Plant eco-physiological strategies of response to drought and the future of the “Campos de Altitude”

Ilaíne Matos discusses the main findings and importance of her recent paper: Three eco‐physiological strategies of response to drought maintain the form and function of a tropical montane grassland. Find out more about the plant species of the ‘Campos de Altitude’ in Southeastern Brazil and how this research aims to help conserve them.

We invite you to climb to the top of the mountains in southeastern Brazil and to observe the “Campos de Altitude”, a peculiar vegetation that grows in this place (Fig 1). Growing in this area may not be an easy task, as plants have to deal with frosts, high winds and irradiation, rocky and acidic soils and, occasionally, fires. Water scarcity is also becoming a problem as climate and land-use changes have been altering the seasonality of precipitation and decreasing the frequency of fog events (which are an important alternative source of water in mountainous environments).

Figure 1. “Campos de Altitude” covering the top of the mountains in southeastern Brazil (Rio de Janeiro state, 22º22’37” S 44º42’28” W; 2, 400 m asl) and the 51 plant species evaluated in *our study.* *Diagram: Ilaíne Matos*

How will intensifying dry conditions affect this plant community? To answer this question, we need to (1) investigate which strategies plants exhibit to deal with drought, and (2) quantify how much each strategy contributes to maintaining community structure and ecosystem functioning. In the ‘worst-case scenario’, the strategies most sensitive to drought will contain dominant (i.e. species with high frequency/abundance), original (i.e. species with a trait-combination that differ from the rest of the community) and functionally-important species (i.e. species that have a high relative contribution to ecosystem functions, such as productivity). Thus, drought events will result in loss of biomass and functional diversity, and in a reduced capacity to provide ecosystem services. Conversely, in the ‘best-case scenario’, the strategies most sensitive to drought will encompass rare (i.e. species with low frequency/abundance) and redundant species (i.e. species with a trait-combination very similar to the rest of the community) with low contribution to functionality, so drought events will have negligible impacts.

Will droughts lead the “Campos de Altitude” to the worst- or to the best-case scenario? Let’s find out…

Identifying the eco-physiological strategies of response to drought

To identify the strategies, we measure several traits that describe the plant size, e.g. how big is a leaf (Fig 2A); plant economics, e.g. what is the cost for the plant to produce a leaf; plant hydraulics, e.g. how much water is lost through transpiration (Fig 2B); and plant regeneration (Fig 2C), e.g. whether the is plant able to regenerate after losing all its aerial biomass.

Figure 2. Example of plant traits. A. Measuring the size of the leaf size. B. Using a leaf porometer to measure how much water a plant loses through transpiration. C. Experiment to assess the plant capacity for regeneration. *Diagram: Ilaíne Matos*

After measuring such traits in 51 plant species (Fig 1), if we apply some statistical analysis, we will discover three main eco-physiological strategies co-existing in the “Campos de Altitude” (Fig 3). S-tolerant/avoiders include shrubs and herbaceous species with a conservative-resource strategy and low resprout capacity. They are the less sensitive to drought, as they can either tolerate or avoid dehydration. CR-escape/avoiders include competitor and ruderal herbaceous species with a resource-acquisitive strategy. They are the most sensitive to drought as they do not downregulate water-loss under water stress, and then, they are more prone to lose turgor during drought. Although they cannot tolerate drought, they can still avoid dehydration by absorbing water through their leaves or escape dehydration by resprouting. CS-escape/tolerant include herbs and shrubs with a mix of competitor and stress-tolerance strategies. They also exhibit intermediate capacity for regeneration.

Figure 3. Three eco-physiological strategies of response to drought. Strategies are shown in the multidimensional trait-space generated by the combination of size, economic, hydraulic and regeneration traits (SUC – leaf succulence, LA – leaf area, SS – seed size, RA – resprout ability, tlp – leaf water potential at the turgor loss point, gs max – maximum midday stomatal conductance, Fsp – fraction of the epidermis allocated to stomatal pores, FWU – foliar water uptake, SSD – stem specific density, SD – stomatal density, VD – vein density, LDMC – leaf dry matter content, slope – iso/anisohydric behaviour). Point shapes indicate the three eco-physiological strategies (S-tolerant/avoider, CS-escape/tolerant and CR-escape/avoider), sizes indicate species frequency in the study area, and colour indicates relative percentage of competitiveness (C% in red), stress-tolerance (S% in green) and ruderalism (R% in blue). Polygons delimit the three eco-physiological strategies according to clustering analysis. 12 species are highlighted: 1. Eryngium glaziovianum, 2. Machaerina ensifolia, 3. Cortaderia modesta, 4. Achyrocline satureioides, 5. Pleroma hospita, 6. Chusquea pinifolia, 7. Baccharis uncinella, 8. Chionolaena capitata, 9. Gamochaeta purpurea, 10. Hypocaeris lutea, 11. Mikania glaziovii, 12. Leptostelma maximun. *Diagram: Ilaíne Matos*

Quantifying strategies’ contribution to community structure and ecosystem functioning

After identifying the strategies, we can simulate distinct scenarios of species-loss to estimate the contribution of each strategy to originality, dominance and functionality. According to our simulations, the CS-strategy contributes more to dominance and functionality, while CR- and S-strategies contribute more to originality. Therefore, intensifying droughts may lead the “Campos de Altitude” to the worst-case scenario, where the loss of more sensitive strategies (i.e. CR- and CS-species) will greatly affect both the structure of the vegetation (decreased originality and dominance) and its capacity to provide ecosystem services (decreased productivity).

Looking into the future

This is not good news either for the plants or for the thousands of people that depend on the ecosystem services provided by the “Campos de Altitude”. So, what could be done to escape this unfortunate fate?

Our study allows us not only to predict how drought events will affect plant communities, but also to specify conservation priorities and to develop more efficient conservation practices. Our results suggest that CR-escape/avoiders should be preserved to maintain originality, while CS-escape/tolerants should be preserved to maintain dominance and functionality. As S-tolerant/avoiders can tolerate dehydration, they would not be an immediate priority for conservation, unless extreme drought events surpass their tolerance limits. Disseminating this information beyond the academia is also a good way to contribute for the conservation of the “Campos de Altitude”; and you can help with that by downloading, playing and sharing our “Functional guide of Campos de Altitude plants”. A set of 32 illustrated cards that we created to describe the eco-physiological strategies in a playful way (Fig 4).