence at the studied scale, with the spatial distribution of P showing higher range (autocorrelation distance) than that of N. The spatial pattern and scale of nutrients in senesced leaves differed from that in green leaves, with senesced leaves having a higher spatial range but lower percentage of variance explained by distance. All soil variables and the light availability index showed spatial dependence at the examined spatial scale. However, only soil water content and extractable-P were significantly correlated with resorption proficiency and efficiency, although these relationships explained a low percentage (<5%) of total variance. The spatial dependence for resorption variables has important implications for sampling design because nearby individuals cannot be considered independent samples. It may also have implications for ecosystem-level processes related to litter quality, which might also exhibit spatial dependence.

Phenological and water-use patterns underlying maximum growing season length at the highest elevations: implications under climate change

Consequences of climate change on tree phenology are readily observable, but little is known about the variations in phenological sensitivity to drought between populations within a species. In this study, we compare the phenological sensitivity to temperature and water availability in Abies pinsapo Boiss., a drought-sensitive Mediterranean fir, across its altitudinal distribution gradient. Twig growth and needle fall were related to temperature, precipitation and plant water status on a daily scale. Stands located at the top edge of the distributional range showed the most favourable water balance, maximum growth rates and little summer defoliation. Towards higher elevations, the observed delay in budburst date due to lower spring temperatures was overcome by a stronger delay in growth cessation date due to the later onset of strong water-deficit conditions in the summer. This explains an extended growing season and the greatest mean growth at the highest elevation. Conversely, lower predawn xylem water potentials and early partial stomatal closure and growth cessation were found in low-elevation A. pinsapo trees. An earlier and higher summer peak of A. pinsapo litterfall was also observed at these water-limited sites. Our results illustrate the ecophysiological background of the ongoing altitudinal shifts reported for this relict tree species under current climatic conditions.

Biotic and abiotic factors associated with altitudinal variation in plant traits and herbivory in a dominant oak species

PREMISE OF THE STUDY It is generally thought that herbivore pressure is higher at lower elevations where climate is warmer and less seasonal, and that this has led to higher levels of plant defense investment at low elevations. However, the generality of this expectation has been called into question by recent studies. METHODS We tested for altitudinal gradients in insect leaf damage, plant defenses (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of the long-lived tree Quercus robur, and further investigated the abiotic factors associated with such gradients. We sampled 20 populations of Q. robur distributed along an altitudinal gradient spanning 35-869 m above sea level, which covered most of the altitudinal range of this species and varied substantially in abiotic conditions, plant traits, and herbivory. KEY RESULTS Univariate regressions showed that leaf herbivory, phenolics, and phosphorus increased toward higher elevations, whereas leaf nitrogen did not vary with altitude. Multiple regression analyses indicated that temperature was the single most important factor associated with herbivory and appears to be strongly associated with altitudinal variation in damage. Leaf phenolics were also correlated with herbivory, but in a manner that suggests these chemical defenses do not underlie altitudinal variation in damage. In addition, we found that variation in leaf traits (phenolics and nutrients) was in turn associated with both climatic and soil variables. CONCLUSIONS Overall, these findings suggest that altitudinal gradients in herbivory and defenses in Q. robur are uncoupled and that elevational variation in herbivory and plant traits responds mainly to abiotic factors.

Spatial pattern and variability in soil N and P availability under the influence of two dominant species in a pine forest

The presence of a legume in a nitrogen (N)-limited forest ecosystem may not only create “islands of N fertility” but also affect the phosphorus (P) availability. The main objective of this study was to compare the effect of a pine (Pinus canariensis) and a leguminous (Adenocarpus viscosus) species on the spatial pattern and variability of different labile organic-N (microbial biomass-N [MB-N] and dissolved organic-N [DON]), as well as inorganic-N (IN) and –P fractions (NH4-N, NO3-N, and PO4-P), in a forest soil of the Canary Islands (Spain). Assuming some litter quantity and quality differences between these two species, we expected to find higher soil labile organic-N concentrations under isolated individuals of P. canariensis than under isolated individuals of A. viscosus. We also expected to find higher concentrations and spatial dependence (percentage of total variance explained by spatial autocorrelation) of NO3-N beneath A. viscosus than beneath P. canariensis canopies, and higher spatial scaling of soil variables under the influence of P. canariensis canopies than under the presence of A. viscosus individuals. Moreover, we tested whether the soil variables measured under isolated individuals of both species showed a different spatial variability than the same soil variables measured under overlapping pine canopies inside a pine forest. To test these hypotheses, soil samples under isolated mature individuals of each species were collected in the winter and summer, whereas under a pine forest canopy, the sampling was performed only in the winter. The winter MB-N and DON concentrations were significantly higher beneath the pine individual, whereas the winter NO3-N, NO3-N-to-IN ratio, and PO4-P were significantly higher under the leguminous individual; these differences were not observed in the summer samples. We found higher spatial ranges under the pine than under the legume canopy in the winter sampling, and the spatial dependence of NO3-N was twice as high beneath the legumes as under the pines at both sampling dates. The soil spatial variability was higher (up to 17 times higher) under isolated individuals than inside the pine forest. The results of this study suggest that both the morphological and physiological characteristics of P. canariensis and A. viscosus, as well as the spatial pattern of P. canariensis, may influence the spatial pattern and variability of soil resources.

