Long-term Experimental Drought Research Articles

Delayed and altered post-fire recovery pathways of Mediterranean shrubland under 20-year drought manipulation

Increasing water deficits and severe droughts are expected to alter the dynamics of vegetation post-disturbance recovery by decreasing new recruitment and limiting growth in semi-arid Mediterranean ecosystems in future. However, which vegetation metrics will be shifted and how they respond over time are not clear, and the experimental evidence is still limited. Here we assessed the impacts of a long-term (20 years) experimental drought (−30% rainfall) on the pathways of vegetation metrics related to species richness, community composition and abundance dynamics for an early-successional Mediterranean shrubland. The results indicate that the pathways of vegetation metrics were differently affected by experimental drought. The abundance of Globularia alypum follows pathway 1 (altered mature state). Simpson diversity and abundance of Erica multiflora follow pathway 2 (delayed succession) while species richness, community abundance and shrub abundance follow pathway 3 (alternative stable state). There were no significances for the resilience to extremely dry years (the ratio between the performance after and before severe events) between control and drought treatment for all vegetation metric. But, their resilience for the metrics (except Simpson diversity) to extremely dry years in 2016–17 were significantly lower than that of 2001 and of 2006–07, possibly caused by the severe water deficits in 2016–17 at mature successional stage. Principal component analysis (PCA) shows that the first two principal components explained 72.3 % of the variance in vegetation metrics. The first axis was mainly related to the changes in community abundance, shrub abundance and species richness while the second axis was related to Simpson diversity and abundance of G. alypum and E. multiflora. Principal component scores along PC1 between control and drought treatment were significantly decreased by long-term experimental drought, but the scores along PC2 were not affected. Further research should focus on successional pathways in more water-deficit conditions in Mediterranean ecosystems and the consequences of changes in vegetation recovery pathways on ecosystem functions such as biomass accumulation and soil properties.

Long-term experimental drought combined with natural extremes accelerate vegetation shift in a Mediterranean holm oak forest

Increasing drought combined with natural extremes are expected to accelerate forest die-off and shifts in vegetation in the Mediterranean Basin. However, fewer studies have explored these climate-driven changes in forest ecosystems.A long-term (17-year) experimental drought (−30% precipitation) was established in a Mediterranean holm oak forest with high (H) and low (L) canopies to determine the changes in stem mortality, recruitment and composition shifts.Experimental drought increased annual stem mortality rate at the community level for both H- and L-canopies. Natural drought amplified the effects of experimental drought on stem mortality at the community level and of Q. ilex for H- and L-canopies. The timescales of natural drought, however, varied substantially with canopy types and species, with shorter timescales in L- than H-canopy and for Q. ilex than P. latifolia. Furthermore, experimental drought combined with natural extremes amplified the increases in stem mortality and decreases in growth for L-canopy. Contrasting responses between Q. ilex and P. latifolia for the relative in abundance and growth were observed in L-canopy and drought treatment reinforced the vegetation shift favoring P. latifolia.These findings suggest continuous drought regimes accelerated a vegetation shift, implying potential consequences for the functions and services for water-limited forest ecosystems.

The combined effects of a long-term experimental drought and an extreme drought on the use of plant-water sources in a Mediterranean forest.

Vegetation in water-limited ecosystems relies strongly on access to deep water reserves to withstand dry periods. Most of these ecosystems have shallow soils over deep groundwater reserves. Understanding the functioning and functional plasticity of species-specific root systems and the patterns of or differences in the use of water sources under more frequent or intense droughts is therefore necessary to properly predict the responses of seasonally dry ecosystems to future climate. We used stable isotopes to investigate the seasonal patterns of water uptake by a sclerophyll forest on sloped terrain with shallow soils. We assessed the effect of a long-term experimental drought (12years) and the added impact of an extreme natural drought that produced widespread tree mortality and crown defoliation. The dominant species, Quercus ilex, Arbutus unedo and Phillyrea latifolia, all have dimorphic root systems enabling them to access different water sources in space and time. The plants extracted water mainly from the soil in the cold and wet seasons but increased their use of groundwater during the summer drought. Interestingly, the plants subjected to the long-term experimental drought shifted water uptake toward deeper (10-35cm) soil layers during the wet season and reduced groundwater uptake in summer, indicating plasticity in the functional distribution of fine roots that dampened the effect of our experimental drought over the long term. An extreme drought in 2011, however, further reduced the contribution of deep soil layers and groundwater to transpiration, which resulted in greater crown defoliation in the drought-affected plants. This study suggests that extreme droughts aggravate moderate but persistent drier conditions (simulated by our manipulation) and may lead to the depletion of water from groundwater reservoirs and weathered bedrock, threatening the preservation of these Mediterranean ecosystems in their current structures and compositions.

