La phénologie des arbres caducifoliés en forêt guyanaise (5° de latitude nord) : illustration d'un déterminisme à composantes endogène et exogène

Abstract

La phbnologie des arbres caducifoliCs en for& guyanaise (So de latitude nord) : illustration d'un dbterminisme h composantes endogkne et exogkne DENIS LOUBRY I Laboratoire d'e'cologie vdgdtale, Institut franqais de la recherche scientifique pour le de'veloppement en coopiration (ORSTOM), B. P. 165, 97323 Cayenne Cddex, Gicyane franqaise Recu le 15 dCcembre 1993 LOUBRY, D. 1994. La phCnologie des arbres caducifoliCs en forCt guyanaise (5" de latitude nord) : illustration d'un dCterminisme & composantes endogkne et exogkne. Can. J. Bot. 72 : 1843 -1857. La forCt guyanaise est une foret sempervirente ombrophile qui renferme plus d'une centaine d'espkces d'arbres caducifoliCes. La distribution taxonomique du caractkre caducifolit? est Ctendue mais sa presence, voire son absence, au sein de certains taxons ainsi que son anciennetC sont des arguments en faveur de son origine endogkne. La pCriodicitC de la dCfeuillaison a Ct C CtudiCe sur un Cchantillon de 500 arbres au cours de deux annCes d'observation. Toutes espkces confondues, la pCriodicitC est annuelle et nettement saisonnikre. Elle n'est pas corrClCe aux variations pluviomCtriques et, en particulier, elle n'est pas li6e i l'existence d'une saison skche. Elle semble plus nettement like aux variations photopCriodiques, bien que celles-ci soient de faible amplitude (35 min ii 5'30' de latitude nord). Toutefois, chaque arbre se dtfeuille selon une periodicit6 qui lui est propre. I1 existe donc un paradoxe entre la saisonnalitC de la dCfeuillaison et son caractkre individuel. L'existence de ce paradoxe nous conduit & considCrer l'hypothkse d'une intCgration des composantes endogkne et exogkne dans le dCterminisme de la pCriodicitC de la dCfeuillaison. Mots cle's : arbre, dCfeuillaison, forCt tropicale humide, Guyane fran~aise, phCnologie, photopCriodisme. LOUBRY, D. 1994. La phCnologie des arbres caducifoliCs en forCt guyanaise (5" de latitude nord) : illustration d'un determinisme & composantes endogkne et exogkne. Can. J. Bot. 72: 1843-1857. The FrenchGuianan forest is an evergreen rain forest that contains approximately 100 deciduous tree species. The taxonomical distribution of the deciduous character is widely spread, and its presence or absence among some taxa, as well as its ancient origin, are arguments in favour of an endogenous origin. Leaf-fall periodicity was studied on a sample of 500 trees during a 2-year period. Periodicity is annual and seasonal. It is not correlated to rainfall and not linked to the occurrence of a dry season. It seems closely correlated to photoperiodical variations, even though those variations are weak (35 min at latitude 5'30'N). However, each tree has its own periodicity for leaves shedding. Therefore, there is a paradox between seasonality and individual periodicity of leaf fall. The existence of this paradox leads us to consider the hypothesis of an integration of endogenous and exogenous components in the determination of leaf-fall periodicity.

Interseasonal comparison of CO 2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana)

“…As a result, total canopy [CO 2 ] gradients during the wet season were about 40 mol mol )1 lower than those during the dry season. Since high decay factors have been reported for tropical forests (turnover rates 1.2-3.2 year )1 ; Whitmore 1990; Wieder and Wright 1995), and maximum litterfall in French Guiana forests generally occurs during July and August (Loubry 1994), lower soil respiration rates during the wet season were unexpected and counterintuitive. However, Wofsy et al (1988) attributed lower soil CO 2 efflux from an Amazonian forest soil during the wet season to a lower solar flux, which could reduce carbon allocation of plants to roots.…”

“…The canopy is stratified in three layers: an upper canopy formed by trees 25-35 m tall, a middle canopy at about 15-20 m, and an understory regeneration with saplings < 2 m tall. Three angiosperm families (out of 73 total) make up 60% of the rainforest in French Guiana, i.e., the Lecythidaceae (genera Eschweilera and Lecytis), the Caesalpiniaceae (genera Eperua and Dycorinia) and the Chrysobalanaceae (genera Licania and Parinari ;Favrichon 1994;Loubry 1994). The soil is a well-drained Oxisol on Precambrian bedrock.…”

Dry‐season decline in tree sapflux is correlated with leaf turgor loss point in a tropical rainforest

“…We therefore suggest that the measurement of π tlp is informative of the responses of plant water use, and hence potentially growth and whole‐plant performance, during dry seasons (Mart, Veneklaas, & Bramley, ). Some of the trees shed their leaves, in species already known to be semi‐deciduous (Loubry, ). However, at our study site, deciduousness was not directly related to leaf drought tolerance as indicated by π tlp , in contrast with drier tropical forests (Sobrado, ; Xu, Medvigy, Powers, Becknell, & Guan, ), suggesting these adaptive strategies may respond to different constraints in rainforests as found elsewhere (Wright & van Schaik, ; Wu et al, ).…”

Tropical forest phenology in French Guiana from MODIS time series

“…However, we only had a clear view of the northerly forested landscape located near populated areas along the coast for which there are both litterfall records (Sabatier and Puig 1986) and quantitative observations on growth (Loubry 1994). Field data are extremely limited for the other forested landscapes, which are still difficult to access.…”