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The Reproductive Response of a Holm Oak Forest to Long-term Drought

Paper Reviewed
Bogdziewicz, M., Fernández-Martínez, M., Espelta, J.M., Ogaya, R. and Penuelas, J. 2020. If forest fecundity resistant to drought? Results from an 18-yr rainfall-reduction experiment. New Phytologist doi: 10.111/nph.16597.

Writing as background for their study, Bogdziewicz et al. (2020) say "ecosystem services, such as mitigating the risk of avalanches, carbon storage, habitat availability and value for the economy and recreation, can suffer if reduced reproduction slows forest expansion or limits the recruitment of merchantable tree species and seed producers that support wildlife." Unfortunately, they add few studies "have compared the impacts of environmental change on the reproductive ecology of trees."

In an attempt to gain insight into this topic, the five forest researchers set out to investigate the impact of artificially-induced drought on the fecundity of three dominant tree species growing in the Prades Mountains of southern Catalonia (northeastern Spain). Since 1999, an experimental site was established in a typical Holm oak (Quercus ilex) forest accompanied by other Mediterranean tree species, including Mock privet (Phillyrea latifolia) and Strawberry tree (Arbutus unedo). A portion of rainwater was intercepted on various study plots, effectively causing long-term drought that decreased soil moisture by 10-30% over the study period, allowing the researchers to examine the effect of long-term drought on the reproduction of the above-mentioned three tree species. And what did that examination reveal?

As shown in the figure below, drought treatment had no statistical impact on fruit dry mass production in any of the three tree species. The authors also report the year-to-year variability and synchrony of reproduction were also not impacted by drought. However, drought "did affect the allocation of resources in Q. ilex and A. unedo but not the more drought-tolerant P. latifolia [such that the] production of larger numbers of fruits by both Q. ilex and A. unedo was associated with a stronger decrease in growth in the rainfall-reducing plots compared with the control plots, suggesting that these species were able to maintain their fecundity by shifting their allocation of resources away from growth [to reproduction]."

Commenting on their findings, Bogdziewicz et al. say their study results "indicated substantial resistance of tree fecundity in a Q. ilex dominated forest subjected to an average 15% (median 13%) decrease in the amount of soil moisture," which findings provide "growing evidence ... that Q. ilex dominated forests are resistant to an increase in drought to some extent, suggesting that these ecosystems may adapt to a progressive increase in arid conditions." And that is good news for the stability of these forests, especially if they ever do experience the model-based predictions of increasing drought.

Figure 1. Fruit production by (a) Quercus ilex, (b) Phillyrea latifolia and (c) Arbutus unedo in the control and drought plots. The solid lines and shaded areas are annual means and associated standard deviations, respectively. Source: Bogdziewicz et al. (2020).

Posted 22 July 2020