Science - USA (2021-12-17)

science.org SCIENCE

PHOTO: LAURA MEREDITH

By Nico Eisenhauer1,2 and Alexandra Weigelt1,3

C

limate and biodiversity change in-

fluence carbon, water, and green-

house gas dynamics, thereby driving

the delicate balance of ecosystems

as carbon sinks or sources ( 1 , 2 ). On

page 1514 of this issue, Werner et al.

( 3 ) show how highly controlled and large-

scale Earth science facilities can reveal the

plant physiological processes, the role of

plant-soil-atmosphere interactions, and the

dynamics of greenhouse gases and volatiles

that underlie tropical forest responses to

drought. Specifically, they used an enclosed

experimental rainforest in the Biosphere 2

facility in Oracle, Arizona ( 4 ) to explore how

drought influences carbon and water fluxes

as well as soil-plant-atmosphere interac-

tions by tracing isotopically labeled^13 CO 2

and deep-water^2 H 2 O, and volatile organic

compounds related to drought stress. The

forest ecosystem displayed highly diverse

drought responses within and among spe-

cies throughout increasing drought and re-

covery phases, reflecting differences in func-

tional plant traits, drought adaptations, and

microclimatic conditions.

Werner et al. observed that drought-tol-

erant and drought-sensitive tree species dif-

fered substantially in how they contributed

to carbon and water fluxes over the course of

the drought and rewetting periods (see the

figure). Drought-sensitive species were the

largest contributors before and after drought,

whereas the importance of drought-tolerant

species increased under severe drought.

These results suggest that different hydrau-

lic strategies are used by distinct plant func-

tional groups and that their changing pro-

portional contributions determine overall

ecosystem fluxes and plant-soil-atmosphere

interactions ( 5 ). These findings further point

to a key role of plant hydrological traits,

which may determine forest responses to

climate extremes ( 6 ), and to the effects of

tree diversity on ecosystem functioning, as

found in subtropical China ( 7 ). Given the

importance of belowground processes and

intimate plant-soil interactions, future work

should consider above-belowground trait

syndromes (i.e., associations between leaf

and root traits) ( 8 ), because root traits are

critical in carbon and water fluxes and pro-

vide a predictive framework for plants and

their environment in a changing world ( 8 ).

Werner et al. also report that drought ef-

fects cascaded through all forest strata and

changed over time. Atmospheric drought was

followed by overstory responses that propa-

gated to the understory and to the soil, from

top layers to deeper layers during late and

severe drought. The authors also observed

decreases in evapotranspiration, ecosystem

respiration, and gross primary productivity.

At the same time, the concentrations, dynam-

ics, and composition of atmospheric vola-

tiles changed substantially during drought.

During predrought conditions, the soil ab-

sorbed volatiles produced by the canopy,

but it did not function as a strong volatile

sink during severe drought. As drought pro-

gressed, several volatiles characteristic for

drought stress of trees were released into the

atmosphere. These findings stress that there

are critical thresholds of soils to buffer the

consequences of drought and underline cli-

mate change effects on plant-soil feedbacks

( 9 ). Moreover, these insights show that the

composition of volatiles can be a powerful

stress indicator as well as an important feed-

back mechanism to climate change. For ex-

ample, isoprene production (a volatile liquid

hydrocarbon synthesized by plants) indicated

the onset of reductions in evapotranspira-

tion and gross primary productivity, whereas

production of the organic volatile hexanal

denoted their final decline under severe

drought and was related to leaf senescence.

Drought-induced release of volatile organic

compounds and particles may contribute to

producing nuclei for cloud formation and

precipitation, potentially influencing the hy-

drological cycle in tropical rainforests ( 10 ).

Moreover, Werner et al. observed pronounced

legacy effects even after several months of re-

wetting after the drought, which were caused

by persistent structural changes in drought-

sensitive species (such as a loss of hydraulic

conductivity and leaves). This indicates the

potential problem of recurrent droughts in

consecutive years and may also be related to

changes in plant-soil interactions ( 11 ).

The Werner et al. study exemplifies the

potential of highly controlled enclosed fa-

cilities to explore the consequences of envi-

ronmental change and unravel insights into

the underlying mechanisms. Such facilities,

including the so-called Ecotrons, enable the

simulation of a wide range of environmental

conditions in replicated experimental units

and the comprehensive assessment of ecosys-

tem processes ( 12 ). Mechanistic experiments

on physiological responses of organisms can

improve our process-based understanding of

reciprocal effects of climate change and eco-

logical communities ( 1 ). Moreover, they en-

Biosphere 2 is a large-scale, fully enclosed Earth science

facility in Arizona, USA, that harbors a well-structured

tropical forest ecosystem with a large number

of different woody and herbaceous plant species.

(^1) German Centre for Integrative Biodiversity Research (iDiv)
Halle-Jena-Leipzig, Puschstraße 4, 04103 Leipzig, Germany.
(^2) Institute of Biology, Leipzig University, Puschstraße 4,
04103 Leipzig, Germany.^3 Institute of Biology, Leipzig
University, Johannisallee 21, 04103 Leipzig, Germany.
Email: [email protected]
ECOLOGY
Ecosystem effects of
environmental extremes
A large-scale experimental facility
reveals tropical rainforest responses to drought
PERSPECTIVES
1442 17 DECEMBER 2021 • VOL 374 ISSUE 6574