Foliar CO2 in a holm oak forest subjected to 15 years of climate change simulation

A long-term experimental drought to simulate future expected climatic conditions for Mediterranean forests, a 15% decrease in soil moisture for the following decades, was conducted in a holm oak forest since 1999. Net photosynthetic rate, stomatal conductance and leaf water potential were measured from 1999 to 2013 in Quercus ilex and Phillyrea latifolia, two co-dominant species of this forest. These measurements were performed in four plots, two of them received the drought treatment and the two other plots were control plots. The three studied variables decreased with increases in VPD and decreases in soil moisture in both species, but the decrease of leaf water potential during summer drought was larger in P. latifolia, whereas Q. ilex reached higher net photosynthetic rates and stomatal conductance values during rainy periods than P. latifolia.The drought treatment decreased ca. 8% the net photosynthetic rates during the overall studied period in both Q. ilex and P. latifolia, whereas there were just non-significant trends toward a decrease in leaf water potential and stomatal conductance induced by drought treatment. Future drier climate may lead to a decrease in the carbon balance of Mediterranean species, and some shrub species well resistant to drought could gain competitive advantage relative to Q. ilex, currently the dominant species of this forest.

Reproductive output in Mediterranean shrubs under climate change experimentally induced by drought and warming

Abstract The effects of climate change on plant reproductive performance affects the sequence of different plant reproductive stages from flowering to seed production and viability, as well as the network of relationships between them. These effects are expected to respond to different components of climate change, such as temperature and water availability, and may be sensitive to differences in species phenology. We used long-term experimental drought and warming treatments to study the effect of climate change on flower production, fruit and seed-set, seed size and seed germination rate (proportion of germinating seeds) in three Mediterranean shrubs coexisting in a coastal shrubland. Larger plants produced significantly more flowers in all three species, and higher fruit-set in Dorycnium pentaphyllum. Flower production was reduced in drought and warming treatments in the spring-flowering species D. pentaphyllum and Helianthemum syriacum, but not in the autumn–winter species Erica multiflora, which increased flowering in the warming treatment. However, the drought treatment eventually resulted in a decreased seed-set in E. multiflora. Structural equation modelling revealed strong correlations between the sequential reproductive stages. Specifically, flower density in inflorescences determined seed-set in H. syriacum, and seed size and germination rate in E. multiflora. Nevertheless, the relevance of relationships between reproductive traits changed between climatic treatments: in D. pentaphyllum a direct relationship between plant size and seed size only arised in the drought treatment, while in H. syriacum climate treatments resulted in a stronger relationship between the number of flowers and seed-set. This experimental study shows the ability of changing climatic variables to determine the reproductive sequential process of woody species. We show that several parameters of the reproductive performance of some Mediterranean species are affected by drought and warming treatments simulating climate change, highlighting the importance of changes in both water availability and temperature, and the sequential relationship between reproductive stages. Phenological patterns also contribute to species’ differential responses to climatic change, due to the relationship of these patterns with resource availability, environmental conditions and plant–pollinator interactions.

Dynamics of non-structural carbohydrates in three Mediterranean woody species following long-term experimental drought

Stored non-structural carbohydrates (NSC) have been proposed as a key determinant of drought resistance in plants. However, the evidence for this role is controversial, as it comes mostly from observational, short-term studies. Here, we take advantage of a long-term experimental throughfall reduction to elucidate the response of NSC to increased drought 14 years after the beginning of the treatment in three Mediterranean resprouter trees (Quercus ilex L., Arbutus unedo L. and Phillyrea latifolia L.). In addition, we selected 20 Q. ilex individuals outside the experimental plots to directly assess the relationship between defoliation and NSC at the individual level. We measured the seasonal course of NSC concentrations in leaves, branches and lignotuber in late winter, late spring, summer, and autumn 2012. Total concentrations of NSC were highest in the lignotuber for all species. In the long-term drought experiment we found significant depletion in concentrations of total NSC in treatment plots only in the lignotuber of A. unedo. At the same time, A. unedo was the only species showing a significant reduction in BAI under the drought treatment during the 14 years of the experiment. By contrast, Q. ilex just reduced stem growth only during the first 4 years of treatment and P. latifolia remained unaffected over the whole study period. However, we found a clear association between the concentrations of NSC and defoliation in Q. ilex individuals sampled outside the experimental plots, with lower total concentrations of NSC and lower proportion of starch in defoliated individuals. Taken together, our results suggest that stabilizing processes, probably at the stand level, may have been operating in the long-term to mitigate any impact of drought on NSC levels, and highlight the necessity to incorporate long-term experimental studies of plant responses to drought.