MCA: Ontogenetic constraints to climate change resilience – Investigating consequences of heteroblasty under increasing aridity in New Zealand forests

Forest trees are experiencing increased physiological stress due to rising temperatures, less predictable rainfall, and more extreme events such as extreme heatwaves, storms, and floods. Because plants change considerably in form and function as they grow from seedlings to juveniles to adults, the effects of climate change on plant survival and growth are expected to change with plant age.

This could have large consequences for the persistence of species on the landscape. For example, an unusually hot or arid climate period after a disturbance that kills adult trees (like a hurricane or a forest fire), could cause high mortality of regenerating tree seedlings thereby limiting forest recovery. Additionally, unusual climate stressors such as prolonged drought, could lead to dieback of adult trees with consequences for watershed functioning, carbon storage and wildlife habitat.

At the individual plant species scale, we do not understand how plants will tolerate climate change as they grow from juveniles to adults, particularly for long-lived plants that experience changing climates within their lifetimes. This project investigates climate change effects on several forest tree species by comparing the performance of distinct juvenile forms versus adult stage trees of several species in New Zealand forests in response to simulated droughts of two different kinds.

These forests have been reduced in area by land use change and introduced animals, and their unique biodiversity is now being threatened by climate change. Island plants may be particularly vulnerable to climate change due to their limited distributions and potentially limited overall flexibility in growth traits compared to mainland forest tree species.

This work will evaluate existing variability within each species across a natural climate gradient and evaluate how different populations respond to drought in a controlled greenhouse setting. The study will improve our understanding of drought tolerance in different life stages of these forest trees, help to identify the climate conditions that might be refugia where these species can continue to exist as both seedlings and adults as climate warms and dries, and also areas on the landscape where populations are likely to be highly susceptible to mortality because of juvenile sensitivity to climate change or adult tree death.

This information will contribute to the conservation of native island species while engaging native Pacific Island students, and Indigenous forest managers in the research process.

Plants change considerably in form and function as they grow, with the result that seedlings and juvenile trees usually differ from adult trees in leaf shape, rates of photosynthesis and water use, and in the production of spines and toxins as defense against herbivores. Such ontogenetic changes are thought to help plants survive and grow through environments that also shift as plants mature.

For example, seedlings grow near the ground where it is colder, drier, shadier, and less windy than the environment adult plants experience, and they may experience different types and intensities of herbivory. For many plant species, the shifts in environmental conditions as plants pass through developmental phases are predictable, leading to the evolution of fixed developmental changes in plant form and function, a life history known as heteroblasty.

Climate change is introducing new variability within short time scales, including more consecutive days without rain, more extreme rainfall events, and directional changes in mean annual rainfall. How these climatic changes will affect plants throughout their lifetimes, especially those with fixed ontogenetic shifts in form and function, is unclear.

This project investigates how species with fixed (homoblastic) versus flexible (heteroblastic) ontogenetic shifts in form and function perform under climate change using a combination of approaches, including field studies to measure morphological and physiological traits in seedlings and adults across different climates, and greenhouse experiments to test the degree to which plant traits are plastic under simulated climate change conditions. Many species with fixed developmental patterns occur on islands where species are particularly vulnerable to environmental change because they have limited ranges that have been reduced by historic land use change.

Hence, this project will be carried out in New Zealand where heteroblasty is common. This work will improve our understanding of climate change effects on plants as they grow up, and contribute to the conservation of native island species under the threat of global change. It will engage native Pacific Island students in research, and will work with Indigenous forest knowledge holders to exchange information on important island tree species and their potential responses to climate change.

This project is jointly funded by the Population and Community Ecology program and the Established Program to Stimulate Competitive Research (EPSCoR).

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.