Epifyttisk belegg på barnåler i Norge i relasjon til nitrogendeposisjon og klima

Sammendrag

Utbredelsen av epifyttisk belegg på nåler av gran (Picea abies) ble undersøkt i forhold til våtdeposisjonen av nitrogen, klimatiske forhold og geografisk spredning. Forekomsten er mest utbredt i den sørlige del av landet, som også mottar størst deposisjon. Belegget består hovedsakelig av grønnalger, spesielt av arten Pleurococcus vulgaris.

Kvantifisering av det epifyttiske belegg ble utført i laboratoriet. Beregninger viste at forekomsten av epifytter var positivt korrelert til deposisjonen av nitrogen og til årsmiddeltemperatur. Epifyttene finnes i den fotosyntetiserende del av kronen på såvel yngre som eldre trær.

Utbredelsen og mengden av epifytter på grannåler som er observert i den foreliggende studien, gir grunn til å anta, at belegget ved å hemme nålenes fotosyntese har betydning for skogen i Sør-Norge.

Abstract

The occurrence of epiphytic growth on needles of Norway spruce (Picea abies) was investigated with regard to nitrogen deposition, climate and geographic distribution. Amount of epiphytes was assessed in four classes: 0=no visible growth, 1=thin growth, 2=medium growth and 3=dense growth.

In order to determine a relation between nitrogen deposition and epiphytic growth, the effect of some climatic factors were evaluated. These included mean annual temperature, normal precipitation and number of days with mean temperatures above +6°C. The epiphytic growth was dominated by green algae of the species Pleurococcus vulgaris.

The distribution and amount of epiphytic growth was investigated in young trees on branches below 2 m, along two transects in southern Norway (Fig. 1). Investigations were also performed on 12 intensive monitoring plots located in different parts of the country (Fig. l). These plots are located in mature spruce forest.

From five trees in each plot branches were systematically taken from 7th and 15th whorl. The amount of epiphytes, in young as well as old trees, was positively correlated to the deposition of nitrogen (r2=0.79, p < 0.01) and (r2=0.62, p < 0.01), respectively and to mean annual temperature (r2=0.80, p < 0.01) and (r2=0.45, p < 0.05), respectively (Fig.2).

The amount of epiphytes was quantified (Table 1). Quantification was carried out as described in the following. The needles were dried for 48 hours at 65°C and weighed. Thereafter centrifugated on a magnetic centrifuge for 3 hours, dried for 48 hours and weighed.

The weight difference between first and second weighing gives the amount of what is removed from the needles. This may be epiphytes/micro-organisms, particles and compounds extracted from the needles. This is recognised in table 1 where needles from class 0 also has lost weight. The method has so far been consistent.

A small number of needles were evaluated with regard to composition of algae species. Dominating species were Pleurococcus vulgaris, Stichoccus bacillaris and Chlorella sp. Epiphytic growth is previously found to reduce photosynthesis (Burg 1990) and weaken the needle epidermis (Pevling et al. 1992).

The distribution and amount of epiphytes on needles of Norway spruce found in this study give reason to assume that it is a serious and widespread phenomenon. The epiphytic growth is found in the photosynthetic active crown in both young and old trees. It is therefore likely that epiphytic growth will add to the weakening in the vitality of the trees.

Referanse

Thomsen, M.G. 1992. Epifyttisk belegg på barnåler i Norge i relasjon til nitrogendeposisjon og klima. Rapport fra Skogforsk 23/92: 11 s.