Latitudinal variation in plant chemical defences drives latitudinal patterns of leaf herbivory

A long-standing paradigm in ecology holds that herbivore pressure and thus plant defences increase towards lower latitudes. However, recent work has challenged this prediction where studies have found no relationship or opposite trends where herbivory or plant defences increase at higher latitudes. Here we tested for latitudinal variation in herbivory, chemical defences (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of a long-lived tree species, the English oak Quercus robur. We further investigated the underlying climatic and soil factors associated with such variation. Across 38 populations of Q. robur distributed along an 18° latitudinal gradient, covering almost the entire latitudinal and climatic range of this species, we observed strong but divergent latitudinal gradients in leaf herbivory and leaf chemical defences and nutrients. As expected, there was a negative relationship between latitude and leaf herbivory where oak populations from lower latitudes exhibited higher levels of leaf herbivory. However, counter to predictions there was a positive relationship between leaf chemical defences and latitude where populations at higher latitudes were better defended. Similarly, leaf phosphorus and nitrogen increased with latitude. Path analysis indicated a significant (negative) effect of plant chemical defences (condensed tannins) on leaf herbivory, suggesting that the latitudinal gradient in leaf herbivory was driven by an inverse gradient in defensive investment. Leaf nutrients had no independent influence on herbivory. Further, we found significant indirect effects of precipitation and soil porosity on leaf herbivory, which were mediated by plant chemical defences. These findings suggest that abiotic factors shape latitudinal variation in plant defences and that these defences in turn underlie latitudinal variation in leaf herbivory. Overall, this study contributes to a better understanding of latitudinal variation in plant–herbivore interactions by determining the identity and modus operandi of abiotic factors concurrently shaping plant defences and herbivory.

Comparing the use of leaf and soil analysis as N and P availability indices in a wildfire chronosequence

Two types of measures have traditionally been used to monitor changes after disturbances in the nutrient availability of forest ecosystems: (1) soil nutrient pools and transformation rates and (2) foliar nutrient content. We used a wildfire chronosequence in natural and unmanaged Pinus canariensis forests to determine which kind of measure is more effective in discriminating between disturbed and undisturbed plots and to determine whether the different availability indices provide comparable and consistent results within the chronosequence and between different sampling dates. The results showed that (1) foliar N and P concentrations were the variables that best discriminated between the plots of the chronosequence, (2) the various soil N availability indices neither showed steady relationships nor predicted the plant nutrient availability, and (3) P availability indices showed steady relationships and predicted plant nutrient availability. Due to the changing nature of the soil N pools, repeated sampling over a long period of time could yield results different from those presented here. However, the large sampling effort required would favor the use of foliar nutrient concentrations as the most desirable first approach to the community’s nutritional status, especially when time or budget constraints are relevant.

Temporal changes in the spatial pattern of leaf traits in a Quercus robur population

BackgroundPlant resources in natural ecosystems are frequently spatially structured at a scale relevant to individuals. This spatial structure can be variable in time, and can even vary within a single growing season. Several leaf traits may show spatial structure at the same scale as light or soil resources. However, whether this spatial structure stays constant from one growing season to the next is still unknown.MethodsWe hypothesize that the spatial structure of the limiting nutrient should be more stable over the years than the non-limiting nutrient. We also hypothesized that single leaf traits [leaf N, leaf P, and leaf mass per area ratio (LMA)] should be less spatially variable than those regulated by complex processes, such as leaf polyphenols or nutrient resorption efficiencies. We studied these different leaf traits in green and senesced young oak leaves sampled during 2000 and 2006, using a spatially explicit design.Results Leaf P, the most limiting nutrient, also showed the most stable spatial structure. Nitrogen and P resorption efficiencies had more variability than green leaf N and P respectively. Leaf polyphenols had the highest spatial and temporal variability of all studied leaf traits.Conclusions Our results suggested that the variability of a leaf trait may increase as more variables control it. The relatively constant patch size makes the space predictable, and this may have important consequences for ecosystem processes such as litter decomposition and nutrient cycling.

Interspecific variation in leaf functional and defensive traits in oak species and its underlying climatic drivers

Plants exhibit a diverse set of functional traits and ecological strategies which reflect an adaptation process to the biotic and abiotic components of the environment. The Plant Economic Spectrum organizes these traits along a continuum from conservative to acquisitive resource use strategies and shows how the abiotic environment governs a species’ position along the continuum. However, this framework does not typically account for leaf traits associated with herbivore resistance, despite fundamental metabolic links (and therefore co-variance) between resource use traits and defensive traits. Here we analyzed a suite of leaf traits associated with either resource use (specific leaf area [SLA], nutrients and water content) or defenses (phenolic compounds) for saplings of 11 species of oaks (Quercus spp.), and further investigated whether climatic variables underlie patterns of trait interspecific variation. An ordination of leaf traits revealed the primary axis of trait variation to be leaf economic spectrum traits associated with resource use (SLA, nitrogen, water content) in conjunction with a defensive trait (condensed tannins). Secondary and tertiary axes of trait variation were mainly associated with other defensive traits (lignins, flavonoids, and hydrolysable tannins). Within the primary axis we found a trade-off between resource use traits and both water content and condensed tannins; species with high SLA and leaf N values invested less in condensed tannins and viceversa. Moreover, temperature and precipitation mediated the trait space occupied by species, such that species distributed in warmer and drier climates had less leaf N, lower SLA, and more defenses (condensed tannins, lignins and flavonoids), whereas opposite values were observed for species distributed in colder and wetter climates. These results emphasize the role of abiotic controls over all-inclusive axes of trait variation and contribute to a more complete understanding of interspecific variation in plant functional strategies.