Passive tree diversity increase after intense forest exploitation? A matter of drought-tolerant and animal-dispersed species

Miriam Selwyn discusses their latest study’s findings, conducted with colleagues. Results find ca. 30 years of passive tree species diversity increase following intense forest management release. This is largely thought to be led by animal-dispersed and higher drought tolerant species in the context of increasing temperatures and decreasing precipitations.

Why does diversity matter?

Tree species diversity is considered one of the main attributes promoting forest resilience as it increases the range of tree responses to disturbances (i.e., the insurance hypothesis by Yachi & Loreau, 1999) and provides functional complementarity fostering the efficient use of resources. Thus, numerous studies have emphasized the benefits of promoting tree species diversity to tackle the climate change crisis.

Yet, it has also been warned that increasing tree species diversity may require long time periods and may be constrained by a myriad of factors influencing recruitment, such as

land-use legacies
forest management
changes in the climatic conditions.

In addition to assessing the factors triggering changes in tree species diversity, it will also be essential to identify the functional traits of the species involved which modulate recruitment success or failure.

Who is in charge?

In our recently published article, we have analyzed ca. 30 years (1989-2016) of tree species diversity shifts in forests of Catalonia (NE Spain), considering the influence of management and environmental factors.

*Turdus spp* are important seed dispersal agents © Pixabay

Our results showed a moderate yet significant passive increase in tree species richness and diversity following the release of intense forest management. This is largely influenced by the life history traits of species. As so, animal-dispersed tree species with higher drought tolerance were the ones mostly contributing to diversity increase.

Interestingly, this process is occurring in a climate change scenario with increasing temperatures and drought events in the region. Thus, a higher presence of highly mobile and drought tolerant species may be crucial to passively increase diversity, in combination with diminishing forest exploitation intensity.

Recruitment events (%) of tree species from inside (local pool) or outside (regional pool) the plots for the tree (A) and regeneration (B) layers. Species were classified according to their dispersal mode (animal vs. other methods) and drought tolerance. Notice that the majority of recruitment events correspond to animal-dispersed and higher-drought tolerant species (green bars). Clusters represent: 1 (yellow) = animal-dispersed species with lower drought tolerance; 2 (green) = animal-dispersed species with higher drought tolerance; 3 (blue) = species with other dispersal methods with lower drought tolerance; and 4 (purple) = species with other dispersal methods with higher drought tolerance © (Selwyn et al., 2024)

Key opportunities

Increasing tree species diversity to adapt forests to climate change and human related disturbances is a primary objective extensively claimed by the scientific community. The higher recruitment of drought-tolerant species reported in our study could be crucial to enhance the resilience of NE Iberian forests under future scenarios of greater aridity.

In addition, the animal seed dispersal habit of these species may help surmount dispersal and environmental filters in forest landscapes secularly affected by intense exploitation and habitat fragmentation.

This study was supported by the wildE Horizon Europe (GAP-101081251) project.

Read the full article “Recent tree diversity increase in NE Iberian forests following intense management release: A task for animal-dispersed and drought-tolerant species” in Journal of Applied Ecology.